Chemistry of Ca(OH)2Leaching on Mineral Matter Removal from Coal

An attempt was made to remove mineral matter from Newstan coal by Ca(OH)2 leaching followed by dilute hydrochloric acid washing. Ash removal of 76% was achieved by using 5% CaO loading at 340 °C for 120 min and an autogenous vapor pressure. CaO loading was found to be very important for optimization of ash removal. Above 5% CaO loading, a significant fraction of calcium was retained in treated coal. X-ray diffraction analysis and inductive coupling plasma spectroscopy analysis were used to study the chemistry of demineralization. The major crystalline minerals in the original coal were quartz and kaolinite. Quartz and kaolinite reacted with Ca(OH)2 to form ill-crystalline calcium silicate hydrates and calcium aluminum silicate hydrates, respectively. At lower CaO loadings, these products were effectively removed by acid washing, whereas at higher CaO loadings, calcium silicate hydrates and/or calcium aluminum silicate hydrates formed were less soluble in acid. Hence, treated coal was enriched with the undissolved calcium. Upon high-temperature ashing, the residual Ca-containing species were allowed to react with SO2 released from coal and capture a fraction of sulfur as a form of calcium sulfate.