A comparison of electrochemical methods and the rotating electrochemical quartz crystal microbalance for measuring hydrometallurgical reaction kinetics

As cementation and most leaching processes are electrochemical in nature, there is an interest in the use of electrochemical methods to estimate the rate at which these reactions occur. Despite this, there has been little work published evaluating methods such as linear polarisation resistance (LPR) in hydrometallurgical applications. In the present paper, the kinetics of a range of different metal leaching and cementation reactions was measured using the rotating electrochemical quartz crystal microbalance. The measured reaction rates were then compared to those calculated using Evans' diagrams and LPR. The accuracy of the Evans' diagrams is limited to instances where the constituent half reactions are independent of each other; examples will be provided where this assumption is valid and invalid. LPR will be shown to be problematic, as the polarisation resistance is a measure of both the Tafel slopes and the reaction rate. Examples will be given where the Tafel slope either varies or is difficult to obtain; in these cases, LPR is of limited value. Another problem with LPR which will be highlighted is when the mixed potential is close to the open circuit potential of one of the constituent half reactions; this leads to the polarisation resistance being a combination of the reaction rate and the exchange current density.