- Email: [email protected]
High capacity fine coal cleaning using an enhanced gravity concentrator
Minerals Engineering, Vol. 11, No. 12, pp. 1191-1199, 1998
Pergamom~ 0892-6875(98)0010$-8
© 1998 Elsevier Science Ltd All fights reserved o s 9 2 - ~ 5 / 9 8 t $ - - see front matter
[HGH CAPACITY FINE COAL CLEANING USING AN ENHANCED GRAVITY CONCENTRATOR
R.Q. HONAKER Department of Mining & Mineral Resources Engineering, Southern Illinois University - Carbondale, C~xbondale, Illinois 62901-6603, U.S.A. E-mail: [email protected] (Received 22 June 1998; accepted 14 August 1998)
ABSTRACT Recent research has shown that enhanced gravity separators provide the opportunity to effectively clean I mm x 37 micron coal. A detailed experimental program was performed on a continuoas Falcon Concentrator, which was found to achieve separations at mass throughput capacity values in excess of 75 tonnes/hour. The apparent particle separation density for I mm x 75 micron coal was about 1.60 with relatively high efficiency (Ep = 0.12). Typical ash and sulfur rejection values of 85% and 70%, respectively, were achieved while recovering 85% of the combustible material. Metallurgical and efficiency data from the evaluation are presented in this publication. © 1998 Elsevier Science Ltd. All rights reserved
Keywords Fine particle processing, gravity concentration, coal
INTRODUCTION Research reported by several investigators have shown that gravity-based separators are more efficient in the treatment of middling-type particles than froth flotation. However, until recently, high capacity gravitybased systems used to treat particles finer than 212 lan were inefficient. Over the past decade, several enhanced gravity separators have evolved which have the ability to effectively treat particles that are normally concentrated in froth flotation processes. In fact, a patent has recently been received which covers a circuit using an enhanced gravity separator to compliment a froth flotation system for the improved rejection of sulfur for fine coal [1]. The continuous Falcon Concentrator is an enhanced gravity separator that has been found to provide efficient separatious for fine coal cleaning [2]. From a quantitative comparison of the available data from the various enhanced gravity separators, the Falcon Concentrator provided the lowest separation density (Dso) as a result of its ability to provide the maximum centrifugal field of 300 g's [3]. However, to date, all available data has been collected from pilot scale units having mass throughput capacity values less than 5 tonnes/hr.
1191
1192
R.Q. Honaker
This publication will present the results obtained from a full-scale C40 Falcon Concentrator, which was constructed to have a mass throughput capacity of I00 tonnes/hr. Metallurgical and process efficiency data are presented and discussed on a particle size-by-size basis.
EXPERIMENTAL
Unit description Currently, the C40 Falcon Concentrator represents the largest enhanced gravity concentrator worldwide, although development of other technologies may result in units of nearly equal size. The C40 unit measures 100 cm in diameter and possesses the ability to provide up to 210 g's of centrifugal force. The separation mechanism is based on flowing film principles and has been described in previous publications [4,5,6 ]. The unit is supported on a 3 x 3 m 2 frame and stands 4.6 m tall. The bowl is sloped at an angle of 10° from vertical to provide a force parallel to the bowl wall which pushes the packed solids bed towards the 36 discharge chutes equally spaced along the circumference near the top of the bowl. The heavy underflow stream can freely flow through the chutes and out of the nozzles or the flow rate can be regulated by fluctuating pinch valves. Complete monitoring and control of the process parameters is provided through a central control system. The 36 discharge chutes that provide passage for the heavy underflow stream are equipped with a 0.55 cm diameter fixed orifice, which limits the upper particle size to I mm and, to some degree, the upper feed solids content due to potential plugging. However, recent develops of the C40 Falcon unit include the elimination of the fixed valves, which allows the treatment of larger particle sizes without the problem of plugging.
Circuit description The circuit consisted of a 20 kL feed tank in which the feed solids content was adjusted to the desired level. The volumetric feed rate was fixed at the required value by manipulation of a 20 cm gate valve on a recirculation line that sent a portion of the stream directly to the product sump. The feed was passed through a 1.4 mm screen prior to entering the C40 unit to remove material that could cause potential valve plugging. The overflow and underflow streams were combined into a 5.5 kL sump and subsequently returned to the feed tank to complete the closed circuit. Approximately 8 tonnes of fine coal solids were used to charge the system. Typical volumetric feed flow rates evaluated were between 3.7 and 6.7 kL at solid concentrations between 10% and 30% by weight. Other operating parameters include bowl rotational speed and underflow volumetric flow rate. After allowing sufficient time for the process to realize steady-state conditions, a representative sample of each product stream was collected. The volumetric flow rate of the underflow stream was measured and recorded. Each of the samples were wet screened using 150 tin1 (100 mesh) and 37 ~ (400 mesh) sieves. The screened sample were filtered, dried and analyzed for weight, ash, total sulfur and, in some cases, pyritic sulfur contents using ASTIVl procedures. Because of the difficulty in accurately measuring the volumetric flow rates of the product streams, the response variables, which include mass yield, recovery, and ash rejection, were quantified using the two-preduct formula. To evaluate the efficiency of the C40 Concentrator, partition curves were obtained by conducting washability analyses on the various particle size fractions in the feed and product samples. All washability analyses were conducted using a centrifugal technique and a lithium metatungstate (LMT) solution as the medium.
Sample description The fine coal slurry sample was collected from the nominal -0.6 nun (28 mesh) fine circuit feed of an operating coal processing plant treating run-of-mine Pittsburgh No. 8 seam coal. A representative sample of the total bulk was collected for a particle size-by-size analysis. As shown in Table 1, the sample was
High capacity fine coal cleaning
1193
found to be relatively fine with 42% of the material having a particle size below 37 tan (400 mesh). However, the majority of the coal particles existed in the 600 x 300 tnn size fraction, i. e., 26% by weight. The sulfur content was relatively high and distributed irregularly within the coal sample.
TABLE 1
Particle size-by-size analysis results of the nominal -0.6 mm Pittsburgh No. 8 seam coal sample
Particle Size :Fraction (pro) +600 600 x 300 300 x 212 212 x 150 150 x 75 75 x 37 -37 Total
Weight
Ash
(%)
(%)
5.95 20.40 10.59 3.26 9.77 7.93 42.10 100.00
11;26 12.70 13.71 12.32 1Z07 16.32 60.40 33.00
Total Sulfur (%) 3.21 3.32 4.26 3.06 3.37 6.52 2.70 3.40
A centrifugal washability analysis was conducted on the sample from each particle size fraction identified in Table 1 and the samples from each density fraction was analyzed for weight, ash and total sulfur contents. Table 2 shows the overall washability results for the +37 pm (+400 mesh) particle size fractions. The coal sample contained a relatively low-to-moderate amount of middling-type particles with only about 5% of the feed partJicles having a density between 1.6 and 2.0. Thus, the coal sample would be classified as easy-to-clean. TABLE 2
Particle density-by-density analysis results of the + 37 tnn (+400 mesh) mterial in the Pittsburgh No. 8 seam coal sample Specific Gravity Fraction 1.3 Float 1.3 x 1.4 1.4 x 1.6 1.6 x 1.8 1.8 x 2.0 2.0 x 2.2 2.2 Sink Total
Weight
Ash
(%)
(%)
53.73 21.30 7.43 3.47 1.67 1.80 10.60 100.00
2.46 6.10 17.10 30.10 42.80 56.50 70.60 14.15
Total Sulfur (%) 1.28 1.90 3.19 3.70 4.29 4.63 17.03 3.42
In addition, the -75 tan (-200 mesh) and -37 gnu (-400 mesh) particle size fractions, as well as, a representative sample of the bulk was subject to ~'aditionalrelease analyses to obtain the ultimate flotation performance for comparison purposes. The procedure used was introduced by Dell in 1964 [7] and modified by Forrest et al. [8] and Mohanty et al. [9].The results are presented in the following section.
RESULTS AND DISCUSSION Metallurgical results An experimental program based on a statistical design was conducted to evaluate the parametric effects and optimize the perfoJ~nance of the C40 Falcon, A totalof 18 experiments were conducted over a range of operating parameter values. The metallurgical results obtained from the treatment of the Pittsburgh No. 8
1194
R.Q.
Honaker
seam coal sample are compared to the washability curve in Figures 1 (a) and (b). On the basis of both ash and sulfur contents, the yield differential increases as the product quality improves, which reflects moderate inefficiencies in the process and is typical of most commercial separators. The relatively efficient separation is indicated by nearly a 75% rejection of ash beating material and 60% of the sulfur to the tailings stream while recovering 80% of the mass yield to the product. In fact, the organic efficiency (i.e., ratio of actual recovery over theoretical recovery.) is about 96%. 100
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Metallurgical results obtained on the basis of a) product ash and b) product total sulfur contents from the treatment of the +37 pm (400 mesh) pa.,ticle size fnmtion of the Pittsburgh No. 8 cx)aL
High capacityfinecoal cleaning
1195
One of the most important uses of enhanced gravity separation in fine coal cleaning involves the rejection of the pyritic sulfur that is normally recovered in flotation. Table 3 shows the total sulfur reductions realized on a particle size-by-size basis from the treatment of the Pittsburgh No. 8 seam coal using the C40 Falcon unit. Excellent sulfur reductions were achieved for the +75 lan (+200) size fractions while a significant but lower reduction was achieved for the 75 x 37 lan (200 x 400 mesh) size fraction. One should note the high sulfur contents in ~ae tailings material, which should be mostly in the form of pyritic sulfur since a high coal recovery is realized.
Total sulfur reductions achieved on a particle size-by.size basis from the treatment olt' the --600 promPittsburgh No. 8 coal by the C40 Falcon concentrator
TABLE 3
Particle Size Fraction 0an) +6CO 600 x 300 300 x 212 212 x 150 150 x 75 75 x 37
Feed 3.21 3.32 4.26 3.06 3.37 6.52
Total Sulfur (%) Product 1.92 1.67 1.71 1.77 1.77 3.12
Tails 12.30 12.60 9.92 13.70 22.20 20.50
Combustible Recovery (%) 90.8 92.7 92.9 93.6 93.7 94.4
The relatively high separation efficiencies achieved by the C40 Falcon Concentrator were realized at mass throughput capacities up to 78 tonnes/hr as shown in Table 4. Factors that limit the mass throughput capacity of the continuous Falcon concentrators include: .
2. 3.
Excessive volumetric flow rates which provide insufficient residence to time for a proper separation; High feed solid contents which prevent efficient particle stratification; Underflow valves having volumetric constraints, which limits the amount of mass that can flow through the ports.
Metallurgical results obtained for the +37 l~m particle size material showing the ability to achieve efficient separations at high mass throughput values
TABLE 4
Test
1
2 3
Feed Mass Flow (tph) 78 6O 52
Feed 19.4 17.5 13.1
Ash (%) Product 4.71 5.10 6.30
Combustible Recovery (%) 80.0 78.3 92.4
Feed Flow Rate (kL/min) 6.7 5.2 6.7
The test program presented in this publication was unable to identify the maximum acceptable feed rate due to constraints of the circuit, which limited the volumetric capacity to about 6.7 kL/min. Flotation compmison It is well known ~aat enhanced gravity concentrators remain ineffective in the treatment of slimes, i.e. -37 lan material [3]. q~us, froth flotation remains to be needed for recovering the coal in the slimes fraction. In addition, it is possible that, for coals having a small amount of middling type particles, froth flotation may be more efficient for treating particles with a size greater than 37 IJm. To evaluate this possibility, release analyses were conducted on the nominal -600 lan Pittsburgh No. 8 coal sample as well as the -75 la'n and-37 lan particle size fractions. The release data were combined with the C40 Falcon results to simulate treatment of the total sample.
1196
R.Q. Honaker
As shown in Figure 2(a), the most effective separation achieved on the basis of product ash content involved the treatment of the -75 pm particle size fractions using a froth flotation process while the +75 pin fractions were better treated by the Falcon Concentrator. On the other hand, the ultimate separation efficiency on the basis of total sulfur reductions was realized by the treatment of the +37 pin size fractions by the Falcon unit while treating the -37 pin fraction with a flotation process (Figure 2b). The use of froth flotation to treat the entire -600 pin Pittsburgh No. 8 coal sample provided a much inferior separation performance.
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A comparison of the metallurgical separation performances obtained from the treatment of the Pittsburgh No. 8 coal sample using different combinations of treatment strategies with the C40 Falcon Concentrator and a froth flotation process on the basis of (a) ash and (b) total sulfur reductions.
High capacity fine coal cleaning
1197
The results indicate that, for the Pittsburgh No. 8 coal sample, the Falcon was more effective in reducing the total sulfur content in the 75 x 37 lain particle size fraction whereas ash rejection was more efficiently achieved by froth flotation. This finding is due to the differences in solid density and floatability between the ash-forming components (average sp. gr. = 2.7) and coal pyrite (average sp. gr. = 4.8). In other words, as the particle size decreases, the heavier and floatable pyrite particles are more readily rejected in the Falcon Concentrator until the particle mass becomes too small to allow transport to the heavy stream within the corresponding residence time. Below this particle size, separation based on differential flotation kinetics is more effective. Since the ash-forming components are lighter, the transition particle size occurs at a larger value. However, it should be noted that this effect could be counteracted with an increase in centrifugal force at the sacrifice of lost coarse coal. The transitional particle size in the above discussion is a function of the amount of middling type particles in the coal and the floatability of the coal pyrite. As shown in Table 2, the Pittsburgh No. 8 coal sample has a relatively low middlings content. Thus, the Falcon Concentrator may be more efficient than flotation over a larger particle size range for a coal having a greater quantity of middlings. Process efficiency Based on the parmnetric steady, a set of operating conditions was identified, which provided the optimum separation performance. Under these conditions, the C40 Falcon Concentrator reduced the ash content in the +37 lavaparticle size fractions from 14.1% to 6.3% while recovering 92.4% of the coal. The total sulfur content was simultaneously decreased from 3.49% to 1.94%. To evaluate the efficiency of the process, centrifugal washability analyses were conducted on the various particle size fractions in the feed and product samples. The partition curves generated from the analyses are shown in Figure 3. It is clear that more efficient separations were achieved on the coarsest particle size fractions. In addition, lower separation density values are also realized for the coarsest size fractions. This trend is more evident by the results shown in Figure 4. Probable error values for the coarsest size fractions were around 0.12 with separation density (Dso) values of approximately 1.50. These values are in agreement with those obtained from the smaller C10 Falcon Concentrator [2]. As the particle mass decreases with decreasing size, the effect of the centrifugal force reduces, which results in higher Dso values and lower efficiencies. Alternatively, it may be the presence of the larger particle:; that causes the reduced efficiency due to the movement of coarse material toward the bowl wall and the resulting increase in water velocity moving away from the wall. The inward water velocity reduces the ability of the finest particles to report to the underflow during the corresponding retention time. Thus, the efficiency and Dso values are negatively effected. SUMMARY AND CONCLUSIONS A full-scale C40 Falcon concentrator has been successfully evaluated for the treatment of fine coal. The experimental program was conducted using a closed-loop circuit, which provided a maximum volumetric feed rate of 6.7 kL/min. Efficient separation performances were achieved at mass throughput capacity values as high as 78 tonnes/hr. Other significant findings include: .
.
.
The metallurgical performances achieved by the C40 Falcon Concentrator were near those obtained from washability analysis of the 600 x 37 lan particle size fraction of a Pittsburgh No. 8 coal. Organic efficiency values greater than 95% were obtained while recovering 80% of the total mass to the product. For a nominal -600 pm Pittsburgh No. 8 seam coal sample, ash reduction for the -75 pra particle size fraction was more efficiently achieved by a froth flotation process. However, the Falcon ConcenUator provided a better sulfur reduction performance for particle sizes as low as 37 gun. Under optimum operating conditions, probable error values of nearly 0.12 were achieved for particle size fractions greater than 150 pan and exponentially increased as the particle size decreas¢cl. Likewise, the separation density (Ds0) increased from a value of around 1.50 at 400 sJm to a relatively constant value of 1.85 at about 250 pro.
1198
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The effect of particle size on the probable error and separation density achieved by the C40 Falcon Concentrator.
ACKNOWLEDGMENTS The funding for the work presented in this publication was funded in part by the Illinois Clean Coal Institute (Project No. 94-1.1A-1) and CONSOL Inc. The author expresses sincere appmcintion for the assistance of Dr. D. Wang and Mr. Richard Voyles for efforts in constructi~ and maintaining the fine coal cleaning circuit. Gratitude for support is also extended to the SIUC Coal Research Center and Steve McAlister of Falcon Concentrators, Inc.
High capacityfinecoal cleaning
1199
REFERENCES
.
.
3.
.
.
6. 7. 8. 9.
Venkatram~m, P., Luttrell, G.H. & Yoon, R.H., Fine coal cleaning using the multi-gravity separator. High Efficiency Coal Preparation: An International Symposium, S. K. K_awatra, ed., SME, Littleton, Colorado, 109 (1995). Honaker, R.Q., Paul, B.C., Wang, D. & Huang, M., Application of centrifugal washing for finecoal cleaning. Minerals and Metallurgical Processing, May, 80 (1995). Lut~ell, G.H., Honaker, R.Q. & Phillips, D.L, Enhanced gravity separators: new alternatives for fine coal cleaning. Proceedings, 12~ International Coal Preparation Conference, Lexington, Kentucky, :May, 281 (1995). Honaker, ~:.Q., Paul, B.C., Wang, D. & Ho. K., Enhanced gravity separation: an alternative to flotation. High Efficiency Coal Preparation: An International Symposium, S. K. Kawatra, ed., SME, Littleton, Colorado, 69 (1995). Honaker, R..Q., Wang, D., & Ho, K., Application of the Falcon Concentrator for fine coal cleaning. Minerals Engineering, 9(11), 1143 (1996). McAlister, S. & Armstrong, K.C., Development of the Falcon Concentrators. Preprint No. 98-172, SME Annual Meeting, Orlando, Florida, March (1998). Dell, C.C., An improved release analysis procedure for determining coal washability. Journal of the Institute of Fuel, 37, 149 (1964). Forest, W.R., Adel, G.T. & Yoon, R.-H., Characterizing coal flotation performance using release analysis. Coal Preparation, 14, 13 (1994). Mohanty, M.K., Honaker, R.Q., Patwardhan, A. & Ho, K., A modified procedure to measure the ultimate flotation response of coal. Preprint No. 97-178, SME Annual Meeting, Denver, Colorado, February (1997).
Correspondence, on papers published in Minerals Engineering is invited, preferably by email to [email protected], or by Fax to +44-(0)1326-318352
Pergamom~ 0892-6875(98)0010$-8
© 1998 Elsevier Science Ltd All fights reserved o s 9 2 - ~ 5 / 9 8 t $ - - see front matter
[HGH CAPACITY FINE COAL CLEANING USING AN ENHANCED GRAVITY CONCENTRATOR
R.Q. HONAKER Department of Mining & Mineral Resources Engineering, Southern Illinois University - Carbondale, C~xbondale, Illinois 62901-6603, U.S.A. E-mail: [email protected] (Received 22 June 1998; accepted 14 August 1998)
ABSTRACT Recent research has shown that enhanced gravity separators provide the opportunity to effectively clean I mm x 37 micron coal. A detailed experimental program was performed on a continuoas Falcon Concentrator, which was found to achieve separations at mass throughput capacity values in excess of 75 tonnes/hour. The apparent particle separation density for I mm x 75 micron coal was about 1.60 with relatively high efficiency (Ep = 0.12). Typical ash and sulfur rejection values of 85% and 70%, respectively, were achieved while recovering 85% of the combustible material. Metallurgical and efficiency data from the evaluation are presented in this publication. © 1998 Elsevier Science Ltd. All rights reserved
Keywords Fine particle processing, gravity concentration, coal
INTRODUCTION Research reported by several investigators have shown that gravity-based separators are more efficient in the treatment of middling-type particles than froth flotation. However, until recently, high capacity gravitybased systems used to treat particles finer than 212 lan were inefficient. Over the past decade, several enhanced gravity separators have evolved which have the ability to effectively treat particles that are normally concentrated in froth flotation processes. In fact, a patent has recently been received which covers a circuit using an enhanced gravity separator to compliment a froth flotation system for the improved rejection of sulfur for fine coal [1]. The continuous Falcon Concentrator is an enhanced gravity separator that has been found to provide efficient separatious for fine coal cleaning [2]. From a quantitative comparison of the available data from the various enhanced gravity separators, the Falcon Concentrator provided the lowest separation density (Dso) as a result of its ability to provide the maximum centrifugal field of 300 g's [3]. However, to date, all available data has been collected from pilot scale units having mass throughput capacity values less than 5 tonnes/hr.
1191
1192
R.Q. Honaker
This publication will present the results obtained from a full-scale C40 Falcon Concentrator, which was constructed to have a mass throughput capacity of I00 tonnes/hr. Metallurgical and process efficiency data are presented and discussed on a particle size-by-size basis.
EXPERIMENTAL
Unit description Currently, the C40 Falcon Concentrator represents the largest enhanced gravity concentrator worldwide, although development of other technologies may result in units of nearly equal size. The C40 unit measures 100 cm in diameter and possesses the ability to provide up to 210 g's of centrifugal force. The separation mechanism is based on flowing film principles and has been described in previous publications [4,5,6 ]. The unit is supported on a 3 x 3 m 2 frame and stands 4.6 m tall. The bowl is sloped at an angle of 10° from vertical to provide a force parallel to the bowl wall which pushes the packed solids bed towards the 36 discharge chutes equally spaced along the circumference near the top of the bowl. The heavy underflow stream can freely flow through the chutes and out of the nozzles or the flow rate can be regulated by fluctuating pinch valves. Complete monitoring and control of the process parameters is provided through a central control system. The 36 discharge chutes that provide passage for the heavy underflow stream are equipped with a 0.55 cm diameter fixed orifice, which limits the upper particle size to I mm and, to some degree, the upper feed solids content due to potential plugging. However, recent develops of the C40 Falcon unit include the elimination of the fixed valves, which allows the treatment of larger particle sizes without the problem of plugging.
Circuit description The circuit consisted of a 20 kL feed tank in which the feed solids content was adjusted to the desired level. The volumetric feed rate was fixed at the required value by manipulation of a 20 cm gate valve on a recirculation line that sent a portion of the stream directly to the product sump. The feed was passed through a 1.4 mm screen prior to entering the C40 unit to remove material that could cause potential valve plugging. The overflow and underflow streams were combined into a 5.5 kL sump and subsequently returned to the feed tank to complete the closed circuit. Approximately 8 tonnes of fine coal solids were used to charge the system. Typical volumetric feed flow rates evaluated were between 3.7 and 6.7 kL at solid concentrations between 10% and 30% by weight. Other operating parameters include bowl rotational speed and underflow volumetric flow rate. After allowing sufficient time for the process to realize steady-state conditions, a representative sample of each product stream was collected. The volumetric flow rate of the underflow stream was measured and recorded. Each of the samples were wet screened using 150 tin1 (100 mesh) and 37 ~ (400 mesh) sieves. The screened sample were filtered, dried and analyzed for weight, ash, total sulfur and, in some cases, pyritic sulfur contents using ASTIVl procedures. Because of the difficulty in accurately measuring the volumetric flow rates of the product streams, the response variables, which include mass yield, recovery, and ash rejection, were quantified using the two-preduct formula. To evaluate the efficiency of the C40 Concentrator, partition curves were obtained by conducting washability analyses on the various particle size fractions in the feed and product samples. All washability analyses were conducted using a centrifugal technique and a lithium metatungstate (LMT) solution as the medium.
Sample description The fine coal slurry sample was collected from the nominal -0.6 nun (28 mesh) fine circuit feed of an operating coal processing plant treating run-of-mine Pittsburgh No. 8 seam coal. A representative sample of the total bulk was collected for a particle size-by-size analysis. As shown in Table 1, the sample was
High capacity fine coal cleaning
1193
found to be relatively fine with 42% of the material having a particle size below 37 tan (400 mesh). However, the majority of the coal particles existed in the 600 x 300 tnn size fraction, i. e., 26% by weight. The sulfur content was relatively high and distributed irregularly within the coal sample.
TABLE 1
Particle size-by-size analysis results of the nominal -0.6 mm Pittsburgh No. 8 seam coal sample
Particle Size :Fraction (pro) +600 600 x 300 300 x 212 212 x 150 150 x 75 75 x 37 -37 Total
Weight
Ash
(%)
(%)
5.95 20.40 10.59 3.26 9.77 7.93 42.10 100.00
11;26 12.70 13.71 12.32 1Z07 16.32 60.40 33.00
Total Sulfur (%) 3.21 3.32 4.26 3.06 3.37 6.52 2.70 3.40
A centrifugal washability analysis was conducted on the sample from each particle size fraction identified in Table 1 and the samples from each density fraction was analyzed for weight, ash and total sulfur contents. Table 2 shows the overall washability results for the +37 pm (+400 mesh) particle size fractions. The coal sample contained a relatively low-to-moderate amount of middling-type particles with only about 5% of the feed partJicles having a density between 1.6 and 2.0. Thus, the coal sample would be classified as easy-to-clean. TABLE 2
Particle density-by-density analysis results of the + 37 tnn (+400 mesh) mterial in the Pittsburgh No. 8 seam coal sample Specific Gravity Fraction 1.3 Float 1.3 x 1.4 1.4 x 1.6 1.6 x 1.8 1.8 x 2.0 2.0 x 2.2 2.2 Sink Total
Weight
Ash
(%)
(%)
53.73 21.30 7.43 3.47 1.67 1.80 10.60 100.00
2.46 6.10 17.10 30.10 42.80 56.50 70.60 14.15
Total Sulfur (%) 1.28 1.90 3.19 3.70 4.29 4.63 17.03 3.42
In addition, the -75 tan (-200 mesh) and -37 gnu (-400 mesh) particle size fractions, as well as, a representative sample of the bulk was subject to ~'aditionalrelease analyses to obtain the ultimate flotation performance for comparison purposes. The procedure used was introduced by Dell in 1964 [7] and modified by Forrest et al. [8] and Mohanty et al. [9].The results are presented in the following section.
RESULTS AND DISCUSSION Metallurgical results An experimental program based on a statistical design was conducted to evaluate the parametric effects and optimize the perfoJ~nance of the C40 Falcon, A totalof 18 experiments were conducted over a range of operating parameter values. The metallurgical results obtained from the treatment of the Pittsburgh No. 8
1194
R.Q.
Honaker
seam coal sample are compared to the washability curve in Figures 1 (a) and (b). On the basis of both ash and sulfur contents, the yield differential increases as the product quality improves, which reflects moderate inefficiencies in the process and is typical of most commercial separators. The relatively efficient separation is indicated by nearly a 75% rejection of ash beating material and 60% of the sulfur to the tailings stream while recovering 80% of the mass yield to the product. In fact, the organic efficiency (i.e., ratio of actual recovery over theoretical recovery.) is about 96%. 100
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Metallurgical results obtained on the basis of a) product ash and b) product total sulfur contents from the treatment of the +37 pm (400 mesh) pa.,ticle size fnmtion of the Pittsburgh No. 8 cx)aL
High capacityfinecoal cleaning
1195
One of the most important uses of enhanced gravity separation in fine coal cleaning involves the rejection of the pyritic sulfur that is normally recovered in flotation. Table 3 shows the total sulfur reductions realized on a particle size-by-size basis from the treatment of the Pittsburgh No. 8 seam coal using the C40 Falcon unit. Excellent sulfur reductions were achieved for the +75 lan (+200) size fractions while a significant but lower reduction was achieved for the 75 x 37 lan (200 x 400 mesh) size fraction. One should note the high sulfur contents in ~ae tailings material, which should be mostly in the form of pyritic sulfur since a high coal recovery is realized.
Total sulfur reductions achieved on a particle size-by.size basis from the treatment olt' the --600 promPittsburgh No. 8 coal by the C40 Falcon concentrator
TABLE 3
Particle Size Fraction 0an) +6CO 600 x 300 300 x 212 212 x 150 150 x 75 75 x 37
Feed 3.21 3.32 4.26 3.06 3.37 6.52
Total Sulfur (%) Product 1.92 1.67 1.71 1.77 1.77 3.12
Tails 12.30 12.60 9.92 13.70 22.20 20.50
Combustible Recovery (%) 90.8 92.7 92.9 93.6 93.7 94.4
The relatively high separation efficiencies achieved by the C40 Falcon Concentrator were realized at mass throughput capacities up to 78 tonnes/hr as shown in Table 4. Factors that limit the mass throughput capacity of the continuous Falcon concentrators include: .
2. 3.
Excessive volumetric flow rates which provide insufficient residence to time for a proper separation; High feed solid contents which prevent efficient particle stratification; Underflow valves having volumetric constraints, which limits the amount of mass that can flow through the ports.
Metallurgical results obtained for the +37 l~m particle size material showing the ability to achieve efficient separations at high mass throughput values
TABLE 4
Test
1
2 3
Feed Mass Flow (tph) 78 6O 52
Feed 19.4 17.5 13.1
Ash (%) Product 4.71 5.10 6.30
Combustible Recovery (%) 80.0 78.3 92.4
Feed Flow Rate (kL/min) 6.7 5.2 6.7
The test program presented in this publication was unable to identify the maximum acceptable feed rate due to constraints of the circuit, which limited the volumetric capacity to about 6.7 kL/min. Flotation compmison It is well known ~aat enhanced gravity concentrators remain ineffective in the treatment of slimes, i.e. -37 lan material [3]. q~us, froth flotation remains to be needed for recovering the coal in the slimes fraction. In addition, it is possible that, for coals having a small amount of middling type particles, froth flotation may be more efficient for treating particles with a size greater than 37 IJm. To evaluate this possibility, release analyses were conducted on the nominal -600 lan Pittsburgh No. 8 coal sample as well as the -75 la'n and-37 lan particle size fractions. The release data were combined with the C40 Falcon results to simulate treatment of the total sample.
1196
R.Q. Honaker
As shown in Figure 2(a), the most effective separation achieved on the basis of product ash content involved the treatment of the -75 pm particle size fractions using a froth flotation process while the +75 pin fractions were better treated by the Falcon Concentrator. On the other hand, the ultimate separation efficiency on the basis of total sulfur reductions was realized by the treatment of the +37 pin size fractions by the Falcon unit while treating the -37 pin fraction with a flotation process (Figure 2b). The use of froth flotation to treat the entire -600 pin Pittsburgh No. 8 coal sample provided a much inferior separation performance.
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A comparison of the metallurgical separation performances obtained from the treatment of the Pittsburgh No. 8 coal sample using different combinations of treatment strategies with the C40 Falcon Concentrator and a froth flotation process on the basis of (a) ash and (b) total sulfur reductions.
High capacity fine coal cleaning
1197
The results indicate that, for the Pittsburgh No. 8 coal sample, the Falcon was more effective in reducing the total sulfur content in the 75 x 37 lain particle size fraction whereas ash rejection was more efficiently achieved by froth flotation. This finding is due to the differences in solid density and floatability between the ash-forming components (average sp. gr. = 2.7) and coal pyrite (average sp. gr. = 4.8). In other words, as the particle size decreases, the heavier and floatable pyrite particles are more readily rejected in the Falcon Concentrator until the particle mass becomes too small to allow transport to the heavy stream within the corresponding residence time. Below this particle size, separation based on differential flotation kinetics is more effective. Since the ash-forming components are lighter, the transition particle size occurs at a larger value. However, it should be noted that this effect could be counteracted with an increase in centrifugal force at the sacrifice of lost coarse coal. The transitional particle size in the above discussion is a function of the amount of middling type particles in the coal and the floatability of the coal pyrite. As shown in Table 2, the Pittsburgh No. 8 coal sample has a relatively low middlings content. Thus, the Falcon Concentrator may be more efficient than flotation over a larger particle size range for a coal having a greater quantity of middlings. Process efficiency Based on the parmnetric steady, a set of operating conditions was identified, which provided the optimum separation performance. Under these conditions, the C40 Falcon Concentrator reduced the ash content in the +37 lavaparticle size fractions from 14.1% to 6.3% while recovering 92.4% of the coal. The total sulfur content was simultaneously decreased from 3.49% to 1.94%. To evaluate the efficiency of the process, centrifugal washability analyses were conducted on the various particle size fractions in the feed and product samples. The partition curves generated from the analyses are shown in Figure 3. It is clear that more efficient separations were achieved on the coarsest particle size fractions. In addition, lower separation density values are also realized for the coarsest size fractions. This trend is more evident by the results shown in Figure 4. Probable error values for the coarsest size fractions were around 0.12 with separation density (Dso) values of approximately 1.50. These values are in agreement with those obtained from the smaller C10 Falcon Concentrator [2]. As the particle mass decreases with decreasing size, the effect of the centrifugal force reduces, which results in higher Dso values and lower efficiencies. Alternatively, it may be the presence of the larger particle:; that causes the reduced efficiency due to the movement of coarse material toward the bowl wall and the resulting increase in water velocity moving away from the wall. The inward water velocity reduces the ability of the finest particles to report to the underflow during the corresponding retention time. Thus, the efficiency and Dso values are negatively effected. SUMMARY AND CONCLUSIONS A full-scale C40 Falcon concentrator has been successfully evaluated for the treatment of fine coal. The experimental program was conducted using a closed-loop circuit, which provided a maximum volumetric feed rate of 6.7 kL/min. Efficient separation performances were achieved at mass throughput capacity values as high as 78 tonnes/hr. Other significant findings include: .
.
.
The metallurgical performances achieved by the C40 Falcon Concentrator were near those obtained from washability analysis of the 600 x 37 lan particle size fraction of a Pittsburgh No. 8 coal. Organic efficiency values greater than 95% were obtained while recovering 80% of the total mass to the product. For a nominal -600 pm Pittsburgh No. 8 seam coal sample, ash reduction for the -75 pra particle size fraction was more efficiently achieved by a froth flotation process. However, the Falcon ConcenUator provided a better sulfur reduction performance for particle sizes as low as 37 gun. Under optimum operating conditions, probable error values of nearly 0.12 were achieved for particle size fractions greater than 150 pan and exponentially increased as the particle size decreas¢cl. Likewise, the separation density (Ds0) increased from a value of around 1.50 at 400 sJm to a relatively constant value of 1.85 at about 250 pro.
1198
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ACKNOWLEDGMENTS The funding for the work presented in this publication was funded in part by the Illinois Clean Coal Institute (Project No. 94-1.1A-1) and CONSOL Inc. The author expresses sincere appmcintion for the assistance of Dr. D. Wang and Mr. Richard Voyles for efforts in constructi~ and maintaining the fine coal cleaning circuit. Gratitude for support is also extended to the SIUC Coal Research Center and Steve McAlister of Falcon Concentrators, Inc.
High capacityfinecoal cleaning
1199
REFERENCES
.
.
3.
.
.
6. 7. 8. 9.
Venkatram~m, P., Luttrell, G.H. & Yoon, R.H., Fine coal cleaning using the multi-gravity separator. High Efficiency Coal Preparation: An International Symposium, S. K. K_awatra, ed., SME, Littleton, Colorado, 109 (1995). Honaker, R.Q., Paul, B.C., Wang, D. & Huang, M., Application of centrifugal washing for finecoal cleaning. Minerals and Metallurgical Processing, May, 80 (1995). Lut~ell, G.H., Honaker, R.Q. & Phillips, D.L, Enhanced gravity separators: new alternatives for fine coal cleaning. Proceedings, 12~ International Coal Preparation Conference, Lexington, Kentucky, :May, 281 (1995). Honaker, ~:.Q., Paul, B.C., Wang, D. & Ho. K., Enhanced gravity separation: an alternative to flotation. High Efficiency Coal Preparation: An International Symposium, S. K. Kawatra, ed., SME, Littleton, Colorado, 69 (1995). Honaker, R..Q., Wang, D., & Ho, K., Application of the Falcon Concentrator for fine coal cleaning. Minerals Engineering, 9(11), 1143 (1996). McAlister, S. & Armstrong, K.C., Development of the Falcon Concentrators. Preprint No. 98-172, SME Annual Meeting, Orlando, Florida, March (1998). Dell, C.C., An improved release analysis procedure for determining coal washability. Journal of the Institute of Fuel, 37, 149 (1964). Forest, W.R., Adel, G.T. & Yoon, R.-H., Characterizing coal flotation performance using release analysis. Coal Preparation, 14, 13 (1994). Mohanty, M.K., Honaker, R.Q., Patwardhan, A. & Ho, K., A modified procedure to measure the ultimate flotation response of coal. Preprint No. 97-178, SME Annual Meeting, Denver, Colorado, February (1997).
Correspondence, on papers published in Minerals Engineering is invited, preferably by email to [email protected], or by Fax to +44-(0)1326-318352