Hydrothermal Reaction of Ca(OH)2with Quartz in Connection with Coal Demineralization

The hydrothermal reaction of Ca(OH) 2 with reagent quartz at temperatures between 175 and 340 °C has been studied to obtain fundamental information on coal demineralization with Ca(OH) 2 . X-ray diffraction analysis was used to measure the conversion of quartz and to identify reaction products. The reaction sequence strongly depended on Ca/Si atomic ratio and on autoclaving conditions. At a Ca/Si atomic ratio of about 1, quartz was first converted to Ca 2 Si 7 O 4 ·H 2 O, which then reacted with residual quartz or residual Ca(OH) 2, forming a silica-rich phase or a calcium-rich phase, and finally all these intermediate compounds were transformed to xonotlite upon a prolonged high-temperature treatment. At higher Ca/Si ratios, a fraction of quartz remained unreacted even after severe treatments, and the final products were jaffeite (Ca/Si = 3) and tobermorite-like compounds (Ca/Si = ∼1) instead of xonotlite. On the basis of the above investigations on model reaction, demineralization reactions to remove quartz from coal through Ca(OH) 2 /HCl leaching were interpreted and a suitable acid-washing method was proposed.