EVALUATION OF THE CHEMICAL AND BIOLOGICAL CHARACTERISTICS OF A FERRIC SULFATE LEACH MEDIUM (SPECIATION, ORE, FERROUS IRON, CHEMICAL MODEL, OXIDATION)

This report presents the results of an investigation to evaluate the chemical and biological factors associated with an acidic (pH = 1 to 2.5) ferric sulfate leach media. The investigation was divided into the following three study phases: (i) Phase I evaluated the equilibrium chemistry of a microbial leach solution containing ferric iron, (ii) Phase II evaluated leach solution chemistry and the effect of ferric iron speciation on Cu(,2)S ore dissolution, and (iii) Phase III evaluated microbial ferrous iron oxidation in a ferrous sulfate leach medium. Phase I entailed modification and calibration of a chemical equilibrium computer model using analysis of synthetic solutions containing ferric sulfate at concentrations representative of microbial leach solutions. Phase II examined the first stage of chalcocite ore dissolution in synthetic ferric sulfate leach solutions of varying chemical compositions and pH. Phase III work examined ferrous iron oxidation by Thiobacillus ferrooxidans in both batch and continuous flow-through reactors. The modified chemical equilibrium model was used to aid in interpretation of Phase II and Phase III results. Phase III results were also compared to a thermodynamic biological model.; The information obtained from this investigation could be used to better control and enhance commerical microbial leaching operations, as well as to provide knowledge of similar chemical and biological reactions that may occur naturally, as in surface waters and groundwaters affected by mine acid drainage. Future work should include evaluation of microbial oxidation of elemental sulfur that forms as a result of ferric iron oxidation of metal sulfide ores such as chalcopyrite and chalcocite.