Investigation of the effect of pH operating conditions on bioleaching of low-grade chalcopyrite in column reactors

Abstract

Bioleaching of low-grade sulphide minerals is now an established process, with much interest in chalcopyrite. However few studies have been carried out on ores containing silicates gangue materials. Chalcopyrite has been reported to be refractory at ambient temperature. Several factors that influence bioleaching kinetics are well documented such as particle size, pH, temperature, galvanic interaction and microbial activity. The purpose of this research was to investigated the effect of pH as well as pre-leaching on bioleaching of silicate rich and low-grade chalcopyrite using mixed thermophilic cultures, with a view to maximize copper solubilization rate in a column reactor operated at 70°C. The column was packed with low-grade chalcopyrite of the size range -20+15 mm. Leaching was monitored at specific time intervals (3 days) by measuring the pH, the redox potential, the copper and iron concentration in the solution. The results of the investigation have shown that copper extracted was highest at pH 1.3 and at moderately low redox potential (410 – 430 mV) using Ag/AgCl electrode, and that pre-leaching contributed insignificantly to the leaching rate. At pH 2.5, the copper extraction was low due to the jarosite. Furthermore, the analysis XRD of leached residues has indicated that the main passivating products were gypsum, jarosite, hexahydrite, and silica. However, although low pH resulted to high copper recovery, the results also showed that the pregnant leach solution (PLS) contained high concentrations of dissolved ions which might have inhibited the microbial activities.