The precipitation process for a rare earth leach solution with calcium oxide in the presence of ascorbate and calcination to oxide of high purity

Abstract

Sulfate ions coprecipitate with rare earth metal ions to form alkaline rare earth sulfate (RE2(OH)4SO4) in the CaO precipitation process, leading to excessive content of sulfate in the mixed rare earth oxide. Due to the difference of coordination ability between ascorbate, sulfate, hydroxyl and rare earth metal ions, ascorbate is introduced into the pregnant leach solution (PLS) during the CaO precipitation process. It realizes the coprecipitation between ascorbate (C6H7O6−) and rare earth metal ions, reducing the formation of RE2(OH)4SO4 and improving the purity of mixed rare earth oxide. In this paper, the standard reaction Gibbs free energy changes of the possible reactions in the precipitation process were calculated using the group contribution method, to draw the species distribution equilibrium diagram with Newton's iteration method. When ascorbate exists, rare earth metal ions in the PLS tend to form the alkaline rare earth ascorbate (RE2(OH)4(C6H7O6)2) instead of RE2(OH)4SO4. Furthermore, the effect of the reaction conditions on the precipitation process were studied. When the molar ratio of rare earth metal ions to ascorbate was 1.17, the purity of the mixed rare earth oxide was 96.2 wt% and the sulfate content was 2.2 wt%. Finally, based on the results of the material balance calculation, FTIR, XRD and TG-DSC tests, it was further confirmed that ascorbate participated in the form of RE2(OH)4(C6H7O6)2, and effectively suppressed the generation of RE2(OH)4SO4. Consequently, the results facilitate the green and efficient extraction of rare earths from the ion-adsorption type ore.