Mills produce solid wastes, tailings too, made up of the finely ground rock that has been separated from the ore minerals in the milling process. The tailings can hold dangerous byproducts the milling process has separated from the useful materials. For instance, gold ores most of the time contain arsenic and uranium ores which almost always contain radioactive decay products like radium. Tailings are disposed of in tailings ponds that must be designed to mantain 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 after be safely disposed of separate from the main portion of the tailings of the mine.
Most if the time the tailings contain sulphides which are worthless, such as the iron sulphides pyrite and pyrrhotite, which were present in the ore along with the useful ones. Like those in the waste rock, sulphides in the tailings must be mantained from forming acid runoff. Again, this means diverting water away from the tailings pond or treating the water which it drains from.
Most iron ores have little or no sulphur. In iron mining, the principal concern is not acid mine drainage, but the water discharged from the gravity and magnetic concentration processes and from pelletizing plants. This water often contains particles of stable iron oxides. These minerals have a high specific gravity and very commonly tend to settle out of water rapidly, affecting waterways only short distances from the point of discharge.
When the tailings ponds are full at last, they must be 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.