Read the text, do the exercises.

Mineral deposits represent concentrations of an element or elements to a level at which they can be profitably extractable. As such these deposits represent concentrations of several elements well above crustal abundance. Such naturally occurring high concentrations are reflected in the chemistry of the soils, waters, sediments, plants, etc., close to the deposit. Indeed, measurement of the concentrations of various metals and non-metals in the media such as soil, sediments and waters has been used to locate mineral deposits.

Although there are likely to be significant natural enrichments of several elements in the vicinity of mineral deposits, mining and extraction of the deposit will add greatly to these enrichments. The mining and subsequent beneficiation of minerals and the separation and refining of their various components is one of the most serious sources of contaminationof soils, waters, and the biosphere.

Humans have extracted minerals, particularly the metalliferous ores, since ancient times, and the extraction and refining of metals have played a major role in human development. The mining and processing of minerals have increased through time, due to population growth and the greater utilization of raw materials for manufacture. Many areas of past mining activity, in both the Old and New Worlds, bear witness to these extractions in the form of abandoned workings and extensive waste tips. Modern mineral extraction technology is generally far more efficient than past practices, and in many countries such processes are heavily regulated to limit the degree of contamination from extractive industries. However, historical mineral extraction involved less efficient technologies and in those times virtually no environmental regulations were in place. Long-abandoned mineral workings are currently the cause of serious environmental pollution in many countries.

A large number of different materials are extracted from the Earth ranging from fuels such as oil and coal, industrial minerals such as clays and silica, aggregates for building and roadstone, and minerals for fertilizers as well as sources of non-metals. However, the major cause of concern is the metalliferous ores that are used as sources of metals and metalloids.

The extraction and subsequent processing of ores can be summarized as follows:

Mining → crushing/grinding

→ concentration of ore mineral

→ smelting/refining.

Ores are extracted from the Earth by either subsurface mining, open pit surface techniques, or in a few cases by solution mining, which carries with it risk of groundwater pollution. Both subsurface and surface extraction result in waste material, which is generally piled on the surface in the vicinity of mine. However, while such waste piles, which frequently contain ore minerals, are sources of environmental contamination, it is the subsequent processing of the ores that result in the greatest environmental problems.

The crushing and grinding (comminution) of mineral processing has the objective of separating the ore minerals from the waste, generally referred to as gangue. To effect separation, the mined ore is finely crushed to liberate individual ore mineral grains to enable concentration of the sought after ore mineral. The very fine waste material left after this concentration process is referred to as tailings, and this material can contain, along with the gangue minerals, residual amounts of the ore minerals and can be a serious source of pollution. The tailingsare very fine hence are subject to wind ablation and can easily transported by surface runoff. At many mine sites tailings have been left open to the environment resulting in serious contamination of surrounding soil and water.

Following the mining and processing of the ores, the resultant concentrate is transported to a smelter. Pyrometallurgical smelting involves roasting of the ore concentrate at high temperatures with the subsequent emission of large quantities of potentially harmful elements. The smelter emissions can be in the form of gases, such as sulphur dioxide, aerosols, and larger particulates. Modern smelter stacks are fitted with electrostatic precipitatorsand other dust recovery mechanisms, which results in the retentionof most of the particulates, but some gaseous and aerosolic emissions are still released into the atmosphere. Although any larger particulates released are likely to be deposited close to the source, aerosols and gases can be transported long distances and as a result the smelting of ores has far wider aerial impact than the mining and processing of these ores.

Date: 2016-01-14; view: 855