Effect of microbial consortium on the biokinetic test for assessing acid rock drainage potential

Abstract

Acid rock drainage (ARD) is one of the most severe environmental challenges currently faced by the mining industry worldwide. ARD is formed from the oxidation of sulphide-bearing minerals, particularly pyrite, in the presence of water and oxygen. ARD generation is accelerated by the presence of naturally occurring iron and sulphur-oxidizing micro-organisms, which regenerate leaching agents that facilitate sulphide mineral oxidation. ARD pollution is characterized by a high concentration of metals and sulphates in solution, low pH and a high salt content (salinity) in the environment, contaminating soil and groundwater. In South Africa, ARD is a major challenge in the gold and coal mining industries, where millions of tons of sulphide waste rock and overburden are generated and discarded. Characterization of these waste materials is required to develop an appropriate disposal strategy to minimise the risk of pollution and the generation of ARD. Potential ARD generation prediction from waste rock depends on the precise characterization of ARD potential using Biokinetic tests. Commonly used ARD prediction methods are static and long-term kinetic tests. Static tests provide data for a worst-case scenario focussing on strong acid chemical leaching potential to give an overall acid forming potential of a sample. Such kinetic tests provide data illustrating the rate of the net acid generation capacity of mine waste. However, these tests are capital intensive and time-consuming and fail to provide adequate information on the effect of micro-organisms on the overall net acid generation capacity of mine waste. The Biokinetic test reported herein and developed at the University of Cape Town, focusses on addressing a worst case scenario provided by static tests in a cost-effective manner and reduced time frames provided for by conventional kinetic tests. This test primarily provides relative rates of ARD generation in the presence of micro-organisms within 90 days. However, the Biokinetic test is at the developmental stage and thus far, has not been consistently used for different waste ores to determine a standardised approach. Therefore, the aim of this study was to investigate the effects of microbial consortia and to develop a standardisation approach for the test for ARD formation potential using gold-bearing and copper-bearing waste rock. Additionally, to refine the Semi-continuous Biokinetic test simulation, a flow-through system where there is minimal seepage in the waste deposit, was also developed. The sulphur content of the gold and copper-bearing samples used in this study was between 2.3 and 3.15%, respectively. These waste rock samples were found to be potentially acid- forming. In the Biokinetic test, finely milled waste rock samples were slurrified, inoculated with consortia and cultured under standard bioleaching conditions. Leaching and acidification rates were monitored.