A short review of improvement in flotation of low rank/oxidized coals by pretreatments

Low rank/oxidized coals are difficult to float with oily collectors. The presence of polar hydroxyl, carbonyl, phenol groups and some peroxide type oxygenated moieties on the surface of low rank/oxidized coals is the main factor which decreases the coal hydrophobicity. Pretreatments may be used to enhance the flotation of low rank/oxidized coals due to improved hydrophobicity. The primary methods of pretreatments are grinding, premixing/preconditioning, ultrasound, thermal, microwave and direct contact mixing of the reagents with dry coal before wetting. Among these pretreatment methods, grinding, premixing/preconditioning may be considered to remove the oxide layer on the oxidized coal surface, which is also reflected by ultrasound treatment. Thermal and microwave treatments are attributed to the removal of pore water, hydration water and some hydroxy functional groups in low rank/oxidized coals. Direct contact procedure makes the oxidized coal avoid the prevention for the collectors absorbing on the coal surface. The prevention usually comes from hydration shell on wet coal surface while the coal is wetted by water. This review is to highlight the effect of different pretreatment methods on the flotation of low rank/oxidized coals.