Development of a method to assay the microbial population in heap bioleaching operations

Abstract

Heap bioleaching is an economically viable approach to the mining of low-grade ores. Oxidation is microbially assisted, involving a consortium of microorganisms that together span the mesophilic to extreme thermophilic range of temperatures (25–80 °C). Temperatures inside the heap are not externally regulated, making the microbial interactions difficult to predict. In order to gain insight into these interactions, a qualitative and quantitative assay of the microorganisms that colonise the ore surface or are present in the liquid phase between the ore clusters at different levels within a heap has been developed. This method was developed using crude ore and liquid samples obtained from the GeoBiotics temperature controlled mesophilic heap operation at the Agnes Gold Mine in Barberton, South Africa, and the high temperature test columns at SGS Lakefield Research, Johannesburg, South Africa. This method of sample analysis may be applied to bioheap leach operations with and without temperature control. Ease of application, reproducibility and turn around time influenced technique design in order to provide a useful assay for commercial bioleaching operations. Following microbial removal from the solid phase using successive washes with detergent and acidified water, the cells were enumerated and genetic DNA was isolated. Microbial identification was achieved via restriction endonuclease analysis of the 16S rRNA genes, as well as 16S rRNA gene sequencing where necessary. Quantification was achieved using species-and genus-specific probes through fluorescent in situ hybridisation (FISH).