Hexavalent chromium mobility in a high amorphous phase Chromite Ore Processing Residue (COPR) in the perspective of a chromium remediation treatment

Abstract

The mineralogical and chemical composition of chromite ore processing residue (COPR) from a site in the north of France (Lille) was investigated. The mineralogical composition was obtained by X ray diffraction and Rietveld analysis. Geochemical characteristics were established based on elemental analysis, acid leaching, sequential extraction and a chemical equilibrium experiment. Remarkably, this COPR material is composed of 65 % amorphous, silica rich phases. Another noticeable result is the presence of about 11 % of quartz. Content in toxic Cr(VI) is about 4.9 g/kg, occuring in the solution phase or fixed in unstable crystalline cement phases. Literature data on most studied COPR materials allowed establishing a classification of the materials into high calcium/low silica, intermediate and low calcium/high silica categories. This calcium to silica relation is indicative of the quality of the original ore and the geochemical changes having occurred in a COPR deposit over time, compared with fresh COPR produced from pure ore. The Lille material belongs to the third category. The high silica content has influenced the phase associations and their stabilities and favours Cr(VI) mobility. Extraction of leachable Cr(VI) from COPR induces formation of a new chemical equilibrium in the material with a recharge in mobile chromium due to dissolution of cement phases. The rate of equilibration however is very slow. 400 days were needed for the high amorphous phase material in this study. Extraction of leachable Cr(VI) is not a suitable remediation method because it will not allow to withdraw the solid bound Cr(VI) from the material in a single treatment.