Tailings Disposal Systems

Mills also produce solid wastes, tailings, made up of the finely ground rock that has been separated from the ore minerals in the milling process. The tailings can contain hazardous byproducts the milling process has separated from the useful materials. For example, gold ores often contain arsenic and uranium ores almost always contain radioactive decay products like radium. Tailings are disposed of in tailings ponds that must be designed to keep these byproducts where they belong, preventing them from reaching the environment. This can mean adding a treatment process that captures the byproducts chemically so that they can then be safely disposed of separate from the main portion of the mine’s tailings.

Often the tailings contain worthless sulphides such as the iron sulphides pyrite and pyrrhotite, that were present in the ore along with the useful ones. Like those in the waste rock, sulphides in the tailings must be kept from forming acid runoff. Again, this means diverting water away from the tailings pond or treating the water from which it drains.

Most iron ores have little or no sulphur. In iron mining, the main concern is not acid mine drainage, but the water discharged from the gravity and magnetic concentration processes and from palletizing plants. This water often contains particles of stable iron oxides. These minerals have a high specific gravity and tend to settle out of water rapidly, affecting waterways only short distances from the point of discharge

When the tailings ponds are finally full, they must he capped with impermeable soils such as clay to prevent water from percolating through the tailings and drawing potentially harmful substances into the groundwater. The capping material is usually seeded so that plants will grow on the surface and prevent wind and water from eroding the cap.