Improved flotation of heat treated lignite with saline solutions containing mono and multivalent ions

Flotation of lignites is inherently difficult. However, pre-heat treatment of coal is also known to make coal surfaces more hydrophobic possibly through removal of water entrapped in the structure of coal. In this context, the objective of this study was, therefore, to determine changes in the hydrophobicity of some lignites under moderately controlled heat treatment, and correlate the flotation response of lignites in different salt solutions of NaCl, KCl, CaCl₂, and MgCl₂ without using any reagent. The results of flotation tests suggested that, under the present test conditions, it was possible to float thermally pre-treated lignite samples of partially hydrophobic character in salt solutions in the absence of collector and frother. In addition, the effect of heat treatment on hydrophobicity, and in turn flotation was explained by a theoretical model based on extended DLVO interactions to quantify the effects of both heat treatment and salt concentration on bubble-particle interactions. The results of theoretical modeling suggested that the removal of hydrogen containing groups from coal surfaces significantly contributed to the electrical double layer and hydrophobic forces that governed the magnitude of energetic barrier and also the extent of bubble-particle attachment.