Removal of ash from Indian Assam coking coal using sodium hydroxide and acid solutions

Mineral matter (ash) removal from Assam coking coal by leaching with different concentrations of sodium hydroxide and acid (HCl, H2SO4, HNO3, and HF) solutions has been investigated at a temperature of 75 degrees C. The parameters tested were concentration of NaOH, type of acid, concentration of acids, and number of acid leaching steps. Total ash removed increased with increase of NaOH and acid concentrations up to the range studied. For the same experimental conditions, treatment of caustic leached coal in HCl acid resulted in better demineralization than in acid concentration (> 20%) of HNO3 acid had aa adverse effect on the de-ashing of coal. The NaOH-HF leaching process has been found to be the most effective method of coal de-ashing. The two acid treatment steps (HCl-H2SO4 / HCl-HNO3) after caustic leaching are the next most effective methods of coal de-ashing. The removal of mineral matter (including S) from coal is expected to decrease the graphite reactivity and thus the atmospheric pollution (due to the generation of smaller quantities of CO and SO2 gases).