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Liberation characteristics of coal middlings comminuted by jaw crusher and ball mill
International Journal of Mining Science and Technology 23 (2013) 669–674
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Liberation characteristics of coal middlings comminuted by jaw crusher and ball mill Xie Weining a, He Yaqun a,b,⇑, Zhu Xiangnan a, Ge Linhan a, Huang Yajun a, Wang Haifeng a a b
School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 23 December 2012 Received in revised form 10 January 2013 Accepted 12 February 2013 Available online 31 August 2013 Keywords: Coal middlings Fragmentation force Liberation Float and sink test Sectional micrograph
a b s t r a c t The associated minerals make coal middlings possess a relatively high ash content. Subsequent liberation through size reduction can cause recovery increase. However, effect of comminution facilities on mineral liberation of middlings was ignored. This paper studied the liberation characteristics of middlings crushed with different kinds of fragmentation forces. Middlings of 3 mm + 0.5 mm sampled from a dense medium cyclone were comminuted by a jaw crusher and a ball mill to 0.5 mm with similar size distribution respectively. The generating mechanism of fines was also analyzed. Full densimetric analyses indicate that mineral liberation of the product crushed by the jaw crusher is better than that by the ball mill at each fraction. For sizes of 0.125 mm + 0.074 mm and 0.074 mm, yields of the product with ash content 11% comminuted by jaw crusher are nearly 20% higher than that by the ball mill. Sectional micrographs observed by the scanning electron microscopy (SEM) also show the same law for these two fractions and some intergrowth particles still exist in the fraction of 0.5 mm + 0.25 mm. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction The rapid growth of economy leads to the tremendous energy challenge that the world will face, especially for China. As a developing country with coal as the main resource, enormous energy consumption directly leads to the shortage of coal. Producers, therefore, are looking to maximize the recovery of saleable coal from raw coal to meet the huge market demand. Generally speaking, raw coal is separated into clean coal, middlings and gangue by different preparation methods. In China, middlings normally accounts for nearly 20% of rough coal and generally has an ash content of 20–40%. Without any retreatment, middlings was directly used as fuel in power plant, which might lead to energy waste and environmental problems due to sulfide ore associated in coal. Associated minerals make middlings possess relatively high ash content, thus it is not suitable for the demand of saleable quality. Liberation of minerals from middlings through size reduction and the subsequent preparation are important to upgrade recovery and to promote economic benefit [1,2]. In general, size reduction will result in mineral liberation [3,4]. As to improve the clean coal recovery, size of comminution product must be suitable for retreatment, otherwise the new generated fine coal will be themselves problematic [5–9]. Compared with the extremely fine liberation size for metal mine, mineral liberation size of coal ⇑ Corresponding author. Tel.: +86 15162116750. E-mail address: [email protected] (Y. He).
middlings is relatively coarse due to the characteristics of the associated minerals, and the low limit of separation size for coal is comparatively higher than that of metal ores [10–13]. Therefore, size of the crushed product should be strictly controlled and preferential breakage which occurs at boundary between the associated minerals and coal is required during the comminution process. Various kinds of comminution devices have been developed according to different fracture mechanisms. For the conventional comminution facilities, it is believed that crusher, ball mill and stirred mill utilize crushing, impact and abrasion forces to realize size reduction respectively [14,15]. In recent years, high voltage pulses and electrical disintegration are also employed for experimental study of coal liberation. Ito et al. compared the comminution behavior of coal using an electrical disintegration and a roll crusher respectively, and reported that cracks were generated at minerals and coal boundaries and good liberation can be achieved even in the coarse fractions with the electrical disintegration [16]. Wang et al. reported the studies on the mineral liberation by high voltage pulses and conventional comminution with the same specific energy levels [17]. The experimental results indicated that the electrical comminution was a potential method to use less energy generating similar liberation degree with selective fragmentation of mineral ores as in the mechanical comminution [18]. To recover more clean coal, some coal preparation plants crushed coal middlings to 3 mm and separated progeny with the combined flow sheet of gravity separation and flotation [19]. Although the retreat-
2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.08.009
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W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
ment of coal middlings can improve the economic benefit, effect of comminution facilities on liberation characteristics of coal is ignored. Compared with the metallic mineral, the value of coal is relatively low, and process cost for clean coal recovery should be considered in the selection of comminution and the subsequent separation devices for middlings. The work outlined in this study is to investigate the effect of different kinds of fragmentation forces on the mineral liberation of coal middlings. Considering the low limit of separation size for coal, a jaw crusher and a ball mill were selected as the experimental devices to achieve the similar size distribution of the product. Full densimetric analyses and sectional micrographs on sized fractions of coal middlings prior to and after comminution were conducted to compare the discrepancy of liberation degree between these two methods. 2. Experimental
In China, the coking coal is paucity and its reserve is only 25.81% of the total. Due to the strict quality requirement of coking coal for steelmaking industry, clean coal with low ash content is produced and at the same time, nearly 20% middlings is generated with 20–40% ash content at the same time. Fig. 1 shows the yields of coking coal middlings in recent years, which reached 0.12 billion ton in 2011, thus it is meaningful for the retreatment of coking coal middlings to save energy resource. So, coking coal middlings was chosen as the experimental material which was sampled from a coking coal preparation plant. In this study, middlings with size of 3 mm + 0.5 mm was selected to represent the sampled middlings as a larger reduction ratio generally means the higher liberation degree when size of the comminution product was 0.5 mm [20]. It was different with the liberation characteristic research of coal middlings undertaken by Oliver et al., in which size of material comminuted by swing hammer crusher ranged from 25 to 150 mm [21]. 2.2. Comminution of coking coal middlings Two kinds of comminution devices, the jaw crusher and ball mill, were utilized to compare the effect of different kinds of fragmentation forces on mineral liberation on the condition of similar size distribution of the product. For the jaw crusher, closed circuit comminution was employed to verify the product size below 0.5 mm at the smallest discharge. The ball mill utilized in this study was a cylindrical mill with size of D350 mm L250 mm. 40 kg steel balls of 35 mm + 20 mm were selected and each experiment was performed at the material mass concentration of
Yields of coking coal middling (ton)
Size fraction (mm)
d (mm)
Kc
1 + 0.5 0.25 + 0.125 0.125 + 0.074 0.074
0.94 1.92 2.67 2.91
3.49 2.00 1.74 1.74
1.2×108 1.0×108 0.8×108 0.6×108 0.4×108 0.2×108
r
qe (g cm3)
D0 (cm)
w
(kgcm2) 80
6.49
31.69
0.91
70%. As size distribution of the product ground by the ball mill was widely effected by the ball diameter, ball diameter ratio, grinding time, rotation rate and adding ball rate, the semi-theoritic formula based on the principle of fragmentation mechanics was applied to guide the grinding process [22]. The semi-theoritic equation is shown as follows:
Db ¼ K c
2.1. Samples
0.0×108
Table 1 Parameters used for the semi-theory formula.
0:5224
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
w2 w6
10qe D0
3
r
d
ð1Þ
where Db is the diameter of certain steel ball which is suitable for a certain particle size d; Kc the comprehensive empirical correction coefficient; w the rotation rate; r the uniaxial compressive strength of the rock; qe the effective density of steel ball in slurry; D0 the diameter of intermediate aggregation layer of steel balls in mill; and d the diameter with the cumulative yield of 95% in each size fraction. Parameters employed in this formula are shown in Table 1. 2.3. Techniques Size distribution of middlings and the comminution product were sieved. The float and sink test was conducted to obtain the variation of density distribution between coking coal middlings and the comminution product. For middlings of 3 mm + 0.5 mm, density solutions of 1.30, 1.40, 1.50, 1.60, 1.80 kg/L with different proportions of Zncl2 and water were utilized according to the National Standard of Float and Sink Analysis of Coal (GB/T 4782001). Float and sink analyses for the comminuted product of sized and unsized were performed in a centrifuge with rotational speed 3000 r/min for 12 min. Density distribution for coal fines was the same, while the heavy liquid was the mixture of benzene, carbon tetrachloride and tribromomethane. Ash contents of coal middlings, sized and unsized particle of the product were measured with a muffle furnace. Sectional micrographs of coal middlings and the sized product were carried out with the FEI Quanta™ 250 scanning electron microscopy (SEM). Both parent and progeny particles were washed firstly with ethyl alcohol to remove the unexpected fines [23]. Then 20 g epoxy resin with a mixed proportion of 70:30 for Epothin epoxy resin (20-8140-128) and Epothin epoxy hardener (20-8142-064) were poured into a plastic container which includeds 4–5 g samples. As epoxy adequately mixed with the samples, Buehler vacuum impregnation pump was employed to remove bubbles from the liquid to prevent negative effect on the further micrographs observed by the SEM. Samples were solidified about 24 h later and were removed from the container to be polished. As the ordinary epoxy resin and pure coal phase were carbonate material with the very close average atomic weight, the SEM parameters were adjusted to achieve the easy distinction between pure coal and background [24,25]. 3. Results and discussion
2005
2006
2007
2008 Year
2009
2010
2011
Fig. 1. Yields of coking coal middlings in China in recent years.
3.1. Particle size distribution Table 2 indicates the size distributions of raw coal and the grinding media which is suitable for each size fraction calculated
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W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
Size fraction (mm)
Weight (g)
Yield (%)
Ash content (%)
Calculated diameter Db (cm)
Practical diameter Db (cm)
3 + 2.8 2.8 + 2 2 + 1 1 + 0.5 Sum
119.51 825.10 469.68 585.71 2000
5.98 41.26 23.48 29.29 100
31.93 28.54 30.85 29.97 29.71
3.50 3.21 2.65 2.27
3.5 3.0 2.5 2.0
Table 3 Size distribution of products comminuted by the two methods.
0 10 20 30 40 50 60 70 80 90 100
Coking coal middling Communited by jaw crusher Communited by ball mill 0
5
10 15 20 Ash content (%)
25
30
Fig. 3. Cumulative floats curves of middlings and comminution products 0.5 mm.
Yield (jaw crusher) (%)
Ash content (%)
Yield (ball mill) (%)
Ash content (%)
0.5 + 0.25 0.25 + 0.125 0.125 + 0.074 0.074 Sum
31.82 25.47 15.09 27.62 100
31.09 29.43 28.53 29.03 29.71
25.53 23.62 17.02 33.83 100
28.33 29.06 29.54 31.31 29.71
by the semi-theoritic formula. The content of steel balls with different parameters was the same with the yield of its suitable size fraction except for the decrease of the smallest ball to prevent over grinding. In order to ensure the representativeness of material, 2 kg coking coal middlings were sampled to be comminuted by the jaw crusher and the ball mill respectively. The new generated 0.5 mm product was classified into 0.074 mm, 0.125 mm + 0.074 mm, 0.25 mm + 0.125 mm and 0.5 mm + 0.25 mm size fractions, and the weight of the each fraction was measured. On the basis of a certain amount of exploratory experiments, similar size distribution was conducted by these two methods. Table 3 shows the yield and ash content of the each fraction. The results show that the ash content almost remains the same as the size changes.
3.2. Float and sink analyses Fig. 2 indicates the cumulative yield with the increase of density for coal middlings and the comminuted products. Compared with no material floating at the density of 1.3 kg/L for middlings and the product ground by the ball mill, yield of this density for the product by the jaw crusher is 3.06%, which suggests that some clean coal with low ash content liberates from middlings. Yields of the products crushed by the jaw crusher and the ball mill with density less than 1.5 kg/L are 35.97% and 27.71% respectively,
100 90 80 70 60 50 Coking coal 40 middling Communited by 30 jaw crusher 20 Communited by ball mill 10 0 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 Density (g/cm3)
Fig. 2. Cumulative yield vs. density of middlings and comminution products 0.5 mm.
100 Communited by jaw crusher
80 60
Cumulative yield (%)
Size fraction (mm)
Cumulative yield (%)
Cumulative yield (%)
Table 2 Size distribution of the raw coal and grinding media which is suitable for each size fraction.
0.5 mm+0.25 mm 0.25 mm+0.125 mm 0.125 mm+0.074 mm 0.074 mm
40 20 0 80 60
Communited by ball mill
40 20 0
1.3
1.4
1.5 1.6 1.7 1.8 Density (g/cm3)
1.9
2.0
Fig. 4. Cumulative yield vs. density of the four size fractions for the two comminution products.
which are much higher than that of middlings. Consequently, the results indicate that the retreatment potential of middlings is much more improved through these two comminution processes. Fig. 3 depicts the cumulative float curves of middlings and the comminuted products. Due to the mineral liberation in the comminution process, yields of the products crushed by the jaw crusher and the ball mill with ash content 11% are 32.45% and 25.01%, which are 22.45% and 15.01% higher than those of middlings respectively. As the ash content of 11% has met the demand for the steelmaking industry, conventional comminution facilities are suitable for the mineral liberation of middlings. It also indicates that liberation of middlings conducted by the jaw crusher is much better than that by the ball mill. Fig. 4 shows the cumulative yields of the four size fractions for these two comminuted products as a function of ash content. For the product ground by the ball mill, curves are nearly coincident except for the finest size fraction which is almost 5% lower than the others. Generally speaking, in the ball mill grinding process, fine particles are partly generated under the effect of abrasion, which causes fine coal stripping from the surface of middlings rather than liberating through the mineral associated boundaries. For the product crushed by the jaw crusher, yield of the each fraction is a little higher than that of particle ground by the ball mill except for the 0.5 + 0.25 mm. While unlike the rules shown by the ball mill grinding product, cumulative yields of the four size fractions are not the same, which also means the different liberation degree for these four size fractions. Cumulative yield of 0.25 mm + 0.125 mm at density
W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
Cumulative yield (%)
Cumulative yield (%)
672
0 20 40
Communited by jaw crusher 0 Communited 20 by ball mill 40
60 80 100
60
0.5 mm+0.25 mm 0 5 10 15 20 25 30 35
80 100
0
0
20
20
40
40
60
60
80 100
0.125 mm+0.074 mm 0 5 10 15 20 25 30 35 Ash content (%)
0.25 mm+0.125 mm 0 5 10 15 20 25 30 35
3.3. Analyses of sectional micrographs
80 100
the jaw crusher show better liberation degree than that of the product ground by the ball mill. At ash content of 11%, cumulative yield of the size fraction of 0.125 mm + 0.074 mm crushed by the jaw crusher is 48.25%, which is nearly 20% higher than that of the product ground by the ball mill. The same phenomenon is also investigated at the size fraction of 0.074 mm. Difference of comminution mechanisms between these two comminution methods lead to the different liberation degree though the size distribution of progeny are nearly the same.
0.074 mm 0
5 10 15 20 25 30 Ash content (%)
Fig. 5. Cumulative floats curves of the four size fractions for the two comminution products.
Fig. 6. Micrograph of middlings observed by SEM.
of 1.5 kg/L is the highest, which indicates a relatively better mineral liberation. The cumulative float curves of the four size fractions for these two comminuted products are plotted in Fig. 5. For the size fractions of 0.5 mm + 0.25 mm and 0.25 mm + 0.125 mm, the curves are nearly coincident, which means the same liberation degree is achieved. While for other fractions, particles crushed by
Before the comminution and liberation experiments, micrograph of the middlings was firstly observed by the SEM, just as shown in Fig. 6. In the micrograph, dark particles with low ash content account for nearly 10% due to the relatively low separation efficiency. While the other particles which contain darkness and brightness simultaneously being minerals are associated with coal. For these minerals, some are along the phase boundaries, or some are transgranular, which are the substances for retreatment. Fig. 7 shows the SEM micrographs of the progeny comminuted by the jaw crusher and the ball mill with the size fractions ranging from 0.5 mm + 0.25 mm to 0.74 mm. Liberated mineral particles with the brightest ores are observed in these four size fractions of progeny. Meanwhile some intergrowth particles are also found in the size fraction of 0.5 mm + 0.25 mm which were crushed by both methods. The intergrowth particles account for nearly 20%, which means relatively low liberation degree compared with the other size fractions. In the mean time dark particles with low ash content occupy comparatively high proportion for the size fractions of 0.125 mm + 0.074 mm and 0.074 mm. Therefore, liberation degree of these two fractions is much higher than that of other two fractions, which is corresponding to the float and sink results of the sized products. 3.4. Fracture mechanism of these two methods The liberation degree of the products comminuted by the jaw crusher and the ball mill are contrasted under the similar size distribution. Combined effects of crushing, splitting and bending are
Fig. 7. Micrographs of progeny comminuted by jaw crusher (above) and ball mill (below) ranging from 0.5 mm + 0.25 mm to 0.074 mm (from left to right).
W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
673
Fig. 8. Size reduction mechanism of jaw crushing (a) and ball milling (b and c).
applied to realize size reduction by the jaw crusher. In these fragmentation forces, crushing is the dominant force which can urge particles to be separated through boundaries and is meaningful for mineral liberation. When an irregular particle is crushed by crushing, product falls into two distinct size ranges: coarse particles resulted from the induced tensile failure, and fines from compressive failure near the points of loading, or by shear at projections just as shown in Fig. 8a. In this comminution process, particles are firstly crushed into intermediate size and size reduction of these new produced particles is according to the same rule. In the end, fines are obtained with time accumulation. Nevertheless, impact and abrasion are the main forces which ball mills utilize. The spherical grinding medium leads to relatively small area to contact with materials and stress concentration, which causes mineral liberating through the interface as shown in Fig. 8b. Therefore, cumulative yields of 0.5 mm + 0.25 mm obtained by these two comminution processes are nearly the same with density increase. Meanwhile, fines are stripping from ore body due to the abrasion effect which is unassisted for mineral liberating through the boundaries as shown in Fig. 8c. Consequently, the grinding process of a ball mill is a combination of volume and surface breakage. Comminution process could improve the retreatment potential of coal middlings. The micrographs and float and sink tests of the progeny of 0.125 mm + 0.074 mm and 0.074 mm show that the product crushed by the jaw crusher possesses a higher mineral liberation degree than that ground by ball mill. Meanwhile, micrographs of progeny also indicate that surface of the ball milling product are some smoother and rounder than those of jaw crushing because of the different generating mechanism of fines. On the other hand, energy efficiency of jaw crusher is much higher than that of ball mill, which the energy efficiency is only 5–10% [26,27]. So jaw crushers are the potential device for comminution process of coal middlings, while the retreatment potential of progeny should be further indicated by the timed-release analysis.
4. Conclusions (1) Through contrasting the effects of different kinds of fragmentation forces on mineral liberation, the jaw crusher with crushing as the main force creates a better mineral liberation of middlings than that by the ball mill with impact and abrasion as the main forces. (2) Cumulative yields of sized product by jaw crushing at density of 1.5 kg/L are 10–15% higher than that by ball milling except for size 0.5 mm + 0.25 mm. On the condition of ash content of 11%, which is suitable for steeling industry, yields of the products comminuted by the jaw crusher and the ball mill are 22.45% and 15.01% higher than that of middlings respectively. (3) For the size fractions of 0.125 mm + 0.074 mm and 0.074 mm, cumulative yields of progeny crushed by the jaw crusher with 11% ash content are nearly 20% and 25%
higher than that ground by the ball mill due to the different fines generating mechanism. Sectional micrographs of these two fractions also show the same law. (4) Through the comminution, liberation, densimetric analyses and section micrographs experiments, different contributions to the liberation of middlings are made by the combinations of fragmentation forces, in which crushing is the most efficiency force. Consequently, jaw crushers are indicated to be the potential device for middlings comminution. Acknowledgments This work is supported financially by the National Natural Science Foundation of China (No. 51074156, 51274196 and 51221462), the Natural Youth Science Foundation of China (No. 51104160). Thanks to the Advanced Analysis & Computation Center of China University of Mining and Technology for the measurement. References [1] Vizcarra TG, Wightman EM, Johnson NW, Manlapig EV. The effect of breakage mechanism on the mineral liberation properties of sulphide ores. Miner Eng 2010;23(5):374–82. [2] Gerson SZ, Gonzalo MA. Errors in the estimation of size-by-liberation flotation rate constants. Miner Eng 2012;27–28(2):1–10. [3] Oki T, Yotsumoto H, Owada S. Calculation of degree of mineral matter liberation in coal from sink-float separation data. Miner Eng 2004;17(1):39–51. [4] Yang XL, Zhao YM, Luo ZF, Chen ZQ, Song SL. Effects of sintered metal distributor on fluidization quality of the air dense medium fluidized bed. Mining Sci Technol 2011;21(5):681–5. [5] Powell MS, Morrison RD. The future of comminution modeling. Int J Miner Process 2007;84(1–4):228–39. [6] Bokányi L, Csöke B. Preparation of clean coal by flotation following ultra fine liberation. Appl Energy 2003;74(3–4):349–58. [7] Xu LJ, Peng TF, Zhang DY, Zhang FK. Fractal desulfurization kinetics of highsulfur coal. Int J Mining Sci Technol 2012;22(1):111–4. [8] Pascoe RD, Power MR, Simpson B. QEMSCAN analysis as a tool for improved understanding of gravity separator performance. Miner Eng 2007;20(5):487–95. [9] Hao J, Zhang H, Yang KY, Lu C, Chen JB, Li YN. Effect of different milling processes on the mineral distribution in a coal powder. Int J Mining Sci Technol 2012;22(2):237–42. [10] Gao MW, Forssberg E. Prediction of product size distributions for a stirred ball mill. Powder Technol 1995;84(2):101–6. [11] Ding ZY, Zhou MH, Liu Y. Effects of coal prices on merchandise prices in China. Mining Sci Technol 2011;21(4):651–4. [12] Zhao YM, Liu JT, Wei XY, Luo ZF, Chen QR, Song SL. New progress in the processing and efficient utilization of coal. Mining Sci Technol 2011;21(4):547–52. [13] Brien G, Gu Y, Adair BJI, Firth B. The use of optical reflected light and SEM imaging systems to provide quantitative coal characterization. Miner Eng 2011;24(12):1299–304. [14] Voltolini M, Zandomeneghi D, Mancini L, Polacci M. Texture analysis of volcanic rock samples: quantitative study of crystals and vesicles shape preferred orientation from X-ray microtomography data. J Volcanol Geoth Res 2011;202(1–2):83–95. [15] Berger A, Herwegh M, Schwarz J, Putlitz B. Quantitative analysis of crystal/ grain sizes and their distributions in 2D and 3D. J Struct Geol 2011;33(12):1751–63. [16] Ito M, Owada S, Nishimura T, Ota T. Experimental study of coal liberation: electrical disintegration versus roll-crusher comminution. Int J Miner Process 2009;92(1–2):7–14.
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Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Liberation characteristics of coal middlings comminuted by jaw crusher and ball mill Xie Weining a, He Yaqun a,b,⇑, Zhu Xiangnan a, Ge Linhan a, Huang Yajun a, Wang Haifeng a a b
School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 23 December 2012 Received in revised form 10 January 2013 Accepted 12 February 2013 Available online 31 August 2013 Keywords: Coal middlings Fragmentation force Liberation Float and sink test Sectional micrograph
a b s t r a c t The associated minerals make coal middlings possess a relatively high ash content. Subsequent liberation through size reduction can cause recovery increase. However, effect of comminution facilities on mineral liberation of middlings was ignored. This paper studied the liberation characteristics of middlings crushed with different kinds of fragmentation forces. Middlings of 3 mm + 0.5 mm sampled from a dense medium cyclone were comminuted by a jaw crusher and a ball mill to 0.5 mm with similar size distribution respectively. The generating mechanism of fines was also analyzed. Full densimetric analyses indicate that mineral liberation of the product crushed by the jaw crusher is better than that by the ball mill at each fraction. For sizes of 0.125 mm + 0.074 mm and 0.074 mm, yields of the product with ash content 11% comminuted by jaw crusher are nearly 20% higher than that by the ball mill. Sectional micrographs observed by the scanning electron microscopy (SEM) also show the same law for these two fractions and some intergrowth particles still exist in the fraction of 0.5 mm + 0.25 mm. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction The rapid growth of economy leads to the tremendous energy challenge that the world will face, especially for China. As a developing country with coal as the main resource, enormous energy consumption directly leads to the shortage of coal. Producers, therefore, are looking to maximize the recovery of saleable coal from raw coal to meet the huge market demand. Generally speaking, raw coal is separated into clean coal, middlings and gangue by different preparation methods. In China, middlings normally accounts for nearly 20% of rough coal and generally has an ash content of 20–40%. Without any retreatment, middlings was directly used as fuel in power plant, which might lead to energy waste and environmental problems due to sulfide ore associated in coal. Associated minerals make middlings possess relatively high ash content, thus it is not suitable for the demand of saleable quality. Liberation of minerals from middlings through size reduction and the subsequent preparation are important to upgrade recovery and to promote economic benefit [1,2]. In general, size reduction will result in mineral liberation [3,4]. As to improve the clean coal recovery, size of comminution product must be suitable for retreatment, otherwise the new generated fine coal will be themselves problematic [5–9]. Compared with the extremely fine liberation size for metal mine, mineral liberation size of coal ⇑ Corresponding author. Tel.: +86 15162116750. E-mail address: [email protected] (Y. He).
middlings is relatively coarse due to the characteristics of the associated minerals, and the low limit of separation size for coal is comparatively higher than that of metal ores [10–13]. Therefore, size of the crushed product should be strictly controlled and preferential breakage which occurs at boundary between the associated minerals and coal is required during the comminution process. Various kinds of comminution devices have been developed according to different fracture mechanisms. For the conventional comminution facilities, it is believed that crusher, ball mill and stirred mill utilize crushing, impact and abrasion forces to realize size reduction respectively [14,15]. In recent years, high voltage pulses and electrical disintegration are also employed for experimental study of coal liberation. Ito et al. compared the comminution behavior of coal using an electrical disintegration and a roll crusher respectively, and reported that cracks were generated at minerals and coal boundaries and good liberation can be achieved even in the coarse fractions with the electrical disintegration [16]. Wang et al. reported the studies on the mineral liberation by high voltage pulses and conventional comminution with the same specific energy levels [17]. The experimental results indicated that the electrical comminution was a potential method to use less energy generating similar liberation degree with selective fragmentation of mineral ores as in the mechanical comminution [18]. To recover more clean coal, some coal preparation plants crushed coal middlings to 3 mm and separated progeny with the combined flow sheet of gravity separation and flotation [19]. Although the retreat-
2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.08.009
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W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
ment of coal middlings can improve the economic benefit, effect of comminution facilities on liberation characteristics of coal is ignored. Compared with the metallic mineral, the value of coal is relatively low, and process cost for clean coal recovery should be considered in the selection of comminution and the subsequent separation devices for middlings. The work outlined in this study is to investigate the effect of different kinds of fragmentation forces on the mineral liberation of coal middlings. Considering the low limit of separation size for coal, a jaw crusher and a ball mill were selected as the experimental devices to achieve the similar size distribution of the product. Full densimetric analyses and sectional micrographs on sized fractions of coal middlings prior to and after comminution were conducted to compare the discrepancy of liberation degree between these two methods. 2. Experimental
In China, the coking coal is paucity and its reserve is only 25.81% of the total. Due to the strict quality requirement of coking coal for steelmaking industry, clean coal with low ash content is produced and at the same time, nearly 20% middlings is generated with 20–40% ash content at the same time. Fig. 1 shows the yields of coking coal middlings in recent years, which reached 0.12 billion ton in 2011, thus it is meaningful for the retreatment of coking coal middlings to save energy resource. So, coking coal middlings was chosen as the experimental material which was sampled from a coking coal preparation plant. In this study, middlings with size of 3 mm + 0.5 mm was selected to represent the sampled middlings as a larger reduction ratio generally means the higher liberation degree when size of the comminution product was 0.5 mm [20]. It was different with the liberation characteristic research of coal middlings undertaken by Oliver et al., in which size of material comminuted by swing hammer crusher ranged from 25 to 150 mm [21]. 2.2. Comminution of coking coal middlings Two kinds of comminution devices, the jaw crusher and ball mill, were utilized to compare the effect of different kinds of fragmentation forces on mineral liberation on the condition of similar size distribution of the product. For the jaw crusher, closed circuit comminution was employed to verify the product size below 0.5 mm at the smallest discharge. The ball mill utilized in this study was a cylindrical mill with size of D350 mm L250 mm. 40 kg steel balls of 35 mm + 20 mm were selected and each experiment was performed at the material mass concentration of
Yields of coking coal middling (ton)
Size fraction (mm)
d (mm)
Kc
1 + 0.5 0.25 + 0.125 0.125 + 0.074 0.074
0.94 1.92 2.67 2.91
3.49 2.00 1.74 1.74
1.2×108 1.0×108 0.8×108 0.6×108 0.4×108 0.2×108
r
qe (g cm3)
D0 (cm)
w
(kgcm2) 80
6.49
31.69
0.91
70%. As size distribution of the product ground by the ball mill was widely effected by the ball diameter, ball diameter ratio, grinding time, rotation rate and adding ball rate, the semi-theoritic formula based on the principle of fragmentation mechanics was applied to guide the grinding process [22]. The semi-theoritic equation is shown as follows:
Db ¼ K c
2.1. Samples
0.0×108
Table 1 Parameters used for the semi-theory formula.
0:5224
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
w2 w6
10qe D0
3
r
d
ð1Þ
where Db is the diameter of certain steel ball which is suitable for a certain particle size d; Kc the comprehensive empirical correction coefficient; w the rotation rate; r the uniaxial compressive strength of the rock; qe the effective density of steel ball in slurry; D0 the diameter of intermediate aggregation layer of steel balls in mill; and d the diameter with the cumulative yield of 95% in each size fraction. Parameters employed in this formula are shown in Table 1. 2.3. Techniques Size distribution of middlings and the comminution product were sieved. The float and sink test was conducted to obtain the variation of density distribution between coking coal middlings and the comminution product. For middlings of 3 mm + 0.5 mm, density solutions of 1.30, 1.40, 1.50, 1.60, 1.80 kg/L with different proportions of Zncl2 and water were utilized according to the National Standard of Float and Sink Analysis of Coal (GB/T 4782001). Float and sink analyses for the comminuted product of sized and unsized were performed in a centrifuge with rotational speed 3000 r/min for 12 min. Density distribution for coal fines was the same, while the heavy liquid was the mixture of benzene, carbon tetrachloride and tribromomethane. Ash contents of coal middlings, sized and unsized particle of the product were measured with a muffle furnace. Sectional micrographs of coal middlings and the sized product were carried out with the FEI Quanta™ 250 scanning electron microscopy (SEM). Both parent and progeny particles were washed firstly with ethyl alcohol to remove the unexpected fines [23]. Then 20 g epoxy resin with a mixed proportion of 70:30 for Epothin epoxy resin (20-8140-128) and Epothin epoxy hardener (20-8142-064) were poured into a plastic container which includeds 4–5 g samples. As epoxy adequately mixed with the samples, Buehler vacuum impregnation pump was employed to remove bubbles from the liquid to prevent negative effect on the further micrographs observed by the SEM. Samples were solidified about 24 h later and were removed from the container to be polished. As the ordinary epoxy resin and pure coal phase were carbonate material with the very close average atomic weight, the SEM parameters were adjusted to achieve the easy distinction between pure coal and background [24,25]. 3. Results and discussion
2005
2006
2007
2008 Year
2009
2010
2011
Fig. 1. Yields of coking coal middlings in China in recent years.
3.1. Particle size distribution Table 2 indicates the size distributions of raw coal and the grinding media which is suitable for each size fraction calculated
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Size fraction (mm)
Weight (g)
Yield (%)
Ash content (%)
Calculated diameter Db (cm)
Practical diameter Db (cm)
3 + 2.8 2.8 + 2 2 + 1 1 + 0.5 Sum
119.51 825.10 469.68 585.71 2000
5.98 41.26 23.48 29.29 100
31.93 28.54 30.85 29.97 29.71
3.50 3.21 2.65 2.27
3.5 3.0 2.5 2.0
Table 3 Size distribution of products comminuted by the two methods.
0 10 20 30 40 50 60 70 80 90 100
Coking coal middling Communited by jaw crusher Communited by ball mill 0
5
10 15 20 Ash content (%)
25
30
Fig. 3. Cumulative floats curves of middlings and comminution products 0.5 mm.
Yield (jaw crusher) (%)
Ash content (%)
Yield (ball mill) (%)
Ash content (%)
0.5 + 0.25 0.25 + 0.125 0.125 + 0.074 0.074 Sum
31.82 25.47 15.09 27.62 100
31.09 29.43 28.53 29.03 29.71
25.53 23.62 17.02 33.83 100
28.33 29.06 29.54 31.31 29.71
by the semi-theoritic formula. The content of steel balls with different parameters was the same with the yield of its suitable size fraction except for the decrease of the smallest ball to prevent over grinding. In order to ensure the representativeness of material, 2 kg coking coal middlings were sampled to be comminuted by the jaw crusher and the ball mill respectively. The new generated 0.5 mm product was classified into 0.074 mm, 0.125 mm + 0.074 mm, 0.25 mm + 0.125 mm and 0.5 mm + 0.25 mm size fractions, and the weight of the each fraction was measured. On the basis of a certain amount of exploratory experiments, similar size distribution was conducted by these two methods. Table 3 shows the yield and ash content of the each fraction. The results show that the ash content almost remains the same as the size changes.
3.2. Float and sink analyses Fig. 2 indicates the cumulative yield with the increase of density for coal middlings and the comminuted products. Compared with no material floating at the density of 1.3 kg/L for middlings and the product ground by the ball mill, yield of this density for the product by the jaw crusher is 3.06%, which suggests that some clean coal with low ash content liberates from middlings. Yields of the products crushed by the jaw crusher and the ball mill with density less than 1.5 kg/L are 35.97% and 27.71% respectively,
100 90 80 70 60 50 Coking coal 40 middling Communited by 30 jaw crusher 20 Communited by ball mill 10 0 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 Density (g/cm3)
Fig. 2. Cumulative yield vs. density of middlings and comminution products 0.5 mm.
100 Communited by jaw crusher
80 60
Cumulative yield (%)
Size fraction (mm)
Cumulative yield (%)
Cumulative yield (%)
Table 2 Size distribution of the raw coal and grinding media which is suitable for each size fraction.
0.5 mm+0.25 mm 0.25 mm+0.125 mm 0.125 mm+0.074 mm 0.074 mm
40 20 0 80 60
Communited by ball mill
40 20 0
1.3
1.4
1.5 1.6 1.7 1.8 Density (g/cm3)
1.9
2.0
Fig. 4. Cumulative yield vs. density of the four size fractions for the two comminution products.
which are much higher than that of middlings. Consequently, the results indicate that the retreatment potential of middlings is much more improved through these two comminution processes. Fig. 3 depicts the cumulative float curves of middlings and the comminuted products. Due to the mineral liberation in the comminution process, yields of the products crushed by the jaw crusher and the ball mill with ash content 11% are 32.45% and 25.01%, which are 22.45% and 15.01% higher than those of middlings respectively. As the ash content of 11% has met the demand for the steelmaking industry, conventional comminution facilities are suitable for the mineral liberation of middlings. It also indicates that liberation of middlings conducted by the jaw crusher is much better than that by the ball mill. Fig. 4 shows the cumulative yields of the four size fractions for these two comminuted products as a function of ash content. For the product ground by the ball mill, curves are nearly coincident except for the finest size fraction which is almost 5% lower than the others. Generally speaking, in the ball mill grinding process, fine particles are partly generated under the effect of abrasion, which causes fine coal stripping from the surface of middlings rather than liberating through the mineral associated boundaries. For the product crushed by the jaw crusher, yield of the each fraction is a little higher than that of particle ground by the ball mill except for the 0.5 + 0.25 mm. While unlike the rules shown by the ball mill grinding product, cumulative yields of the four size fractions are not the same, which also means the different liberation degree for these four size fractions. Cumulative yield of 0.25 mm + 0.125 mm at density
W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
Cumulative yield (%)
Cumulative yield (%)
672
0 20 40
Communited by jaw crusher 0 Communited 20 by ball mill 40
60 80 100
60
0.5 mm+0.25 mm 0 5 10 15 20 25 30 35
80 100
0
0
20
20
40
40
60
60
80 100
0.125 mm+0.074 mm 0 5 10 15 20 25 30 35 Ash content (%)
0.25 mm+0.125 mm 0 5 10 15 20 25 30 35
3.3. Analyses of sectional micrographs
80 100
the jaw crusher show better liberation degree than that of the product ground by the ball mill. At ash content of 11%, cumulative yield of the size fraction of 0.125 mm + 0.074 mm crushed by the jaw crusher is 48.25%, which is nearly 20% higher than that of the product ground by the ball mill. The same phenomenon is also investigated at the size fraction of 0.074 mm. Difference of comminution mechanisms between these two comminution methods lead to the different liberation degree though the size distribution of progeny are nearly the same.
0.074 mm 0
5 10 15 20 25 30 Ash content (%)
Fig. 5. Cumulative floats curves of the four size fractions for the two comminution products.
Fig. 6. Micrograph of middlings observed by SEM.
of 1.5 kg/L is the highest, which indicates a relatively better mineral liberation. The cumulative float curves of the four size fractions for these two comminuted products are plotted in Fig. 5. For the size fractions of 0.5 mm + 0.25 mm and 0.25 mm + 0.125 mm, the curves are nearly coincident, which means the same liberation degree is achieved. While for other fractions, particles crushed by
Before the comminution and liberation experiments, micrograph of the middlings was firstly observed by the SEM, just as shown in Fig. 6. In the micrograph, dark particles with low ash content account for nearly 10% due to the relatively low separation efficiency. While the other particles which contain darkness and brightness simultaneously being minerals are associated with coal. For these minerals, some are along the phase boundaries, or some are transgranular, which are the substances for retreatment. Fig. 7 shows the SEM micrographs of the progeny comminuted by the jaw crusher and the ball mill with the size fractions ranging from 0.5 mm + 0.25 mm to 0.74 mm. Liberated mineral particles with the brightest ores are observed in these four size fractions of progeny. Meanwhile some intergrowth particles are also found in the size fraction of 0.5 mm + 0.25 mm which were crushed by both methods. The intergrowth particles account for nearly 20%, which means relatively low liberation degree compared with the other size fractions. In the mean time dark particles with low ash content occupy comparatively high proportion for the size fractions of 0.125 mm + 0.074 mm and 0.074 mm. Therefore, liberation degree of these two fractions is much higher than that of other two fractions, which is corresponding to the float and sink results of the sized products. 3.4. Fracture mechanism of these two methods The liberation degree of the products comminuted by the jaw crusher and the ball mill are contrasted under the similar size distribution. Combined effects of crushing, splitting and bending are
Fig. 7. Micrographs of progeny comminuted by jaw crusher (above) and ball mill (below) ranging from 0.5 mm + 0.25 mm to 0.074 mm (from left to right).
W. Xie et al. / International Journal of Mining Science and Technology 23 (2013) 669–674
673
Fig. 8. Size reduction mechanism of jaw crushing (a) and ball milling (b and c).
applied to realize size reduction by the jaw crusher. In these fragmentation forces, crushing is the dominant force which can urge particles to be separated through boundaries and is meaningful for mineral liberation. When an irregular particle is crushed by crushing, product falls into two distinct size ranges: coarse particles resulted from the induced tensile failure, and fines from compressive failure near the points of loading, or by shear at projections just as shown in Fig. 8a. In this comminution process, particles are firstly crushed into intermediate size and size reduction of these new produced particles is according to the same rule. In the end, fines are obtained with time accumulation. Nevertheless, impact and abrasion are the main forces which ball mills utilize. The spherical grinding medium leads to relatively small area to contact with materials and stress concentration, which causes mineral liberating through the interface as shown in Fig. 8b. Therefore, cumulative yields of 0.5 mm + 0.25 mm obtained by these two comminution processes are nearly the same with density increase. Meanwhile, fines are stripping from ore body due to the abrasion effect which is unassisted for mineral liberating through the boundaries as shown in Fig. 8c. Consequently, the grinding process of a ball mill is a combination of volume and surface breakage. Comminution process could improve the retreatment potential of coal middlings. The micrographs and float and sink tests of the progeny of 0.125 mm + 0.074 mm and 0.074 mm show that the product crushed by the jaw crusher possesses a higher mineral liberation degree than that ground by ball mill. Meanwhile, micrographs of progeny also indicate that surface of the ball milling product are some smoother and rounder than those of jaw crushing because of the different generating mechanism of fines. On the other hand, energy efficiency of jaw crusher is much higher than that of ball mill, which the energy efficiency is only 5–10% [26,27]. So jaw crushers are the potential device for comminution process of coal middlings, while the retreatment potential of progeny should be further indicated by the timed-release analysis.
4. Conclusions (1) Through contrasting the effects of different kinds of fragmentation forces on mineral liberation, the jaw crusher with crushing as the main force creates a better mineral liberation of middlings than that by the ball mill with impact and abrasion as the main forces. (2) Cumulative yields of sized product by jaw crushing at density of 1.5 kg/L are 10–15% higher than that by ball milling except for size 0.5 mm + 0.25 mm. On the condition of ash content of 11%, which is suitable for steeling industry, yields of the products comminuted by the jaw crusher and the ball mill are 22.45% and 15.01% higher than that of middlings respectively. (3) For the size fractions of 0.125 mm + 0.074 mm and 0.074 mm, cumulative yields of progeny crushed by the jaw crusher with 11% ash content are nearly 20% and 25%
higher than that ground by the ball mill due to the different fines generating mechanism. Sectional micrographs of these two fractions also show the same law. (4) Through the comminution, liberation, densimetric analyses and section micrographs experiments, different contributions to the liberation of middlings are made by the combinations of fragmentation forces, in which crushing is the most efficiency force. Consequently, jaw crushers are indicated to be the potential device for middlings comminution. Acknowledgments This work is supported financially by the National Natural Science Foundation of China (No. 51074156, 51274196 and 51221462), the Natural Youth Science Foundation of China (No. 51104160). Thanks to the Advanced Analysis & Computation Center of China University of Mining and Technology for the measurement. References [1] Vizcarra TG, Wightman EM, Johnson NW, Manlapig EV. The effect of breakage mechanism on the mineral liberation properties of sulphide ores. Miner Eng 2010;23(5):374–82. [2] Gerson SZ, Gonzalo MA. Errors in the estimation of size-by-liberation flotation rate constants. Miner Eng 2012;27–28(2):1–10. [3] Oki T, Yotsumoto H, Owada S. Calculation of degree of mineral matter liberation in coal from sink-float separation data. Miner Eng 2004;17(1):39–51. [4] Yang XL, Zhao YM, Luo ZF, Chen ZQ, Song SL. Effects of sintered metal distributor on fluidization quality of the air dense medium fluidized bed. Mining Sci Technol 2011;21(5):681–5. [5] Powell MS, Morrison RD. The future of comminution modeling. Int J Miner Process 2007;84(1–4):228–39. [6] Bokányi L, Csöke B. Preparation of clean coal by flotation following ultra fine liberation. Appl Energy 2003;74(3–4):349–58. [7] Xu LJ, Peng TF, Zhang DY, Zhang FK. Fractal desulfurization kinetics of highsulfur coal. Int J Mining Sci Technol 2012;22(1):111–4. [8] Pascoe RD, Power MR, Simpson B. QEMSCAN analysis as a tool for improved understanding of gravity separator performance. Miner Eng 2007;20(5):487–95. [9] Hao J, Zhang H, Yang KY, Lu C, Chen JB, Li YN. Effect of different milling processes on the mineral distribution in a coal powder. Int J Mining Sci Technol 2012;22(2):237–42. [10] Gao MW, Forssberg E. Prediction of product size distributions for a stirred ball mill. Powder Technol 1995;84(2):101–6. [11] Ding ZY, Zhou MH, Liu Y. Effects of coal prices on merchandise prices in China. Mining Sci Technol 2011;21(4):651–4. [12] Zhao YM, Liu JT, Wei XY, Luo ZF, Chen QR, Song SL. New progress in the processing and efficient utilization of coal. Mining Sci Technol 2011;21(4):547–52. [13] Brien G, Gu Y, Adair BJI, Firth B. The use of optical reflected light and SEM imaging systems to provide quantitative coal characterization. Miner Eng 2011;24(12):1299–304. [14] Voltolini M, Zandomeneghi D, Mancini L, Polacci M. Texture analysis of volcanic rock samples: quantitative study of crystals and vesicles shape preferred orientation from X-ray microtomography data. J Volcanol Geoth Res 2011;202(1–2):83–95. [15] Berger A, Herwegh M, Schwarz J, Putlitz B. Quantitative analysis of crystal/ grain sizes and their distributions in 2D and 3D. J Struct Geol 2011;33(12):1751–63. [16] Ito M, Owada S, Nishimura T, Ota T. Experimental study of coal liberation: electrical disintegration versus roll-crusher comminution. Int J Miner Process 2009;92(1–2):7–14.
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