Waste management influences mine layout

The vexed question of tailings management on mine sites is coming to the fore with mine owners increasingly having to account for the social and environmental outcomes of their management decisions. World expert in tailings dam design, construction and maintenance, Professor David Williams from University of Queensland says waste management is the single biggest factor influencing community perception of the industry. So much so it’s forced a rethink of the traditional mine layout. This is the fifth in a series of stories developed from an extended interview with Professor Williams who is director of the UQ Geotechnical Engineering Centre and manager of the Large Open Pit Project, which is made up of representatives of the world’s major mining companies. In it he argues that tailing dams located closer to the mine reduces both the costs and the amount of water used in transporting the gangue, or waste, to the tailings dam. It also presented options for dry dumping. Here’s the thing about tailings dams. They are ideally built on geography with some relief that helps containment and are often removed from the processing plant and mine. Tailings are pumped from site to dam in a slurry as a cheaper form of removal. Tailings dams can hold millions of cubic metres of waste which is often toxic. These dams fail at a rate hundred times greater than water reservoirs for a number of reasons but principally because they are not fit for use over the longer term. Cadia tailings dam failure. Tailings dams are usually located further away from the pit and waste rock and plant located closer. That’s counterintuitive in the longer term when tailings dams get larger to accommodate mine growth, Professor Williams said. “Then you think a little bit more laterally than that, you think of, well, what materials do we use to build the tailings dam from? And often, it comprises to a large degree, waste rock,” he said. “So, you have to transport the waste rock to the more distant tailings dam, further. Therefore, the cost of building the tailings dam goes up progressively over time.” “When you start thinking about that. Is there a better location for the tailings dam to be located relative to the pit? “And it may be that if you built a very sturdy buttress-like waste rock dump as your tailings dam, your tailings containment, you might well put that closer to the pit and then put your tailings in behind that.” Designing the tailings dam closer to the pit and plant would also present options for using less water which typically took up half the volume of the dam and also complicated the rehabilitation process said Professor Williams. More reduced costs It also meant a reduced cost of transporting the waste in a solid state, he said. “The more you can reduce that water volume, the better use you make of the storage, the cheaper the storage becomes for your tailings. “And then, down the track, it's so much easier to rehabilitate the tailings surface so you can actually do something with it. “You can actually get to a post-mining land use or a post-mining ecological value of some sort or ecological function. Much easier to do that. “It's very difficult to cover tailings that have been placed as a slurry that aren't properly desiccated (moisture removed). Very difficult to cover them and use them for any purpose down the track. And they'll go unsettling for a long period of time.” The Australasian Institute of Mining and Metallurgy’s Mine Water and Tailings conference will be held in Brisbane from July 14-16. Photo at top of page: Tailings dams at the QNI Yabulu plant north of Townsville

Waste management influences mine layout

Industry Queensland

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