Guidance for reporting tailings and waste rock: chapter 2
The main activities associated with the mine life cycle may include:1
- line cutting, drilling, trenching and bulk sampling;
- development of mine workings and construction of associated infrastructure;
- extraction of ore;
- management of mine and site drainage;
- ore processing;
- disposal of waste rock, tailings and other wastes; and
- site reclamation activities.
Mining facilities are faced with the challenge of managing large volumes of ore, waste rock, and tailings. Figure 1 describes the typical activities of the mine operations phase.
Figure 1 - Typical activities of the mine operations phase ( Environmental Code of Practice for Metal Mines, 2009)
1. Underground mines
In underground mines, the ore is extracted through a series of vertical shafts and ramps and horizontal drifts and adits (see Figure 2). Extraction is more selective than in open pit mining, and the ratio of waste rock to ore generated is much lower. In about one half of Canadian underground mines, waste rock is used as mine backfill to provide roof and wall support underground. Waste rock that is not used for construction or as backfill is disposed of on the surface.
Figure 2 - Series of vertical shafts and ramps and horizontal drifts and adits to extract the ore ( Environmental Code of Practice for Metal Mines, 2009)
2. Ore processing
Once ore is extracted from a mine it is processed to recover the valuable minerals. Ore typically consists of small amounts of valuable minerals in close association with much larger amounts of waste minerals of no economic value (gangue). The valuable ore minerals are separated (liberated) from the gangue in milling operations to obtain higher quality metal. Major steps in ore processing include grinding and crushing, chemical/physical separation and dewatering.
3. Grinding and crushing
Grinding and crushing of ore is undertaken to physically liberate valuable minerals prior to separation by physical and chemical processes. Crushing is done dry, and is used for coarse size reduction. Grinding is used to achieve finer size reduction. Grinding is conducted wet, and chemicals such as lime, soda ash, sodium cyanide, and sulphur dioxide may be added in the grinding circuit in preparation for ore separation. Ore must be ground fine enough to liberate the ore minerals from the gangue or subsequent separation methods will not be as effective.
4. Ore separation
Ore separation may be done using physical or chemical separation methods. The end product of ore separation is an ore concentrate. After separation, some ore concentrates are sent for further processing, such as smelting, to produce pure metal for sale.
A by-product of ore separation is tailings, which are a mixture of water and finely ground rock from which most of the minerals of value have been removed. Tailings may still contain metal-bearing minerals, and the mixture may also contain residues of reagents used in ore processing.
Physical Separation Processes: Physical separation processes exploit differences in the physical properties or behaviour of mineral particles, such as size, density and surface energy. The bulk of the mineral is not chemically altered, although chemical reagents may be used to help in the separation process. Commonly used physical separation processes are as follows:
- Gravity Separation: Minerals can be separated on the basis of differences in density, particularly for iron ore and gold, as well as tungsten, tantalum and niobium. Gravity separation may also be used to pre-concentrate metallic minerals prior to further processing. Gravity separation tends to require the use of smaller amounts of process reagents than some other ore separation methods.
- Magnetic Separation: Minerals can be separated on the basis of differences in magnetic susceptibility. Magnetic separation has been used in Canada to separate iron ore from waste minerals, to remove magnetite (iron oxide) and pyrrhotite (iron sulphide) from base metal ores prior to flotation, and to recover magnetite from copper concentrates. Like gravity separation, magnetic separation tends to require the use of smaller amounts of process reagents than some other ore separation methods.
- Flotation Separation: Flotation is used for the separation of a wide variety of minerals on the basis of differences in surface properties of minerals in contact with air and water. It is the dominant process for the recovery of base metal ores and is also used in uranium and gold processing operations. To separate minerals using flotation, fine air bubbles are introduced into a mixture of ground ore in water, known as a slurry. In this slurry, mineral particles collide with air bubbles, and minerals that favour contact with air attach to the air bubbles and float to the surface of the flotation cell. As air bubbles accumulate at the surface, a froth forms and eventually overflows as the flotation cell concentrate. Minerals that favour contact with water remain in the slurry and go to the flotation cell tailings. A number of chemical reagents are used to aid the process.
Chemical Separation Processes: Chemical separation processes involve the preferential leaching of one or more minerals, particularly for the recovery of gold, silver and uranium and in some cases copper. A number of chemical processes are used for ore separation:
- Leaching with Cyanide: This is the dominant method for recovery of metallic gold or silver. A dilute solution of calcium or sodium cyanide is used to dissolve the metal. Following leaching, metals are recovered from the solution by adsorption directly from the leach slurry onto activated carbon granules or by the addition of zinc dust to the solution which causes the precious metals to precipitate from the solution.
- Leaching with Sulphuric Acid: Uranium ores are processed using sulphuric acid to dissolve the uranium. The uranium is then removed from the solution using ion exchange or solvent extraction, which results in the adsorption of uranium on a resin or organic solvent. The uranium is then removed from the resin or solvent. In some cases, copper ores are also leached with sulphuric acid.
The ore concentrates obtained from most physical ore separation processes are slurries with high water content that must be dewatered prior to further processing. Dewatering involves two processes, i.e., thickening and filtration. In thickening, slurries are thickened by gravity settling. The excess water is decanted off and may be recycled in the milling processes. After thickening, the slurry is passed through a vacuum filter, which traps the particulates. Most of the remaining water is removed.
6. Mine closure
Mines are closed when the ore minerals are completely exhausted or when it is no longer profitable to recover the minerals that remain. In some cases, mines may be closed temporarily and put into a status called “care and maintenance,” also known as temporary suspension.
This is frequently done during periods of low commodity prices in the expectation that higher prices in the future will make further commercial operations financially viable. Eventually, ore reserves are depleted, and mines are permanently closed.
The above list is illustrative and not exhaustive.
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