Automated and Quantitative Mineralogy Applied to Chromite Ore Characterization and Beneficiation

A characterization study of chromite ore from South Africa was conducted using bulk assays, X-ray diffraction, optical, scanning electron microscopy (SEM), automated electron probe microanalysis (EPMA) and quantitative evaluation of mineral by scanning electron microscopy (QEMSCAN) mineralogical techniques, and quantitative EPMA. The aim was to identify all major gangue impurities, the degree of chromite liberation, and possible beneficiation options. The bulk material assayed 40.5% Cr2O3 with the major impurities being Al2O3 (13.2%), MgO (12.1%), and SiO2 (7.5%). Quantitative mineral phase analysis showed that the sample mineralogy was dominated by a chrome-rich spinel phase with an average chemical composition (in wt.%) of: Cr2O3—47.8; FeO—26.0; Al2O3—15.4; and MgO—11.0. Contaminant phases included siliceous minerals enstatite, anorthite-rich plagioclase (bytownite), Cr-rich diopside (containing 1–2 wt.% Cr2O3), and phlogopite mica. QEMSCAN analysis of sized fractions indicated that (a) most silicate gangue species were in the +850 µm fractions, (b) the chrome-rich spinel in all fractions was >80% liberated, and (c) the most common mineral association for chromite was with enstatite. Based on the results, upgrading test work demonstrated that stage crushing followed by wet gravity concentration produced a chemical–metallurgical-grade ‘chromite’ product containing >46% Cr2O3 and <1% SiO2.