Control of Acid Generation from Pyrite Oxidation in a Highly Reactive Natural Waste: A Laboratory Case Study

Laboratory kinetic leach column (KLC) tests were carried out to define the conditions required to control acid generation from a highly reactive, potentially acid-forming (PAF) iron ore waste rock. It was found that lime addition (0.1 wt% blended) plus either blending of silicates (25 wt% K-feldspar and 25 wt % chlorite), or addition of a non-acid forming (NAF) top cover containing about 10% dolomite (PAF: NAF = 5: 1 wt %), when watered/flushed with lime-saturated water, greatly reduced acid generation as compared to the control KLC (PAF alone, watered/flushed with Milli-Q water), but did not result in circum-neutral pH as required for pyrite surface passivation and effective acid and metalliferous drainage (AMD) mitigation. In contrast, the combined use of these treatments-blended lime and silicates with an NAF cover and watering/flushing with lime-saturated water-resulted in leachate pH of approximate to 12 (up to 24 weeks). Mass balance calculations for Ca2+ and scanning electron microscopy (SEM) analyses suggest that calcite or gypsum may have formed in the NAF-amended KLCs and lime with added silicate KLC. Although the combined approach in the form trialled here may not be practical or cost-effective, control of a highly reactive natural PAF waste by pyrite surface passivation appears to be possible, and an improved treatment methodology (e.g., slightly increased lime blending without the need for further lime watering/flushing) could usefully be examined in the future.