Development of a new coal washability index

A new coal washability index, termed as ‘near gravity material index (NGMI)’, has been developed based on the sink-float data. This new index incorporates the effect of ash distribution in the near gravity material at various specific gravities of separation. The optimum specific gravity of separation of a particular coal may be determined by plotting NGMI as a function of various specific gravities of separation or the desired clean coal ash content. The concept and the method of calculation of this index have been discussed in detail. Using some published float-sink analytical data, a quick comparison of the washability characteristics of several different coals is demonstrated utilizing the NGMI curves for these coals.