Evaluation of PGE Liberation and Chromium Isolation in a Solid UG2 Chromitite Concentrates at Moderate Temperatures Using ICP-OES

Abstract

Complete sample digestion is a prerequisite in achieving accurate and reproducible results in wet chemical analysis as well as effective element recovery in hydrometallurgical beneficiation processes. Inductively coupled plasma-optical emission spectroscopy was used to evaluate the efficiency of (NH₄)₂HPO₄/(NH₄)H₂PO₄, Na₂HPO₄/NaH₂PO₄·H₂O (800°C), NH₄F·HF flux (250°C), microwave dissolution using HCl and aqua regia acids (240°C) to dissolve and liberate the platinum group metals (PGE) in a Upper Group 2 (UG2) chromitite concentrate sample. Complete digestion of the UG2 chromitite ore was achieved using Na₂HPO₄/NaH₂PO₄·H₂O and (NH₄)₂HPO₄/(NH₄)H₂PO₄ flux mixtures and average PGE (Ru, Os and Pt) yields of 1.90 g/kg (Ru), 0.88 g/kg (Os), 2.52 g/kg (Pt) were obtained using Sc as internal standard. Fusion with NH₄F·HF yielded 0.85 g/kg (Ru), 0.72 g/kg (Os) and 0.95 g/kg (Pt) whilst microwave dissolution using HCl and aqua regia yielded an average of 0.77 g/kg (Ru), 0.08 g/kg (Os) and 0.35 g/kg (Pt). Sodium phosphate flux, however, introduced Na⁺ ions as easily ionised elements, which affected the emission intensities to yield slightly inflated PGE (Ru, Os and Pt) yields. The use of ammonium phosphate and sodium phosphate at 800°C (after the selective removal of Na⁺ ions) proved to better the fluxes and produced higher and consistent PGE yields. The use of ammonium phosphate flux was also shown to facilitate the isolation of a green chromium precipitate with a 98.9% purity, which may assist in a hydrometallurgical beneficiation process of the UG2 chromitite concentrate ore and may also have important implications for the ferro-chrome industry.