The replacement of chalcopyrite by copper sulphides and its application in Cu extraction

Author: Zhen He

He, Zhen, 2022 The replacement of chalcopyrite by copper sulphides and its application in Cu extraction, Flinders University, College of Science and Engineering

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Mineral replacement reactions occur widely in Nature and result in the formation of a range of sulphide minerals in the presence of hydrothermal fluids. These mineral replacement reactions under hydrothermal conditions have been intensively studied over past two decades, but only a few studies have focused on the replacement of chalcopyrite by copper sulphides. In Nature, chalcopyrite is replaced by covellite, and digenite in the supergene zone of ore deposits, but the mechanism and kinetics of these reactions are not fully understood. Moreover, the dissolution of chalcopyrite at low temperature is one of biggest challenges in the copper mining sector and previous studies suggest that covellite and digenite more readily dissolve at low temperatures. The purpose of this project was to investigate the mechanisms and kinetics of the replacement of chalcopyrite by covellite and digenite and assess potential applications in ¬in-situ¬ covellite dissolution at low temperature. In this project, the replacement reaction of chalcopyrite by covellite or digenite were investigated in detail under hydrothermal conditions and the kinetic data were extrapolated to lower temperatures. An environmentally friendly chemical reagent (glycine) was chosen for the covellite dissolution study at low temperature.

Chalcopyrite was successfully replaced by covellite in the temperature range of 140-200 °C under acidic condition (pH 0.7-3) via a coupled dissolution-reprecipitation reaction mechanism. Covellite was identified by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis (EDAX). The replacement reaction rate was controlled by solution chemistry (pH values), temperature, availability of dissolved oxygen and the surface area of chalcopyrite grains. Covellite precipitation is the rate-determining step of the overall mineral replacement reaction at temperatures above 140 °C, with elemental sulphur found to be an intermediate product in initial stage of the replacement reaction. At 120 °C, only elemental sulphur was formed after reaction of 18 days without covellite. X-ray photoelectron spectroscopy (XPS) was used to assist in the identification of the reaction mechanism during the chalcopyrite dissolution step. The results show 89±5% of chalcopyrite was replaced by covellite at pH 0.7 and 200 °C after 16 h in the presence of 0.44 mmol H2O2. The average activation energies of replacement reaction at pH 0.7, 1 and 1.5 are 90±21 kJ/mol, 86±17 kJ/mol and 27±7 kJ/mol respectively.

Covellite precipitated as the dominant product in the replacement of chalcopyrite by copper sulphides in the presence of added Cu2+. Digenite was formed by replacing chalcopyrite only at or above 180 °C in pH 2 and pH 2.5 solutions with 0.22 mmol CuCl2 and 0.44 mmol H2O2. At pH 3, chalcopyrite was replaced by digenite and covellite at temperatures between 140-180 °C with same added amount of Cu (II) ions and oxidants. The replacement reaction proceeds via dissolution of chalcopyrite and reprecipitation of digenite and covellite. The surface morphology of digenite was imaged by SEM. The fraction of digenite in the final solid residues of replacement reaction was found to increase with: increasing concentration of CuCl2 (up to 0.66 mmol), increasing temperature (up to 200 °C); decreasing acidic condition (pH 1-pH 2.5) or increasing specific surface area. The activation energies of chalcopyrite dissolution during the replacement reactions at pH 2 and 2.5 are 44±6 kJ/mol and 46±14 kJ/mol respectively.

The dissolution of covellite was investigated by using alkaline glycine solution at low temperatures (25-55 °C). The Cu concentration in the solution was measured by inductively coupled plasma optical emission spectrometry (ICP-OES). The results revealed that 57±5% of covellite, 38-75 µm size fraction was dissolved and Cu-glycinate complexes were formed at 55 °C in pH 11 solutions with 0.5 M glycine after 107 h of reaction. The dissolution rate of covellite was influenced by OH- concentration, availability of dissolved oxygen and the glycine concentration. The shrinking core model was used to determine the dissolution rate and controlled by diffusion through product layer. The activation energy of covellite dissolution is 33±7 kJ/mol.  

Keywords: chalcopyrite, covellite, digenite, mineral replacement reaction, glycine, copper extraction

Subject: Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Professor Sarah Harmer