TECK and Vale operate their medium temperature copper and nickel sulfide concentrate leaching processes at 150°C. "Medium temperature" is used to describe a variety of processes that operate above the melting point of sulfur (119°C) but below the temperature where sulfur become highly viscous (159°C). During leaching, and depending upon various process parameters, iron (Fe) may precipitate as hematite, goethite, jarosite or other oxyhydroxide compounds. Hematite is the favored precipitate

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... e it is the most environmentally (thermodynamically) stable and does not ad/absorb as much copper (Cu), nickel (Ni), or other solution constituents during precipitation. A better understanding of the formation and structure of these iron precipitates may elucidate key factors that would ultimately result in lower valuable metal losses and more stable leach residues. This thesis details the experimental work performed to clarify the conditions under which the precipitation of highly crystalline hematite occurs during medium temperature leaching of copper sulfide concentrates. Various process parameters at the lab scale were studied and classical, as well as newly developed, methods to identify the optimal conditions for hematite precipitation were employed. Higher acid concentrations resulted in increased copper extractions and favor the formation of hematite during concentrate leaching, rather than other metastable phases. Seeding with synthetic hematite resulted in more crystalline residues. Furthermore, commercially available water displacement formula 'WD40®' and other novel reagents (benzene sulfonic acid, phenyl phosphonic acid, decane, mineral oil) affect Fe precipitation and sulfur chemistry, leading to very different process outcomes such as improved extractions (from 98.0 to 99.2%) and larger, more easily separated, sulfur particles (from 20 µm to 1 mm). The solubility of ferrihydrite (and its main transformation product, hematite) increased with increasing acid concentration. The solid-state transformation of fer [...]