Dissolution of bornite in sulfuric acid using oxygen as oxidant

Abstract

Leaching of natural bornite in a sulfuric acid solution with oxygen as oxidant was investigated using the parameters: temperature, particle size, initial concentration of ferrous, ferric and cupric ions, and using microscopic, X-ray and electronprobe microanalysis to characterize the reaction products. Additionally, stirring rate, pH and P O 2 were varied. Dissolution curves for percent copper extracted as a function of time were sigmoidal in shape with three distinct periods of reaction: induction, autocatalytic and post-autocatalytic which levelled off at 28% dissolution of copper. The length of the induction period was not reproducible, causing the dissolution curves to be shifted with respect to time. The dissolution curves in the autocatalytic and post-autocatalytic regions were reproducible, and this property was utilized to treat much of the kinetic data. The iron dissolution curves had four dissolution regions. An initial small but rapid release of iron to solution preceded the three periods just given for copper dissolution. Aside from this initial iron release, the iron and copper dissolution curves were almost identical. Stirring rate had no effect on dissolution of copper above 400 min −1 nor did oxygen flow rate in the range 20–40 cm 3/min. Dissolution rate was slightly dependent on oxygen partial pressure for P O 2 < 0.67. Hydrogen ion concentration had no effect except that sufficient acid was required to prevent hydrolysis and precipitation of iron salts. The dissolution rate was directly dependent on the reciprocal of particle diameter indicating possible surface chemical reaction control, but the activation energy of 35.9 kJ/mol (8.58 kcal/mol) for the autocatalytic region of copper dissolution is slightly too small for that, though not unreasonable. Initial addition of Fe 2+ had a rather complex effect and markedly enhanced dissolution of copper, as also did initial addition of Fe 3+. Microscopic analysis showed nuclei of two new phases, covellite and Cu 3FeS 4, in the induction region. The new phases grow rapidly in the autocatalytic stage, which is controlled by nuclei formation and chemical reaction. The post-autocatalytic region is characterized by complete transformation of bornite into covellite on the particle surfaces and Cu 3FeS 4 as an internal product with an X-ray spectrum very similar to that of chalcopyrite. The post-autocatalytic region is controlled by autocatalytic growth of newly formed phases. Further reaction beyond the autocatalytic region (percent copper dissolution > 28%) occurs so slowly with oxygen as oxidant that it was not studied. The rate of copper dissolution appears to be controlled by the rate of iron dissolution. Using that and the other experimental evidence a mechanism for reaction is proposed in which iron-deficient bornite, Cu 5Fe ▭S 4, is formed on the surface by initial preferential iron dissolution. Labile Cu + diffuses into this from Cu 5Fe ▭SO 4 and unreacted bornite to produce CuS on the surface. Depletion of labile Cu + ions from Cu 5FeS 4 produces Cu 3FeS 4 in the interior of the mineral particles.