Kinetics of leaching: a review

Abstract

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.

The Kinetics of Pyrite Dissolution in Nitric Acid Solution

“…To examine the pyrite dissolution in nitric acid, a 10 g pyrite sample (particle size +75-106 µm) was added to the 200 mL of 3 M nitric acid (solid to liquid ratio of 1:20) when the temperature inside the leaching solution reached the desired temperature (25 to 85 • C). During the experimental runs, samples were withdrawn from the reaction vessel at regular time intervals (10,20,30,60,90, and 120 min) with a pipette and filtered using a 45 µm syringe filter. The iron content of the pregnant leached solution (PLS) was analysed using atomic absorption spectroscopy (AAS) (Agilent, Santa Clara, CA, USA) and used as an indication of pyrite dissolution.…”

“…Leaching is a heterogeneous reaction between solid particles and a liquid [30]. In the case of pyrite leaching in nitric acid solution, the reaction(s) took place at the interface of pyrite (solid phase) and nitric acid (liquid), and the leaching reaction could be expressed as follows:…”

“…HNO 3(l) + FeS 2(s) → Products Generally, during leaching, the following sequence of events occurs: the diffusion of the leaching agent (nitric acid) through a thin liquid film surrounding the pyrite particles; anodic and cathodic reactions occur on the surface of the pyrite particles (including charge transfer); then, the diffusion of the produced products, i.e., Fe 2+ , S 0 , SO 4 2− , and NO x, into the bulk solution takes place. The slowest process(es) acts as a limiting step(s) and controls the kinetic model [30].…”

“…In the case of pyrite leaching in nitric acid solution, the reaction(s) took place at the interface of pyrite (solid phase) and nitric acid (liquid), and the leaching reaction could be expressed as follows: HNO3(l) + FeS2(s) → Products Generally, during leaching, the following sequence of events occurs: the diffusion of the leaching agent (nitric acid) through a thin liquid film surrounding the pyrite particles; anodic and cathodic reactions occur on the surface of the pyrite particles (including charge transfer); then, the diffusion of the produced products, i.e., Fe 2+ , S 0 , SO4 2− , and NOx, into the bulk solution takes place. The slowest process(es) acts as a limiting step(s) and controls the kinetic model [30].…”

Electrochemical Approaches for the Recovery of Metals from Electronic Waste: A Critical Review

“…A metal leaching processes step is required, which depends on the particle size of the grinded waste as well as on the lixiviant nature, concentration, treatment time, temperature, pH, solid-liquid ratio, agitation, and redox potential. These have been reviewed elsewhere [10][11][12].…”