A review of rate equations proposed for microbial ferrous-iron oxidation with a view to application to heap bioleaching

Abstract

In view of the fact that the microbial oxidation of ferrous iron to the ferric form is an essential sub-process in the bioleaching of sulphide minerals, the development of a comprehensive rate equation for this sub-process is critical. Such a rate equation is necessary for the design and modelling of both tank and heap bioleach systems. Most of the rate equations presented in the literature define the specific microbial growth rate using a Monod-type form for ferrous substrate limitation, with further terms added to account for ferric product inhibition, ferrous substrate limitation and inhibition. A few of the published rate equations describe the specific substrate utilization rate in terms of a modified Michaelis–Menten equation and include the maximum yield constant and cell maintenance via the Pirt equation. Other rate equations are based on chemiosmotic theory or an analogy with an electrochemical cell. In the present paper a selection of rate equations are compared against each other by calibrating them against the same set of data and comparing the fits. It was found that none fits the data particularly well and that some of the underlying assumptions need to be questioned. In particular, it appears that ferric inhibition is perhaps not as significant a factor than previously assumed and that rate control by the availability of ferrous is more significant. Some rate equations include terms to account for the effects of temperature, pH, biomass concentration, ionic strength as well as inhibition due to arsenic. In general these effects have been studied in isolation and in ranges not too far off the optimum. Few rate equations combine more than 2 effects and there is no clarity on how a comprehensive model to account for all effects should be constructed. Rate equations have been applied to tank bioleach systems, which usually operate under controlled conditions near the optimum. Heap bioleach systems, on the other hand, often operate far from optimum conditions with respect to temperature, pH, solution conditions, etc., at the same time. The kinetics of such sub-optimal systems are still poorly understood. Future studies should be directed towards the development of a comprehensive rate equation useful for describing the kinetics of heap bioleaching over a wide range of conditions.