Solubility of ammonium metal fluorides in aqueous ethanol mixtures – implications for scandium recovery by antisolvent crystallization

Abstract

The recovery of scandium from waste streams of other mining and metallurgical processing industries is gaining research interest due to the scarcity of scandium-containing ores. Hydrometallurgical techniques such as leaching, solvent extraction and crystallization amongst others have been successfully applied to recover scandium salts from such waste streams. Scandium can be recovered as (NH₄)₃ScF₆ by antisolvent crystallization from NH₄F strip liquors obtained after solvent extraction. The coextraction of metal impurities such as Fe, Al, Zr and Ti causes contamination of the final solid product. The extent of coprecipitation of ammonium metal fluorides depends on their initial concentration in the strip liquor and their solubility in the NH₄F–antisolvent mixtures. Here, the solubility of ammonium metal fluorides of Sc, Zr, Fe, Al and Ti is reported separately in 3 mol L⁻¹ NH₄F–ethanol mixtures at 25 °C as well as in a system containing all five solid phases. The solubility of (NH₄)₃ZrF₇ is slightly higher than that of (NH₄)₃ScF₆, while the solubilities of (NH₄)₃FeF₆ and (NH₄)₃AlF₆ are significantly lower in comparison to (NH₄)₃ScF₆. The solubility of (NH₄)₂TiF₆ is 1–2 orders of magnitude higher than those of other ammonium metal fluorides. When a mixture of ammonium metal fluoride salts is dissolved in the same 3 mol L⁻¹ NH₄F–ethanol mixture as for the individual salts, the resultant solubility of the ammonium metal fluorides of Sc, Zr and Fe decreases significantly, while the resultant solubility of ammonium aluminum hexafluoride increases. This is likely due to changes in solution speciation with increased NH₄F concentration and ionic strength.