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Study on regeneration of spent hydrochloric acid in chemical coal leaching process

For Indian coals, chemical leaching process has better advantage over the conventional physical cleaning techniques due to its drift origin. Coal leaching is a two-stage process where alkali and acid leaching cycles are used, where most of the ash constituents are accumulated in the spent acid and hence purification of spent acid containing silica, alumina, iron, calcium and magnesium is of great importance. Four different methods namely adsorption using activated carbon, ion exchange with anion exchange resin, pyrohydrolysis and stage-wise neutralisation are studied. Results with adsorption and ion exchange techniques are poor and a maximum of 50% removal is observed. Results indicate that, pyrohydrolysis shows better performance with > 99% pure hydrochloric acid as product but it is energy intensive. Stage-wise neutralisation technique is promising for dilute acid, where silica is precipitated by digestion and it produces magnesium oxide and calcium salt with 86.7% and 93.86% purity respectively.