Physical and chemical mechanism underlying ultrasonically enhanced hydrochloric acid leaching of non-oxidative roasting of bastnaesite

In this study, we investigated an alternative to the conventional hydrochloric acid leaching of roasted bastnaesite. The studies suggested that the rare earth oxyfluorides in non-oxidatively roasted bastnaesite can be selectively leached only at elevated temperatures Further, the Ce(IV) in oxidatively roasted bastnaesite does not leach readily at low temperatures, and it is difficult to induce it to form a complex with F− ions in order to increase the leaching efficiency. Moreover, it is inevitably reduced to Ce(III) at elevated temperatures. Thus, the ultrasonically-assisted hydrochloric acid leaching of non-oxidatively roasted bastnaesite was studied in detail, including, the effects of several process factors and the, physical and chemical mechanisms underlying the leaching process. The results show that the leaching rate for the ultrasonically assisted process at 55 °C (65% rare earth oxides) is almost the same as that for the conventional leaching process at 85 °C. Based on the obtained results, it is concluded that ultrasonic cavitation plays a key role in the proposed process, resulting not only in a high shear stress, which damages the solid surface, but also in the formation of hydroxyl radicals (OH) and hydrogen peroxide (H2O2). Standard electrode potential analysis and experimental results indicate that Ce(III) isoxidized by the hydroxyl radicals to Ce(IV), which can be leached with F− ions in the form of a complex, and that the Ce(IV) can subsequently be reduced to Ce(III) by the H2O2. This prevents the Cl− ions in the solution from being oxidized to form chlorine. These results imply that the ultrasonically-assisted process can be used for the leaching of non-oxidatively roasted bastnaesite at low temperatures in the absence of a reductant.