The dissolution and leaching of minerals: Mechanisms, myths and misunderstandings

The dissolution of minerals is of importance to a number of fields of endeavour. In particular, it is the rate of dissolution that is important. Knowledge of the kinetics might allow the rate to be accelerated or retarded, depending on the field of endeavour. In understanding the mechanism of dissolution, it is the order of reaction that is of particular interest. The kinetics of dissolution of minerals are frequently found to be close to one-half order in the oxidant. The electrochemical mechanism of dissolution describes this dependence. However, a number of misunderstandings about the dissolution of minerals and the electrochemical mechanism recur, and need to be addressed. This paper addresses seven of these misunderstandings, and makes the following conclusions: (i) mechanism is not the same as chemical pathway, (ii) there is no separation of the surface into anodic sites and cathodic sites, (iii) there is no flow of electrons across the bulk of the mineral, (iv) the oxidation and reduction reactions are coupled by the transfer of electrons, not by a chemically bonded activated state, (v) polysulphides do not passivate the surface, (vi) the first step of the dissolution reaction is not by acid, and (vii) the solids do not need to be electrical conductors to dissolve by the electrochemical mechanism.