Gangue is the unwanted material that is mined with valuable minerals and must be separated. It can be discharged as tailings, but sometimes new valuable materials are discovered and can be extracted from the tailings. Mining technology has improved to better separate economic and uneconomic fractions. Tailings may contain hazardous materials, so environmentally safe landfills are used to prevent contamination.
Gangue is the substance that has to be mined together with the desired minerals or ores due to their close association. These materials are usually removed together with the ores or target minerals and must therefore be separated. The gangue can then be discharged as tailings. In some cases, there may be additional uses for the materials present in the tailings. When situations like these arise, the tailings can be further processed to remove the new valuable materials.
During the mining process, the materials that are unearthed are generally classified into two groups. The cheap fraction is the desirable material that has value, while the uneconomic fraction, otherwise known as gangue, is everything else. When determining whether a mining operation will be economically feasible, the costs of separating the cheap fraction from the useless gangue are usually considered. Sometimes, a desirable material will be so integrated with useless gangue that it would be too expensive to extract.
Mining technology has developed over the years and more sophisticated methods have been created to separate the economic and uneconomic fractions. Early mining operations could be inefficient and often left relatively large quantities of useful minerals in the mine tailings. This was often due to the poor ability of older techniques to effectively separate useful minerals from the gangue, or to the minerals being suspended in particularly difficult configurations. As new techniques are created, old tailings dumps are sometimes revisited for their ore deposits and useful ores.
Tailings can also be re-examined if new uses are discovered for old materials. Arsenopyrite was once considered a useless gangue ore and dumped in the tailings as part of the uneconomical fraction. Subsequent developments led to arsenic becoming a popular insecticide, at which time the arsenopyrite contained in mine tailings could be considered an economic fraction. When developments like this occur, the tailings are often reworked to extract the newly desired minerals and ores.
In some mining operations, gangue can contain hazardous materials. This can be the result of the mining of hazardous materials, some of which invariably ends up in the tailings due to imperfect processing. Hazardous or poisonous chemicals, such as cyanide, can also be used in the processing of minerals such as gold and silver, in which case certain quantities often end up in the tailings. When this occurs, the mining operation will often include an environmentally safe landfill to prevent poisonous or hazardous materials in the tailings from leaching into the groundwater.
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