Rosemont Copper is planning to use newer technology at its proposed mine in the Santa Rita Mountains south of Tucson to help mitigate the environmental impact of the mine. One of those newer techniques is to use dry-stacked tailings as opposed to the conventional wet tailing stacks that are used at the mines near Green Valley.
Before getting into a comparison of dry versus wet stacking, I will explain what mine tailings are. In the copper deposits south of Tucson, most of the copper is contained within sulfide minerals that usually comprise less than 5% of the rock mass. Copper ore containing chalcopyrite, bornite, or chalcocite is mined and crushed in giant ball or roller mills. The crushed ore is sent to the concentrator plant for flotation extraction. The flotation mill consists of a series of tanks where the crushed ore is stirred into a water solution to make a slurry, and air bubbles are injected into the bottom of the tank. The chemistry is adjusted to gain the desired specific gravity of the solution and set the surface tension of the air bubbles. Sulfide grains stick to the bubbles, which float to the top and are skimmed off. Rock materials sink to the bottom and must be disposed of as tailings.
The photo shows the mines near Green Valley and their associated tailings piles. In the photo the Mission mine is at the top, the idle Twin Buttes mine in the middle, and the Sierrita mine is on the bottom. These mines use the conventional wet tailing stacks. You can see the blue water on the piles.
The principal advantage of wet stacking is that it is less expensive than dry stacking. Nevertheless, there is a price to pay. First, lots of water is tied up in the tailing ponds and in this arid climate, much is lost through evaporation. The rock-powder slurry must be contained within dams which sometimes fail. (Worldwide, there are 2 to 5 tailings dam failure incidents per year.) Because wet tails contain so much water, there may be seepage problems that must be addressed. Finally, if the wet tailings are allowed to dry, there is a fugitive dust problem. Usually reclamation must wait until the tailing pile is completed and no longer used.
In the dry stacking method, filters and vacuum presses remove more of the water, which can be recycled for process water, reused in the flotation step for instance. Typically, dry tails contain less than 25% water. Dry tails are transported to the tailing piles by conveyor belt or truck, then dumped and compacted. Dry tails don’t need a containment structure and are more stable. Dry stacked tailings are essentially immune to geotechnical “failure” and can be designed to withstand static and seismic forces. Because there is much less water in the tails, there is less chance of seepage. Dry stacks usually have a smaller footprint than conventional wet tailings. Fugitive dust can occur from dry stacks also but is minimized because reclamation can commence very early in the project life. Dust is also minimized by compaction, and use of binders or wetting agents, or agglomeration. Progressive reclamation often includes covers and re-vegetation of the tailings slopes and surface as part of the annual operating cycle.
For more details, see a report prepared for Rosemont by AMEC Earth & Environmental, Inc here.
The Mission and Sierrita mines used the best technology available at the time those mines were designed, but now, Rosemont has a chance to do it better.
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