Effect of heat treatment on the flotation of Turkish lignites in brine solution

Literature on the flotation of bituminous coal particles in salt solutions in the absence of collector and frother is reported to be possible. On the other hand, flotation of lignitic coals is inherently difficult. Meanwhile, 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 matter, the objective of this study was therefore to determine changes in the hydrophobicity of some lignites under modestly controlled heat treatment and correlate the flotation response of lignite coals in NaCl salt solution without using any flotation chemicals. The thermal treatment of lignite sample showed significant improvement in the floatability and hydrophobicity of coal samples. While the effect of salt addition on the floatability ROM lignite was marginal, that of heat treated coal was rather significant. Several methods were used to correlate the flotation response and the quantity of hydrophobicity and hydrophilic groups in coal. For example, FTIR measurements demonstrated a direct correlation between the flotation recoveries and hydrophobic and hydrophilic FTIR peaks in coal. The improvement in the floatability of lignite upon heat treatment was ascribed to the removal of trapped water molecules in the pores and at the coal surface and to the removal of oxygen containing groups. The flotation results suggest that under the present test conditions it is possible to float thermally treated lignite samples of partially hydrophobic character in salt solutions in the absence of collector and frother.