- Email: [email protected]
Counter-current ion exchange for the removal of organically bound sodium from low-rank coals
Minerals Engineering, Vol. 13, No. 13, pp. 1423-1428, 2000
Pergamon 0892-6875(00)00124-2
© 2000 Published by Elsevier Science Lid All rights reserved 0892-6875/00/$ - see front matter
TECHNICAL NOTE COUNTF, R-CURRENT ION EXCHANGE FOR THE REMOVAL OF ORGANICALLY BOUND SODIUM FROM LOW-RANK COALS* K. B. QUAST School of Geoscience, Minerals and Civil Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia. Email: [email protected] (Received 19 April 2000; accepted 25 August 2000)
ABSTRACT South Australia~ low-rank coals contain high levels of water-soluble and ion-exchangeable ions that can be deleterious to coal combustion and gasification. The presence of sodium causes the formation of low melting point bonded deposits that can produce severe fouling and slagging in many processes using these coals. Approximately 70 % of the sodium present in Bowmans coal is water-soluble and can therefore be removed by simple water leaching, however the remaining 30 % is organically bound, predominantly to carboxyl groups. Ion exchange using sulphuric acid at pH 2 is necessary to remove this sodium, but this can generate large volumes of acidic wastewater if multiple washing stages are used with fresh water each time. Using a simple laboratory procedure, the continuous counter-current ion exchange leaching of sodium from this coal was simulated which reduced the sodium level from 1.30 % on a dry basis (db) to 0.15 % (db) after three stages of counter-current washing at a total coal:water ratio of 1:2. The effectiveness of interstage dewatering on tke efficiency of sodium removal was highlighted. The agreement between predicted and measured values of moisture and sodium in coal was excellent. © 2000 Published by Elsevier Science Ltd. All i~ights reserved.
Keywords Leaching; ion exchange; modelling INTRODUCTION South Australian low-rank coals contain high levels of water-soluble impurities (e.g. sodium chloride) and ion-exchangeable cations (e.g. organically bound sodium ions). Pilot plant gasification studies conducted by the former Eleelxicity Trust of South Australia (now Flinders Power Pty. Ltd.) have shown that the presence of particularly sodium and chloride ions (referred to as sodium and chloride in this paper) is deleterious to coal gasification. Sodium and chloride precipitate to form halite (NaCI) (Manzoori and Plastow, 1988) which blocks gas passes and dust extraction equipment and sodium causes low melting point bonded deposits during subsequent combustion. In pulverised fuel (pf) combustion, the presence of sodium forms "sticky" deposits (Wardle, 1991); in the ease of Bowmans coal these are probably sodium sulphates and sodium silicates. Even the partial removal of soluble salts from Bowmans coal can have a
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
1423
1424
K.B. Qu~t
very significant effect on combustion characteristics, in particular on ash deposit formation characteristics (Quast et al., 1990). For Lochiel coal, another "salty" South Australian low-rank coal, Manzoori (1990) found a linear relationship between rate of deposition of coating in circulating fluid bed combustion with sodium content. Recent work by Vuthaluru et aL (1999) showed that water washing Bowmans coal reduced the mass increase for the fluidised bed combustion compared to the ROM coal. This paper is an extension of the work reported at the 7 th Australian Coal Science Conference by the author on the counter-current water leaching of Bowmans coal at the natural pH (3.5) (Quast, 1996). For the previous study, the counter-current removal of water-soluble impurities (primarily NaCI) was investigated and modelled. Counter-current leaching at a 1:2 coal:water ratio reduced chloride levels to 0.2 % (db) after two stages and 0.1% (db) after three stages from a run-of-mine (ROM) value of 1.4 % (db). Corresponding sodium levels for counter-current washing were 0.35 % (db) after two stages and 0.3 % (db) after three stages. For the procedure to work it is necessary to generate wash water streams of the same chemical composition as the subsequent effluent stream. This can be done by assuming that the leaching stage reaches equilibrium prior to flocculation and filtration. In the current study, the same technique was applied to ion exchange leaching at pH 2.
L I T E R A T U R E REVIEW ON ION EXCHANGE L E A C H I N G Allen (.1981) used 0.1N sulphuric acid to leach -25 mm and -150 lam Bowmans coal samples at a coal:water ratio of 1:2. For the -25 mm coal, acid leaching gave a residual sodium concentration of 0.98 % (db), while for the -150 ~tm coal, sodium in leached residue was 0.34 % (db), highlighting the influence of particle size on leaching efficiency. Acid leaching has also been modelled by Readett et al. (1986). In Bowmans coal, approximately 70 % of the sodium was water-soluble, but the remaining 30 % could only be removed by ion exchange, as it was believed o be organically bound to humates. Test work was conducted on -8+4 mm, -4+2 mm and - 2 mm Bowmans coal, and modelling of the concentration of sodium in leach liquor was successful using a simple diaphragm cell model and two rate constants. One of these rate constants was related to liquid film diffusion and the other related to particle diffusion.
EXPERIMENTAL
Material examined The coal sample used in this test work was obtained from the Bowmans pit stockpile and assayed 51.8 % moisture, 14 % ash, 1.35 % (db) chloride and 1.30 % (db) sodium. It had been crushed to 9 1 % passing 4 mm (41% passing 0.5 mm) and double sealed in plastic prior to use.
Ion exchange leaching A 300 g sample of crushed coal was leached with 600 ml of demineralised water acidified with sulphuric acid to pH 2 for 30 minutes. Previous test work reported by Readett et al. (1986) and Readett and Quast (1989) had shown no further leaching occurred after this time for this coal at this size. Agitation was supplied by a variable speed stirrer fitted with a nylon shaft and propeller driven sufficiently quickly to ensure that all the coal particles remained in suspension. After the leaching was completed, 9 ml of 0.1% non-ionic flocculant was added and dispersed through the slurry by agitation for 10 seconds. The slurry was then allowed to settle for 30 minutes and vacuum filtered in a Buchner filter on Whatman No. 1 filter paper until the last traces of free water just disappeared from the surface of the filter cake. The volume of filtrate was measured and retained for analysis. For multi-stage leaching, a volume of leachant equal to that removed in the previous stage was added to maintain a constant coal:water ratio in the leach stages.
Counter-currention exchangefor removalof organicallyboundsodiumfromlow-rankcoals
1425
For the multi-stage counter-current leach tests it was necessary to generate an inlet stream of equivalent sodium (and chloride) composition to the outlet stream in the subsequent stage. The composition of these inlet streams could be estimated by assuming that the sodium (and chloride) contents of the coal are in equilibrium with the sodium (and chloride) contents of the liquor at the end of the leach stage.
Analysis The moisture content of the coal was determined by weight loss of samples in a muffle furnace held at 105°C for two hours. Sodium in solution was determined via Atomic Absorption Spectrophotometry (AAS). To inhibit the ionisation of the sodium, leach filtrates were diluted 50:50 with a 4 g/l K + solution prior to aspiration. All sodium standards were prepared in 2 g/l K+ solutions. Sodium in coal was determined by leaching a 2g sample in 50 ml of 2M nitric acid, filtration and determination of the sodium content of the filtrate using AAS. This was then converted to sodium in coal on a dry basis. Chloride analyses had previously been done using the standard Mohr's titration technique (Quast, 1996), but this technique is not suitable for solutions of pH 2. Since these tests paralleled those reported earlier by the author using the same water:coal ratio, and knowing that all the chloride is water-soluble, hence acidsoluble, it was assumed that chloride would behave similarly in the two series of tests.
RESULTS The results for (one), two and three stage counter-current ion-exchange leaching at pH 2 are given in Figures 1, 2 and 3 respectively. A comparison between calculated and analysed values for the ion-exchange leaching of Bowmans coal is given in Table 1.
Water 600 ml Sodium 0.02 g
Coal 144 g Water 156 g
I "-
Sodium 1.87 g
7
I 1
Coal 144 g Water 216 g
[
Sodium 0.58 g
,1
Water 540 ml Sodium 1.25 g Fig. 1 Single stage ion exchange leaching test.
1426
K.B. Quast
Water 520 ml Sodium 0.02 g
Water 600 ml Sodium 0.5 g
Coal 144 g J
Coal 144 g .-I
Water156 g "] 1 Sodium 1.87 g [
Water 236 g "1 Sodium 0.89 g I
Coal 144 g
2
Water 246 g Sodium 0.37 g
rl L~
Water 510 ml Sodium 0.54 g
Water 520 ml Sodium 1.48 g
Fig.2 Two stage ion exchange leaching test. Coal 144 g Water 156 g Sodium 1.87 g
Water 600 ml Sodium 0.65 g
P
Water 530 ml Sodium 1.58 g
Coal 144 g Water 226 g Sodium 0.94 g
Water 530 ml Sodium 0.24 g
2 l
Water 520 ml "~ Sodium 0.70 g
Coal~44 g Water 236 g Sodium 0.48 g
Water 520 ml Sodium 0.02 g
Water 500 ml " Sodium 0.28 g
Coal 144 g Water 256 g Sodium 0.22 g Fig.3 Three stage ion exchange leaching.
Counter-currention exchangefor removalof organicallyboundsodiumfromlow-rankcoals
1427
TABLE 1 Corrdation between calculated and analysed values of moisture and sodium content Stages Single Two Three
Moisture content (%) Calculated Measured 60 60 64 63 64 64
Sodium content (%)(db) Measured Calculated 0.41 0.40 0.27 0.26 0.15 0.15
DISCUSSION To the best of the author's knowledge, this is the first time that the counter-current ion exchange leaching of an Australian low rank coal has been simulated in laboratory batch testing. These results should therefore be used as a guide to large scale behaviour, and pilot plant testing, at least, should be conducted to verify the batch data. The basis for the leaching procedure is shown in Figure 1. Water:coal ratio was maintained at 2:1 at each stage and the concentrations of sodium in coal were calculated from a mass balance using the feed values and leach liquor cortcentration values. A single stage ion exchange leach stage followed by flocculation and filtration gave the calculated coal product shown in Table 1. The efficiency of the test depends on the amount of water removed as the sodium in all water (filtrate and water in coal) is assumed to be in equilibrium. A simple settling test gave a solids density in the sludge of 23.5 % after 30 minutes settling time from a feed solids density of 16 %. This means that the effluent water volume (supernatant) would only be 287 ml. Under these conditions, the concentration of sodium would remain constant in the effluent water, and the calculated coal product would be: moisture 76.5 % and sodium 0.81% (db). This highlights the crucial effect of dewatering on the efficiency of sodium removal. This has been examined in an earlier publication by Quast and Readett (1989), and points to interstage filtration being necessary to achieve rapid removal of soluble and ion-exchangeable impurities. Using a two-stage ion exchange leaching process, the calculated coal product contained 63 % moisture and 0.26 % sodium on a dry basis. This means that the sodium content has been reduced by 80 %. The correlation between calculated and analysed coal products is excellent (see Table 1). Three stage ion exchange leaching reduced the sodium content to 0.15 % (db), a reduction of 88.5 % which is excellent bearing in mind tha~Lonly a total of 2 litres of fresh water are used per 1 kg of ROM coal. The process does, however, generate an acidic waste water stream containing 3 g/1 sodium and associated chloride and sulphate, which would need to be treated prior to disposal or reuse.
CONCLUSIONS It is possible to simulate the counter-current ion exchange leaching of sodium from Bowmans coal using a simple laboratory procedure. The technique works by allowing the leaching stage to reach equilibrium prior to flocculation and filtration. An efficient dewatering stage is crucial to the efficiency of impurity removal. For the procedure to work it is necessary to generate wash water streams of the same chemical composition as the subsequent effluent stream. A highly saline effluent stream is generated, however its volume was much less than that for washing with fresh water each time.
ACKNOWLEDGEMENT The author is gratetid to Flinders Power Pty. Ltd. for permission to publish this paper.
REFERENCES Allen, R.J., Sodium removal from Wakefield coal--Stage 2, Amdel report No. 1435. 1981, (unpublished).
1428
K.B. Quast
Manzoori, A.R., Role of the inorganic matter in agglomeration and defluidisation during the circulating fluid bed combustion of low-rank coals, Ph.D. thesis, University of Adelaide, 1990, 314p. Manzoori, A.R. and Plastow, K.M., Gasification of South Australian salty coals using the High Temperature Winkler process, Paper A4.7, Australian Coal Science Conference, Adelaide, 1988. Quast, K.B., Counter-Current leaching of Bowmans coal, Proceedings, 7th AlE Coal Science Conference, Churchill, Victoria, 1996, pp. 409-415. Quast, K.B., Pescott, E.R., Readett, D.J., Williams, R. and Kosminski, A., The partial removal of soluble salts from Bowmans coal and its effect on combustion, Proceedings, AlE Coal Science Conference 4, Brisbane, Queensland, 1990, pp. 331-338 Quast, K.B. and Readett, D.J., Surfactant assisted filtration of lignite slurries, Proceedings, AuslMM Dewatering Technology and Practice Conference, Brisbane, Queensland, 1989, pp. 31-35. Readett, D.J., Hall, S.F., Quast, K.B. and Ketteridge, I.B. Beneficiation of Bowmans lignite-optimisation of sodium reduction, Proceedings, CHEMECA '86, 1986, Adelaide, pp. 325-329. Readett, D.J. and Quast, K.B., Beneficiation of Bowmans coal-water leaching of soluble salts, Proceedings, CHEMECA '89, Gold Coast, Queensland, 1989, pp. 456-463. Vuthaluru, H.B.,Linjewile, T.M., Zhang, D-k. and Manzoori, A.R., Investigations into the control of agglomeration and defluidisation during fluidised-bed combustion of low-rank coals, Fuel, 1999, 78, 419-425. Wardle, D.G., The release of sodium and potassium during pulverised coal combustion, Proceedings, 1991 International conference on coal science, University of Newcastle-upon-Tyne, Butterworth-Heinemann, 1991, pp. 299-302.
Correspondence on papers published in Minerals Engineering is invited by e-mail to bwills @min-eng.com
Pergamon 0892-6875(00)00124-2
© 2000 Published by Elsevier Science Lid All rights reserved 0892-6875/00/$ - see front matter
TECHNICAL NOTE COUNTF, R-CURRENT ION EXCHANGE FOR THE REMOVAL OF ORGANICALLY BOUND SODIUM FROM LOW-RANK COALS* K. B. QUAST School of Geoscience, Minerals and Civil Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia. Email: [email protected] (Received 19 April 2000; accepted 25 August 2000)
ABSTRACT South Australia~ low-rank coals contain high levels of water-soluble and ion-exchangeable ions that can be deleterious to coal combustion and gasification. The presence of sodium causes the formation of low melting point bonded deposits that can produce severe fouling and slagging in many processes using these coals. Approximately 70 % of the sodium present in Bowmans coal is water-soluble and can therefore be removed by simple water leaching, however the remaining 30 % is organically bound, predominantly to carboxyl groups. Ion exchange using sulphuric acid at pH 2 is necessary to remove this sodium, but this can generate large volumes of acidic wastewater if multiple washing stages are used with fresh water each time. Using a simple laboratory procedure, the continuous counter-current ion exchange leaching of sodium from this coal was simulated which reduced the sodium level from 1.30 % on a dry basis (db) to 0.15 % (db) after three stages of counter-current washing at a total coal:water ratio of 1:2. The effectiveness of interstage dewatering on tke efficiency of sodium removal was highlighted. The agreement between predicted and measured values of moisture and sodium in coal was excellent. © 2000 Published by Elsevier Science Ltd. All i~ights reserved.
Keywords Leaching; ion exchange; modelling INTRODUCTION South Australian low-rank coals contain high levels of water-soluble impurities (e.g. sodium chloride) and ion-exchangeable cations (e.g. organically bound sodium ions). Pilot plant gasification studies conducted by the former Eleelxicity Trust of South Australia (now Flinders Power Pty. Ltd.) have shown that the presence of particularly sodium and chloride ions (referred to as sodium and chloride in this paper) is deleterious to coal gasification. Sodium and chloride precipitate to form halite (NaCI) (Manzoori and Plastow, 1988) which blocks gas passes and dust extraction equipment and sodium causes low melting point bonded deposits during subsequent combustion. In pulverised fuel (pf) combustion, the presence of sodium forms "sticky" deposits (Wardle, 1991); in the ease of Bowmans coal these are probably sodium sulphates and sodium silicates. Even the partial removal of soluble salts from Bowmans coal can have a
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
1423
1424
K.B. Qu~t
very significant effect on combustion characteristics, in particular on ash deposit formation characteristics (Quast et al., 1990). For Lochiel coal, another "salty" South Australian low-rank coal, Manzoori (1990) found a linear relationship between rate of deposition of coating in circulating fluid bed combustion with sodium content. Recent work by Vuthaluru et aL (1999) showed that water washing Bowmans coal reduced the mass increase for the fluidised bed combustion compared to the ROM coal. This paper is an extension of the work reported at the 7 th Australian Coal Science Conference by the author on the counter-current water leaching of Bowmans coal at the natural pH (3.5) (Quast, 1996). For the previous study, the counter-current removal of water-soluble impurities (primarily NaCI) was investigated and modelled. Counter-current leaching at a 1:2 coal:water ratio reduced chloride levels to 0.2 % (db) after two stages and 0.1% (db) after three stages from a run-of-mine (ROM) value of 1.4 % (db). Corresponding sodium levels for counter-current washing were 0.35 % (db) after two stages and 0.3 % (db) after three stages. For the procedure to work it is necessary to generate wash water streams of the same chemical composition as the subsequent effluent stream. This can be done by assuming that the leaching stage reaches equilibrium prior to flocculation and filtration. In the current study, the same technique was applied to ion exchange leaching at pH 2.
L I T E R A T U R E REVIEW ON ION EXCHANGE L E A C H I N G Allen (.1981) used 0.1N sulphuric acid to leach -25 mm and -150 lam Bowmans coal samples at a coal:water ratio of 1:2. For the -25 mm coal, acid leaching gave a residual sodium concentration of 0.98 % (db), while for the -150 ~tm coal, sodium in leached residue was 0.34 % (db), highlighting the influence of particle size on leaching efficiency. Acid leaching has also been modelled by Readett et al. (1986). In Bowmans coal, approximately 70 % of the sodium was water-soluble, but the remaining 30 % could only be removed by ion exchange, as it was believed o be organically bound to humates. Test work was conducted on -8+4 mm, -4+2 mm and - 2 mm Bowmans coal, and modelling of the concentration of sodium in leach liquor was successful using a simple diaphragm cell model and two rate constants. One of these rate constants was related to liquid film diffusion and the other related to particle diffusion.
EXPERIMENTAL
Material examined The coal sample used in this test work was obtained from the Bowmans pit stockpile and assayed 51.8 % moisture, 14 % ash, 1.35 % (db) chloride and 1.30 % (db) sodium. It had been crushed to 9 1 % passing 4 mm (41% passing 0.5 mm) and double sealed in plastic prior to use.
Ion exchange leaching A 300 g sample of crushed coal was leached with 600 ml of demineralised water acidified with sulphuric acid to pH 2 for 30 minutes. Previous test work reported by Readett et al. (1986) and Readett and Quast (1989) had shown no further leaching occurred after this time for this coal at this size. Agitation was supplied by a variable speed stirrer fitted with a nylon shaft and propeller driven sufficiently quickly to ensure that all the coal particles remained in suspension. After the leaching was completed, 9 ml of 0.1% non-ionic flocculant was added and dispersed through the slurry by agitation for 10 seconds. The slurry was then allowed to settle for 30 minutes and vacuum filtered in a Buchner filter on Whatman No. 1 filter paper until the last traces of free water just disappeared from the surface of the filter cake. The volume of filtrate was measured and retained for analysis. For multi-stage leaching, a volume of leachant equal to that removed in the previous stage was added to maintain a constant coal:water ratio in the leach stages.
Counter-currention exchangefor removalof organicallyboundsodiumfromlow-rankcoals
1425
For the multi-stage counter-current leach tests it was necessary to generate an inlet stream of equivalent sodium (and chloride) composition to the outlet stream in the subsequent stage. The composition of these inlet streams could be estimated by assuming that the sodium (and chloride) contents of the coal are in equilibrium with the sodium (and chloride) contents of the liquor at the end of the leach stage.
Analysis The moisture content of the coal was determined by weight loss of samples in a muffle furnace held at 105°C for two hours. Sodium in solution was determined via Atomic Absorption Spectrophotometry (AAS). To inhibit the ionisation of the sodium, leach filtrates were diluted 50:50 with a 4 g/l K + solution prior to aspiration. All sodium standards were prepared in 2 g/l K+ solutions. Sodium in coal was determined by leaching a 2g sample in 50 ml of 2M nitric acid, filtration and determination of the sodium content of the filtrate using AAS. This was then converted to sodium in coal on a dry basis. Chloride analyses had previously been done using the standard Mohr's titration technique (Quast, 1996), but this technique is not suitable for solutions of pH 2. Since these tests paralleled those reported earlier by the author using the same water:coal ratio, and knowing that all the chloride is water-soluble, hence acidsoluble, it was assumed that chloride would behave similarly in the two series of tests.
RESULTS The results for (one), two and three stage counter-current ion-exchange leaching at pH 2 are given in Figures 1, 2 and 3 respectively. A comparison between calculated and analysed values for the ion-exchange leaching of Bowmans coal is given in Table 1.
Water 600 ml Sodium 0.02 g
Coal 144 g Water 156 g
I "-
Sodium 1.87 g
7
I 1
Coal 144 g Water 216 g
[
Sodium 0.58 g
,1
Water 540 ml Sodium 1.25 g Fig. 1 Single stage ion exchange leaching test.
1426
K.B. Quast
Water 520 ml Sodium 0.02 g
Water 600 ml Sodium 0.5 g
Coal 144 g J
Coal 144 g .-I
Water156 g "] 1 Sodium 1.87 g [
Water 236 g "1 Sodium 0.89 g I
Coal 144 g
2
Water 246 g Sodium 0.37 g
rl L~
Water 510 ml Sodium 0.54 g
Water 520 ml Sodium 1.48 g
Fig.2 Two stage ion exchange leaching test. Coal 144 g Water 156 g Sodium 1.87 g
Water 600 ml Sodium 0.65 g
P
Water 530 ml Sodium 1.58 g
Coal 144 g Water 226 g Sodium 0.94 g
Water 530 ml Sodium 0.24 g
2 l
Water 520 ml "~ Sodium 0.70 g
Coal~44 g Water 236 g Sodium 0.48 g
Water 520 ml Sodium 0.02 g
Water 500 ml " Sodium 0.28 g
Coal 144 g Water 256 g Sodium 0.22 g Fig.3 Three stage ion exchange leaching.
Counter-currention exchangefor removalof organicallyboundsodiumfromlow-rankcoals
1427
TABLE 1 Corrdation between calculated and analysed values of moisture and sodium content Stages Single Two Three
Moisture content (%) Calculated Measured 60 60 64 63 64 64
Sodium content (%)(db) Measured Calculated 0.41 0.40 0.27 0.26 0.15 0.15
DISCUSSION To the best of the author's knowledge, this is the first time that the counter-current ion exchange leaching of an Australian low rank coal has been simulated in laboratory batch testing. These results should therefore be used as a guide to large scale behaviour, and pilot plant testing, at least, should be conducted to verify the batch data. The basis for the leaching procedure is shown in Figure 1. Water:coal ratio was maintained at 2:1 at each stage and the concentrations of sodium in coal were calculated from a mass balance using the feed values and leach liquor cortcentration values. A single stage ion exchange leach stage followed by flocculation and filtration gave the calculated coal product shown in Table 1. The efficiency of the test depends on the amount of water removed as the sodium in all water (filtrate and water in coal) is assumed to be in equilibrium. A simple settling test gave a solids density in the sludge of 23.5 % after 30 minutes settling time from a feed solids density of 16 %. This means that the effluent water volume (supernatant) would only be 287 ml. Under these conditions, the concentration of sodium would remain constant in the effluent water, and the calculated coal product would be: moisture 76.5 % and sodium 0.81% (db). This highlights the crucial effect of dewatering on the efficiency of sodium removal. This has been examined in an earlier publication by Quast and Readett (1989), and points to interstage filtration being necessary to achieve rapid removal of soluble and ion-exchangeable impurities. Using a two-stage ion exchange leaching process, the calculated coal product contained 63 % moisture and 0.26 % sodium on a dry basis. This means that the sodium content has been reduced by 80 %. The correlation between calculated and analysed coal products is excellent (see Table 1). Three stage ion exchange leaching reduced the sodium content to 0.15 % (db), a reduction of 88.5 % which is excellent bearing in mind tha~Lonly a total of 2 litres of fresh water are used per 1 kg of ROM coal. The process does, however, generate an acidic waste water stream containing 3 g/1 sodium and associated chloride and sulphate, which would need to be treated prior to disposal or reuse.
CONCLUSIONS It is possible to simulate the counter-current ion exchange leaching of sodium from Bowmans coal using a simple laboratory procedure. The technique works by allowing the leaching stage to reach equilibrium prior to flocculation and filtration. An efficient dewatering stage is crucial to the efficiency of impurity removal. For the procedure to work it is necessary to generate wash water streams of the same chemical composition as the subsequent effluent stream. A highly saline effluent stream is generated, however its volume was much less than that for washing with fresh water each time.
ACKNOWLEDGEMENT The author is gratetid to Flinders Power Pty. Ltd. for permission to publish this paper.
REFERENCES Allen, R.J., Sodium removal from Wakefield coal--Stage 2, Amdel report No. 1435. 1981, (unpublished).
1428
K.B. Quast
Manzoori, A.R., Role of the inorganic matter in agglomeration and defluidisation during the circulating fluid bed combustion of low-rank coals, Ph.D. thesis, University of Adelaide, 1990, 314p. Manzoori, A.R. and Plastow, K.M., Gasification of South Australian salty coals using the High Temperature Winkler process, Paper A4.7, Australian Coal Science Conference, Adelaide, 1988. Quast, K.B., Counter-Current leaching of Bowmans coal, Proceedings, 7th AlE Coal Science Conference, Churchill, Victoria, 1996, pp. 409-415. Quast, K.B., Pescott, E.R., Readett, D.J., Williams, R. and Kosminski, A., The partial removal of soluble salts from Bowmans coal and its effect on combustion, Proceedings, AlE Coal Science Conference 4, Brisbane, Queensland, 1990, pp. 331-338 Quast, K.B. and Readett, D.J., Surfactant assisted filtration of lignite slurries, Proceedings, AuslMM Dewatering Technology and Practice Conference, Brisbane, Queensland, 1989, pp. 31-35. Readett, D.J., Hall, S.F., Quast, K.B. and Ketteridge, I.B. Beneficiation of Bowmans lignite-optimisation of sodium reduction, Proceedings, CHEMECA '86, 1986, Adelaide, pp. 325-329. Readett, D.J. and Quast, K.B., Beneficiation of Bowmans coal-water leaching of soluble salts, Proceedings, CHEMECA '89, Gold Coast, Queensland, 1989, pp. 456-463. Vuthaluru, H.B.,Linjewile, T.M., Zhang, D-k. and Manzoori, A.R., Investigations into the control of agglomeration and defluidisation during fluidised-bed combustion of low-rank coals, Fuel, 1999, 78, 419-425. Wardle, D.G., The release of sodium and potassium during pulverised coal combustion, Proceedings, 1991 International conference on coal science, University of Newcastle-upon-Tyne, Butterworth-Heinemann, 1991, pp. 299-302.
Correspondence on papers published in Minerals Engineering is invited by e-mail to bwills @min-eng.com