Tailings are the product of processing ore to recover the contained metals. After extraction from the mine, the ore is crushed and subsequently ground to a particle size that allows for the liberation of the metals from the host rock. Most mineral deposits are composed of many different minerals, some of which contain metals. A mineral deposit may be the host to more than one metal but not all metals may exist in sufficient quantities to be of economic interest. The mineralogy of the deposit and the method used to recover the metals of interest define the tailings management system.
The Mt Todd project will grind the ore to a nominal particle size of ~100 micrometers and then use sodium cyanide in solution to recover the precious metals. The process of recovering the precious metals from the finely ground ore is called leaching and will occur in large steel tanks. This type of recovery technique is commonly used throughout the world. Following the recovery of the precious metals, the pulp (a mixture of finely ground ore and water) will be thickened to recover part of the water containing the cyanide. The processing plant will produce tailings as a slurry at a density at about 55% solids and 45% water. The thickened pulp, or tailings, will then be treated to destroy the remaining cyanide prior to the tailings leaving the processing plant. The tailings will be pumped to the tailings storage facility which will consist of an engineered earth dam and a water recovery system. The earth dam will be constructed in a manner that ensures that the fluids inside the tailings storage facility are contained and not able to leak into the surrounding environment. The structural integrity of the dam and the containment system, which will be composed of synthetic liners and clay, will be a priority throughout the life of the facility and beyond. Tailings will be discharged from one or more moveable discharge points located around the perimeter. Once placed in the tailings impoundment facility, the fine ground ore will quickly settle out and leave a pool of water that will be pumped to the process plant.
Over the life of the tailings storage facility the settling of the finely ground ore will result in a dense, layered and relatively impermeable mass. The Mt Todd project will have two tailings storage facilities, the first will be the existing storage facility which will be raised to contain an additional 60 million tonnes of tailings and a second new storage facility will be constructed later in the project life to contain the remainder of the tailings. The tailings storage facilities will be designed to permanently contain the processed ore from the project. In addition, they will be designed and operated to ensure that there is sufficient extra capacity to contain the water normally present plus additional water that will accumulate during the cyclonic events of each wet season.
Once a tailings storage facility reaches its design capacity, the remaining water will be pumped from the tailings storage facility to either be used in the process plant or treated in the water treatment plant prior to release as clean water. After removing the water from the tailings storage facility, the surface will be covered with layers of rock, clay and topsoil to ensure that water will drain from the rehabilitated surface rather than infiltrate into the tailings. Long term monitoring will be conducted to ensure the integrity of the cap and rehabilitated surface. The goal of the rehabilitation will be to restore the area of the tailings impoundment facility to a natural and productive state.