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

Abstract

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 CaF 2, MgF 2 and a compound containing Na, which is believed to be NaAlF 4. It is proposed that HF reacts preferentially with the aluminosilicates in the mineral matter to form largely AlF 2 +, AlF 3 and SiF 4, and that the concentrations of free fluoride (F −) and AlF 4 − are not high enough to complex cations such as Ca 2+, Mg 2+ and Na +. When the mineral matter is treated with HF concentrations greater than that required to dissolve all of the aluminosilicates, AlF 3, AlF 4 − and SiF 6 2− form, the concentration of F − is high enough to complex Ca 2+ and Mg 2+ and form insoluble CaF 2 and MgF 2, and the concentration of AlF 4 − is high enough to complex Na + and form insoluble NaAlF 4. This work has application toward the development of a process for producing Ultra Clean Coal with less than 0.1% by weight mineral matter.