Using experimental design and response surface methodology (RSM) to optimize gold extraction from refractory sulphidic gold tailings with ionic liquids

© 2020 South African Institute of Mining and Metallurgy. All rights reserved. This work examined the feasibility of gold extraction from a pyrite flotation concentrate sample using an ionic liquid and water mixture as solvent, thiourea complexing agent, and iron(III) sulphate oxidant. A design of experiment (DOE) methodology was used to optimize the process parameters. The purpose of the investigation was to determine how feasible it would be to replace the traditional cyanide extraction process by using an alternative approach, and compare the yield that could be obtained with a less environmentally damaging and hazardous combination of chemicals. Test parameters such as ionic liquid concentration, pulp density, time, and temperature were varied using two imidazolium-based ionic liquids: 1-butyl-3-methylimidazolium hydrogen sulphate [Bmim+HSO4–] and 1-butyl-3-methylimidazolium trifluoromethansulphonate [Bmim+CF3SO3–]. The effects on gold extraction were assessed and screened using a half fractional factorial design (25-1) approach. The ionic liquid concentration, pulp density, and temperature had a statistically significant effect on gold extraction, while the type of ionic liquid and extraction time did not affect the gold extraction as much within the operating range investigated. A high gold extraction was obtained at low ionic liquid concentration, low pulp density, and high temperature. A central composite design in conjunction with response surface methodology were used to create an optimization design with the statistically significant parameters in an attempt to establish the optimal gold extraction conditions. It was found that the optimum concentration of ionic liquid [Bmim+HSO4–] in the aqueous solution was 15% (v/v), pulp density was 15% (w/v), and the temperature 60°C, with a gold extraction of 35.7% under these conditions. This, sadly, was only about half of the yield achieved with the cyanide process. In order to compete with the traditional approach, a way will have to be found to completely destroy the pyrite component in the material, in which a substantial portion of the gold was locked up. This work, and similar studies reported in the literature, indicates that cyanide technology for gold recovery will remain the process of choice in the gold industry for the immediate future

Case study modelling for an ettringite treatment process

Several technologies have been developed to treat acid mine drainage (AMD) and attention is shifting towards the removal of sulphate. The formation of ettringite, a hydrous calcium aluminium sulphate mineral, is an option to treat AMD and can reduce the sulphate concentration to well below the discharge specifications. The process modelled in this study includes the formation of ettringite and the recovery of gibbsite through the decomposition of recycled ettringite. The modelling of this process was done using PHREEQC and the results presented in this paper are based on the outcome of different case studies that investigated how the process is affected by a change in parameters. These include, changing the feed water pH, altering the split fractions in the hydrocyclone for the gibbsite recovery and varying the pH for ettringite formation

Oxidative leaching of refractory sulphidic gold tailings with an ionic liquid

© 2020 Elsevier Ltd The effect of temperature in the range of 20–65 °C on gold extraction from refractory gold tailings containing pyrite in 20% (v/v) of 1-butyl-3-methylimidazolium hydrogen sulphate [Bmim+HSO4−]-water solution with thiourea as complexing agent and iron(III) sulphate oxidant, at pH of 1 was examined for 12 h leaching time intervals. The experimental results revealed that the extraction of gold improved by increasing the temperature. The effect of time showed that a substantial amount of gold extraction took place during the first day of extraction and thereafter very little gold extraction occurred. The ionic liquid [Bmim+HSO4−] is water soluble and in aquatic solution can act like a Brønsted acid by releasing H+ ions. The apparent activation energy (Ea) was calculated based on the Arrhenius theory and it suggests that the kinetic process of gold leaching with this acidic ionic liquid follows shrinking core model with mixed and diffusion controlled reaction regions having Ea of 17.97 kJ/mol and 27.17 kJ/mol, respectively. In the SEM image of the solid residue of the leached sample some pores appeared on the reactive pyrite particles, whereas the pyrite particles in the unleached sample looked smooth. Additionally, Raman spectra detected sulphur shifts in the recorded spectra of the solid residue after leaching which can be an indication of the formation of a product surface layer of elemental sulphur, which supports the diffusion through a product layer in the kinetic model