Milling, smelting and the environment

— The following is an excerpt from Mining Explained, published by The Northern Miner.

After the ore has been brought to surface, the process of getting the metal out also can create harmful substances. Apart from the ores they use and the wastes they produce, milling and smelting run on fuels and use chemicals to extract the metals. These substances also can present a hazard to the environment.

The milling process uses plenty of water, and large mills may use several hundred litres per minute. This water contains small concentrations of various organic and inorganic reagents used in the milling process. Many companies recycle all or part of the water back into their mills instead of discharging it into the environment. If it has to be discharged, then it also must be treated by capturing or destroying the chemicals.

One particularly important example is gold extraction. Most mills extract gold using weak solutions of sodium cyanide under slightly alkaline conditions. Traces of cyanide are left in gold mill effluents. Cyanide is not a single chemical element, so it can be broken down into the elements that make it up, namely carbon and nitrogen. The waste waters from the mill can be held in a pond where sunlight and contact with air break the cyanide down. Another choice is to incorporate a cyanide destruction process into the mill circuit, in which waste waters are aerated or chemicals are added that react with the cyanide to form less hazardous substances.

The use of cyanide in mining sometimes raises public concern. It is a dangerous poison and can be fatal if ingested. But since cyanide is also a compound, and an unstable one at that, it is relatively easy to destroy any excess in waste waters, leaving them suitable for discharge to the environment.

Tailings

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 pelletizing 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 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.

Mining smelter gases and dusts

When sulphide ores are oxidized during the smelting stage, huge amounts of sulphur dioxide are produced. If the sulphur dioxide goes up the smokestack of a smelter, it can react with the water vapor in the air to form sulphuric acid, the acid in acid rain or snow. It is estimated that about 60% of all sulphur emitted to the atmosphere comes from smelting and other industrial activity. Ores that can be treated by hydrometallurgical methods do not produce air emissions, but process waters may have to be treated before they can be discharged.

Many smelters now trap their sulphur dioxide to make sulphuric acid, which is a useful product in itself. A large smelter can produce thousands of tonnes of sulphuric acid a day. The gases are drawn through a catalytic oxidation process that turns the sulphur dioxide into sulphur trioxide. Dissolved in water, this chemical reacts to form sulphuric acid which can be sold to the general bulk chemical trade or used to make agricultural fertilizers.

Dusts in the smelter exhausts can be recovered by passing them through cyclones and electrostatic precipitators. These units can recover up to 95% of particulate dusts, returning them for processing to recover their metal content.

After mining ends

Mining is always but a temporary use of land, and an important goal of the operating company is to return the mine site to a natural and stable state, thereby making it available for other uses.

Towards this end, the industry has embraced mine reclamation techniques that consist of removing, relocating or demolishing buildings and physical infrastructure; closing pits and shafts; stabilizing underground workings, soil and slopes; treating tailings and waste water appropriately; and revegetating land.

Reclamation can be carried out at various stages of mining activity — after exploration; after surface or underground mining; or after treatment and processing facilities have been closed.