Mining. Underground Extraction

Mining is the extraction of useful materials from surface or underground operations. However, mining can also encompass downstream processing to recover and refine the valuable fraction contained in the extracted material. Mining is distinct from other industries because the raw materials that are extracted—such as sand, gravel, stone and rock, coal, or metal-bearing ores—cannot in general be worked indefinitely because deposits are finite.

In the past the availability of rich geological resources in tandem with market conditions largely outside the control of metal-producing enterprises resulted in an industry characterized by a low level of technological innovation. Corporate strategies traditionally focused on the discovery and acquisition of high-grade and easily accessible mineral deposits and on increased scale of production to offset declining ore grades.

In the past, health and safety regulation was a major driver behind technological innovation alongside the need to improve process energy efficiency. However, the focus has now altered: After a period of limited technological innovation a spur to technology development in mining has been applied by public concern over environmental damage and the design of environmental regulation that obliges firms to mitigate or prevent such damage.

Metals and other mineral resources are rarely found in a sufficiently pure state to be sold in an "as- mined" form. Metals are often found in chemical combination with oxygen (as oxides), sulfur (as sulfides), or other elements (e.g., chlorides, carbonates, arsenates, phosphates, etc.) and physically mixed with less valuable or valueless minerals (e.g., silicates). Nonmetal mineral resources (e.g., coal, industrial minerals) also normally contain impurities in their undisturbed state.

Valueless or low-value minerals associated with valuable minerals are generally known as "gangue." Inputs from mining (extraction) to mineral processing/extractive metallurgy contain gangue that exits the process in a number of forms (e.g., as contaminants in the primary product, as separate salable by-products, solid wastes, gases, fumes, and suspended particulates).

During the extraction and processing of ore, mass is conserved. However, the physical and chemical characteristics of inputs may be modified by interactions with process chemicals and/or the process itself. For example, depending on the point at which it is rejected from the process, gangue may be disposed of in an as-mined state (e.g., waste rock), as tailings (residues) (e.g., mineral processing), as glass-like slags (e.g., after smelting), or as other waste products (e.g., dusts, sludges from water treatment, spent ore from leaching, etc.).

These wastes may also contain significant quantities of the target mineral or metal due to inefficient processing, technological limitations, or mineralogical factors. Therefore, the disposal of solid and liquid wastes in the mining industry is a major issue. Chemicals are also widely used in the process of moving from as-mined material to the finished salable product, and these chemicals may also give rise to environmental and social issues and impacts.

Extraction is the first operation in the commercial exploitation of a mineral resource. Extraction is the act of removing one or more components of the target material. There are three types of extraction: surface, underground, and in-situ (solution mining). The latter is somewhat limited in its application, although it is sometimes used to exploit residual mineralization as grades drop at surface or underground mines.

Surface extraction is dominated by open-pit (e.g., base and precious metal ore) or open-cast (e.g., coal) operations. Surface and underground extraction usually occur independently of one another, although open-pit extraction does occasionally occur in areas already partly worked by underground methods. Similarly, underground methods are sometimes used to extract ore from beneath or in the vicinity of pits where further extension of the pit itself is not economically or technically feasible.

Underground Extraction. Underground extraction methods are usually employed to exploit richer, deeper, and smaller ore bodies where open-pit methods would be impractical. Underground extraction operations are a complex combination of tunnels, rock support, ventilation, electrical systems, water control, and hoists for the transportation of people, ore, and materials. Consequently, underground extraction is less flexible than surface extraction, and deviations in the production rate are more difficult to accommodate. Because extraction may proceed in several underground locations to ensure an appropriate level of production, planning and control of mine development are important.

Compared to underground extraction methods, surface extraction methods allow a higher degree of worker safety, greater flexibility in extraction (the capacity to practice selective mining and grade control), and lower development and maintenance costs (due to the requirement for fewer specialized systems). However, the major benefit lies in economy of scale because large-capacity earth-moving equipment can be used to generate high productivity.

The decline in ore grades over the last ten years has also increased competitive pressures in the mining industry. It has forced many companies to reassess corporate strategy, prompting the adoption of more sophisticated technologies and increased mine sizes. Indeed, the major thrust of innovation in extraction aspects of mining has been the development of open- pit techniques and exploitation of the economies of scale created by developing larger mining equipment and larger-scale extraction techniques.

For example, major developments have included improved ventilation equipment and improved visibility; larger and more efficient hoisting machinery, compressors, pumps, and so forth; improved mine development equipment and procedures for mechanized shaft sinking, shaft freezing, raising, and drifting; larger and heavier equipment with better drill-bit and blastholedrilling techniques; trackless mining equipment, initially to mechanize ore loading and transport; improved blasting agents and techniques for greater efficiency and safety; and electric mining shovels and draglines with significantly greater bucket capacities.

Other developments have been 181-metric-ton diesel-electric haul trucks and front-loading equipment; portable and mobile in-pit ore-crushing and overland conveyor systems, allowing more continuous mining operations and the elimination of truck haulage where hauls are long and adverse; self-propelled scalpers and scrapers to integrate extraction and transport operations; online/automated production monitoring and control to optimize extraction and truck and shovel capacity utilization through scheduling/dispatching and route selection; and computerized and remote control systems for underground train haulage and mine pumps.

Some of these developments, of course, also apply to underground mining. In underground mining, advances have also been made in the use of "right-inspace" mining methods, which seek to ensure that drill holes, stopes (steplike excavations for the removal of ore), and other underground workings are placed more accurately in relation to the ore body during both development and production. These methods allow high ore recovery and reduce dilution by unwanted gangue, reducing waste that requires disposal and management.

Regardless of the method employed, mining is always accompanied by processing. For relatively pure materials processing may be limited to crushing and sizing (e.g., quarried rock) or washing (e.g., some coal operations). Such simple processing is possible only when the target mineral forms the majority of the material mined. In such cases the main environmental impacts are associated with the mining itself rather than with subsequent processing.

Although any mining operation has the potential to produce environmental impacts, typically the potential arises from discharges of solid, liquid, and gaseous waste products. The characteristics of the discharges, the nature of the receiving environment, and the distance over which the discharges are transported are major factors in determining the magnitude of the impacts.

Societal values and preferences also play a significant role in determining how certain discharges are viewed by various stakeholders (those affected by the actions of another). This more subjective aspect of the discharges and receiving environment characteristics therefore determines, in part, the site-specific environmental footprint of an operation.

Ore Extraction. Ore extraction and processing are the sources of major environmental impacts by releasing solid, liquid, and gaseous wastes into soil, air, and water. With the possible exception of water contamination, arguably the most significant impact arises from the disposal of solid wastes, particularly in relation to soil and water pollution. In mining, as in most industries, less than 100 percent of the raw material is the target product.

Average figures (based on a survey of Canadian metal mines) indicate that 42 percent of the total mined material is rejected as waste rock, 52 percent is rejected from the mill as tailings, 4 percent is rejected from the smelter as slag, leaving as a valuable component only 2 percent of the originally mined tonnage. At many gold operations, the concentration of valuable material is so low that effectively all of the mined ore is disposed of as waste. In effect, such operations are as much about waste disposal as they are about resource extraction.

The significance of this dual extraction/disposal role in mining of base and precious metals is seen in global estimates of the annual generation of solid and gaseous wastes from copper and lead production, with the amounts being dominated by solid waste.

Although waste disposal directly sterilizes a significant land area, it is the presence of potentially harmful elements, minerals, and other contaminants in the solid (and liquid) wastes generated during mining, mineral processing, and other downstream processes (e.g., waste rock, tailings, slags, smelter flue dusts, and precipitates from the chemical treatment of metal-contaminated liquid effluents) that determines the wider environmental impact.

The contaminants may be organic (e.g., flotation reagents [substances used because of their chemical or biological activity]) or inorganic (e.g., metals). Their original source may be the ore body itself or chemicals applied during extraction and processing of the ore. The contaminants may cause a number of related environmental impacts, including contamination of ground and surface waters and soil resources, sustained ecosystem degradation, and atmospheric suspension of respirable (breathable) dusts.

These impacts can combine to extend the spatial and temporal footprint of a mine site, an example being the long-term generation of acid rock drainage (ARD) from surface wastes and from underground and open- pit workings. ARD is one of the most intractable problems in the mining of nonferrous metals and coal, a fact reflected by the increasing volume of research published in conference proceedings, journals, and books. ARD is both the most serious environmental impact caused by mining and the industry's greatest environmentally related technical challenge.

At a site-specific level, the extent of the environmental impacts of mining and mineral processing depends on a number of variables, including size of the site, method of ore extraction, ore mineralogy, processing and treatment routes, process efficiency, volume and nature of wastes, hydrology (the properties, distribution, and circulation of water), hydrogeology, local environmental conditions, nature of the regulatory framework, and the timing of any remediation.