Behavior of Coal Particles in Column Flotation.

Abstract

The behavior of Datong coal particles in a flotation column was investigated in a batchwise operation using a gas distributor evolving uniform bubbles by varying the superficial gas velocity and column height. The value of superficial gas velocity was controlled at 0.66 or 1.99 cm s–1.The time course of ash content in floats was measured when column height was 1.85 m. The ash content in clean coal was larger for higher superficial gas velocity. The hydrophobicity of coal particles recovered from the top of the column was evaluated by means of a film flotation method. Hydrophilic particles having the critical wetting surface tension of particles, γc, greater than 60 mN m–1 floated well for higher superficial gas velocity. The weight percent of fractions having γc above 60 mN m–1 was 29.5, 31.9 and 40.5% for 0.25, 2.00 and 4.25 min, respectively. These values increased with sampling time, and were much greater than those for the lower superficial gas velocity. At lower superficial gas velocity, the hydrophobic coal particles seem to be selectively floated by colliding and attaching to the bubble surfaces. On the other hand, at higher superficial gas velocity, hydrophilic particles that are difficult to attach to bubbles seem to be entrained in the turbulent wake of bubble swarms and float.The grade of clean coal was better for the lower superficial gas velocity, however, carbonaceous materials recovery was lower than that for higher superficial gas velocity. Carbonaceous materials recovery was improved by increasing the recovery zone height.