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Study on the washability of the Kaitai coal, Guizhou Province China
Fuel Processing Technology 92 (2011) 692–698
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Study on the washability of the Kaitai coal, Guizhou Province China Qin Zhang a, Yingzhong Tian a, Yueqin Qiu a, Jianxin Cao a, Tiancun Xiao a,b,⁎ a b
CSCSTUK Clean Energy Centre, School of Mines, Guizhou University, Caijiaguan Campus, Guiyang City, 550003, Guizhou, China Inorganic Chemistry Laboratory, Oxford University, South Parks Road, OX1 3QR United Kingdom
a r t i c l e
i n f o
Article history: Received 5 July 2010 Received in revised form 28 November 2010 Accepted 29 November 2010 Available online 28 December 2010 Keywords: Float–sink Kaitai coal Different sizes Desulfurisation
a b s t r a c t Tonnage of coal samples were collected from Kaitai Coal field, Puan County, Guizhou Province and sieved into different particle size catalogs. The analysis of the overall coal suggested that the coal has low ash but high sulfur content of 3.17 wt.% with medium to high volatile content. The heating value of the coal is 31.668 mJ/ kg. The coal sample with different particle size ranges were tested for float–sink using gravity separation method, in which ZnCl2 solutions with different density are used. It is showed that decreasing sulfur to 2.12 wt.% can give coal yield of 94.55%, suggesting that the coal's floatability is good with sulfur. The coal yield is only 85.4% when reducing sulfur to 1.5 wt.%, and 76.7% when sulfur is decreased to 1.2% through the ZnCl2 float–sink process. The δ ± 0.1 is 20.55, which is in the 20.1–30 range, suggest that Kaitai coal is difficult to float–sink for depyritisation. Characterization of the floated coal at different sizes showed that the organic sulfur may mainly be present in the small size, the pyrite sulfur is mainly present in the coal with bigger particle size, which can be easily removed through float–sink process. The ash in the small particle sized coal is mainly from kaolinite and quartz, while the pyrite is the main ash contributor to the coal with big size. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Coal is the major primary energy source in China and is forecast to account for over 60% of the primary energy consumption mix, and the total coal demand will reach 2.3–2.9 billion tons in 2020 [1]. Sulfur and mineral matter contents of a coal are important factors, deciding the coal properties and utilisation, because direct utilization of high ash and highsulfur content coals leads to serious technological and environmental problems. With the many year mining and consumption of coal, the coal reserve with high quality, e.g., low sulfur and less ash is depleting. Carbon is the main component in coal, which also contains various hetero atoms such as sulfur, nitrogen, chlorine, minerals such as silica, kaolionite, titania, quartz and various trace metals such as Mo, W, alkali metal and toxic elements such as Hg, As. Because of their high content, mineral matter and sulfur exhibit harmful effects on the utilization of coal for purposes of combustion, carbonization, gasification, liquefaction, etc. These impurities poison the catalyst that is used in coal conversion processes. SO2 produced during combustion leads to atmospheric pollution, acid rain, and the corrosion of boilers, pipelines, and other machinery. Therefore, it is necessary to remove mineral matter and sulfur from coal prior to utilization, the demineralization and/or desulfurization of low-grade high-ash and/or high-sulfur coals to obtain environmentally acceptable clean fuels has attracted greater attention [1–6]. ⁎ Corresponding author. Inorganic Chemistry Laboratory, Oxford University, South Parks Road, OX1 3QR, United Kingdom. Tel.: +44 1865272660. E-mail address: [email protected] (T. Xiao). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.030
Guizhou has the most coal reserve in South China, with a reserve of 241.9 billion tons of coal, it is therefore planned as the Southern base for power generation, and exporting the electricity to the Eastern China. However, the coal in Guizhou rank ranges widely from high volatile bituminous to low volatile bituminous and anthracite. The sulfur content is from 1 wt.% to 5 wt.%. The Major mineral phases present in the Liupanshui coal are kaolinite, quartz, pyrite, and calcite. Traces of other primary minerals are marcasite, gypsum, and dolomite. The Shuicheng coal usually has higher kaolinite and quartz contents than the Liuzhi coal. Marcasite occurs indiscriminately in the different coal seams without a clear distribution pattern. The presence of other minerals, such as rutile, anatase, tourmaline, zircon, and phosphates, was also noted. The total sulfur content of Liupanshui coals is higher in the marineinfluenced coal seams (up to 7.5% dry), and lower in the non-marineinfluenced coals (as low as 0.3%). In the Liuzhi coal field, the coal is characterized by a high sulfur and iron content, whereas in the Shuicheng coal field, contents varied from low to high, depending on the coal seams [7–9]. However, generally speaking, with increasing consumption, the coal quality deteriorates, especially sulfur content increases in all the coal field, which is beyond the increasing strict national environmental regulation limit. To meet the market and environmental regulations, there have been lots of studies on coal desulfurisation and de-ashing [2,10–24], among which float–sink has been widely used to remove minerals and inorganic sulfur, while organic sulfur is removed through pyrolysis or microbiological floatation.
Q. Zhang et al. / Fuel Processing Technology 92 (2011) 692–698
The most efficient industrial gravity-based separators used for coal cleaning employ the use of a dense-medium, which is most commonly comprised of an aqueous suspension of ultrafine magnetite. The density of the suspension is adjusted to a value that is between the densities of coal and the associated mineral matter. As such, the light coal particles float while the heavy particles sink. Although the separation performance is generally superior to water-only systems, the overall efficiency of the process declines with a decrease in particle size due to process kinetics. However, to provide the basic information for the gravity-based separator so as to adjust the aqueous suspension of the ultrafine magnetite, the coal washability is normally first studied in lab scale using coal sample of up to tonnage scale. To have a more accurate density adjustment, the lab scale washbility study is normally operated using ZnCl2 solution, whose density ccan be continuous adjusted. In this work, tonnage of coal samples were collected from Kaitai Coal field, one of Puan County coal seams, and sieved into different size range. Each sized portion has been processed using float–sink method to study the floatability of the coal samples. The samples at various stages have been characterized and some interesting results have been obtained. 2. Experimental Each time, about 1000 kg of the coal sample were randomly collected from the storage site of Kaitai Coal field, which supplied coal to two main thermal power stations located in the Southwest of Guizhou Province. The location of Puan, and Guizhou province in China is shown in Fig. 1. The samples were run-of-mine samples. Normally the coal underground was mined and dumped in a storage site, the coal samples were randomly taken from 30 different points and height of coal pile. About 40 kg coal were collected at each point and mixed as it is. The collected coal samples were screened and fractionated into size classes from −53 um to 50 mm. Selected fractions of the coal samples were analyzed regarding mineralogical and chemical composition. Also, selected fractions were used for washability analysis. 2.1. Washability study China has set out National Standard Method for Coal Float-andSink Analaysis in 1987. Thereforfe in this work. All the experimental procedures follows the GB478-1987 fo r the washability study.
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In the float–sink experiments, the coal sample from each size grade is dried at 100 °C, then cooled down to room temperature, weighed and the particle smaller than 0.5 were washed away, fired and weighed again. The samples were placed in a barrel bottomed with different size of mesh (N 0.5 mm). They are dipped into ZnCl2 water solutions starting with lower ZnCl2 density, to higher density, step by step. The gravity solution densities were adjusted by the ZnCl2 content in the solution. The floatation solution with density (kg/L) of 1.40, 1.50, 1.60, 1.70 and 1.80 were prepared, with the pH of 3.8, 3.26, 2.66, 2.02, and 1.6. The components whose density is lower than the dipped solution would float, and sieved out by the specific sized mesh, and the sink part would be put in the heavier solutions and the floated component were meshed out. This process is repeated until the highest density media. The floated coal parts were washed until the chlorine content in the sample is less than 0.01 wt.%, dried and weighed and then analysed. 2.2. Chemical analysis The proximate analysis (moisture, volatile matter, fixed carbon, and ash) and ultimate analysis (carbon, hydrogen, nitrogen, and sulfur) of collected coal samples were done in the CSCST-UK—Guizhou University Clean Energy Lab. The proximate analyses of the coal samples were done by standard methods (IS: 1350 (Part I) 1984). The elements C, H, N and S were analysed by the PerkinElmer 2400 Series II CHNS Elemental Analyzer. The calorific values were determined using Bomb calorimeter (ASTM D 3286). The analyses of the coal samples are given in Table 1. The ash analysis of the coal samples was carried out by standard methods. All analyses for the samples were carried out in quadruplicate and mean values have been reported. The crystalline structure of the samples were determined by X-ray diffraction using a Philips X' PeRT Pro Alpha 1 diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at a tube current of 40 kV and a voltage of 40 Ma. Data were collected over 2θ values from 20° to 80°, at a scan speed of 1°/min. 3. Results and discussion The tonnage coal sample were sieved and divided into various portions, and a general sample is taken through mixing the proportional parts of each sized sample, mixed and ground into powder, and then analysed. The results of the general Kaitai coal sample are shown in Table 1. It is seen that the moisture is 2.22%, ash
Fig. 1. Location of Kaitai Coal field which lies in Puan County, Guizhou Province, China.
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Table 1 Property of Kaitai Coal with natural sized particle.
Natural particle size
Mad/%
Aad/%
Vdaf/%
FCad/%
Stad/%
Cdaf/%
Hdaf/%
Ndaf/%
(S + O)af/%
Qgr,
2.22
9.45
11.67
76.66
3.17
91.42
3.34
1.26
3.98
31.669
content is 9.45 wt.%, and volatile content is 11.67 wt.%, within the smoke coal range. The fixed carbon content is as high as 76.66 wt.%. The total sulfur content is 3.17 wt.%, in the high sulfur class coal. The total content of oxygen and sulfur is 3.98 wt.%, and the nitrogen content is 1.26 wt.%. From the functional group composition analysis, it can be inferred that the carbon may be present in the form of elemental substance or its iso-form of the same element. The heating value of the coal is 32.669 MJ/kg, which is high heating value coal, and well-correlated with its low ash content [25]. The sulfur status is further studied and the results are shown in Table 2. Among the 3.17 wt.% of sulfur in the coal sample, sulfate S content is 0.12 wt.%, which accounts for 3.78% of the total sulfur, organic sulfur is 0.67%, which is bond to the carbon, and may be in the form of volatile or fixed carbon compounds. The sulfide S is 2.38 wt.%, which is mainly inorganic sulfur existing as mineral, and one of the main ash contributions. The amount the sulfur compounds, the sulfate sulfur and sulfide sulfur are in the inorganic form, which are part of the ash after combustion. The sulfur status analysis showed that it might be possible to separate most of sulfur from the Kaitai coal though physical method, because of the density different between the inorganic minerals and the carbon materials [26–28]. The as-mined coal has a broad range of lump or particle sizes, so 100 kg of the coal sample was taken randomly and then sieved to different size range. The sample from each size range has been analysed for sulfur and ash content. The results are shown in Table 3. It is seen that the coal with size range of 25–50 mm or above account for 33.85 wt.% of the total sample, with ash content of 9.92% and sulfur 3.76%. The coal with size range of 13–25 mm is 21.66% of the total sample, its ash and sulfur contents are 9.66% and 3.28 wt.% respectively, slightly lower than those with bigger size. With the coal particle size dropping to 6–13 mm and 3–6 mm, the total amount of the coal in these two ranges drop to 15.6% and 10.06%, with ash and sulfur content decreasing too. The coal portion with the size range of 0.5–3 mm amount increase slightly to 13.4% and the ash content rises to 8.68%, although the sulfur content in this size range decreases to 2.43 wt.%. This suggests that more ash in the coal parts sized 0.5–3 mm may come from non-sulfide minerals, which may be kaolinite or quartz. The fine coal powder (b 0.5 mm) accounts for 5.42% of the Kaitai coal, however it has more or less the same level of ash content and sulfur level with the coal with size range of 13–25 mm. From the sulfur column in Table 3, it can be seen that the overall trend of sulfur decrease with the coal size range, which suggests that the sulfur may be mainly embedded or contained in the big coal lumps. The Kaitai coal with different sieved sizes are analysed using XRD, and the results are shown in Fig. 2. The broad peak from 2 theta of 22– 28o is assigned to the crystalline or semi-crystalline carbon, which suggesting that the coal is mature with high fixed carbon. The sharp diffraction peaks may be due to the minerals presenting in the coal sample. The main inorganic impurities existing in the coal samples include pyrite, quartz, kaolinite, mullite and calcite. Generally Table 2 Sulfur status in natural sized Kaitai coal. Sulfur status
Sulfur content %
Proportion/%
Organic sulfur Sulfate sulfur Sulfide sulfur Total sulfur content
0.67 0.12 2.38 3.17
21.14 3.78 75.08 100
ad/%
MJ/kg
speaking, the higher content of minerals would give stronger intensity. It is seen that the pyrite diffraction peak intensities increase with the size of the coal sample, suggesting that the pyrite is mainly present in the bigger coal particles, the quartz and kaolinite diffraction peak almost unchanged among all the coal sample with different sizes, suggesting that the kaolinite and quartz are evenly distributed in the coal samples. The float–sink analysis results of the coal sample size ranging from 50 to 25 mm are given in Table 4. Generally the coal with less mineral but higher carbon has lower density, so easy to float with lower density media. The portion with density lower than 1.4 accounts for 21.65% in this size range coal, and only 7.27% in the whole sample. As expected, the sulfur content in the sample is much less, only 0.86%, which can be considered mainly from the organic sulfur. The floated portion from the media of 1.4–1.5 is the majority of the 50–25 mm sized coal sample, accounting for 56.63%, its ash content increases to 8.5%, and sulfur increases more sharply to 2.14 wt.%, the accumulated float yield increases to 78.28%, and the overall sulfur in the samples at density lower than 1.6 is 1.79%. The accumulated sink sample yield is 78.35%, with total sulfur 4.53 wt.%. When the float solution density increases to 1.5–1.6, the floated sample in this density range is about 13.55% of this sized sample, the ash content increases to 13.79%, and sulfur increases to 6.51 wt.% in this density range. The accumulated yield of the float sample, mainly coal and light minerals increase at density lower than 1.6 is 91.84%, and the total sulfur of the floated sample is 2.48%, the sink portion accumulated sulfur is 10.75%. The portion with density range of 1.6–1.7 accounts for 3.49 wt.% of the 50– 25 mm sample, the ash content increase to 18.76%, while sulfur to 11.78 wt.%. With the density increase to 1.7–1.8, the coal portion with this density range account for only 1.97% of the N 25 mm sized coal, ash increases to 23.67%, while sulfur content in this density range sample is 14.61%. The total accumulated float yield of the coal is 97.29%, the accumulated sink portion, e.g., the sample with density higher than 1.8 is yielded at 4.67%, while the sulfur content is 22.26%. For the materials contained in the coal with density higher than 1.8, which can be considered as all mineral materials, especially the pyrite, as detected by XRD, accounts for 2.71 wt.% of the N 25 mm sized coal sample, the ash content in this portion of sample is 34.03%, and sulfur is 27.82 wt.%, which exist in the form of FeS2 (S/Fe = 1.14). Table 5 summarizes the float–sink experimental results of the Kaitai coal portion with size of 25–13 mm. Similar to the coal portion of 50–25 mm, the sample with density lower than 1.4 accounts for 22.54% of this ranged coal, with ash content of 4.35%, and sulfur of 0.59. The coal content with density of1.4–1.5 accounts for nearly 60% of this sized range of coal sample, with ash increasing to 7.97%, and sulfur to 1.58%. The accumulated coal yield with density lower than 1.5 is 81.89%, in which sulfur content is 1.31%. The coal with density
Table 3 Sieving results of Kaitai natural sized coal. Size range
Content in the overall sample %
Ad/%
Sulfur %
50–25 25–13 13–6 6–3 3–0.5 0.5–0 Summary
33.85 21.66 15.6 10.06 13.41 5.42 100
9.92 9.66 8.33 7.74 8.68 13.85 9.44
3.76 3.28 2.73 2.46 2.43 2.35 3.11
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Table 5 Float–sink test of the 25–13 mm coal sample sieved from KAITAI natural sized coal. Density
b 1.4 1.40–1.5
Fig. 2. XRD patterns the sieved samples of Kaitai natural coal.
range of 1.5–1.6 is 10.7 wt.% of this sized range coal, but its ash increasing to 14.53% and sulfur to 6.78%. The coal portion in the 25– 13 mm size range with density of 1.7–1.7 is only 3.33% of the sized range, although the ash and sulfur content increasing significantly. The coal content in the 25 mm–13 mm size decreases with the density increasing, suggesting less coal but more minerals such as pyrite is sunk and separated. Using 1.8 solution for the floatation, 3.14% of the sample is separated from the 13–25 mm sized coal, its sulfur content is 29.38%, and the ash content is 42.59%. The coal sludge in this sample is only 0.91%. These results showed that heavy minerals such as pyrite is present in the coal sample with density above 1.4, and increase with the density., while other impurities such as quartz is present in all the samples. The float–sink test results of the 13–6 mm sized Kaitai coal sample are shown in Table 6. The coal with density lower than 1.4 account for 31.23% of this sized range, while the sample in the density range of 1.4–1.5 account for 56.12%. The ash content increases from 3.85% to 7.4% and sulfur from 0.77 wt.% to 1.55%. The coal with density of 1.5– 1.6 only accounts for 6.58 wt.% in the 13–6 mm sized range samples, but the ash content nearly double the one with the density of 1.4–1.5, and sulfur increases by nearly 4 times to 6.81 wt.%. The accumulated yield of the coal with density lower than 1.6 is 93.52% with sulfur about 1.66%, suggesting that the recovery ratio of coal can be more than 93.92% with sulfur less than 2 wt.% in this sized range sample. Only 2.26 wt.% of the coal in the 13–6 mm size coal sample has the density of 1.6–1.7, whose ash content is 19.47%, and sulfur is 11.35%. Only 0.98% of this size range sample density of 1.7–1.8, with ash content of 24% and total sulfur of 13.75%. The accumulated yield is
Table 4 Float–sink test of the 50–25 mm coal sample sieved from KAITAI natural sized coal. Density range
Overall Ash Ratio in this content % % size sample
b 1.4 21.65 1.40–1.50 56.63 1.50–1.60 13.55 1.60–1.70 3.49 1.70–1.80 1.97 N 1.8 2.71 Sub-sum 100 Coal sludge 0.81 Summary 100
7.27 19.02 4.55 1.17 0.66 0.91 33.58 0.28 33.85
Total Accumulated sulfur float % Yield Total % sulfur %
4.7 0.86 8.5 2.14 13.79 6.51 18.61 11.78 23.67 14.61 34.03 27.82 9.74 3.73 24.06 2.04 9.85 3.72
21.65 78.28 91.84 95.33 97.29 100
0.86 1.79 2.48 2.82 3.06 3.73
Accumulated sink Yield %
Total sulfur %
100 78.35 21.72 8.16 4.67 2.71
3.73 4.53 10.75 17.78 22.26 27.82
Coal yield in this sizerange %
Ash Yield in % the overall sample %
22.54 59.35
4.84 12.74
1.50–1.6 10.7 1.6–1.70 3.33 1.70–1.8 0.94 N 1.8 3.14 Sub-total 100 Coal sludge 0.91 Total 100
2.3 0.72 0.2 0.67 21.46 0.2 21.66
4.35 7.97
Total S%
0.59 1.58
Float yield %
Sulfur Sink in float yield % %
22.54 0.59 81.89 1.31
14.53 6.78 92.59 19.66 11.68 95.92 24.92 14.61 96.86 42.59 29.38 100 9.49 3.24 27.88 2.43 9.66 3.24
1.94 2.28 2.4 3.24
100 77.4
Total sulfur in sink % 3.24 4.02
18.1 12 7.41 19.54 4.08 25.97 3.14 29.38
about 97.17% in this size range, with sulfur about 2.01%. The substance with density more than 1.8 is 2.83%, and ash content of 40.58%, sulfur 25.8%, and overall sulfur of 2.68%. This suggests that Kaitai coal with size range of 13–6 mm has overall relatively low sulfur content. The coal sludge content is less than 1 wt.% in this coal size range sample, with high ash content and relatively low sulfur content. As shown in Table 3, the coal sample with size range of 6–3 mm is about 10.06 wt.% of the total Kaitai coal. The float–sink test of this sized sample showed that the coal with density lower than 1.4 is about 52.99 wt.%, which has ash content of 3.93% and sulfur of 0.84% (Table 7). The portion with density of 1.4–1.5 is about 37.01 wt.% in this sized sample, and ash content increases to 8.24%, sulfur to 1.86 wt.%. The total yield is about 90%, with sulfur as low as 1.26. The coal with density range of 1.5–1.6 in this sized sample is about 5.08 wt.% of the sample, and ash content increases to 14.69 wt.% and sulfur to 6.9 wt.%. The total yield with density lower than 1.6 is 95.08 wt.%, and sulfur less than 1.56 wt.%. This suggests that the coal sample with smaller size range can give high coal yield with less sulfur containing, which can meet the fuel regulations. The substance with density of 1.6–1.7 in this sized coal sample accounts only 1.68 wt.%, with ash content of 20.64% and sulfur 11.74%. The total coal yield with density lower than 1.7 increases slightly to 96.85%, with sulfur content reducing to about 1.75%. The substance with density of 1.7–1.8 in this sized range coal has the smallest portion, only 0.71% with ash content of 26.38% and sulfur 14.19 wt.%. The materials with density higher than 1.8 in this sized range is about 2.44%, which is in fact most pyrite, has ash content of 44.61% and sulfur 25.64%. This sample has slight more coal sludge, about 1.3 wt.%. The Kaitai coal with the size of 3–0.5 mm range only accounts for 12.76% of the total coal. The float–sink test results of this sized range coal are shown in Table 8. The component with density lower than 1.4 accounts for 47.3% of this sized range coal, which is significantly
Table 6 Float–sink test of the 13–6 mm coal sample sieved from KAITAI natural sized coal. Density range
Float Toal Ash Content To content sulfur total in this Yield % sample % size % % ratio %
b 1.4 31.23 1.40–1.50 56.12 1.50–1.60 6.58 1.60–1.70 2.26 1.70–1.80 0.98 N 1.8 2.83 Sub-total 100 Coal sludge 1 Summary 100
4.8 8.62 1.01 0.35 0.15 0.43 15.36 0.24 15.6
3.85 7.4 14.71 19.47 24 40.58 8.15 31 8.37
0.77 1.55 6.81 11.35 13.75 25.8 2.68 2.39 2.68
31.23 87.35 93.92 96.19 97.17 100
Sink Sulfur Yield % %
Sulfur %
0.77 1.27 1.66 1.89 2.01 2.68
2.68 3.55 12.41 18.47 22.69 25.8
100 68.77 12.65 6.08 3.81 2.83
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Table 7 Float–sink test of the 6–3 mm coal sample sieved from KAITAI natural sized coal. Density range
Content In total Ash sample % in this % sized sample %
b 1.4 52.99 1.40–1.50 37.01 1.50–1.60 5.08 1.60–1.70 1.78 1.70–1.80 0.71 N 1.8 2.44 Sub-total 100 Coal sludge 1.3 Summary 100
5.24 3.66 0.5 0.18 0.07 0.24 9.89 0.17 10.06
Total Float sulfur Yield % %
3.93 0.84 8.24 1.86 14.69 6.9 20.64 11.74 26.38 14.19 44.61 25.64 7.52 2.42 26.14 2.98 7.76 2.43
52.99 90 95.08 96.85 97.56 100
Sink Total S Yield % %
Total S %
0.84 1.26 1.56 1.75 1.84 2.42
2.42 4.2 12.84 18.97 23.05 25.64
100 47.01 10 4.92 3.15 2.44
higher the coal portion with bigger particle size. The ash content is also low, only 2.69%, and sulfur content is 0.7 wt.%. The coal portion in this size range with density at 1.4–.1.5 accounts for 40.68% of the coal in this size. The ash content increases sharply to 6.83%, and sulfur to 1.36% with the density increase. The portion with density range of 1.6–1.7 is only 5.29% of this sized coal, but ash content further increase, so does the sulfur. There is much less coal with the density of 1.7–1.8, which is only 1.07% of the 3–0.5 mm size range coal. The accumulated yield of the coal sample is 95.86% with total sulfur of 1.6%. The sunk portion yield is 5.39%, with sulfur content of 19.19%. Interestingly, this sized range coal also contain the substance with density higher than 1.4, which accounts for 4.32% of the coal in this size range, its ash content is up to 57.2%, while sulfur is 20.55%. The increase of sulfur is less accelerated than the ash compared to the coal sample with bigger size range such as 50–25 mm (Table 4). This suggests that there may be more other heavy minerals existing in the smaller sized coal samples. The coal sludge content in the 3–0.5 mm sized coal is about 4.83% of the coal sample, which is higher than the coal with bigger sizes, it might result from that some fine coal powder may be removed from the floatation. The summary of the whole coal float–sink results is shown in Table 8. It is shown that the portion in the portion with density less than 1.4 accounts for 28.23% of the total coal, its sulfur content is 0.76%, and ash content about 3.94%. The sink part using 1.4 media to float has yield of 100%, and its sulfur content is 3.13, δ is 83.2%. About 49.35% of Kaitai coal has density of 1.4–1.5, with sulfur content of 1.79%, and ash content of 7.99%. The total coal yield with density lower than 1.5 is 83.2%. The portion of the Kaitai coal with the density of 1.5– 1.6 decrease to 9.04% of the total coal sample, with sulfur increasing rapidly to 6.65%, and ash content to 14.1%. The accumulated coal yield is 92.91% with average sulfur content of 1.96%, lower than the national fuel limit. The δ value decreases to 12.48%. The substance with density
of 1.6–1.7 in the Kaitai is 2.58% of the total sample, and 1.22% for that with density of 1.7–1.8. Generally the portion in the Kaitai coal sample decrease with the density lowering, its sulfur content and ash contents increase. The portion with density higher than 1.8 accounts for 2.81%, with sulfur of 26.27%, it has ash content of 42.6%, which can be assigned most to pyrite. The coal sludge generated is about 1.41%, containing sulfur of 2.31%, and ash content of 26.9%, which may come from non-pyrite mineral. The changes of sulfur to ash content ratio with the density and size of the Kaitai coal is shown in Fig. 3. For the coal sample with size of 25–50 mm, 13–25 mm, 13–6 mm and 6–3 mm ranges, the sulfur/ash ratio increase with the floatation media density, suggesting that the pyrite content in these coal sample increase with the size, which may be embedded in the coal lump. The sulfur to ash ratio increase more sharply in the 50–25 mm sized coal range, suggesting it has more sulfur content. However, according to the coal utilization in China, the coal with bigger size may not be used for fuel, but other applications, which therefore does not require the sulfur to be lower than 2%. Therefore, it is suggested the coal with bigger lump size should be mechanically separated and not sold as fuel as to maximize its value. The other fragile minerals may not be enriched in the big coal lumps. For the coal with size range of 0.5–3 mm, the sulfur to ash ratio has maximum value for the density range of 1.5–18, the ones with density of 1.4–1.5 has the lowest sulfur/ash ratio, suggesting that these coal may have different form with those in the big lump coal. The washability study results of Kaitai coal sample using float–sink analysis method is summarised in Fig. 4. It is seen that if more than 94.5% of coal can be floated and recovered when its total sulfur is decreased to 2.12% from 3.17 wt.% in the raw coal. This means that if the required sulfur content is about 2.12%, the coal can be considered as easily-floated. If the sulfur content is required to decrease to 1.2 wt. %, only 76.71 wt.% of coal can be recovered through float–sink method, which is therefore considered as the difficultly floated. When the sulfur in the floated coal is required to be about 1.5 wt.%, the overall coal yield is 85.39%, and δ ± 0.1 is 20.55, which is within 20.1– 30. Hence the coal with such required coal level is relatively difficult float. Comparing the float–sink results of the Kaitai coal with the literature results [2,29,30], it is seen that the float–sink condition in this paper is proper, the limited capability to remove sulfur to less than 2% through the float–sink is due to the organic sulfur, while these ash and inorganic sulfur can be mostly removed through ZnCl2
Table 8 Float–sink test of the 3–0.5 mm coal sample sieved from KAITAI natural sized coal. Density
Content in the sized coal
b 1.4 47.3 1.40–1.50 40.68 1.50–1.60 5.29 1.60–1.70 1.33 1.70–1.80 1.07 N 1.8 4.32 Sub-total 100 Coal sludge 4.83 Summary 100
In the Ash overall % sample % 6.04 5.19 0.67 0.17 0.14 0.55 12.76 0.65 13.41
Sulfur Float Sink % Accumulated Total Yield yield % S% %
2.69 0.7 6.83 1.36 13.3 6.76 19.5 10.86 24.2 13.7 57.2 20.55 7.75 2.42 26.9 2.23 8.67 2.41
47.3 87.99 93.27 94.61 95.68 100
0.7 1.01 1.33 1.47 1.6 2.42
100 52.7 12.01 6.73 5.39 4.32
Total S% 2.42 3.97 12.8 17.54 19.19 20.55 Fig. 3. The sulfur to ash ratio in the coal sample separated from different sizes and densities.
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Fig. 5. XRD patterns of the float–sink samples products of Kaitai Ground lump coal.
Fig. 4. Float–sink curves of the Kaitai natural sized coal samples.
solution float–sink, and the yield to low sulfur coal is relatively high (Table 9). The XRD patterns of the Kaitai coal separated through float–sink are shown in Fig. 5. It is seen that the lowest density (1.4) coal portion has strong peak at 2 theta at 12.4, 24–26, and one at 43.5o, which can be assigned to the graphite coal or semi-crystalline carbon in the coal samples. The weak peak at about 20 and 26.63o can be assigned to the low grade quartz, suggesting that quartz is present in the coal with low density. With the density increasing, these peak intensities due to the graphite or crystalline carbon become weaker, while other narrow and sharp diffraction peaks due to pyrite, kaolinite, dolomite, and pyrite gradually increase [31–33], in the sample with density from 1.6 to 1.8, the peak due to carbon materials almost disappears, but the pyrite and kaolinite increases. These results are in agreement with the floated sample results.
seen that Kaitai coal has high carbon content, very high heating value, relatively low ash content and medium to higher sulfur. The total content of sulphate and organic sulfur in Kaitai coal is 24.92%, which makes it relatively difficulty to reduce the sulfur content through float–sink method. ZnCl2 solution with various densities has been applied to the float– sink of the Kaitai coal with various sizes, the coal with bigger size contains more pyrite at higher density. However, the overall desulfurisation efficiency in the bigger coal lump sizes tends to be lower due to the higher sulfur content. The float–sink method can recover more than 94 wt.% of coal with sulfur around 2.12%, in this case, it can be considered as easily-floated coal. Further reducing sulfur to 1.2 wt.% only gives coal yield of 76.71 wt.% in the float–sink process, which is therefore considered as the difficult floated. When the sulfur in the floated coal is required to be about 1.5 wt.%, the overall coal yield is 85.39%, and δ ± 0.1 is 20.55%, which is within 20.1–30%. Hence the coal with such required coal level is relatively difficult float. Acknowledgements
4. Conclusions Kaitai coal in Puan County, Guizhou Province, Southwest China has been sampled and sieved into different sized grades and analysed. It is
This work is financially supported by the Ministry of Science Technology of China through IC-UK program. We would like to thank General Secretary Long of Guizhou University for her support.
Table 9 The float–sink results of the KAITAI natural sized coal overall samples ranging from 50 to 0.5 mm. Density grade
b 1.4 1.40–1.50 1.50–1.60 1.60–1.70 1.70–1.80 N 1.8 Sub-total Coal sludge Total
To the total sample amount
Sulfur %
28.23 49.28 9.04 2.58 1.22 2.81 93.17 1.41 94.58
0.76 1.79 6.65 11.63 14.38 26.27 3.12 2.31 3.11
Ash %
3.94 7.99 14.1 19.2 24.1 42.6 8.92 26.9 9.19
Float
Sink
Yield %
S%
Yield %
Sulfur %
30.3 83.2 92.91 95.68 96.99 100
0.76 1.41 1.96 2.24 2.41 3.13
100 69.7 16.8 7.1 4.33 3.02
3.13 4.15 11.6 18.36 22.67 26.27
Float density δP
δ ± 0.1 content
1.4 1.5 1.6 1.7 1.8
83.2 62.61 12.48 4.08 4.33
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Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Study on the washability of the Kaitai coal, Guizhou Province China Qin Zhang a, Yingzhong Tian a, Yueqin Qiu a, Jianxin Cao a, Tiancun Xiao a,b,⁎ a b
CSCSTUK Clean Energy Centre, School of Mines, Guizhou University, Caijiaguan Campus, Guiyang City, 550003, Guizhou, China Inorganic Chemistry Laboratory, Oxford University, South Parks Road, OX1 3QR United Kingdom
a r t i c l e
i n f o
Article history: Received 5 July 2010 Received in revised form 28 November 2010 Accepted 29 November 2010 Available online 28 December 2010 Keywords: Float–sink Kaitai coal Different sizes Desulfurisation
a b s t r a c t Tonnage of coal samples were collected from Kaitai Coal field, Puan County, Guizhou Province and sieved into different particle size catalogs. The analysis of the overall coal suggested that the coal has low ash but high sulfur content of 3.17 wt.% with medium to high volatile content. The heating value of the coal is 31.668 mJ/ kg. The coal sample with different particle size ranges were tested for float–sink using gravity separation method, in which ZnCl2 solutions with different density are used. It is showed that decreasing sulfur to 2.12 wt.% can give coal yield of 94.55%, suggesting that the coal's floatability is good with sulfur. The coal yield is only 85.4% when reducing sulfur to 1.5 wt.%, and 76.7% when sulfur is decreased to 1.2% through the ZnCl2 float–sink process. The δ ± 0.1 is 20.55, which is in the 20.1–30 range, suggest that Kaitai coal is difficult to float–sink for depyritisation. Characterization of the floated coal at different sizes showed that the organic sulfur may mainly be present in the small size, the pyrite sulfur is mainly present in the coal with bigger particle size, which can be easily removed through float–sink process. The ash in the small particle sized coal is mainly from kaolinite and quartz, while the pyrite is the main ash contributor to the coal with big size. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Coal is the major primary energy source in China and is forecast to account for over 60% of the primary energy consumption mix, and the total coal demand will reach 2.3–2.9 billion tons in 2020 [1]. Sulfur and mineral matter contents of a coal are important factors, deciding the coal properties and utilisation, because direct utilization of high ash and highsulfur content coals leads to serious technological and environmental problems. With the many year mining and consumption of coal, the coal reserve with high quality, e.g., low sulfur and less ash is depleting. Carbon is the main component in coal, which also contains various hetero atoms such as sulfur, nitrogen, chlorine, minerals such as silica, kaolionite, titania, quartz and various trace metals such as Mo, W, alkali metal and toxic elements such as Hg, As. Because of their high content, mineral matter and sulfur exhibit harmful effects on the utilization of coal for purposes of combustion, carbonization, gasification, liquefaction, etc. These impurities poison the catalyst that is used in coal conversion processes. SO2 produced during combustion leads to atmospheric pollution, acid rain, and the corrosion of boilers, pipelines, and other machinery. Therefore, it is necessary to remove mineral matter and sulfur from coal prior to utilization, the demineralization and/or desulfurization of low-grade high-ash and/or high-sulfur coals to obtain environmentally acceptable clean fuels has attracted greater attention [1–6]. ⁎ Corresponding author. Inorganic Chemistry Laboratory, Oxford University, South Parks Road, OX1 3QR, United Kingdom. Tel.: +44 1865272660. E-mail address: [email protected] (T. Xiao). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.030
Guizhou has the most coal reserve in South China, with a reserve of 241.9 billion tons of coal, it is therefore planned as the Southern base for power generation, and exporting the electricity to the Eastern China. However, the coal in Guizhou rank ranges widely from high volatile bituminous to low volatile bituminous and anthracite. The sulfur content is from 1 wt.% to 5 wt.%. The Major mineral phases present in the Liupanshui coal are kaolinite, quartz, pyrite, and calcite. Traces of other primary minerals are marcasite, gypsum, and dolomite. The Shuicheng coal usually has higher kaolinite and quartz contents than the Liuzhi coal. Marcasite occurs indiscriminately in the different coal seams without a clear distribution pattern. The presence of other minerals, such as rutile, anatase, tourmaline, zircon, and phosphates, was also noted. The total sulfur content of Liupanshui coals is higher in the marineinfluenced coal seams (up to 7.5% dry), and lower in the non-marineinfluenced coals (as low as 0.3%). In the Liuzhi coal field, the coal is characterized by a high sulfur and iron content, whereas in the Shuicheng coal field, contents varied from low to high, depending on the coal seams [7–9]. However, generally speaking, with increasing consumption, the coal quality deteriorates, especially sulfur content increases in all the coal field, which is beyond the increasing strict national environmental regulation limit. To meet the market and environmental regulations, there have been lots of studies on coal desulfurisation and de-ashing [2,10–24], among which float–sink has been widely used to remove minerals and inorganic sulfur, while organic sulfur is removed through pyrolysis or microbiological floatation.
Q. Zhang et al. / Fuel Processing Technology 92 (2011) 692–698
The most efficient industrial gravity-based separators used for coal cleaning employ the use of a dense-medium, which is most commonly comprised of an aqueous suspension of ultrafine magnetite. The density of the suspension is adjusted to a value that is between the densities of coal and the associated mineral matter. As such, the light coal particles float while the heavy particles sink. Although the separation performance is generally superior to water-only systems, the overall efficiency of the process declines with a decrease in particle size due to process kinetics. However, to provide the basic information for the gravity-based separator so as to adjust the aqueous suspension of the ultrafine magnetite, the coal washability is normally first studied in lab scale using coal sample of up to tonnage scale. To have a more accurate density adjustment, the lab scale washbility study is normally operated using ZnCl2 solution, whose density ccan be continuous adjusted. In this work, tonnage of coal samples were collected from Kaitai Coal field, one of Puan County coal seams, and sieved into different size range. Each sized portion has been processed using float–sink method to study the floatability of the coal samples. The samples at various stages have been characterized and some interesting results have been obtained. 2. Experimental Each time, about 1000 kg of the coal sample were randomly collected from the storage site of Kaitai Coal field, which supplied coal to two main thermal power stations located in the Southwest of Guizhou Province. The location of Puan, and Guizhou province in China is shown in Fig. 1. The samples were run-of-mine samples. Normally the coal underground was mined and dumped in a storage site, the coal samples were randomly taken from 30 different points and height of coal pile. About 40 kg coal were collected at each point and mixed as it is. The collected coal samples were screened and fractionated into size classes from −53 um to 50 mm. Selected fractions of the coal samples were analyzed regarding mineralogical and chemical composition. Also, selected fractions were used for washability analysis. 2.1. Washability study China has set out National Standard Method for Coal Float-andSink Analaysis in 1987. Thereforfe in this work. All the experimental procedures follows the GB478-1987 fo r the washability study.
693
In the float–sink experiments, the coal sample from each size grade is dried at 100 °C, then cooled down to room temperature, weighed and the particle smaller than 0.5 were washed away, fired and weighed again. The samples were placed in a barrel bottomed with different size of mesh (N 0.5 mm). They are dipped into ZnCl2 water solutions starting with lower ZnCl2 density, to higher density, step by step. The gravity solution densities were adjusted by the ZnCl2 content in the solution. The floatation solution with density (kg/L) of 1.40, 1.50, 1.60, 1.70 and 1.80 were prepared, with the pH of 3.8, 3.26, 2.66, 2.02, and 1.6. The components whose density is lower than the dipped solution would float, and sieved out by the specific sized mesh, and the sink part would be put in the heavier solutions and the floated component were meshed out. This process is repeated until the highest density media. The floated coal parts were washed until the chlorine content in the sample is less than 0.01 wt.%, dried and weighed and then analysed. 2.2. Chemical analysis The proximate analysis (moisture, volatile matter, fixed carbon, and ash) and ultimate analysis (carbon, hydrogen, nitrogen, and sulfur) of collected coal samples were done in the CSCST-UK—Guizhou University Clean Energy Lab. The proximate analyses of the coal samples were done by standard methods (IS: 1350 (Part I) 1984). The elements C, H, N and S were analysed by the PerkinElmer 2400 Series II CHNS Elemental Analyzer. The calorific values were determined using Bomb calorimeter (ASTM D 3286). The analyses of the coal samples are given in Table 1. The ash analysis of the coal samples was carried out by standard methods. All analyses for the samples were carried out in quadruplicate and mean values have been reported. The crystalline structure of the samples were determined by X-ray diffraction using a Philips X' PeRT Pro Alpha 1 diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at a tube current of 40 kV and a voltage of 40 Ma. Data were collected over 2θ values from 20° to 80°, at a scan speed of 1°/min. 3. Results and discussion The tonnage coal sample were sieved and divided into various portions, and a general sample is taken through mixing the proportional parts of each sized sample, mixed and ground into powder, and then analysed. The results of the general Kaitai coal sample are shown in Table 1. It is seen that the moisture is 2.22%, ash
Fig. 1. Location of Kaitai Coal field which lies in Puan County, Guizhou Province, China.
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Table 1 Property of Kaitai Coal with natural sized particle.
Natural particle size
Mad/%
Aad/%
Vdaf/%
FCad/%
Stad/%
Cdaf/%
Hdaf/%
Ndaf/%
(S + O)af/%
Qgr,
2.22
9.45
11.67
76.66
3.17
91.42
3.34
1.26
3.98
31.669
content is 9.45 wt.%, and volatile content is 11.67 wt.%, within the smoke coal range. The fixed carbon content is as high as 76.66 wt.%. The total sulfur content is 3.17 wt.%, in the high sulfur class coal. The total content of oxygen and sulfur is 3.98 wt.%, and the nitrogen content is 1.26 wt.%. From the functional group composition analysis, it can be inferred that the carbon may be present in the form of elemental substance or its iso-form of the same element. The heating value of the coal is 32.669 MJ/kg, which is high heating value coal, and well-correlated with its low ash content [25]. The sulfur status is further studied and the results are shown in Table 2. Among the 3.17 wt.% of sulfur in the coal sample, sulfate S content is 0.12 wt.%, which accounts for 3.78% of the total sulfur, organic sulfur is 0.67%, which is bond to the carbon, and may be in the form of volatile or fixed carbon compounds. The sulfide S is 2.38 wt.%, which is mainly inorganic sulfur existing as mineral, and one of the main ash contributions. The amount the sulfur compounds, the sulfate sulfur and sulfide sulfur are in the inorganic form, which are part of the ash after combustion. The sulfur status analysis showed that it might be possible to separate most of sulfur from the Kaitai coal though physical method, because of the density different between the inorganic minerals and the carbon materials [26–28]. The as-mined coal has a broad range of lump or particle sizes, so 100 kg of the coal sample was taken randomly and then sieved to different size range. The sample from each size range has been analysed for sulfur and ash content. The results are shown in Table 3. It is seen that the coal with size range of 25–50 mm or above account for 33.85 wt.% of the total sample, with ash content of 9.92% and sulfur 3.76%. The coal with size range of 13–25 mm is 21.66% of the total sample, its ash and sulfur contents are 9.66% and 3.28 wt.% respectively, slightly lower than those with bigger size. With the coal particle size dropping to 6–13 mm and 3–6 mm, the total amount of the coal in these two ranges drop to 15.6% and 10.06%, with ash and sulfur content decreasing too. The coal portion with the size range of 0.5–3 mm amount increase slightly to 13.4% and the ash content rises to 8.68%, although the sulfur content in this size range decreases to 2.43 wt.%. This suggests that more ash in the coal parts sized 0.5–3 mm may come from non-sulfide minerals, which may be kaolinite or quartz. The fine coal powder (b 0.5 mm) accounts for 5.42% of the Kaitai coal, however it has more or less the same level of ash content and sulfur level with the coal with size range of 13–25 mm. From the sulfur column in Table 3, it can be seen that the overall trend of sulfur decrease with the coal size range, which suggests that the sulfur may be mainly embedded or contained in the big coal lumps. The Kaitai coal with different sieved sizes are analysed using XRD, and the results are shown in Fig. 2. The broad peak from 2 theta of 22– 28o is assigned to the crystalline or semi-crystalline carbon, which suggesting that the coal is mature with high fixed carbon. The sharp diffraction peaks may be due to the minerals presenting in the coal sample. The main inorganic impurities existing in the coal samples include pyrite, quartz, kaolinite, mullite and calcite. Generally Table 2 Sulfur status in natural sized Kaitai coal. Sulfur status
Sulfur content %
Proportion/%
Organic sulfur Sulfate sulfur Sulfide sulfur Total sulfur content
0.67 0.12 2.38 3.17
21.14 3.78 75.08 100
ad/%
MJ/kg
speaking, the higher content of minerals would give stronger intensity. It is seen that the pyrite diffraction peak intensities increase with the size of the coal sample, suggesting that the pyrite is mainly present in the bigger coal particles, the quartz and kaolinite diffraction peak almost unchanged among all the coal sample with different sizes, suggesting that the kaolinite and quartz are evenly distributed in the coal samples. The float–sink analysis results of the coal sample size ranging from 50 to 25 mm are given in Table 4. Generally the coal with less mineral but higher carbon has lower density, so easy to float with lower density media. The portion with density lower than 1.4 accounts for 21.65% in this size range coal, and only 7.27% in the whole sample. As expected, the sulfur content in the sample is much less, only 0.86%, which can be considered mainly from the organic sulfur. The floated portion from the media of 1.4–1.5 is the majority of the 50–25 mm sized coal sample, accounting for 56.63%, its ash content increases to 8.5%, and sulfur increases more sharply to 2.14 wt.%, the accumulated float yield increases to 78.28%, and the overall sulfur in the samples at density lower than 1.6 is 1.79%. The accumulated sink sample yield is 78.35%, with total sulfur 4.53 wt.%. When the float solution density increases to 1.5–1.6, the floated sample in this density range is about 13.55% of this sized sample, the ash content increases to 13.79%, and sulfur increases to 6.51 wt.% in this density range. The accumulated yield of the float sample, mainly coal and light minerals increase at density lower than 1.6 is 91.84%, and the total sulfur of the floated sample is 2.48%, the sink portion accumulated sulfur is 10.75%. The portion with density range of 1.6–1.7 accounts for 3.49 wt.% of the 50– 25 mm sample, the ash content increase to 18.76%, while sulfur to 11.78 wt.%. With the density increase to 1.7–1.8, the coal portion with this density range account for only 1.97% of the N 25 mm sized coal, ash increases to 23.67%, while sulfur content in this density range sample is 14.61%. The total accumulated float yield of the coal is 97.29%, the accumulated sink portion, e.g., the sample with density higher than 1.8 is yielded at 4.67%, while the sulfur content is 22.26%. For the materials contained in the coal with density higher than 1.8, which can be considered as all mineral materials, especially the pyrite, as detected by XRD, accounts for 2.71 wt.% of the N 25 mm sized coal sample, the ash content in this portion of sample is 34.03%, and sulfur is 27.82 wt.%, which exist in the form of FeS2 (S/Fe = 1.14). Table 5 summarizes the float–sink experimental results of the Kaitai coal portion with size of 25–13 mm. Similar to the coal portion of 50–25 mm, the sample with density lower than 1.4 accounts for 22.54% of this ranged coal, with ash content of 4.35%, and sulfur of 0.59. The coal content with density of1.4–1.5 accounts for nearly 60% of this sized range of coal sample, with ash increasing to 7.97%, and sulfur to 1.58%. The accumulated coal yield with density lower than 1.5 is 81.89%, in which sulfur content is 1.31%. The coal with density
Table 3 Sieving results of Kaitai natural sized coal. Size range
Content in the overall sample %
Ad/%
Sulfur %
50–25 25–13 13–6 6–3 3–0.5 0.5–0 Summary
33.85 21.66 15.6 10.06 13.41 5.42 100
9.92 9.66 8.33 7.74 8.68 13.85 9.44
3.76 3.28 2.73 2.46 2.43 2.35 3.11
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Table 5 Float–sink test of the 25–13 mm coal sample sieved from KAITAI natural sized coal. Density
b 1.4 1.40–1.5
Fig. 2. XRD patterns the sieved samples of Kaitai natural coal.
range of 1.5–1.6 is 10.7 wt.% of this sized range coal, but its ash increasing to 14.53% and sulfur to 6.78%. The coal portion in the 25– 13 mm size range with density of 1.7–1.7 is only 3.33% of the sized range, although the ash and sulfur content increasing significantly. The coal content in the 25 mm–13 mm size decreases with the density increasing, suggesting less coal but more minerals such as pyrite is sunk and separated. Using 1.8 solution for the floatation, 3.14% of the sample is separated from the 13–25 mm sized coal, its sulfur content is 29.38%, and the ash content is 42.59%. The coal sludge in this sample is only 0.91%. These results showed that heavy minerals such as pyrite is present in the coal sample with density above 1.4, and increase with the density., while other impurities such as quartz is present in all the samples. The float–sink test results of the 13–6 mm sized Kaitai coal sample are shown in Table 6. The coal with density lower than 1.4 account for 31.23% of this sized range, while the sample in the density range of 1.4–1.5 account for 56.12%. The ash content increases from 3.85% to 7.4% and sulfur from 0.77 wt.% to 1.55%. The coal with density of 1.5– 1.6 only accounts for 6.58 wt.% in the 13–6 mm sized range samples, but the ash content nearly double the one with the density of 1.4–1.5, and sulfur increases by nearly 4 times to 6.81 wt.%. The accumulated yield of the coal with density lower than 1.6 is 93.52% with sulfur about 1.66%, suggesting that the recovery ratio of coal can be more than 93.92% with sulfur less than 2 wt.% in this sized range sample. Only 2.26 wt.% of the coal in the 13–6 mm size coal sample has the density of 1.6–1.7, whose ash content is 19.47%, and sulfur is 11.35%. Only 0.98% of this size range sample density of 1.7–1.8, with ash content of 24% and total sulfur of 13.75%. The accumulated yield is
Table 4 Float–sink test of the 50–25 mm coal sample sieved from KAITAI natural sized coal. Density range
Overall Ash Ratio in this content % % size sample
b 1.4 21.65 1.40–1.50 56.63 1.50–1.60 13.55 1.60–1.70 3.49 1.70–1.80 1.97 N 1.8 2.71 Sub-sum 100 Coal sludge 0.81 Summary 100
7.27 19.02 4.55 1.17 0.66 0.91 33.58 0.28 33.85
Total Accumulated sulfur float % Yield Total % sulfur %
4.7 0.86 8.5 2.14 13.79 6.51 18.61 11.78 23.67 14.61 34.03 27.82 9.74 3.73 24.06 2.04 9.85 3.72
21.65 78.28 91.84 95.33 97.29 100
0.86 1.79 2.48 2.82 3.06 3.73
Accumulated sink Yield %
Total sulfur %
100 78.35 21.72 8.16 4.67 2.71
3.73 4.53 10.75 17.78 22.26 27.82
Coal yield in this sizerange %
Ash Yield in % the overall sample %
22.54 59.35
4.84 12.74
1.50–1.6 10.7 1.6–1.70 3.33 1.70–1.8 0.94 N 1.8 3.14 Sub-total 100 Coal sludge 0.91 Total 100
2.3 0.72 0.2 0.67 21.46 0.2 21.66
4.35 7.97
Total S%
0.59 1.58
Float yield %
Sulfur Sink in float yield % %
22.54 0.59 81.89 1.31
14.53 6.78 92.59 19.66 11.68 95.92 24.92 14.61 96.86 42.59 29.38 100 9.49 3.24 27.88 2.43 9.66 3.24
1.94 2.28 2.4 3.24
100 77.4
Total sulfur in sink % 3.24 4.02
18.1 12 7.41 19.54 4.08 25.97 3.14 29.38
about 97.17% in this size range, with sulfur about 2.01%. The substance with density more than 1.8 is 2.83%, and ash content of 40.58%, sulfur 25.8%, and overall sulfur of 2.68%. This suggests that Kaitai coal with size range of 13–6 mm has overall relatively low sulfur content. The coal sludge content is less than 1 wt.% in this coal size range sample, with high ash content and relatively low sulfur content. As shown in Table 3, the coal sample with size range of 6–3 mm is about 10.06 wt.% of the total Kaitai coal. The float–sink test of this sized sample showed that the coal with density lower than 1.4 is about 52.99 wt.%, which has ash content of 3.93% and sulfur of 0.84% (Table 7). The portion with density of 1.4–1.5 is about 37.01 wt.% in this sized sample, and ash content increases to 8.24%, sulfur to 1.86 wt.%. The total yield is about 90%, with sulfur as low as 1.26. The coal with density range of 1.5–1.6 in this sized sample is about 5.08 wt.% of the sample, and ash content increases to 14.69 wt.% and sulfur to 6.9 wt.%. The total yield with density lower than 1.6 is 95.08 wt.%, and sulfur less than 1.56 wt.%. This suggests that the coal sample with smaller size range can give high coal yield with less sulfur containing, which can meet the fuel regulations. The substance with density of 1.6–1.7 in this sized coal sample accounts only 1.68 wt.%, with ash content of 20.64% and sulfur 11.74%. The total coal yield with density lower than 1.7 increases slightly to 96.85%, with sulfur content reducing to about 1.75%. The substance with density of 1.7–1.8 in this sized range coal has the smallest portion, only 0.71% with ash content of 26.38% and sulfur 14.19 wt.%. The materials with density higher than 1.8 in this sized range is about 2.44%, which is in fact most pyrite, has ash content of 44.61% and sulfur 25.64%. This sample has slight more coal sludge, about 1.3 wt.%. The Kaitai coal with the size of 3–0.5 mm range only accounts for 12.76% of the total coal. The float–sink test results of this sized range coal are shown in Table 8. The component with density lower than 1.4 accounts for 47.3% of this sized range coal, which is significantly
Table 6 Float–sink test of the 13–6 mm coal sample sieved from KAITAI natural sized coal. Density range
Float Toal Ash Content To content sulfur total in this Yield % sample % size % % ratio %
b 1.4 31.23 1.40–1.50 56.12 1.50–1.60 6.58 1.60–1.70 2.26 1.70–1.80 0.98 N 1.8 2.83 Sub-total 100 Coal sludge 1 Summary 100
4.8 8.62 1.01 0.35 0.15 0.43 15.36 0.24 15.6
3.85 7.4 14.71 19.47 24 40.58 8.15 31 8.37
0.77 1.55 6.81 11.35 13.75 25.8 2.68 2.39 2.68
31.23 87.35 93.92 96.19 97.17 100
Sink Sulfur Yield % %
Sulfur %
0.77 1.27 1.66 1.89 2.01 2.68
2.68 3.55 12.41 18.47 22.69 25.8
100 68.77 12.65 6.08 3.81 2.83
696
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Table 7 Float–sink test of the 6–3 mm coal sample sieved from KAITAI natural sized coal. Density range
Content In total Ash sample % in this % sized sample %
b 1.4 52.99 1.40–1.50 37.01 1.50–1.60 5.08 1.60–1.70 1.78 1.70–1.80 0.71 N 1.8 2.44 Sub-total 100 Coal sludge 1.3 Summary 100
5.24 3.66 0.5 0.18 0.07 0.24 9.89 0.17 10.06
Total Float sulfur Yield % %
3.93 0.84 8.24 1.86 14.69 6.9 20.64 11.74 26.38 14.19 44.61 25.64 7.52 2.42 26.14 2.98 7.76 2.43
52.99 90 95.08 96.85 97.56 100
Sink Total S Yield % %
Total S %
0.84 1.26 1.56 1.75 1.84 2.42
2.42 4.2 12.84 18.97 23.05 25.64
100 47.01 10 4.92 3.15 2.44
higher the coal portion with bigger particle size. The ash content is also low, only 2.69%, and sulfur content is 0.7 wt.%. The coal portion in this size range with density at 1.4–.1.5 accounts for 40.68% of the coal in this size. The ash content increases sharply to 6.83%, and sulfur to 1.36% with the density increase. The portion with density range of 1.6–1.7 is only 5.29% of this sized coal, but ash content further increase, so does the sulfur. There is much less coal with the density of 1.7–1.8, which is only 1.07% of the 3–0.5 mm size range coal. The accumulated yield of the coal sample is 95.86% with total sulfur of 1.6%. The sunk portion yield is 5.39%, with sulfur content of 19.19%. Interestingly, this sized range coal also contain the substance with density higher than 1.4, which accounts for 4.32% of the coal in this size range, its ash content is up to 57.2%, while sulfur is 20.55%. The increase of sulfur is less accelerated than the ash compared to the coal sample with bigger size range such as 50–25 mm (Table 4). This suggests that there may be more other heavy minerals existing in the smaller sized coal samples. The coal sludge content in the 3–0.5 mm sized coal is about 4.83% of the coal sample, which is higher than the coal with bigger sizes, it might result from that some fine coal powder may be removed from the floatation. The summary of the whole coal float–sink results is shown in Table 8. It is shown that the portion in the portion with density less than 1.4 accounts for 28.23% of the total coal, its sulfur content is 0.76%, and ash content about 3.94%. The sink part using 1.4 media to float has yield of 100%, and its sulfur content is 3.13, δ is 83.2%. About 49.35% of Kaitai coal has density of 1.4–1.5, with sulfur content of 1.79%, and ash content of 7.99%. The total coal yield with density lower than 1.5 is 83.2%. The portion of the Kaitai coal with the density of 1.5– 1.6 decrease to 9.04% of the total coal sample, with sulfur increasing rapidly to 6.65%, and ash content to 14.1%. The accumulated coal yield is 92.91% with average sulfur content of 1.96%, lower than the national fuel limit. The δ value decreases to 12.48%. The substance with density
of 1.6–1.7 in the Kaitai is 2.58% of the total sample, and 1.22% for that with density of 1.7–1.8. Generally the portion in the Kaitai coal sample decrease with the density lowering, its sulfur content and ash contents increase. The portion with density higher than 1.8 accounts for 2.81%, with sulfur of 26.27%, it has ash content of 42.6%, which can be assigned most to pyrite. The coal sludge generated is about 1.41%, containing sulfur of 2.31%, and ash content of 26.9%, which may come from non-pyrite mineral. The changes of sulfur to ash content ratio with the density and size of the Kaitai coal is shown in Fig. 3. For the coal sample with size of 25–50 mm, 13–25 mm, 13–6 mm and 6–3 mm ranges, the sulfur/ash ratio increase with the floatation media density, suggesting that the pyrite content in these coal sample increase with the size, which may be embedded in the coal lump. The sulfur to ash ratio increase more sharply in the 50–25 mm sized coal range, suggesting it has more sulfur content. However, according to the coal utilization in China, the coal with bigger size may not be used for fuel, but other applications, which therefore does not require the sulfur to be lower than 2%. Therefore, it is suggested the coal with bigger lump size should be mechanically separated and not sold as fuel as to maximize its value. The other fragile minerals may not be enriched in the big coal lumps. For the coal with size range of 0.5–3 mm, the sulfur to ash ratio has maximum value for the density range of 1.5–18, the ones with density of 1.4–1.5 has the lowest sulfur/ash ratio, suggesting that these coal may have different form with those in the big lump coal. The washability study results of Kaitai coal sample using float–sink analysis method is summarised in Fig. 4. It is seen that if more than 94.5% of coal can be floated and recovered when its total sulfur is decreased to 2.12% from 3.17 wt.% in the raw coal. This means that if the required sulfur content is about 2.12%, the coal can be considered as easily-floated. If the sulfur content is required to decrease to 1.2 wt. %, only 76.71 wt.% of coal can be recovered through float–sink method, which is therefore considered as the difficultly floated. When the sulfur in the floated coal is required to be about 1.5 wt.%, the overall coal yield is 85.39%, and δ ± 0.1 is 20.55, which is within 20.1– 30. Hence the coal with such required coal level is relatively difficult float. Comparing the float–sink results of the Kaitai coal with the literature results [2,29,30], it is seen that the float–sink condition in this paper is proper, the limited capability to remove sulfur to less than 2% through the float–sink is due to the organic sulfur, while these ash and inorganic sulfur can be mostly removed through ZnCl2
Table 8 Float–sink test of the 3–0.5 mm coal sample sieved from KAITAI natural sized coal. Density
Content in the sized coal
b 1.4 47.3 1.40–1.50 40.68 1.50–1.60 5.29 1.60–1.70 1.33 1.70–1.80 1.07 N 1.8 4.32 Sub-total 100 Coal sludge 4.83 Summary 100
In the Ash overall % sample % 6.04 5.19 0.67 0.17 0.14 0.55 12.76 0.65 13.41
Sulfur Float Sink % Accumulated Total Yield yield % S% %
2.69 0.7 6.83 1.36 13.3 6.76 19.5 10.86 24.2 13.7 57.2 20.55 7.75 2.42 26.9 2.23 8.67 2.41
47.3 87.99 93.27 94.61 95.68 100
0.7 1.01 1.33 1.47 1.6 2.42
100 52.7 12.01 6.73 5.39 4.32
Total S% 2.42 3.97 12.8 17.54 19.19 20.55 Fig. 3. The sulfur to ash ratio in the coal sample separated from different sizes and densities.
Q. Zhang et al. / Fuel Processing Technology 92 (2011) 692–698
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Fig. 5. XRD patterns of the float–sink samples products of Kaitai Ground lump coal.
Fig. 4. Float–sink curves of the Kaitai natural sized coal samples.
solution float–sink, and the yield to low sulfur coal is relatively high (Table 9). The XRD patterns of the Kaitai coal separated through float–sink are shown in Fig. 5. It is seen that the lowest density (1.4) coal portion has strong peak at 2 theta at 12.4, 24–26, and one at 43.5o, which can be assigned to the graphite coal or semi-crystalline carbon in the coal samples. The weak peak at about 20 and 26.63o can be assigned to the low grade quartz, suggesting that quartz is present in the coal with low density. With the density increasing, these peak intensities due to the graphite or crystalline carbon become weaker, while other narrow and sharp diffraction peaks due to pyrite, kaolinite, dolomite, and pyrite gradually increase [31–33], in the sample with density from 1.6 to 1.8, the peak due to carbon materials almost disappears, but the pyrite and kaolinite increases. These results are in agreement with the floated sample results.
seen that Kaitai coal has high carbon content, very high heating value, relatively low ash content and medium to higher sulfur. The total content of sulphate and organic sulfur in Kaitai coal is 24.92%, which makes it relatively difficulty to reduce the sulfur content through float–sink method. ZnCl2 solution with various densities has been applied to the float– sink of the Kaitai coal with various sizes, the coal with bigger size contains more pyrite at higher density. However, the overall desulfurisation efficiency in the bigger coal lump sizes tends to be lower due to the higher sulfur content. The float–sink method can recover more than 94 wt.% of coal with sulfur around 2.12%, in this case, it can be considered as easily-floated coal. Further reducing sulfur to 1.2 wt.% only gives coal yield of 76.71 wt.% in the float–sink process, which is therefore considered as the difficult floated. When the sulfur in the floated coal is required to be about 1.5 wt.%, the overall coal yield is 85.39%, and δ ± 0.1 is 20.55%, which is within 20.1–30%. Hence the coal with such required coal level is relatively difficult float. Acknowledgements
4. Conclusions Kaitai coal in Puan County, Guizhou Province, Southwest China has been sampled and sieved into different sized grades and analysed. It is
This work is financially supported by the Ministry of Science Technology of China through IC-UK program. We would like to thank General Secretary Long of Guizhou University for her support.
Table 9 The float–sink results of the KAITAI natural sized coal overall samples ranging from 50 to 0.5 mm. Density grade
b 1.4 1.40–1.50 1.50–1.60 1.60–1.70 1.70–1.80 N 1.8 Sub-total Coal sludge Total
To the total sample amount
Sulfur %
28.23 49.28 9.04 2.58 1.22 2.81 93.17 1.41 94.58
0.76 1.79 6.65 11.63 14.38 26.27 3.12 2.31 3.11
Ash %
3.94 7.99 14.1 19.2 24.1 42.6 8.92 26.9 9.19
Float
Sink
Yield %
S%
Yield %
Sulfur %
30.3 83.2 92.91 95.68 96.99 100
0.76 1.41 1.96 2.24 2.41 3.13
100 69.7 16.8 7.1 4.33 3.02
3.13 4.15 11.6 18.36 22.67 26.27
Float density δP
δ ± 0.1 content
1.4 1.5 1.6 1.7 1.8
83.2 62.61 12.48 4.08 4.33
698
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