Production of Ultra Clean Coal: Part I—Dissolution behaviour of mineral matter in black coal toward hydrochloric and hydrofluoric acids

The mineral matter in an Australian black coal has been isolated using a low-temperature ashing (LTA) procedure. This LTA procedure is a modification of the Australian Standard for LTA at 370°C, and alleviates adverse effects to the minerals caused by the heat of combustion. The leaching behaviour of the mineral matter towards aqueous HCl and hydrofluoric acid (HF) is presented. HCl can dissolve simple compounds such as phosphates and carbonates, yet it cannot completely dissolve the clays. HF reacts with almost every mineral in the mineral matter, except pyrite, and most of the reaction products are water soluble. However, at HF concentrations greater than that required to dissolve the aluminosilicate compounds in the mineral matter, insoluble compounds form. These compounds include CaF2, MgF2 and a compound containing Na, which is believed to be NaAlF4. It is proposed that HF reacts preferentially with the aluminosilicates in the mineral matter to form largely AlF2+, AlF3 and SiF4, and that the concentrations of free fluoride (F−) and AlF4− are not high enough to complex cations such as Ca2+, Mg2+ and Na+. When the mineral matter is treated with HF concentrations greater than that required to dissolve all of the aluminosilicates, AlF3, AlF4− and SiF62− form, the concentration of F− is high enough to complex Ca2+ and Mg2+ and form insoluble CaF2 and MgF2, and the concentration of AlF4− is high enough to complex Na+ and form insoluble NaAlF4. This work has application toward the development of a process for producing Ultra Clean Coal with less than 0.1% by weight mineral matter.