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Effect of wet screening on particle size distribution and coal properties☆
Fuel 82 (2003) 2231–2237 www.fuelfirst.com
Effect of wet screening on particle size distribution and coal propertiesq A. Govender*, J.C. van Dyk Sasol Technology (Pty) Ltd, R&D Division, Syngas and Coal Technologies, 1 Klasie Havenga Street, P.O. Box 1, Sasolburg 1947, South Africa Received 19 December 2002; revised 3 June 2003; accepted 3 June 2003; available online 21 June 2003
Abstract Wet screening is one of the methods used to remove fine material from the coal feed to gasification. Sasol Synfuels in South Africa undertook an investigation to quantify fine coal generation in the coal supply to gasification. Coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (20.5 mm) increased almost five times when the coal was wet screened compared to dry screening. This study was subsequently initiated by Sasol Technology R&D to establish the mechanism of fine coal generation during wet screening, as well as the effect of wet screening on particle size distribution (PSD) and chemical properties of coal. Changes in the PSD and chemical properties of coal from individual coal sources used at Sasol Synfuels were compared. Composite coal samples with a predetermined PSD of all individual coal sources used at Sasol Synfuels were screened under wet and dry conditions. The PSD was again determined after screening, as well as the mineral composition (by X-ray diffraction) of the fines. Results indicated that wet screening caused clay minerals to be removed from the coal structure leading to an increase in the fines. This removal of minerals weakened the coal structure causing further size degradation of coarser fractions. q 2003 Elsevier Ltd. All rights reserved. Keywords: Wet screening; Particle size distribution; Chemical properties of coal
1. Introduction An investigation was undertaken by Sasol Synfuels to quantify fine coal generation in the coal supply to gasification. The coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (2 0.5 mm) were almost five times higher when wet screened than with dry screening. A conclusion from the study was that the laboratory wet screening process removed all 2 0.5 mm material from the surface of wet or semi-wet coarser particles, which is not removed by normal dry screening. The concern was raised by Sasol Technology R&D that the percentage of 2 0.5 mm material appeared unrealistically high and that it was possibly generated by clay minerals being washed from the coal, as well as fragmentation occurring during the screening process [4]. The clay mineral, kaolinite, is predominantly present as small particles in the channels and pores of the coal. Water could easily gain entry to these channels and pores and * Corresponding author. Tel.: þ 27-16-960-4098; fax: þ27-11-522-1349. E-mail address: [email protected] (A. Govender). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00193-5
remove the clay mineral—in effect removing the ‘glue’ that binds the coal particle together. The aims of this study were to establish the causes of fine coal generation during wet screening as well as the effect of wet screening on particle size distribution (PSD) and chemical properties.
2. Background The Sasol – Lurgi fixed bed dry bottom gasification process operates on lump sized coal. The coal bed has to be porous and permeable to allow the rising reactant gases to pass through the bed. If the concentration of fine material is too high, the permeability of the bed is severely reduced and channel flow is enhanced leading to operational instabilities [1]. When lump coal, such as South African low-rank inertinite-rich coal, enters a fixed-bed gasifier and is exposed to high temperatures, it tends to undergo fragmentation (the breaking of a single particle into two or more pieces), thus changing the PSD of the coal [6]. A study conducted by Van Dyk [3] determined the thermal fragmentation of individual Sasol Synfuels coal
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sources (the same coal sources used in this study) under wet and dry conditions. The finding was that thermal fragmentation of wet coal was more than 50% higher than thermal fragmentation of dry coal. Furthermore, the fragmentation of coal sources B and C were the highest, while that of coal source F was the lowest [3]. It can therefore be concluded that it is necessary to keep the amount of fines in the coal feed to the gasifier, as well as the amount of thermal fragmentation in the gasifier, as low as possible. Gasification at Sasol Synfuels in Secunda currently utilizes a wet screening process to remove all fine material from the coal feed to gasification. Although a wet screening process is considered to be more effective at removing fine material than a dry screening process, a number of negative impacts on coal properties have been observed and will be addressed in this paper. A large number of minerals have been identified in coal. Most of these minerals occur only in trace amounts and are of minor importance for practical purposes [2]. The abundant minerals are predominantly the clays, silicates and carbonates.
3. Experimental Batch coal samples from individual sources used at Sasol Synfuels were obtained for this study. Representative samples of each were obtained by coning and quartering and then passing through a splitter. These samples were first homogenised and crushed to 2 19 mm and then reconstituted to a fixed PSD of 2 19 mm þ 6.7 mm. The exact compositions may be found in Table A1. 3.1. Identification of possible trends by screening of individual coal sources Composite samples of coal sources A to F were used to highlight trends in PSD and chemical properties of the coal that result from the dry and wet screening. 3.1.1. Dry screening One composited 0.5 kg coal sample from each source was dry screened. The sample was passed through a set of eight screens with aperture sizes ranging from 13.2 to 0.5 mm as listed in Table A1. Each screen fraction was weighed to determine the new PSD after screening as noted in Table A1. An X-ray diffraction (XRD) analysis was conducted on the 2 0.5 mm fraction to determine the mineral composition which is listed in Table A2. 3.1.2. Wet screening One composited 0.5 kg coal sample from each source was wet screened. This involved a similar experimental set-up as for dry screening with the set of eight screens.
The coal was immersed in 1 l of distilled water and then poured over the screens. A further 2 l of distilled water was also passed through to aid the screening of the coal. The different screen fractions of coal were dried and weighed to determine the new PSD after wet screening as noted in Table A1. The slurry containing the 2 0.5 mm fraction was filtered. The 2 0.5 mm fraction collected was air dried and analysed by XRD to determine the mineral composition which is given in Table A2. 3.1.3. Clay test A clay test [5] was carried out to quantify fine (2 6.7 mm) coal generation of the various coal sources in order to compare with results from the wet screening process. The clay test involves a composite coal sample of known mass being immersed in water and stirred for 20 min. The coal is then screened and dried. 3.2. Identification of specific trends of coal source C In order to determine the specific changes in PSD and chemical properties of the coal that are affected by dry and wet screening, composite samples of coal source C were prepared. The mineral composition of a composited sample was determined by XRD to compare with the results obtained after the samples were dry and wet screened. 3.2.1. Dry screening One composite sample was dry screened as in Section 3.1.1. The different particle size fractions were combined then weighed to determine the PSD after screening as given in Table A3 and analysed for mineral composition (by XRD) as listed in Table 1. 3.2.2. Wet screening One composited sample was wet screened as in Section 3.1.2. The different particle size fractions were dried, weighed in order to determine the change in PSD and analysed as in Section 3.2.1. The slurry was filtered to separate the 2 0.5 mm fraction from the water. The 2 0.5 mm fraction was air dried and analysed by XRD to determine the mineral composition which is shown in Table 1. The analysis on the water is discussed in Section 3.2.3. 3.2.3. Analysis of water The distilled water was analysed, both prior to screening and after screening by Inductively Coupled Plasma (ICP) to identify the elements present as given in Table 2. This would help identify which water-soluble minerals were being removed by wet screening. 4. Results and discussion The experimental results are presented with the corresponding discussions.
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Table 1 Mineral distribution of coal source C as determined by XRD Sample
Fraction (mm)
Mineral (mass%) Quartz
Pyrite
Calcite
Dolomite
Aragonite
Alunite
Mica
K-feldspar
Illite–smectite
Kaolinite
Composite sample
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7
28 26 34
– – 2
3 3 4
10 11 8
– 9 –
5 7 5
– – –
– – –
– – –
54 44 47
Dry screening
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 0.5 20.5
45 29 34 29 23 21
4 2 2 3 – –
2 3 3 5 10 18
6 8 7 9 8 8
4 – – 6 – –
4 5 6 6 8 4
– – – – – 7
– – – – – –
– 5 – – – –
35 48 48 42 51 42
Wet screening
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 0.5 20.5
32 33 27 34 54 29
4 3 2 2 1 1
3 8 4 2 1 3
7 9 11 6 1 7
5 – – 5 – –
4 4 6 5 – 5
5 – – – – 8
– – – – 4 –
– – – – – –
40 43 50 46 39 47
4.1. Identification of possible trends by screening of individual coal sources This section focuses on the results of the experiments conducted on coal sources A to F. 4.1.1. Particle size distribution The PSD of the individual coal sources as listed in Table A1 displayed similar trends after wet and dry screening. The average of the PSD results was plotted and is depicted in Fig. 2. The trend shows that the coarse fraction decreased while the fine fraction (2 0.5 mm) increased after wet screening. This is indicative of possible fragmentation occurring during wet screening and mineral (i.e. clay) removal from the coal structure. There are differences in particle size between dry and wet screened coal all along the PSD curves with the differences at the two extremes of the curve in Fig. 1 being the most pronounced. Since the original sample was composed of 2 19 mm þ 6.7 mm material, all 2 6.7 mm material present after screening was generated as a result of the screening process. The variation in the 2 6.7 mm fractions after dry and wet screening of the individual coal sources is depicted in Fig. 2. The graph illustrates that wet screening significantly increases the amount of fine material. Coal sources C and B showed the greatest amount of fines generation, while F produced the least. The reason for this could be that F is a high caking coal, whereas C and B are medium to low caking coals. 4.1.2. Mineral composition Various minerals were identified in the different coal sources from the XRD analysis performed on the 2 0.5 mm fractions as listed in Table A2. Only five of the minerals (quartz, calcite, mica, dolomite and kaolinite) were common to all of the sources. The variation in the distribution of
those minerals after dry and wet screening is depicted in Fig. 3. The scattered graph indicates distinct differences in the concentrations of the various minerals when comparing dry and wet screening. The most abundant mineral in the coals, kaolinite (a clay mineral) showed a higher average concentration in the fines after wet screening (51 mass%) than after dry screening (42 mass%). This confirms the removal of kaolinite from pores and layers within the coal structure during wet screening. 4.1.3. Clay test The results of the clay test are given in Fig. 4. This correlates with the observations from the wet screening, Table 2 Concentration of elements present in distilled water as determined by ICP Element
Al As Ca Cd Co Cr Cu Fe K Mg Mn Mo Na Ni Pb Zn
Water sample (ppm) Original distilled water
Water collected after wet screeninga
,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1
,1 ,1 1.4 (std dev. ¼ 0.40) ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 4 (std dev. ¼ 0.58) ,1 ,1 ,1
a Values are accurate to the standard deviation obtained from triplicate measurements.
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Fig. 1. Average particle size distribution after dry and wet screening of individual coal sources.
depicted in Fig. 2, confirming that coal sources B and C generated the most slimes (clays) and coal source F generated the least. These results are also in fair agreement with those of Van Dyk [3].
screening than after dry screening. Similarly, kaolinite is 12% higher after wet screening. These results correlate with those in Section 4.1.2 and confirm that wet screening causes the clays to be present in higher concentrations in the fines.
4.2. Identification of specific trends of coal source C This section focuses on the results of the experiments conducted on coal source C. 4.2.1. Particle size distribution The changes in PSD of coal source C as plotted in Fig. 5 with the values listed in Table A3. They correspond closely with the average results of the coal sources shown in Fig. 2, indicating a decrease in coarse material and an increase in the percentage of fines after wet screening. 4.2.2. Mineral composition Comparison of the values from Table 1 shows 38% more quartz present in the 0.5 mm fraction after wet
4.2.3. Analysis of water The distilled water used in the wet screening was analysed for a range of 16 elements by ICP. The results are listed in Table 2. Calcium and sodium were not present in the original water, but were detected after it came into contact with the coal particles and minerals during wet screening. The sodium is due to the known presence of sodium salts such as NaCl in coal [7]. The calcium is most likely from the watersoluble carbonates such as calcite present in the coal. It would explain why the concentration of calcite is 83% lower in the 2 0.5 mm fraction of the wet screened coal.
Fig. 2. Variation in the 26.7 mm fractions after dry and wet screening of the individual coal sources.
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Fig. 3. Variation in mineral distribution of the 20.5 mm fractions of the individual coal sources after wet and dry screening as determined by XRD.
4.3. Summary of results In summary, the percentage 2 0.5 mm material increased significantly during wet screening and is possibly related to clays removed from the pores of coal particles. Fig. 6, an
optical micrograph of a polished coal sample in reflected light, shows the argument proposed in this study. Clay minerals are located in the channels and pores within the structure. It is predominantly the clay mineral, kaolinite, which is present as small particles in the channels and pores
Fig. 4. 2 6.7 mm fractions of the individual coal sources as determined by Clay Test.
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Fig. 5. Particle size distribution of individual coal source C.
5. Conclusions
Fig. 6. An optical micrograph of a polished coal sample in reflected light.
of the coal. Water could easily gain entry to these channels and pores and remove the clay mineral—in effect removing the ‘glue’ that binds the coal particle together. This would lead to a weakening of the coal structure and result in increased fragmentation.
† During wet screening, the coarse fraction decreased while the fine fraction (2 0.5 mm) increased, indicating that fragmentation occurs during wet screening. † The concentration of clays (predominantly kaolinite) present in the 2 0.5 mm fraction increased in the wet screened samples in comparison to the dry screened samples and this indicates the removal of minerals during wet screening. † Water solubility of minerals is also a factor in concentrations after screening. † Wet screening causes clays (predominantly kaolinite) to be removed from the coal structure which leads to an increase in the 2 0.5 mm material. The removal of the minerals weakens the coal structure, leading to further size degradation of the coarser coal fractions, thus resulting in fragmentation.
Table A1 Particle size distributions after wet and dry screening of individual coal sources Screen size (mm)
Initial PSD
Coal source (mass%) A
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 3.35 23.35 þ 2.36 22.36 þ 1.7 21.7 þ 0.5 20.5
42.1 33.3 24.6
B
C
D
E
F
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
42.3 30.5 25.2 1.8 0.0 0.0 0.0 0.0 0.1
37.6 31.7 24.5 2.5 0.6 0.2 0.1 0.2 2.6
39.4 33.4 25.4 1.5 0.0 0.0 0.0 0.0 0.1
36.3 30.7 25.0 1.3 0.2 0.1 0.1 0.1 6.1
40.5 32.9 25.4 1.1 0.0 0.0 0.0 0.0 0.1
32.6 35.2 22.3 1.8 2.1 1.2 0.6 0.6 3.6
40.6 33.8 23.6 1.8 0.1 0.0 0.0 0.0 0.0
37.1 33.7 23.0 1.0 0.3 0.3 0.1 0.1 4.3
42.4 32.2 23.8 1.4 0.0 0.0 0.0 0.0 0.1
38.9 29.6 24.1 1.1 0.5 0.2 0.1 0.3 5.3
41.3 33.2 24.7 0.7 0.0 0.0 0.0 0.0 0.1
40.3 31.5 23.9 1.4 0.1 0.0 0.0 0.1 2.6
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Table A2 Mineral distribution of the 20.5 mm fractions of individual coal sources after dry and wet screening as determined by XRD Mineral
Coal source (mass%) A
Pyrite Quartz Hematite Spinel Calcite Dolomite K-feldspar Alunite Analcime Mica Kaolinite Smectite
B
C
D
E
F
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
– 15 5 – 13 6 – 4 22 3 32 –
– 24 – – 2 4 – – – 18 48 4
– 22 3 – 26 6 – – – 4 39 –
5 24 – – 5 3 4 – – 7 52 –
– 20 3 – 9 8 – 4 9 8 39 –
2 27 4 – 12 5 – – – 5 45 –
– 23 5 – 6 11 – 6 – – 49 –
– 20 – 9 5 5 – – – 11 50 –
– 30 4 – 18 7 – – – 4 37 –
2 28 – – 7 5 – – – 11 47 –
– 15 4 – 10 8 – 2 – 5 56 –
2 7 – – 4 4 – – – 19 64 –
Appendix A
Table A3 Particle size distribution of individual coal source C Screen size (mm)
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 3.35 23.35 þ 2.36 22.36 þ 1.7 21.7 þ 0.5 20.5
Original PSD
42.1 33.3 24.6
Tables A1 – A3.
Coal source C (mass%) Dry
Wet
32.2 38.7 23.9 5.0 0.0 0.0 0.0 0.0 0.1
28.7 36.5 25.0 5.7 1.1 0.4 0.2 0.2 2.3
Acknowledgements The author gratefully acknowledges support and input received from the co-author J.C. Van Dyk as well as M.J. Keyser and also wishes to thank B. Ashton and S. du Plessis for their work contribution to the successful completion of this paper.
References [1] Slaghuis JH. Coal gasification: a study guide for the National Diploma in Fuel Technology. Coal Process III: Part A 1993. [2] Slaghuis JH. Fuel science: a study guide for the National Diploma in Fuel Technology. Fuel Sci III 1995. [3] Van Dyk JC. Caking propensity of secunda coal sources. Private Communication; March 2001. [4] Van Dyk JC. Concerns w.r.t. wet and dry screening. Private Communication; December 2001. [5] Van Dyk JC. Ondersoek van Steenkool van Brandspruit, Middelbult en Herwinner 3 & 4, wat verband hou met die onstabiliteit by Sasol Wes in Secunda. Private Communication; January 1995. [6] Van Dyk JC. Thermal friability of coal sources used by Sasol Chemical Industries (SCI) for gasification–quantification and statistical evaluation. MSc Thesis. University of the Witwatersrand; 1999. [7] Vassilev SV, Vassileva CG. Relations between ash yield and chemical and mineral composition of coals. Fuel 1997;76:3–8.
Effect of wet screening on particle size distribution and coal propertiesq A. Govender*, J.C. van Dyk Sasol Technology (Pty) Ltd, R&D Division, Syngas and Coal Technologies, 1 Klasie Havenga Street, P.O. Box 1, Sasolburg 1947, South Africa Received 19 December 2002; revised 3 June 2003; accepted 3 June 2003; available online 21 June 2003
Abstract Wet screening is one of the methods used to remove fine material from the coal feed to gasification. Sasol Synfuels in South Africa undertook an investigation to quantify fine coal generation in the coal supply to gasification. Coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (20.5 mm) increased almost five times when the coal was wet screened compared to dry screening. This study was subsequently initiated by Sasol Technology R&D to establish the mechanism of fine coal generation during wet screening, as well as the effect of wet screening on particle size distribution (PSD) and chemical properties of coal. Changes in the PSD and chemical properties of coal from individual coal sources used at Sasol Synfuels were compared. Composite coal samples with a predetermined PSD of all individual coal sources used at Sasol Synfuels were screened under wet and dry conditions. The PSD was again determined after screening, as well as the mineral composition (by X-ray diffraction) of the fines. Results indicated that wet screening caused clay minerals to be removed from the coal structure leading to an increase in the fines. This removal of minerals weakened the coal structure causing further size degradation of coarser fractions. q 2003 Elsevier Ltd. All rights reserved. Keywords: Wet screening; Particle size distribution; Chemical properties of coal
1. Introduction An investigation was undertaken by Sasol Synfuels to quantify fine coal generation in the coal supply to gasification. The coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (2 0.5 mm) were almost five times higher when wet screened than with dry screening. A conclusion from the study was that the laboratory wet screening process removed all 2 0.5 mm material from the surface of wet or semi-wet coarser particles, which is not removed by normal dry screening. The concern was raised by Sasol Technology R&D that the percentage of 2 0.5 mm material appeared unrealistically high and that it was possibly generated by clay minerals being washed from the coal, as well as fragmentation occurring during the screening process [4]. The clay mineral, kaolinite, is predominantly present as small particles in the channels and pores of the coal. Water could easily gain entry to these channels and pores and * Corresponding author. Tel.: þ 27-16-960-4098; fax: þ27-11-522-1349. E-mail address: [email protected] (A. Govender). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00193-5
remove the clay mineral—in effect removing the ‘glue’ that binds the coal particle together. The aims of this study were to establish the causes of fine coal generation during wet screening as well as the effect of wet screening on particle size distribution (PSD) and chemical properties.
2. Background The Sasol – Lurgi fixed bed dry bottom gasification process operates on lump sized coal. The coal bed has to be porous and permeable to allow the rising reactant gases to pass through the bed. If the concentration of fine material is too high, the permeability of the bed is severely reduced and channel flow is enhanced leading to operational instabilities [1]. When lump coal, such as South African low-rank inertinite-rich coal, enters a fixed-bed gasifier and is exposed to high temperatures, it tends to undergo fragmentation (the breaking of a single particle into two or more pieces), thus changing the PSD of the coal [6]. A study conducted by Van Dyk [3] determined the thermal fragmentation of individual Sasol Synfuels coal
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sources (the same coal sources used in this study) under wet and dry conditions. The finding was that thermal fragmentation of wet coal was more than 50% higher than thermal fragmentation of dry coal. Furthermore, the fragmentation of coal sources B and C were the highest, while that of coal source F was the lowest [3]. It can therefore be concluded that it is necessary to keep the amount of fines in the coal feed to the gasifier, as well as the amount of thermal fragmentation in the gasifier, as low as possible. Gasification at Sasol Synfuels in Secunda currently utilizes a wet screening process to remove all fine material from the coal feed to gasification. Although a wet screening process is considered to be more effective at removing fine material than a dry screening process, a number of negative impacts on coal properties have been observed and will be addressed in this paper. A large number of minerals have been identified in coal. Most of these minerals occur only in trace amounts and are of minor importance for practical purposes [2]. The abundant minerals are predominantly the clays, silicates and carbonates.
3. Experimental Batch coal samples from individual sources used at Sasol Synfuels were obtained for this study. Representative samples of each were obtained by coning and quartering and then passing through a splitter. These samples were first homogenised and crushed to 2 19 mm and then reconstituted to a fixed PSD of 2 19 mm þ 6.7 mm. The exact compositions may be found in Table A1. 3.1. Identification of possible trends by screening of individual coal sources Composite samples of coal sources A to F were used to highlight trends in PSD and chemical properties of the coal that result from the dry and wet screening. 3.1.1. Dry screening One composited 0.5 kg coal sample from each source was dry screened. The sample was passed through a set of eight screens with aperture sizes ranging from 13.2 to 0.5 mm as listed in Table A1. Each screen fraction was weighed to determine the new PSD after screening as noted in Table A1. An X-ray diffraction (XRD) analysis was conducted on the 2 0.5 mm fraction to determine the mineral composition which is listed in Table A2. 3.1.2. Wet screening One composited 0.5 kg coal sample from each source was wet screened. This involved a similar experimental set-up as for dry screening with the set of eight screens.
The coal was immersed in 1 l of distilled water and then poured over the screens. A further 2 l of distilled water was also passed through to aid the screening of the coal. The different screen fractions of coal were dried and weighed to determine the new PSD after wet screening as noted in Table A1. The slurry containing the 2 0.5 mm fraction was filtered. The 2 0.5 mm fraction collected was air dried and analysed by XRD to determine the mineral composition which is given in Table A2. 3.1.3. Clay test A clay test [5] was carried out to quantify fine (2 6.7 mm) coal generation of the various coal sources in order to compare with results from the wet screening process. The clay test involves a composite coal sample of known mass being immersed in water and stirred for 20 min. The coal is then screened and dried. 3.2. Identification of specific trends of coal source C In order to determine the specific changes in PSD and chemical properties of the coal that are affected by dry and wet screening, composite samples of coal source C were prepared. The mineral composition of a composited sample was determined by XRD to compare with the results obtained after the samples were dry and wet screened. 3.2.1. Dry screening One composite sample was dry screened as in Section 3.1.1. The different particle size fractions were combined then weighed to determine the PSD after screening as given in Table A3 and analysed for mineral composition (by XRD) as listed in Table 1. 3.2.2. Wet screening One composited sample was wet screened as in Section 3.1.2. The different particle size fractions were dried, weighed in order to determine the change in PSD and analysed as in Section 3.2.1. The slurry was filtered to separate the 2 0.5 mm fraction from the water. The 2 0.5 mm fraction was air dried and analysed by XRD to determine the mineral composition which is shown in Table 1. The analysis on the water is discussed in Section 3.2.3. 3.2.3. Analysis of water The distilled water was analysed, both prior to screening and after screening by Inductively Coupled Plasma (ICP) to identify the elements present as given in Table 2. This would help identify which water-soluble minerals were being removed by wet screening. 4. Results and discussion The experimental results are presented with the corresponding discussions.
A. Govender, J.C. van Dyk / Fuel 82 (2003) 2231–2237
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Table 1 Mineral distribution of coal source C as determined by XRD Sample
Fraction (mm)
Mineral (mass%) Quartz
Pyrite
Calcite
Dolomite
Aragonite
Alunite
Mica
K-feldspar
Illite–smectite
Kaolinite
Composite sample
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7
28 26 34
– – 2
3 3 4
10 11 8
– 9 –
5 7 5
– – –
– – –
– – –
54 44 47
Dry screening
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 0.5 20.5
45 29 34 29 23 21
4 2 2 3 – –
2 3 3 5 10 18
6 8 7 9 8 8
4 – – 6 – –
4 5 6 6 8 4
– – – – – 7
– – – – – –
– 5 – – – –
35 48 48 42 51 42
Wet screening
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 0.5 20.5
32 33 27 34 54 29
4 3 2 2 1 1
3 8 4 2 1 3
7 9 11 6 1 7
5 – – 5 – –
4 4 6 5 – 5
5 – – – – 8
– – – – 4 –
– – – – – –
40 43 50 46 39 47
4.1. Identification of possible trends by screening of individual coal sources This section focuses on the results of the experiments conducted on coal sources A to F. 4.1.1. Particle size distribution The PSD of the individual coal sources as listed in Table A1 displayed similar trends after wet and dry screening. The average of the PSD results was plotted and is depicted in Fig. 2. The trend shows that the coarse fraction decreased while the fine fraction (2 0.5 mm) increased after wet screening. This is indicative of possible fragmentation occurring during wet screening and mineral (i.e. clay) removal from the coal structure. There are differences in particle size between dry and wet screened coal all along the PSD curves with the differences at the two extremes of the curve in Fig. 1 being the most pronounced. Since the original sample was composed of 2 19 mm þ 6.7 mm material, all 2 6.7 mm material present after screening was generated as a result of the screening process. The variation in the 2 6.7 mm fractions after dry and wet screening of the individual coal sources is depicted in Fig. 2. The graph illustrates that wet screening significantly increases the amount of fine material. Coal sources C and B showed the greatest amount of fines generation, while F produced the least. The reason for this could be that F is a high caking coal, whereas C and B are medium to low caking coals. 4.1.2. Mineral composition Various minerals were identified in the different coal sources from the XRD analysis performed on the 2 0.5 mm fractions as listed in Table A2. Only five of the minerals (quartz, calcite, mica, dolomite and kaolinite) were common to all of the sources. The variation in the distribution of
those minerals after dry and wet screening is depicted in Fig. 3. The scattered graph indicates distinct differences in the concentrations of the various minerals when comparing dry and wet screening. The most abundant mineral in the coals, kaolinite (a clay mineral) showed a higher average concentration in the fines after wet screening (51 mass%) than after dry screening (42 mass%). This confirms the removal of kaolinite from pores and layers within the coal structure during wet screening. 4.1.3. Clay test The results of the clay test are given in Fig. 4. This correlates with the observations from the wet screening, Table 2 Concentration of elements present in distilled water as determined by ICP Element
Al As Ca Cd Co Cr Cu Fe K Mg Mn Mo Na Ni Pb Zn
Water sample (ppm) Original distilled water
Water collected after wet screeninga
,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1
,1 ,1 1.4 (std dev. ¼ 0.40) ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 4 (std dev. ¼ 0.58) ,1 ,1 ,1
a Values are accurate to the standard deviation obtained from triplicate measurements.
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Fig. 1. Average particle size distribution after dry and wet screening of individual coal sources.
depicted in Fig. 2, confirming that coal sources B and C generated the most slimes (clays) and coal source F generated the least. These results are also in fair agreement with those of Van Dyk [3].
screening than after dry screening. Similarly, kaolinite is 12% higher after wet screening. These results correlate with those in Section 4.1.2 and confirm that wet screening causes the clays to be present in higher concentrations in the fines.
4.2. Identification of specific trends of coal source C This section focuses on the results of the experiments conducted on coal source C. 4.2.1. Particle size distribution The changes in PSD of coal source C as plotted in Fig. 5 with the values listed in Table A3. They correspond closely with the average results of the coal sources shown in Fig. 2, indicating a decrease in coarse material and an increase in the percentage of fines after wet screening. 4.2.2. Mineral composition Comparison of the values from Table 1 shows 38% more quartz present in the 0.5 mm fraction after wet
4.2.3. Analysis of water The distilled water used in the wet screening was analysed for a range of 16 elements by ICP. The results are listed in Table 2. Calcium and sodium were not present in the original water, but were detected after it came into contact with the coal particles and minerals during wet screening. The sodium is due to the known presence of sodium salts such as NaCl in coal [7]. The calcium is most likely from the watersoluble carbonates such as calcite present in the coal. It would explain why the concentration of calcite is 83% lower in the 2 0.5 mm fraction of the wet screened coal.
Fig. 2. Variation in the 26.7 mm fractions after dry and wet screening of the individual coal sources.
A. Govender, J.C. van Dyk / Fuel 82 (2003) 2231–2237
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Fig. 3. Variation in mineral distribution of the 20.5 mm fractions of the individual coal sources after wet and dry screening as determined by XRD.
4.3. Summary of results In summary, the percentage 2 0.5 mm material increased significantly during wet screening and is possibly related to clays removed from the pores of coal particles. Fig. 6, an
optical micrograph of a polished coal sample in reflected light, shows the argument proposed in this study. Clay minerals are located in the channels and pores within the structure. It is predominantly the clay mineral, kaolinite, which is present as small particles in the channels and pores
Fig. 4. 2 6.7 mm fractions of the individual coal sources as determined by Clay Test.
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Fig. 5. Particle size distribution of individual coal source C.
5. Conclusions
Fig. 6. An optical micrograph of a polished coal sample in reflected light.
of the coal. Water could easily gain entry to these channels and pores and remove the clay mineral—in effect removing the ‘glue’ that binds the coal particle together. This would lead to a weakening of the coal structure and result in increased fragmentation.
† During wet screening, the coarse fraction decreased while the fine fraction (2 0.5 mm) increased, indicating that fragmentation occurs during wet screening. † The concentration of clays (predominantly kaolinite) present in the 2 0.5 mm fraction increased in the wet screened samples in comparison to the dry screened samples and this indicates the removal of minerals during wet screening. † Water solubility of minerals is also a factor in concentrations after screening. † Wet screening causes clays (predominantly kaolinite) to be removed from the coal structure which leads to an increase in the 2 0.5 mm material. The removal of the minerals weakens the coal structure, leading to further size degradation of the coarser coal fractions, thus resulting in fragmentation.
Table A1 Particle size distributions after wet and dry screening of individual coal sources Screen size (mm)
Initial PSD
Coal source (mass%) A
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 3.35 23.35 þ 2.36 22.36 þ 1.7 21.7 þ 0.5 20.5
42.1 33.3 24.6
B
C
D
E
F
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
42.3 30.5 25.2 1.8 0.0 0.0 0.0 0.0 0.1
37.6 31.7 24.5 2.5 0.6 0.2 0.1 0.2 2.6
39.4 33.4 25.4 1.5 0.0 0.0 0.0 0.0 0.1
36.3 30.7 25.0 1.3 0.2 0.1 0.1 0.1 6.1
40.5 32.9 25.4 1.1 0.0 0.0 0.0 0.0 0.1
32.6 35.2 22.3 1.8 2.1 1.2 0.6 0.6 3.6
40.6 33.8 23.6 1.8 0.1 0.0 0.0 0.0 0.0
37.1 33.7 23.0 1.0 0.3 0.3 0.1 0.1 4.3
42.4 32.2 23.8 1.4 0.0 0.0 0.0 0.0 0.1
38.9 29.6 24.1 1.1 0.5 0.2 0.1 0.3 5.3
41.3 33.2 24.7 0.7 0.0 0.0 0.0 0.0 0.1
40.3 31.5 23.9 1.4 0.1 0.0 0.0 0.1 2.6
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Table A2 Mineral distribution of the 20.5 mm fractions of individual coal sources after dry and wet screening as determined by XRD Mineral
Coal source (mass%) A
Pyrite Quartz Hematite Spinel Calcite Dolomite K-feldspar Alunite Analcime Mica Kaolinite Smectite
B
C
D
E
F
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Wet
– 15 5 – 13 6 – 4 22 3 32 –
– 24 – – 2 4 – – – 18 48 4
– 22 3 – 26 6 – – – 4 39 –
5 24 – – 5 3 4 – – 7 52 –
– 20 3 – 9 8 – 4 9 8 39 –
2 27 4 – 12 5 – – – 5 45 –
– 23 5 – 6 11 – 6 – – 49 –
– 20 – 9 5 5 – – – 11 50 –
– 30 4 – 18 7 – – – 4 37 –
2 28 – – 7 5 – – – 11 47 –
– 15 4 – 10 8 – 2 – 5 56 –
2 7 – – 4 4 – – – 19 64 –
Appendix A
Table A3 Particle size distribution of individual coal source C Screen size (mm)
219 þ 13.2 213.2 þ 9.5 29.5 þ 6.7 26.7 þ 4.75 24.75 þ 3.35 23.35 þ 2.36 22.36 þ 1.7 21.7 þ 0.5 20.5
Original PSD
42.1 33.3 24.6
Tables A1 – A3.
Coal source C (mass%) Dry
Wet
32.2 38.7 23.9 5.0 0.0 0.0 0.0 0.0 0.1
28.7 36.5 25.0 5.7 1.1 0.4 0.2 0.2 2.3
Acknowledgements The author gratefully acknowledges support and input received from the co-author J.C. Van Dyk as well as M.J. Keyser and also wishes to thank B. Ashton and S. du Plessis for their work contribution to the successful completion of this paper.
References [1] Slaghuis JH. Coal gasification: a study guide for the National Diploma in Fuel Technology. Coal Process III: Part A 1993. [2] Slaghuis JH. Fuel science: a study guide for the National Diploma in Fuel Technology. Fuel Sci III 1995. [3] Van Dyk JC. Caking propensity of secunda coal sources. Private Communication; March 2001. [4] Van Dyk JC. Concerns w.r.t. wet and dry screening. Private Communication; December 2001. [5] Van Dyk JC. Ondersoek van Steenkool van Brandspruit, Middelbult en Herwinner 3 & 4, wat verband hou met die onstabiliteit by Sasol Wes in Secunda. Private Communication; January 1995. [6] Van Dyk JC. Thermal friability of coal sources used by Sasol Chemical Industries (SCI) for gasification–quantification and statistical evaluation. MSc Thesis. University of the Witwatersrand; 1999. [7] Vassilev SV, Vassileva CG. Relations between ash yield and chemical and mineral composition of coals. Fuel 1997;76:3–8.