Lower size limit of raw coal for efficient beneficiation in air-fluidized bed with magnetite particles

A feasible criterion was established to determine the lower size limit of raw coal (d_pRm) for efficient beneficiation in the air-fluidized bed with magnetite particles. The feasibility of using small magnetite particles to accommodate the fine raw coal was demonstrated from the experimental perspective. The minimum size for the magnetite particles to be fluidized smoothly was clarified as 47.1 μm, which corresponded to the border between Geldart-B and -A groups. Since the gangue and coal components in the raw coal were crushed into the same size, d_pRm depended on the greater one between d_pGm (minimum size required for the gangue particles to sink towards the bottom) and d_pCm (minimum size required for the coal particles to float towards the top). d_pGm was determined as 259 μm by supposing that provided the gangue particles accumulated in the lower half bed, they could be potentially extracted from the bottom. On the other hand, it was observed that the coal particles could always accumulate in the upper half bed. Under such circumstances, d_pCm was revealed as 9.8 μm since finer coal particles would be blown out by air before the 47.1 μm sized magnetite particles became fluidized. Eventually, d_pRm was clarified as 259 μm, agreeing with the common view that raw coal coarser than 6 mm could be effectively beneficiated in the air-fluidized bed with magnetite particles. Additionally, the difficulty in beneficiating the fine raw coal was revealed to arise more from the remixing of sorted gangue particles than that of separated coal particles.