Enhanced release of Fe from mine wastes receiving acid pulse

The potential for contaminant mobilisation is often assessed by leaching techniques. In this study, metalliferrous mill tailings from South Crofty tin mine, West Cornwall, UK were column leached using five chemical leachants, each designed to represent potential post-disposal environmental conditions. The release of copper, iron, zinc, sulphate, and leachate pH, and the occurrence of iron oxidising acidophilic bacteria was monitored during 18 month period. The solubilisation of metals, particularly Fe, was found to be largely influenced by the solution pH and chelating properties of the leachant. Iron released during 18 months of leaching was less than 1% of the total Fe content of each sample. In a leaching test simulating an acid pulse, the Fe released into solution was higher than leaching with deionised water despite the leachate pH returning to that observed from deionised water columns only after three weeks. The confirmed presence of iron oxidising bacteria in all treatments may have had an effect on the release of Fe despite the leachate pH being consistently higher than 4.9. The results indicate that a slight acid pulse through the waste could have a significant detrimental effect on the long term release of iron to the environment.