Surface characterization of alkali-treated coals

Surface coals

characterization

John P. Baltrus,

J. Rodney

of alkali-treated

Diehl, Joseph R. D’Este and Edward

Pittsburgh Energy Technology Center, US Department Pittsburgh, PA 15236-0940, USA (Received 28 March 1989; revised 22 August 1989)

P. Ladner

of Energy, PO Box 10940,

The surface interactions of alkali with coal and mineral matter have been studied by electron spectroscopy for chemical analysis (ESCA) to establish a basis for predicting the relative reactivity of alkali with sulphur during coal beneficiation processes. The speciation of sulphur in coals doped with various alkali salts was determined and quantified after heating the mixtures to 648K in air or N,, then washing with water. Organic and pyritic sulphur on the surface react with alkali when heated, and the sulphide or oxidized sulphur product can then be washed from the coal. The extent of reaction between the alkali salt and surface sulphur is governed by the size of the cation and the electronic properties of the anion; larger alkali cations are more effective in promoting the reactivity of surface sulphur, as are anions with stronger nucleophilic properties. (Keywords:

alkali; coal; surface properties)

Over the years, several processes have been used for removing sulphur from coal’. These methods include flotation separation, chemical comminution, electrostatic separation, chemical coal cleaning, and, most recently, microbial treatments. While all of these methods remove pyritic sulphur from coal, only chemical coal-cleaning methods employing caustic have been successful in removing significant amounts of organosulphur as well as pyrite from coal. Molten-caustic leaching (MCL) has been shown to be one of the most effective chemical methods for removing sulphur and ash from coal14. Research has been conducted to determine the effects of process variables on the efficiency of the MCL system and to develop methods for analysing and regenerating spent causti&‘. More fundamental studies have attempted to determine the mechanisms for interaction between Na and K hydroxides and the components of coal, especially pyrite’ and organosulphur”,“. Previous studies of caustic methods employed for the removal of sulphur from coal have concentrated on the bulk properties of coal with respect to sulphur reactivity and the extent of sulphur removal. However, increasingly stringent demands for sulphur removal have required that coal be ground to liner particle sizes, to liberate pyrite and mineral matter. The continuous decrease in particle size is accompanied by an increase in surface area, which means that surface properties may play a more influential role in determining the ultimate success of chemical beneliciation processes. Significant differences may exist between the reactivity of sulphur in the bulk and on the surface of coal. The aim of this study was to focus on the treatment of finely ground coals with alkali to better understand the surface interactions of alkali with coal and mineral matter. The reactivity of surface organosulphur compounds is of particular importance. Such interactions may be important not only in MCL, but in other beneliciation processes that employ alkali for the removal 001~2361/90/010117~5$3.00 0 1990 Butterworth & Co. (Publishers)

Ltd.

of sulphur from coal. This was not intended to be a direct study of MCL, as not all of the reactions were carried out under N,, and an excess of caustic was not used. Changes in the chemical state of the sulphur components on the surface of coal were monitored following deposition of various alkali hydroxides and sodium and potassium salts, and after subjecting the coals to various thermal treatments and washes. EXPERIMENTAL Preparation

of alkali-doped

samples

A Pittsburgh No. 8 hvAb coal and Rasa lignite (Yugoslavia) were ground in air to - 100 mesh ( < 150 ,um). Analyses of the coals are reported in Table I. A sample of coal (0.75 g) was mixed with 1 ml of an aqueous solution containing enough NaOH or KOH to produce the desired alkali concentration in the coal. Low concentrations of alkali were necessary so that the coal surface could be observed after alkali deposition. The alkali-doped coals were placed in a ceramic boat and dried overnight at 333 K in a flow of N,, then stored under air in a desiccator. No changes in the chemical forms of sulphur in any sample were observed during storage for prolonged periods of time. Table 1

Chemical

Analysis

(wt% d.b.)

C H 0 N S Ash Sulphate S Pyritic S Organic S

analyses

of coal samples Pittsburgh

No. 8 coal

71.8 4.7 7.6 1.3 4.7 11.2 0.0 1.7 3.0

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Surface

characterization

of alkali-treated coals: J. P. Baltrus et al. Instrumentation

The coals were analysed as powders mounted on double-sided tape. The magnesium anode was operated at 12 kV and 20mA, with the analysis chamber at a vacuum 62 x lo-‘mbar.

RESULTS Thermal treatments

/

/j--+

73.0

j \

i

170.0 BINDING

167.0

-G48K

L,...,

164.0

ENERGY,

a.0 eV

Figure 1 ESCA S 2p spectra for a non-doped Pittsburgh No. 8 coal heated for 1 h in air at various temperatures

Four Pittsburgh No. 8 coal samples, the first containing 0.5% Na, the second containing 0.5% K, the third containing 0.25% Na and 0.25% K, and a fourth containing no added Na or K, were analysed after heating up to 648 K for 1 h in flowing air or N,. To determine the effects of washing, samples of Na-doped Pittsburgh No. 8 coal, which had been heated for 1 h at 648 K in air or Nz, were placed in a beaker containing x 50ml of deionized water. The slurry was stirred with a magnetic bar for 30 min and then filtered. This washing process was repeated three times, then the filtered coal was placed in a ceramic boat and dried in a tube furnace at 333 K under a flow of N,. A quantitative study of the effects of washing on sulphur removal was carried out on Na-doped and (Na+K)-doped Rasa lignite that had been heated at 648 K for 1 h in air or N,. The lignite samples were doped with alkali using the procedure described above, so that the final concentrations of alkali were ~5% Na, 2.5% Na+2.5% K, and 5% Na+ 5% K, respectively. After the samples were heated, they were mixed with l&40% high-surface-area gamma alumina, which was used as an internal standard for monitoring changes in sulphur content during the wash procedure. The Pittsburgh No. 8 coal was also doped with aqueous solutions of Na, K, Rb and Cs hydroxides, so that 0.75 g of coal contained 1.62 x 10e4 moles of one of the alkali metals. Another series of coal samples was impregnated with aqueous solutions of various Na and K salts (fluorides, chlorides, bromides, nitrates, acetates, and carbonates); the concentration of Na or K was 0.5% in each sample. All of these samples were heated either in air or in nitrogen at 648 K for 1 h.

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The sulphur 2p region was curve-fitted with two peaks, one at 169.7 eV due to oxidized sulphur, SoI, and another at 164.0eV due to unoxidized sulphur, SC”) (organic sulphur, pyrite, and elemental sulphur)“. Based on the binding energy measured for So), oxidized sulphur is predominantly SO:-, although minor concentrations of organic sulphur-oxygen compounds, such as sulphones, may also be present 12. Representative S 2p spectra are shown in Figure1 for the non-doped Pittsburgh No. 8 coal after heating to various temperatures in air. Table 2 shows the results of curve-fitting analyses of the S 2p spectra for the non-doped, Na-doped, K-doped, and (Na + K)-doped coals, after heating them in air or N,. Heating the coals to higher temperatures in air resulted in the conversion of an increasing percentage of S,,, to SC,, (%S,,,= 100-S&; the amount of S,,, was always greater for alkali-doped coals. The coal containing no alkali showed little change in the percentage of So) during heating under N,. This is in contrast to the decrease in So) observed on heating the alkali-doped coals under N, (Figure 1 and Table 2). Bulk ASTM analysis showed no difference in the total sulphur concentration of the as-prepared and heated coal samples. A curve-fitting analysis of the C Is ESCA spectra for coals that were heated in air to 648 K showed that less than 20% of the total C Is peak area could be assigned to carbon-oxygen functionalities (binding energies of 286.1 eV, 287.7eV, and 289.1 eV). Oxidation of carboncarbon bonds was lessened by heating the coals in N,. Carbonate formation was not observed for coals heated in air or N,. ESCA was used to monitor changes in the relative surface concentrations of sulphur forms after washing the Na-doped coals that were heated in air or N, (Figure 2). The effectiveness of the wash procedure was determined for a separate sample by ESCA analysis of the Na 1s region; no Na signal was observed after the washings were completed. The predominant species of sulphur prior to washing the coals heated in air and N, were So) Percentages of S,,, as determined by ESCA for a Pittsburgh No. 8 coal heated for 1 h at various temperatures

Table 2

Temperature

Non-doped coal

Na-doped coal

K-doped coal

(Na + K)-doped coal

Not heated 373 K” 423 K” 523 K” 623 K” 648 K” 648 K*

31 34 38 47 59 67 32

38 37 43 71 83 87 15

33 38 43 61 77 81 16

46 49 57 81 84 88 24

0 Heated in air “Heated in N, Values are reproducible

to + 3%, absolute

Surface 1 169.7

1164.0

S2P

Na - Doped Pgh # 8 Coal

A

173.0

170.0

BINDING

167.0

164.0

ENERGY,

161.0

eV

Figure 2 ESCA S 2p spectra for a Na-doped Pittsburgh No. 8 coal heated for 1 hat 648 K: a, N, + washed; b, N,; c, air + washed; d, air

and St”),respectively. Washing of the coal heated in air led primarily to the removal of sulphate species, so that the surface ratio of SjO, to SC”)decreased from 12:l to 1:3. Conversely, washmg the coal that was heated in N, caused a small increase in the S~Oj:S~Uj ratio, presumably due to leaching of Na,S formed by reaction of NaOH with FeS, and organic sulphur. Therefore, it appears that washing removes sulphur from the air-heated coal, but Figure 2 does not show that washing the N,-heated coal is effective in removing sulphur. Quantitative determination of sulphur removal

Experiments employing Rasa lignite (which has a high organic-sulphur content and no St,,) and Na or (Na + K) hydroxide were carried out to determine whether the alkali hydroxides were reactive toward organic sulphur, and how much of the reaction products could be removed from the lignite by washing. The S,, 2p/Al 2s (So,= total sulphur) intensity ratios before washing and SC”) 2p/Al 2s intensity ratios after washing, are reported m Table3. No more than 19% of the total sulphur remaining on the surface after washing was in the form of S,,, for any of the alkali-doped lignites. The results for the Na-doped lignite heated in N, show that washing led to a 34% decrease in the total amount of unoxidized sulphur in the starting lignite, at least on the surface. A large fraction of the decrease in S,,, was due to its removal from the lignite. If all of the Na was converted to Na,S upon heating the lignite in N,, then 35% of the total sulphur in the lignite would have reacted. Complementary reductions in unoxidized sulphur content were observed upon washing the air-heated lignite (Table3); here the reaction product would most likely be Na2S0,, not

characterization

of alkali-treated

coals: J. P. Balms

et al.

Na,S. Since reactions of alkali with the mineral matter in coal could not be measured, such reactions were not considered in calculations used to predict the amount of sulphur that would react*. The fact that alkali is necessary for the reaction and removal of organic sulphur was verified by heating the non-doped lignite in air at 648 K for 1 h; no sulphur could be removed by washing. Further evidence for the stoichiometric reaction of Na, K, and surface organosulphur under N, at 648 K, and the subsequent removal of sulphur by washing, is illustrated in Table 3. A decrease of z 31% in the amount of unoxidized sulphur was observed for lignite doped with 2.5% Na and 2.5% K after washing. Calculations based on the formation of Na,S and K,S predicted that 26% of the sulphur in the lignite should have reacted. Similarly, for the lignite doped with 5% Na and 5% K, calculations predicted that 52% of the sulphur would react; the results showed a 50% decrease in the amount of unoxidized sulphur after washing. The percentages of SC,, for a Pittsburgh No. 8 coal doped with various Na and K salts and heated in air at 648 K for 1 h are reported in Table 4. Coals doped with Na or K salts and heated in N, showed no significant difference in the percentages of S,,, and St”) (%S(,,= 14.3 f 2.4) because formation of an alkali sulphide, if such a reaction occurs, renders the reacted sulphur indistinguishable by ESCA from unreacted organosulphur and pyrite. Table4 shows that for coals doped with different Na salts and heated in air, those coals doped with salts containing the anions F-, NO,-, and CO,‘- formed the least amount of S,,,, while those doped with the acetate salt formed the most SC,,. For coals doped with different K salts, those salts containing F-, Cl-, and NO,- were least effective for SC,, formation compared with the acetate salt, which was the most reactive. The effects of alkali cation size on the reactivity of the sulphur-containing components of coal heated in air are Table 3 ESCA S 2p/Al 2s intensity ratios for a heat-treated (648 K, 1 h) Rasa lignite Before washing So, 2p/At 2s ______ Na-doped (5% Na) in aif 0.80 Na-doped (5% Na) in Nzb 1.40 (Na+K)-doped (2.5% Na 0.77 +2.5% K) in N,” (Na +K)-doped (5% Na+ 0.30 5% K) in N,’

After washing S,,, 2p/AI 2s

A%

0.49 0.92 0.53

-39% -34% -31%

0.15

- 50%

“Mixed with x20% Al,O, b Mixed with z 10% Al,O, ‘Mixed with -40% AI,O,

Percentages of S,,, as determined by ESCA for heat-treated”, alkali-doped Pittsburgh No. 8 coal Table 4

Anion OHFclBrNO, (C,H,O,)co,z-

Nat cation _____ _____. 87 77 85 88 80 95 81

K+ cation

______~~

_

81 71 74 83 78 90 79

a Heated at 648 K for 1 h in air Values are reproducible to + 3%, absolute

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Table 5 Percentages of S,,, as determined by ESCA for heat-treated”, alkali-doped Pittsburgh No. 8 coal Cation

S,,, (%I

None Na+ K+ Rb+ CS+

67 87 87 91 100

’ Heated at 648 K for 1 h in air Values are reproducible to If:3%, absolute

reported in Tuble5. The greatest conversion of, S!“, and So) occurred for coal doped with the salt contammg the largest cation.

DISCUSSION The ease with which sulphur on coal surfaces can be oxidized is demonstrated in Figure 1 and Table 2. While heating in N, does little to alter the percentage of oxidized sulphur, as expected, heating in air brings about a significant increase in sulphate concentration at higher temperatures. Pyritic sulphur is much easier to oxidize than organic sulphur l3 . Furthermore, the percentage of sulphate on the surfaces of the heated coals increases in the presence of alkali. This observation, and the fact that sulphur on the surface of Rasa lignite, which is almost exclusively organic sulphur, is not oxidized after heating to 648 K in air, suggests that sulphate formation in the non-doped coals can be attributed to pyrite oxidation, while organosulphur remains inert. The sulphate fraction determined by ESCA does not have to equal the pyritic sulphur fraction determined by chemical analysis, because the sulphate fraction measured by ESCA is a function of the relative dispersion of the pyrite and organic sulphur phases. The increase in sulphate formation in the presence of alkali would then be due to oxidation of organic sulphur. Other hypotheses may also explain the increase in sulphate formation on the surface of the alkali-doped coals heated in air. The alkali may capture SO, evolved from oxidation of either surface organosulphur or bulk pyrite. Organosulphur was not reactive under these conditions. Capture of SO, evolved from oxidation of bulk pyrite would lead to an increase in the S 2p/C 1s intensity ratios, due to a nett increase in surface sulphur. This behaviour was not observed. Therefore, it is proposed that the alkali hydroxide reacts directly with surface organosulphur-containing structures, and catalyses their oxidation in air to produce sulphate. It is not possible to directly compare the relative formation of sulphate for the Na-, K-, and (Na+ K)-doped coals, since an equal weight basis rather than an equal molar basis was used. However, based on the evidence in Table 2, doping the coal with the mixed alkali hydroxides was most effective in promoting sulphate formation. The reason for this behaviour is not clear, although creation of an eutectic mixture of Na and K hydroxides on heating the alkali-doped coal may enhance their contact with surface sulphur and promote formation of So). The cause of the decrease in So) for the alkali-doped coals heated in N, is also uncertain. Since neither the surface nor bulk concentration of sulphur was observed to change on heating in N,, loss of sulphate by

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volatilization is unlikely. A possible explanation is that in the presence of caustic, SC,, may dissolve on heating to 648 K and diffuse from the surface to the bulk. Once it was established that alkali promotes the reactivity of surface sulphur, it was valuable to determine whether the products of the reaction could be removed by washing. Removal of soluble sulphate from the surface of coal heated in air is evidenced by a decrease in the relative intensity of the peak due to SC,,,in Figure2. The residual So) after washing may be jarositic sulphate, which is insoluble under these washing conditions. Since the alkali-sulphide product expected from the reaction of alkali and sulphur under N, is indistinguishable from pyrite and organosulphur, based on their ESCA S 2p spectra, it is impossible to determine whether any reaction has occurred. Similarly, subsequent removal of the water-soluble alkali-sulphide product along with some of the SC,,,by washing may have little effect on the relative proportions of SC,, and S,,, in the S 2p spectra. To avoid problems in determining the reactivity of sulphur with alkali under N,, the washing procedure was modified. At the same time, the approximate stoichiometry of the reaction products was determined, and the reactivity of organosulphur was examined more closely. The first two goals were accomplished by using an internal standard during the wash procedure, and the latter goal was accomplished by employing Rasa lignite in the experiments. Evidence supporting the reaction of organosulphur with alkali in both N, and air is given in Tuble3. The removal of surface sulphur from the Na-doped lignite heated in air is confirmed by the decrease in the S 2p/Al 2s intensity ratio after washing. The same is true for the Na-doped lignite heated in N,. This is consistent with previous observations that bulk organosulphur reacts with alkali under N, and produces a product that can be removed by washingiO~“. The stoichiometry of the reaction products formed under air or N, can be determined by comparing the values for the amount of sulphur predicted to react, based on the amount of alkali, with the A% values reported in Table3. Good agreement between the two values indicates that the predicted reaction product is probably the product formed. This is the case for both the Na-doped lignite heated in air and that heated in N,. Formation of Na,S has been demonstrated in earlier studies9 of the reactions between alkali hydroxides and pyrite, and suggested in studies employing model organic compounds’ ‘. Similarly, the oxidizing environment of air leads to formation of Na,SO,. The importance of the alkali hydroxide for promoting the reactivity of organosulphur is illustrated by the experiments with Rasa lignite. To further evaluate the effect of the alkali cation and its associated anion on the reactivity of sulphur in coal, two sets of experiments were performed. First, the effects of cation size on the reactivity of sulphur in coal were examined. Previous studies of MCL showed that the extent of reaction of model organosulphur compounds increased with cation size’0~“*‘4. The results reported in Table5 show a cation-size effect when comparing the percentages of So) formed for the largest cation, Cs+, with those formed for the smaller cations, Na+ and K+. In the MCL studies, it was proposed that the larger cation was more effective in stabilizing an intermediate, leading to sulphur elimination from the organosulphur molecule. However, for the present experiments, a surface phenomenon may

Surface characterization be responsible for the observed behaviour. The reactivity of sulphur on the surface may be related to the ability of the alkali cation to move below the surface of the coal (5OA). A greater fraction of the smaller cations may be able to move below the surface and react with sulphur. Therefore, some of the reaction products would not be detected by ESCA. The larger cations, which cannot penetrate the surface of the coal as easily due to steric effects, may remain closer to the surface, where the products of reaction with sulphur are more easily detected. Also, a decrease in the melting point of the alkali hydroxides with an increase in cation size may enhance the contact between the larger cations and the coal surface, and facilitate their reaction with sulphur. The role of the anion in determining the reactivity of sulphur on the surface of coals heated in air initially appears to be governed by size as well. As demonstrated in Tuble5, salts containing the smaller anions, such as fluoride, are least reactive. Those containing the largest anions, such as bromide and acetate, are most effective for promoting oxidation of sulphur. However, salts containing the larger nitrate and carbonate anions are among those showing lower reactivity. Therefore other factors, such as electronic properties, may also influence the anion reactivity. Those anions that are more highly electrophilic (less nucleophilic), such as fluoride, chloride, nitrate, and carbonate, are least reactive, while anions that are both larger and relatively less electrophilic, such as bromide and acetate, are most reactive. The high reactivity of the acetate salts with coal is especially interesting, because the acetate anion decomposed when heated, as shown by the absence of the ESCA C 1s peak due to acetate after heating. ESCA analysis of the unheated coal showed no abnormal changes in sulphur forms that may have been introduced simply by doping the coal with acetate. It is possible that the low melting points of the acetate salts (< 598 K) allow them to spread alkali over the surface of the coal before decomposing, thereby promoting intimate contact between the alkali and sulphur. Also, the acetate anion may be more effective for dissolving organic functional groups containing sulphur. Finally, based on the reactivity towards sulphur of the salts used in later experiments involving a Pittsburgh No. 8 coal heated in air, it was suspected that the same salts would be equally reactive in N,. Therefore, coals doped with either sodium acetate or sodium bromide were heated in N, for 1 h at 648 K. They were then mixed with the Al,O, internal standard and washed according to the procedure used earlier. Subsequent comparison of S 2p/Al 2s intensity ratios before and after washing showed that no sulphur was removed from the coal. These results are consistent with model compound studies in nitrogen (S. Friedman and B. R. Utz,

of alkali-treated

coals: J. P. Baltrus et al.

unpublished results). The interactions and surface sulphur differ depending in which the coals are heated.

between these salts on the atmosphere

CONCLUSIONS These experiments have shown that removal of sulphur from coal surfaces may be accomplished by doping the coal with alkali hydroxides, heating to moderate temperatures, and washing. The types of alkali salts used for treating coal surfaces, as well as the atmosphere in which they are heated, are important in determining the nature and extent of sulphur reactivity on the surface of coal. ACKNOWLEDGEMENTS The authors thank B. Utz, S. Friedman, and Y. Miron for helpful discussions. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favouring by the US Department of Energy. REFERENCES 1

2 3

4

8 9 10

Wheelock, T. D. and Markuszewski, R. in ‘The Science and Technology of Coal and Coal Utilization’ (Eds. B. R. Cooper. and W. A. Ellington), Plenum Press, New York, USA, 1984, pp. 477123 Masciantonio, P. X. Fuel 1965, 44, 269 ‘Gravimelt Process Development’, TRW Energy Development Group, Final Report, DOE/PC/42295-T7 (DE84013743) June 1983 Meyeres, R. A., Jones, J. F. and McClanathan, L. C. paper presented at Coal Utilization and Environmental Control Contractors’ Review Meeting, Pittsburgh, PA, USA, July 1987 Markuszewski, R., Mroch, D. R., Norton, G. A. and Straszheim, W. E. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1985,30,41 Criswell, C. D., Mroch, D. R. and Markuszewski, R. Anal. Chem. 1986,58, 319 Markuszewski, R. paper presented at Coal Utilization and Environmental Control Contractors’ Review Meeting, Pittsburgh, PA, USA, July 1987 Kaushik, S. M., Norton, G. A. and Markuszewski, R. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1988, 33(2), 18 Chiotti, P. and Markuszewski, R. Ind. Eng. Chem. Proc. Da. Dev. 1985, 24, 1137 Utz, B. R., Friedman, S. and Sobaczenski, S. K. in ‘Fossil Fuel Utilization’ (Eds. R. Markuszewski and B. D. Blaustein). Am. Chem. Sot., Washington, DC, USA, 1986, pp. 51-61 ” Friedman, S., Utz, B. R., Nowak, M. A. et al. Coal Sri. Technol. 1987, 11, 435 Wagner, C. D., Riggs, W. M., Davis, L. E. et al. ‘Handbook of X-Ray Photoelectron Spectroscopy’, Perkin-Elmer Corporation Physical Electronics Division, Eden Prairie, MN, USA, 1979 Frost, D. C., Leeder, W. R., Tapping, R. L. and Wallbank, B. Fuel 1977, 54, 277 Wallace, T. J., Hofmann, J. F. and Schriesheim, A. J. Am. Chem. Sot. 1963, 85, 2739

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