Recovery of Scandium by Crystallization Techniques

摘要

Bauxite residues, i.e., red mud, can be processed to recover various valuable end products, while reducing the environmental impact of the waste. Scandium is one of the valuable elements in bauxite residues. It is possible to extract and enrich scandium from red mud by leaching and solvent extraction. Scandium can then be recovered from the pregnant strip liquor by crystallization. Different crystallization techniques can be used to generate the supersaturation required for scandium to crystallize out as a salt. In the present study, the crystallization of an ammonium scandium fluoride phase by cooling and antisolvent crystallization techniques is presented. Cooling crystallization gave a low yield of ammonium scandium hex-afluoride, (NH_4)_3ScF_6, below 50% at the lowest temperature of 1 °C investigated. Antisolvent crystallization using ethanol gave almost complete recovery with precipitation efficiency greater than 98% for an ethanol-to-strip liquor volumetric ratio of 0.8. Solubility data of (NH_4)_3ScF_6 under different temperatures and in different ethanol-strip liquor mixtures is herein presented. The product obtained by antisolvent crystallization had very minute crystals (< 2 urn) due to the high supersaturation generated upon adding ethanol to the strip liquor, while it was easier to obtain larger crystals by cooling crystallization. Fe and Ti impurities were detected in the solid product, and an insight into the mechanism of impurity uptake is discussed.