An Investigation into Cerium Oxidation Under Acidic Conditions
Separation of rare earth elements (REE) is often achieved by acid leaching of mineral concentrates followed by subsequent solvent extraction, to produce individual rare earth metals. Cerium, the most abundant of these elements, is usually the largest component of commercial ores. Demand for less prevalent REE, such as neodymium and dysprosium; have led to a market saturation and price collapse of cerium. One method of improving the economics of REE separation is to remove cerium from solution prior to solvent extraction. This is accomplished via the oxidation of cerium (III) to cerium (IV), which precipitates as cerium hydroxides. In general this process is undertaken at basic pH, however this requires large quantities of base to raise the solution of leach liquors to levels suitable for these processes. Oxidation in acidic solutions would allow for savings on reagents as well as lower water use, increasing the financial viability and environmental profile of rare earth purification processes. This study probes the efficacy of four oxidants; hydrogen peroxide, sodium hypochlorite, potassium permanganate, and Caro’s (peroxymonosulfuric) acid in this endeavor. Cerium oxidation was tested using these oxidants at varying stoichiometry, a pH range of 1.0 to 4.0, temperatures of 25, 45, and 65 °C, varying cerium concentrations, and in the presence of other REE. Kinetic constants were calculated from time-sampled data. 99.9% cerium removal is achievable using potassium permanganate and Caro’s acid in the absence of other REE. The greatest cerium removal, 98.4%, was achieved using permanganate in multi REE solutions leaving a product with a cerium to total REE (Ce/TREE) quiotient of 0.62. Varying levels of cerium removal were achieved with lower selectivity using the other oxidants. To investigate adsorption of other REE to cerium, adsorption isotherms of neodymium and terbium were created using ceric hydroxide and fitted to the Langmuir and Freundlich isotherms. The cerium solids produces were examined using transmission electron microscopy, scanning electron microscopy, thermogravemetric analysis, and x-ray diffraction to gain a better understanding of their form.