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The mechanism of bornite leaching: Insights from mineralogical and textural characterisation

Pal-ing, Kevin John (2018) The mechanism of bornite leaching: Insights from mineralogical and textural characterisation. Honours thesis, Murdoch University.

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Copper is a versatile metal used in a range of applications including electronics, electrical and tele-communications industries. The long-term demand for copper has been promising, especially in the development of electric vehicles and renewable energies. To meet the increasing demand of copper, it is important to be able to effectively extract copper from its ores. But the majority of the current reserves for copper minerals are hovering to low grades. Heap leaching is a well-established extractive metallurgy for the processing of various low-grade ores. Additionally, in situ leaching has been suggested as a profitable alternative in recovering inaccessible, deep-seated copper ore bodies, but these processes highly depend on comprehensive understanding of the underlying fundamental leaching mechanism and how it interacts with the mineralogy of the ore body. Bornite (Cu5FeS4) is a major copper sulphide in a wide range of copper orebodies, so understanding the mechanism and kinetics of bornite under heap and in situ leaching conditions is therefore important.

Although, there have been various studies in the past that have helped define the phases and reaction sequences involving oxygen as oxidant and ferric ion leaching of bornite, there are still many uncertainties associated with the effect of various parameters on the leaching mechanism and kinetics.

The mechanisms of bornite leaching in this study were revealed by a thorough kinetic, mineralogical and textural study on its grains at 70 oC and 90 oC for particle sizes of -355 + 150 μm and -53 + 38 μm. Different oxidant types were investigated including O2, Fe3+ and H2O2 in a sulphuric acid solution. The leached solutions and residues were analysed using atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), reflective light optical microscope and scanning electron microscope (SEM). Based on the quantitative and qualitative analysis of the results, it is found that ferric (III) sulphate as oxidant produced the fastest kinetics and led to the highest recovery out of all the three oxidants that has been investigated. Likewise, using hydrogen peroxide as oxidant produced a faster kinetics than using oxygen at shorter leaching times (<24 hours). Nevertheless, at longer leaching times (≥24 hours) oxygen produced a faster kinetics than using hydrogen peroxide. Furthermore, using temperature of 90 oC produced higher recovery and faster kinetics than 70 oC for all conditions. The extent of kinetics at 90 oC, however, is only 2.17% Cu extracted using oxygen as oxidants than at 70 oC. Furthermore, after 192 hours of leaching using ferric (III) sulphate and hydrogen peroxide there are only differences of 0.85% and 2.03% Cu extracted, respectively, between two temperatures. From a mineral processing perspective, leaching at 70 oC may be economically desirable than leaching at 90 oC. It is therefore recommended from these results that leaching at 70 oC may be beneficial to save more money as leaching to higher temperatures requires a lot of energy. Moreover, smaller particle size range of -53 + 38 μm produced a higher recovery than -355 + 150 μm, the difference in the recoveries are not that huge. The difference in copper extraction using oxygen, ferric (III) sulphate, and hydrogen peroxide as oxidants between two particle size ranges at 70 oC are 5.93%, 14.84%, and 3.96% and at 90 oC are 6.73%, 15.04%, and 3.12%, respectively. It is therefore recommended for industries to use larger particles of -355 + 150 μm than -53 + 38 μm as it will be more economically beneficial. This is because grinding to finer sizes in the comminution circuit requires a lot of energy, as this can entail high costs in terms of energy consumption and media use. These costs can be minimised by selecting appropriate operating conditions.

Based on quantitative and qualitative analysis, a mechanism has been proposed for the reactions taking place during the leaching of bornite in this study. For oxygen as oxidant, transformation of the original orthorhombic crystal bornite structure to secondary bornite of a cubic crystal system belonging to F-43m space group with unit cell parameter of a=10.70 Å, then to covellite and chalcopyrite and finally to sulphur has been proposed. For ferric (III) sulphate as oxidant, the bornite evolution from primary to secondary, and then the formation of chalcopyrite lamellae, and finally the transformation into sulphur has been proposed. For hydrogen peroxide as oxidant, mineral transformation is the evolution of initial bornite to a secondary bornite, and then the formation of the covellite, and finally the slow formation of chalcopyrite exsolution lamellae has been proposed. Such mineral replacement processes have been very well recognised in Earth sciences, but they are less known to extractive metallurgists, which makes this study more significant as these same processes prevail in hydrometallurgical processing. The clear mechanism and the associated kinetic data in this study can be further applied to optimise the operation conditions in industrial leaching to improve process efficiency and Cu recovery from bornite.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Xia, Fang
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