Removal of Sulfate Ions from Calcium Oxide Precipitation Enrichment of a Rare Earth Leaching Liquor by Stirring Washing with Sodium Hydroxide

Abstract

The use of calcium oxide as a precipitant can achieve a nonammonia enrichment of a rare earth leaching liquor. However, an alkaline rare earth sulfate forms during the precipitation process, thus resulting in excessive SO42– content in the mixed rare earth oxides. Therefore, a stirring washing process for precipitation enrichment, which was obtained from calcium oxide precipitation, was investigated using a sodium hydroxide solution. It was determined that the Gibbs free energy of the stirring washing reaction, which was calculated by a group contribution method, was between −60 and −300 kJ/mol, depending on the different rare earth elements. The above results indicated that the reaction was thermodynamically feasible. The optimum conditions of the washing process were obtained, namely, a feed ratio of 2.85, a liquid–solid ratio of 6.5 mL/g, a stirring washing temperature of 35 °C, and a stirring washing time of 20 min. Under the optimal conditions, the purity and the SO42– content of the mixed rare earth oxides were 94.38% and 3.48%, respectively, and the stirring washing process with the sodium hydroxide solution had good recyclability. Moreover, the washing product was tested using thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), which verified that the stirring washing process with NaOH could effectively remove SO42– from the precipitation enrichment into solution. On this basis, a new extraction process of the ion-adsorption-type rare earth ore by magnesium salt leaching–calcium oxide precipitation–sodium hydroxide stirring washing is proposed. This new process can eliminate the traditional aluminum-removal process and effectively reduces the rare earth loss in the process. It can also solve the problem of the excessive SO42– content in the mixed rare earth oxides caused by the calcium oxide precipitation process. The research in this paper can have great significance for green, efficient extraction of the ion-adsorption-type rare earth ore and the improvement of resource utilization.