Process engineering of demineralisation of moderate to high ash Indian coals through NaOH-HCl leaching and HF leaching

Abstract Chemical leaching of coals would be required to produce cleaner coals for some special applications where physical beneficiation may not be effective enough. This would also help in recovering Li and rare earth metals besides in the sequestration of CO 2 . About 20 Indian coals having complexly distributed moderate to high ash contents were sequentially treated with various alkali–acid such as NaOH-HCl, HF, HCl, HCl-HF, and NaOH-HCl-HF leaching. This aimed to establish and design the best stepwise sequential process for the highest degree of demineralisation through a chemical leaching process. Kinetics and process intensification studies were carried out. More than 80% demineralisation of Madhaipur and Neemcha coals was observed using the best sequential treatment designed presently. The repeated stepwise treatment of the alkali and the acid was also studied, which was found to significantly enhance the degree of demineralisation of coals. The integrated process of alkali–acid leaching followed by solvent extraction (Organo-refining) and vice versa of the treated coal was also studied for producing cleaner coals.