Chapter 5 The inorganic constituents of lignites

218

Chapter 5

THE I N O R G A N I C CONSTITUENTS OF LIGNITES

5.1 INCORPORATION OF MAJOR ELEMENTS 5.1.1 Accumulation of inorganic components The average ash value of peat, on a dry basis, is 8.8% [1]. On the same basis, the ash value of Fort Union lignites ranges from 8 to 9% [1]. The similarity suggests that, for these lignites, most of the accumulation of inorganic constituents occurred during the formation of the precursor peat. Separated samples of anthraxylon yielded 2.5-4% ash on an as-received basis. The mean value of ash of common tree woods is 3.6% [2]. The agreement of ash values for lignitederived anthraxylon and modern wood, and the fact that most lignites have ash values well above 4%, suggest that most of the inorganic components associate with the non-anthraxylous (i.e., nonwoody) portion of the lignite. Silicified samples of lignites were found in the Wood Mountain uplands of south-central Saskatchewan [3]. The silicified plant debris consists mainly of compressed stems and grass blades. In Rosenbach (German) lignite, silicification arises from colloidal silica derived from sedimentary rocks [4]. The variation of elemental concentration as function of ash value indicates both the source and the fate of elements in the depositional system. Coals appear to be closed systems for most inorganic elements; that is, once an element enters the depositional system, it tends to stay and become incorporated in the coal [5,6]. Comparison of elemental concentrations in lignites with those in higher rank coals determines the elements for which the concept of a closed system is valid. Compared to higher rank coals, lignites are relatively enriched in Ba, B, Ca, Mg, Na, and Sr [5,7]. These elements are associated with the organic portion of the lignite and their concentrations in coals decrease as a consequence of the increase in rank. All of these elements, except boron, associate largely or wholly with carboxyl groups in lignite; loss of these groups on coalification to higher ranks removes the ion-exchange sites that are the principal means of incorporating these elements. As a cautionary note, the organic affinity of a given element may vary among coals from the eastern United States, Illinois Basin, and the western United States [8]; a generalization about the organic affinity of an element in a coal sample that has not been analyzed may be inaccurate. Data for a wide range of elements, including such chemically diverse species as Ba, Cu, Ga, Hf, Pb, Mo, K, Sc, Ag, Ta, Th, Ti, V, Y, Zr, and the rare earths, show that elemental concentration increases with ash value. This relationship indicates a detrital source for these

219 elements [5]. Calcium and strontium, on the other hand, decrease in concentration with increasing ash value. These two elements derive from the original plant material; consequently, their original concentration in the coalifying organic matter becomes "diluted" with increasing influx of detrital inorganic matter [5]. In some South Australian lignites, the ash value and mineral matter content are highest where the detrital inertinite is highest [9]. In this instance, the local depositional environment had a direct influence on the accumulation of inorganic species by the lignite, particularly the inherent mineral matter and syngenetic minerals. The organic functional groups in lignite act as traps in concentrating elements from groundwater, mainly by ion-exchange processes. In addition, some coordination or chelation occurs, as evidenced by the presence of acid-soluble forms of elements which cannot be accounted for as carbonates or other acid-soluble minerals. The association of exchangeable cations with carboxyl groups can be inferred from pKa data. Lignites contain two acidic functional groups: carboxyl and phenolic. The pKa values of the carboxyl groups range from 4 to 5, and thus they undergo ion-exchange, whereas the pKa's of phenols are in the range of 10.5-12, hence phenols more likely participate in complex formation [ 10]. Ion exchange of cations in groundwater with carboxylate groups in lignite may be affected by the specificity of cations for coordinating with carboxylate as a ligand [ 11]. If two cations occur in solution (e.g., groundwater) at equal activities, one will be preferred to the other; consequently the amount of one held in association with the carboxylate groups will be greater than the amount of the other. The ionic potential (charge to radius ratio) of the cations governs this selectivity. A cation of large ionic potential will displace one having smaller ionic potential. The practical implication is that, assuming equal activities in solution, a higher quantity of an alkaline earth cation, such as Ca+2, will be incorporated relative to an alkali cation such as Na+. If only a series of alkali metal cations, or only alkaline earth cations, are compared (so that ionic charge is not a factor) then the cations undergo exchange in order of radii, i.e., Li+
220 capacities determined in this way are about twice the amount of cations actually extracted from the lignite with ammonium acetate, then it appears that a large fraction (roughly half) of the carboxyl groups in the lignite exist in the free acid form. Electron microprobe analysis of Baukol-Noonan (North Dakota) lignite, tracking elemental concentrations across a strip of the sample, indicated that some of the iron, aluminum, and magnesium were associated with the organic portion, presumably as carboxylate salts [ 13]. The deduction is based on the existence of a uniform background signal for these elements across the sample; by contrast, elements present in discrete mineral grains would show signal spikes at locations in which the particular mineral grain was present. 5.1.2 Chemical fractionation The inorganic components of lignite occur in several forms: discrete minerals, ions bonded to carboxylate groups in the coal structure, ions associated on the ionic sites of clays, or coordinated to oxygen-, nitrogen-, or sulfur-containing functional groups in the lignite structure. Chemical fractionation is the procedure for determining the proportions of an element present in each of these modes of occurrence [ 14]. Chemical fractionation uses extraction with 1M ammonium acetate solution to remove cations present on ion-exchange sites, followed by extraction with 1M HC1. Hydrochloric acid removes elements present in acid-soluble minerals, such as carbonates and hydrous oxides, and may also remove some elements present as coordination complexes. The elements remaining after treatment with both reagents are considered to be present in insoluble minerals such as quartz or pyrite. Analysis of the extracts and of the residue at each step provides the data needed to determine the proportions of a given element present in each form. The specific details of chemical fractionation have been modified from time to time since the original publication; Fig. 5.1 is a flow sheet of chemical fractionation as it has now evolved [15]. Detailed procedures have been published [ 16]. Specific gravity fractionation in combination with chemical fractionation distinguishes between organically and inorganically associated elements. Sodium is almost completely ionexchangeable and virtually no sodium is found in the heavy fractions [14]. In comparison, potassium in the lighter fractions is ion-exchangeable, hence likely bound to carboxyl groups, while the potassium in heavy fractions is acid-insoluble, occurring in illite [ 14]. The trace elements Be, V, Y, Yb, Sc, Cr, Cu, and Zn, as well as the acid-soluble portion of the Ti, concentrate in the lower specific gravity fractions [ 14]. This behavior suggests that these elements are likely held in the lignite as coordination complexes. A portion of the aluminum showed similar behavior, suggesting that some of the aluminum in lignite might also be present as a coordination complex. Susceptibility to removal of cations by ammonium acetate treatment relates inversely to the ionic radii, within an isovalent group. The susceptibility to removal of a suite of five elements from Gascoyne (North Dakota) lignite is Na§ > Mg+2 > Ca+2 = Sr+2 > Ba+2 [17]. For the divalent elements, the percentages remaining in the lignite after three extractions with ammonium acetate are

221 coal

-325 mesh coal Vacuum dried 48h

Analysis

~ cc~al Analysis

~,M filtrate

1 extraction 100 m L H20

250C - 24 h ~ residue

Analysis

~,9 filtrate t

3 extractions 100 mL 1M NH4OAc 70~ 24 h Analysis

residue~ 2 extractions 100 m L 1M HCI

70~ - 24 h I

Analysis ~

filtrate

I

I

I

v

v_

residue

Analysis

Figure 5.1. Flow chart for chemical fractionation procedure [15].

Mg, 3%; Ca, 10%, Sr, 9%; and Ba, 50%. Removal of alkaline earth elements by extraction of three lignites with 1Mammonium acetate produced the results shown in Table 5.1, where the data show the percentage of the initial concentration of each element extracted from the lignite by the ammonium acetate solution [ 17]. Assuming that two carboxyl groups associate with each divalent cation and one with each monovalent cation, the percentage of total carboxyl groups associated with cations ranges from 43% for the Hagel (North Dakota) lignite to 54-60% for the Montana lignite [16,18]. The effect of ammonium acetate extraction on both major and trace elements in BaukolNoonan lignite is shown in Table 5.2 [19]. The twelve elements shown in Table 5.2 are those for which the greatest extractability in ammonium acetate solution was observed; data for 31 other major and trace elements are given in the original literature [19]. A comparison of the amounts of ion-exchangeable cations in lignite sampled from two pits of the same mine (Gascoyne) is provided in Table 5.3 [20]. (Additional data on these samples are shown later in this chapter.)

222 TABLE 5.1 Percentage of elements extracted by aqueous ammonium acetate [ 17]. Source of lignite State Seam N. Dakota Hagel Texas Darco Montana Fort Union

Percentage extracted C___~a M_M_g B___~a Sl" 70 60 100 78 79 62 68 82 79 84 71 74

TABLE 5.2 Percentage of elements extracted from Baukol-Noonan lignite [19]. Element % Extracted AI 10.6 Ba 20.0 Br 59.8 Ca 63.4 K 33.3 Mg 80.8

Element Mn Na Ni P Rb Sr

% Extracted 27.1 98.5 62.5 27.5 29.6 58.3

TABLE 5.3 Comparison of exchangeable cations in two Gascoyne lignite samples from different pits [20]

Element Aluminum Barium Calcium Iron Magnesium Manganese Potassium Silicon Sodium Titanium Carboxyl, meq/g

Red Pit Blue Pit Initial conc. % Removed Initial conc. % Removed lxg/g mf NH4Cg_H_3Q2 ~ / g mf N H 4 ~ Q 2 8740 0 7300 0 593 39 1268 61 17370 78 22790 85 3890 0 2540 0 2991 74 2588 75 123 34 163 39 1260 22 1430 19 28640 0 10920 1 1317 100 2694 100 1180 0 546 0 2.46

2.65

A concern in chemical fractionation is the potential solubility of various minerals in ammonium acetate solution, since cations contributed by these soluble minerals would be counted as exchangeable cations. Observed weight losses for common lignite minerals treated with 1M

223 ammonium acetate solution are shown in Table 5.4 [ 16]. Furthermore, exchangeable ions occur in clay minerals. The potential contribution of exchangeable cations from clays is shown in Table 5.5 [16]. TABLE 5.4 Weight loss of minerals treated in ammonium acetate solution [ 16]. Mineral Calcite Dolomite Gypsum Illite Kaolinite Montmorillonite Pyrite Quartz

% Weight loss 9 10

100 4 6 18 1 0

TABLE 5.5 Exchangeable cations in clays [ 16].

Clay Kaolinite Illite Montmorillonite

Exchange capacity 10-3 mol/g clay 0.02-0.10 0.13-0.42 0.80-1.50

Contribution to exchangeable cation content l_._Q0-6mol/g dmmf coal 0.50-2.50 3.25-10.5 20.0-37.5

Optical microscopy and mineralogical analysis of three lignites indicated that gypsum amounted to less than 10% of the total mineral matter [ 16]. The possible error contributed by the complete solubilization of gypsum in ammonium acetate solution for a lignite of 10% mineral matter and 1.5% actual exchangeable calcium is about 10% of the measured exchangeable calcium [16]. Since gypsum forms during lignite weathering, the amount of exchangeable calcium contributed by gypsum in unweathered lignite would be less than this estimate of 10%. The role of 0.1M hydrochloric acid in extracting elements from co3rdination complexes is not clear. The suitability of hydrochloric acid in this role depends on the stability constants of the complexes to be disrupted. Some complexes may not even dissociate at pH 1 [ 10]. Acid extraction is by no means selective for complexed metal ions, since dilute hydrochloric acid will also dissolve carbonates and some oxides and sulfides, as well as leach cations from clays. Chemical fractionation data for a high-sodium Beulah (North Dakota) lignite and its constituent lithotypes are shown in Table 5.6 [21]. Vitrains from a single layer in the Beulah-Zap seam have uniform elemental composition;

224 TABLE 5.6 Chemical fractionation data for Beulah lignite and its constituent lithotypes, ~g/g dry basis [21].

Element Na Mg A1 Si K Ca Ti Mn Fe Ni Ba

Lignite Removed by NH4OAc HCI 5412 26 1913 179 0 2323 13 423 31 166 3135 7136 0 11 13 32 5 2144 0 1 346 428

Attritus Removed by NH4OAc HCI 2685 18 1644 410 0 3064 66 1418 0 0 5434 6536 0 23 14 70 5 1659 0 1 156 668

Fusain Removed by NH4OAc HCI 3854 28 2355 312 0 2292 53 963 31 0 8041 4876 0 7 17 60 3 1613 0 2 299 451

Vitrain Removed by NH4OAc HCI 5569 80 2095 104 0 3044 42 318 31 0 7132 2747 0 3 13 37 0 1411 0 2 338 350

but samples of vitrain from different levels in the seam have different inorganic compositions, especially with regard to calcium, magnesium, and sulfur [22]. Fusain and attritus have inorganic compositions significantly different from the vitrain, particularly higher quantities of calcium, magnesium, and silicon, and lower amounts of aluminum, iron, and sulfur. Microprobe analysis of ulminite from the Beulah-Zap bed showed 1.2% S, 0.7% Ca, 0.4% Na, 0.3% Fe, 0.3% Mg and smaller amounts of Si, K, Ba, and Sr [23]. The lithotypes of Beulah lignite show strong differences in inorganic composition [24]. Vitrain contains major amounts of calcium and sulfur, significant sodium and magnesium, and lesser amounts of aluminum, silicon, potassium, strontium, and barium. Fusain also contains major amounts of calcium, but with significantly lesssodium and sulfur compared to vitrain. Attritus contains major amounts of calcium, magnesium, and sodium. Various grains of vitrain from Beulah lignite showed a consistent chemical pattern of high sulfur and calcium, moderate sodium, magnesium, and aluminum, and low silicon, potassium, iron, and strontium [25]. Several subtypes of vitrain grains have been identified on the basis of S/Ca and Mg/AI ratios. Major differences exist among ulminite macerals, both from within a particular lignite seam, as well as in different seams. The data in Table 5.7 illustrate these differences [22]. Generally, the ulminites of the Beulah-Zap have high iron and sodium and low sulfur. Hagel seam ulminites have high calcium but low sodium and sulfur. Martin Lake (Texas) ulminites have low sodium and high magnesium and sulfur. These differences reflect the effects of hydrogeochemical processes during diagenesis. Maceral samples from eastern Alabama lignite showed definite changes in inorganic composition [11]. Calcium and sulfur were homogeneously dispersed, suggesting that both elements are organically associated. Humodetrinites showed S/Ca ratios of 2. Fusinites showed S/Ca of nearly 1, but with some variations in X-ray intensity for Ca from one particle to another.

225 TABLE 5.7 Inorganic compositions of ulminites (weight percent) determined by electron microprobe analysis [22]. Lignite Beulah-Zap Hagel Martin Lake

Sample 1 2 1 2 1 2

AI 0.22 0.18 0.16 0.24 0.32 0.16

Ca 0.49 1.54 1.03 2.40 1.69 1.67

Fe 0.51 0.79 0.24 0.68 0.45 0.36

Mg 0.11 0.38 0.15 0.42 0.38 0.42

Na 0.32 0.54 0.06 0.07 0.15 0.06

S 0.45 0.43 0.59 0.51 1.25 1.16

Si 0.10 0.12 0.08 0.17 0.28 0.07

Sr 0.04 0.06 0.01 0.05 0.06

High-calcium fusinites displayed Ca intensities exceeding any of the humodetrinites. Gelinite gave the highest observed S/Ca ratio, 3, and a calcium intensity comparable to humodetrinite. Inertodetrinite gave the highest calcium intensity (about triple that of humodetrinite) but the lowest S/Ca ratio, 0.5. The content of organically bound calcium varies from one maceral to another and can be high in the inert macerals. Variations in the S/Ca ratio indicate chemical differences among macerals. A high concentration of ion-exchangeable calcium in fusinite is unusual because fusinite is usually considered to have low oxygen content [26], so it would be expected that fusinite would have few functional groups capable of acting as ion-exchange sites. (This observation relates to suggestions that lignites may contain highly reflecting materials that may be identified as fusinite or inertodetrinite, but that are also highly oxidized, thus containing large amounts of oxygen functional groups [27,28].) Chemical fractionation of two samples of Beulah lignite, a high-sodium lignite with 2.32 meq/g carboxyl, and a low-sodium lignite with 1.46 meq/g carboxyl, are shown in Tables 5.8 and 5.9, respectively [29]. Although the absolute amounts of the various elements change markedly from one lignite to the other, the percentages of each element in exchangeable, acid-soluble, or residual forms are quite similar. TABLE 5.8 Chemical fractionation of high-sodium Beulah lignite [29].

Element Aluminum Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. opm (dry) 2880 720 7770 4880 980 24 7760 4620 104

% Removed by .NH4_Q2_~O~ HCI 0 81 48 40 73 18 0 45 85 15 22 75 0 6 100 0 0 11

% in Residue 19 12 8 55 0 3 94 0 89

226 TABLE 5.9 Chemical fractionation of low-sodium Beulah lignite [29].

Element Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. oom (dry) 370 7500 10560 1480 52 7990 1380 280

% Removed by NH4_~2_~Q2 HCI 27 24 78 11 0 36 77 16 29 66 0 16 97 1 0 9

% in Residue 49 11 64 7 5 84 2 91

Additional data on the comparative behavior of high- and low-sodium samples of the same lignites (Beulah and Gascoyne) is presented in [30]. A higher percentage of barium was removed by ammonium acetate from the high-sodium lignites than from the low-sodium lignites. With hydrochloric acid, a higher percentage of aluminum was removed from the high-sodium than from the low-sodium lignites. The difference in the behavior of aluminum seems related to the presence of micaceous clays, with the aluminum removed by hydrochloric acid resulting from selective attack on gibbsite layers in the micaceous clays. However, in comparing the two pairs of lignites, no significant differences exist between the modes of occurrence of the inorganic constituents, the major differences being in the amounts of the inorganic elements present in each lignite. Chemical fractionation data for lignite from the Blue and Red pits of the Gascoyne mine are shown in Tables 5.10 and 5.11, respectively [31]. TABLE 5.10 Chemical fractionation of Gascoyne lignite, Blue pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc. % Extracted % Extracted ppm, mf by NH4OAc by HC1 7300 22.6 38.8 22790 85.1 14.3 11 0 0 2540 0 42.5 17890 98.0 2.0 25 28.0 72.0 1430 89.0 0 10920 11.6 16.7 313 94.6 5.4 546 0 57.0

% Remaining in Lignite 38.6 0.6 100 57.5 0 0 11.0 71.7 0 43.0

227 TABLE 5.11 Chemical fractionation of Gascoyne lignite, Red pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc., ppm, mf 8740 17370 10 3890 20540 18 1260 28640 276 1180

% Extracted % Extracted by NH4OAc by HCI 22.2 27.6 78.1 19.8 0 0 0 55.8 95.4 4.6 22.0 78.0 62.8 3.7 17.2 8.1 77.5 22.5 0 13.6

% Remaining in Lignite 50.2 2.1 100 44.2 0 0 33.5 74.7 0 86.4

Chemical fractionation data for Choctaw (Alabama) lignite are shown in Table 5.12 [32]. This is one of the most extensive sets of data available on chemical fractionation results for trace elements. This sample of Choctaw lignite had been washed prior to shipment, but details of the washing process are not known. TABLE 5.12 Chemical fractionation of Choctaw lignite (dry basis) [32].

Element Aluminum Antimony Barium Bromine Calcium Cesium Cobalt Europium Gold Iodine Iron Lanthanum Magnesium Manganese Molybdenum Potassium Rubidium Samarium Scandium Selenium Silicon Sodium Thorium Titanium Vanadium

Initial conc. % Removed by ppm (dry) NI-t4_~2_H~Q2_._ HC1 5907 0 37 0.87 5 54 87 80 0 6 15 0 13958 48 44 0.15 0 0 3 52 36 0.17 6 24 0.0011 36 11 0.81 0 12 14092 8 70 2 0 22 1098 34 27 112 14 85 10 23 46 746 62 0 4 0 0 0.46 4 15 3 41 0 2 4 0 11305 0 0 864 87 1 0.80 0 0 356 0 0 13 0 42

% in Residue 63 42 20 85 8 100 12 70 54 88 22 78 38 1 31 38 100 81 59 96 100 12 100 100 58

228 Chemical fractionation data for Center (North Dakota) lignite are provided in Table 5.13 [20]. These data illustrate the effect of a water leaching prior to ammonium acetate extraction. This source [20] also contains additional data on Beulah and Gascoyne (Yellow pit) lignites. TABLE 5.13 Chemical fractionation of Center lignite (moisture-free basis) [20].

Element Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~/lz mf 40 27301 19754 3747 691 11630 2329 400 138 -

v

% Removed by

H:,0

N'H4C~HH~O~

0 0 0 0 4 1 21 0 0

32 82 0 78 21 1 76 94 0

_

HCI 68 18 26 8 15 21 1 16 8

% in Residue 0 0 74 14 60 77 2 0 92

The chemical fractionation data for Indian Head (North Dakota) lignite are shown in Table 5.14 [21]. TABLE 5.14 Chemical fractionation of Indian Head lignite [21].

Element Aluminum Barium Calcium Iron Magnesium Nickel Potassium Silicon Sodium Titanium

Initial conc .... % Removed by ~t~/~z, mf NHaOAc HC1 4940 0 51 519 38 61 15560 39 12 6810 0 40 9690 19 1 5 0 18 1150 2 1 7060 1 14 6221 76 0 402 0 2

% in Residue 49 1 49 60 80 82 97 85 24 98

Chemical fractionation results for Bryan (Texas) lignite are given in Table 5.15 [33]. Additional data on Beulah lignite are available in this source [33]. Despite differences in geographical source and ash content, extraction of alkali and alkaline earth elements is reasonably consistent between the Bryan lignite data shown in Table 5.15 and Beulah lignite. Larger differences between the lignites are observed when comparing the results of the hydrochloric acid extraction. The solubility of aluminum may reflect its occurrence as a complex with humic acid structures in ways analogous to the reported behavior of aluminum in peat [34] or soil [3 5].

229 TABLE 5.15 Chemical fractionation of Bryan lignite [33].

Element Aluminum Barium Calcium Chromium Copper Iron Magnesium Manganese Nickel Potassium Silicon Sodium Strontium Sulfur Titanium

Initial conc. ~ / ~ , mf 12360 190 7130 21 24 20950 2000 300 40 1970 69400 310 80 19500 1410

% Removed by NI-Lt.OAc HC1 0 22 28 55 62 18 14 43 0 42 0 68 94 6 43 51 0 52 9 8 0 0 75 3 81 19 0 0 0 29

% in Residue 78 17 20 43 58 32 0 6 48 83 100 22 0 100 71

Data for another Texas lignite, Monticello, are given in Table 5.16 [36]. TABLE 5.16 Chemical fractionation of Monticello lignite [36].

Element Aluminum Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~,l~, mf 9110 100 7320 2200 1180 335 23600 315 130 395

HgO 0 0 0 0 0 21 0 31 0 0

% Removed by NH4OAc 0 52 77 0 70 5 0 31 72 0

HC1 5 26 13 53 4 0 1 0 11 15

% in Residue 95 22 10 47 26 74 99 18 17 85

A procedure for estimating the amount of alkali and alkaline earth oxides in ash which derived from cations associated with the organic matter defines [37] "net calcium oxide" as the amount of acid-soluble calcium not stoichiometrically equivalent to the amount of carbonate estimated from the CO2 yield, viz.,

CaOnet = CaOa.s.- 1.274 CO2

230 where the calcium oxide term on the right-hand side is determined from the acid-soluble calcium. Then the organically derived alkali and alkaline earth oxides is found from ALK = CaOnet + (MgO + Na20 + K20)a.s. where again the subscript a.s. indicates the acid-soluble amounts of the respective species. When a low-rank coal containing significant alkali or alkaline earth elements associated with the carboxyl groups is ashed, some of the oxygen in the ash will derive from the organic functional groups. The amount of this oxygen can be estimated by calculating the amount that would be associated with the exchangeable cations. Oxygen associated with ion-exchangeable elements, Oie, is found from

Ole- (Ocao)ne t + (OMgO + ONa20 + OK20)a.s.

[37] where the subscripts have the same meaning as before. 5.1.3 The major elements (i) Aluminum. A portion of the aluminum in Beulah and Savage (Montana) lignites was found to be organically bound, on the basis of electron microprobe line scans [38]. In a North Dakota lignite, approximately one-third of the aluminum was ion-exchangeable, the balance present in clays [ 11 ]. 27AI nuclear magnetic resonance may provide an opportunity for determining how aluminum is associated with the organic portion of the lignite. The data reported so far indicate the possibility of distinguishing between tetrahedral and octahedral coordination, but no specific details of the organic environment [39]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed about a third of the total aluminum to be extractable with acid, the amount increasing as the gravity of the fraction decreased [10]. This suggests some aluminum complexation by the organic matter, since the aluminum leached from clays with dilute hydrochloric acid is small, and acid-soluble aluminum minerals such as gibbsite and boehmite have specific gravities in the range of 2.8-3.0 (thus concentrating in the heavier fractions). In some Alabama lignites the acid-soluble aluminum amounts to 50% of the total [ 11]. Acid-soluble aluminum does not correlate with acid-insoluble aluminum, indicating that the aluminum dissolving in acid does not represent acid leaching of the clays. Gascoyne lignite (previously treated with ammonium acetate to remove ion-exchangeable cations) was leached with 0.1M disodium EDTA at pH's of 4.5, 5.5, and 7.0. In addition, kaolinite, montmorillonite, halloysite, and illite were also treated with this reagent. The lignite extracts contained up to ten times more aluminum than the mineral extracts [40,41], with little variation due to pH. The higher levels of aluminum removed from the lignite represent aluminum contained in some type of complex with the organic structure. (Some potassium, calcium, and iron

231 were also removed from the lignite by the EDTA procedure, but the nature of the association of these elements was not elucidated.) Aluminum is associated with detrital minerals in Alabama lignite [ 11]. Aluminum occurs in clays in the Calvert Bluff (Texas) lignites [42] and Monticello lignite [36]. An electron microprobe study of Gascoyne and Beulah lignites showed aluminum present only in combination with silica in kaolinite, illite, and mica [30]. As in the case of silicon, some acid-insoluble aluminum is present in all specific gravity fractions, but occurs in maximum amount in the 1.5-1.8 sp. gr. fraction [ 10]. This fraction also contains the highest amount of clays. The acid-insoluble aluminum is accounted for by the kaolinite content [ 11]. (ii) Calcium. Electron microprobe line scans of Beulah and Savage lignites show the calcium to be predominantly organically bound [38]. The dispersion of calcium through lignite as salts of carboxyl groups has been confirmed by extended X-ray absorption fine structure (EXAFS) spectroscopy [43--45] and X-ray absorption near-edge spectroscopy (XANES) [45,46]. Infrared spectra of humic acids extracted from Spanish lignites show calcium ions complexed with the carboxyl groups [47]. The average bond distances in the calcium sites are 0.24 nm [45]. The calcium is coordinated by six oxygen atoms. There is little long-range order around the calcium ions, suggesting that the calcium sites are separated from each other and are well dispersed throughout the samples [48]. Calcium is present in ion-exchangeable form in an Alabama lignite, and is the major exchangeable cation in these lignites [11]. Among Alabama lignites the Ca+2/Mg+2 molar ratios vary from 2.0 to 9.1 [11]. Chemical fractionation of three lignites showed calcium to be the predominant cation [ 16]. This observation was substantiated in studies on a larger number of lowrank coals; calcium represented 1-2.5% of the dry coal, and 75-95% was exchangeable with ammonium acetate [ 14]. About 90% of the calcium in the lignite of the Calvert Bluff Formation occurs in organic association [42]. The vertical distribution of ion-exchangeable calcium in Alabama lignite varies little with position in the seam [14], but correlates with variations in petrographic composition. Specific gravity fractions of Glenharold lignite show that up to 92% of the calcium was extracted into ammonium acetate [ 11 ], but the proportion of calcium soluble in ammonium acetate decreased with increasing specific gravity. Some calcium in Alabama lignite is present in carbonate minerals (calcite) [11,16] and associated with detrital minerals. Although most of the ion-exchangeable calcium is present as the counterion of carboxyl groups, some exchangeable calcium may also be present in gypsum, which is partly soluble in ammonium acetate solution. The non-exchangeable calcium in Calvert Bluff lignites [42] occurs as calcite, clays and phosphates. About 8% of the total calcium in Glenharold lignite is present as gypsum or calcite. The acid-soluble and acid-insoluble calcium may not be different forms of the element, but rather residual material soluble, but incompletely dissolved, in ammonium acetate. (iii) Iron. Some ion-exchangeable iron was observed in a North Dakota lignite [11]. Analyses of Beulah and Savage lignites by electron microprobe line suggest that a portion of the

232 iron in these samples may be organically bound, on the basis of line scans that indicate an appreciable, nearly constant background in addition to occasional spikes indicative of discrete mineral grains [38]. No iron was detected in ammonium acetate extracts of specific gravity fractions of Glenharold lignite [ 11 ]. Supporting evidence also comes from studies of iron salts ionexchanged onto lignite, in investigations of dispersed iron catalysts for direct liquefaction [49]. M6ssbauer spectroscopy of lignite exchanged with iron(II) chloride solution shows about 55% of the iron in clusters of 0-2 nm [49]. After exchange with iron(II) acetate, about 45% of the iron is in clusters >6.5 nm, and about 30% in 0-2 nm clusters. XANES indicates a coordination number of 4.8_+1 in the third shell of atoms around a central iron atom, and 7.6-2_1 in the fourth [49]. Iron soluble in 2M HCI increases with increasing specific gravity of fractions of Glenharold lignite, and may represent dissolution of goethite, jarosite, and melanterite. Some acidsoluble iron, especially in the lighter specific gravity fractions, may exist as complexes which are dissociated by acid [11]. In the Calvert Bluff lignites iron is found as siderite, as well as pyrite and marcasite [42]. Chemical fractionation of an Alabama lignite showed iron to be associated with pyrite [11]. The iron in the 1.80 sp. gr. sink fraction of Glenharold lignite is more than triple the amount calculated from the pyritic sulfur (1854 and 560 ppm, respectively). This implies a second, highspecific gravity iron mineral in the lignite, but it was not specifically identified. Iron is present in this lignite in minerals that concentrate in the heaviest specific gravity fractions [ 11 ]. Most of the iron in Beulah and San Miguel (Texas) lignites is present as pyrite, as indicated by MOssbauer spectroscopy [46]; the same technique showed 100% of the iron in a sample of Beulah-Zap lignite to be present as pyrite, with no indication of iron sulfates, iron-bearing clays, or siderite [50]. Determination of pyritic sulfur by extraction with dilute nitric acid, evaporation of the extract to dryness, dissolution of the solid in hydrochloric acid, and analysis of the resulting solution for iron and sulfur [51] allows calculation of pyritic sulfur from the equation

Spy-" 1.145(FEN- Fell) where Spy is the pyritic sulfur, FeN the percentage of iron in the nitric acid extraction, and Fe H the percentage of iron soluble in the hydrochloric acid. For most coals the sulfur determined directly as pyritic sulfur and that calculated from the so-called pyritic iron agreed quite well. For lignites, however, the difference between determined and calculated pyritic sulfur was greater than for any other coals, leading to the inference that not all of the iron in lignite was in the form of pyrite, but that a portion of the iron is present in some other form soluble in dilute HNO3. This observation was not followed up to investigate the nature of the acid-soluble iron. In Alabama lignite the acid-soluble iron correlated with the pyrite, suggesting that the acidsoluble material was oxidation products of the pyrite [11 ]. However, the correlation does not exist for Darco (Texas) lignite, suggesting that in this lignite some other form of acid-soluble iron

Occurs.

233 (iv)

Magnesium. Electron microprobe line scans of Beulah and Savage lignites suggest the

magnesium to be present in organically bound form, based on a relatively constant composition across the sample rather than sharp peaks indicative of mineral grains [38]. Chemical fractionation of three lignites showed magnesium to be the second-most dominant cation [16]. Magnesium is present primarily in ion-exchangeable form in Alabama lignite, second only to exchangeable calcium in these lignites [11]. In a North Dakota lignite the ion-exchangeable form predominated [11]. Most of the magnesium in specific gravity fractions of Glenharold lignite is present as ionexchangeable form. In some lignites magnesium is present totally in an ion-exchangeable form [16]. Electron microprobe analyses of Gascoyne and Beulah lignites showed magnesium in combination with aluminosilicates and with calcium carbonate [30]. Some magnesium in Alabama lignites also occurred in carbonate minerals and with detrital minerals [11]. The presence of magnesium in detrital minerals results from its substitution in the clay structures. The partial insolubility of magnesium and its presence in heavier specific gravity fractions was also attributed to its presence in the lattices of clays [10]. The acid-insoluble magnesium in specific gravity fractions of Glenharold lignite had a similar distribution as the acid-insoluble silicon and aluminum, suggesting that it is tightly bound in clays. In Alabama lignite the acid-insoluble magnesium correlated with the clays [ 11]. Magnesium occurs mainly with clays and micas in the lignite of the Calvert Bluff [42]. (v)

Nitrogen. The possible occurrence of inorganic nitrogen as nitrate has been a subject of

controversy. Nitrates can be extracted from lignites with water. The extraction process appears to be complex, and depends on the interactions among the ion-exchange behavior and concentrations of the various ions present in the lignite, and on the adsorption properties of the lignite itself [52]. The difficulty of water extraction of nitrates suggests some special interaction, such as preferential adsorption of nitrate on clays or encapsulation of nitrates in coalified plant structures. X-ray powder patterns of Beulah lignite include several lines characteristic of calcium nitrate, corroborating the presence of alkaline earth nitrates in water extracts of Beulah lignites [52]. Nitrate has been reported in the low-temperature ash of lignites [53,54]. Sodium and alkaline earth nitrates have been identified in the low temperature ashes of lignites, but not in lowtemperature ashes of bituminous coals or anthracites [55]. The possibility of this nitrate being mainly an artifact of the ashing process was first raised because of the inability to detect sodium nitrate in the water extracts of North Dakota lignites [.56]. Fourier transform infrared spectroscopy (FTIR) of the low-temperature ashes of lignites, washed lignites, lignite extracts, and model compounds led to the assignment of the band at 1384 cm-1 as evidence that the fixation of organic nitrogen occurs during the ashing process to produce nitrate. The formation of nitrate from organic nitrogen during low temperature ashing was demonstrated by ashing a mixture of 3-hydroxyl-6methylpyridine, sodium carbonate, and graphite. The presence of nitrate in the resulting ash was confirmed by FTIR analysis, specifically by the presence of the 1384 cm-1 peak [57]. Fixation depends on the presence of the carboxyl groups in the lignite and is affected by the type of metal

234 ion associated with the acid groups. Spectra of water extracts show that a small quantity of nitrate may be present in the parent lignite, although most of the nitrate in the ash is formed as an artifact of the ashing process [57]. (vi) Phosphorus. The average phosphorus content in five Montana lignite samples was 0.32% on a dry basis, with a range of values of 0.004% to 0.058% [58]. For eight North Dakota lignites, the comparable values were 0.009% and 0.002-0.017%, respectively [58]. The low concentration of phosphorus in most lignites, and its evident lack of significant affect on any lignite technology, likely are the reasons why the geochemistry of phosphorus in North American lignites has been studied so little. (vii) Potassium. In many lignites potassium is present both in detrital minerals and in ionexchangeable form [11,16]. EXAFS shows that some of the potassium in lignite is bonded to carboxyl groups [48]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed that ion-exchangeable potassium decreases sharply with increasing gravity, but the amount of acid-soluble potassium increased with gravity [10]. In this lignite about 44% of the total potassium was ion-exchangeable and 34% bound in minerals such as illite. Potassium is associated primarily with the detrital minerals in an Alabama lignite [ 11]. Acidinsoluble potassium in Alabama lignite correlated with the clays, suggesting that potassium is present in illite or in a mixed-layer clay [11]. Potassium also associates with clay minerals in Monticello lignite [36]. In the Calvert Bluff lignites potassium is found in clays, mica, and feldspars [42]. XANES of San Miguel and Beulah lignites shows potassium present almost entirely as illite clays [46] Potassium in Gascoyne and Beulah lignites was present in aluminosilicates, illite, mica, and potassium feldspar [30]. (viii) Silicon. Chemical fractionation of specific gravity fractions of Glenharold lignite showed some acid-insoluble silicon present in all gravity fractions [ 10]. The highest concentration of acid-insoluble silicon occurred in the 1.5-1.8 sp. gr. fraction; this fraction also contains the highest concentration of clays. The highest concentration of quartz was found in the 1.80 sp. gr. sink fraction. The acid-insoluble silicon is contained in quartz and kaolinite [11]. Silicon is associated with detrital minerals in an Alabama lignite [11]. Most of the silicon in Calvert Bluff lignites occurs in quartz and clays [42], as is also true for Monticello lignite [36]. (ix) Sodium. Sodium is the only element extracted from lignites in appreciable quantities by water leaching [59]. It was assumed to be present as the chloride, sulfate, or as water-soluble humate salts. Some data on the amounts of the total sodium extracted by water are shown in Table 5.17 [59]. Press dewatering of Falkirk (North Dakota) lignite at 1 l0 MPa expressed 20 mL of water from a 550 g sample. The water had a pH of 8.1 with sodium the dominant cation, at 3600 ppm [60--62]. (Other cations observed were Ca, at 430 ppm, and Mg, 210 ppm.) The dominant anion was sulfate (10480 ppm); carbonate was not analyzed. A similar experiment with Gascoyne lignite expressed water containing 3200 ppm Na, 510 ppm Ca, and 360 ppm Mg [63]. These water analyses, which presumably reflect the composition of water contained in the pores of the lignite,

235 TABLE 5.17 Water-extractable sodium in lignites [59]. Lignite Beulah Center Gascoyne (White) Gascoyne (Yellow)

Initial Na Content, ppm 5800 2300 5100 700

% Extracted by Water 14 21 18 42

are remarkably similar, especially in light of very significant differences in the ash value and ash composition between the lignites. Falkirk lignite had 8.2% ash containing 30.3% CaO, and supposedly 0.0% reported Na20, whereas the Gascoyne sample had 13.5% ash containing 12.4% CaO and 3.6% Na20. The complete analyses of the expressed pore water from the Falkirk and Gascoyne lignites are shown in Table 5.18 [59]. TABLE 5.18 Analyses of water expressed from Falkirk and Gascoyne lignites, ~tg/mL [62].

Aluminum Barium Boron Calcium Chloride Copper Iron

Magnesium Manganese Molybdenum Nitrate Potassium Silicon Sodium Strontium Sulfate Zinc pH

Falkirk 2 0.1 10 430 13 0.1

Gascoyne <0.3 0.1 24 510 17 0.1

<0.1

<0.1

210 0.3 0.7 4 25 19 3600 8 10500 0.1 8

360 0.7 0.7 * 26 22 3200 13 10080 0.2 7.5

*Not reported Groundwater compositions in the Kinneman Creek (North Dakota) and Hagel beds as well as sand and silt units below the Hagel show a genetic similarity among the water compositions in the various units [64]. The groundwater types range from Na to Na,Ca-HCO3 and from SO4 to SO4-HCO3 types. Distilled, demineralized water removed essentially all of the sodium from an unidentified lignite from the area of Minot, North Dakota [52]. Sodium is entirely or very largely present in ion-exchangeable form in lignites [11,16,56].

236 Electron microprobe analyses of Beulah and Savage lignites show the sodium to be organically bound [38]. 23Na nuclear magnetic resonance has shown that the sodium ion is strongly hydrated and associated with a single organic ligand [39]. The major mode of occurrence of sodium in lignite is as the salt of carboxylic acids. This sodium likely entered the lignite by ion exchange with groundwater, either in the original coal-forming environment or after burial. Distinct differences among vitrain, attritus, and fusain are shown in Table 5.19 [65]. TABLE 5.19 Sodium content of ashes from lignite lithotypes [65]. Lignite Glenharold

Baukol-Noonan

Lithotype Vitmin Vitmin Attritus Fusain Vitrain Attritus*

% Na~O 10.3 14.8 7.4 5.0 3.9 1.5

* Fusain-rich

Generally vitrain tends to contain more sodium than the other lithotypes. Since vitrain also usually has the lowest ash value, these observations provide further evidence for the organic association of sodium. Comparisons of the infrared spectra of a high-sodium vitrain and a low-sodium fusain show peaks in the spectrum of the vitrain at 1450 and 1520 cm-1, which are assigned respectively to the symmetrical and asymmetrical vibrations of the ionized carboxyl group, --COO- [65]. The observation suggests that a higher proportion of the carboxyl groups in the vitrain may be in the ionized (i.e., salt) form. Chemical fractionation of specific gravity fractions of Glenharold lignite showed a marked concentration of sodium in the lighter fractions [10]. Only about 1% of the total sodium occurred in the 1.80 sp. gr. sink fraction. About 96% of the total sodium was extractable in ammonium acetate, but for a series of specific gravity fractions the proportion soluble in ammonium acetate decreased as specific gravity increased [11]. Nevertheless, only about 2% of the sodium is acidinsoluble, and, of that, most was in the heaviest specific gravity fraction. The observation that 96% of the sodium is extracted into ammonium acetate is consistent with the virtual absence of sodiumcontaining minerals (except for traces of plagioclase) from this lignite. Small amounts of sodium (generally less than 5% of the total sodium) were observed by scanning electron microscopy to be in association with aluminosilicates in Gascoyne and Beulah lignites [30]. Sodium in mineral matter occurs principally in association with clays, principally smectites, although some may be bound in tectosilicates such as albite, NaAISi308, or analcime, NaAISi206.H20. In clays, sodium is predominantly associated with ion-exchange sites, so its concentration in the clay is affected by the composition of groundwater. In tectosilicates, sodium is

237 much more tightly held so is not affected by groundwater. Sodium in tectosilicates arrived in the coal-forming environment as part of detrital tectosilicate particles. (x)

Sulfur. Electron microprobe analyses of Beulah and Savage lignites show that the

sulfur is largely well dispersed through the lignite particles, on the basis of line scans taken across the samples [38]. Unfortunately the data were not correlated with the relative amounts of organic and pyritic sulfur in the samples. Data on the forms of sulfur in a suite of North Dakota lignites are presented in Table 5.20 [66]. TABLE 5.20 Forms of sulfur in North Dakota lignites [66]. Lignite Gascoyne Dunn County Beulah-Zap Center Stanton Dickinson Velva Williams County

Sulfatic 0.04 0.18 0.05 0.02 0.01 0.08 0.02 0.32

F'yritic 0.71 0.38 0.72 0.21 0.04 4.73 0.11 2.82

Organic 1.29 1.01 0.42 0.47 0.65 0.78 0.34 0.66

Distribution of sulfur forms in Dakota Star (North Dakota) lignite is shown in Table 5.21, on an asanalyzed basis, to illustrate the variability among different samples from the same mine [67]. TABLE 5.21 Variation of sulfur forms for lignite samples from the same mine [67]. Sample 1 2 3

Total,% 0.99 1.00 0.88

Sulfides

Pyritic, % 0.28 0.38 0.42

Sulfatic, % 0.38 0.34 0.08

Organic, % 0.32 0.32 0.27

(i.e., as distinct from pyrite) were not found in the untreated lignite. Sulfatic sulfur is a

minor component of the sulfur in Texas lignites [68]. Retention of sulfur during ashing complicates both the determination of ash and of sulfur. German lignites heated with the Eschka mixture (a 2:1 mixture of magnesium carbonate or oxide and sodium oxide) lost both organic and inorganic sulfur compounds due to smoldering during heating [69]. Uniform heating in an electric muffle furnace eliminated the problem. Only the sodium carbonate in the Eschka mixture was active in retaining sulfur, and indeed accurate sulfur determinations could be performed using sodium carbonate alone with slightly modified technique.

238 Magnesium oxide in the Eschka mixture appears simply to be helping provide a more uniform melt. Beulah lignite contains 128+4 ~tg of elemental sulfur per gram of lignite (dry basis) [70]. The quantity represents about 1.6% of the total sulfur in the sample, or 2.5% of the organic and 3.7% of the inorganic sulfur. (xi) Titanium. Titanium has been found to be present in organic complexes in lignites. Acidsoluble titanium, presumed to be present as a coordination complex, was enriched at the seam margins. This enrichment is the basis for presuming that at least this form of titanium came in to the lignite in solution and was trapped at the seam margins by reaction with the organic functional groups in the lignite [ 11]. Principal components analysis for data from Alabama lignites has shown that acid-soluble and acid-insoluble titanium both load on the same factor, suggesting that the acidsoluble titanium arose from alteration of ilmenite [ 11 ]. Elemental associations in five lignite drill cores showed that 30-50% of the titanium was extractable by dilute acid [ 11]. This extractable portion of the titanium was inferred to be incorporated in an organic complex. A major portion of the titanium was acid-soluble in four low-rank coals, including Texas, Alabama, and North Dakota lignites [ 14]. The acid-soluble titanium concentration peaks at seam margins, but is low or even zero in floor and roof rocks. In contrast, acid-insoluble titanium also shows peak concentrations at seam margins, but has high concentrations in the floor and roof as well, further substantiating the idea that acid-soluble titanium has been complexed by the organic portion of the lignite. Specific gravity fractions of Glenharold lignite showed no ion-exchangeable or acid-soluble titanium [10]. Common titanium minerals--rutile, brookite, anatase, and ilmenite--are all insoluble in dilute hydrochloric acid. However, the amount of titanium increased with decreasing specific gravity of the fraction, which could not be the case if the principal source of titanium were one or more of these minerals. The inverse relationship of acid-insoluble titanium concentration was attributed to the existence of a stable titanium complex with the organic matter. A very small fraction of the titanium in Texas lignite (Zavala County) can be extracted into various organic solvents [71]. The amount ranges from 0.03% of the total titanium extracted into dioxane to 0.47% taken into dimethyl sulfoxide. The fraction present in ion-exchangeable form is less than 0.2% of the total, suggesting that only a very small portion of the titanium is present as carboxylate salts. When this lignite is separated into density fractions, the titanium concentrations increase with increasing density, such that 93% of the titanium is present (cumulatively) in the fractions of density > 1.86. These observations suggest that titanium is associated with the mineral matter. Titanium is associated with detrital minerals in Alabama lignite [11]. Titanium can replace aluminum in aluminosilicate minerals, and was observed combined with aluminosilicates in Beulah and Gascoyne lignites, though it was thought that the association may have arisen from rutile intrusions [30]. Titanium is found in rutile and anatase in the lignite of the Calvert Bluff [42], and in rutile in Monticello lignite [36]. Titanium occurs as elongated grains of TiO2 less than 10 ~tm long, and associated with quartz and other minerals. A significant amount of the titanium was

239 associated with the aluminosilicates in the low temperature ash, associated with the aluminosilicates as finely dispersed TiO2. This finely dispersed TiO2 represents anatase produced authigenically, while the discrete grains represent detrital rutile. 5.2 MINERALS IN LIGNITES

5.2.1 Introductory overview Float-sink separation of fourteen low-rank coals--lignite and subbituminous--using 50x140 mesh material in carbon tetrachloride (specific gravity 1.55) allows identification of minerals by optical microscopic point count analysis of the sink fraction. Commonly occurring minerals were quartz, calcite, kaolinite, pyrite and gypsum; minor constituents included illite, montmorillonite, jarosite, hornblende, muscovite, barite, oligoclase, andesine, albite, orthoclase, dolomite, aragonite, hematite, biotite, szomolnokite, and marcasite [72]. The relationship of minerals in lignites of the Sentinel Butte Formation (North Dakota) lignite to the minerals in overburden and underclay is illustrated by the data in Table 5.22 [73,74]. TABLE 5.22 Minerals in overburden, lignite, and underclay of Sentinel Butte Formation [73,7"

Alkali feldspars Augite Barite Biotite Calcite/dolomite Chlorite Gypsum Hematite Hornblende Illite Kaolinite Montmorillonite Plagioclase Pyrite Quartz Volcanic glass

Overburden Common Minor Common Common Minor Common Common Abundant Abundant Abundant Minor

Lignite Minor Minor Minor Minor Minor Abundant Common Minor Abundant Minor Minor Common Abundant

Underclay Minor

Minor

Minor Abundant Abundant Minor Abundant

The major detrital minerals include montmorillonite, quartz, plagioclase, alkali feldspar, biotite, chlorite, and volcanic glass. Pyrite, gypsum, hematite, siderite, and possibly calcite formed via post-depositional processes. Minerals observed most frequently in Beulah lignite (using scanning electron microscopy, SEM) were quartz, clays (particularly kaolinite, montmorillonite, and illite) and pyrite [75]. Less common minerals were magnesium silicates (talc), rutile, hematite, and various carbonates. Rare occurrences included zircon, feldspars, barite, and gypsum. Minerals observed to occur in four lignites, on the basis of SEM examination [5], are listed in Table 5.23.

240

TABLE 5.23 Minerals observed in four lignite samples by scanning electron microscopy [5]. Anatase Apailte Barite Calcite Ca-Montmorillonite Cassiterite Celestite Chalcopyrite

Clausthalite Crandallite Diaspore Epidote Feldspars Galena Gypsum Illite

Ilmenite Kaolinite Marcasite Micas Mixed layer clays Monazite Pyrite Quartz

Rutile Siderite Sphalerite Sphene Tourmaline Xenotime Zircon

Lignites are notoriously difficult to decompose during low-temperature ashing. Suggested reasons include the possibility of surface water destroying peroxides which are responsible for oxidation of the organic structure, or the possible blocking of reactive sites by water molecules [76]. Identification of minerals in the low-temperature ash can be performed by combining X-ray diffraction and infrared spectroscopy. However, the bassanite and calcite X-ray peaks, at 29.8 ~ and 29.4* 2 0 respectively, overlap. This problem can be remedied by heating the low-temperature ash to 500 ~ to convert bassanite to anhydrite. Quartz, calcite, and pyrite can be determined by X-ray diffraction; kaolinite and anhydrite by infrared spectroscopy. Other clay minerals (e.g., illite and montmorillonite) can be estimated by a normative calculation. The amounts of SiO2 and A1203 in kaolinite and quartz are calculated and subtracted from the total amounts; the balance remaining can then be apportioned among the other clay minerals by presuming that they contain 25% A1203 and 50% SiO2. Even after the normative determination of these so-called "other clays," there may still remain a portion of the low-temperature ash unaccounted for. Results of applying this procedure to three lignites are shown in Table 5.24 [ 16,17]. TABLE 5.24 Mineral matter in low-temperature ashes [ 16,17]. State Seam LTA (% of dry coal) Kaolinite Quartz Pyrite Calcite Anhydrite Other clays Unaccounted for

N. Dakota Hagel 11.5 5 9 * 11 21 17 37

Texas Darco 20.5 41 12 * 2 14 7 14

*Pyrite was detected in amounts too small to quantify.

Montana Fort Union 17.5 20 19 * 16 l0 8 27

241 Qualitative analysis of the low-temperature ash of Baukol-Noonan lignite showed the following minerals: kaolinite, illite, chlorite, calcite, coquimbite, barite, bassanite, quartz, and plagioclase [ 19]. Computer-controlled scanning electron microscopy analysis of Beulah lignite indicated the minerals listed in Table 5.25, shown as a percentage of the total minerals in the lignite [77]. The total mineral content was 4.8%. TABLE 5.25 Minerals identified in Beulah lignite by computer-controlled scanning electron microscopy [77]. Quartz Iron oxide Aluminosilicates Ca-aluminosilicates Fe-aluminosilicates K-aluminosilicates Pyrite

17.5% 1.6 40.8 0.2 0.1 0.9 27.5

Gypsum Barite Calcite Rutile Pyrrhotite Si-rich minerals Unknown

1.6% 0.9 0.1 0.3 0.7 0.4 6.7

Minerals identified in a sample of Scranton (North Dakota) lignite by Fourier transform infrared analysis of the low-temperature ash are shown in Table 5.26 [78], The data are shown as weight percent of the low-temperature ash. TABLE 5.26 Minerals identified in Scranton lignite by FrlR of low-temperature ash [78]. Kaolin Mica Illite Mixed layer clays Montmorillonite Feldspar Chlorite Miscellaneous clays

1.1-2.9% 0 0.0-1.2 0 15.5-20.4 0-13.5 0 0.0-2.7

Quartz Fe sulfide Fe oxide Fe sulfate Siderite Calcite Gypsum Dolomite

15.5-20.9% 4.8-13.1 0.0-0.3 0 2.4-5.8 0 34.4-49.2 0

A survey of 86 coals was used to assess the potential of calculating the distribution of minerals from the high temperature (i.e., 750"C) ash composition, the results being compared with mineralogical analysis by X-ray diffraction and infrared spectrometry on the low-temperature ash [52]. It was concluded that the process is not applicable to lignites (or, for that matter, to subbituminous coals) because 30 to 50% of the ash-forming constituents are present as cations associated with carboxyl groups or are present as coordination complexes. In the discussion which follows, the order of treatment of mineral families follows standard

242 mineralogical texts (e.g., [79].) 5.2.2 Sulfide minerals (i) Pyrite. Pyrite is the predominant sulfide mineral in lignites. Pyrite contents are low in North American lignites [16], relative to coals of higher rank. Pyrite begins forming syngenetically as small framboids during the peat stage of coal formation [80,81]. The framboids occur as isolated blebs and do not follow cracks or fissures. Epigenetic pyrite mineralization occurs after the coal has been formed, and is brought about by ironcontaining solutions passing through the coal. Some evidence for epigenetic pyrite mineralization is evident as pyrite deposited in woody tissue in the lignite [31,75]. Epigenetic pyrite is concentrated in the basal sections of the seam, is massive in form, and follows cracks and fissures in the lignite [75]. Pyrite nodules in the 6 to 13 ~tm size range have been observed in Beulah lignite; these nodules had grown between layers in the lignite [82]. Massive epigenetic pyrite occurs also in Ravenscrag lignite [83]. Pyrite occurs in Fort Union lignites as material deposited in cracks or cleats. The size of pyrite particles varies widely, from microscopic crystals to nodules of more than 2.5 cm diameter [1]. The pyrite originates from bacterial action on solutions containing iron and sulfur. Sulfur balls, which contain pyrite, gypsum, and lignite, range from 2.5 to over 15 cm in diameter [1]. The Fort Union lignites developed primarily in fresh water environments and, consequently, contain relatively small amounts of pyrite [84]. Pyrite is an authigenic component of the lignites of the Sentinel Butte Formation because it is not present in the overburden or underclay (cf. Table 5.22) [73]. Pyrite was frequently observed in the 1.55 sp. gr. sink fractions Beulah, Glenharold, Savage, and Gascoyne lignites [10,85-88]. Nearly all of the pyrite observed (by scanning electron microscopy with electron microprobe, SEM-EDX) was syngenetic [80,81], seen as small rounded blebs or isolated framboids throughout the seam. Similarly, most of the pyrite in Hat Creek (British Columbia) lignites is syngenetic [89]. Pyrite was found primarily as 20-30 ~m diameter framboids in the North Dakota lignites [29,31,88]. Most of the framboids are well dispersed through the lignite, so it is likely that framboidal pyrite is authigenic. Epigenetic pyrite occurring in cracks, crevices, and cell cavities exists in both microscopic (30-300 ~tm) and megascopic (10-50 mm) sizes. Some bands of massive pyrite occur in basal sections of the Beulah-Zap seam, where the pyrite follows the woody structure of the lignite [31]. Pyrite was most commonly found associated with vitrain, as cell fillings or 20-60 lxm diameter framboids. In vitrain, pyrite is predominant among the Minerals, accounting for about 60% of the total Minerals [90]. (The capitalized terms Minerals and Inorganics follow Australian brown coal nomenclature, e.g., [91].) Pyrite in vitrain is framboidal and arose syngenetically in the peat or early diagenesis stage [80]. In fusain, epigenetic pyrite is found filling cell cavities, and formed from aqueous solutions migrating through the relatively porous fusain [90]. Pyrite is observed filling cell voids in fusinite in Saskatchewan lignites [92], and, in Moose River Basin (Ontario) lignite, not only infilling cell lumens but also replacing cell

243 walls and infilling cracks [93]. Pyrite distributes in the Beulah lignite fairly evenly throughout the seam [75,88], with occasional spikes of concentration, as, in this case, a very high frequency of pyrite 1 m above the base of the seam [75]. In samples collected from the Freedom (North Dakota) mine, pyrite in the uppermost lithologic layer was found as small framboidal masses about 50 ~tm in diameter [41]. This pyrite may be the result of microbial activity [41]. Near the base of the seam, pyrite is found as fillings in cell cavities and fissures, suggesting possible precipitation from groundwater. Pyrite was a major mineral in the 1.80 sp. gr. fraction of Glenharold lignite [11]. In this sample, pyrite occurred predominantly as impregnations in the lignite. Many of the lignite particles impregnated with pyrite were coated with melanterite (FeSO4"7H20), formed by weathering of the pyrite. The melanterite may be an artifact of the sampling process, since it was observed to form rapidly in the laboratory. In comparison, jarosite and goethite likely arose from oxidation of the pyrite in situ. The Hagel seam lignite at the Baukol-Noonan mine is characterized by abundant, thick vitrain lenses which derive from coalified logs and stumps of conifers. Often the tree rings in these coalified pieces are characterized by pyrite mineralization. The pyrite usually associates with wood or ray cells. Iron sulfides can generally occur as joint fillings, nodules, or irregularly shaped bodies in the seam. On a microscopic level some iron sulfides are observed in association with medullary ray cells and summer wood of conifers [94]. Infilling of medullary ray cells by pyrite may represent a significant fraction of the total pyrite accumulated in a lignite seam. Pyrite occurs in less than half the samples of Moose River Basin lignite [93]. In pyrite-rich samples of this lignite, the pyrite may replace the whole wood structure, infill cell cavities, coat organic surfaces, or occur in cracks. Pyrite is commonly associated with gypsum. Authigenic pyrite occurs as syngenetic deposits replacing the original plant structure by infilling cell cavities. Pyrite may also occur as infillings of shrinkage cracks or fractures, in which case there is no relationship to cell morphology or structure of the organic portion of the lignite. In such instances the pyrite is rather coarse grained compared to syngenetic pyrite. The Sabine River--Stone Coal Bluff lignites of Louisiana contain abundant pyrite in the form of nodules and pyritized wood. A pyritized log 90 cm long and 30 cm in diameter was found in this seam [95]. (ii) Pyrrhotite. Pyrrhotite has been identified by SEM-EDX analysis in Martin Lake lignite [96]. This mineral generally occurred in the vicinity of pyrite and was found as lenticular masses, spherical nodules, or framboids. An unusual occurrence was a framboid having a pyrite core rimmed with pyrrhotite. (iii) Minor sulfides and selenides. Millerite was reported in isolated samples of Moose River Basin lignite [93]. Sulfide or selenide minerals tentatively identified in Calvert Bluff lignites included chalcopyrite, clausthalite, galena, marcasite, and sphalerite [42]. Greigite, Fe3S4, sometimes called the thiospinel of iron, occurs as a trace component in one of nineteen samples of Poplar River (Hart seam, Saskatchewan) lignite [92].

244 5.2.3 Oxide minerals Although quartz is of course an oxide, it is discussed below in the subsection on silicate minerals. (i) Hematite and magnetite. Hematite was observed in the 1.55 sp. gr. sink fractions of Beulah and Velva (North Dakota) lignites [85,86]. It was not found in comparable samples from four other Northern Great Plains lignites. Thin plates of hematite, about 15 ~tm diameter, were found as a minor constituent of Beulah-Zap lignite [29,31]. Some hematite particles were seen lining pores and cell cavities, an occurrence which suggests an authigenic origin. Hematite occurs in significant quantities in Beulah lignite [88]. Magnetite occurs with high frequency in the CC14 sink fraction of Beulah lignite [86]. (ii) Gibbsite and boehmite. The presence of gibbsite in Moose River Basin lignite was inferred from unusually high values of AI:Si (8:1) determined in the X-ray photoelectron spectroscopic examination of some samples [93]. Gibbsite may be an indicator of intense weathering. Gibbsite is found only in isolated samples, associated with quartz [93]. It appears to be an authigenic coating on the quartz grain surface [93]. Boehmite has been reported in some samples of Estevan (Saskatchewan) lignite, associated with the calcium oxalate mineral weddellite in six of seventeen samples analyzed, but at appreciable amounts (4-6% of ash) in only three [83,92]. (iii) Rutile and anatase. Small amounts of rutile and rutilated quartz were observed as weathered grains 5-25 l~m diameter in Beulah-Zap lignite [29,31]. The rutile grains are probably heavy mineral detritus. Rutile has been reported as a trace mineral in some Beulah-Zap lignite samples [87] and as a significant occurrence in others [88]. Rutile usually occurs as an accessory mineral in detrital mineral matter [88]. Needle-shaped crystals of rutile (and possibly anatase) were observed in Texas lignite samples obtained from drill cores in four Texas counties [97]. Rutile was observed in the Calvert Bluff lignites of Texas [42]. Discrete grains of detrital rutile, of about 10 ~tm size, have been observed in Wilcox Group lignite from Zavala County, Texas [71]. Anatase was observed in Alabama lignite [11]. Its formation may be due to the removal of iron from ilmenite (which, for example, has been observed in the roof of Darco lignite) by leaching with mildly acidic water, leaving a hydrated titanium dioxide which is then converted to anatase. Anatase was tentatively identified in a Texas lignite from the Calvert Bluff formation [42]. Finely disseminated particles of authigenic anatase are associated with the clays and biogenic structures in Zavala County lignite [71]. Anatase occurs infrequently in Saskatchewan lignites. In seventeen samples of Estevan lignite, anatase was observed as a trace component of the ash in one sample, and at 7% in a second [83,92]. It was also observed at 7% of the ash in one of thirteen samples from the Ferris seam outcrop [92]. (iv) Zircon. Zircon is a rare occurrence in Beulah lignite [88]. Small (10-25 0m) weathered grains of zircon were observed in Beulah-Zap lignite [29], including a 17 ~tm particle that still retained its evident crystal morphology [31]. Zircon is well known to have a high-temperature igneous origin [98]. Euhedral zircon crystals are found as a component of volcanic ash in coal

245 partings, and it is usually associated with sedimentary rocks as a heavy mineral detrital constituent. Thus the zircon observed in the Beulah-Zap lignite is almost certainly detrital. Zircon was tentatively identified in the lignite of the Calvert Bluff [42]. Zircon has also been observed in Tertiary brown coals of the Weisse Elster Basin in Germany [99]. 5.2.4 Carbonate minerals (i) Calcite. Calcite is the dominant carbonate mineral in most North American lignites [100]. For example, it constitutes 10--48% of the ash of Estevan lignites, and 4--42% of the ash of Poplar River lignites [92]. Exceptions exist, however. Thus, no calcite was observed in the lowtemperature ash of Scranton lignite [78], nor was it detected in a sample of Beulah-Zap [ 101] or in most samples of Moose River Basin lignites [93]. Calcite occurs in Fort Union lignites in cracks or cleats in the lignite. It originated by precipitation from solution after the lignite had been formed [1]. Carbonate minerals in Beulah lignite are authigenic because they can be seen lining cracks in the lignite or in cavities and pores in the carbonaceous structure [31,75,88]. Similarly, calcite, when it occurs, infills cell lumens in Moose River Basin lignite [93]. Calcite is the predominant mineral phase in the 1.55 sp. gr. sink fraction of Baukol-Noonan lignite, and also occurs in Glenharold lignite [85]. It was not observed in four other lignites in the same study. Large calcite nodules are present in Freedom lignite [ 102]. Calcite occurs as plates 25-40 gm in diameter in crevices or cracks in Beulah lignite, and in massive patches on the surface of the lignite [88]. Calcite was observed in a lignite of the Calvert Bluff formation [42]. Small amounts of calcite occur in Moose River Basin lignites, in isolated samples [93]. Detrital calcite and dolomite were observed in Glenharold lignite [10]. Calcite and dolomite both occur with low frequency in the CC14 sink fraction of Beulah lignite, and with medium frequency in the same fraction of Savage lignite [86]. Calcite, magnesite, and dolomite particles of 25-40 ~m diameter were found as crevice or cleat growths in Beulah-Zap lignite [29,31]. The textural form suggests an authigenic origin. Trace amounts of aragonite occur in samples of Estevan lignite [83,92]. (ii) Dolomite. Dolomite was found as a major constituent of the low-temperature ash of a Montana lignite [103]. Large lenticular inclusions of dolomite have been observed in European lignites, with MgO contents of 12-38% and CaO/MgO ratios of 1.30-1.60 [104]. Calcite, magnesite, and dolomite occur in small amounts in Beulah lignite. Magnesite and dolomite occur mainly as crevice or cleat growths [88]. Dolomite is seen in only a few samples of Saskatchewan lignites, but in those cases it accounts for a large percentage of the ash, e.g., 28% of the ash in one of seventeen samples of Estevan lignite, and 50% of the ash in one of nineteen samples of Poplar River lignite [83,92]. Dolomite is also observed in some Hat Creek lignites [89]. (iii) Siderite. Siderite occurs as a trace component of Moose River Basin lignite, being found in isolated samples [93]. It also occurs in a few samples of Estevan and Poplar River lignites, accounting for up to 7% of the ash in occasional samples of Estevan lignite [83,92].

246 Siderite is also occasionally a major constituent of Hat Creek lignite [89]. Siderite was observed by X-ray diffraction in samples of overburden from the Beulah mine; its presence in the lignite itself would account for the observation of iron soluble in dilute hydrochloric acid during chemical fractionation [82]. Siderite was observed in the lignite of the Calvert Bluff formation of Texas [42]. (iv) Minor carbonates. Witherite [83,92], rhodocrosite [89], and manganoan calcite [89] have been reported to occur in Canadian lignites. 5.2.5 Phosphate minerals Monazite has been reported as a trace mineral in Beulah-Zap lignites [87]. It was tentatively identified in the lignite of the Calvert Bluff formation of Texas, as were the phosphate minerals crandallite and xenotime [42]. Apatite was found filling the cell voids in a sample of Magothy lignite [105]. In this particular lignite, the amount of apatite appears to be significant. 5.2.6 Sulfate minerals (i) Gypsum, bassanite, anhydrite, and selenite. In the Fort Union lignite, gypsum and selenite are among the most abundant of minerals deposited by precipitation from solution into cracks or cleats in the lignite after coalification was well advanced. Gypsum crystals of 5-20 mm are common occurrences at the Beulah mine [29]. The development of gypsum is authigenic, via conversion of organic sulfur and calcium carboxylates. Gypsum is commonly encountered in the North Dakota lignites [84], though occasional reports have asserted the contrary [88]. Gypsum has been observed by X-ray diffraction in the sink fraction of CC14float-sink separation of Glenharold lignite [85]. It is also the most common sulfate in Saskatchewan lignites [92], and is common in Hat Creek lignites [89]. Gypsum was observed in samples of lignite from the Calvert Bluff Formation [42]. Gypsum occurs rarely in Moose River Basin lignite, being found in less than half the samples examined. Its presence is attributed to recent weathering of pyrite [93]. Because of this origin, the occurrences of gypsum and pyrite are spatially related. The origin of gypsum by pyrite weathering may account for the very high concentrations of gypsum occasionally reported. For example, the minerals of the Ferris seam outcrop contain up to 81% gypsum [92]. Selenite occurs in sediments when opportunities exist for slow crystallization from solution, the slow crystallization thus providing the necessary time for the large, elongate crystals to form. The occurrence of selenite in Glenharold lignite was considered inferential evidence that during the development of this particular seam (part of the Hagel bed) extensive dehydration had occurred [ 10]. Selenite crystals 7.5 cm long have been reported from the Harmon (North Dakota) bed [I]. The upper portion of the lignite in the Freedom mine contains unusually large amounts of selenite in vertical fractures, though selenite deposits occur throughout the mine [ 102]. Bassanite, which may be an artifact of the low-temperature ashing of gypsum, was identified as a major component of 27 Texas lignite samples taken from four drill cores [97].

247 Bassanite was also observed in the low-temperature ash of a Montana lignite, again formed from the parent gypsum [ 103]. Bassanite is reported along with gypsum in Poplar River lignite [92]. On the other hand, no bassanite was reported from the Hat Creek lignites, even though gypsum is a common constituent [89]. Anhydrite was reported in one of thirteen samples of the Ferris seam outcrop; gypsum is common in these samples [92]. (ii) Sodium sulfate. Lignite from Ward County, North Dakota contains extensive pockets of a white material which contained 47% sodium sulfate [ 106]. The sodium sulfate was deposited by groundwater percolation; an excellent correlation was observed between the elevation of this particular lignite bed and the sodium oxide content of the ash, with samples taken from lower elevations having higher sodium contents [106]. Thenardite has been reported in North Dakota lignites [65], forming epigenetically by evaporation of groundwaters having high concentrations of sulfate ions. (iii) Barite. Barite was a minor component in the 1.55 sp. gr. sink fractions of Beulah, Gascoyne, and Velva lignites [85], and is generally rare in Beulah lignite [88]. Barite was tentatively identified in a Calvert Bluff formation lignite [42]. Scanning transmission electron microscopy of sub-micron mineral matter in Hagel lignite tentatively identified barite as a major component of the mineral matter below 100 nm in diameter [107]. This finding would not have been predicted from the composition of the ash of the lignite, since barium is always a minor to trace component of the ash. This also shows that the distribution of elements among different particle sizes in the mineral matter is not uniform. Barite generally occurs only as a trace constituent of Saskatchewan lignites, though in occasional samples it can be an appreciable contributor to the mineral content, up to 16% in one sample of Estevan lignite [83,92]. (iv) Minor sulfates. Jarosite is commonly encountered in North Dakota lignites [84]. Celestite was tentatively identified in Calvert Bluff lignite [42] and was observed in the low temperature ash of Zavala County lignite [108]. 5.2.7 Silicate minerals (i) Quartz. Along with clays, quartz is one of the most significant components of mineral matter in American lignites [16]. Quartz was the most frequently encountered mineral in Beulah lignite [29,88], in Monticello lignite [36], and in Saskatchewan lignites [92]. It is a major mineral constituent throughout the Beulah-Zap seam [87], and was a ubiquitous component of the 1.55 sp. gr. sink fractions of six Northern Great Plains lignites [85]. It occurs with medium frequency in the CC14sink fraction of Beulah lignite, and with high frequency in the same fraction of Savage lignite [86]. Quartz was found as a major component of the low temperature ashes of Texas lignites [42,97] About 80% of the Moose River Basin lignite samples contain quartz, and it is a dominant component in these lignites [93]. In Beulah lignite the quartz is fairly well size-sorted with a slight skew to the larger sizes [31,75,88]. The distribution is similar to that expected for detrital quartz transported by wind or

248 water. Typical quartz groins were 20-30 ~tm in diameter with subrounded to angular form [88]. The sub-rounded to angular nature indicates a detrital origin [31,75,88]. Authigenic quartz was the primary mineral, with smaller amounts of detrital quartz, in 27 samples of Texas lignites from four drill cores from Nacogdoches, Panola, Rusk, and Shelby counties. Quartz in Moose River Basin lignite occurs mainly as detrital grains, in grains less than 190 ~tm, subrounded to subangular [93]. In a single sample of laminated fusain and sandstone, cell infillings of authigenic silica were noted. The grain size of authigenic silica depends on the size of the cell cavities, but averages about 125 ~tm [93]. Authigenic silica has no crystal faces. Detrital quartz is associated with kaolinite and mica, usually in mineral-rich bands in the lignite. Quartz in Saskatchewan lignites is also mainly detrital [83,92]. Other evidence for the detrital origin of quartz in Beulah-Zap lignite is the presence of rutile inclusions [29,31]. Rutile associated with quartz is usually a high-temperature polymorph [ 109]. The high-temperature origin also indicates that the rutilated quartz is detrital. Some quartz grains showed distinct euhedral faces; these crystalline quartz grains may represent wind-blown volcanic ash. A possible volcanic origin is also ascribed to cristobalite and tridymite in Hat Creek lignite [89]. In the Freedom mine in the Beulah-Zap bed, quartz is more abundant in the uppermost lithologic layer than lower in the seam. All of the quartz grains observed were less than 30 ~m diameter; these quartz grains are a result of wind-blown sedimentation [41]. Detrital quartz was observed in Glenharold lignite [10]. The quartz distribution shows higher concentrations in the middle and upper portions of the Beulah-Zap seam [88]. Quartz (cristobalite and tridymite) also shows concentration in the upper half of the Hat Creek lignite [89]. It was most abundant in attritus, as angular to sub-rounded grains of 10-60 ~m size. Quartz makes up about 20% of the Minerals present in vitrain in the Beulah-Zap lignite [90]. Quartz and other phases of SiO2 are present as a result of inclusion of attrital quartz and of preferential silicification of woody material. Quartz constitutes about 20% of the Minerals in the vitrain of Beulah-Zap lignite [90], present as a result both of preferential silicification of woody material and of inclusion of attrital quartz. (ii) Kaolinite. Clays in general are one of the most significant fractions of mineral matter in American lignites [ 16]. Kaolinite represents virtually all of the clay occurrences in Beulah lignite [29,87,88], along with small amounts of halloysite, a hydrated form of kaolinite. It is a major constituent of the Hat Creek lignites, occurring in 74 of 76 samples analyzed, and increasing with depth [89]. About 50% of the low-temperature ash of a Montana lignite was kaolinite [103]. Kaolinite is a major component of the low-temperature ashes of 27 Texas lignite samples selected from four drill cores [97]. Kaolinite has been reported in the lignite of the Calvert Bluff formation [42]. Clay minerals in Beulah lignite tend to occur as massive mineralized areas some 30-400 ~m in diameter and flat to platy in form [88]. In the 13eulah-Zap bed kaolinite generally forms dull, massive bands tens of microns thick. The banded appearance implies a detrital origin. Clay minerals tend to concentrate at the margins of the seam in Beulah lignite [75,88]. Kaolinite and

249 halloysite occur most often near the top of the seam in fusinite cell cavities. Kaolinite occurs as irregular aggregates or in structures described as "book-like" [87], whereas halloysite typically occurs with 1 ~n diameter circular cross sections. Clays of detrital origin predominate among the Minerals in attritus [90]. Kaolinite is a major component of some Moose River Basin lignites, occurring both in association with the organic components and in clastic horizons in the lignite [93]. About 80% of the samples had kaolinite scattered throughout the organic portion. Kaolinite is the most common clay mineral in this lignite. Authigenic kaolinite in crystals of about 5 ~m in diameter is observed in fusinite cell lumens [93]. It likely arises from degradation of feldspar. Kaolinite in Saskatchewan lignites may have been introduced in situ by flocculation from ground waters in the peat swamp, or by breakdown of feldspars [83]. (iii) Mica. In Moose River Basin lignite, mica usually occurs with quartz [93]. It is a minor accessory mineral in this lignite [93]. Mica is mainly detrital [93]. Mica is sometimes observed in Saskatchewan lignites, occurring in measurable quantities in three of nineteen samples of Poplar River lignite, and two of eleven of Ferris lignite [92], and two of seventeen of Estevan lignite [83,92]. Mica has also been reported in the lignite of the Calvert Bluff formation of Texas [42]. (iv) lllite. Illite occurs as a minor accessory detrital mineral in relatively low amounts in Moose River Basin lignite [93]. It is observed in variable amounts in Beulah-Zap lignite, reported occurrences ranging from very low [29] to frequent [88]. It is similarly observed in variable amounts in Hat Creek lignites, being present in only twenty of 76 samples, and in major concentration in only three of those twenty [89]. In comparison, illite occurred, in trace amounts, in only one of seventeen samples of Estevan lignite [83], and at most in trace amounts in Moose River Basin lignite [93]. Illite has been reported in the Calvert Bluff lignite of Texas [42]. Potassium aluminosilicate grains in Monticello lignite are considered to be illite [36]. Since illites are weathering products of feldspars [110], illite in lignite may represent weathered feldspars. (v) Nacrite. Nacrite is an uncommon clay mineral of the kaolin group. Its composition is the same as kaolinite, AlzSizO5(OH)4, but it has a distinct structure. Nacrite has been reported, on the basis of X-ray diffraction and electron microprobe analyses, in the 1.55 sp. gr. sink fractions of six Northern Great Plains lignites: Beulah, Baukol-Noonan, Glenharold, Savage, Velva, and Gascoyne [85]. Nacrite also occurs in association with quartz. Nacrite occurs with "medium-high" frequency in the CC14 sink fraction of Beulah lignite and with high frequency in this fraction of Savage lignite [86]. (vi) Volcanic ash. Volcanic ash can be a significant component of low-rank coal deposits. The deposits are recognizable as distinct, light colored partings, but in some cases the volcanic ash may be disseminated throughout the coal [5]. Nineteenth century big-game hunting expeditions in Africa revealed some lignitic coal associated with massive deposits of volcanic ash [ 111 ]. A useful monograph on volcanic ash is available [ 112]. (vii) Minor silicates. Halloysite was a major component of the low-temperature ashes of 27 Texas lignite samples, selected from four drill cores [97]. Albite, NaA1Si308, occurs with very

250 low frequency in the CC14sink fraction of Beulah lignite [86]. Lawsonite, CaAI 2SIO4" H20, was found in the low-temperature ash of a Montana lignite [ 103]. Epidote, feldspars, pyroxenes, and tourmaline have been identified in lignite from the Calvert Bluff formation [42]. In addition, tentative identification was made of Ca-montmorillonite. Iron- and calcium-containing montmorillonite have been identified in Monticello lignite [36]. Montmorillonite was frequently encountered in Beulah lignite [88], but is essentially absent from Hat Creek lignites [89]. Talc was occasionally encountered in Beulah lignite [88] as thin flakes 15-35 ~n in diameter. Feldspars are rare in Beulah lignite [88], and are encountered in some samples of Saskatchewan lignites [83,92]. 5.2.8 Oxalate minerals Two forms of calcium oxalate, weddellite and whewellite, have reported as minor components of Saskatchewan lignites [83,92]. Weddellite is usually associated with deep-sea deposits. 5.3 T R A C E E L E M E N T S

5.3.1 Introduction The measurement of trace element concentrations in lignite is of interest for at least three reasons. First, the data are important in developing an understanding of the inorganic geochemistry of lignite

(e.g., [74]). Second, at least seven trace elements--As, Be, Cd, F, Pb, Hg, and

Se--may be toxic. Their release to the environment is of concern during lignite utilization or by extraction from products (such as groundwater leaching of ash used as mine fill). Third, interest in occasionally expressed in recovering valuable elements from lignite or its ash. At one time, the prospect of recovering uranium from lignite ash received serious consideration. More recently some attention has shifted to the potential extraction of gallium and germanium. Much of the early work on trace element concentrations in lignite was done on ashes. This approach was dictated in part by the detection limits of the analytical methods used. Since ashing causes roughly a ten-fold increase in concentration of the elements, in some cases it became possible to determine an element in the ash, even if its concentration in the lignite itself was below the detection limit of the analytical method. When the purpose of analyzing ash is to determine the concentration of the element in lignite, one must presume that no volatilization loss occurred during ashing, a presumption not necessarily valid. The determination of trace element concentrations in ash is important in its own fight for environmental studies, as, for example, to assess the potential for leaching of toxic elements by groundwater from ash. Trace element concentrations in North Dakota lignite and lignite ash have been well studied [8,54,74,113-121], because of the state's already extensive electric power industry and a perceived potential for eventually supporting an equally extensive lignite gasification industry. Useful summaries of the data available prior to 1975 have been published [121,122]. About sixty trace elements have been detected in North Dakota lignite. (Another twelve

251 elements occur in larger concentrations in the lignite or its inorganic components; disregarding the noble gases and the highly unstable radioactive elements, lignite contains in some quantity almost 90% of all the known elements.) The concentrations of different trace elements in lignite vary greatly. A difference of an order of magnitude between the lowest and highest reported values can be encountered frequently for some elements. Differences of two orders of magnitude are not uncommon. Even in the case of a single vertical slice through one seam, the concentration of trace elements can vary by at least an order of magnitude [74,113]. For example, within 2 m in the Center mine, the uranium concentration varied from 0.35 to 6.1 ppm [74]. Data for concentrations of trace elements in eastern and western Alabama and Darco lignites are presented in Table 5.27 [11]. Six Fort Union lignites, five from North Dakota and one from Montana (but otherwise unidentified as to source) were included in a major survey of the trace elements in coal [8]. The ranges of values observed are shown in Table 5.28. Trace element concentrations of three Saskatchewan lignites (from the Bienfait, Hart, and Shaunavon seams) are shown in Table 5.29 [92].

TABLE 5.27 Trace element concentrations* in Gulf Coast lignites [11]. Element Beryllium Chromium Gallium Lanthanum Nickel Scandium Vanadium Yttrium Ytterbium Zinc

Eastern Alabama 0.4-1.3 4-12 0.8-3 9-17 2--6 2-5 16-39 7-11 0.5-1.2 11--49

Western Alabama 1- 4 2- 24 3- 9 2-7 1-25 0.7-5 1-78 7-21 2--4 4--43

Darco 0.04-2.7 2-16 0.3-6 0.7-12 2-22 0.8-8 5--50 3-13 0.3-1.1 7-55

*ppm, dry coal basis

Although the order of magnitude of concentration of a given element is often the same from one lignite to another, there can nevertheless be significant exceptions. For example, the highest concentrations of germanium, cesium, and molybdenum are greater than the lowest concentrations by factors of 10, 15, and 20, respectively. Generalizations about the concentration of a particular trace element in lignite should at best be made and used only with great caution. 5.3.2 Alkali metals Lithium occurs at 28-30 vtg/g in Beulah-Zap lignite ash, and 2.7-2.9 ~tg/g on a "whole coal" basis [123]. Lithium, rubidium, and cesium are inorganically associated in Ravenscrag lignites [83].

252 TABLE 5.28 Trace element concentrations* in six Fort Union lignites [8]. Element Ag As B Ba Be Br Cd Ce Co Cr Cs Cu Dy Eu F

Range*_ Element 0.02-0.07 Ga 1.8-9.8 Ge 44-100 Hf 480-1600 Hg 0.12-0.70 I 1.0-1.9 In <0.10 La 3.3-11 Lu 0.80-1.1 Mn 6.0-17 Mo 0.02-0.30 Ni 3.1-6.1 P 0.36-4).61 Pb 0.07-0.13 Rb 19---64 Sb

Range*_ 0.90--4.2 0.10-0.95 0.37-1.2 0.04-0.16 <0.30-0.95 <0.01-0.25 2.0-5.7 <0.01-0.06 33--86 0.10-2.0 2.2-5.6 27-200 1.1-5.5 <1.0-1.9 0.20-0.75

Element Sc Se Sm Sn Sr Ta Tb Th U V W Yb Zn Zr

Range*_ 0.70-1.8 0.40-1.5 0.30-0.50 <0.30 240-500 0.05-0.18 0.10-0.23 0.82-3.1 0.74-1.7 4.8-7.7 0.24-1.1 0.16-0.55 <0.30-4.0 14-36

*Data reported as parts per million, moisture-free whole coal basis. Only three samples were analyzed for Tb.

TABLE 5.29 Trace element concentrations* in three Saskatchewan lignites [92]. Element As B Ba Br Ce C1 Co Cr Cs Cu Dy Eu

Range*_ 0.5-9.9 27-233 577-969 1.4-4.1 10.4-51.9 39-65 1-11 6-13 0.1-1.3 7-13 1.2-4.3 0.2-1.0

Element Hf Ho La Lu Mn Mo Nb Nd Ni Pb Rb Sb

Range*_ 1-3.6 0.2-0.9 6.3-28.6 0.1-0.4 13-145 1.9-12.9 2-13 5-18 4-20 6.4-17.3 11-14 0.8-1.5

Element Sc Se Sm Sr Ta Th Tm U V W Yb Zn

Range*_ 1.7-2.2 2.3-4.1 0.7-5.3 288-537 0.2-0.8 2.9-11.6 0.3-0.6 1.6-7.1 9-28 1.0-2.2 0.4-2.8 62

*Data reported as parts per million, whole coal basis.

The mean rubidium content of Moose River lignites is 5.9 ppm [ 124], with a range of 0.0-20.0. In these lignites rubidium is associated with the detrital minerals [93]. Rubidium is associated with clays in Calvert Bluff formation lignite of Texas [42]. Roughly the same rubidium concentration, 6.3 ppm, is seen in the Hat Creek No. 2 lignite [125]; a value of 5.0 ppm is reported for Ravenscrag lignite [83]. Higher rubidium contents, 11-14 ppm, are seen in

253 Saskatchewan lignites [92]. The rubidium content of Beulah-Zap lignite is 0.9_+0.2 lag/g on a whole coal basis [126]; an independent determination gives the value 0.90_+0.065 [127]. The possibility of rubidium occurring on ion-exchange sites in clay structures has been suggested [11 ]. Cesium was also associated with clays in Calvert Bluff lignite [42]. The cesium concentration of Beulah-Zap lignite is 0.09_+0.032 ~tg/g [ 127]; the Saskatchewan lignites show cesium concentrations in the range 0.1-1.3 ppm [92]. Hat Creek lignite is at the high end of this range, 1.0 ppm [125] 5.3.3. Alkaline earths Beryllium in Glenharold lignite is clearly associated with the organic portion of the lignite [ 10]. The lighter specific gravity fractions of this lignite are enriched with beryllium in the same manner as they are with the organically associated major elements. In fact, beryllium occurs in Bulgarian lignites as a complex with humic and fulvic acids [128]. However, beryllium seems to be associated with inorganic components of Bohemian lignites, based on a correlation of beryllium content and ash value [ 129]. Beryllium accumulation appears to occur via a variety of processes [1291. The beryllium content of Beulah-Zap lignite is 0.17-0.18 ~tg/g on a whole coal basis, and is 1.8-1.9 ~g/g in the ash [123]. Other work reports a beryllium content of 2.4-3.5 ppm for this ash [ 130]. A survey of numerous samples of North and South Dakota lignites indicates a beryllium content of the ash in the range <1-18 ppm [131]. Both strontium and barium have high concentrations in low-rank coals, and were especially pronounced in North Dakota lignites [54]. Strontium generally occurs largely [11] or almost totally [16] in ion-exchangeable form in lignites. This observation has been made for Alabama [11], Texas [ 11,36], and North Dakota [11] lignites. Strontium has a dominantly organic association in Calvert Bluff lignite, but is also found in celestite [42]. It is also organically bound in Ravenscrag lignite [83]. The mean strontium content of 17 Moose River samples is 109 ppm [124]. Strontium has several associations--organically bound, with the clays, and as a carbonate [93]. The strontium concentration of Beulah-Zap lignite is 650+_50 ~g/g [ 126]; also reported as 600_+67 ~g/g [127]. Other workers report a somewhat lower value, 500-510 ~g/g in the lignite, and 5300-5400 ~g/g in the ash [123]. A range of 6200-7300 is also reported for the ash of this lignite [130], in reasonable agreement with other work [132]. Saskatchewan lignites have strontium contents in the range 288-537 ppm [92], with the high end of the range (537.7 ppm) observed in Ravenscrag lignite [83]. An even lower concentration, 116 ppm, is seen in Hat Creek No. 2 lignite [125]. Strontium ranges from 0 to 1000 ppm in Moose River Basin lignites, with a mean value of 108.7 [93].In the ashes of lignites from North and South Dakota, strontium concentrations are in the range 100--4700 ppm [ 131]. The distribution of strontium in specific gravity fractions of Glenharold lignite follows the behavior of calcium [10]. Most is ion-exchangeable with ammonium acetate, and the amount is generally higher in the lighter fractions. About 2-3% of the total strontium in this lignite

254 concentrated in the acid-insoluble portion of the 1.80 sp. gr. sink fraction, presumably occurring as strontium sulfate. Strontium is associated with clays and carbonates as well as with the organic fraction of Moose River Basin lignite [93]. In Hicas (Hungary) lignite, strontium is absorbed into the crystal lattice of aragonite [133]. Strontium is believed to be syngenetic. In many lignites barium is present primarily in ion-exchangeable form. Similarly, both strontium and barium are associated with organic matter in Indian lignites [134]. However, in Ravenscrag lignite barium is associated with both inorganic and organic materials [83]. In a North Dakota lignite it was present as a carbonate tentatively identified as witherite [ 11]. In Monticello lignite, 52% of the barium was removed as an exchangeable cation, 26% was removed by hydrochloric acid, and 22% was insoluble in acid [36]. Barium was found associated with detrital minerals in an Alabama lignite. The barium content of Beulah-Zap lignite is 4 8 ~

~tg/g on a whole coal basis [126]; other

workers give a range 390-450 ~tg/g in the coal, and 410(O4700 ~tg/g in the ash [123]. Higher ranges, 4700-5(K~ [132] and 5300-7400 ppm [ 130], are also reported for the ash; and a higher value, 680+-50, for the lignite [127]. Saskatchewan lignites have barium contents in the range 577-969 ppm [92]. In various lignite samples from the Dakotas, the barium content of the ash ranges from less than 10 ppm to a high of 1.22% [131]. In Glenharold lignite, a small amount (15 ppm) of ion-exchangeable barium was found in all specific gravity fractions [10]. Most of occurred in the 1.80 sp. gr. sink fraction as the acidsoluble carbonate and the acid-insoluble sulfate, with the latter predominating. In this lignite the prevalence of barium in mineral forms is different from the behavior of strontium, which was mostly found in the ion-exchangeable form [ 11]. Acid-insoluble barium was detected by neutron activation analysis in gypsum, suggesting that the barium is present as a sulfate [11 ]. Barium-containing minerals predominate among the submicron mineral matter in a North Dakota lignite examined by scanning transmission electron microscopy [ 135]. Barium occurs in barite in Calvert Bluff lignite [42]. 5.3.4 First-row transition metals Scandium was found with detrital minerals in an Alabama lignite [11]. It has a strong inorganic association in Calvert Bluff lignites, being associated with clays [42]. In contrast, scandium has a predominantly organic association with Glenharold lignite [10]. In Ravenscrag lignite it is associated both with minerals and organic structures [83]. The scandium content of Beulah-Zap lignite ash is 9.1-10 ppm [130]; the value in the lignite is 0.85+_0.047 ~tg/g [127]. Scandium contents of Saskatchewan lignites are higher, in the range 1.7-7.2 ppm [92]. Hat Creek No. 2 lignite contains 6.6 ppm scandium [ 125]. The occurrence of vanadium is believed to be due to its ability to exist in multiple oxidation states, this ability facilitating its inclusion in biogeochemical reactions of oxidizing or reducing nature [136]. Vanadium is well known to occur in petroleum as vanadyl porphyrins, but these compounds are not thought to be present in coals [14]. The porphyrin contents of coals are

255 generally very low [ 137,138], and probably too low to account for the vanadium content [139]. Thus the organically associated vanadium must be present in some other complex than porphyrins [139]. Vanadium is enriched at the seam margins in lignites, suggesting that it may have been trapped at the seam margins by the formation of coordination complexes with functional groups in the lignite structure [11]. Vanadium is associated with inorganic components of Ravenscrag lignites [83]. The vanadium content of Beulah-Zap lignite is 3.4--3.6 ~tg/g on a whole basis, and 36-38 ~tg/g in the ash [ 123,130]. Higher vanadium contents are seen in Saskatchewan lignites, 9-28 ppm [92]. Very high (comparatively speaking!) vanadium is observed in Hat Creek No. 2 lignite, 122.2 ppm [125]. The vanadium content of the ashes of a variety of Dakota lignites is 10-550 ppm [131]. Vanadium has a mainly organic association in Glenharold lignite [10]. Vanadium is also primarily organically associated in Indian lignites [134]. Organically associated vanadium could be present as a coordination complex or as ion-exchangeable V§ ions [ 11 ]. Vanadium in the highest gravity fraction of Glenharold lignite is present in clay minerals [ 11]. Vanadium occurs with the clays in Calvert Bluff lignite [42], and with the inorganic fraction in the Tertiary lignites of Northern Ireland [ 140]. The mean chromium content of Moose River lignite is 65.6 ppm [124], with a range of 0--4975 [93]. The chromium is associated with heavy minerals, possibly as grains of chromite [93]. Saskatchewan lignites have less chromium, 6--13 ppm [92]. A lower value, 4_+1 ~tg/g, is reported for Beulah-Zap lignite [126]; other work reports 2.2-2.6 0g/g in the lignite [123,127] and 24--29 ~tg/g [123,130,132] in the ash. The chromium content in Hat Creek No. 2 lignite is about the same, 30 ppm [ 125]. Chromium in Glenharold lignite has a predominantly organic association [ 10], as it does in Indian lignites [ 134]. Chromium in the Calvert Bluff lignite occurs with the clays and oxide minerals [42]. In the Tertiary lignites of Northern Ireland chromium was introduced with the inorganic fraction [ 140]. Manganese was found primarily in ion-exchangeable form in an Alabama lignite, but also in carbonate minerals and associated with pyrite [11]. Manganese also associates with both the organic and inorganic portions of Indian lignites [ 134]. Manganese-containing metalloporphyrins have been isolated from a Turkish lignite [141]. Manganese is inorganically combined in Ravenscrag lignite [83]. The manganese content of Beulah-Zap lignite is 85_+9 ~tg/g [ 126]. Other work gives a comparable range, 79-81 ~tg/g in the lignite, and 830-850 in its ash [123]. The Saskatchewan lignites bracket that range, having 13-145 ppm manganese on a whole coal basis [92]. Other Dakota lignites have manganese contents in the ash from <10 ppm to 700 ppm [131]. In Glenharold lignite, 25% of the total manganese was exchangeable with ammonium acetate, 72% was soluble in dilute hydrochloric acid, and 3% remained in the insoluble residue [10]. Chemical fractionation of the specific gravity fractions of this lignite showed that the ion-exchangable manganese first decreased and then increased with increasing gravity, suggesting that ionexchangable manganese may be held in the lignite both as a counterion to the carboxyl groups and

256 in clays. Acid-soluble manganese was presumed present in Glenharold lignite as the carbonate, and occurs in siderite in lignite of the Calvert Bluff formation [42]. Manganese may be present in Alabama lignites in coordination complexes. If manganese were present as the sulfide, carbonate or as an acid-soluble form in clay, the amount of acid-soluble manganese would increase in samples from horizons with high pyrite or high clay concentrations, but in fact acid-soluble manganese is not increased in such samples [11]. Manganese was associated with iron oxides in spherical particles of about 20 nm [97] in lignite cores from four counties in Texas. This manganese would report to the acid-soluble fraction on chemical fractionation. Insoluble manganese may be present in pyrolusite, or associated with iron sulfides or oxides. Cobalt adsorption on lignite proceeds more slowly than that of nickel, but 99% of the cobalt in ammoniacal solutions can be absorbed on Indian lignites [142]. Cobalt has a strong organic association with lignite of the Calvert Bluff formation, and occurs with the sulfide minerals as well [42]. Similarly, cobalt is associated both with the mineral and organic portions of Ravenscrag lignite [83]. It is organically associated in Indian lignites [134]. The mean cobalt content of Moose River lignite is 35.9 ppm [124], associated with sulfides and the clay minerals [93]. The range of cobalt concentrations in this lignite is 5.0-197.5 ppm [93]. Cobalt occurs at 0.78_+0.5 ~tg/g in Beulah-Zap lignite [127], with 5.7-6.0 ppm in the ash [ 130]. A wide range of values, <1-200 ppm, is observed for ashes of various Dakota lignites [131]. Saskatchewan lignites show somewhat higher cobalt contents, 1-11 ppm [92]. Hat Creek No. 2 lignite is in this range, with 9.1 ppm cobalt [ 125]. The mean nickel content of Moose River lignites is 47.4 ppm [124]. The range is 4.2-190.7 ppm [93]. It occurs in several modes: associated with the organic matter, with the clay minerals, and with pyrite [93]. The nickel associated with sulfides may also occur as millerite [93]. Nickel is inorganically associated in Ravenscrag lignites [83]. Nickel is organically associated in Indian lignites [134]. Saskatchewan lignites have less nickel, 4-20 ppm [83]. A lower value, 3.3_+0.1 ~tg/g, is observed in Beulah-Zap lignite [ 126]. Other work puts the nickel content of this lignite even lower, 1.1-1.4 ~tg/g, and 12-15 ~tg/g in the ash [123]. Ranges of 17-22 [130] and 17-25 [132] ppm are also reported for this ash. An enormous range of nickel contents, 2-1500 ppm, has been reported for a variety of other Dakota lignites [131]. In Glenharold lignite, nickel has a mainly organic association [10,11]. Indian lignites are very effective at adsorbing nickel from solutions containing about 1% nickel [143], suggesting that nickel might enter lignites by concentration from groundwater percolating through the lignite. Copper is enriched at seam margins of some lignites, implying that it had come into the seam in solution and had been trapped by the functional groups of the lignite. In a North Dakota lignite copper was found to be mainly present as a coordination complex [11]. In Glenharold lignite, copper was found primarily in an acid-soluble form, and particularly in the lighter specific gravity fractions [10]. This behavior suggests an organic association, presumably in the form of coordination complexes. The mean copper content of Moose River lignite samples was 20.0 ppm

257 [124], with a range 2.0-44.0 [93]. In this lignite, copper associates with the clays [93]. A value of 4.2_+1.1 ~tg/g is reported for Beulah-Zap lignite [126]. Other work gives a comparable range, 3.5-5.6 ~tg/g in the lignite, and 37-59 ~tg/g in its ash [123]. The ash analysis is also reported as 34--36 [ 132] and 40-75 ppm copper [ 130]. The copper content of ashes of other Dakota lignites ranges widely, from <1 ppm to 700 ppm [131]. As seen with other elements, the copper content of Saskatchewan lignites seems intermediate between the northern Ontario Moose River Basin lignite and the North Dakota Beulah-Zap lignite. Saskatchewan lignites contain 7-13 ppm copper [92]. Copper associates with clay minerals [93]. Copper was found in chalcopyrite in lignite of the Calvert Bluff formation [42]. 5.3.5 Heavy transition metals The yttrium content of Beulah-Zap lignite is 2.4_+1.3 ~tg/g on a whole coal basis [ 126]. A slightly lower range, 1.8-1.9 ~tg/g on a whole coal basis, corresponding to 19-20 ~tg/g in the ash, has been reported [ 123]. Other work places the yttrium content of the ash as 24--27 ppm [ 130]. In Glenharold lignite, yttrium has mainly an organic association [10], as it does in Indian lignites [ 134]. However, in the Calvert Bluff lignite of Texas, yttrium has a strong inorganic association in the mineral xenotime [42]. Zirconium was found with detrital minerals in an Alabama lignite, and in many other lignites, but has an organic affinity in a North Dakota lignite [11]. In a suite of lignites from the Dakotas, the zirconium content of the ash is 40-3000 ppm [131]. The zirconium content of BeulahZap lignite is 16_+5 ~tg/g on a whole coal basis [126]. The zirconium content of the ash of this lignite is 130-140 ppm [130], though a lower range, 47-74 ppm, has also been reported [132]. In Glenharold lignite, zirconium was enriched in the lighter specific gravity fractions [10]. This behavior suggests an organic association for zirconium in this lignite [11]. The mean zirconium concentration in Moose River lignite is 93.5 ppm [124], with a range of 0-530 ppm [93] associated with the pyrite [93]. Zirconium occurs as zircon in the Calvert Bluff lignites [42]. Hafnium also occurs in zircon in these lignites [42]. Hafnium is associated with clays in Saskatchewan lignites [83]. The hafnium content of Beulah-Zap lignite is 0.341_+0.0025 ~tg/g [127]. Higher values, 1-3.6 ppm, are seen in Saskatchewan lignites [92]; Hat Creek No. 2 lignite is also in this range, with a hafnium content of 2.3 ppm [ 125]. The niobium content of Beulah-Zap lignite ash is 5.3--6.6 ppm [ 130]. Little is known about the association of niobium in North American lignites; in Indian lignites it is thought to have an organic association [ 134]. In Calvert Bluff lignites, tantalum occurs with oxide minerals [42]. The tantalum content of Beulah-Zap lignite is very low, being 0.093-+0.001 ~tg/g [127]. Tantalum in Saskatchewan lignites is in the range 0.2--0.8 ppm, on a whole coal basis [92]. In core samples of North and South Dakota lignites molybdenum was more concentrated near the tops of the lignite beds than in any other sections [144]. In this respect the behavior of molybdenum parallels that of uranium. Molybdenum is associated with pyrite in some lignites [14]. The sulfur in pyrite originates with the bacterial reduction of aqueous sulfate to H2S. The

258 H2S converts molybdate ions to thiomolybdate, which eventually decomposes to MoS 3 and then to MoS2. In a North Dakota lignite, molybdenum and pyrite were concentrated in the heaviest specific gravity fraction [14]. Molybdenum also occurs with the sulfide minerals in the Calvert Bluff lignites [42]. The mean molybdenum content of Moose River lignite samples is 5.9 ppm [124], associated with the organic portion of the lignite [93]. The range of molybdenum values for this lignite is 5--15 ppm [93]. In Bulgarian lignites, molybdenum also occurs in association with humic and fulvic acids [ 128]. Molybdenum associates with both the organic and mineral portions of the Saskatchewan lignites [83]. A suite of lignite ashes from the Dakotas have molybdenum concentrations in the very wide range 2-7200 ppm [131].The molybdenum content of Beulah-Zap lignite ash is 8.2-8.7 ppm [130]. The Saskatchewan lignites contain 1.9-12.9 ppm molybdenum on a whole coal basis [92]. Hat Creek No. 2 lignite has a molybdenum content of 3.5 ppm [ 125]. Tungsten in Moose River Basin lignites occurs with the sulfide minerals; a portion may be associated with the organic structure [124]. It may also associate with carbonates [93]. The tungsten values range from 1.0 to 3.0 ppm, with a mean of 1.7 ppm [93]. Tungsten associates with both organic and inorganic portions of the Saskatchewan lignites [83]. In Bulgarian lignites tungsten is associated with the humic and fulvic acids [ 128]. The tungsten content of Beulah-Zap lignite is 0.35•

~tg/g [127]. Higher values, 1.0-2.2 ppm, occur in the Saskatchewan lignites

[92]. The platinum group elements (Pt, Pd, Rh and Ir, collectively indicated as PGE) show increasing concentrations with decreasing ash contents in Moose River Basin lignite [ 124]. This is an indication of organic association of these elements. In addition, some of the PGE's may occur with the detrital minerals [93]. Concentrations of PGE's are very low, being, e.g., 13.3 ppb for platinum, 16.1 ppb for palladium, and 0.07 ppb for iridium [93]. PGE's also occur at very low concentrations in Beulah-Zap lignite ash [ 130]. Silver in the ash of North and South Dakota lignites ranges from a high of 1.7 ppm to less than 0.1 ppm [131]. The mean gold content of Moose River Basin lignite is 13.6 ppb, with a range of 1-96 ppb [93]. 5.3.6 Lanthanides The lanthanum concentration in Beulah-Zap lignite ash is 37--40 ppm [130]; the concentration in the lignite, on a whole coal basis, is 2.82•

~tg/g [ 127]. A somewhat lower

range of concentrations, 6.3-28.6 ppm on a whole coal basis, is seen in Saskatchewan lignites [92]. The cerium concentration of Beulah-Zap lignite is 4.4•

ppm, on a whole coal basis

[127]. Much higher cerium concentrations are seen in the Saskatchewan lignites, in the range 10.4-51.9 ppm, on the same basis [92]. In Beulah-Zap lignite ash, the praseodymium concentration ranges from 7.3 ppm to less than 6.8 [ 130]. The neodymium concentration of Beulah-Zap lignite ash is 5.3-6.6 ppm [130]; in the

259 lignite itself, on a whole coal basis, the concentration is 2.3+0.8 ppm [127]. These values are at the low end of the range seen for Saskatchewan lignites, which is 5-18 ppm, on a whole coal basis [92]. Samarium, in Beulah-Zap lignite ash, ranges from less than 3.2 ppm to 4.9 ppm [130]. The concentration in the whole lignite is 0.41+0.046 l*g/g [127]. The samarium content of the lignite is within the range reported for Saskatchewan lignites, 0.7-5.3 ppm [92]. Europium is present in Beulah-Zap lignite at a concentration of 0.081+0.012 ~g/g, on a whole coal basis [ 127]. Europium occurs at similar concentration in the Saskatchewan lignites, 0.2-1.0 ppm [92]. Terbium, along with lutetium, seem to be the least plentiful of the lanthanides. The terbium concentration in Beulah-Zap lignite is 0.056-~0.014 ~g/g on a whole coal basis [ 127]. Dysprosium occurs at concentrations of 1.2--4.3 ppm in Saskatchewan lignites [92]. The holmium content is 0.2-0.9 ppm, on a whole coal basis [92]. Thulium is found in concentrations in the range of 0.3--0.6 ppm, on a whole coal basis [92]. The enrichment of ytterbium at seam margins implies trapping by the functional groups in the lignite structure [ 11]. In Glenharold lignite, ytterbium has predominantly an organic association [10]. The ytterbium content of Beulah-Zap lignite is 0.29+0.092 l*g/g [127]. Ytterbium concentrations in Saskatchewan lignites are higher, being in the range 0.4-2.8 ppm on a whole coal basis [92]. Lutetium in Beulah-Zap lignite occurs in concentration 0.036_+0.005 ~g/g [127]. Higher lutetium concentrations are observed in the Saskatchewan lignites, 0.1-0.4 ppm [92]. All of the data on lutetium are on a whole coal basis. Most of the rare earth elements have inorganic associations with the Saskatchewan lignites [83]. However, cerium, dysprosium, and lutetium are associated with both the minerals and the organic structure [83]. 5.3.7 Actinides The thorium content of Moose River outcrop and drill core samples [ 145] ranges from <0.3 ppm to I 1.0 ppm, the latter being recorded for a black earthy and woody lignite. The median value is 2.5 ppm. Thorium in this lignite occurs both in clay minerals and the heavy minerals [93]. Thorium in Beulah-Zap lignite occurs at 1.07_+0.11 ~tg/g, on a whole coal basis [127]. It has a higher range in Saskatchewan lignites, 2.9-11.6 ppm [92], where it is associated with clays [83]. The Hat Creek No. 2 lignite is at the low end of this range, 2.4 ppm [125]. Thorium occurs in clays and monazite in the lignites of the Calvert Bluff formation [42]. Lignites of the Rhineland contain 0.4-2.7 ppm thorium, with an average of 1.1 ppm [ 146]. One concept of the origin of uranium in Fort Union lignites is that it was brought into the lignite from the precursor peat [ 147]. No correlation of uranium with the petrographic constituents was observed for samples of Slim Buttes (South Dakota) lignites, nor is there a significant correlation of uranium with degraded humic matter [148]; however, layers of a lignite bed with

260 highest uranium concentration also contained particulate plant debris and decayed plant matter [149]. Furthermore, the high-uranium layers were usually overlain by a layer high in detrital material [149]. The highest concentrations of waxy material in Goose Creek (South Dakota) lignite correlated with the highest radioactivity [ 148]. An alternative theory of the origin of uranium in lignite is that it brought in with percolating groundwater [147,150-153], possibly by lateral movement of the water [154], but more likely by leaching from overlying tuffs followed by epigenetic mineralization in the lignite [151,155]. Support for this concept is based on the following observations: 1) in all of areas studied in North and South Dakota, Wyoming, Idaho, and New Mexico, uraniferous lignite beds were overlain by radioactive volcanic rocks; 2) springs from these volcanic rocks show high levels of uranium in the water; 3) the extent of mineralization of the lignite can be correlated with variations in the permeability of the overlying rock; 4) the topographically higher beds in a series are more radioactive than lower beds in the same series; and 5) the uranium content is independent of the age of the coal but correlates with topographic location and the permeability of the overlying rocks [150]. Furthermore, twenty cores of North and South Dakota lignites showed uranium more highly concentrated at the tops of the beds than in other sections [ 144,148]. Uranium is trapped in lignite by reduction of hexavalent uranium in solution to insoluble tetravalent uranium by the lignite. A consequence of the concentration of uranium in this manner is that some lignites will give unusually high readings during 7-ray logging (usually most coals will show very low radioactivity levels, comparable, for example, to limestone). South Dakota lignites are an example of unusually high 7 radiation [ 156]. In North Dakota, uranium generally occurs in lignite or carbonaceous shales overlain or underlain by a sandstone aquifer. Uranium in the groundwater of these aquifers derives from leaching from volcanic ash by meteoric waters. Water of meteoric origin leaches uranium from the ash, the uranium-bearing water then flows downward and laterally until it contacts lignite or other organic sediments which capture the uranium as coordination complexes. The ability of uranium to exist in multiple oxidation states enhances its ability to participate in either oxidation or reduction reactions with the lignite. Uranium in Wilcox and Jackson lignites appears concentrated near the contact of the lignite with a sandstone or shale [157]. This again suggests that uranium, being transported by groundwater as soluble complexes, accumulates at the contact via reaction with the organic portion of the lignite. A variation of the pyrite or silicates content of lignite will cause a similar variation in the amount of uranium present; however, a change in the carbon content of the lignite will cause an even greater change in uranium [158]. This implies that the geochemical control of uranium content through some factor common to uranium and carbon (i.e., the organic portion of the lignite) is more important than control through factors common to uranium and the minerals. Carbon is the most important component in relating the uranium content to the bulk composition of the sample. A multiple linear regression equation originally worked out for black shales is

261 0 = -107.1686 + 1.2182 X 1 + 2.6168 X2 + 4.4610 X3 where 0 is the uranium content in parts per million, X: the percent silicates, X 2 the percent pyrite, and X3 the percent carbon in the sample [ 158]. The uncertainty in the predicted value of uranium is +12.4 ppm. French

(e.g., Arjuzanx) lignites having high concentrations of carboxyl groups will rapidly

remove uranium from aqueous solution by ion exchange [159]. Infrared spectral evidence suggests that the uranium forms complexes with the carboxyl groups. Such complexes subsequently undergo thermal decomposition, forming uranitite upon decarboxylation of the lignite structure. About half the uranyl ion in solution can be precipitated as uranitite in about 130 days [160]. For French lignites that do not have high concentrations of carboxyl groups

(e.g, Gardanne) other

uranium complexes form without ion exchange [159]. The overall mechanism of uranium incorporation involves complexation of uranyl species without reduction in low-temperature environments, followed later by reduction in diagenetic or hydrothermal environments. The ratedetermining step at 100-200~ may be the formation of uranium oxides [ 160]. Uranium can incorporated in the lignite as uranitite or coffinite at favorable Eh-pH conditions [161]. These minerals occur together with the uranium complexed to the organic structure. When groundwater containing [UO2(CO3)2]-2 enters an acidic environment, the uranyl ions are scavenged by the organic material, or are precipitated as minerals such as autunite. Some microenvironments may have an Eh sufficiently negative to permit formation of minerals containing U(IV). Any U(IV) minerals that precipitate in reducing zones might be recycled by subsequent oxidation and returned to the system as uranyl ions, which then may be trapped by the organic matter. Uranium is present in many low-rank coals as an organic complex [157,159,162]. About 60--80% of the total uranium in Texas lignite is bound to the humic portion [161]. The remainder is associated with fine-grained minerals. Uranium has a dominant organic association in Calvert Bluff lignite, though some may also be found with zircon [42]. Density gradient fractionation of a Texas lignite containing about 2500 ppm uranium suggests that the uranium is associated with the organic matter [ 157]. X-ray mapping of a Texas lignite shows a homogeneous distribution of the uranium through the lignite [157]. Extraction of this lignite with dimethyl sulfoxide removes 40-50% of the uranium in molecular structures which have molecular weights below 1500 [157]. Addition of benzene, water, or acetone to the dimethyl sulfoxide extracts results in formation of a precipitate having infrared spectra similar to lignite humic acids. This humic acid-like precipitate is enriched in uranium. The uranium content of Moose River lignite correlates with the nickel content. Since nickel is thought to be associated with humic acids in coals (e.g., [163]) the correlation of uranium with nickel may reflect in turn an organic association for the uranium. Uranium in a weathered sample of Mendenhall (South Dakota) lignite was associated with the organic matter as a coordination complex [ 164]. Some uranium minerals, such as metaautunite, metatorbernite, metazeunerite, novacekite,

262 saleeite, and abernathyite, were found as coatings on fracture surfaces of South Dakota lignites [154]. Megascopically visible uranium minerals, mostly phosphates and arsenates such as metatorbernite, have been reported in North and South Dakota lignites [165]. However, in a Jackson lignite from Karnes County, Texas, about 10% of the lignite is present in the form of 10-30 ~tm minerals in the >2.90 sp. gr. fraction of the low-temperature ash [ 161]. These particles are coffinite, or products of its alteration. Other minerals included sodium autunite, metauranocirite and sabugalite [ 165]. The uranium may be associated with clay minerals or carbonates in Moose River Basin lignite [93]. Typical uranium-bearing lignites are very high in ash. For example, samples from former National Lead Co. holdings in Slope and Billings Counties, North Dakota, have ash contents of 30-60% with uranium contents, expressed as U308 on a dry basis, as high as 2% [166]. However, some lignite samples of unknown provenance contained only 9% ash on a dry basis with up to 0.5% U308 [166]. These data contrast with substantially lower values for many other North Dakota lignites [74,113,117,119,120], although most lignites for which uranium data have been obtained were not specially selected to be uraniferous. A Saskatchewan lignite containing 0.08% U (on a coal basis) was considered to be of comparable quality to uranium ores [167]. Montana lignites contain an average of 0.005% uranium, reported as U308 [155]. The average uranium content of Moose River lignite is 1.63 ppm [145]; the range is 0.14-7.20 ppm [93]. A similar value, 1.4 ppm, is seen in Hat Creek No. 2 lignite [ 125]. These values are at the low end of the range seen in Saskatchewan lignites, 1.6-7.1 ppm [92]. The value for Beulah-Zap lignite is 0.49+_0.06 ~tg/g [ 127]. Analyses of three lignites, Savage, Gascoyne and Beulah, show average contents of 238U of 6.0, 8.4, and 3.7 mBq/g, respectively. The content of 234U in the same samples was 6.1, 8.9, and 3.9 mBq/g, respectively [168]. The maximum concentrations of uranium in Texas lignites are 20 ppm in the Wilcox formation to 7800 ppm in the upper Jackson formation [157]. The uranium content of Keuper (Vosges region of France) lignite of Triassic age is 12.7-18.8 ppm [136]. Lignites of the Rhineland have uranium contents in the range of 0.2-1.4 ppm, averaging about 0.4 ppm [ 146]. An extensive investigation of United States lignites, involving over 500 samples from the western U.S., shows that, for 423 lignites in the Dakotas, the uranium content of the ash ranged from a low of 0.001% to a high of 6.3862% [131]. Sixteen of the 423 ashes contained uranium in excess of 1%. The highest value occurred in a sample from the Lower Ludlow Formation, Slim Buttes area, Harding County, South Dakota [131]. California lignites, in contrast, generally have much lower uranium contents. Of 171 samples analyzed, only 28 contained uranium in concentration greater than 0.001% in the ash, and the highest value was about 0.05% [131]. Lowrank coals of the southwestern United States also contain, in some cases, high uranium contents. Fourteen ash samples contained from 0.0008% to 5.5880% [131]. The highest value was observed in a sample of woody lignite from the Big Bend strip mine, near San Rafael, New Mexico [ 131 ].

263 5.3.8 Group lib elements Zinc is present primarily in complexes in a North Dakota lignite [11]. In Glenharold lignite most of the zinc is present in an acid-soluble form and concentrates in the lighter specific gravity fractions [ 10], presumably as a coordination compound. In western Alabama lignite, much of the zinc is found in acid-soluble form, but the abundance of pyrite in this lignite suggests that zinc is present as sphalerite [11]. Zinc is associated with the inorganic components of Saskatchewan lignites [83]. In Beulah-Zap lignite, the zinc concentration is 5.7_+2.5 ~tg/g on a whole coal basis [126,127]; other work shows comparable, albeit slightly lower values, 4.5-5.1 ~tg/g on a whole coal basis, equivalent to 47-54 ~tg/g in the ash [123]. A range of 54-61 ppm in the ash has also been reported [132]. A suite of lignites from the Dakotas contain 2-510 ppm zinc in the ash [131]. Zinc occurs in Hat Creek No. 2 lignite at 26.4 ppm [ 125]. Even higher zinc concentrations occur in Saskatchewan lignites, about 62 ppm [92]. Zinc has an organic association in Moose River lignite [93,124]; its mean concentration is 159.2 ppm [93], with a range of 5.2-311.2 ppm [93]. In soils, cadmium is incorporated in humic acids, with about half the cadmium present in exchangeable form and the remainder present in coordination complexes [169], suggesting that coalifying organic matter could trap and retain cadmium during its conversion to lignite. The mean cadmium content of Moose River lignites is 0.76 ppm [93,124], present in sphalerite [93]. The range of cadmium concentrations in this lignite is 0--4.77 ppm [93]. Cadmium occurs in quite low concentrations in Beulah-Zap lignite, 0.044,4).048 ~tg/g on a whole coal basis, or 0.46-0.50 ~tg/g in the ash [ 123]. 5.3.9 Group III elements Boron has a dominantly organic association with the lignite of the Calvert Bluff formation, and also occurs with tourmaline [42]. Boron is organically bound in Saskatchewan lignites; high concentrations were likely introduced into the lignite by circulating groundwater [83]. In BeulahZap lignite, the boron content is 50(O810 ppm [130]. Somewhat lower values, 27-233 ppm on a whole coal basis, are seen in the Saskatchewan lignites [92], and an even lower value, 9.4 ppm, in Hat Creek No. 2 lignite [ 125]. Boron contents of the ashes of a variety of North and South Dakota lignites are in the range 9-500 ppm [ 131]. Gallium has been observed to be enriched at seam margins in lignites, implying that it was trapped there by interactions with the functional groups in the lignite structure [11]. Galliumcontaining metalloporphyrins have been isolated from a Turkish lignite [ 141 ]. However, gallium might also substitute for aluminum in clay structures. Gallium in the Calvert Bluff lignites is associated with the clays [42], and is associated exclusively with mineral matter in Indian lignites [134]. The gallium content of Beulah-Zap lignite is 16--20 ppm [130]. In the ash of a suite of Dakota lignites, gallium ranges from 1 ppm to 38 ppm [131]. 5.3.10 Group IV elements The retention of germanium is due to interaction with the hydroxyl groups in humic acids

264 [ 170]. In lignites of the former Soviet Union, 12% of the germanium content is adsorbed, and the remainder is chemically bonded to the lignite [171]. With these lignites also interaction with the humic acids is responsible for chemical incorporation of the germanium [171]. Germanium is introduced from solution during coalification. It interacts with lignin structures in the early stages of coalification [ 172]. Acid-soluble germanium is likely present as organic complexes [139]. Germanium is enriched in the lighter specific gravity fractions of Indian lignites, suggesting that it is largely associated with the organic matter [ 173] in coordination complexes. There was no correlation of germanium content with vertical distribution in the seam [ 173], though with Russian brown coals a high germanium content is noticed at the seam margins [ 174]. An inverse relationship between iron and germanium was noted in Russian brown coals, the presence of siderite indicating an absence of germanium, and marcasite and pyrite indicating a decreased germanium content. In ashes, a high germanium content correlates with high calcium and aluminum and low iron. In addition, germanium correlated directly with gallium and vanadium and inversely with nickel, cobalt, and lanthanum. In these coals only 0.6-2.3% of the germanium was bonded to humic acids [174]. However, in some Bulgarian lignites the germanium is associated with the humic and fulvic acids [128]. Occasional reports have indicated extremely high concentrations of germanium in some lignite ashes. Patuxent (District of Columbia) lignite ash contains 6.0% Ge [175], while other lignitic logs of similar age (Cretaceous) along the Atlantic seaboard are have as much as 7.5% Ge in the ash [ 176]. Patuxent lignite ash is well endowed with rare elements which, in this instance, scarcely merit the term "trace." The ash of this lignite can contain up to 5.0% vanadium, 0.8% chromium, and 0.2% gallium [175]. North Dakota lignite ashes used for comparison show maximum values, on a percentage basis, of .0065% Ge, .02% V, .088% Cr, and .006% Ga. A suite of lignites from the Dakotas have germanium contents, in the ash, in the range <1-90 ppm [1311. Tin concentrations in Beulah-Zap lignite ash range from 13 ppm to less than 4.6 ppm [130]. In other Dakota lignites similarly low concentrations occur, in the range <1-9 ppm [131]. Lead is associated with selenium in small particles of clausthalite in drill cores from four counties in Texas [97]. In addition, some 30 nm spherical particles rich in lead and tin were observed. Lead occurs as galena in the Calvert Bluff lignites [42]. The mean lead content of Moose River lignite is 25.1 ppm [93,124], with a range 5-155 ppm [93], possibly associated with the organic structure. A much lower lead concentration occurs in Beulah-Zap lignite, 0.2•

~g/g

[ 126], although other work indicates a lead content of 1.5 ~g/g on a whole coal basis, or 16 ~g/g in the ash [123]. An even higher lead concentration in the ash, 22-32 ppm, has also been reported [130]. These results with Beulah-Zap lignite fall in the range observed for other Dakota lignites, <1-57 ppm in the ash [131]. Saskatchewan lignites contain 6.4-17.3 ppm lead [92], inorganically associated [83].

265 5.3.11 Group V elements In Texas lignites, arsenic contents vary only slightly as a function of depth in the seam [161]. Arsenic is associated with the organic fraction, as indicated by its concentrating in the lighter density fractions. The arsenic content of these samples ranged from 1 to 5.5 ppm [177]. BeulahZap lignite has an arsenic content in this range, 3.3_+0.1 ~g/g [126]. Other work places the value at 2.63_+0.19 ~tg/g [127]. A wide range of arsenic concentrations is seen in ashes of various Dakota lignites, ranging from less than 0.001% to a high of 0.24% [131]. The range of arsenic concentrations in Saskatchewan lignites, 0.5-9.9 ppm, is somewhat wider, but otherwise comparable to, the values seen for the Texas and North Dakota lignites [92]. A higher value is observed for the Hat Creek No. 2 lignite, 12.7 ppm [ 125]. In Moose River Basin lignites arsenic is associated with sulfide minerals [93,124]. The mean arsenic content in Moose River Basin lignite is 4.0 ppm, with a range 3.0--6.0 [93]. In the Calvert Bluff lignites, arsenic is strongly associated with the organic fraction of the lignite, as well as with sulfides [42]. Both antimony and arsenic are associated with the inorganic portion of the Saskatchewan lignites [83]. The antimony content of Beulah-Zap lignite is 0.154_+0.012 ~tg/g on a whole coal basis [ 127]. Ravenscrag lignite has a higher antimony concentration, 0.8 ppm [83]. 5.3.12 Chalcogens The selenium content of some Texas lignite samples ranged from 3.9 to 22.9 ppm [177]. The association of selenium with the lower density fractions suggests that it is incorporated in the lignite with the organic portion. However, in the Calvert Bluff lignite, selenium has both organic and inorganic associations, being found in clausthalite as well as with the organic portion [42]. Selenium has been observed in other Texas lignites in association with lead in particles of clausthalite [97]. Selenium in Beulah-Zap lignite has a concentration of 2.2_+1.2 ~tg/g [ 126]; other work places the value somewhat lower, at 0.58_+0.15 0.g/g [127]. The range of concentrations in Saskatchewan lignites is 2.3-4.1 ppm [92]. The polonium in samples of Savage, Gascoyne, and Beulah lignites averaged, for two determinations, respectively 7.4, 10.2, and 4.5 mBq/g [168]. 5.3.13 Halogens Fluorine concentration in Beulah-Zap lignite is 35_+6 ~tg/g on a whole coal basis [126]. Bromine has a dominantly organic association in Calvert Bluff lignite [42], as it does in Saskatchewan lignites [83]. Chlorine also has a dominantly organic association in Calvert Bluff lignite [42] and in Moose River Basin lignites [93], though associates with both mineral and organic components in Saskatchewan lignites [92]. In Saskatchewan lignites, the chlorine content ranges from 39 to 65 ppm [92]. A similar concentration, 42.3 ppm, occurs in Hat Creek No. 2 lignite [125]. The bromine content of Beulah-Zap lignite is 0.9_+0.4 ~tg/g [126], also reported as 1.42_+0.19 ~tg/g [127]. This is at the low end of the range seen for Saskatchewan lignites, 1.4--4.1 ppm [92]. The bromine concentration is higher in Hat Creek No. 2 lignite, 11.8 ppm [125].

266 5.4 VARIABILITY OF INORGANIC C O M P O S I T I O N 5.4.1 Introduction As the foregoing sections have shown, the inorganic chemistry of lignites is very complex. Many of the major inorganic elements in coal distribute among two or more possible modes of occurrence, including ion-exchange sites on carboxyl groups, coordination sites on heteroatoms, and in a variety of minerals. Analysis of lignites to the trace level shows that virtually all elements, except the noble gases and highly unstable radioactive elements, occur at least to some extent. Furthermore, surveys of the variation of elemental concentrations vertically or horizontally within a seam or comparing one seam with another show that the inorganic composition of lignites is highly variable. Nevertheless, despite this complexity, it is important to realize that all of these aspects of the inorganic chemistry of lignite--the concentration of elements, their distribution among possible modes of occurrence, and their spatial distribution--are all governed by, and consequences of, the laws of chemistry and geology. Geochemical factors such as source rocks, tectonics, depositional environment, nature of the plant material and its degradation or alteration, and pH, Eh, and composition of ground water will vary over some range of possible conditions or values. It would not be unreasonable to expect, consequently, that an extraordinary variability in inorganic composition and mineralogy would be observed for coals. Remarkably, even if one considers coals of all ranks, geological ages, and geographic origins, rather than just lignites, the differences in mineralogy and in trace element concentrations are primarily differences of amount or degree rather than fundamental differences of kinds [5]. In fact, virtually all major minerals in all coals belong to one of four categories: oxides, carbonates, sulfides, and silicates [178]. The minerals that constitute the major components of all North American coals are in the relatively short list of pyrite, calcite, quartz, kaolinite, illite and mixed-layer clays, and siderite [5]. Even among accessory minerals, the only significant differences observed in a suite of 100 coals was a slightly higher frequency of occurrence of apatite and barite in coals from the western U. S. [ 179]. Trace element data for U. S. coals [7] show that the only significant difference among ranks is a decrease in the concentrations of six elements--Ba, B, Ca, Mg, Na, and Sr--as rank increases [5]. The practical implication of the variability of elemental concentrations in lignite is that if it is desired to know the average concentration of an element in a lignite seam, it is vital that the analysis sample be representative of the whole seam. 5.4.2 Factors affecting the inorganic composition of lignites (i)

Geologicalfactors.

The first factor affecting the observed inorganic composition of

lignite is the nature of the geological conditions prevailing in the depositional setting. These conditions may include the kind of depositional environment, the kinds of source rocks, and the tectonic characteristics of the region. The amount of detrital material brought into and deposited in the region where lignite is being laid down will be determined by these geological conditions. At

267 the same time, the chemical nature of the plant material and the extent to which the original molecular structures of the plant components are altered or degraded during coalification may affect the ability of the organic material to trap inorganic species in coordination complexes. The formation of authigenic material will be affected by the characteristics of the groundwater coming into the coalifying plant material. The important groundwater characteristics include its composition, pH, and Eh. The locations of the inorganic constituents in a lignite seam are determined by the ways in which they were accumulated in the lignite. Detrital constituents, carried in by water or wind, will most likely be enriched at the margins of the seam and at any partings within the seam. Examples of detrital constituents include quartz, feldspar, mica, clays, and volcanic ash [43]. Authigenic minerals are mainly formed by precipitation from solution. As groundwater moves through the accumulating plant debris or through the lignite, ion-exchange processes with carboxylic acid groups or other minerals can also occur. Depending on the composition of the groundwater, changes in its composition over time, and changes in patterns of groundwater movement, distribution patterns reflecting the influence of groundwater could involve concentration of elements at the seam margins or enrichment at the center of the seam. Some inorganic elements may also be contributed by the plant matter; the vertical distribution patterns of such elements may be related to the depositional history of the original swamp. The variations in elemental concentrations in a particular lignite deposit will result from the combined effects of several factors. One is the geochemical behavior of the element, i.e., its likelihood of occurring as, for example, an insoluble hydrolyzate or a soluble cation, as determined by ionic potential. A second factor is the characteristics of the lignite, such as the availability of ionexchange sites for binding cations. A third factor is the geological setting of the deposit, which might determine, for example, the potential for, and extent of, authigenic mineralization. The observed patterns of distribution can be explained by changes in depositional conditions during accumulation, changes during diagenesis and post-diagenetic processes. For example, the addition of detrital clay and silt at the beginning and end of peat deposition would increase Si and A1 concentrations at the margins of the seam. Other depositional factors include changes of Eh or pH, influx of volcanic ash, or changes in the flora during peat accumulation. Lateral flow of water through the deposit after deposition could selectively concentrate elements in the margins of the seam. Similarly, vertical flow of water might concentrate elements at either the upper or lower margin. Other post-depositional factors which might affect the distribution of the inorganic components include the extent of degradation of plant material, degree of compaction (influencing the permeability of the lignite or its surrounding sediments), and changes in groundwater chemistry. (ii) Organic vs. inorganic affinity. The organic or inorganic affinity of an element is determined by the relationship between the concentration of that element in the moisture-free coal and the ash value of the coal [8,180]. For lignites having an ash value greater than about 5%, an increase in mineral matter content generally reduces proportion of the trace element content that is

268 organically bound [ 181]. Linear least squares analysis of data from the Center mine [ 113] showed that Na, Ca, Mn, Br, Sr, Y, and Ba had organic affinity; Mg, K, Cu, As, Rb, Ce, and Eu showed both organic and inorganic affinity; and all the remaining elements had inorganic affinity. In the Calvert Bluff Formation lignite the concentrations of most elements correlate with the ash, suggesting a detrital origin. Elements displaying this behavior are AI, Ba, Cr, Mg, Ti, V, Si, Dy, Hf, La, Sc, Se, Ta, and Th [42]. Ca and Sr show a negative correlation with ash, indicating that they associate with the organic portion of the lignite. Co and As were also thought to have an organic association. A correlation of elemental concentration with specific gravity of densityseparated samples indicates the elemental association with the mineral constituents. The elements in these samples that show this behavior are Na, AI, Cr, Fe, Mo, Sb, Ba, W, U, Th, Sc, Hf and most of the rare earths [42]. The only elements whose concentrations decrease with increasing specific gravity are Ca and Co. No trend was evident for V, As, Se, Cs, Ag, Ta, C1, and Br. Samples of high ash tended to have a higher proportion of the detrital minerals quartz and illite and a lower proportion of authigenic minerals kaolinite, calcite, siderite, and pyrite [42]. About 10% of the rare earth elements associate with the organic fraction [5]. In Saskatchewan lignites Na, Sr, S, Br, and B are entirely organically bound [92]. Ba, Ca, Co, Mo, Ce, Dy, Lu, C1, Sc, and W associate with both inorganic and organic components. The remainder of the elements are almost entirely inorganically bound. Generally, patterns of elemental distribution showing little vertical variation (sometimes called regular or even distributions within the seam) are observed for those elements associated with the organic components of lignite. More irregular pattems, reflecting concentration at seam margins or at one or more locations within the seam, may be characteristic of elements related to detrital and authigenic minerals. Elements displaying an even distribution are characterized by ion-exchangeable behavior during chemical fractionation, an organic affinity, and an ionic potential less than 3 [30]. Generally these elements include Ba, Ca, Mg, Mn, Na, and Sr. Elements showing enrichment at one or both margins are characterized by association with the acid-soluble fraction or the residue in chemical fractionation, an inorganic affinity, and ionic potential between 3 and 12 [30]. Elements which usually fall into this group include Al, Br, Ce, Cl, Cr, Eu, Rb, Sm, Sc, Si, Th, Ti, U, V, Yb, and Zr. The elements having a random or irregular distribution remain in the residue during chemical fractionation, have an inorganic affinity, and tend to form by authigenic mineralization. In addition, most are chalcophilic. These elements include Sb, As, Cd, Fe, Ni, Se, and Zn. The irregular distribution of these elements is a result of the syngenetic and epigenetic mineralization by formation of sulfides. (iii)

Effects of groundwater. In

North Dakota, the most common subsurface water

composition in Tertiary deposits has dominant concentrations of Na+, HCO3-, and SO4-2 [ 182]. As meteoric water infiltrates below the surface, mineral dissolution and ion-exchange processes occur. The composition of the water at equilibrium, and the concentrations of the inorganic species in it, depend on the initial pH, the extent of calcium (from dissolution of calcite, dolomite, or gypsum)

269 exchange for sodium on ion-exchange sites in clays, and whether pyrite is dissolved. As the meteoric water first infiltrates below the surface, it becomes charged with CO2. Dissolution of calcite in this water results in the dominant ions being Ca+2 and HCO3--. Some Mg+2 may be added from dissolution of dolomite. Ion exchange of Ca+2 for Na+ increases the concentration of Na + in the water. Dissolution of pyrite or gypsum provides a source of SO4-2. In North Dakota, subsurface water movement is generally slow. The hydraulic conductivity of Tertiary sand is 10-5 to 10-6m/s; of glacial till, 10-8 to 10-10 m/s, and of lignite, 10-5 to 10-7 m/s [182]. The average compositions of pore water from the spoils of two North Dakota lignite mines are shown in Table 5.30 (calculated from data in [ 182]). The composition of the pore water in the spoils may be affected by hydrogeological processes accompanying mining and thus may not be representative of groundwater percolating through undisturbed overburden. TABLE 5.30 Average compositions of pore water from spoils at two North Dakota lignite mines, mg/L [ 182].

No. of samples Magnesium Sodium Potassium Bicarbonate Sulfate

Center Average Std. Dev. 8* 215 105 433 355 34 22 854 421 1775 755

Indian Head Average Std. Dev. 4 172 35 1083 92 36 13 1286 46 2493 315

*Seven samples analyzed for potassium, six for the anions.

The ratio of sodium to (calcium + magnesium) in the groundwater determines the sodium distribution in the lignites near Underwood, North Dakota [183]. The factors controlling the Na/(Ca+Mg) ratio include the texture, lithologic composition, and the thickness of the overburden. Ion-exchange processes can only occur when the lignite is saturated with water. Consequently, the position of the water table relative to the lignite is also important. If the lignite becomes unsaturated, the elemental distribution (particularly the sodium distribution) established during saturation will be preserved into the unsaturated condition. Sodium-rich lignites are characterized by low hydraulic conductivity in the lignite and the overburden, and by a clayey overburden rich in exchangeable sodium, whereas low sodium lignites have thin, coarse-textured overburden [ 183]. The correlation of the sodium content of the lignite with the Na/(Ca+Mg) ratio of the groundwater suggests that groundwater establishes an ion-exchange equilibrium with the lignite [183]. It is important to determine whether the composition of the groundwater is established before it enters the lignite, or whether the groundwater composition is established after being in contact with the lignite. That is, does the groundwater composition establish the inorganic content

270 of the lignite, or does the inorganic content of the lignite establish the groundwater composition? A linear correlation between the percentage of sodium in dry lignite and the Na/(Ca + Mg) ratio of a saturated paste extract of the sediment above the lignite establishes the former case: the composition of the groundwater is determined before it enters the lignite and therefore controls the inorganic composition of the lignite, and not vice versa. Sodium in groundwater most likely derives from clay in the overburden. Water moving downward first becomes enriched in calcium and magnesium by dissolution of calcite and dolomite, followed by dissolution of gypsum. As this calcium- and magnesium-rich water then moves through clayey sediments, exchange of calcium and magnesium with clays enriches the water in sodium. The dominant clay in the lignite overburdens in the Underwood area is sodium smectite. High-sodium lignites are almost invariably overlain by these clayey sediments. High-sodium lignite also occurs in areas of thick overburden [ 183]. An inverse relationship exists between the thickness of the overburden and the hydraulic conductivity of the lignite; the hydraulic conductivity decreasing by an order of magnitude for each 17 m increase in depth [183]. The reduction of hydraulic conductivity could reduce the groundwater flow, and in turn the reduction in groundwater flow could reduce the rate of sodium removal from the overburden and sodium exchange with the lignite. However, thickness of overburden alone does not guarantee a high sodium lignite. If the overburden is not a significant reservoir of sodium (e.g., a sand overburden), the lignite will be low sodium regardless of overburden thickness. Several factors affect the Na/(Ca+Mg) ratio. A reservoir of sodium-rich clay in the overburden is necessary to provide a large Na/(Ca+Mg) ratio in the groundwater and thus to prevent a calcium- and magnesium-rich groundwater from removing sodium from the lignite. A high-sodium lignite would require a clay overburden several hundred meters wide and several meters thick [ 183]. If the lignite is above the water table, no exchange will occur. Taken in combination, these factors affect the sodium content of the lignite in the following ways [183]: A lignite below the water table, with clayey overburden greater than 25 m thick, will be high sodium. A similar lignite but with a sandy overburden will be low in sodium, because the groundwater reaching the lignite will have a high content of calcium and magnesium and will, therefore, exchange sodium from, rather than into, the lignite. A lignite below the watertable but overlain by thin sandy, silty, or clayey overburden will be low in sodium. A lignite above the water table, regardless of the thickness and lithology of the overburden, will have whatever sodium content was established at the time the lignite was beneath the water table. At depths below 60 m the water flow in the lignite will be very sluggish. The Na/(Ca+Mg) ratio in the groundwater will be high [183]. An inverse relationship between sodium and calcium, and between sodium and (calcium + magnesium) occurs in North Dakota lignites and their lithotypes, suggesting some petrographic control or relationship with inorganic composition [ 184]. Low-sodium lignite of the Williston basin is generally overlain by loosely consolidated sand or silt [ 184]. More generally, low-sodium lignite is always overlain by immediately (i.e., within about 60 cm) sandy or silty roof rock or by thin, weathered overburden of any lithology [184].

271 5.4.3 Vertical variability within seams (i) Introduction. Elements with inorganic association generally show distribution patterns having a concentration at one of both margins of the seam. In a few cases, mainly with minor elements, an irregular or undefinable distribution pattern is observed. These elements are primarily unaffected by chemical fractionation, remaining in the residue. Such elements also have ionic potentials in the range of 3-12, characteristic of elements normally expected to form insoluble hydrolysates in geochemical processes [185]. This group of elements includes Si, Sc, Ti, V, Cr, Co, Zr, Sb, Cs, La, Sm, Eu, Yb, Th, and U. They are frequently found to occur in detrital or authigenic mineral grains in the upper and lower margins of the seams. In comparison, elements having an organic affinity are ones having ionic potentials less than 3, and expected to occur as hydrated cations [ 185]. The distribution patterns show a slight concentration in the center of the seam, or, again in some cases, an irregular distribution. During chemical fractionation, they are almost completely removed either in the ion-exchange step or by ion exchange and HC1 extraction. Electron microprobe analysis of individual lithotype or maceral samples shows that the elements having organic affinity are disseminated through the organic components with no evidence of occurrence as discrete mineral grains. The main elements in this category are Na, Ca, Sr, and Ba. Many trace elements (e.g., Be, Ge, Yb, Y, and Sc) tend to concentrate at the seam margins [14]. Some others have fairly even distribution profiles (e.g., Cu and V) [14]. The concentrations of trace elements appear to vary widely from one seam to another. This variability reflects differences in the availability of the elements from nearby source rocks. Since many trace elements are in low concentration or even undetectable in roof rocks and seam partings, the organic material of the lignite likely had a role in trapping these elements from circulating groundwaters. (ii) Northern Great Plains lignites. Beulah-Zap lignite typically has a higher ash value at the seam margins compared to the inner portions of the seam [87]. This relationship persists laterally through a wide area encompassing the Beulah, Indian Head, and Freedom mines. The distributions in a vertical section of the Beulah seam follow the following trends: Co, Se, Eu, Sm, Sb, and Br are concentrated at the margins; V, Cr, and Sb are concentrated at the base; Ba, Yb, U, V, Cr are concentrated at the top; Mg, Ti, Ru, Cu, Zn, Ni, K, A1 and Si have an even distribution throughout the section; and Na, Ca, and Se are concentrated in the center of the seam [31 ]. Other studies of the Beulah-Zap bed show that the elements concentrated at one or both margins include Sb, Br, Ce, Cr, Co, Eu, La, P, Sm, Sc, Si, Th, Ti, U, V, Yb, Y, and Zr [41 ]. Elements showing concentration in the central portion of the seam, sometimes somewhat irregularly, are Ba, Ca, Mg, Na, and Sr [41 ]. Elements with irregular distribution patterns (which may in fact be a superposition of two or more patterns) include A1, As, Cd, Cu, Ge, Fe, Mn, K, Se, S, and Zn [41 ]. Patterns of elemental distributions for 36 elements in three sections of Beulah and one of Center lignites have been published [30,41]. For the four sets of data, the pattern of elemental distribution (e.g., enrichment at both margins, enrichment at the center, etc.) for any given element

272

is almost never the same in the four sections. Only Eu and Sc consistently show enrichment at both seam margins. A larger group of elements--A1, Sb, Cr, Co, Cu, Sm, Si, S, Ti, U, V, and Yb---are enriched in all cases either at one of the margins or at both. Iron consistently shows an irregular distribution, presumably reflecting concentration of pyrite in different portions of each of the seams. The variability of inorganic composition of stratigraphic sequences in the Beulah and Center mines was determined by sampling overburden, lignite, seam partings, and underclay, with analyses for 35 elements were by neutron activation or X-ray fluorescence techniques. Extensive tables of the data from this study have been published [74,113]. Examples of the patterns of distribution in the Beulah mine (Beulah-Zap bed) are shown in Figure 5.2, and patterns in the Center mine are shown in Figure 5.3. '

3.5

~, . . .

.

.

I .

i

.

3

2.5

,=..,,.,

,,"

Z ~

o

!

1.5 1

m I-! 17/1 !i i II

i w=

.~

0.5 0

IRON CALCIUM SILICON ALUMINUM MAGNESIUM SODIUM

-0.5 9

0

,

I

,

I

,

I

,

10000

Elemental concentration, ppm dry basis Figure 5.2. Vertical distribution of major elements in Orange pit, Beulah lignite [74, 113]. In Center lignite the elements concentrating at or near the margins of the seam are A1, Ti, Fe, C1, Sc, Cr, Co, Ni, Zn, As, Ru, Ag, Cs, Ba, La, Ce, Sm, Eu, and U [186]. These elements are, in most cases, associated with detrital constituents such as clays. These elements have ionic potentials in the range of 3 to 12, expected geochemically to form insoluble hydrolyzates, and usually occur as acid-insoluble species in chemical fractionation. Elements with ionic potentials less than 3 occur as hydrated cations and generally show even distribution through the seam.These elements usually appear in the ion-exchangeable fraction during chemical fractionation. They are Cd, Mn, Mg, Na, and Ca [186]. These elements are associated with the organic portion of the

273

2.15 1.85 1.55 1.25

am 1"7 !~1 !~ WI II

0.95 0

0.65

Iron Calcium Silicon Aluminum Magnesium Sodium

0.35 0.05

0

10000 Elemental concentration, ppm dry basis

20000

Figure 5.3. Vertical distribution of major elements in Center lignite [74,1113].

lignite or with authigenic minerals. V, K, and Sb increase toward the base of the seam [186]. Elements having no clear pattern of distribution are Se, Br, Cs, and Yb. The concentrations of the rare earth elements in this lignite agree with the rare earth abundance pattern in sedimentary rocks. The addition of detrital clay and silt at the beginning and end of peat deposition would increase Si, AI, Mg, Ca, Na, and K at the margins of the lignite. Elemental redistribution (particularly of those elements which are exchangeable) might then result from the flows of meteoric water or groundwater. Lignite of the Kinneman Creek bed shows the same major trends of elemental distribution: concentration at the margins, concentration in the lower part of the seam, an even distribution, and irregular distribution [74]. A1, Si, S, Sc, Fe, Co, Ni, Zn, As, Rb, Y, Zr, Ag, Ba, Ce, Sm, Eu, Yb, Th, and U concentrate at the margins. Elements concentrated in the lower part of the seam are C1, K, Ti, V, Cr, Cu, Ge, Se, Ru, Sb, Cs, and La. Elements showing an even distribution (sometimes tending toward a slight increase in concentration at the center of the seam) include Na, Mg, Ca, Sr, and Mn. Elements with irregular distribution are P, Br, and Cd. In the Baukol-Noonan mine sodium, which exists predominantly or wholly in ionexchangeable form, tends to concentrate near the center of the seam [187]. In contrast, arsenic concentrates at the seam margins. Estevan (Saskatchewan) lignite shows higher uranium values at the top and bottom of the

274 seam [ 188]. This behavior suggests uranium sorption by interaction of the organic matter in the lignite with circulating groundwaters after the lignite had been buried. Vanadium concentrates at the top of the seam [188], similar to North Dakota lignites [189]. Other work with Saskatchewan lignites shows that Zn, S, and Se concentrate near the top of the seam; Sb and Be show concentration toward the bottom [83]. (iii) Gulf Coast lignites. Ion-exchangeable barium and manganese decrease by a factor of two with increase in depth, whereas calcium, magnesium, strontium, and sodium do not show this behavior in Darco lignite [11]. The concentration gradient for barium and manganese may be a result of a chromatographic effect caused by the ions being carried downward by groundwater. Both barium and manganese have a high degree of ion exchange specificity. The molar ratio of ionexchangeable Ca/Ba increases from 64 at the upper margin of the seam to 100 at the lower margin [ 11]. This is typical of an ion-exchange process from groundwater, since Ba§ should replace Ca§ as long as the concentration of Ba§ in solution is adequate. The sodium content of two Texas lignites increases with increasing depth from the surface

(i.e., sodium content increases near the bottom margin of the seam) [ 190]. The percentage of the total sodium in a water-soluble form decreases as depth increases. The patterns of vertical variability of minerals in Martin Lake lignite reflect changes in the depositional conditions during accumulation of the lignite, and subsequent chemical changes during and after diagenesis [96]. The environment of deposition was characterized by abundant quartz (sand) prior to the development of the peat swamp. Clay was deposited at the end of peat accumulation and afterward. An increase of pyrite in the center of the seam may reflect changes caused by flow of meteoric water. The vertical variability of elements in Darco, Wall (Wyoming), and eastern Alabama lignite shows an even distribution for the ion-exchangeable portions of calcium (all three seams), magnesium (Wall and eastern Alabama), sodium (Wall), strontium (eastern Alabama), barium (Darco), and manganese (Darco) [191]. In comparison, the acid-insoluble portions of aluminum, potassium, silicon, and magnesium in the Darco seam showed high concentrations at the margins and near partings, with a sharp increase in concentration in approximately the center of the thickest lignite [ 191 ]. In four lignites, three from the Gulf Coast and one from the Northern Great Plains [11], some trace elements are enriched with respect to at least one seam boundary, i.e., a roof, floor, or parting. All four lignites showed enrichment of Be, Cu, Ga, Sc, Y, and Yb. Three of the lignites showed enrichment of Cr, Ni, and V; two, Ge, La, Pb, and Zr; and one lignite showed enrichment of Ce relative to surrounding sediments. These four coals, including Texas, Alabama, and North Dakota lignites, show little variation of ion-exchangeable calcium with seam height [14]. The variation in calcium correlated with petrographic composition. Similar behavior was observed for the ion-exchangeable portions of magnesium, strontium, barium, manganese, sodium, and potassium. This behavior is attributed to the influence of circulating groundwater on establishing the concentrations of the ion-

275 exchangeable cations. In particular, calcium is the major cation in most groundwaters, and the divalent calcium cation will preferentially replace monovalent cations in ion-exchange processes; calcium is the predominant cation in the coals studied [ 14]. Little vertical variation was observed for the trace elements in an Alabama lignite [139]. In eastern Alabama lignite the exchangeable elements showed minor fluctuations with depth [11]. Zones of high concentrations of fusinite and inertodetrinite also showed higher concentrations of exchangeable elements. 5.4.4 In-mine variability The variation of total sulfur between different sections of the same coal bed is very pronounced, except for cases of very low sulfur lignites [ 192]. This variability is attributed to an irregular distribution of pyrite [192]. Data from a survey of the variability of ash composition and fusibility of lignites from ten major mines in western North Dakota and eastern Montana [ 106,193] were treated statistically by determining the range and 2a (twice the estimated standard deviation) limits. For samples taken from a single mine, either at 6.1 m horizontal intervals or vertically through the bed, the 2a limits were of about the same order of magnitude as the 2~ limits for all the samples (i.e., representing the entire mine). This result suggests that much of the variation within a mine occurs within relatively short distances. In commercial operation, however, much of this variability may be eliminated as blending occurs during mining and subsequent handling and transportation. One exception to these findings is sodium, where samples taken close together showed smaller limits of variation than did the data for the entire mine [193]. Within a single vertical section much of the apparent variability of sodium content could be eliminated by expressing the sodium content as a percentage of coal rather than of ash. The observation is in keeping with more recent work [ 11,16,56] which shows that for most lignites most of the sodium is associated with the carbonaceous portion on ion-exchange sites. The organic association of sodium indicates that the concentration of sodium does not correlate directly with ash content. The composition of ash also varied significantly between mines. The variability within a mine was sufficiently unique that it could be considered to be a characteristic of that mine. Lignite samples from three mines in the Hagel bed showed a correlation between the percentage of ash and several kinds of colpate and porate palynomorphs [194]. These palynomorphs are of detrital origin, suggesting that much of the inorganic material contributing to ash is also of detrital origin. An extensive study has been made of the variability in inorganic composition and mineralogy of lignite from three pits in the Gascoyne mine [ 195], identified as the Red, White, and Blue pits. The elemental composition of the lignites is shown in Table 5.31 [195]. A qualitative comparison of the mineral components of these lignites is shown in Table 5.32; the major and minor minerals are listed according to apparent decreasing abundance, based on peak heights in the TABLE 5.31

276 TABLE 5.31 Variability of inorganic composition of lignite from three pits of the Gascoyne mine. (Data are shown as ~tg/g, dry basis) [195]. Element Aluminum Calcium Chromium Copper Iron Magnesium Molybdenum Potassium Silicon Sodium Strontium Titanium Zinc

Red Pit 5240 15290 9.4 5.6 5300 4719 13.1 367 13690 2041 172 429 10

Blue Pit White Pit 11480 8140 14070 19550 13.4 13.9 13.4 9.9 2810 3420 4885 6000 23.0 22 1299 358 33060 12550 5479 6119 129 94 1060 370 <20 9

X-ray diffractogram of the low-temperature ash.

TABLE 5.32 Variability of mineral constituents of lignite from three pits of the Gascoyne mine [195]. Red Pit Quartz Kaolinite*

Blue Pit Quartz Bassanite Kaolinite*

White Pit Quartz Bassanite Kaolinite*

Minor

Pyrite Bassanite Illite ? Barite

Pyrite Illite ? Barite

Pyrite Calcite

Rare

Muscovite Zircon

Siderite Chromite Zircon SrSO4

Barite Rutile SrSO4

Major

*Identified as the hydrated form, halloysite.

Carboxyl contents of the three lignites are similar, ranging from 2.2 meq/g for the White Pit lignite to 2.6 meq/g for the Blue Pit lignite. Generally the percentage of each element removed in chemical fractionation by ammonium acetate extraction is similar among the three lignites. Furthermore, those elements customarily removed in large percentages by ammonium acetate solution, such as sodium and magnesium, are similarly removed in large percentage from these

277 52% for the White. Extraction with disodium EDTA removes 40% of the aluminum from the Red Pit lignite, but only 16-18% from the other two lignites, suggesting that a much higher percentage of the aluminum may be present in coordination complexes in the Red Pit lignite. Chemical fractionation results are summarized in Table 5.33 [195]. In general the behavior of the major elements is similar among the three samples, and, of the minor elements, only strontium shows marked differences (compare the White Pit lignite with the other two). TABLE 5.33 Chemical fractionation results for lignite from three pits of the Gascoyne mine. (Results expressed as percent of element originally in the lignite.) [195].

Element Aluminum Calcium Chromium Copper Iron Magnesium Molybdenum Potassium Silicon Sodium Strontium

Red Pit NH4OAc HCI 0 52 74 14 12 38 5 0 0 38 97 0 23 52 10 0 0 2 82 1 73 21

Blue Pit White Pit Res. NH4.,OAc HCI Res. NIqa.OAc HCI 48 0 31 69 0 32 12 75 24 1 71 29 50 18 22 60 15 14 95 0 16 84 0 0 62 0 29 71 0 32 3 91 5 4 80 6 25 13 30 57 15 21 90 4 0 96 14 0 98 0 2 98 0 5 17 100 0 0 91 0 6 17 83 0 69 31

Res.* 68 0 71 100 68 14 64 86 95 9 0

*Percentage of element remaining in residue (i.e., not extracted by either reagent)

The comparison of the compositions of low- and high-sodium lignites from the Gascoyne and Beulah mines is shown in Table 5.34 [196]. In the high-sodium lignites the carboxyl, calcium, and barium are higher than in the lowsodium lignites from the same mine. Magnesium, aluminum, silicon, iron, and ash contents are lower in the high-sodium lignites than in the corresponding low-sodium lignites. The low temperature ashes of the lignites are similar. Chemical fractionation of these four samples shows a higher removal of aluminum by hydrochloric acid from the high-sodium lignites than from the low-sodium lignites from the same mine [196]. In all four samples the ratio of the amounts of aluminum removed to silicon removed range from 4:1 to 14:1 [ 196]. This value does not correspond to the AI/Si ratio in any common clay mineral, and suggests that hydrochloric acid selectively attacks the aluminum-containing layer in the clay. In the residue insoluble potassium is present in micaceous clays, titanium in futile, and barium as barite.

278 TABLE 5.34 Comparison of compositions (dry basis) of low- and high-sodium lignites from Gascoyne and Beulah mines [196].

Component Aluminum (a) Barium Calcium Iron Magnesium Manganese Potassium Silicon Sodium Ti tani um Carbon (c) Hydrogen Nitrogen Sulfur Oxygen Ash Carboxyl (d)

Gascoyne Low-sodium High-sodium 8740 7300 593 1268 17370 22790 3890 2540 2991 2588 123 163 1260 1430 28640 10920 1317 2694 1180 546 58.3 54.5 4.0 5.2 0.88 0.84 1.7 1.4 19.6 19.3 15.5 8.8 2.46 2.54

Beulah Low-sodium High-sodium 5420 2890 179 397 15610 18110 10240 5160 1476 979 52 24 916 ND (b) 8950 3530 1379 4625 503 583 61.4 66.4 4.1 3.6 0.42 0.87 3.2 1.2 18.2 19.5 12.6 8.4 2.47 2.76

Notes: a) Elements aluminum through titanium reported as ~tg/g, dry basis; b) Not determined; c) Elements C through O and Ash reported as weight percent, dry basis (oxygen by difference); d) meq/g dry basis.

5.4.5 State and regional variability Data on the composition of 413 samples of ashes from North Dakota lignites have been tabulated [ 197]. Average CaO content is high, at 31%. The highest concentrations occur in samples from the eastern part of the state's lignite fields (east of Range 97W). Na20 averages 6.5% but with a very high variability, having a standard deviation of 5, and a range of 0.1-27%. The highest SiO2 contents occur west of Range 97W. The average for the state is 27%; variability is high. The average A1203 content is 14%; that of Fe203, 12%. Neither of these components shows great variability or evident geographic trends. The variability among samples from a given mine was large for all of the mines sampled in the study. The variability of ash composition for samples taken from the Gascoyne mine was remarkable. For example, SiO2 ranged from 61.6% to 18.0%, CaO from 44.7% to 14.7%, and Na20 from 10.2% to 1.1%. Similar data for 82 samples of Montana lignite ash has been published [197]. The CaO content averaged 27%, with high variability but no evident trend with location. The average SiO2 content was 31%. The A1203 content averaged 20% with no location trend. Fe203 averaged 8% but with high variability. The average Na20 content was 2.2% with high variability. Even allowing for variability, this sodium content is markedly lower than that found in North Dakota lignites. Analyses of eighteen Northern Great Plains province samples show that this province has the highest mean values of Mg, Ca, Ba, and Na, and the lowest mean values of Si, Ti, K, Cr, Cu,

279 the highest mean values of Mg, Ca, Ba, and Na, and the lowest mean values of Si, Ti, K, Cr, Cu, Ga, La, Ni, Rb, Sc, Yb, and Zn of coals from the western coal provinces (Northern Great Plains, Gulf, Rocky Mountain, and Pacific) in the United States [198], when the data are expressed on an ash basis. On a whole-coal basis, this province has the highest means of Mg, Ca, Na, and Ba, and lowest means of Si, Al, K, Cr, Ga, La, Ni, Sc, U, Y, Yb, Zn, and Zr; pyritic and organic sulfur; and high-temperature ash. (In this work, high-temperature ash is prepared by heating a 10 g sample of coal slowly to 750"C, holding at that temperature over night, grinding the ash to -200 mesh, and drying at 750* for three hours [ 198].) A ternary composition diagram for Northern Great Plains province ashes is shown in Fig. 5.4 [ 198]. This diagram is prepared by normalizing SiO2, A1203, and the sum of Fe203, CaO, and MgO to 100%. These samples have SIO2/A1203 ranging from 0.73 to 3.76 and 20-59% of the other elements, which is a moderately high amount in comparison to the ashes of coals from other provinces in the United States.A survey of the inorganic composition of American coals includes data on the major element concentrations in six Fort Union lignites [8]. Five of these lignites were from North Dakota and the other was from Montana, but the origin of the samples was not otherwise specified. The ranges of concentrations are shown in Table 5.35. (Trace element data on the same samples were provided in Table 5.28.) TABLE 5.35 Major element concentrations in six Fort Union lignites [8]. Element Aluminum Calcium Chlorine Iron Magnesium

Range 0.31-0.89 1.70-3.80 0.01-0.03 0.40-O. 60 0.18-0.3 9

Element Potassium Silicon Sodium Titanium

Range 0.01-0.07 0.58-1.30 0.02-0.46 0.02-0.04

Analyses of 18 Gulf province samples show that on a whole-coal basis, the Gulf province coals have the highest mean values of Be, Ga, Mn, Sc, U, and pyritic, organic, and sulfate sulfur, and the lowest means of P and Ba [ 198] among the four western coal provinces. A ternary composition diagram for Gulf province ashes is shown in Fig. 5.5 [198]. The SIO2/A1203 ratio is fairly high, ranging from 1.78 to 4.88, and the other elements have a very wide range, 3-52%. For most major and trace elements, the ranges of concentrations in the Fort Union and Gulf Coast lignites overlap [21]. Comparison of mean concentrations plus or minus one standard deviation shows that aluminum, bromine, europium, lanthanum, nickel, scandium, selenium, and titanium concentrations tend to be higher for the Gulf Coast lignites in this data set, while the silver concentration is higher in the Fort Union lignites [199]. The ranges of concentrations for the other elements overlapped between the two sets of lignites.

280

SiO2

5O

A1203

Fe203 + CaO + MgO

Figure 5.4. Ternary composition diagram for ashes of Northern Great Plains province coals, normalized to 100% [198]. Compositions of 18 ash samples lie within shaded region.

SiO2

A1203

Fe203 + CaO + MgO

Figure 5.5. Ternary composition diagram for ashes of Gulf province coals, normalized to 100% [198]. Compositions of 18 ash samples lie within shaded region.

Useful surveys of data on ash composition have been published [115,131,200]. An extensive compilation of data on variability in the Beulah mine for about 30 elements has been

281 published [41]. A survey of variability of ash composition and fusibility of lignites from ten major mines in North Dakota and Montana, together with supplemental data on ash fusibility and information on the location and mining practices of each of the ten mines, has been published [106,193]. REFERENCES

10 11 12 13 14 15 16 17 18 19 20 21

U.S. Bureau of Mines, Technology of Lignitic Coals, U.S. Bur. Mines Inf. Circ. 7691, (1954). G.A. Richter, Cellulose from hardwood, Ind. Eng. Chem., 33 (1941) 75-83. P.L. Broughton, Silicified lignite from the Tertiary of south-central Saskatchewan, Can. J. Earth Sci. 13 (1976) 1719-1722. K. Hoehne, Late Tertiary silicified wood in the Rosenback coal seams near Latschach, Lake Faaker, Carinthia, Geologie 2 (1953) 185-189. R.B. Finkleman, The origin, occurrence, and distribution of the inorganic constituents in low-rank coals, in: H.H. Schobert (Ed.), Proceedings of the Low-rank Coal Basic Coal Science Workshop, U.S. Dept. Energy Rept. CONF-811268, (1982), pp. 69-90. R.B. Finkelman, Mode of occurrence of accessory sulfide and selenide minerals in coal, Proc. IX Intl. Carboniferous Conf., 1982. V.E. Swanson, J.H. Medlin, J.R. Hatch, S.L. Coleman, G.H. Wood Jr., S.D. Woodruff, and R.T. Hildebrand, Collection, chemical analysis, and evaluation of coal samples in 1975, U.S. Geol. Surv. Rept. 76-468, (1976). H.J. Gluskoter, R.R. Ruchm W.G. Miller, R.A. Cahill, G.B. Dreher, and J.K. Kuhn, Trace elements in coal: Occurrence and distribution. Ill. State Geol. Surv. Circ. 499, (1977). D. Readett, G. Springbett, L. Green, K. Quast and S. Hall, Borecore analysis of South Australian lignites, Proc. 2d Coal Res. Conf. New Zealand, 1987, Paper R5.3. R.N. Miller and P.H. Given, The association of major, minor and trace inorganic elements with lignites. I. Experimental approach and study of a North Dakota lignite, Geochim. Cosmochim. Acta, 50 (1986) 2033-2043. R.N. Miller and P.H. Given, A geochemical study of the inorganic constituents in some low-rank coals, U.S. Energy Res.Devel. Admin. Rept. FE-2494-TR-1, (1978). H.N.S. Schafer, Carboxyl groups and ion exchange in low-rank coals, Fuel, 49 (1970) 197-213. W. Beckering, H.L. Haight, and W.W. Fowkes, Examination of coal and coal ash by xray techniques, in: J.L. Elder and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Inf. Circ. 8471, (1970), pp. 89-102. R.N. Miller and P.H. Given, Variations in organic [sic.] constituents of some low rank goals [sic.], in: R.H. Bryers (Ed.), Ash Deposits and Corrosion Due to Impurities in Combustion Gases, Hemisphere, Washington, 1977, pp. 39-50. .1.19. Hurley, personal communication, Grand Forks, ND, May 1986. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic constituents in American lignites, U.S. Dept. Energy Rept. FE-2030-TR21, (1980). M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic constituents in American lignites, Fuel, 60 ( 1981) 189-193. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Analysis of the inorganic constituents in American lignites, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 219222. J.K. Kuhn, F.L. Fiene, R.A. Cahill, H.J. Gluskoter, and N.F. Shimp, Abundance of trace and minor elements in organic and mineral fractions of coal, Ill. State Geol. Surv. Environ. Geol. Notes EGN88, (1980). J.P. Hurley, Oral presentation, Project Sodium semiannual sponsors' meeting, Grand Forks, N.D., November 1985. S.A. Benson, J.P. Hurley, E.N. Steadman, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984),

282 22 23 24 25 26 27 28 29

30 31 32 33 34 35 36 37 38

39 40 41 42 43 44

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