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A novel and sustainable technology to recover gold from thiosulfate medium using ionic liquids, i.e., Cyphos IL 101 (Cy IL 101) and Cyphos IL 102 (Cy IL 102) diluted in toluene, has been developed. Gold was extracted into the ionic liquid phase as [{P66614 +}3{Au­(S2O3)2 3–}] and stripped using NaCl solution. The recyclability of ionic liquids has shown promising recirculation of the solvents for the extraction of gold from thiosulfate medium. Gold was quantitatively extracted from 0.2 mol/L sodium thiosulfate initially containing 10 mg/L gold at pH = 9.0 with 1.25 mmol/L ionic liquid (Cy IL 101 or Cy IL 102) in one stage at A/O = 2, whereas total gold stripping with 1.5 mol/L NaCl also needed one stage at A/O = 1. Using a high A/O ratio in the extraction stage (A/O = 10) and low (A/O = 1/10) in the stripping stage confirmed the economic and environmental viability of the process. The results revealed that Cy IL 101 presents slightly better behavior toward gold recovery than Cy IL 102 and is a viable and promising alternative to recover gold from the thiosulfate medium on a pilot scale. The overall study confirmed the suitability of the developed scheme for industrial application to provide economic and environmental benefits.

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Thiocyanate (SCN–) is a promising alternative to cyanide as a lixiviant for gold extraction and is 1000 times less toxic than cyanide. In this study, the following leaching parameters were tested to optimize the gold recovery for the first time from an oxide ore using the response surface methodology: initial thiocyanate concentration (10–500 mM), initial Fe3+ concentration (10–500 mM), and pulp density (10–50% w/v). The maximum gold recovery (96%) was achieved with 500 mM thiocyanate, 100 mM Fe3+, and 50% pulp density at 25 °C and pH = 2 for 24 h. A kinetic study on the optimum leaching condition showed that it followed the shrinking core model, in which the rate-controlling mechanism was the diffusion process. These results are discussed in the context of the published literature.

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The decreasing grade of gold deposits and environmental regulations concerning the use of cyanide, a conventional extraction agent used in gold recovery, has highlighted the challenge in the field of gold extraction. The bioleaching of gold using microorganisms for in situ cyanide production to recover gold is a promising new approach for prohibiting the use of conventional chemical cyanide. For the first time, this study reports a comprehensive investigation on the role of pH on individual steps of Bacillus megaterium bacterial growth, cyanide production, and gold recovery. This novel way of maximizing biogenic cyanide is highly efficient and demonstrates biocyanidation as a potentially viable technique for direct treatment of sulfidic gold ores, i.e., eliminating the step of biooxidation for the first time. The low-grade sources were treated with both chemical cyanide and biogenic cyanide generated by B. megaterium. Results indicate that the bacteria generated a maximum of 61.89 ppm cyanide, which correspondingly recovered over 87% and 43% gold from O1 and O2 sources, respectively, comparable to gold recoveries by chemical leaching. The pure gold powder leaching was also performed to evaluate the loss in gold recovery due to the biosorption of gold to bacterial cells. The feasibility of the work is also supported by green chemistry metrics, and a comparison has been made between conventional and biocyanide leaching. The results revealed the potential of biocyanidation for industrial applications.

Activated carbon has been used for gold recovery in the gold mining industry commercially for decades. The high specific surface area and porosity, good affinity to aurocyanide ions, and abundant resources make activated carbon an efficient and economical material for the adsorption of aurocyanide. However, the separation of activated carbon from the slurry is usually a challenge, and the adsorption rate of activated carbon is limited by the coarse particle size. Herein, a simple and sustainable way to recover gold from cyanide solution using magnetic activated carbon synthesized via a solvothermal method has been developed. The synthesized magnetic activated carbon possesses good magnetism (44.57 emu/g) and specific surface area equal to 249.7 m2/g. The magnetic activated carbon showed 99.1% recovery efficiency of gold from 10 mg/L solution within 5 h, which is much faster compared to the commercial granular activated carbon, and the magnetic activated carbon can be easily separated from the solution with an external magnet. The adsorption ability of this magnetic activated carbon has been tested under different conditions in the cyanide solution, the adsorption isotherm and kinetics are also investigated. The magnetic activated carbon was also recycled in the adsorption–desorption tests and showed good reusability.

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