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Understanding the origins of mine drainage is crucial. Here’s a quick overview of the primary sources.
- Drainage from Underground Workings: When underground mines are excavated, they often intersect the water table. Water then flows into these mine voids. If sulfide minerals are present in the exposed rock walls and floors, they react with the water and air (oxygen) to produce acidic solutions.
- Surface Runoff from Open Pit Mine Faces and Pit Workings: Open pit mining creates large, exposed surfaces of rock. Rainwater and snowmelt runoff over these surfaces, leaching out minerals. If the exposed rock contains sulfides, this runoff becomes acidic and carries dissolved metals.
- Waste Rock Dumps from Metal Mines and Spoil Piles from Coal Mines: Waste rock dumps are piles of rock removed to access the ore. Spoil piles are the equivalent in coal mining. Both contain material that was not economically viable to process. These piles can contain significant amounts of sulfide minerals. When exposed to air and water, they generate AMD.
- Tailings Deposits: Tailings are the finely ground waste material left over after ore processing. They often contain residual sulfides and are typically stored in tailings ponds or impoundments. If these tailings are not properly managed and become exposed to air and water, they can be a major source of AMD.
- Ore Stockpiles and Spent Ore Piles from Heap-Leach Operations: Ore stockpiles, awaiting processing, and spent ore piles (after leaching) can also generate AMD. In heap leaching, for example, the ore is leached with a solution (often acidic) and the remaining rock can still contain sulfides.
How can we best mitigate these drainage sources? Share your solutions below.
