Co-precipitation of Impurity (Ti, Fe, Al, Zr, U, Th) Phases During the Recovery of (NH_4)_3ScF_6 from Strip Liquors by Anti-solvent Crystallization

摘要

Scandium can be extracted from waste streams of other industrial processes, particularly the bauxite residue and TiO_2 acid waste, by acidic leaching and solvent extraction of the leach solutions. Stripping of the organic phase using NH_4F solutions produces strip liquors containing Sc (＞2000 mg/L). Scandium can be separated from these liquors by anti-solvent crystallization of (NH_4)_3ScF_6. In this study, the extent to which impurities co-precipitate as separate crystalline phases or are incorporated into the crystal lattice of (NH_4)_3ScF_6 was investigated. The impurity metals Fe, Zr, and U co-precipitated with the Sc phase. Moderate Ti precipitation was only observed from strip liquors containing mainly Fe and Ti impurities. Detection of these phases by powder XRD was difficult due to almost similar peak positions of the ammonium metal hexafluoride salts. However, EDS confirmed that the impurity metals were present in the precipitates in relative abundances that matched non-proportionally those of the initial strip liquors, except for Ti. SEM images showed that (NH_4)_3ScF_6 crystals obtained from strip liquors containing predominantly scandium were bigger (2-3 μm) compared to crystals of the mixed precipitate samples (＜2 μm) obtained from strip liquors containing relatively high impurity levels. This could be attributed to surface diffusion impediment of one metal ion by other metal ions at the solid-liquid interface and surface incorporation of foreign metal ions on the growth steps or kinks of one solid phase, thereby reducing the crystal growth rate of that phase. The excess supersaturation is then consumed by crystal nucleation as observed.