Selective Separation of Scandium from Titanium White Waste Acid Using the HDEHPA-TBP Extraction System

The solvent extraction and separation process for scandium(III) (Sc) and titanium(IV) (Ti) from titanium white waste acid (TWWA) was studied. The influences of process parameters, including extractant type, feed solution acidity, and di(2-ethylhexyl) phosphoric acid (HDEHPA) concentration, on the extraction efficiency were systematically investigated. The results demonstrated that under optimized conditions, the extraction efficiency of scandium(III) reached 99.98% via two-stage countercurrent extraction, with a co-extraction ratio of titanium(IV) of 13%, enabling efficient extraction and separation of scandium(III) from titanium(IV). Furthermore, the extraction mechanism was elucidated based on saturation capacity measurements and infrared spectroscopy analyses. It was confirmed that scandium(III) was extracted into the organic phase in the form of an HSc(SO4)2·4HA complex. An aqueous TSC (trisodium citrate)-H2O2 mixture was used as the washing agent to selectively remove co-extracted titanium(IV). Three-stage crosscurrent washing experiments revealed that the washing efficiency of titanium(IV) reached 100%, whereas the co-washing loss of scandium(III) was merely 0.06%. After washing to eliminate titanium(IV), scandium(III) in the loaded organic phase was stripped with NaOH solution, and a cumulative stripping efficiency of 96.80% was obtained via a two-stage crosscurrent stripping process. This work provides a feasible process for scandium resource recovery from TWWA. On this basis, an innovative conceptual flowchart for scandium recovery combining solvent extraction and pyrometallurgical chlorination was proposed.