The Century open-pit zinc mine is near Lawn Hill in the remote lower Gulf region of Queensland.
The Century mine tailings dam has a surface area of 320 ha. The tailings contain high concentrations of sulfides and metals, including lead, zinc, cadmium and manganese. The potential for acid rock drainage from the tailings dam has been managed during operation by maintaining it at near-saturated conditions.
After the mine closes the tailings will dry out, which will increase the potential for acid rock drainage. The problem is further compounded because there are only a few sources of material to use to build a layer to reduce permeability, which will prevent fluids from leaking out of the dam, and prevent water from infiltrating and moving acid rock drainage to the receiving environment.
EMM’s closure, rehabilitation and soils team worked with ATC Williams to develop conceptual cover designs. ATC Williams modelled the cover designs and the three best designs were selected for scaled-up field trials that are being built. EMM developed the instrumentation and monitoring program to measure how the covers perform, which includes volumetric and matric suction sensors to measure the depth and rate of wetting-up and drying-out in the cover. EMM has developed a way to calibrate the matric suction sensors that extends the range of measurement and can be done quickly. A weather station calculates potential evapotranspiration.
The cover trials will be monitored over the next ten years. Based on the results, a final cover design will be selected to close the tailings dam.