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

The geoenvironmental characterization of chromite ore processing residue (COPR) at two deposition sites in New Jersey and Maryland involved geotechnical, chemical, mineralogical, and leaching analyses of three main COPR types: gray-black, hard brown, and clayey. Quantitative mineralogical analyses were essential in identifying the geochemical differences among these types, leading to a framework for predicting COPR responses to remediation. The study revealed that the mineralogy of COPR resembled cement, with hydration and pozzolanic reactions dominating its geochemistry. The gray-black COPR remained largely unreacted despite prolonged exposure to humid conditions, while the hard brown COPR was fully hydrated and possessed high chromium (Cr(VI)) concentrations. Both materials exhibited chemical similarity, with dilution being a factor for differences in chemical and density properties. Although the total acid neutralization capacity of gray-black and hard brown COPR was identical, the latter displayed higher neutralization capacity at high pH levels due to the presence of hydrating materials, which resulted in enhanced buffering capacity and reduced chromium leaching levels. The findings conclude that gray-black and hard brown materials originated from the same ore and processing methods, with postdepositional transformations contributing to the development of hard brown layers in COPR sites, complicating in situ reductive treatment due to specific physicochemical properties.