Recovery of scandium from white waste acid generated from the titanium sulphate process using solvent extraction with TRPO

Scandium (Sc), a rare earth element, generally lacks independent large-scale deposits and is mainly recovered as a by-product in mineral processing. Herein, we systematically investigated the extraction of Sc by trialkyl phosphine oxide (TRPO) from both a scandium sulfate solution and leaching solution of Sc concentrate generated from titanium white waste acid in the sulfuric medium. The results showed that the optimum extraction conditions of Sc from scandium sulfate by TRPO were as follows: 10% TRPO, 5 mol/L H2SO4, and an A/O ratio of 1 for 20 min. The equation of scandium extraction by TRPO was calculated based on the log[c(TRPO)] - log[D] fitting results coupled with Fourier transform infrared spectroscopy (FT-IR) analysis, to which the extracted complex was deduced to be HSc(SO4)2•2TRPO. Furthermore, for Sc extraction from leaching solution of its concentrate from titanium white waste acid, a relatively high extraction rate of up to 95.5% was obtained on the conditions of 20% TRPO, 7 mol/L H2SO4, 0.5% of H2O2, and at an A/O ratio of 10. Stripping with H2C2O4 produced a high stripping rate of 99.9%, and the extraction and stripping system still presented excellent extraction performances even after five treatment cycles. Through the subsequent calcination of the stripped Sc2(C2O4)3 at 1000 °C for 2 h, we obtained the final product of Sc2O3 with a high purity of over 99.34% with a total recovery yield of Sc as high as 95% in the whole process. Our findings contribute novel insights into the extraction and purification of Sc for its recovery from the secondary wastes.