Effect of crushing on near-gravity material distribution in different size fractions of an Indian non-coking coal

remove physical impurities as effectively and economically as possible. The beneficiation process includes various operations on the asmined coal to make it more suitable for enduse application without destroying the physical identity of the coal. There are several coal beneficiation processes, but gravity-based separators remain the most efficient unit operation for removing the undesirable gangue material from run-of-mine coal. These separators exploit the difference in density between coal particles and gangue particles to effect a separation. The feed is introduced into an aqueous suspension of ultrafine magnetite with a particular density. The feed particles with a relative density less than that of the suspension (clean coal) float to the top surface of the suspension, whereas particles denser than the suspension density (gangue material) sink to the bottom. When the difference in density between the particle and the aqueous suspension is large, the separation is easy, but when this difference is less the separation becomes much more difficult since the settling velocity of the particles is very low. This lower settling velocity increases the probability that particles that should report to floats report to sinks, and vice versa, hence increasing the amount of misplaced material. Therefore, the degree of difficulty in beneficiating a typical coal at a particular specific gravity depends on the amount of material occurring within ±0.1 specific gravity range. This is known as ‘neargravity’ material, also termed ‘near-dense’ material, at that particular specific gravity of separation. As coal is heterogeneous in nature, the density of a daughter coal particle depends on the nature of the parent coal particle from which it has been produced and the fracture patterns involved during crushing, as shown in Figure 1. As the fracturing of particulate material is a random process and many factors contribute to the fracturing of any single particle, the result is never exactly predictable. Particles of the same material and similar in size and shape show a wide variation in individual fracture patterns. Therefore, when a bigger heterogeneous particle of particular density is subjected to crushing a broad spectrum of particles of various sizes and densities is produced. Similarly, when run-ofmine coal is crushed, a wide variety of smaller particles are generated, which alters the amount of near-gravity material present in a particular density class and accordingly changes the degree of difficulty in washing. Therefore, it is imperative that the change in mass of the near-gravity material at a particular specific gravity on crushing should be determined accurately. The difficulty in washing a particular coal can be interpreted from the steepness of the ‘characteristic curve’, which is derived from the sink-and-float data (Corriveau and Effect of crushing on near-gravity material distribution in different size fractions of an Indian non-coking coal