From trace to pure: Recovery of scandium from the waste acid of titanium pigment production by solvent extraction

The waste acid from the titanium pigment production process contains scandium ion at low concentration. The waste is thus an extra resource for scandium. To recover the trace scandium from the waste acid, a process combining pre-enrichment and purification steps via solvent extraction using the extractant of bis-(2-ethylhexyl) phosphoric acid was reported. In the pre-enrichment step, to obtain high extraction efficiency at low extractant consumption, the volume fraction of bis-(2-ethylhexyl) phosphoric (as extractant), tributyl phosphate (as synergistic agent) and the ratio of aqueous-to-organic extractant (A/O) were optimized. Afterwards, to remove the main impurities from the enriched liquor, the further purification step was investigated. Herein, to improve the separation between scandium and titanium, (TiO)2+ was transformed to [TiO(H2O2)]2+ by addition of H2O2 into the pre-enriched liquor prior to extraction. The co-extracted metal ions were subsequently scrubbed with 3 mol/L H2SO4/6% H2O2 by multi-stage cross-current. Further, the extractant was reused for four cycles and the extraction efficiency for scandium was found to be stable after the third cycle. Based on current results, a process to recover the trace scandium from real titanium white waste acid was introduced on lab scale as a proof of principle.