Effect of fluidizing characteristics on upgrading of lignitic coals in gravity based air jig

•Presence of pulse air affects the fluidizing characteristics.•Critical pulsation frequency — for appropriate stratification of hard to wash coals•Fluidizing characteristics affect the stratification degree of particles.•“Two layer regime” occurs in complete stratification.•Cohesive forces should be broken for appropriate stratification.

Two lignitic coals characterized by easy and difficult to wash were upgraded on a pneumatic semi-pilot scale Allair stratification jig. The fluidizing characteristics of lignites, i.e. apparent and cohesive minimum fluidizing air velocities, and Umfa and Umf-coh values were initially determined in a continuous system involving discharging units of the products (light and dense materials) in the presence of pulsed air. The effect of fluidizing conditions on separation conditions for − 15 + 4 mm coal samples was systematically investigated. Fluidizing air velocity and pulsation frequency were selected as the main parameters to determine the optimum separation efficiencies. The results reveal that separations at fluidizing air velocities of 1000–1200 cm/s (approximately %100 of Umf-coh) yielded high separation performance. In addition, although easy to float coals exhibited good separation performance in a wide range of pulsation frequencies of 3–4 s− 1, particularly the difficult to float coal required a specific pulsation frequency of 3.5 s− 1. Under the optimum operational conditions, the ash contents of clean coal products were reduced down to the levels of 16–18% from 44% feed ash with the separation efficiencies of 41 and 53% for difficult and easy to float coals, respectively.

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