Selective cerium removal by thermal treatment of mixed rare earth oxalates or carbonates obtained from non-purified rare earth sulphate liquor

Abstract

One of the world́s most abundant of all the rare earth elements (REE) is cerium. It represents about 50 wt% of total REE content in the main rare earth minerals used industrially (monazite and bastnaesite). However, cerium is not a valuable product if compared with the other REE. In the most current pyro-hydrometallurgical processes routes, a high amount of cerium goes to the rare earth liquor, incurring in high operational and capital costs in the solvent extraction step used in the separation of each individual rare earth element. In this context, this study evaluates the use of thermal treatment for rare earth oxalates and carbonates and also hydrochloric acid leaching of calcination products for selective separation of cerium from other REE. This investigation also sheds light on the mechanisms involved in the processes proposed. For this purpose, mixed rare earth and cerium oxalates, also mixed rare earth and cerium carbonates were used as precursors. The mixed rare earth salts used in the experiments were generated from a non-purified rare earth sulphate liquor. The results indicated that the minimum required temperature to fully decompose the mixed rare earth oxalates, cerium oxalate, mixed rare earth carbonates and cerium carbonate to their respective rare earth oxides (REE2O3) or CeO2, was 1100 °C, with 1 h of residence time. In the experiments carried out with mixed rare earth oxalates, Ce3+ was not fully oxidized to Ce4+, it formed a mixture of Ce2O3 and CeO2 represented by Ce4O7, which was completely dissolved in the hydrochloric acid solution (37 wt%). Therefore, in such case, cerium was not selectively removed from the mixed rare earth oxalates by thermal treatment and hydrochloric acid leaching. On the other hand, the Ce3+ presented in the composition of the pure cerium oxalate or carbonate was fully oxidized to Ce4+ (CeO2) under the same condition of calcination. As expected, CeO2 was not dissolved by the hydrochloric acid solution (37 wt%). It was concluded that mixed rare earth carbonates generated Ce0.60Nd0.40O1.80 (Ce1-xNdxO2-x/2, x = 0.40) when calcined under the same condition. This crystalline structure was not also dissolved by hydrochloric acid solution (37 wt%). As a result, a process flowsheet was proposed to remove cerium selectively from a rare earth ore.