Reactive Grinding of Coal Ash for Enhanced Rare Earth Extraction

The size of rare earth minerals in coal and coal by-products tend to be on the order of 1 - 20μm, and intensive grinding may be required to liberate these minerals for subsequent beneficiation and extraction. By adding reactants to the grinder, such as sodium hydroxide, the rare earth minerals could react to form rare earth hydroxides, which are more amenable to leaching in acid. Coal ash was subjected to grinding in a standard laboratory ball mill with, and without reactants, such as sodium hydroxide, in order to enhance subsequent leaching of the rare earth elements in hydrochloric acid.
In the case of sodium hydroxide, it is hypothesized that the rare earths in the coal ash are converted to rare earth hydroxides, which are more amenable to leaching in hydrochloric acid verses rare earth phosphates. Various grinding times were studied to determine the extraction yield of the rare earth elements. Grinding materials to increase overall surface area of the sample can also increase the leaching efficiency, and this was also examined. Grinding fly ash with 25wt% NaOH for 3 hours showed significantly enhanced extraction of the rare earth elements compared to grinding for 3 hours without NaOH. The extraction of other elements such as phosphorous will also be discussed.