Study of pyrite oxidation with chlorine dioxide under mild conditions

Pyrite, usually associated with gold deposit, is disadvantageous to gold leaching. In this study, an environmentally friendly oxidizing agent, chlorine dioxide (ClO 2 ), was used for the first time to efficiently oxidize pyrite under mild experimental conditions, so as to remove the adverse effects of pyrite on gold leaching. Firstly, with established content determination method, the prepared pure ClO 2 solutions with different concentrations were investigated on their impacts on pyrite oxidation efficiency. Secondly, solid NaClO 2 was used to oxidize pyrite under acidic condition, providing a new oxidation approach for practical application. In fact, the key of the approach is the use of solid NaClO 2 in acidic environment to generate ClO 2 and then oxidize pyrite. For the pyrite pulp with 400 mL volume with an initial concentration of 0.025 M, the oxidation efficiencies of Fe and S in pyrite reached 99.8% and 98% at a NaClO 2 concentration of 0.125 M and C 4 H 6 O 6 concentration of 0.0025 M with magnetic stirring for 1 h at 30 °C in a constant temperature water bath. The addition of C 4 H 6 O 6 can increase the acidity of the system and form a complex with iron ions, thereby preventing the precipitation of ferrihydrite from acidity changes caused by overdose NaClO 2 . After redox reaction, the acidity and oxidation–reduction potential (ORP, E h ) of the system were noticeably increased. The results of chemical composition analysis and X ray diffraction (XRD) analysis of pyrite oxidation residue confirmed that pyrite was oxidized to ions by ClO 2 , and no solid by-products such as Fe 2 O 3 or Fe(OH) 3 were formed. The scanning electron microscope (SEM) analysis of residual pyrite indicated that small pyrite particles were oxidized first, and then oxidants eroded the defects of large particles, gradually oxidizing all pyrites. This method is expected to be used as a pretreatment approach for pyrite oxidation before gold leaching process.