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Ultrafine coal cleaning using spiral concentrators
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Minerals Engineering 20 (2007) 1315–1319 This article is also available online at: www.elsevier.com/locate/mineng
Ultrafine coal cleaning using spiral concentrators R.Q. Honaker *, M. Jain, B.K. Parekh, M. Saracoglu University of Kentucky, Department of Mining Engineering, 234-B Mining and Mineral Resources Building, Lexington, KY 40506-0107, USA Received 29 June 2007; accepted 11 August 2007
Abstract The conventional circuit fine coal circuit typically incorporates spiral concentrators to treat nominal 1 · 0.15 mm material. Pilot and in-plant studies have been performed to determine the operating parameter values needed to achieve optimum separation performances when extending the lower limit of a conventional spiral concentrator to 44 lm. Based on experimental and empirical data, a feed solids concentration of about 12% by weight is required with a feed rate of around 60 l/min per start. Under these conditions, 60% of the ashforming minerals and 48% of the sulfur was rejected from the 210 · 44 lm size fraction of a given coal source. 2007 Elsevier Ltd. All rights reserved. Keywords: Coal; Fine particle processing; Gravity concentration; Process optimization
1. Introduction In a recent survey of worldwide coal preparation practices, it was reported that spiral concentrators treat 6% of the coal treated in processing plants (Kempnich, 2003). The application is typically associated with cleaning the 1 · 0.15 mm fraction of the plant feed. The popularity of spiral concentrators is due to their simplicity, low cost and ability to maintain minimal by-pass of low density particles to the tailings stream. Based on a typical coal cleaning application, spirals effectively treat between 2 and 3 t/h per start at a recommended solid concentration of 30% by weight (Osborne, 1986). The separation performance of a spiral is determined by both feed characteristics and operating parameters associated with the spiral. Feed characteristics include particle sizeby-size weight and density distribution, volumetric flow rate and the solid concentration. Significant research effort has focused on the development of density-based separators over the past two decades for cleaning coal having a particle size below 150 lm. Density-based separators have the potential to be more selec*
Corresponding author. Tel.: +1 859 257 1108; fax: +1 859 323 1962. E-mail address: [email protected] (R.Q. Honaker).
0892-6875/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.08.006
tive than alternative technologies such as froth flotation and the separation principles are better understood by the typical plant operator. Based on research reported by Richards et al. (2000), spiral concentrators have the ability to achieve effective density-based separations for 100 lm materials. Optimization of spiral concentrator performance has been achieved for the cleaning of ultrafine coal using empirical models derived from data obtained from an experimental program. The optimized test conditions were used as a guide in a test program which quantified the separation efficiency achieved over a range of product ash contents for a coal that is classified as relatively difficult-to-clean. The results are presented and discussed in this publication. 2. Experimental 2.1. Material The fine coal was collected from the underflow stream of a deslime screen in an operating preparation plant treating coal extracted from the Coalberg seam in the central Appalachian coalfields (USA). The coal slurry was screened using a 210 lm sieve and the underflow used as feed in the spiral tests. Based on a wet sieve analysis, about 58.89% of
1316
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
Table 1 Particle size-by-size and density-by-density (210 · 44 lm fraction only) results obtained from analyses performed on the fine Coalberg seam coal Incremental particle size analysis
Incremental washability analysis
Particle size fraction (lm) 210 · 150 150 · 75 74 · 45 45
Total
Weight (%)
Ash (%)
Particle density fraction
23.51 23.82 11.56 41.11
26.57 33.87 53.18 71.54
1.3 Float 1.3 · 1.4 1.4 · 1.6 1.6 · 1.8 1.8 · 2.0 2.0 Sink
100.00
49.87
Weight (%)
Ash (%)
47.37 9.28 6.33 2.02 1.37 33.63
4.20 6.05 14.32 27.49 44.58 90.69
100.00
31.48
Table 2 Parameter value ranges used in the 3-level Box–Behnken test program Independent variables
Parameter values 1 Level
0 Level
+1 Level
Volume feed rate (l/min) Feed solids content (%) Splitter position (cm)
46 6 16
76 12 23
106 18 30
validation purposes. The samples from the optimized tests were subjected to washability analysis to develop a partition curve and quantify additional efficiency measurements. 3. Results and discussion
the sample had a particle size in the range of 210 · 25 lm as shown in Table 1. The overall ash content was 49.87%. Density-based washability and flotation release performance data were obtained on the 210 · 44 lm particle size fraction. The washability results were obtained using a solution of lithium metatungstate (LMT) and represent the theoretically optimum performance for a density-based separation. Likewise, release analysis results provide the ultimate performance expected from any froth flotation device and were obtain by a series of rougher-cleaner flotation steps using a laboratory Denver conventional flotation cell.
3.1. Experimental results The experimental program provided a broad range of product ash values as shown in Fig. 1. The spiral concentrator provided a reduction in ash content from 33.5% to a minimum of 11.71%, while recovering 84% of the combustible material. The separation performances are significantly lower than both the theoretical best performance predicted for gravity concentration and froth flotation (release analysis data). However, the simplicity and low cost characteristics of the spiral compared to other alternatives may override the inferior performance.
2.2. Methods All experiments were conducted using a single-start LD4 spiral (MDL, Australia). The spiral was operated in closedcircuit arrangement whereby the product, middling and tailing streams were recycled back to the feed tank. A valve located on the feed line controlled the amount of slurry returned to the feed tank, which, in turn, allowed the adjustment to the desired feed rate. The splitter position was measured from the outside of the center column to the tip of the splitter.
3.2. Model evaluation Table 3 is the ANOVA for the combustible recovery and product ash models. All major statistics indicate that the models can be used for effectively describing the operating parameter effects on the response variables. The models do not have a significant lack of fit (F-value less than F0.05-value and the Prob > F greater than 0.1). The product
2.3. Experimental program A statistically designed test program was performed to obtain the necessary data needed to develop empirical models that accurately describe the effect of three key operating parameter values and their interactive relationships when treating coal in the size range of 210 · 44 lm using a commercially available spiral concentrator. The models were further used to identify sets of optimum parameter values that will maximize combustible recovery over a range of product ash values. The range of operating values for each parameter tested is shown in Table 2. The experimental program was based on a Box–Behnken design and the samples taken of each process stream were screened to remove material having a particle size below 44 lm. After model development, optimum operating conditions were identified by maximizing recovery over a range of product ash values. Subsequent spiral tests were conducted for
Fig. 1. Test results obtained on the basis of combustible recovery versus product ash content from the treatment of 210 · 44 lm coal as compared with density-based washability and flotation release analysis performances.
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
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Table 3 ANOVA table derived for the recovery and product ash models derived from the results achieved from treating 210 · 44 lm coal F value
F0.05 value
Prob > F
R2
Adjusted R2
Combustible recovery model Model 257 9 Lack of 1.51 3 fit
36.8 0.51
3.68 6.59
<0.0001 0.7
0.97
0.95
Product ash model Model 132 7 Lack of 3.51 5 fit
22 0.67
3.29 6.26
<0.0001 0.67
0.94
0.90
Source
Sum of squares
Degrees of freedom
ash model has a lower degree of freedom for model term (5 instead of 7) due to the elimination of two terms (i.e. double interaction of splitter position (C2) and interaction between volumetric feed rate and splitter position (AC)) based on lack of significance evaluation. Removing the two terms improved model prediction as indicated by higher R2 and adjusted R2 values. The coefficient estimate and significance level of the parameter and parameter interactions are provided in Table 4 for the two performance models. The data clearly shows the importance of residence time (governed by volumetric feed rate) in determining combustible recovery and product ash for the fine 210 lm coal, as indicated by the Prob > jtj values less than 0.1 when assessing the coefficient. Also, the feed solid concentration has a significant effect on product ash content, especially in regards to an interactive effect with volumetric feed rate. This finding indicates that particle population has a role in determining spiral performance. Also, similar to previous findings, splitter position and volumetric flow rate jointly determine the product grade. The predicted variation in combustible recovery over the range of volumetric feed flow rates and solid concentrations was relatively small (i.e., 90–95%) while the change in product ash content was significant. Maximum recovery was predicted at the highest volumetric feed rates and lowTable 4 Estimated coefficient values for the parameter and parameter interaction effects; A = volumetric feed rate (l/min), B = feed solids concentration (% by weight), C = splitter position (cm) Factor
A B C A2 B2 C2 AB AC BC
Combustible recovery
Product ash
Coefficient estimate
t-Value
Prob > t
Coefficient t-Value Prob > t estimate
1.67 0.37 5.20 0.62 0.28 1.70 0.09 0.81 0.17
5.35 1.20 16.68 1.43 0.66 3.97 0.20 1.83 0.83
0.0011 0.2692 <0.0001 0.1950 0.5312 0.0054 0.8439 0.1103 0.7150
3.09 0.33 2.34 0.62 0.81 – 0.97 – 0.75
9.46 1.01 7.15 1.38 1.81 – 2.09 – 1.62
<0.0001 0.3378 <0.0001 0.2015 0.1045 – 0.0661 – 0.1395
Fig. 2. Simulated effects of volumetric feed rate and solid concentration on (a) recovery and (b) product ash content when treating 210 · 44 lm coal using a 23 cm outer splitter position.
est feed solid concentration while the minimum product ash content is realized at the opposite end of the parameter value ranges. As shown in Fig. 2, the largest change in product ash content was realized when the flow rate was reduced from 106 l/min to 46 l/min at a feed solid concentration of 18% by weight. The flow rate reduction resulted in a decrease in product ash content from 22% to 14%. The product ash values realized at high feed rates (greater than 90 l/min) and feed solid concentrations show a marked difference when the product splitter is placed at its outermost position. 3.3. Optimized performance The optimum performance curve was identified by the maximum combustible recovery at a given product ash value. The obvious goal is to identify conditions under which the optimum performance curve mimics the washability curve to the best extent possible. The corresponding set of conditions provides operational information for achieving separations along the recovery–grade curve when required. Optimum performance characteristics (Separation density and probable error) can be assessed at different points on the curve. The optimized separation performances and the corresponding operating parameter values were determined using the empirical models are provided in Table 5. An interesting finding is that feed solid concentration and splitter position played the most significant role in achieving the performances on the vertical portion of the recovery–product ash curve, where product ash values less than 15% were predicted. It should be noted, however, that the volumetric
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R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
Table 5 Operating conditions derived from the non-linear optimization study to maximize combustible recovery at a desired product ash value for 210 · 44 lm coal cleaning
Table 7 In-plant separation performance achieved by the SX7 spiral when cleaning nominally 150 lm coal; middling stream assays and weights not provided
Feed rate (l/min)
Solids (%)
Splitter (cm)
Desired product ash (%)
Maximum recovery (%)
Mass flow (t/h)
Particle size fraction (lm)
Feed Ash (%)
Sulfur (%)
Ash (%)
Sulfur (%)
Ash (%)
Sulfur (%)
46 46 46 46 49 58 67 75 83 90
13.2 14.5 15.8 17.1 16.1 14.9 13.7 12.6 11.6 10.7
30 27 23 19 16 16 16 16 16 16
12 13 14 15 16 17 18 19 20 21
83.50 87.70 91.30 94.17 95.75 96.40 96.89 97.23 97.46 97.58
0.38 0.42 0.46 0.50 0.51 0.55 0.58 0.59 0.60 0.61
1000 · 150 150 · 44 44
8.44 19.33 53.74
2.64 3.37 5.77
4.67 11.80 45.66
2.45 2.64 4.44
36.87 56.33 68.68
4.38 5.86 9.91
% of Feed
100.00
feed rates in these conditions were at the lowest value as a result of the optimization constraint. Notably, to increase product ash content, feed solid concentration needs to be increased. Beyond the elbow of the recovery–product ash curve and along the horizontal portion, optimized conditions require a decrease in solid content and an increase in the volumetric flow rate. Five additional experiments were conducted based on the optimized test conditions identified by the empirical models. As shown in Table 6, the process efficiency values from the tests are comparable to the predicted values in Table 5. The role of splitter position in determining separation density is clearly seen from the results achieved for Tests 3 and 4, where elevated separation density values were realized due to locating the splitter at the innermost position. It is also interesting to note that, in both tests, the by-pass of low-density material was negligible. According to Richards et al. (1985) and Hornsby et al. (1993), shifting the splitter position to the inner region increases the separation density and, for most coals, this position is typically associated with a low amount of near density material. Efficiency (Ep) values obtained while treating 210 · 44 lm coal under the optimum conditions appear reasonable and are not far from that obtained while treating 1 · 0.15 mm coal (Osborne, 1986). Table 6 Process performance and efficiency data achieved from the cleaning of 210 · 44 lm coal under optimum conditions in a spiral concentrator Efficiency parameters
Test 1
Test 2
Test 3
Test 4
Test 5
Feed ash (%) Product ash (%) Tailings ash (%) Mass yield (%) Combustible recovery (%) Ash rejection (%) Separation efficiency (%) Separation density (d50) Probable error (Ep) High-density by-pass (%) Low-density by-pass (%) Organic efficiency (%)
36.82 11.71 74.92 60.28 84.23 80.83 65.06 1.89 0.23 15.05 9.77 86.88
35.26 13.30 85.61 69.63 93.25 73.73 66.98 2.06 0.22 19.95 2.00 96.13
36.04 16.14 90.51 73.24 96.03 67.21 63.24 2.17 0.13 27.55 0 98.49
38.45 19.73 91.10 83.02 97.78 48.58 46.36 2.25 0.13 42.36 0 99.78
31.85 8.04 69.10 50.19 74.99 89.50 64.49 1.65 0.22 5.03 16.93 78.11
Product
65.08
Tailings
13.03
The data in Table 6 clearly indicates that it is possible to obtain product ash values between 8% and 12% by maintaining feed rates between 30 and 46 l/min, the feed solid concentration at 12% by weight and the splitter location at 30 cm from the center column. This corresponds to a mass flow rate per spiral start between 0.25 and 0.40 t/h. 3.4. In-plant test An in-plant study was performed to evaluate the separation performance of an SX7 Multotec spiral when applied to treat the underflow of a secondary 15 cm diameter classifying cyclone at a preparation plant that processes Illinois No. 6 seam coal (USA). The nominal 150 · 44 lm spiral feed contained about 12.0% solids by weight. The volumetric and solid mass feed flow rates were 70 l/min and 0.6 t/h, respectively. As shown in Table 7, significant ash and total sulfur reductions were achieved for all size fractions in the feed. For the +44 micron coal, 60% of the ash-forming minerals were rejected while 48.3% rejection of ash was obtained for the overall coal. Likewise, 47.6% total sulfur rejection was achieved. To achieve an acceptable final product, the 44 lm fraction will need to be removed by classification or screening. 4. Conclusions In pilot and in-plant studies, the spiral concentrator was found to be an effective technology for cleaning coal as fine as 44 lm. Separation density values of around 1.8 were achieved with probable error values in the range of 0.2– 0.25. However, optimization of the process parameter values using empirical models developed from experimental data indicates that the mass throughput capacity must be substantially reduced from the typical values reportedly used when cleaning the 1000 · 150 lm fraction. The optimum volumetric flow rate was below 60 l/min while the optimum solids concentration was approximately 12% by weight, which equates to a mass flow rate of 0.6 t/h. All three parameters evaluated, i.e., feed volumetric flow rate, feed solids concentration and splitter position, are significant in regards to achieving the maximum yield possible to the product stream over a range of product ash contents.
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
References Hornsby, D.T., Watson, S.J., Clarkson, C.J., 1993. Fine coal cleaning by spiral and water washing cyclone. Coal Preparation 12, 133–161. Kempnich, R.J., 2003. Coal preparation – a world review. In: Proceedings of the 20th International Coal Preparation Conference, Lexington, Kentucky, pp. 15–40.
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Osborne, D.G., 1986. Fine coal cleaning by gravity methods: a review of current practice. Coal Preparation 2, 207–242. Richards, R.G., MacHunter, D.M., Gates, P.J., Palmer, M.K., 2000. Gravity separation of ultra-fine (0.1 mm) minerals using spiral separators. Minerals Engineering 13 (1), 65–77. Richards, R.G., Hunter, J.L., Holland-Batt, A.B., 1985. Spiral concentrators for fine coal treatment. Coal Preparation 1, 207–229.
Minerals Engineering 20 (2007) 1315–1319 This article is also available online at: www.elsevier.com/locate/mineng
Ultrafine coal cleaning using spiral concentrators R.Q. Honaker *, M. Jain, B.K. Parekh, M. Saracoglu University of Kentucky, Department of Mining Engineering, 234-B Mining and Mineral Resources Building, Lexington, KY 40506-0107, USA Received 29 June 2007; accepted 11 August 2007
Abstract The conventional circuit fine coal circuit typically incorporates spiral concentrators to treat nominal 1 · 0.15 mm material. Pilot and in-plant studies have been performed to determine the operating parameter values needed to achieve optimum separation performances when extending the lower limit of a conventional spiral concentrator to 44 lm. Based on experimental and empirical data, a feed solids concentration of about 12% by weight is required with a feed rate of around 60 l/min per start. Under these conditions, 60% of the ashforming minerals and 48% of the sulfur was rejected from the 210 · 44 lm size fraction of a given coal source. 2007 Elsevier Ltd. All rights reserved. Keywords: Coal; Fine particle processing; Gravity concentration; Process optimization
1. Introduction In a recent survey of worldwide coal preparation practices, it was reported that spiral concentrators treat 6% of the coal treated in processing plants (Kempnich, 2003). The application is typically associated with cleaning the 1 · 0.15 mm fraction of the plant feed. The popularity of spiral concentrators is due to their simplicity, low cost and ability to maintain minimal by-pass of low density particles to the tailings stream. Based on a typical coal cleaning application, spirals effectively treat between 2 and 3 t/h per start at a recommended solid concentration of 30% by weight (Osborne, 1986). The separation performance of a spiral is determined by both feed characteristics and operating parameters associated with the spiral. Feed characteristics include particle sizeby-size weight and density distribution, volumetric flow rate and the solid concentration. Significant research effort has focused on the development of density-based separators over the past two decades for cleaning coal having a particle size below 150 lm. Density-based separators have the potential to be more selec*
Corresponding author. Tel.: +1 859 257 1108; fax: +1 859 323 1962. E-mail address: [email protected] (R.Q. Honaker).
0892-6875/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.08.006
tive than alternative technologies such as froth flotation and the separation principles are better understood by the typical plant operator. Based on research reported by Richards et al. (2000), spiral concentrators have the ability to achieve effective density-based separations for 100 lm materials. Optimization of spiral concentrator performance has been achieved for the cleaning of ultrafine coal using empirical models derived from data obtained from an experimental program. The optimized test conditions were used as a guide in a test program which quantified the separation efficiency achieved over a range of product ash contents for a coal that is classified as relatively difficult-to-clean. The results are presented and discussed in this publication. 2. Experimental 2.1. Material The fine coal was collected from the underflow stream of a deslime screen in an operating preparation plant treating coal extracted from the Coalberg seam in the central Appalachian coalfields (USA). The coal slurry was screened using a 210 lm sieve and the underflow used as feed in the spiral tests. Based on a wet sieve analysis, about 58.89% of
1316
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
Table 1 Particle size-by-size and density-by-density (210 · 44 lm fraction only) results obtained from analyses performed on the fine Coalberg seam coal Incremental particle size analysis
Incremental washability analysis
Particle size fraction (lm) 210 · 150 150 · 75 74 · 45 45
Total
Weight (%)
Ash (%)
Particle density fraction
23.51 23.82 11.56 41.11
26.57 33.87 53.18 71.54
1.3 Float 1.3 · 1.4 1.4 · 1.6 1.6 · 1.8 1.8 · 2.0 2.0 Sink
100.00
49.87
Weight (%)
Ash (%)
47.37 9.28 6.33 2.02 1.37 33.63
4.20 6.05 14.32 27.49 44.58 90.69
100.00
31.48
Table 2 Parameter value ranges used in the 3-level Box–Behnken test program Independent variables
Parameter values 1 Level
0 Level
+1 Level
Volume feed rate (l/min) Feed solids content (%) Splitter position (cm)
46 6 16
76 12 23
106 18 30
validation purposes. The samples from the optimized tests were subjected to washability analysis to develop a partition curve and quantify additional efficiency measurements. 3. Results and discussion
the sample had a particle size in the range of 210 · 25 lm as shown in Table 1. The overall ash content was 49.87%. Density-based washability and flotation release performance data were obtained on the 210 · 44 lm particle size fraction. The washability results were obtained using a solution of lithium metatungstate (LMT) and represent the theoretically optimum performance for a density-based separation. Likewise, release analysis results provide the ultimate performance expected from any froth flotation device and were obtain by a series of rougher-cleaner flotation steps using a laboratory Denver conventional flotation cell.
3.1. Experimental results The experimental program provided a broad range of product ash values as shown in Fig. 1. The spiral concentrator provided a reduction in ash content from 33.5% to a minimum of 11.71%, while recovering 84% of the combustible material. The separation performances are significantly lower than both the theoretical best performance predicted for gravity concentration and froth flotation (release analysis data). However, the simplicity and low cost characteristics of the spiral compared to other alternatives may override the inferior performance.
2.2. Methods All experiments were conducted using a single-start LD4 spiral (MDL, Australia). The spiral was operated in closedcircuit arrangement whereby the product, middling and tailing streams were recycled back to the feed tank. A valve located on the feed line controlled the amount of slurry returned to the feed tank, which, in turn, allowed the adjustment to the desired feed rate. The splitter position was measured from the outside of the center column to the tip of the splitter.
3.2. Model evaluation Table 3 is the ANOVA for the combustible recovery and product ash models. All major statistics indicate that the models can be used for effectively describing the operating parameter effects on the response variables. The models do not have a significant lack of fit (F-value less than F0.05-value and the Prob > F greater than 0.1). The product
2.3. Experimental program A statistically designed test program was performed to obtain the necessary data needed to develop empirical models that accurately describe the effect of three key operating parameter values and their interactive relationships when treating coal in the size range of 210 · 44 lm using a commercially available spiral concentrator. The models were further used to identify sets of optimum parameter values that will maximize combustible recovery over a range of product ash values. The range of operating values for each parameter tested is shown in Table 2. The experimental program was based on a Box–Behnken design and the samples taken of each process stream were screened to remove material having a particle size below 44 lm. After model development, optimum operating conditions were identified by maximizing recovery over a range of product ash values. Subsequent spiral tests were conducted for
Fig. 1. Test results obtained on the basis of combustible recovery versus product ash content from the treatment of 210 · 44 lm coal as compared with density-based washability and flotation release analysis performances.
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
1317
Table 3 ANOVA table derived for the recovery and product ash models derived from the results achieved from treating 210 · 44 lm coal F value
F0.05 value
Prob > F
R2
Adjusted R2
Combustible recovery model Model 257 9 Lack of 1.51 3 fit
36.8 0.51
3.68 6.59
<0.0001 0.7
0.97
0.95
Product ash model Model 132 7 Lack of 3.51 5 fit
22 0.67
3.29 6.26
<0.0001 0.67
0.94
0.90
Source
Sum of squares
Degrees of freedom
ash model has a lower degree of freedom for model term (5 instead of 7) due to the elimination of two terms (i.e. double interaction of splitter position (C2) and interaction between volumetric feed rate and splitter position (AC)) based on lack of significance evaluation. Removing the two terms improved model prediction as indicated by higher R2 and adjusted R2 values. The coefficient estimate and significance level of the parameter and parameter interactions are provided in Table 4 for the two performance models. The data clearly shows the importance of residence time (governed by volumetric feed rate) in determining combustible recovery and product ash for the fine 210 lm coal, as indicated by the Prob > jtj values less than 0.1 when assessing the coefficient. Also, the feed solid concentration has a significant effect on product ash content, especially in regards to an interactive effect with volumetric feed rate. This finding indicates that particle population has a role in determining spiral performance. Also, similar to previous findings, splitter position and volumetric flow rate jointly determine the product grade. The predicted variation in combustible recovery over the range of volumetric feed flow rates and solid concentrations was relatively small (i.e., 90–95%) while the change in product ash content was significant. Maximum recovery was predicted at the highest volumetric feed rates and lowTable 4 Estimated coefficient values for the parameter and parameter interaction effects; A = volumetric feed rate (l/min), B = feed solids concentration (% by weight), C = splitter position (cm) Factor
A B C A2 B2 C2 AB AC BC
Combustible recovery
Product ash
Coefficient estimate
t-Value
Prob > t
Coefficient t-Value Prob > t estimate
1.67 0.37 5.20 0.62 0.28 1.70 0.09 0.81 0.17
5.35 1.20 16.68 1.43 0.66 3.97 0.20 1.83 0.83
0.0011 0.2692 <0.0001 0.1950 0.5312 0.0054 0.8439 0.1103 0.7150
3.09 0.33 2.34 0.62 0.81 – 0.97 – 0.75
9.46 1.01 7.15 1.38 1.81 – 2.09 – 1.62
<0.0001 0.3378 <0.0001 0.2015 0.1045 – 0.0661 – 0.1395
Fig. 2. Simulated effects of volumetric feed rate and solid concentration on (a) recovery and (b) product ash content when treating 210 · 44 lm coal using a 23 cm outer splitter position.
est feed solid concentration while the minimum product ash content is realized at the opposite end of the parameter value ranges. As shown in Fig. 2, the largest change in product ash content was realized when the flow rate was reduced from 106 l/min to 46 l/min at a feed solid concentration of 18% by weight. The flow rate reduction resulted in a decrease in product ash content from 22% to 14%. The product ash values realized at high feed rates (greater than 90 l/min) and feed solid concentrations show a marked difference when the product splitter is placed at its outermost position. 3.3. Optimized performance The optimum performance curve was identified by the maximum combustible recovery at a given product ash value. The obvious goal is to identify conditions under which the optimum performance curve mimics the washability curve to the best extent possible. The corresponding set of conditions provides operational information for achieving separations along the recovery–grade curve when required. Optimum performance characteristics (Separation density and probable error) can be assessed at different points on the curve. The optimized separation performances and the corresponding operating parameter values were determined using the empirical models are provided in Table 5. An interesting finding is that feed solid concentration and splitter position played the most significant role in achieving the performances on the vertical portion of the recovery–product ash curve, where product ash values less than 15% were predicted. It should be noted, however, that the volumetric
1318
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
Table 5 Operating conditions derived from the non-linear optimization study to maximize combustible recovery at a desired product ash value for 210 · 44 lm coal cleaning
Table 7 In-plant separation performance achieved by the SX7 spiral when cleaning nominally 150 lm coal; middling stream assays and weights not provided
Feed rate (l/min)
Solids (%)
Splitter (cm)
Desired product ash (%)
Maximum recovery (%)
Mass flow (t/h)
Particle size fraction (lm)
Feed Ash (%)
Sulfur (%)
Ash (%)
Sulfur (%)
Ash (%)
Sulfur (%)
46 46 46 46 49 58 67 75 83 90
13.2 14.5 15.8 17.1 16.1 14.9 13.7 12.6 11.6 10.7
30 27 23 19 16 16 16 16 16 16
12 13 14 15 16 17 18 19 20 21
83.50 87.70 91.30 94.17 95.75 96.40 96.89 97.23 97.46 97.58
0.38 0.42 0.46 0.50 0.51 0.55 0.58 0.59 0.60 0.61
1000 · 150 150 · 44 44
8.44 19.33 53.74
2.64 3.37 5.77
4.67 11.80 45.66
2.45 2.64 4.44
36.87 56.33 68.68
4.38 5.86 9.91
% of Feed
100.00
feed rates in these conditions were at the lowest value as a result of the optimization constraint. Notably, to increase product ash content, feed solid concentration needs to be increased. Beyond the elbow of the recovery–product ash curve and along the horizontal portion, optimized conditions require a decrease in solid content and an increase in the volumetric flow rate. Five additional experiments were conducted based on the optimized test conditions identified by the empirical models. As shown in Table 6, the process efficiency values from the tests are comparable to the predicted values in Table 5. The role of splitter position in determining separation density is clearly seen from the results achieved for Tests 3 and 4, where elevated separation density values were realized due to locating the splitter at the innermost position. It is also interesting to note that, in both tests, the by-pass of low-density material was negligible. According to Richards et al. (1985) and Hornsby et al. (1993), shifting the splitter position to the inner region increases the separation density and, for most coals, this position is typically associated with a low amount of near density material. Efficiency (Ep) values obtained while treating 210 · 44 lm coal under the optimum conditions appear reasonable and are not far from that obtained while treating 1 · 0.15 mm coal (Osborne, 1986). Table 6 Process performance and efficiency data achieved from the cleaning of 210 · 44 lm coal under optimum conditions in a spiral concentrator Efficiency parameters
Test 1
Test 2
Test 3
Test 4
Test 5
Feed ash (%) Product ash (%) Tailings ash (%) Mass yield (%) Combustible recovery (%) Ash rejection (%) Separation efficiency (%) Separation density (d50) Probable error (Ep) High-density by-pass (%) Low-density by-pass (%) Organic efficiency (%)
36.82 11.71 74.92 60.28 84.23 80.83 65.06 1.89 0.23 15.05 9.77 86.88
35.26 13.30 85.61 69.63 93.25 73.73 66.98 2.06 0.22 19.95 2.00 96.13
36.04 16.14 90.51 73.24 96.03 67.21 63.24 2.17 0.13 27.55 0 98.49
38.45 19.73 91.10 83.02 97.78 48.58 46.36 2.25 0.13 42.36 0 99.78
31.85 8.04 69.10 50.19 74.99 89.50 64.49 1.65 0.22 5.03 16.93 78.11
Product
65.08
Tailings
13.03
The data in Table 6 clearly indicates that it is possible to obtain product ash values between 8% and 12% by maintaining feed rates between 30 and 46 l/min, the feed solid concentration at 12% by weight and the splitter location at 30 cm from the center column. This corresponds to a mass flow rate per spiral start between 0.25 and 0.40 t/h. 3.4. In-plant test An in-plant study was performed to evaluate the separation performance of an SX7 Multotec spiral when applied to treat the underflow of a secondary 15 cm diameter classifying cyclone at a preparation plant that processes Illinois No. 6 seam coal (USA). The nominal 150 · 44 lm spiral feed contained about 12.0% solids by weight. The volumetric and solid mass feed flow rates were 70 l/min and 0.6 t/h, respectively. As shown in Table 7, significant ash and total sulfur reductions were achieved for all size fractions in the feed. For the +44 micron coal, 60% of the ash-forming minerals were rejected while 48.3% rejection of ash was obtained for the overall coal. Likewise, 47.6% total sulfur rejection was achieved. To achieve an acceptable final product, the 44 lm fraction will need to be removed by classification or screening. 4. Conclusions In pilot and in-plant studies, the spiral concentrator was found to be an effective technology for cleaning coal as fine as 44 lm. Separation density values of around 1.8 were achieved with probable error values in the range of 0.2– 0.25. However, optimization of the process parameter values using empirical models developed from experimental data indicates that the mass throughput capacity must be substantially reduced from the typical values reportedly used when cleaning the 1000 · 150 lm fraction. The optimum volumetric flow rate was below 60 l/min while the optimum solids concentration was approximately 12% by weight, which equates to a mass flow rate of 0.6 t/h. All three parameters evaluated, i.e., feed volumetric flow rate, feed solids concentration and splitter position, are significant in regards to achieving the maximum yield possible to the product stream over a range of product ash contents.
R.Q. Honaker et al. / Minerals Engineering 20 (2007) 1315–1319
References Hornsby, D.T., Watson, S.J., Clarkson, C.J., 1993. Fine coal cleaning by spiral and water washing cyclone. Coal Preparation 12, 133–161. Kempnich, R.J., 2003. Coal preparation – a world review. In: Proceedings of the 20th International Coal Preparation Conference, Lexington, Kentucky, pp. 15–40.
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Osborne, D.G., 1986. Fine coal cleaning by gravity methods: a review of current practice. Coal Preparation 2, 207–242. Richards, R.G., MacHunter, D.M., Gates, P.J., Palmer, M.K., 2000. Gravity separation of ultra-fine (0.1 mm) minerals using spiral separators. Minerals Engineering 13 (1), 65–77. Richards, R.G., Hunter, J.L., Holland-Batt, A.B., 1985. Spiral concentrators for fine coal treatment. Coal Preparation 1, 207–229.