Solvent Extraction of Scandium from the Waste Solution of TiO₂ Production Process

Abstract

Extraction and stripping properties of Sc³⁺ were investigated by using a mixed extractant of Versatic Acid 10 (VA10) and Tri-n-butyl phosphate (TBP). The effect of adding TBP to VA10 was considered by analyzing the binding state of VA10 and TBP by FT-IR. Solvent extraction of scandium from an aqueous solution of Sc³⁺–Ti⁴⁺–Zr⁴⁺, which simulated waste water from the titanium oxide manufacturing process, the Bayers process and others, was performed, and the separation factor of Sc³⁺ over Ti⁴⁺ and Zr⁴⁺ was investigated. To clarify the operating conditions of a countercurrent multistage process for Sc³⁺, McCabe–Thiele analysis was applied to the extraction isotherm and stripping isotherm of Sc³⁺. The extraction pH of Sc³⁺ was found to shift to higher pH, and Sc³⁺ was stripped at a lower concentration of H₂SO₄ by adding TBP to VA10. Addition of TBP to VA10 suppresses the extraction of Sc³⁺ and promotes the stripping of Sc³⁺. IR spectra of the organic phase suggested that the dimeric portion of VA10 formed hydrogen bonds with TBP. The extractions of Sc³⁺, Ti⁴⁺ and Zr⁴⁺ were 77, 3 and 8%, respectively. The separation factors of these metal ions were β_Sc/Ti = 100 and β_Sc/Zr = 38 at pH4.6 on extraction with VA10 + TBP from aqueous solutionof Sc³⁺–Ti⁴⁺–Zr⁴⁺. Stripping efficiency of 100, 8 and 22% could be achieved for Sc³⁺, Ti⁴⁺ and Zr⁴⁺, respectively by using 0.5 mol/dm³ H₂SO₄. From these results, it was possible to separate Sc³⁺from impurities such as Ti⁴⁺ and Zr⁴⁺ by countercurrent multistage extraction, and Sc³⁺ was recovered by one-stage stripping with 0.5 mol/dm³ H₂SO₄. Extraction of Sc³⁺from the dilute aqueous solution containing 500 mg/dm³ of Sc³⁺reached 99% at the flow ratio (A/O) = 1.01 in the two-stage countercurrent extraction, and Sc³⁺ in the organic phase was stripped to attain a six-fold concentration by applying one-stage stripping with flow ratio of (O/A) = 6.00.