Selective gold recovery from complex solutions via electrochemical deposition with redox replacement

Global transition toward a carbon-neutral economy and broader use of renewable energy increases demand for raw materials and metals in particular, including gold. Despite gold cyanidation being the de facto industrial standard, concerns about its adverse effects on the environment and human health stimulated the search for alternatives. The electrodeposition-redox replacement (EDRR) method provides a unique possibility for selective, additive-free, and fully electrified metal recovery from complex industrial process streams. This dissertation investigates the recovery of gold from multimetal chloride solution by EDRR as a basis for non-cyanide technology for processing refractory gold ores.

Initial EDRR experiments aimed to outline the detailed reaction mechanism of EDRR and study the effect of process variables using model gold and copper chloride solutions. Electroanalytical techniques, such as cyclic voltammetry, electrochemical quartz crystal microbalance, and rotating ring-disk electrode, were employed for this purpose. The quantity and quality of the produced gold deposits were analyzed with scanning electron microscopy, X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry. The experiments with two-component solutions demonstrated the viability of EDRR: gold recovery of 94.4% was achieved in an 8.5-hour experiment, with the purity of the final product reaching 93.7 wt% Au.

As part of the process upscaling effort, four alloys were evaluated as potential cathode materials for a large-scale process. The high corrosion resistance of the cathode material in the chloride leaching solution was found to inversely correlate with gold recovery and energy efficiency of the EDRR process due to electrode surface passivation. In a trade-off between performance and durability under typical EDRR conditions, the highly alloyed superaustenitic stainless steel 654SMO was selected as the optimal cathode material.

In order to validate EDRR as a relevant gold recovery process, a continuous mini-pilot test was conducted involving leaching of refractory gold ore, filtration of the leach solution, recovery of metallic gold by EDRR and recycling of spent electrolyte. After 150 h of continuous operation, about 83% of the dissolved gold was recovered from solution onto the cathode. According to the process simulation results obtained using HSC Chemistry 10 software, the recirculation of intermediate streams within the flowsheet would increase the total gold recovery from the ore to the cathode up to 84%, exceeding that of the conventional cyanidation process.

With the increasing affordability and availability of renewable energy, electrochemical metal recovery methods such as EDRR will become a feasible option to advance cyanide-free gold extraction technologies and reduce the carbon footprint of the extractive industry.