Development of a characteristic flotation cleaning ...

ELSEVIER

Int. J. Miner. Process. 55 (1999) 231–243

Development of a characteristic flotation cleaning index for fine coal M.K. Mohanty a,Ł , R.Q. Honaker b , B. Govindarajan b a

Richwood Industries, Inc., 707, West 7th Street, Huntington, WV 25704, USA of Mining Engineering, Southern Illinois University, Carbondale, IL 62901-6603, USA

b Department

Received 20 April 1998; accepted 20 October 1998

Abstract Cleaning index values provide valuable information for assessing the ability to concentrate a mineral in an ore using a given physical property and for selecting the appropriate separation technology. In response to the recent developments of advanced froth flotation technologies and flotation characterization procedures, a flotation index (FI) has been developed which can be utilized to quantitatively assess the flotation cleaning potential of a given coal sample. Along with an overall FI value, two separate FI values have been determined for each coal to separately characterize the negative selectivity effects caused by the presence of mixed-phase particles and the suppressed recovery rates caused by a lower degree of hydrophobicity of the coal particles. Separate FI values have been determined for ash and sulfur cleaning, which indicate the superior ash cleaning and relatively inferior sulfur cleaning performances achievable by froth flotation.  1999 Elsevier Science B.V. All rights reserved. Keywords: flotation index; cleaning index; advanced flotation; washability; regression analysis

1. Introduction and background Recently, froth flotation characterization procedures have been investigated and a new procedure, referred to as the advanced flotation washability (AFW) analysis, developed to estimate the ultimate flotation response for a given coal sample (Mohanty et al., 1997, 1998). This procedure can be used to assess the separation efficiency of different flotation technologies at a given product grade. A more useful tool to measure both the cleanability and flotation technology efficiencies should consider the performance over the entire range of product quality values. This measure may be indicated by a single value known as flotation cleaning index or, in short, flotation index (FI). Ł

Corresponding author. Tel.: C1 (304) 525-5436; Fax: C1 (304) 525-8018; E-mail: [email protected]

0301-7516/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 9 8 ) 0 0 0 3 5 - 0

232

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

Table 1 A summarized list of the index values developed by the past investigators Index Types

Investigators (year)

Definition

Yield index

Ityokumbul et al. (1997)

Index of washability (IW)

Govindarajan and Rao (1994)

Floatability index (F1)

Vanangamundi and Rao (1989)

Selectivity index (SI)

Yoon and Luttrell (1989)

Efficiency index

Swanson et al. (1978)

Washability number

Sarkar and Das (1974)

Efficiency index

Tsiperovich and Evtushenko (1959)

Recovery of valuables Ł rejection of gangues A function of the area under the combustible recovery vs. yield and ash recovery vs. yield curve Maximum true recovery of combustible material Ł ash reduction in the concentrate at this maximum recovery 1 (probability of adhesion, Pa of weakly floating particles=Pa of strongly floating particles) Combustible recovery Ł tailings ash=product ash Optimum degree of washability=product ash content at the optimum level Yield Ł tailings ash=product ash

Similar indices have been developed in the past to assess the cleaning potential of coal samples using both flotation and gravity processes, which have been summarized in Table 1. Some of the indices, such as the yield index and the separation efficiency index, are used to compare the relative performances of the flotation or gravity based cleaning operations under various operating conditions, and thus, to determine the optimum separation achievable from any process. However, these indices do not provide any characteristic information on the coal sample. On the other hand, the floatability index developed by Vanangamundi and Rao (1989) and the index of washability developed by Govindarajan and Rao (1994) provide the characteristic information describing the ease of beneficiating a specific sample using flotation and gravity processes, respectively. The floatability index, as defined in Table 1, is only a function of the highest point in the combustible recovery-grade curve and does not recognize the combustible mass in coal as a composition of several species with varying degrees of hydrophobicity. Hence, the significance of the entire selectivity portion of the curve, which will have a characteristic shape for each coal sample because of its unique composition, is completely overlooked. On the other hand, the index of washability, which considers the entire portion of the component recovery versus mass yield relationship, provides a more comprehensive picture of the cleanability of a sample. Based on a 0 to 100 scale, a higher index of washability is indicative of the easier cleanability of a sample and vice versa. However, the approach used to determine the index of washability was found not to be suitable for the flotation data generated from the AFW analysis of most of the samples (B.G. Govindarajan and T.C. Rao, pers. commun., 1998). It was difficult to fit an appropriate equation to the component recovery versus mass yield relationship, especially in the high-recovery region. In addition, the index of washability was originally developed only to assess the separation potential of ash material from the non-ash (combustible) material and, thus, relationships associated with the separation of other components

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

233

such as pyritic sulfur were neglected. Furthermore, the index of washability is based on the overall separation performance with the underlying concept that the separation of pure ash-based and mixed-phase particles from coal occurs in a gradual fashion, thereby providing a nearly constant slope to the entire range of the recovery-grade relationship. This phenomenon may be true for many Gondwana-type coals having very difficult cleaning characteristics and insignificant amounts of free mineral particles. However, most North American coals exhibit different cleaning characteristics due to significantly smaller proportions of middling particles. The pure ash particles are easily separated from the remaining coal mass followed by a relatively difficult separation of middling particles, which provides two distinctly different slopes to the recovery-grade relationship developed from a froth flotation process. Two different flotation cleaning index values, i.e., FI1 and FI2 have been developed to individually characterize each of these regions of the recovery-grade relationship. This publication presents and discusses the results of an investigation aimed at developing a flotation index based on the characteristic performance curve generated from the AFW analysis. Multiple flotation index values have been generated to characterize the ease of cleaning in two different regions (i.e., pure ash separation and middling separation regions) of the performance curve. These indices will also help identify the difference in the cleaning potential of the samples having almost the same overall flotation index values. Separate flotation index values have been calculated to characterize the ash and sulfur cleaning potential of multiple coal samples of a wide range of feed characteristics, ranks and origin. Several potential uses of the flotation index have been suggested.

2. Experimental 2.1. Sample Several coal samples varying in rank, origin and composition were collected to obtain a variety of flotation index values. Some samples were collected from the coal banks maintained by the U.S. Department of Energy and State of Illinois, whereas others were collected from operating mines and refuse ponds to obtain a variety in the nature of the samples. Upon arrival, the dry samples were crushed and ground to the desired particle size using a laboratory jaw crusher followed by a hammer mill. The ground samples were split into representative fractions and stored in sample bags at a temperature of 20ºC to minimize surface oxidation. The flotation feed slurry samples collected from the operating plants were stored in sealed barrels. The ash and total sulfur contents were analyzed according to ASTM procedures for each sample, and the results are listed in Table 2. The coal rank varied from high-volatile A of bituminous type to anthracite, which were collected from the coal seams located in the eastern, mid-western and western part of North America. As shown in Table 2, the ash and total sulfur contents of the samples varied over a wide range from about 10.7% to 49.1% and 0.50% to 4.01%, respectively.

234

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

Table 2 A list of the coal seams with corresponding ranks and characteristics Seam #

Coalberg, WV Garmeada, WV Buckeye Springs, WV Pittsburgh No. 8, PA Upper Coalberg, WV Illinois No. 6, IL Illinois No. 5, IL Blind Canyon, UT Lykens Valley No. 2, PA

Rank

High Vol. A Medium Vol Low Vol. High Vol A. High Vol. A High Vol. B High Vol. A High Vol. A Anthracite

80% passing size (µm)

53 43 105 125 69 125, 20 140, 37 149 177

Feed assays Ash (%)

Total sulfur (%)

49.12 47.68 41.47 23.40 44.57 15.96 18.80 13.10 10.70

1.22 0.86 1.20 2.58 0.87 4.01 2.25 0.47 0.50

2.2. Experimental procedure A representative fraction of each coal sample was subjected to AFW analysis to produce the characteristic flotation performance curves. The first phase of the procedure, which separates the hydrophobic material from the purely hydrophilic minerals, was conducted using a laboratory Denver cell. In the second phase, as prescribed by the AFW procedure, a batch-operated flotation column with a diameter of 5 cm and a length of 140 cm was used to achieve the selective separation among the floatable particles having varying degrees of hydrophobicity. The hydrophobic particles were segregated into seven different fractions according to a decreasing order of hydrophobicity by varying the aeration rate from about 1.0 to nearly 2.5 l=min and collecting separate samples at each air setting. The sample remaining in the cell after the collection of the sixth concentrate fraction forms the seventh fraction, which was collected by flushing the entire cell. The chemical reagents used for the flotation process included kerosene and a poly-glycol as collector and frother, respectively. The reagent package was kept constant for all the samples; however, the dosages varied from sample to sample based on the size distribution and characteristics of the samples. Although the reagent addition was mainly restricted to the first phase of the procedure, addition of a few drops of frother was sometimes necessary in the second phase to facilitate a smooth flow of froth in the batch-operated column.

3. Results and discussion Based on the AFW performance curve generated on a representative coal sample, a ‘flotation index’ (FI) has been developed on a 0 to 100 scale to assign a characteristic quantitative value representing the ease of flotation cleanability of the coal. A high flotation index value suggests an easier cleaning characteristic, whereas a low index value indicates a difficult flotation-cleaning characteristic.

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

235

3.1. Development of the flotation cleaning index To determine the flotation index, characteristic flotation response data were generated by conducting AFW analysis on each coal sample. Initially, an attempt was made to fit a second-order linear equation to the component recovery versus mass yield relationship. However, because of the limited experimental data points in the high-recovery and high mass yield region, fitting an appropriate linear regression equation for most of the coal samples was not possible. Hence, the possibility of fitting exponential equations to several other performance relationships was evaluated. Ultimately, exponential decay equations provided the best fit for the (100-combustible recovery) versus (100-ash rejection) and (100-combustible recovery) versus (100-total sulfur rejection) relationships. The generalized exponential decay equation can be expressed as: y D 100 exp. ax b /

(1)

where y is the (100-combustible recovery), x is the (100-ash or total sulfur rejection) and a and b are the fitting constants. The constant values were determined by fitting the equations to the experimental data obtained from the AFW analysis of each coal sample. As shown in Table 3, high coefficient of correlation (r ) values were obtained from the comparison of the experimental and empirical model results, which indicate the appropriateness for each model fitting. Empirical equations were developed for both ash and total sulfur rejection performances to develop separate flotation indices for ash and sulfur cleaning. The equations generated from each representative sample were compared to the two extreme cleaning possibilities, namely ‘ideal separation’ and ‘no separation’. As shown in the left panel of Fig. 1, for the combustible recovery versus ash rejection relationship for the Illinois No. 5 coal, point A represents the ideal flotation cleaning case, for which a combustible recovery value of 100% will be achieved while rejecting 100% of the ash material. Since the ideal curve must satisfy the boundary conditions, i.e., 100% Table 3 A list of the constant values that best fit the exponential decay equation describing (100-combustible recovery) as a function of (100-ash rejection) and (100-total sulfur rejection) Seam #

Coalberg, WV Garmeada, WV Buckeye Springs, WV Pittsburgh No. 8, PA Upper Coalberg, WV Illinois No. 6, IL Illinois No. 6, IL Illinois No. 5, IL Illinois No. 5, IL Blind Canyon, UT Lykens Valley No. 2, PA

Ash

Total sulfur

a

b

r

a

b

r

0.1595 0.2193 0.1816 0.0456 0.2777 0.0439 0.1070 0.0900 0.1355 0.1376 0.0348

0.8955 0.9794 0.9583 1.0369 0.8115 1.1213 1.0793 1.0258 1.0973 0.8230 1.1576

0.999 0.995 0.995 0.997 0.991 0.995 0.999 0.998 0.998 0.981 0.994

0.0153 0.0086 0.0075 0.0095 0.0175 0.0109 0.0057 0.0097 0.0131 0.0031 0.0101

1.1606 1.2135 1.2738 1.2829 1.1636 1.2353 1.4608 1.3603 1.3207 1.4544 1.2287

0.999 0.995 0.998 0.995 0.994 0.989 0.992 0.989 0.996 0.985 0.989

236

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

Fig. 1. A comparison of the advanced flotation washability performance curve generated on the basis of ash and total sulfur rejection values for the Illinois No. 5 sample with the theoretically ‘best’ and ‘worst’ flotation cleaning scenarios.

combustible recovery at 0% ash rejection and 0% combustible recovery at 100% ash rejection, this curve can be represented by the right-angled curve CAB, shown in Fig. 1, left. On the other hand, the dotted line CB represents the worst scenario where no separation occurs among the combustibles and the ash forming mineral particles. Thus, the combustible recovery and the ash recovery (i.e., 100-ash rejection) values are the same along the ‘no separation’ curve CB. Therefore, the triangular area ABC represents the maximum flotation cleaning that is ideally possible irrespective of the nature of the coal sample. Since the advanced flotation washability curve CDB represents the ultimate ash cleaning performance achievable from the Illinois No. 5 coal, the flotation index (FIash ) for this coal on the basis of ash rejection can be derived using the following equation: FIash D

.area of the triangle ABC/ .area under the curve CDB/ area of the triangle ABC

(2)

R 100 100 exp. ax b / dx .0:5 ð 100 ð 100/ 0 (3) D 0:5 ð 100 ð 100 where x is the (100-ash rejection) and a and b are the fitted constants previously shown in Table 2. The integration term can be further simplified to Z 1 100 n at t e dt (4) b 0 where t D x b and n D .1 b/=b. An analytical integration of the Eq. 4 is not feasible since, upon expansion, the expression provides an infinite series, which does not converge. Therefore, to determine the area under the AFW curve, a numerical integration procedure can be adopted using ‘Simpson’s rule’. The generalized form of the integration-solution-derived using

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

237

Simpson’s rule is as follows: Z l f .x/ dx k 2 3 f .k/ C 4 f .k C h/ C 2 f .k C 2h/ C 4 f .k C 3h/ C 2 f .k C 4h/ C ::: h 5 ³ 4 3 C2 f .k C .2m 2/h/ C 4 f .k C .2m l/h/ C f .l/ (5) where h D .k l/=n and n D 2m discrete strips. Several commercially available mathematical software packages can also be used to integrate such equations and determine the area under the curve. Upon successful calculation of the area under the curve CDB, the flotation index value can be determined by using Eq. 3. A similar exercise can be conducted to calculate a value for the flotation index (FIsulfur ) to describe the ease or difficulty of total sulfur cleaning. The flotation index values for nine different coal samples of varying origin, rank, and feed characteristics were determined by using the aforementioned procedure to describe both ash and sulfur cleaning potential for each coal, which are summarized in Tables 4 and 5, respectively. Predictably, the flotation index values based on total sulfur rejection are consistently lower than the index value based on ash rejection, indicating the superior de-sliming and relatively inferior de-sulfurizing abilities of the froth flotation process. As shown, the coal samples used in this investigation were found to have a wide range of flotation index (FIash ) values with a minimum of 61 to a maximum of 91. The minimum index value corresponded to a coal sample collected from the middling stream of a jig from a coal preparation plant treating the Pittsburgh No. 8 coal seam. The difficult cleaning characteristic of the sample was obvious from its AFW analysis, which provided a reduction in ash content from 23.4% to 18.0% at a combustible recovery value of about 80%. On the other hand, the easy-to-clean sample with an index Table 4 Flotation Index values calculated based on the ash rejection values for several coal samples collected from various coal seams having a wide range of feed characteristics Seam #

Coalberg, WV Garmeada, WV Buckeye Springs, WV Pittsburgh No. 8, PA Upper Coalberg, WV Illinois No. 6, 1BC-105, IL Illinois No. 6, 1BC-105, IL Illinois No. 5, IL Illinois No. 5, IL Blind Canyon, UT Lykens Valley No. 2, PA

Feed ash (%)

49.12 47.68 41.47 23.40 44.57 15.96 15.96 18.80 18.80 13.10 10.70

Flotation index (FI ash ) FI1

FIoverall

FI2

62 64 64 51 63 56 63 61 64 57 55

84 91 88 61 89 69 85 79 88 75 66

22 27 24 11 26 12 21 18 24 18 11

238

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

Table 5 A list of the flotation index values calculated based on the total sulfur rejection values for several coal samples collected from various coal seams having a wide range of feed characteristics Seam #

Coalberg, WV Garmeada, WV Buckeye Springs, WV Pittsburgh No. 8, PA Upper Coalberg, WV Illinois No. 6, 1BC-105, IL Illinois No. 6, 1BC-105, IL Illinois No. 5, IL Illinois No. 5, IL Blind Canyon, UT Lykens Valley No. 2, PA

Feed total sulfur (%) 1.22 0.86 1.20 2.58 0.87 4.01 4.01 2.25 2.25 0.47 0.50

Flotation index (FIsulfur ) FI1

FIoverall

FI2

29 14 17 29 35 27 36 36 46 9.0 23

32 15 19 32 40 30 38 40 51 10 25

3.9 1.3 1.4 2.8 4.8 2.9 2.4 4.2 5.6 0.8 2.3

value of 91, which was collected from a West Virginia coal seam having an ash content of 47.7%, provided a product ash content of about 8.30% at a combustible recovery value of 80%. Although the performance at one point is only compared for illustration purpose, the flotation index is based on the performance generated over the entire range of grade-recovery values. Grinding of the coal to obtain a better separation performance by improving the liberation characteristics is a common procedure, especially with difficult-to-clean coal samples. The enhancement in the floatability of a difficult-to-clean coal is demonstrated by the increased flotation index (FIash ) values for Illinois No. 5 and No. 6 seams shown in Table 4. The Illinois No. 6 coal sample, which was known to be a difficult-to-clean sample, was found to have FIash and FIsulfur values of 69 and 30, respectively, at a d80 particle size of about 125 µm. The index values improved to 85 and 38, respectively, by grinding the sample to a d80 particle size of nearly 20 µm. Similar findings were also obtained for the Illinois No. 5 coal sample, in which case the index values increased from 79 and 40 to 88 and 51, respectively, with a decrease in the d80 from about 140 µm to 37 µm. In light of the above discussion, it may be concluded that the flotation index can be used as a suitable ‘yardstick’ to measure the ease of flotation cleaning of any coal. However, the overall flotation index may fail to identify the subtle differences in the shape of the characteristic flotation response curves obtained from the samples of almost the same flotation index values. For example, it is possible to obtain the performance curves of the shape BEC and BFC, as illustrated in Fig. 2, subject to the nature of the liberation and composition of the feed coal samples having almost the same overall flotation index value. Although the curves BEC and BFC representing two different coal samples appear to be distinctly different, their overall flotation index values will be equal due to the equivalent area under the curves. In addition, a careful observation of the sample AFW performance curve shown in Fig. 1, left, reveals that the entire curve consists of two distinct regions having different slopes. The inflexion point occurs at

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

239

Fig. 2. A simple illustration of the limitation of the overall flotation index value evaluated to characterize the ultimate flotation response of a coal sample.

about a 75% ash rejection level. The differential change in recovery above this point is small whereas the change is large below the 75% ash rejection level. Therefore, it will not be completely appropriate to assess the cleanability of a sample merely from the overall flotation index value. With a view to obtaining a more precise evaluation pertaining to the characteristic of a sample, two different flotation index values can be generated separately, which characterize the two distinctly different areas of separation performance curves, as shown in Fig. 3. FI1 characterizes the area where most of the pure ash particles will be rejected at a minimal loss of combustible recovery, whereas

Fig. 3. An illustration of the concept of multiple flotation indices.

240

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

FI2 represents the area where most of the middling particles will be rejected with a considerable loss of combustible recovery. The performance curves generated from the AFW analysis of coal were found to have an inflexion point between 70% and 90% ash rejection levels. Therefore, an average ash rejection value of 80% was selected as the demarcation between the two flotation index values, i.e., FI1 and FI2 , as shown in Fig. 3. Using the same approach as previously described, the flotation index values were calculated using the following expressions: FI1 D

.area of the triangle BDE/ .area under the curve BF/ area of the triangle ABC

R 100 b .0:5 ð 80 ð 80/ 20 100 exp. ax / dx D 0:5 ð 100 ð 100

(6)

(7)

and FI2 D

.area of the trapezium ACDE/ .area under the curve CF/ area of the triangle ABC

(8)

R 20 b .0:5 ð 180 ð 20/ 0 100 exp. ax / dx (9) D 0:5 ð 100 ð 100 As indicated by analysis of Eqs. 7 and 9, FI 1 and FI2 can have a maximum value of 64 and 36, respectively, for an extremely easy-to-clean coal, which can be considered as the ideal case. The actual values for each coal sample studied in this investigation are listed in Table 4, which shows FI1 values close to 64 for many of the coal samples studied. This finding may indicate the presence of a significant amount of free ash particles in the respective sample. Ideally, a FI1 value close to 64.0 indicates that the free ash particles present in the respective sample constitute about 80% of the total ash content of the sample. The remaining 20% of the ash content are contributed by the middling particles, whose difficulty of cleaning is assessed by FI2 . FI1 is reflective of the degree of hydrophobicity, which is a function of the pure coal surface hydrophobicity and the type of middling particles, whereas FI2 is reflective of the inherent ash content and the amount of middlings and maceral components. As shown in Table 4, the FI2 values for the coal samples have a wide variance from a minimum of 11 for the Pittsburgh No. 8 sample to a maximum of 27 for the Garmeada sample versus an ideal value of 36. Thus, this flotation index value describes the difficulty of cleaning the middling particles present in the individual samples. Similar multiple index values were also developed for characterizing the sulfur cleaning potential, as shown in Table 5. The maximum FI1 value realized for sulfur cleaning was only 46, against the ideal value of 64, indicating the limitation of sulfur cleaning achievable using the flotation process. 3.2. Potential use of flotation index As described previously, several advanced flotation technologies have been commercialized to achieve superior performance for cleaning the fine coal fraction in the coal preparation plants. However, many of these technologies are still in the process of being

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

241

evaluated. Since these technologies are quite established, there may be a well-defined functional relationship describing the separation performance achievable from any technology as a function of its operating parameters. However, this relationship will not be appropriate for all the coal types because of their varying degrees of floatability. Thus, a more appropriate functional relationship will be the one that incorporates the variability in the feed characteristics. In other words, the varying floatability of coal samples should be incorporated into the empirical models developed to predict the metallurgical performances achievable by a specific technology. As described previously, the flotation index (FI), can be suitably used as one of the independent parameters along with the important operating parameters of the concerned technology while developing a functional relationship for a separation performance parameter. Mathematically, the functional relationship will have a generalized form as follows: separation efficiency D f .FI; technology operating parameters/

(10)

This will require conducting experiment at several operating parameter values as well as with several samples having a range of flotation index values. The empirical models developed for the separation efficiency performance will be an appropriate tool to predict the separation performance achievable by the desired technology for cleaning a new coal on the basis of its flotation index, which can be easily obtained by conducting an AFW analysis on a representative sample. Thus, without conducting any actual test work, a precise prediction of performance achievable from any flotation technology may be possible using the flotation index concept and the aforementioned procedure. The third important application of the flotation index may be in deciding an appropriate particle size for achieving a reasonable flotation performance. A study was conducted to evaluate the effect of reducing particle size on the flotation index of an Illinois No. 6 (IBC-105) sample. As illustrated in Fig. 4, a commensurate improvement in both FIash and FIsulfur values was realized with a decrease in the d80 particle size from 125 µm to about 30 µm, apparently due to the improvement in the liberation

Fig. 4. An illustration of the effect of particle size on the flotation index values, both FIash and FIsulfur , for an Illinois No. 6 (IBC-105) coal sample.

242

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

characteristic of the sample. However, reducing the d80 particle size below 30 µm, which required excessive grinding using an attrition mill, did not result in any significant improvement in the flotation index value. Therefore, grinding the IBC-105 coal beyond a d80 particle size of about 30 µm for improving the flotation performance may not be desirable. Thus a flotation index versus particles size relationship generated for a given sample may assist in deciding the optimum particle size necessary, which will have a bearing on the process economics. The other important use for the flotation index is to provide a simplistic tool for the plant managers to decide which coals in their possession would be most suitable or easy to treat using flotation. Furthermore, the reagent package for obtaining the best separation performance from a given coal sample can also be optimized using the flotation index concept. Based on the same procedure described to calculate the flotation index, performance index values can also be calculated to evaluate any physical separation technology for a given coal or mineral sample. These performance index values developed for individual unit operations will provide a convenient tool to the plant manager in comparing the performance achievable from one technology to the other and thus, decide on their suitability for the desired task.

4. Conclusions Based on the ultimate flotation performance provided by the AFW analysis, a characteristic flotation index has been developed which can be utilized to precisely assess flotation cleaning potential of a given coal sample. Based on a 0 to 100 scale, a higher flotation index value will suggest the easier cleaning characteristic, whereas a lower index value will indicate the difficult flotation-cleaning characteristic of a given coal. Multiple flotation index values have been determined to characterize pure ash and middling cleaning potential of the coal samples. Separate flotation index values have been determined to characterize the ash cleaning and total sulfur cleaning potential of several coals that are widely varying in rank, origin and feed characteristics. The flotation index of a coal can be used as a convenient tool to allow a precise prediction of the flotation performance from any technology. A functional relationship can be developed describing the separation performance achievable from a technology as a function of the technological parameters and the flotation index of several coals. This empirical relationship can be used to predict the performance of a new coal on the basis of its flotation index value alone. Another important application of the flotation index may be in the area of flotation technology selection. To clean a coal with a low flotation index, it may be necessary to use the most selective flotation technology. On the other hand, other flotation technologies, which may achieve the desired performances at a higher throughput capacity, may suitably clean a coal with a relatively high flotation index. The flotation index may also prove useful in determining an appropriate particle size necessary to achieve a reasonable flotation performance without allowing excessive grinding.

M.K. Mohanty et al. / Int. J. Miner. Process. 55 (1999) 231–243

243

References Govindarajan, B.G., Rao, T.C., 1994. Indexing the washability characteristics of coal. Int. J. Miner. Process. 42, 285–293. Ityokumbul, M.T., Arnold, B.J., Trubelija, M.P., 1997. Yield Index: a new method for the analysis of flotation results. SME Annual Meeting, Denver, CO, Preprint 97-151. Mohanty, M.K., Honaker, R.Q., Patwardhan, A., Ho, K., 1997. A modified procedure to measure the true flotation response of coal. SME Annual Meeting, Denver, CO, Preprint 97-178. Mohanty, M.K., Honaker, R.Q., Ho, K., 1998. Coal flotation washability: development of an advanced procedure. Coal Preparation 19, 51–67. Sarkar, G.G., Das, H.P., 1974. A world pattern of the optimum ash levels of coals from the washability data of typical coal seams. Fuel 53, 74–84. Swanson, A.R., Firth, B.A., Nicol, S.K., 1978. A convenient index for the assessment of coal cleaning processes. Australas. Inst. Min. Metall. Proc. 268, 57 pp. Tsiperovich, M.V., Evtushenko, V.Ya., 1959. Preparation and Carbonization of Coals. Metallurgizdat, Sverdlovsk, 72 pp. Vanangamundi, M., Rao, T.C., 1989. Development of an index for the floatability of Indian coal fines. Miner. Eng. 2 (4), 511–519. Yoon, R.-H., Luttrell, G.H., 1989. The effect of bubble size on fine particle flotation. Miner. Process. Extract. Metal., pp. 101–122.