Studies on washability characteristics of low-volatile Indian coking coal by differential crushing

The effect of differential crushing on liberation characteristics has been studied for a low volatile coking coal of Indian origin through washability studies. Two parameters, namely “Index of Washability” (IW) and “Near Gravity Material Index” (NGMI), are used to describe the ease of washability. The ROM Coal is crushed to four different top sizes namely, 75 mm, 25 mm, 13 mm and 6 mm.On the basis of calculated IW it is observed that relative ease of washabiltiy increases with decrease in top size. From calculated IW values it may be said that this coal can be economically beneciated using gravity process after crushing to -6 mm size. From the calculated NGMI values, the critical specic gravities have been estimated and the values for crushing to -75 mm,-25 mm,-13 mm and − 6 mm are 1.65, 1.68, 1.53 and 1.58 respectively. These critical specic gravity values suggest the separation at this specic gravity range is most dicult task using gravity methods. From NGMI analysis, it may be said that the NGMI values for coals crushed to -25 mm & -6 mm are identical ( ≈ 0.18) at 17% clean coal ash content. This suggests that with similar degree of diculty, clean coal of 17% ash can be produced from these two different crushing sizes. In order to increase the yield for the clean coal of 17% ash, the decision on blending these two size coals may need to be taken.


Introduction
The top size of the coal to be washed usually has a bearing on the bene ciation process. Optimum crushing of the coal results in adequate liberation such that the combustibles can be easily separated by simple gravity separation methods [3]. It is well accepted practice to carry out the sink-oat analysis of the coal to estimate its amenability towards gravity concentration methods. The data obtained from the sinkoat analysis is used to plot the "washability curves" to extract valuable information regarding the clean coal that can be obtained from a given coal under ideal conditions [2]. From the sink-oat analysis of a given coal, the amount of material oating at a particular speci c gravity can be hypothetically considered as the recovery of two constituents, namely non-ash and ash forming materials. The coal which is easily washable will have complete liberation of non-ash material from ash material and the recovery of all nonash material present in the feed is possible without recovering ash material in the clean coal. On the contrary, the di cult-to-wash coal will have intimate locking of non-ash and ash materials and the ash content can not be reduced by physical bene ciation. Numerical indices such as 'washibility index' [4] and 'washibility number' [5] have been developed to quantify the amenability of given coal towards washing.
The non-availability of any explicit mathematical expressions to estimate the ease of washing often posed a problem [1]. Keeping the above di culty in view Govindarajan et al. [1] and Majumder et al. [2] developed mathematical expressions to calculate the 'Index of washbility (IW)' and 'Near gravity material index (NGMI)' respectively. The values of 'Index of washbility (IW)' vary from 0 to 100 for the 'di cult to wash' to 'easy to wash' [1]. The values of 'Near gravity material index (NGMI)' vary from 0 to 1 for the 'easy to wash' to 'di cult to wash' [2].

Sample preparation and Characterization:
13mm and 6mm top size levels. The crushed coals were subjected to screening to ascertain the size distribution. The size wise ash analysis is shown in Table 1 for each top size level. The sink-oat tests were carried out for each top size level using Zinc-chloride, Bromoform and Benzene solutions as heavy media at different speci c gravities. The detailed washability analysis is shown in Fig.1 Table 1: Size wise ash analysis of Kuju coal at different crushing size levels. 3.0 Estimation of Index of washability: For the calculation of Index of Washability (IW) of a given coal the recovery curves of non-ash and ash material have been drawn based on the assumption that recovery is a continuous function of cumulative fractional weight of the feed coal oated (X) [1]. The typical recovery curves of normal and hypothetical coal crushed to below 75mm size are shown in Fig.1. In this Figure, Table, it is noted that the values of IW increases with the decrease in top size of the coal. This means that the relative ease of washability increases with crushing to lower sizes. The increase is not so signi cant for lowering the top size from 75mm to 25mm. A signi cant improvement is observed when the sample is crushed to -6mm size. According to IW values shown in Table 2, it may be said that this coal can be economically bene ciated using gravity process after crushing to -6mm size. Majumder et al. [2] has developed a method for calculating the near gravity material index (NGMI) from recovery curves of non-ash and ash forming materials.
In Fig. 3, curves ABC and ADC represent the recovery curves for non-ash and ash forming materials, respectively. The quantity of near gravity material index (NGMI) at a given speci c gravity is de ned as the difference in cumulative fractional weights oated at ±0.1 speci c gravity of that speci c gravity. In terms of recovery curves of non-ash and ash forming materials, the NGM distribution at a particular speci c gravity may be geometrically represented by the area under the ±0.1speci c gravity of that speci c gravity. This area will be different at different speci c gravities of separation.
By the above de nition, from  Fig. 4 Critical speci c gravity is a speci c gravity at which the value of NGMI is highest. From Fig. 4 the values of critical speci c gravity are 1.65, 1.68, 1.53 and 1.58 for coal crushed to -75mm, -25mm, -13mm and -6mm, respectively. These values suggest that the separation at this speci c gravity range is most di cult task using gravity methods.
From the washability data, the theoretical clean coal ash contents at those speci c gravities are calculated and NGMI values are plotted as a function of clean coal ash contents as shown in Fig. 5   Recovery curves for normal and hypothetical coal crushed to -75mm size Recovery curves for a given coal at crushed to -75mm size