Importance of Mineralogy in the Geoenvironmental Characterization and Treatment of Chromite Ore Processing Residue

The geoenvironmental characterization of COPR at two deposition sites (New Jersey and Maryland) included geotechnical, chemical, mineralogical, and leaching analyses of three main chromite ore processing residue (COPR) types [gray-black (GB), hard brown (HB), clayey (C)]. Quantitative mineralogical analyses were instrumental in the delineation of the geochemical differences between the three COPR types, which enabled a framework to predict COPR response to potential remediation schemes. Overall, COPR mineralogy resembled cement, with hydration and pozzolanic reactions dominating its geochemistry. GB COPR was largely unreacted despite its prolonged exposure to humid conditions, while HB COPR was completely hydrated and contained high Cr(VI) concentrations. The two materials were chemically similar, with dilution accounting for the chemical and density differences. While the total acid neutralization capacity (ANC) of GB and HB was the same, the ANC at high pH (8-12) was higher in HB due to the dominance of hydrating materials, leading to more buffering capacity and lower Cr(VI) leaching levels. It is concluded that GB and HB were derived from the same ore and process and that postdepositional transformations account for the emergence of HB layers in COPR sites. The physicochemical properties of HB [hardness, high and inaccessible Cr(VI), high ANC] are complicating factors for in situ COPR reductive treatment in the presence of HB.