Effect of particle size on the reverse flotation of subbituminous coal

The surface of subbituminous coal contains a significant amount of oxygen containing functional groups which contribute to a low degree of hydrophobicity. As such, upgrading of the fine fractions is difficult using conventional flotation practices which rely on bubble-coal particle attachment and separation based on density. Alternatively, reverse flotation provides a viable option whereby the mineral matter is made hydrophobic through the addition of a collector while a depressant is added to make the coal particle non-floatable. Bench-scale reverse flotation experiments were conducted using artificial mixtures of subbituminous coal and quartz of varying particle size to assess the flotation performance. The process provided a reduction on the ash content from values around 35% to while recovering nearly 85% of the combustible material. The performance was achieved using mixtures of coarse coal with fine quartz and medium-size coal with fine silica. The performance was significantly better than that achieved by the conventional practice of floating the coal and rejecting the mineral matter to the underflow stream. The separation efficiency obtained when evaluating a mixture of fine coal and quartz particles was not as favorable due to the effect of hydraulic entrainment. (C) 2016 Elsevier B.V. All rights reserved.