Treatment of monazite by organic acids I: Solution conversion of rare earths

Abstract

© 2017 Elsevier B.V. In the present study, the potential for using organic acids to enhance the dissolution of monazite (nominally CePO 4 ) was examined. Oxalic acid was found to release a significant amount of phosphorus ( > 30%) at pH ≤ 1.3 and room temperature over periods of up to 72 h. However, only very small proportions of rare earths were present in solution. The lower than expected iron level present in solution (and thus dissolution of iron oxides) displays the selectivity of the process given the established ability of oxalic acid to complex iron. Analysis of the solids determined that the rare earths were re-precipitating as oxalate salts. This is significant as rare earth oxalates have been shown to be amenable to dissolution. Leach tests conducted over 168 h displayed slowing of the reaction to the point where little further leaching was occurring. This was confirmed to be due to oxalate consumption and not passivation via the re-precipitation of the oxalate species. While a slight increase in temperature (25 to 45 °C) had little impact, elevation to ≥ 65 °C greatly accelerated the reaction achieving similar results in only 10 h. However, iron dissolution also increased markedly. Further temperature elevation to 85 °C significantly increased the dissolution of iron to the detriment of phosphorus release. This is a significant consideration in terms of both extraction efficiency and reagent consumption. This process presents the opportunity to remove the greatest hazards, both safety and environmental, and decrease the energy and potentially reagent requirements for monazite treatment. The second paper of this series discusses the subsequent stage of processing the treated monazite for rare earth extraction.