Crystallization behavior of coal gasification ash

Crystallization behavior of coal gasification ash

NUCLEAR AND C?iEMICAL WASTE MANAGEMENT, Vol. 5, Printed in the USA. All rights reserved. pp.221429.1985 0191-815X/85 $3.00 + .OO Pergamon Press Ltd...

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NUCLEAR AND C?iEMICAL WASTE MANAGEMENT, Vol. 5, Printed in the USA. All rights reserved.


0191-815X/85 $3.00 + .OO Pergamon Press Ltd.

CRYSTALLIZATION BEHAVIOR OF COAL GASIFICATION ASH J. I. Federer R. J. Lauf Oak Ridge National Laboratory, Oak Ridge, Tennessee37831, USA

ABSTRACT. Large-scale commercialization of coal gasification would result in large volumes of ash or slag requiring disposal. Burial of ash or slag raises environmental concerns because of the possible leachability of harmful trace elements into groundwater. In this study, Hygas ash was melted as a means of reducing the volume for disposal. The crystallization behavior of melted ash (slag) and other slags modified to lower the melting temperatures was determined. The principal constituents of Hygas ash were A120J, SiOl, and Fe,Oj with low alkali and alkaline earth content and small concentrations of other elements. This material melted at 1400 “C and recrystallized by rejection of iron oxides from a silicate @ss. Increasing the Na,O content to 37 wt% resulted in a totally glassy structure and a lowering of the meltine,temperature to about 850 “C. Limited leaching tests indicated that the leachability of melted Hygas ash and modified slags increased substantially with increasing NaaO content.

respect to leachability (2). The work involved several principal activities:

INTRODUCTION Renewed interest in coal gasification has developed in the United States in recent years as a supplemental energy source for foreign oil and domestic natural gas. Most gasification processes involve reaction of crushed coal with steam and air to produce gas and byproduct ash or slag (1). Low gasification temperatures produce ash particulates, while higher temperatures fuse the ash to form slag. A commerical plant might process 27 Gg coal/day (30,000 tons/ day). If the coal contained 10% ash, then 2.7 Gg ash/day (3,000 tons/day) would be produced. The ash will contain potentially hazardous elements, probably at higher concentrations than were present in the original coal. This ash must be disposed of in an environmentally acceptable manner, which includes restrictions on leachability of hazardous elements into soil and groundwater. The physical form of the ash might significantly affect leaching characteristics. Ash particulates, for example, might leach more than a compact slag of the same composition. Thus, consolidation of ash would not only decrease the volume of material for disposal, but might also decrease its leachability. The purpose of the work described herein was to investigate methods for stabilizing solid waste with RECEIVED 11/24/M;


consolidation of ash by melting; modification of original ash composition to lower the melting temperature; and ?? leaching of ash and melted ash with pure and acidified water. ?? ??

In conducting these activities we investigated the crystallization behavior of melted ash (slag) as a function of cooling rate and aging time, and identified phases in the slags. The leachability of Pb, Se, and Te dopants was investigated as a function of composition of the slags. CHARACTERIZATION


As-Received Ash The starting material was Hygas Process ash in the form of an aqueous slurry containing about 5 wtolo solids. (The Hygas Process was investigated by Institute of Gas Technology, Chicago, IL, as a method to produce high-BTU gas from coal.) The solids, after drying, consisted of black granules containing about 63 wt% carbonaceous material. The latter was oxidized in air at 800 “C, resulting in a tan granular material having the composition shown in Table 1. The principal equivalent oxides were Al,O,, Fe,OJ, and SiO,. Table 2 shows that principal phases deter-



J. I. FEDERER AND R. J. LAUF TABLE 1 Major Constituents of Hygas AsW’ Content (wtW) Element



Al Ca Fe K Mg Na S Si Ti

1.2 2.1 15.8 1.1 0.2 0.6 0.7 23.6 0.3

7.8 2.3 12.9 1.3 0.2 0.7
Content (wtV0) Equivalent Oxide ALO, CaO

FeZOJ KzO MgO Na,O SO, SiOl TiO,



13.6 2.9 22.6 1.3 0.3 0.8 1.4 50.6 0.5

14.7 3.2 18.4 1.6 0.3 0.9 54.6 0.5

aAlso contained approximately 40 other elements having concentrations of 1 to 700 ppm, including about 60 ppm Pb, 10 ppm Se, and 15 ppm Te. bHeated at 800 ‘C for 1 h in air to remove carbonaceous material. ‘Melted at 1400 “C for 1 h in air.

mined by x-ray diffraction were o-quartz, hematite, and glass, the latter being inferred by diffuse intensity in the low angle region of the diffraction pattern. Melted Ash Dried and decarbonized ash was pressed into pellets weighing about 1 g, and heated in platinum crucibles in air until melting occurred at about 1400 “C. Evidence to be presented indicated that some solid FezOJ was in the melt at 1400 “C. Table 1 shows that the composition of melted ash (hereafter termed slag) was not significantly different from the original ash. Table 2 shows that the phases identified by x-ray diffraction were hematite, magnetite, and glass. The appearance of magnetite was not unexpected since hematite and magnetite coexist at 1400 “C in air (3). Neither quartz nor any crystalline silicate phases were detected in the slags. Molten slags were cooled by quenching in water, by cooling in still air ( - 25 “C), and by decreasing the furnace temperature to 1000 “C in 3 or 4 h. The microstructures of slags cooled at different rates were substantially different. Water quenching, as shown in Fig. la, produced mostly glass with some residual (unmelted) hematite crystals. Figure lb shows that holding the molten slag at 1400 “C for 18 h resulted in larger crystals, some with faceted surfaces, con-

TABLE 2 Phases in Hygas Ash HA-la


a-quartz hematite glass

hematite magnetite glass

aDecarbonized at 800 “C. bMelted at 1400 “C.

firming that the crystals were solids in the melt. Apparently, the melted ash is saturated with iron oxides, which further precipitate on cooling. Figure 2 shows that air cooling caused incipient dendrite formation at the melt surface, on the crucible wall, and on preexisting hematite crystals. Furnace cooling, as shown in Fig. 3, resulted in a welldefined dendritic structure throughout the melt. Growth of original crystals with increasing time at 1400 “C is evident in Fig. 2 and 3. Electron microprobe analysis revealed the distribution of phases within the microstructure. Typical electron microprobe displays for a furnace-cooled slag (Fig. 4) show clearly that Fe is located in the dendrites and large crystals and that Si is in the matrix. In addition, we found that Ti partitions with Fe, and that the matrix contains Al, Ca, K, and Na in addition to Si. The lattice parameters of FezOJ determined by x-ray diffraction were slightly smaller than previously published values (4). The presence of Ti in the Fez03 crystals might have caused slight contraction of the lattice. Since x-ray diffraction did not reveal any crystalline silicate phases, we concluded that solidification and crystallization occur by rejection of iron oxides (containing Ti) from a silicate glass containing Al, Ca, K, and Na. This result appears to conflict with the AI,03-FeZOJ-Si02 phase diagram, which indicates that equilibrium phases are mulhte (3AI,0J* 2Si02), cristobalite (SiOJ, and a solid solution of hematite (FezOJ in mullite (5). It is possible that the slags did not attain equilibrium even during furnace cooling, or other constituents of the slags (particularly the alkali metals) affected the ternary equilibrium. Toxic trace elements in the ash might partition to the iron oxide crystals during crystallization, as did Ti. More likely these elements would remain uni-





Slag water quenched from 1400 “C. (a) Held for 1 h at 1400 “C, (b) held for 18 h at 1400 “C.

formly distributed in solution in the glass as the iron oxides crystallized. If crystallization of silicate compounds occurred, toxic elements might be incorporated into the crystalline structure. The resistance of the crystalline compounds to leaching might be significantly higher than that of glass. Therefore, slags were aged at 950 “C to determine if crystallization of

silicate compounds occurred. X-ray diffraction of water quenched slags subsequently aged at 950 “C for up to 100 h revealed an increase in crystallinity, but detected no crystalline phases other than iron oxides. We concluded, therefore, that crystalline silicate phases cannot be developed in the slag by aging at 950 “C for reasonable time periods.




Slag air-cooled from 1400 “C. (a) Held for 1 h at 1400 “C, (b) held for 18 h at 1400 “C.

Modif ied Ash

Reduction of ash volume, which simplifies disposal, could be accomplished by melting. Since Hygas ash melts at about 1400 “C, modifications of the ash

composition were investigated. In theory, relatively abundant and inexpensive materials could be added to the ash to lower the melting temperature. In this work the ash composition was adjusted approxi-




Slag furnace-cooled



from 1400 “C. (a) Held for 1 h at 1400 “C, (b) held for 18 h at 1400 T.

mately on the basis of lower melting ternary quarternary systems:


1. albite (NaAlSi,OJ-fayalite (Fe,SiO,) eutectic, which melts at 16 wt% fayalite (6);

2. NazO-ALO,-SiO, ternary system, which indicates melting at about 850 “C at 37 wt% Na,O-11 wt% Al,OJ-52 wt% SiOz (7). The higher soda (NazO) content of these systems




Electron microprobe displays of furnace-cooled slag. (a) Back-scattered electron image, (b) FeKa, (c) SiKcy.

relative to Hygas ash results in substantially lower melting temperatures. Although the Na,O-Al,OJSiOl ternary melts at about 850 “C, the presence of iron oxides would increase the melting temperature if true equilibrium occurred. Nevertheless, we adjusted the Na,O, A1203, and SiO, contents of several slags

as if iron oxides were not present, then examined the slags to determine the location of iron oxides. The nominal compositions of melted Hygas ash, synthetic Hygas slag, and modified slags are compared in Table 3. The synthetic slag and the modified slags were synthesized from oxides and carbonates.

221 TABLE 3 Nominal Composition of Melted Hygas Ash and Modified Slags Content (wt@/o) HYW

Synthetic modified Equivalent Oxide Al,03 CaO Fe,O, LO Na,O SiO,

Original HA-2 (1400 “C)”

Synthetic HA-6 (1400 “C)”

HA-16b (1050 QC)d

HA-17C (850 aC)d

HA-21C (850 oC)d

HA-22’ (850 oC)d

16 4 20 2 1 58

16 4 20 2 1 58

16 2 11 1 9 61

10 2 13 1 31 43

10 2 13 1 31 43

10 2 13 1 31 43

aMelting temperature. bBased on albite-fayalite eutectic (1050 “C). ‘Based on low melting region (850 “C) of Na&LAl,O,-SiOl ternary system. If Fe,O, is neglected, the nominal composition is 11 wt% Al,Os-37 wt% NalO-52 wt% SiO,. dExpected melting temperature.

The Na,O and K,O were derived from carbonates, which offered the possible advantage of forming a liquid at about 850 “C, a temperature considerably lower than the melting point of the oxides. This liquid might have hastened the dissolution process required to produce a homogeneous melt. Evolution of CO, was controlled by the heating rate. Some melted Hygas ash, synthetic Hygas slags, and modified slags were doped with Pb, Se, and Te for subsequent leaching tests. Dopants were added during melting either as the elements or as PbO, PbSO+ PbSe, and NazTeO*2H,0. These elements were assumed to be incorporated into the glass matrix. Electron microprobe analysis did not indicate any tendency for concentration of the elements in iron oxide crystals. The microstructure of synthetic Hygas ash was similar to that of melted Hygas ash. The microstructure of modified slag based on the albitefayalite eutectic ( - 9 wt% Na,O) consisted of crystals in a glass matrix, as shown in Fig. 5a. Electron microprobe analysis revealed that the crystals contained only iron (within the limit of detection of the microprobe) and that the glass contained the other elements of the original constituents. Slags based on a low melting composition in the Na20-Al,O,-Si02 ternary system ( - 31 wt% Na,O) contained no crystalline phases (as shown in Fig. 5b), and electron microprobe analysis revealed the presence of all of the original metallic elements. The high soda content of these slags resulted in formation of a silicate glass containing the other oxides in solution. The expected equilibrium phases were albite and fayalite in the slags containing about 9 wt% Na,O and albite, nepheline (Na,0*A1,03* 2SiOJ, and NazO*2Si02 in the slags containing about

37 wt% Na,O. None of these were detected by X-ray diffraction or by ceramographic examination. LEACHING TESTS The leachability of Pb, Se, and Te from slags of different compositions was investigated using a Soxhlet extraction apparatus. In this test the solvent is recycled by a distillation-condensation process. A sample is contacted with condensate, which carries solutes into the reservoir. As a result the sample is continuously contacted with relatively pure solvent, while the solute concentration in the reservoir gradually increases. Subsequently, the solvent is quantitatively analyzed for solutes of interest. In this work the samples were 1 to 2 g of powdered slag and the solvent was 500 mL of pure water. We assumed that a sample that yielded more lead, for example, than another sample in the leaching test would likewise yield more lead to the soil if buried slag were contacted by groundwater. The results of leaching tests are shown in Table 4. Leaching of Hygas ash (HA-l) and melted Hygas ash (HA-3, -4, and -11) yielded no solutes within the limits of detection (0.01, 0.001, and 0.05 mg/mL of Pb, Se, and Te, respectively). Leaching of Hygas ash doped with Pb (HA-18, -19, and -20) yielded no Pb except for an anomalous result for HA-19 leached for 24 h. Leaching of synthetic Hygas slag doped with Pb and Te (HA-6) yielded both Pb and Te. The last four slags in Table 4 were modified to lower the melting temperatures. The leaching results indicate a substantial increase in leachability of the modified slags compared to slags of lower Na,O content. Sufficient data were not obtained to adequately describe the effect of composition on leachability.



5. Modified slags of lower melting temperatures, Na,O-AlaOs-SiO, system (m.p. - 850 “C).

(a) Based on albite-fayalite

An increase in leachability with increasing Na,O content is not unexpected, since several sodium silicates (Na,O*SiOt, NazO*2Si02, 2Na,O*SiO,) exhibit considerable solubility in water (8). Although these compounds were not identified in the slags, the higher NazO contents has apparently affected the leachability of the slags in water.

eutectic (m.p.

- 1050 “C), (b) based on

SUMMARY AND CONCLUSIONS Hygas ash represents the byproduct of several coal gasification processes. If these processes attain a high degree of commercialization, large volumes of ash would require disposal. Land burial, the most likely form of disposal, would be simplified by reducing the


CRYSTALLIZATION OF COAL GASIFICATION ASH TABLE 4 Results of Soxhlet Extraction of Hygas Ash and Slag Using Pure Waler (pH = 7)a Extracted Concentration (mg/mL)

Dopant (wtvo) Number


Na,O (wt%)

HA- 1 HA-3 HA-4 HA-11 HA-18 HA-19 HA-lgd HA-lge HA-20 HA-6 HA-16 HA-17 HA-21 HA-22

Blank Hygas ash Hygas, FC Hygas, AC Hyga.% WQ Hygas, FC Hygas, AC Hygas, AC Hygas, AC Hygas, WQ Synthetic Hygas, FC Synthetic modified, FC Synthetic modified, FC Synthetic modified, FC Synthetic modified, WQ

1 1 1 1 1 1 1 1 1 1 9 31 31 31



















1 1 1 1 1 2 0.5 0.4 0.4 0.4












0.8 0.3 0.3 0.2 0.1

0.02 0.06 0.02 0.02




NDC ND ND ND ND ND 0.02 ND ND ND 0.03 0.02 0.08 0.09 0.14

ND ND ND ND ND ND ND ND ND ND ND 0.08 0.24 0.18 0.11

ND ND ND ND ND ND ND ND ND ND 0.14 0.33 2.58 3.99 0.63

%I0 mL pure water for 24 h except as noted. bNone added; concentration < co.01 wt%. CNot detected. Detection limits are: Pb, 0.01; Se, 0.001; Te, 0.05 mg/mL. dExtracted for 96 h. eExtracted for 168 h. ‘Concentration < CO.01 wtqo.

volume of the ash. Reducing the ash surface area by melting might also decrease the tendency for potentially harmful trace elements to be leached by groundwater. In this work we determined the crystallization behavior of melted Hygas ash (slag) and other slags modified in composition to lower the melting temperatures, then investigated the leachability of slags doped with Pb, Se, and Te. The results that have been presented can be summarized as follows. The inorganic fraction of Hygas ash consists of quartz, hematite, and silicate glass. The ash melts at about 1400 “C in air. The resultant slag consists of hematite, magnetite, and silicate glass saturated with iron. Slow cooling from the molten state resulted in crystallization of dendritic hematite. Crystalline silicate compounds formed neither during slow cooling from the molten state nor during aging of water quenched slags at 950 “C for 100 h. We concluded that in the presence of air melted Hygas slag crystallized simply by rejection of iron oxides from a silicate glass. The Hygas composition was modified to lower the melting temperature. This was essentially accomlished by increasing the Na,O content of the slags. Melting temperatures of 1050 and 850 “C were obtained with slags containing 9 and 37 wt% NazO, respectively. Slag containing 37 wt% Na,O contained no crystalline phases and consisted only of glass. Both melted Hygas and modified slags were doped

with Pb, Se, and Te for leachability tests. Leaching was conducted with pure water in a Soxhlet extraction apparatus. We concluded that the leachability of the slags increased substantially with increasing Na,O content. Decreasing the melting temperature by increasing the NazO content, therefore, had the unwanted effect of increasing the leachability of the slags. REFERENCES 1. Bradley, R. A. and Judkins, R. R. Advanced research and technology development fossil energy materials program plan for FY 1981, ORNL/TM-7612 Oak Ridge National Laboratory, Oak Ridge, TN (1981). 2. Lauf, R. J., Federer, J. I., and Tennery, V. J. Stabilization of coal wastes by crystallization. ORNWTM-7071, Oak Ridxe National Labdratoiy, Oak Ridge, TN (1980). 3. Muan, A. Phase equilibria at high temperatures in iron silicate systems. Am. Ceram. Sot. Bull. 317:81 (1958). 4. Aravindakshan and Ali. Council of Scientific and Industrial Research, Central Fuel Research Institute, Bihat, India. See Card 13-534 in Powder Diffraction File Search Manual, Alphabetical Listing and Search Section of Frequently Encountered Phases, Inorganic 1976. Joint Committee on Powder Diffraction Standards, Swarthmore, PA (1976). 5. Levin, E. M.. Robbins. C. R., and McMurdie, H. F. Phase Diagrams for Ceramists, p. 260. The American Ceramic Society, Columbus, OH (1964). 6. Ibid., p. 284. 7. Ibid., p. 181. 8. Weast, Robert C., ed. Handbook of Chemistry and Physics, 56th ed. p. B-144 (1975).