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Estimation of grindability from sink-float test data for two different coals
tma~mmat mmut w
mlnERnL PRO(ESSln6 ELSEVIER
Int. J. Miner. Process. 53 (1998) 99-106
Estimation of grindability from sink-float test data for two different coals S. Bhattacharya *, V. Anand 1, p. Banerjee 2 Department of Fuel and Mineral Engineering, Indian School of Mines, Dhanbad 826 004, India Accepted 15 April 1997
Abstract
An investigation conducted with a coking and non-coking coal, shows that grindability of the coals is d,~termined by the grindabilities of its constituent specific gravity fractions and the weight percentage distribution of these same fractions. Though the grindability of the constituent specific gravity fractions of both the coals varies in a systematic manner, certain differences, however, can be observed in the variation pattern between the two coals. It is easier to grind the heaviest fraction cf the coking coal than some of its neighbouring heavy fractions. The observed variation indicates the possibility of accurate estimation of the grindability of both the coals studied from routine sink-float data on the basis of the empirical relationship between cumulative mean specific ~Tavity and HGI of cumulative floats at successive specific gravities. © 1998 Elsevier Science B.V. Keywords: coal preparation; grindability index estimation
1. I n t r o d u c t i o n
Run-of-mine coal requires a certain treatment before it is marketed as end product. The treatment may be simple size preparation involving crushing and screening or elaborate beneficiation involving physical cleaning of coal a n d / o r chemical processing. Crushing is an essential step in all these methods of treatment. Crushers, therefore, can be found to be in operation in coal handling plants, coal preparation plants, coke ovens, etc.
* Corre:~ponding author. Fax: +91 (326) 203042//202380. t Presently with Khetri Copper Concentrator of Hindustan Copper Ltd. 2 Presently with Regional Research Laboratory, Bhopal. 0301-7516/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S03G 1-75 1 6 ( 9 7 ) 0 0 0 6 0 - 4
1O0
S. Bhattacharya et al. / Int. J. Miner. Process. 53 (1998) 99-106
Grindability is an important physical property of coal. It determines to a great extent, among others, the crusher selection, crushing circuit configuration and crushed coal size distribution. Changes in grindability of raw coal affect the crusher performance and the size of the crushed coal. Being a lengthy and complex process, determination of grindability for the operating crusher feeds, is usually not done on a regular basis. Sink-float test, however, is a routine exercise in coal preparation plants to evaluate and cross-check the washability characteristics of the coal treated and the washer performance. Some work has already been carded out on the variation of grindability with specific gravity, in particular for some selected specific gravity fractions or for some simulated washing products. Little work has so far been reported on the variation of grindability with all the constituent specific gravity fractions of any coal and on the estimation of grindability from sink-float data. Objectives of this investigation have, therefore, been to study the variation of grindability, if any, with constituent specific gravity fractions of a coking and a non-coking coal and to explore the possibility of estimation of grindability from sink-float data obtained from routine plant sampling. Grindability has been measured by the Hardgrove grindability index.
2. Previous work Way back in the thirties, it was reported that grindability decreased with the increase in specific gravity of the fraction with the constant weight sample for the coals studied (Yancey and Geer, 1936). It was also shown that for the coal tested, the grindability of a sample composed of a mixture of different specific gravity fractions could be calculated from the grindabilities of the individual fractions (Yancey et al., 1934). Fitton et al. (1957) observed a high HGI value for fractions floating at a specific gravity of less than 1.4, and that a grindability of specific gravity fractions of more than about 1.4, reporting to the middling and tailing, could vary to a large extent. Ghosal et al. (1958) observed that with higher specific gravity fractions, there was a gradual decrease in the HGI with the exception of the highest gravity fraction which generally recorded a higher index value than the values for intermediate fractions. In the large number of coals, mostly coking, studied by Sinha et al. (1969) the grindability of clean coal was highest followed by the middlings and it again rose in the case of sinks. However, they also noted an exception to this trend. In the subsequent years, however, little work has been reported in the literature on the variation of grindability with specific gravity and on the relationship between the two. 3. Experimental work Two coals obtained from two different sources, Bagdigi and Lalmatia, have been used for this investigation. Bagdigi coal is prime-coking in nature, obtained from an underground pit using a semi-mechanised board-and-piUar method of mining. This coal is a major feed to the Patherdih Central Washery. Bagdigi coal samples were collected from the feed and the product of the single roll crusher of the washery. Lalmatia coal is
s. Bhattacharya et al. / Int. J. Miner. Process. 53 (1998) 99-106
101
non-coki~ag in nature, obtained from a mechanised open-cast pit. Lalmatia coal samples were collected from the despatch point of the pit-head coal handling plant. Each sample weighed about 2.0 tonnes. The raw coals were first sieved through 0.6 m m to reject the naturally occurring fines. A head sample was collected from both samples for the determination o f head ash and HGI. To ensure accuracy in the s i n k - f l o a t tests the original samples were separately subjected to size analysis, maintaining a top to bottom ratio o f about 1.4-2.0 to 1.0. A small head sample was collected from each size fraction for ash analysis to cross-check the accu:racy of subsequent s i n k - f l o a t tests. Each size fraction was then subjected to s i n k - f l o a t tests with six specific gravity cuts. Zinc chloride and carbon tetrachloride were used as medium. Each size-specific gravity fraction thus obtained was then subjected to ash analysis and H G I determination. In addition, the average specific gravity was determined for the final sink product of both coals, by using specific gravity bottles. Since the variation of grindability with size is not the subject matter of this paper, results presented hereafter include only those relevant for the variation o f grindability with specific gravity.
3.1. Vari!ation o f H G l with specific gravity Table 1 shows the grindability and weight percentage distribution obtained for the constituent specific gravity fractions for the three coal samples combined. The H G I determined for the head samples of both coals are found to be in excellent agreement with the H G I values calculated by mass balance for the same coals. It can be argued that H G I values obtained for the constituent specific gravity fractions of the Bagdigi coal are
Table 1 Hardgrove grindability indices of Bagdigi and Lalmatia coals - - specific gravity wise variation Specific gravity
Bagdigi
Lalmatia
feed < 1.3 1.3-1.4 1.4-1.5 1.5-1.6 1.6-1.7 1.7-1.8
> 1.8 Total Average Calculated Determined
product
wt.%
HGI
wt.%
HGI
wt.%
HGI
1.0 22.2 28.1 20.8 14.3 6.2 7.4 100.0
95 78 76 73 70 69 71 -
0.5 11.0 42.3 18.3 11.6 6.9 9.4 100.0
95 80 76 74 72 70 72 -
11.6 22.4 24.1 16.9 9.5 15.5 100.0
133 122 117 105 90 74 -
-
74 76
-
75 76
-
109 110
The calcuZ[atedHGI values have been obtained by taking the weighted average of the HGI of the constituent specific gravity fractions; values determined have been obtained experimentally.
S. Bhattacharya et al. ~~Int. J. Miner. Process. 53 (1998) 99-106
102
quite close to each other except for the lightest fraction constituting only 0.5-1.0% of the whole mass of coal. Therefore, such an agreement is not unusual. In the case of the Lalmatia coal, however, substantial differences can be observed in the HGI values obtained for the constituent specific gravity fractions, each of which constitutes about 10% or more of the whole mass of coal. A definite trend can be observed in the variation of HGI with specific gravity for both Bagdigi coal samples. HGI values monotonically decrease with the increase in specific gravity for the lighter and intermediate specific gravity fractions till the minimum values are obtained for the floats at 1.8 followed by an increase in the HGI values for the heaviest fractions which have been arbitrarily assumed to be floating at 2.3 for both the coals. Because of crushing and consequent liberation and migration of particles from one specific gravity fraction to another, HGI values obtained for the same specific gravity fraction of the crusher's feed and product do not often repeat themselves. Variation of HGI with specific gravity for the Lalmatia coal broadly follows the same pattern with the difference that the variation appears to be more consistent and the minimum HGI value is obtained for the heaviest fraction. 3.2. Estimation of HGI It is evident, therefore, that the HGI of the constituent specific gravity fractions of Bagdigi coking coal monotonically decreases with the increase in specific gravity for most of the specific gravity fractions followed by a nominal or moderate increase in HGI values for the heaviest fractions (Table 1). Since the HGI values change their trend in the range of higher specific gravities and the minima obtained do slightly vary between the crusher's feed and product samples, it may be difficult to fit an equation to such data. The relationship obtained for the Lalmatia non-coking coal is more consistent as the HGI of constituent specific gravity fractions steadily decreases with the increase in specific gravity. But in this case too, fitting a linear equation may not lead to an accurate estimation, because of the large scatter of the points. However, on the basis of the sink-float test data, generated through routine plant sampling for washability studies, if cumulative HGI are calculated for the successively higher specific gravity fractions, as is done to calculate cumulative float ash, and these values are plotted against the specific gravities of the same cumulative floats, straight lines do emerge (Fig. 1). But fitting an equation to this kind of curves may not be a scientific proposition. In contrast, on the basis of the same sink-float test results (Table 1) when cumulative mean (arithmetic) specific gravity and HGI of the same cumulative floats are calculated (Table 2) on the basis of weighted average and plotted, it is observed that an excellent linear relationship is obtained between the two for both the head samples of Bagdigi coal and Lalmatia coal (Fig. 2). Equations obtained are as follows: HGIcu m = - 65.5582 SGcum + 172.6564
(1)
HGIcu m = - 27.6325 SGcum + 116.6265
(2)
HGIcu m = -
83.1395
SGeu m + 2 4 3 . 5 4 5 6
(3)
S. Bhattacharya et al. l i n t . J. Miner. Process. 53 (1998) 99-106 140
~
X
~
103
X
Lalmatia coal Bagdigi coal
0
Crusher teed
A Crusherproduct
120
0 .J t, tfl
100 ~E U
-r
80
60
,
1.30
1.50
,
1~70
1.80
r?./
2/30
CUMULATIVE FLOATS AT SPECIFIC GRAVITY Fig. 1. Relationship between HGI of cumulative floats and specific gravities of the same cumulative floats.
Table 2 Cumulative mean specific gravity and HGI of cumulative floats for Bagdigi and Lalmatia coals Specific gravity
Bagdigi
Lalmada
fraction
arithmetic
crusher feed cumulativea
crusher product cumulativea cumulativea
mean
wt.%
wt.%
sp.
HGI
grm < 1.3 1.3-1.4 1.4-1.5 1.5-1.6 1.6-1.7 1.7-1.8 > 1.8
1.25 1.35 1.45 1.55 1.65 1.75 1.95/2.21 b
1.0 23.2 51.3 72.1 86.4 92.6 100.0
1.25 1.35 1.40 1.45 1.48 1.50 1.53
sp.
HGI
wt.%
gym 95 79 77 76 75 75 74
0.5 11.5 53.8 72.1 83.7 90.6 100.0
1.25 1.35 1.43 1.46 1.49 1.51 1.55
sp.
HGI
grm 95 81 77 76 76 75 75
11.6 34.0 58.1 75.0 84.5 100.0
1.33 1.41 1.47 1.51 1.54 1.62
133 126 122 118 115 109
aCumulative values of mean specific gravity and HGI have been calculated on the basis of weighted average of the values of successive specific gravity fractions considered as float products. bl.95 and 2.21 for Bagdigi and Lalmatia coals, respectively.
104
S. Bhattacharya et al. lint. J. Miner. Process. 53 (1998) 99-106 Estimated curves actuat data X Lalmatia coat O Bagdigi coat
lt.O-
130 •
=
o
)
120
It.
110 ,<
100 O Z
9o- ~Eq.~
! !- o. so36~
,o- "'"~'--~...~=~_ 70
,.z
~'.3
,'.4
Eq. Z (',': O.9,a?, l'.S
~'.6
¢.?
CUMULATIVE MEAN SPECIFIC GRAVITY
Fig. 2. Linear relationship between cumulative mean specific gravity and HGI of cumulative floats.
Eqs. (1) and (2) have been obtained for the head sample of Bagdigi coal, the first one obtained by considering all the constituent specific gravity fractions (Table 1), whereas in case of Eq. (2), the float at 1.3 has been combined with the float at 1.4 and the lightest specific gravity fraction has been assumed to be the float at 1.4. This has been done in view of the fact that the float at 1.3 has a HGI value distinctly different from those of the remaining specific gravity fractions and its weight percentage is negligible in comparison to those of the remaining fractions. Moreover, the purpose of this investigation is to develop an equation so as to accurately estimate the grindability of Bagdigi raw coal. It can be observed from Fig. 1 that Eq. (2) obtained by ignoring the float at 1.3 gives a better fit, which is further corroborated by the coefficients of correlation ( r ) between the cumulative mean specific gravity and the HGI of cumulative floats obtained for the two equations. Coefficient r can be found to be 0.9036 and 0.9187 for Eqs. (1) and (2), respectively. It is evident from Fig. 1 that the best relationship is obtained for Lalmatia coal with a correlation coefficient as high as 0.9988.
3.3. Benefits Estimation of HGI for the two coals studied by the statistical relationship described above has the following advantages. As it appears, any change in the specific gravity
s. Bhattacharya et al. ~Int. J. Miner. Process. 53 (1998) 99-106
105
distribution of the raw coal may lead to changes in grindability of the same, and the routine sink-float data can, therefore, be monitored closely to detect any possible change in HGI. Since., sink-float tests are routine exercises in coal preparation plants to evaluate and cross-check the washability characteristics, data generated from the same tests can be used to estimate the grindability without actually determining it, thereby saving considerable time and labour. Since changes in grindability affect the crusher performance and the size of the crushed coal, such a quick estimation would enable the operator to quickly intervene in the process to get the desired process performance. Since, the products obtained from washing the Bagdigi and Lalmatia coal are further crushed in steel and power plants, sink-float data generated from a routine washer performance analysis can also be used to estimate the grindability of washed coal and middling products.
4. Conclusion The Hardgrove grindability indices of Bagdigi and Lalmatia coals appear to be entirely determined by the HGI of constituent specific gravity fractions of these coals and the weight percentage distributions of the same fractions. Two distinct trends can be observed in the variation of HGI with specific gravity. At the maintained level of top-size:z, Lalmatia non-coking coal shows the grindability to be monotonically decreasing with the increase in specific gravity, with the smallest HGI for the heaviest fraction. Bagdigi coking coal though, shows the same broad trend but does not record the smallest HGI for the heaviest fractions. A linear relationship, however, appears to exist for both coals between the cumulative mean specific gravity and the HGI of the same cumulative float products. As a result, it appears to be possible to accurately estimate through empirical relationship the HGI of both coals from the routine sink-float data. Such an estimation offers a number of practical advantages.
Acknowledgements The presentation of this paper would not have been possible without the kind cooperation of Prof. D.D. Mishra of our department.
References Fitton, A., Hughes, T.H., Hurtley, T.F., 1957. Grindability of British coals - - a laboratory examination. J. Inst. Fuels 30, 54-65. Ghosal, A., Roy, L.C., Bose, R.N., Lahiri, A., 1958. A study on grindability of coal. J. Inst. Fuels 31, 34-43.
106
S. Bhattacharya et al. l i n t . J. Miner. Process. 53 (1998) 99-106
Sinha, N.C., Nair, C.P.P., Bagchi, S., 1969. Studies on the Hardgrove grindability of coals. J. Min. Met. Fuels 18, 265-273. Yancey, H.F., Geer, M.R., 1936. Further investigation of methods for estimating the grindability of coal. Trans. A1ME 119, 353-357. Yancey, H.F., Furse, O.L., Blackburn, R.A., 1934. Estimation of the grindability of coal. Trans. AIME 108, 267-275.
mlnERnL PRO(ESSln6 ELSEVIER
Int. J. Miner. Process. 53 (1998) 99-106
Estimation of grindability from sink-float test data for two different coals S. Bhattacharya *, V. Anand 1, p. Banerjee 2 Department of Fuel and Mineral Engineering, Indian School of Mines, Dhanbad 826 004, India Accepted 15 April 1997
Abstract
An investigation conducted with a coking and non-coking coal, shows that grindability of the coals is d,~termined by the grindabilities of its constituent specific gravity fractions and the weight percentage distribution of these same fractions. Though the grindability of the constituent specific gravity fractions of both the coals varies in a systematic manner, certain differences, however, can be observed in the variation pattern between the two coals. It is easier to grind the heaviest fraction cf the coking coal than some of its neighbouring heavy fractions. The observed variation indicates the possibility of accurate estimation of the grindability of both the coals studied from routine sink-float data on the basis of the empirical relationship between cumulative mean specific ~Tavity and HGI of cumulative floats at successive specific gravities. © 1998 Elsevier Science B.V. Keywords: coal preparation; grindability index estimation
1. I n t r o d u c t i o n
Run-of-mine coal requires a certain treatment before it is marketed as end product. The treatment may be simple size preparation involving crushing and screening or elaborate beneficiation involving physical cleaning of coal a n d / o r chemical processing. Crushing is an essential step in all these methods of treatment. Crushers, therefore, can be found to be in operation in coal handling plants, coal preparation plants, coke ovens, etc.
* Corre:~ponding author. Fax: +91 (326) 203042//202380. t Presently with Khetri Copper Concentrator of Hindustan Copper Ltd. 2 Presently with Regional Research Laboratory, Bhopal. 0301-7516/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S03G 1-75 1 6 ( 9 7 ) 0 0 0 6 0 - 4
1O0
S. Bhattacharya et al. / Int. J. Miner. Process. 53 (1998) 99-106
Grindability is an important physical property of coal. It determines to a great extent, among others, the crusher selection, crushing circuit configuration and crushed coal size distribution. Changes in grindability of raw coal affect the crusher performance and the size of the crushed coal. Being a lengthy and complex process, determination of grindability for the operating crusher feeds, is usually not done on a regular basis. Sink-float test, however, is a routine exercise in coal preparation plants to evaluate and cross-check the washability characteristics of the coal treated and the washer performance. Some work has already been carded out on the variation of grindability with specific gravity, in particular for some selected specific gravity fractions or for some simulated washing products. Little work has so far been reported on the variation of grindability with all the constituent specific gravity fractions of any coal and on the estimation of grindability from sink-float data. Objectives of this investigation have, therefore, been to study the variation of grindability, if any, with constituent specific gravity fractions of a coking and a non-coking coal and to explore the possibility of estimation of grindability from sink-float data obtained from routine plant sampling. Grindability has been measured by the Hardgrove grindability index.
2. Previous work Way back in the thirties, it was reported that grindability decreased with the increase in specific gravity of the fraction with the constant weight sample for the coals studied (Yancey and Geer, 1936). It was also shown that for the coal tested, the grindability of a sample composed of a mixture of different specific gravity fractions could be calculated from the grindabilities of the individual fractions (Yancey et al., 1934). Fitton et al. (1957) observed a high HGI value for fractions floating at a specific gravity of less than 1.4, and that a grindability of specific gravity fractions of more than about 1.4, reporting to the middling and tailing, could vary to a large extent. Ghosal et al. (1958) observed that with higher specific gravity fractions, there was a gradual decrease in the HGI with the exception of the highest gravity fraction which generally recorded a higher index value than the values for intermediate fractions. In the large number of coals, mostly coking, studied by Sinha et al. (1969) the grindability of clean coal was highest followed by the middlings and it again rose in the case of sinks. However, they also noted an exception to this trend. In the subsequent years, however, little work has been reported in the literature on the variation of grindability with specific gravity and on the relationship between the two. 3. Experimental work Two coals obtained from two different sources, Bagdigi and Lalmatia, have been used for this investigation. Bagdigi coal is prime-coking in nature, obtained from an underground pit using a semi-mechanised board-and-piUar method of mining. This coal is a major feed to the Patherdih Central Washery. Bagdigi coal samples were collected from the feed and the product of the single roll crusher of the washery. Lalmatia coal is
s. Bhattacharya et al. / Int. J. Miner. Process. 53 (1998) 99-106
101
non-coki~ag in nature, obtained from a mechanised open-cast pit. Lalmatia coal samples were collected from the despatch point of the pit-head coal handling plant. Each sample weighed about 2.0 tonnes. The raw coals were first sieved through 0.6 m m to reject the naturally occurring fines. A head sample was collected from both samples for the determination o f head ash and HGI. To ensure accuracy in the s i n k - f l o a t tests the original samples were separately subjected to size analysis, maintaining a top to bottom ratio o f about 1.4-2.0 to 1.0. A small head sample was collected from each size fraction for ash analysis to cross-check the accu:racy of subsequent s i n k - f l o a t tests. Each size fraction was then subjected to s i n k - f l o a t tests with six specific gravity cuts. Zinc chloride and carbon tetrachloride were used as medium. Each size-specific gravity fraction thus obtained was then subjected to ash analysis and H G I determination. In addition, the average specific gravity was determined for the final sink product of both coals, by using specific gravity bottles. Since the variation of grindability with size is not the subject matter of this paper, results presented hereafter include only those relevant for the variation o f grindability with specific gravity.
3.1. Vari!ation o f H G l with specific gravity Table 1 shows the grindability and weight percentage distribution obtained for the constituent specific gravity fractions for the three coal samples combined. The H G I determined for the head samples of both coals are found to be in excellent agreement with the H G I values calculated by mass balance for the same coals. It can be argued that H G I values obtained for the constituent specific gravity fractions of the Bagdigi coal are
Table 1 Hardgrove grindability indices of Bagdigi and Lalmatia coals - - specific gravity wise variation Specific gravity
Bagdigi
Lalmatia
feed < 1.3 1.3-1.4 1.4-1.5 1.5-1.6 1.6-1.7 1.7-1.8
> 1.8 Total Average Calculated Determined
product
wt.%
HGI
wt.%
HGI
wt.%
HGI
1.0 22.2 28.1 20.8 14.3 6.2 7.4 100.0
95 78 76 73 70 69 71 -
0.5 11.0 42.3 18.3 11.6 6.9 9.4 100.0
95 80 76 74 72 70 72 -
11.6 22.4 24.1 16.9 9.5 15.5 100.0
133 122 117 105 90 74 -
-
74 76
-
75 76
-
109 110
The calcuZ[atedHGI values have been obtained by taking the weighted average of the HGI of the constituent specific gravity fractions; values determined have been obtained experimentally.
S. Bhattacharya et al. ~~Int. J. Miner. Process. 53 (1998) 99-106
102
quite close to each other except for the lightest fraction constituting only 0.5-1.0% of the whole mass of coal. Therefore, such an agreement is not unusual. In the case of the Lalmatia coal, however, substantial differences can be observed in the HGI values obtained for the constituent specific gravity fractions, each of which constitutes about 10% or more of the whole mass of coal. A definite trend can be observed in the variation of HGI with specific gravity for both Bagdigi coal samples. HGI values monotonically decrease with the increase in specific gravity for the lighter and intermediate specific gravity fractions till the minimum values are obtained for the floats at 1.8 followed by an increase in the HGI values for the heaviest fractions which have been arbitrarily assumed to be floating at 2.3 for both the coals. Because of crushing and consequent liberation and migration of particles from one specific gravity fraction to another, HGI values obtained for the same specific gravity fraction of the crusher's feed and product do not often repeat themselves. Variation of HGI with specific gravity for the Lalmatia coal broadly follows the same pattern with the difference that the variation appears to be more consistent and the minimum HGI value is obtained for the heaviest fraction. 3.2. Estimation of HGI It is evident, therefore, that the HGI of the constituent specific gravity fractions of Bagdigi coking coal monotonically decreases with the increase in specific gravity for most of the specific gravity fractions followed by a nominal or moderate increase in HGI values for the heaviest fractions (Table 1). Since the HGI values change their trend in the range of higher specific gravities and the minima obtained do slightly vary between the crusher's feed and product samples, it may be difficult to fit an equation to such data. The relationship obtained for the Lalmatia non-coking coal is more consistent as the HGI of constituent specific gravity fractions steadily decreases with the increase in specific gravity. But in this case too, fitting a linear equation may not lead to an accurate estimation, because of the large scatter of the points. However, on the basis of the sink-float test data, generated through routine plant sampling for washability studies, if cumulative HGI are calculated for the successively higher specific gravity fractions, as is done to calculate cumulative float ash, and these values are plotted against the specific gravities of the same cumulative floats, straight lines do emerge (Fig. 1). But fitting an equation to this kind of curves may not be a scientific proposition. In contrast, on the basis of the same sink-float test results (Table 1) when cumulative mean (arithmetic) specific gravity and HGI of the same cumulative floats are calculated (Table 2) on the basis of weighted average and plotted, it is observed that an excellent linear relationship is obtained between the two for both the head samples of Bagdigi coal and Lalmatia coal (Fig. 2). Equations obtained are as follows: HGIcu m = - 65.5582 SGcum + 172.6564
(1)
HGIcu m = - 27.6325 SGcum + 116.6265
(2)
HGIcu m = -
83.1395
SGeu m + 2 4 3 . 5 4 5 6
(3)
S. Bhattacharya et al. l i n t . J. Miner. Process. 53 (1998) 99-106 140
~
X
~
103
X
Lalmatia coal Bagdigi coal
0
Crusher teed
A Crusherproduct
120
0 .J t, tfl
100 ~E U
-r
80
60
,
1.30
1.50
,
1~70
1.80
r?./
2/30
CUMULATIVE FLOATS AT SPECIFIC GRAVITY Fig. 1. Relationship between HGI of cumulative floats and specific gravities of the same cumulative floats.
Table 2 Cumulative mean specific gravity and HGI of cumulative floats for Bagdigi and Lalmatia coals Specific gravity
Bagdigi
Lalmada
fraction
arithmetic
crusher feed cumulativea
crusher product cumulativea cumulativea
mean
wt.%
wt.%
sp.
HGI
grm < 1.3 1.3-1.4 1.4-1.5 1.5-1.6 1.6-1.7 1.7-1.8 > 1.8
1.25 1.35 1.45 1.55 1.65 1.75 1.95/2.21 b
1.0 23.2 51.3 72.1 86.4 92.6 100.0
1.25 1.35 1.40 1.45 1.48 1.50 1.53
sp.
HGI
wt.%
gym 95 79 77 76 75 75 74
0.5 11.5 53.8 72.1 83.7 90.6 100.0
1.25 1.35 1.43 1.46 1.49 1.51 1.55
sp.
HGI
grm 95 81 77 76 76 75 75
11.6 34.0 58.1 75.0 84.5 100.0
1.33 1.41 1.47 1.51 1.54 1.62
133 126 122 118 115 109
aCumulative values of mean specific gravity and HGI have been calculated on the basis of weighted average of the values of successive specific gravity fractions considered as float products. bl.95 and 2.21 for Bagdigi and Lalmatia coals, respectively.
104
S. Bhattacharya et al. lint. J. Miner. Process. 53 (1998) 99-106 Estimated curves actuat data X Lalmatia coat O Bagdigi coat
lt.O-
130 •
=
o
)
120
It.
110 ,<
100 O Z
9o- ~Eq.~
! !- o. so36~
,o- "'"~'--~...~=~_ 70
,.z
~'.3
,'.4
Eq. Z (',': O.9,a?, l'.S
~'.6
¢.?
CUMULATIVE MEAN SPECIFIC GRAVITY
Fig. 2. Linear relationship between cumulative mean specific gravity and HGI of cumulative floats.
Eqs. (1) and (2) have been obtained for the head sample of Bagdigi coal, the first one obtained by considering all the constituent specific gravity fractions (Table 1), whereas in case of Eq. (2), the float at 1.3 has been combined with the float at 1.4 and the lightest specific gravity fraction has been assumed to be the float at 1.4. This has been done in view of the fact that the float at 1.3 has a HGI value distinctly different from those of the remaining specific gravity fractions and its weight percentage is negligible in comparison to those of the remaining fractions. Moreover, the purpose of this investigation is to develop an equation so as to accurately estimate the grindability of Bagdigi raw coal. It can be observed from Fig. 1 that Eq. (2) obtained by ignoring the float at 1.3 gives a better fit, which is further corroborated by the coefficients of correlation ( r ) between the cumulative mean specific gravity and the HGI of cumulative floats obtained for the two equations. Coefficient r can be found to be 0.9036 and 0.9187 for Eqs. (1) and (2), respectively. It is evident from Fig. 1 that the best relationship is obtained for Lalmatia coal with a correlation coefficient as high as 0.9988.
3.3. Benefits Estimation of HGI for the two coals studied by the statistical relationship described above has the following advantages. As it appears, any change in the specific gravity
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distribution of the raw coal may lead to changes in grindability of the same, and the routine sink-float data can, therefore, be monitored closely to detect any possible change in HGI. Since., sink-float tests are routine exercises in coal preparation plants to evaluate and cross-check the washability characteristics, data generated from the same tests can be used to estimate the grindability without actually determining it, thereby saving considerable time and labour. Since changes in grindability affect the crusher performance and the size of the crushed coal, such a quick estimation would enable the operator to quickly intervene in the process to get the desired process performance. Since, the products obtained from washing the Bagdigi and Lalmatia coal are further crushed in steel and power plants, sink-float data generated from a routine washer performance analysis can also be used to estimate the grindability of washed coal and middling products.
4. Conclusion The Hardgrove grindability indices of Bagdigi and Lalmatia coals appear to be entirely determined by the HGI of constituent specific gravity fractions of these coals and the weight percentage distributions of the same fractions. Two distinct trends can be observed in the variation of HGI with specific gravity. At the maintained level of top-size:z, Lalmatia non-coking coal shows the grindability to be monotonically decreasing with the increase in specific gravity, with the smallest HGI for the heaviest fraction. Bagdigi coking coal though, shows the same broad trend but does not record the smallest HGI for the heaviest fractions. A linear relationship, however, appears to exist for both coals between the cumulative mean specific gravity and the HGI of the same cumulative float products. As a result, it appears to be possible to accurately estimate through empirical relationship the HGI of both coals from the routine sink-float data. Such an estimation offers a number of practical advantages.
Acknowledgements The presentation of this paper would not have been possible without the kind cooperation of Prof. D.D. Mishra of our department.
References Fitton, A., Hughes, T.H., Hurtley, T.F., 1957. Grindability of British coals - - a laboratory examination. J. Inst. Fuels 30, 54-65. Ghosal, A., Roy, L.C., Bose, R.N., Lahiri, A., 1958. A study on grindability of coal. J. Inst. Fuels 31, 34-43.
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Sinha, N.C., Nair, C.P.P., Bagchi, S., 1969. Studies on the Hardgrove grindability of coals. J. Min. Met. Fuels 18, 265-273. Yancey, H.F., Geer, M.R., 1936. Further investigation of methods for estimating the grindability of coal. Trans. A1ME 119, 353-357. Yancey, H.F., Furse, O.L., Blackburn, R.A., 1934. Estimation of the grindability of coal. Trans. AIME 108, 267-275.