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Modderfontein, South Africa

The dissolution of the feldspars, one of the most common of the rock-forming minerals, is important in fields as diverse as hydrometallurgy, environment sciences and geochemistry. The rate of dissolution of feldspars is dependent on the pH of the solution. The data from published sources on the rate of dissolution have been reassessed. This analysis indicates that there are three regions of interest in the pH range between 1 and 12. At low values of pH, below 3, the order of reaction with respect to H+ is close to 0.5. In the pH region between 3 and 7, the analysis of the experimental data shows that the order of reaction with respect to H+ changes to 0.25. In the alkaline region above a pH of 7, the rate of reaction is dependent on the concentration of the OH- ions. The order of reaction in this alkaline region is 0.5 with respect to OH-. A novel theory of dissolution is proposed to account for these orders of reaction, which are derived from first principles with no adjustable parameters. The mechanism posits that the removal of aluminium and silicate ions from the surface occurs in parallel and that these parallel reactions are dependent on the potential difference across the Helmholtz layer at the surface. The relationship of the proposed mechanism with other models is discussed. © 2014 Elsevier B.V.

Crundwell F.K.,CM Solutions Pty Ltd
Hydrometallurgy | Year: 2013

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. © 2013 Elsevier B.V.

Crundwell F.K.,CM Solutions Pty Ltd
Hydrometallurgy | Year: 2014

The dissolution of forsterite and other minerals of the olivine and phenakite silicate groups are described by a novel mechanism of dissolution. For many of these minerals, the order of reaction with respect to H+ is close to 0.5 in the acidic region. The mechanismof dissolution proposed here correctly predicts these orders of reaction without any adjustable parameters. Recentwork has shown that the order of reaction of forsterite with respect to H+ changes from0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. Previously proposed models of dissolution cannot predict this change in order of reaction, whereas the mechanism proposed here does predict this change in reaction order, again without any adjustable parameters. The mechanism proposes that the reason for the change in order of reaction is that the H+ needs to be positioned at the inner Helmholtz plane to be effective at higher values of pH. The acceleration of the rate of dissolution by organic acids and the retardation of the rate by dissolved silica and carbon dioxide are also correctly predicted. The mechanism predicts a change in the interfacial potential difference at the same value of pH that the order of reaction changes. This prediction is verified by measurements of the zeta potential,which reflects the predicted change in surface potential at a pH of approximately 6. The proposed mechanism provides a framework for the interpretation of the correlation between the rate of dissolution of the orthosilicates and the exchanges rates ofwater in the inner sphere of the corresponding metal aqua-ion. © 2014 Elsevier B.V. All rights reserved.

Feldspars are one of the most common minerals on the earth's crust. The rate of dissolution of these minerals is inhibited as the concentrations of the products, particularly aluminium and silica, increase. Simple arguments based on both classical and irreversible thermodynamics fail to properly describe the experimental results. In this work, the mechanism presented in Part 1 of this series of papers is extended to account for the equilibrium conditions. The mechanism of dissolution proposed in Part 1 envisages that the reaction occurs by the parallel removal of aluminium and silica components from the surface. The parallel nature of this proposal gives rise to the possibility of partial equilibrium, due to either the removal of aluminium approaching equilibrium or the removal of silica approaching equilibrium. It is shown by the analysis presented in this paper that the available experimental data can be described by the proposed mechanism, in particular, by the phenomenon of partial equilibrium. © 2014 Elsevier B.V.

The reactions of oxide and sulfide minerals with acids are among the most straight-forward of chemical reactions. Despite this, there are still aspects which are not fully understood or explained. The rate of dissolution of these minerals is remarkable, in the sense that their orders of reaction with respect to H+ are most often either 0.5 or 1. In addition, the rate of dissolution is strongly dependent on the metal-oxide bond strength. It is proposed that the breaking of the metal-oxygen or metal-sulfur bond under the influence of the interfacial potential difference determines the rate of dissolution. Both metal atoms and oxygen or sulfur atoms at the surface react independently with species in the solution. The rates of these independent processes are coupled by the potential difference across the Helmholtz layer. The mechanism of dissolution proposed here correctly predicts the observed orders of reaction. © 2014 Elsevier B.V.

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