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Kinetics, as a fundamental requirement of nearly all industrial activities and engineering researches, plays a great role in leaching processes. Although there are many pieces of research on its application, there is not a clear pathway for investigating the kinetics of leaching and researchers usually follow different strategies in their studies. The conventional investigation techniques, which usually do not consider the mixed mechanisms and possibility of any change in the mechanism, normally include many calculations, plots, and inadequate capabilities to detect changes in the controlling mechanism of leaching. In this review, the main mathematical models of leaching and all possible scenarios are presented and discussed. The effect of various leaching parameters (including leaching agent, temperature, particle size, agitation, and solid to liquid ratio) on the rate of dissolution is summarized. Besides, two main approaches of rate determination step (single controlling mechanism and combined resistances method) are described and compared by reporting related equations and suitable examples. A technique to detect any changes in the leaching controlling mechanism is introduced and the alternatives to confirm the results are described. Additional models and equations were suggested for the cases that there is no agreement between data and the conventional models. Also, situations which are ignored in simple models (e.g., reversibility of the leaching reactions, adsorption and desorption of leached species, influence of charge and surface potential, existence of multiple reactants in the solid, galvanic effect, wide particle size distribution, etc.) to develop more legalistic models are discussed. Considering various possible mechanisms in the kinetics of leaching, equations are derived for industrial leaching reactors.

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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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.

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