Article pubs.acs.org/EF
Separation Development Studies on the Beneficiation of Fine Lignite Coal Tailings by the Knelson Concentrator Eyüp Sabah* and Selçuk Koltka Mining Engineering Department, Afyon Kocatepe University, 03200 Afyonkarahisar, Turkey ABSTRACT: While disposing of fine coal without use was cheaper and more permissible previously, recovering some of the fine coal changes the mass balance of wastes and makes co-disposal or integrated dumps possible. The present study aims to investigate the processing of fine lignite coal tailings with 59.08% ash content from Soma tailings ponds in Turkey using a Knelson concentrator and to identify in both numerical and experimental methods the fundamentals of the reason why the Knelson concentrator must be operated in hindered settling conditions for high efficiency separation of high ash-bearing fine lignite coal tailings. The lignite coal tailings were treated by a two-stage concentration scheme for the recovery of fine clean coal. Pre-enrichment experiment parameters were determined by the Taguchi experimental design method, and the results were interpreted by the SPSS 17.0 program to evaluate the optimum parameter values. After the pre-enrichment process, a coal concentrate (42.60% ash, 0.71% total sulfur, and 3350 kcal/kg calorific value) was obtained with a 37.67% yield of coal using a 44 mm hydrocyclone. Under optimum conditions, an underflow product of hydrocyclone was reprocessed in the Knelson concentrator to reduce the coal ash content. The process yielded a coal product containing 30.51% ash and 4259 kcal/kg base calorific value with a separation efficiency of 19.50% using the Knelson concentrator. Without coal tailings being classified in hydrocyclone, better separation efficiency was achieved under the hindered settling conditions when direct feeding was made to the Knelson concentration at solids concentrations as high as 40%.
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INTRODUCTION Technology is the main source of momentum for the progress of the mining industry today. Therefore, researchers must focus on the production phases for this technology, including exploration, production, and environmental issues. Significant proportions of world mineral sources are disposed as slime during mineral processing. In recent years, especially the late 1990s, some public enterprises and private sectors in Turkey have applied full and hemi mechanized coal production methods. These increasingly popular methods produced coal containing inorganic impurities and very fine-sized grains. Lignite coal mining has showed great progress in Turkey after 1980 because of the increased demand from the industry and heating sectors,1 but with increasing coal usage, the topic of air pollution has become more prominent on official agendas. This has created a need for lignite washing methods. The quality or heating value of domestic lignite coal must be increased to compete with the growing tonnage of imported coal. For similar reasons, there has been a recent increase in the number of coal washing plants in service. The old waste dam of Ege Linyitleri Iṡ ļ etmeleri (ELI)̇ in the Soma-Manisa region of Turkey contains over a million tons of fine and very fine coal waste. Preliminary studies performed on those coal wastes have demonstrated that clay minerals (smectite, illite, and kaolinite), quartz, cristobalite, feldspar, calcite, and dolomite are present. The clay minerals and other inorganic impurities in the coal are very fine, and more than 40% of the waste is under 38 μm. Because minerals of the montmorillonite group are not present in the above-mentioned clays, swelling properties are not expected. The flotation method is commonly used in the industry if physical separation methods, such as gravity, present difficulty in the removal of clay and pyrite minerals from the fine coal. © 2014 American Chemical Society
However, this method is ineffective for the separation of very fine particles, insufficient for oxidized coal processes, and produces lower efficiency for coals with significant clay content. These difficulties have led to the development of new coal technologies. The rapidly increasing advances in the coal preparation industry have enabled separation of very fine coal (ultra-fine coal) enrichment with hydrocyclones (hydrocyclones for desliming), the Falcon and Knelson concentrators, the Kelsey jig, the multi-gravity separator (MGS), and the jet Jameson flotation cell. Luttrell et al.2 tried the Falcon concentrator, Kelsey jig, and MGS as new methods to enrich fine coal. Meanwhile, Honaker et al.3 enriched fine coal with a Falcon concentrator and were able to produce clean coal with a recovery rate of 75%. Rubiere et al.4 also tried the MGS, Knelson concentrator, hydrocyclone, and flotation to remove pyrite from fine coal. Their results showed that flotation is the most effective way for the removal of sulfur. Mohanty and Honaker5 and Cowburn et al.6 used the Jameson flotation cell to enrich fine coal. Honaker and Patil7 used a heavy media drum and optimized the parameters required for enrichment. Honaker et al.8 used an air Knelson concentrator to enrich fine coal with 18% ash to obtain a clean coal with 8% ash content with 80% efficiency. Ö ney and Tanrıverdi9 investigated the application of central composite design (CCD) for modeling and optimization of some operating variables on the Knelson concentrator for fine bituminous coal cleaning. Honaker10 used high-capacity (75 tonne/h) Falcon concentrators running continuously to enrich Received: March 31, 2014 Revised: June 18, 2014 Published: June 18, 2014 4819
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fine coal and removed ash and sulfur with 85 and 70% efficiency, respectively. Free and hindered settling conditions for fine lignite coal using response surface methodology (RSM) through a Falcon concentrator and the effect of the solids concentration of the coal slurry having clayey minerals and centrifugal force to identify the optimum separation conditions for the Falcon concentrator have been proposed in a series of papers.11,12 The application of multiple linear regressions and the Taguchi experimental design method for modeling and optimizing of some operation variables of the MGS and Falcon concentrator for lignite coal cleaning was discussed by Can and Sabah.13 Koca et al.14 enriched fine coal using a shaking table and MGS, reduced the ash content of the coal from 49.19 to 16.78% with 53.12% combustible efficiency using a shaking table, and obtained clean coal of 20.53% ash with 83.94% combustible efficiency using the MGS. Ç içek et al.15 tried the combination of the MGS and shaking table as an effective way to enrich fine coal. They produced a clean coal of 18% ash with a 63% recovery rate from a coal sample of 35.9% ash. Falconer16 studied the old and new technologies and compared the advantages relative to each other. Yerriswamy et al.17 enriched Indian fine coals with a Kelsey jig and investigated the effects of the jig operation parameters on enrichment. Hirajima et al.18 investigated the effect of operation parameters of MGS on fine coal enrichment in terms of clean coal ash and combustible efficiency. They obtained clean coal with 18% ash and a combustible efficiency of 57%. In addition, they determined that, when the amount of wash water was decreased and the drum speed and feed rate were increased, the ash content of the coal products and the yield of the process decreased. Majumder et al.19 used MGS to enrich fine coal and received 14.67% ash containing clean coal with a 71.23% yield from 24.61% ash containing coal. Menendez et al.20 used MGS to enrich fine coal and obtained clean coal containing 20.6% ash. In another study, Majumder et al.21 tried to characterize the separation process in the Knelson concentrator with fine coals and obtained a clean coal containing 17% ash with a recovery percentage of 40% and a yield of 36% ash containing coal. In the recently published work by Uslu et al.,22 desulfurization and cleaning of oxidized fine coal classified into the size fractions of −106, −300 + 106, and −500 + 300 μm by the Knelson concentrator have also been investigated and reported that recovery of the maximum combustible matter, pyritic sulfur rejection, and ash rejection were achieved to be 99.13, 91.60, and 60.94%, respectively; the highest separation efficiencies for the pyritic sulfur and ash were determined to be 67.91 and 39.53%, respectively. The objective of this study was therefore to determine the cleaning possibility of fine lignite coal tailings using a Knelson concentrator and to identify the effect of some operational parameters on the cleaning process. Toward this aim, we first performed pre-concentration tests using first a hydrocyclone followed by the Knelson concentrator. On the basis of the optimum results obtained from the previous experiments, separation tests were conducted using the Knelson concentrator and classification tests were performed. In addition, the study aimed to optimize the operational variables of the hydrocyclone and Knelson concentrator, so that the minimum ash level and the maximum recovery of the clean coal could be achieved using a regression analysis equipped with the mathematical software package SPSS 17.0.
Article
KNELSON CONCENTRATOR: BACKGROUND A new generation of enhanced gravity separators has been developed through artificial gravitational fields for commercial use in the minerals processing industry. Gravity separation technology had been decreasing in popularity for a number of years before the Knelson concentrator was first introduced on a large scale in 1986.23 The Knelson concentrator is a centrifuge device invented in 1978 by Byron Knelson. As shown in Figure 1, the unit of the Knelson concentrator is basically a high-speed,
Figure 1. Schematic of the Knelson concentrator.
ribbed, rotating cone with a drive unit. Fluidization water is fed into the concentrate cone through a series of fluidization holes. Pulp is sieved to separate particles coarser than 4 mm. The material in slurry form is introduced centrally to the bottom of a ribbed inner cone that rotates at high speed. Centrifugal force causes the slurry to sequentially fill the ribs from the bottom to the top within a few seconds. Next, the feed moves upward as a thin film over the conical surface of the sand. Compaction of solids trapped in the cone ribs is prevented by adding water through a series of holes in the cone wall. The resultant trapped solids are gradually fluidized. The Knelson concentrator uses the principles of centrifugation to enhance the gravitational force experienced by feed particles, which aids in the separation process based on particle density. A central perforated cone containing horizontal ribs welded along the inside wall rotates at 400 rpm to generate a force of 60 times the gravitational force. The introduction of further feed starts sorting stages where heavy particles are forced out against the wall and trapped between the ribs, while the lighter particles are carried away by water to the top of the unit. More details of the process are given elsewhere.24
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MATERIALS AND METHODS
Material. The lignite fine coal tailings used in the experiments were taken from slime ponds of Soma ELI ̇ (Manisa/Turkey) discharged of fine refuse of the lignite washing plant. Approximately 1 600 000 m3 of waste has been generated from beneficiation of coal in five slime ponds in ELI ̇ since the 1990s. Because of the segregation of the fine tailings occurred during deposition in the slime ponds, a total of 100 kg of tailings was sampled from five different points from the second slime pond, as seen in Figure 2. These samples were mixed and prepared as representative coal slime samples. Methods. The chemical analysis of the tailings was made by X-ray fluorescence, and the particle size distribution was determined using a Retsch AS200 sieve shaker. Determination of the mineral composition of the sample was carried out by X-ray diffraction (XRD) techniques using a Shimadzu, Rigaku-Giger Flex diffractometer. The tailings density was measured via gas pycnometry using a Quantachrome Ultrapycnometer 1000. Ash and sulfur analyses were carried out according to ISO 117125 and ISO 35126 standard methods, 4820
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Figure 2. Waste ponds. where f, c, and t are ash contents (%) of feed, clean coal, and tailings and F, C, and T are the mass of feed, clean coal, and tailings.
respectively. In addition, the lower heating value (also known as the net calorific value) was determined on the basis of ISO 1928.27 Test Procedure. A series of classification tests were carried out using hydrocyclone to separate the clay and/or carbonate minerals from the coal prior to the Knelson concentrator studies. In these tests, a small hydrocyclone, 44 mm in diameter, was used because of the large amounts of fine and ultrafine particles in the tailings, which present a low cut point. A number of classification tests were carried out to determine the effect of the pulp ratio, feed pressure, and apex and vortex finder diameters. The results were analyzed with SPSS 17 software. The model included a set of equations relevant to the variables, i.e., ash content in the clean coal and coal recovery yield. The underflow product received from the hydrocyclone tests was treated using a laboratory-scale Knelson MD3 concentrator. Prior to initiation of the test, approximately 5 L of slurry with different weight percentages of solids was introduced into a sump. A stirrer equipped into the sump kept the solids in suspension. While the Knelson concentrator was in operation, the feed slurry was pumped into the Knelson concentrator at a flow rate of 1 L/min16 using a peristaltic pump. The products from the clean coal and tailing streams were collected in a steady-state condition. The slurry samples were then collected from the overflow and underflow in a systematic manner. Following each experiment, the products were filtered, dried, and analyzed for ash content to calculate the yield of the coal recovery yield and separation efficiency as follows: t−f coal recovery yield = × 100 t−c
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RESULTS AND DISCUSSION Characterization Tests. The chemical composition of the fine lignite coal tailings is shown in Table 1. As seen in Table 1, Table 1. Chemical Analysis of the Fine Lignite Coal Tailings
⎛ ⎞ C(100 − c) Tt × 100 − ⎜100 − × 100⎟ F(100 − f ) Ff ⎝ ⎠
tailings (%)
SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO LOI
22.20 11.34 1.85 1.12 11.62 0.18 0.81 0.31 0.137 0.02 49.00
the loss on ignition (LOI) of the tailings is 49%. This reveals that the amount of organic material (coal) in the tailings is significant compared to the amount of inorganic impurities in the tailings. The high LOI found in the tailings can be ascribed to organic matter (coal) and CaCO3. The X-ray diffraction (XRD) pattern of the tailings sample shows that the main associated minerals are kaolinite, smectite-type clay minerals, mica/illite, quartz, feldspar, calcite, dolomite, and pyrite with
(1)
separation efficiency =
composition
(2) 4821
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ultrafine particles in the tailings. As shown in Figure 4, the original tailings containing 59.08% ash were pre-concentrated with a 44 mm hydrocyclone in one stage. The process flow diagram illustrates the material balance of products obtained for optimum separation. It is found that the best result is achieved with a 44 mm diameter hydrocyclone at 10% solids. The optimized hydrocyclone featured an apex diameter of 4.5 mm and a vortex finder diameter of 14.3 mm and operated at 0.5 bar pressure. Once optimum hydrocyclone conditions were determined, the concentrate was again treated in the Knelson concentrator. Classification experiments with hydrocyclone yielded a product with an ash value of 42.6% and a combustible recovery of 49.51% upon using the two-stage process advanced before. Hydrocyclone experiment parameters were determined by Taguchi experimental design, and the results were interpreted by the SPSS 17.0 program to evaluate optimum parameter values. Hydrocyclone Modeling. Regression analysis was designed to develop a relationship between the response functions (ash value and coal recovery yield) and operating conditions for the hydrocyclone (feed solids, inlet pressure, and vortex and apex diameters). The experimental results are listed in Table 4. for the ash content of the clean coal
different features of size, density, shape, and surface properties assuming to be of coal. Figure 3 presents the particle size
Figure 3. Particle size distribution curve of the fine lignite coal tailings.
distribution of the tailings with a d80 of 450 μm for tailings. The maximum particle size of the tailings is less than 2 mm. Variation of the ash and sulfur contents of the tailings against the particle size is presented in Table 2. It is clear that the Table 2. Ash and Sulfur Contents as a Function of the Particle Size of the Fine Lignite Coal Tailings particle size (mm)
weight (%)
ash (%)
total sulfur (%)
+1 −1 + 0.5 −0.5 + 0.425 −0.425 + 0.3 −0.3 + 0.150 −0.150 + 0.106 −0.106 + 0.075 −0.075 + 0.038 −0.038 total
6.82 11.41 3.84 7.31 12.08 7.04 4.53 11.33 35.64 100.00
30.24 31.41 39.13 39.85 46.05 53.23 56.72 63.85 75.24 55.03
0.79 0.85 0.78 0.79 0.74 0.61 0.61 0.57 0.31 0.81
Xash = 47.254 − 0.715X vortex + 1.113Xapex − 0.135 Xsolid ratio + 0.930X pressure R2 AH = 56.1% (regression coefficients for ash in the hydrocyclone)
for the recovery of the clean coal Xcoal recovery yield = 1.5 + 3.79X vortex − 3.91Xapex + 0.886Xsolid ratio − 2.95X pressure
(regression coefficients for recovery in the hydrocyclone)
The R2 changes of the used regression model for ash content and coal recovery were calculated as 0.561 and 0.638, respectively. This means that the relation between the apex, vortex, solid ratio, and inlet pressure independent variables to the ash rate dependent variable with the variance of 56.1% and the coal recovery dependent variable is capable of explaining the change in variance at the 63.8% level. A correlation coefficient between 50 and 69% is defined as a “medium level relationship”.28 Therefore, there is a medium level relationship between the obtained results. To test the accuracy of the performed experiment results, the analysis of variation (ANOVA) is applied. Coefficients for the ash level of clean coal and recovery are given in Table 5. When the relationship degree between independent variables and dependent variables is investigated on an ANOVA table, p values for the ash level of clean coal and recovery are 0.022 and 0.007, respectively. In both cases, the values are lesser than 0.05. Additionally, F values of the F test related to the ash content of clean coal and recovery are 4.158 and 5.734, and they are greater than Fchart (4.00). It can be concluded that there is a significant relation between variables. In the model the contributions of vortex and solid ratio variables to the outcome are avoided. The increase is an
Table 3. Characteristics of the Fine Lignite Coal Tailings from Soma Waste Pond parameters
(4)
R2 RH = 63.8%
coarser particles have a higher sulfur content than the finer particles. This may be due to the fact that the coal is more friable than sulfur-bearing mineral. However, the ash content highly increases with the decreasing size. Qualitative and quantitative analysis results were used to interpret the characteristics of the plant tailings, as shown in Table 3. Hydrocyclone Tests. A hydrocyclone with a relatively small diameter (Ø = 44 mm) was chosen for separating
specific gravity (g/cm3) moisture (%) ash (%) total sulfur (%) volatile matter (%) lower calorific value (kcal/kg) higher calorific value (kcal/kg) mineral content
(3)
tailings 1.64 5.33 55.03 0.81 25.16 2472 2591 clay minerals (kaolinite, smectite, and illite), quartz, feldspar, calcite, dolomite, and pyrite 4822
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Figure 4. Schematic diagram of the experiential setup of fine lignite coal tailings.
model. In this stage, the ash values of the realized and unrealized (54 − 18 = 36) investigations are estimated and the optimum input variables are determined. Accordingly, the estimated ash amount is calculated as 39.74% on the basis of 14.3 mm vortex, 3.2 mm apex, 10% solid ratio, and 0.5 bar feeding pressure. On the basis of the results, the lowest ash content and highest coal recovery obtained were 42.60 and 49.51% (experiment 6), respectively (Table 4). The coal sample contained 59.08% ash and 0.58% total sulfur and had a net calorific value of 2148 kcal/kg. After using the hydrocylone at optimum operation conditions, a clean coal sample that was 37.67% weight of the feed was obtained with 42.60% ash, 0.71% total sulfur, and 3350 kcal/kg calorific values, with a recovery rate of 49.51% (Figure 4). Knelson Concentrator Tests. The best underflow product obtained from the hydrocyclone experiments based on the ash level and coal recovery of pre-concentration tests (−0.5 + 0.038 mm) was subjected to the final beneficiation process using the Knelson concentrator. The effect of the fluidization pressure, bowl speed, solid concentrator, and sample collection time on the ash rate of coal was investigated in detail. The parameters used in these tests are presented in Table 6. Modeling and Separation Efficiency of the Knelson Concentrator. A similar regression analysis was performed to the test results for the ash content of the clean coal. The regression model could not be used for the coal recovery yield because the sample could not be collected from 10 of 16
Table 4. Tested Operating Parameters and Their Results for Hydrocyclone Experiments number
vortex (mm)
apex (mm)
solid ratio (%)
feed pressure (bar)
ash (%)
coal recovery yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
14.3 14.3 14.3 14.3 14.3 14.3 14.3 14.3 14.3 11 11 11 11 11 11 11 11 11
3.2 3.2 3.2 4.5 4.5 4.5 6.4 6.4 6.4 3.2 3.2 3.2 4.5 4.5 4.5 6.4 6.4 6.4
3 7 10 3 7 10 3 7 10 3 7 10 3 7 10 3 7 10
0.5 1.5 1 1.5 1 0.5 1 0.5 1.5 0.5 1.5 1 1.5 1 0.5 1 0.5 1.5
42.42 41.43 40.20 42.70 42.75 42.60 41.53 43.61 47.04 44.34 41.42 39.89 47.57 44.87 44.51 44.70 47.66 46.87
34.61 47.61 47.82 37.5 42.85 49.51 40.00 36.36 22.22 26.92 34.48 48.00 19.04 26.92 29.16 26.92 18.18 20.83
important clue for the reason for a decrease in the ash amount. Oruc et al.29 also indicated in their study of the hydrocyclone that the vortex and solid ratio affected the ash rate negatively. An estimated value of ash output is obtained by introducing this 4823
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Table 5. Change in R2 and Coefficients for the Ash Level of Clean Coal and Recovery R
R2
corrected R2
standard deviation
Durbin−Watson
0.749 sum of squares 68.839 53.808 122.646
0.561 df 4 13 17
0.426 average of squares 17.210 4.139
2.036 F 4.158
1.85 p 0.022
model ash
a
1 1
model regression remaining total
R
R2
corrected R2
standard deviation
Durbin−Watson
0.799 sum of squares 1322.051 749.301 2071.352
0.638 df 4 13 17
0.527 average of squares 330.513 57.639
7.592 F 5.734
1.78 p 0.007
model achievement of coal yield
b
1 model regression remaining total
1
a
Ash = dependent variable. bCoal recovery efficiency = dependent variable.
concentrator processing tests for fine lignite coal tailings, and the outcome of test number 13 was designated as the optimal based on the production of the ash content (30.51%) and separation efficiency (19.50%) (Table 7). By taking into consideration the data available in Table 6, the regression model is formulated in eq 5. ash content of the clean coal on the Knelson concentrator
Table 6. Tests Conditions of the Knelson Concentrator number
bowl speed (rpm)
fluidization pressure (psi)
sample collection time (s)
solids concentrator (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
800 800 800 800 1000 1000 1000 1000 1200 1200 1200 1200 1400 1400 1400 1400
3 6 8 10 3 6 8 10 3 6 8 10 3 6 8 10
10 20 30 40 20 10 40 30 30 40 10 20 40 30 20 10
5 10 15 20 15 20 5 10 20 15 10 5 10 5 20 15
ashKC = 37.667 − 0.004Xbowl speed + 0.241X fluidization pressure − 0.045Xsample collection time − 0.050X solid concentration (5)
R2 AKC = 52.5% (regression coefficients for ash in the Knelson concentrator)
In this model, R2 = 0.525 and the fluidization pressure, the bowl speed, the solid concentrator, and the sample collection time in the position of independent variables represent the variance belonging to the ash percent when the dependent variable is 52.5%. Accordingly, the correlation between dependent and
beneficiation experiments using the Knelson concentrator. The variables and results were achieved from the Knelson Table 7. Knelson Parameters and Test Results pre-concentrated coala
clean coal
number
bowl speed (rpm)
fluidization pressure (psi)
sample collection time (s)
solid ratio (%)
ash (%)
amount (g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
800 800 800 800 1000 1000 1000 1000 1200 1200 1200 1200 1400 1400 1400 1400
3 6 8 10 3 6 8 10 3 6 8 10 3 6 8 10
10 20 30 40 20 10 40 30 30 40 10 20 40 30 20 10
5 10 15 20 15 20 5 10 20 15 10 5 10 5 20 15
34.65 37.37 34.73 33.08 33.15 33.00 33.09 35.60 31.20 34.34 33.18 33.77 30.51 32.45 33.18 34.98
5.9 2.6 51.8 18.4 25.8 26.5 20.0 41.1 55.2 71.8 14.3 10.2 37.0 13.7 62.0 15.0
a
tailings ash (%)
45.25
48.91 51.52
48.66 45.72 44.48
amount (g) 0.3 0.8 0.7 1.0 5.3 0.1 0.2 0.3 12.8 1.8 0.2 0.1 15.9 0.4 2.8 1.7
separation efficiency (%)
14.72
18.91 14.55
19.50 16.02 12.84
Particle size, −0.5 + 0.038 mm; ash, 42.60%; total sulfur, 0.71%; and heat value, 3350 kcal/kg. 4824
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separation did not take place depending upon the particle density but the particle size. In this way, the Knelson concentrator works like a hydrocyclone. In other words, the coal sample with low density and higher ash content is unable to achieve g-force and will be reported to the overflow parts under the effect of low and/or high fluidization water pressure. This situation indicated that, in the presence of fine and ultrafine ash-bearing minerals, on the contrary of studies performed with metallurgical and/or coke coals,8,21,31 the Knelson concentrator must be run at higher solid concentrations, lower fluidization water pressure, and high bowl speeds. This led to larger increases in solid ratios (up to 40%) and better separation efficiency because of an increase in the hindered settling in the Knelson concentrator. As a result, it created an autogenous medium. In addition, higher sample collection times up to 120 s in general result in increased separation efficiency and decreased production of the ash content (Figure 6). The density of the autogenous medium is
independent variables is at a moderate level. The model predictions indicated that the bowl speed, the sample collection time, and the solid concentrator variables to the outcome are avoided and that their increase while reducing the fludiziation water improves the product ash level, as shown in Figure 5. The
Figure 5. Effect of variable parameters on product ash.
experiments conducted in the MGS on the same coal sample, Soma lignite, by Ö zgen et al.30 have indicated that the R2 value for the ash level of the clean coal was 0.843. In this study, the ash rate of MGS feed (the optimum underflow product of hydrocyclone) was reduced from 52.65 to 22.89% with a recovery of 60.01% because of short circuiting in the desliming with the hydrocyclone before MGS. The detrimental effect of clay size particles on the separation efficiency of MGS, because of the decreased feed solids content and especially feed pulp viscosity, is well-reduced. In the present study, however, low ash clean coal was not obtained from coal tailings with high separation efficiency using the Knelson concentrator; even the flowchart was the same as our previous study. This indicates that the Knelson concentrator should be operated under hindered settling conditions. In this case, a dense and viscous medium was created after changing the separation conditions from size to the specific gravity dependent that is from free settling to hindered settling. As a result, satisfactory separation with the Knelson concentrator was achieved at higher solids concentrations. Boylu12 also reported that the presence of clay minerals on feed creates the autogenous medium separation conditions with an increase in the solid concentration. It is clear from the model predictions that the enhanced gravity force, which is related to the fluidization water pressure, solid concentration, and sample collection time, has a greater impact on the separation performance of the lignite fine coal tailings in the Knelson concentrator, as shown in Figure 5 and Table 6. Decreasing the bowl speed to less than 1000 rpm reduces the particle velocity toward the underflow discharge points as a result of the decreased centrifugal force. As a consequence, particles of increasing density and ash rate report to the product stream, which elevates the clean coal ash level, as given in Table 6. From this, it can be concluded that the separation was not successful at low or high fluidization water pressures and low gravitational speed. To improve the separation efficiency, the bowl speed must be high and the fluidization water pressure must be lower. Because of ultrafine high ash particles in the feed and a suspended solid ratio, the Knelson concentrator was run at low bowl speeds and the
Figure 6. Effect of the sample collection time on separation and product ash of −0.5 mm tailings (solid ratio, 40%; bowl speed, 1400 rpm; fludization pressure, 3 psi; and flow rate, 1 L/min).
adjusted to a value slightly greater than that of coal, and this causes coal particles to rise to the surface, while the ash-bearing mineral particles sink to the fluidized bed formed in the tailing rings. In this case, the specific gravity of solid suspension reached levels from 1.13 to 1.41 g/cm3 and deteriorated the separation because of the increased pulp viscosity. This was overcome by increasing centrifugal force. If the settling speed of coarse particles is delayed at hindered settling conditions, the settling speed of fine particles will be increased. Therefore, the effect of the particle size will decrease, and the effect of the particle density will increase. As a result, fine and ultrafine highdensity particles will acquire an ability to reach the fluidized bed within the given retention time and, consequently, move toward the underflow parts, while coal particles are unable to force their way into the fluidized bed in each ring and simply pass over the top of the inner bowl because of their low specific 4825
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gravities. For a particle collected within the rings, the required centripetal force (Fc) must be provided by the drag (Fd) and Bagnold (Fb) forces. Fc = Fb + Fd
Article
AUTHOR INFORMATION
Corresponding Author
*Telephone: +90-272-2281423. Fax: +90-2281422. E-mail:
[email protected].
(6)
Notes
Because both Fc and Fb are strongly related to angular velocity (ω), their combined effect may be represented by a net force Fc*. It may therefore be postulated that the probability of a particle being retained within the rings must depend upon the relative extents of this net force and the fluid drag force.32 With the help of fine and ultrafine clay particles, an autogenous medium will be created. At hindered settling conditions, heavy high ash content particles must be retained in the tailing ring for the net centripetal force (Fc) required for a particle orbit. In these conditions, a critical path is not supplied by the drag force. For this reason, the magnitude of the net force varies with bowl speed, particle size, and density and, therefore, must be kept low (Fd < Fc*). Both gold processing and low ash content metallurgical and/ or oxidized fine coal beneficiation applications most commonly sampled are classified on the basis of the particle size and fed into the Knelson concentrator. On the basis of the high ash content, fine lignite coal tailings, and the results obtained from the Knelson concentrator, it is suggested that the sample can be beneficiated directly using the Knelson concentrator followed by hydrocyclone. If the Knelson concentration tests are performed at higher centrifugal forces and hindering settling conditions, the optimum separation conditions will be obtained and, hence, a clean lignite coal with a low ash content will be produced with high separation efficiency. Similar results were also reported for the separation of lignite coal fines under 1 mm size fraction from the Tunçbilek region of Turkey using the Falcon enhanced gravity separator L40 by Boylu et al.33
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge the Scientific and Technological ̇ AK) for the support for Research Council of Turkey (TÜ BIT the investigation presented in this publication.
(1) Sabah, E.; Mart, U.; Ç elik, M. S. The share of coal in Turkey’s primary energy consumption between 1970 and 2000. Madencilik 2007, 41, 31−42 (in Turkish). (2) Luttrell, G. H.; Honaker, R. Q.; Phillips, D. I. Enhanced gravity separators: New alternatives for fine coal cleaning. Proceedings of the 12th International Coal Preparation Exhibition and Conference; Lexington, KY, May 2−4, 1995; pp 281−292. (3) Honaker, R. Q.; Wang, D.; Ho, K. Application of the Falcon concentrator for fine coal cleaning. Miner. Eng. 1996, 9 (11), 1143− 1156. (4) Rubiera, F.; Hall, S. T.; Shab, C. L. Sulfur removal by fine coal cleaning processes. Fuel 1997, 76 (13), 1187−1194. (5) Mohanty, M. K.; Honaker, R. Q. Performance optimization of jameson flotation technology for fine coal cleaning. Miner. Eng. 1999, 12 (4), 367−381. (6) Cowburn, J.; Stona, R.; Bourke, S. Hill, B. Design development of the Jameson cell. Proceedings of the Centenary of Flotation Symposium; Brisbane, Queensland, Australia, June 6−9, 2005; pp 1−24. (7) Honaker, R. Q.; Patil, D. P. Parametric evaluation of a dense medium process using an enhanced gravity separator. Coal. Prep. 2002, 22 (1), 1−17. (8) Honaker, R. Q.; Das, A.; Nombe, M. Improving the separation efficiency of the Knelson concentrator using air injection. Coal. Prep. 2005, 25, 99−116. (9) Ö ney, Ö ; Tanrıverdi, M. Optimization and modeling of fine coal beneficiation by Knelson concentrator using central composite design (CCD). J. Ore Dressing 2012, 14, 11−18. (10) Honaker, R. Q. High capacity fine coal cleaning using an enhanced gravity concentrator. Miner. Eng. 1998, 11 (12), 1191−1199. (11) Boylu, F. Modeling of free and hindered settling conditions for fine coal benefication through a Falcon concentrator. Int. J. Coal Prep. Util. 2013, 33, 277−289. (12) Boylu, F. Autogenous medium fine coal washing through Falcon concentrator. Sep. Sci. Technol. 2014, 49, 627−633. (13) Can, M. F.; Sabah, E. Application of multiple linear regressions and Taguchi design method in clean coal recovery from lignite fine coal tailings: Comparison of multi-gravity separator (MGS) and Falcon concentrator. J. Ore Dressing 2012, 14, 26−32. (14) Koca, H.; Koca, S.; Karaoglu, M. Recovering of fine coal particles from tailing ponds of Ankara−Alpagut−Dodurga coal washing plant. Proceedings of the 11th International Conference on Environmental Issues and Management of Waste in Energy and Mineral Production; Calgary, Alberta, Canada, May 30−June 2, 2000. (15) Ç içek, T.; Cöcen, I.̇ ; Engin, V. T.; Demir, S. Recovery of fine coal from tailings of coal washing plants. Proceedings of the 9th International Mineral Processing Symposium; Cappadocia, Turkey, Sept 18−20, 2002. (16) Falconer, A. Gravity seperation: Old technique/new methods. Phys. Sep. Sci. Eng. 2003, 12 (1), 31−48. (17) Yerriswamy, P.; Majumder, A. K.; Barnwal, J. P.; Govindarajan, B.; Rao, T. C. Study on Kelsey jig treating Indian coal fines. Miner. Process. Extr. Metall. Rev. 2003, 112 (3), 206−248.
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CONCLUSION In this study, the recovery of
