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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.