Chalcopyrite Dissolution in Sulphate-Based Leaching and Bioleaching Systems

Sammanfattning: Chalcopyrite (CuFeS2) is the most abundant and the most economically important copper mineral. Increasing worldwide demand for copper accompanied by exhaustion of copper resources necessitate the development of new processes for treating lower-grade copper ores. Heap (bio)leaching of copper oxides and secondary sulphides (covellite (CuS) and chalcocite (Cu2S)) is a proven technology and a convenient process due to its simplicity and relatively low capital investment requirement. As a result, the technology appears to be an attractive process option for treatment of low-grade chalcopyrite ores as well. However, chalcopyrite is recalcitrant to leaching and bioleaching in conventional heaps: its dissolution is slow and it halts on a low level of copper extraction usually unacceptable for a commercial practice. Slow dissolution of chalcopyrite is attributed to the formation of compounds on the surface of the mineral during its oxidative dissolution, often termed “passivation”. There is still no consensus about the nature of the passivation layer. There are, however, four proposed candidates suggested in the literature: metal deficient sulphides, polysulphides, jarosite and elemental sulphur. This project was aimed to further investigate the chalcopyrite dissolution and its passivation.In this thesis, dissolution of a pyritic and a pure chalcopyrite concentrate was studied in stirred tank reactors in the absence and presence of moderately thermophilic microorganisms. The abiotic experiments were performed under accurately controlled redox potential conditions to reproduce the same oxidising conditions recorded during the bioleaching experiments. The results showed that the microorganisms did not have any effect in the copper leaching efficiency other than oxidation of ferrous ions to ferric ions. Comparing the copper dissolution rates in the experiments where bulk elemental sulphur was formed with those experiments where the elemental sulphur was oxidised to sulphate due to microbial activity showed that the bulk elemental sulphur did not hinder the dissolution. The same phenomenon was observed in case of bulk jarosite. Under otherwise identical leaching conditions, the presence of bulk jarosite did not decrease the leaching efficiency compared to those experiments were bulk jarosite was not formed. It was also shown that surface spectroscopic methodologies such as X-ray photoelectron spectroscopy (XPS) cannot be applied on powder leached samples due to interfering data from the bulk precipitated species. As a result, massive natural chalcopyrite samples were II prepared and used in the leaching experiments for XPS measurements. Different samples in different stages of leaching were extracted from the biotic and abiotic experiments and analysed by XPS. Results indicated that the surface elemental sulphur was not oxidised by bacterial activity. The data revealed that the common phases on the surface of the samples leached biotically and abiotically for different durations were elemental sulphur and iron-(oxy)hydroxides. The surface elemental sulphur was rigidly bound to the surface and did not volatise in the room temperature XPS measurements. Jarosite was observed in only one sample from the abiotic experiment, but no correlation between its presence and the slow dissolution could be made. Other minor surface compounds such as iron-sulphate was also observed in some samples with no contribution to the leaching behaviour. It was concluded that a multi-component surface phase consisting of mainly sulphur and iron-hydroxides were responsible for the slow chalcopyrite dissolution.