Liberation Characteristics of a Coal Middlings

The recovery and quality of product coal is closely associated with the distribution of mineral matter within the feed coal and its subsequent liberation. One area where the benefits of liberation could be realised is in the treatment of coal middlings fractions where relatively high ash contents are not uncommon The work outlined in this paper is concerned with the breakage characteristics of middlings, their liberation properties and the potential for retreatment. The material used was a “typical” middlings from a dense medium separator and breakage was achieved by swing hammer crushing. 16 tests were conducted covering a range of operating conditions. Full densimetric analyses on size fractions of material prior to and after breakage were conducted with the data being presented in the form M-curves. Although the original middlings did not display any increase in liberation with reducing particle size, there was ample evidence that crushing increased the liberation of both the clean coal and the mineral matter. Comparison of normalised M-curves representing the crushed products, strongly suggested that a single M-curve may adequately represent one single closely sized fraction regardless of the crushing regime which it has experienced. Any change in liberation of the crushed products as a whole is then merely due to the difference in size distributions.