Evaluating applications of bed and fly ash for controlling acid and metalliferous drainage - examples from Tasmanian mine sites

The use of alkaline materials (e.g., limestone (CaCO3), lime (CaO) to neutralise and control acid and metalliferous drainage (AMD) is a well-established global practice. However, these materials are costly therefore alternative cost effective ameliorants for AMD are needed. Alkaline-rich industrial by-products from paper and pulp mills could, in theory, be used. However, the success of such materials is dependent on their physical (e.g., plasticity) and chemical (i.e., chromium, cadmium and copper content) properties.

Hundreds of mine-impacted sites in Tasmania are affected by AMD, and, as many are under the care of the State Government, a cost-effective approach to rehabilitation is needed. This study evaluated the AMD mitigation potential of boiler ash collected from the Boyer Pulp and Paper Mill operated by Norske Skog in Tasmania. These materials were combined in free-draining column leach cells with sulfidic mine waste (tailings and waste rock) from six legacy sites in Tasmania. Two types of ash were used, a fine fly ash (currently landfilled) and a coarser bed ash. Mineralogically, both were Class F ash comprising of mullite, quartz and carbon, minor gypsum and trace CaO.

The column tests were conducted for 24 weeks using a combination of mine waste capped with both types of boiler ash, fly ash blended with lime, and for the tailings, cells using just an organic cover were tested. The use of fly ash as a capping layer was the least effective cover, however its performance was improved when intermingled through waste materials. The use of bed ash as a capping layer was more effective, particularly for tailings. Overall, the best performing cover was blended lime with boiler ash, particularly for low-pyrite, low-As (< 1 wt. %) wastes. These results suggest that boiler ash has a potential application for controlling AMD in bespoke rehabilitation projects.