Open-pit mining (also known as open-cut mining or opencast mining) is a type of strip mining in which the ore deposit extends very deep in the ground, necessitating the removal of layer upon layer of overburden and ore.
- An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining and then refined to extract the valuable element(s).
In many cases, logging of trees and clear-cutting or burning of vegetation above the ore deposit may precede removal of the overburden. The use of heavy machinery, usually bulldozers and dump trucks, is the most common means of removing overburden. Open-pit mining often involves the removal of natively vegetated areas, and is therefore among the most environmentally destructive types of mining, especially within tropical forests.
- Overburden (also called waste or spoil) is the material that lies above an area with economic or scientific interest. In mining, it is most commonly the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is removed during surface mining.
Because open-pit mining is employed for ore deposits at a substantial depth underground, it usually involves the creation of a pit that extends below the groundwater table. In this case, groundwater must be pumped out of the pit to allow mining to take place. A pit lake usually forms at some point in time after mining stops and the groundwater pumps are turned off.
Corta Atalaya is the largest open-pit mine in Europe.
Typical phases of a mining project.
Each phase of mining is associated with different sets of environmental impacts. Threfore, before and during the actual exploitation environmental impact assessments (EIAs) are needed to ensure the safety of the project.
An environmental impact assessment (EIA) is an assessment of the possible positive or negative impact that a proposed project may have on the environment, together consisting of the environmental, social and economic aspects.
This phase includes surveys, field studies, and drilling test boreholes and other exploratory excavations. The aim is to obtain information about the location and value of the mineral ore deposit. The exploratory phase may involve clearing of wide areas of vegetation, to allow the entry of heavy vehicles mounted wit drilling rigs.
Many countries require a separate EIA for the exploratory phase of a mining project because the impacts of this phase can be profound and because further phases of mining may not ensue if exploration fails to find sufficient quantities of high-grade mineral ore deposits.
The construction of access roads, either to provide heavy equipment and supplies to the mine site or to ship out processed metals and ores, can have substantial environmental impacts, especially if access roads cut through ecologically sensitive areas or are near previously isolated communities.
Site preparation and clearing. If a mine site is located in a remote, undeveloped area, the project proponent may need to begin by clearing land for the construction of staging areas that would house project personnel and equipment. Even before any land is mined, activities associated with site preparation and clearing can have significant environmental impacts, especially if they are within or adjacen to ecologically sensitive areas.
The EIA must assess, separately, the impacts associated with site preparation and clearing.
3. Active Mining
The active mining consists of two phases: the extraction and concentration (or beneficiation) of a metal from the earth. Metallic ores are buried under a layer of ordinary soil or rock (the overburden) that must be moved or excavated to allow access to the ore deposit.
4. Disposal of overburden and waste rock
The overburden must be moved or excavated to allow access to the metallic ore deposit. The quantity of overburden generated by mining is enormous. These high-volume wastes, sometimes containing significant levels of toxic substances, are usually deposited on-site, either in piles on the surface or as backfill in open pits. The EIA for a proposed mining project must carefully assess the management options and associated impacts of overburden disposal.from the earth. Metallic ores are buried under a layer of ordinary soil or rock (the overburden) that must be moved or excavated to allow access to the ore deposit.
5. Ore extraction
Extraction of the mineral ore is made using specialized heavy equipment and machinery, such as loaders, haulers, and dump trucks, which transport the ore to processing facilities using haul roads.This activity creates a unique set of environmental impacts, such as emissions of fugitive dust from haul roads.
Coal mining at Garzweiler, Germany. Photo – Raimond Spekking.
The next step in mining is grinding (or milling) the ore and separating the relatively small quantities of metal from the non-metallic material of the ore in a process called ‘beneficiation.’ Milling is one of the most costly parts of beneficiation, and results in veryfine particles that allow better extraction of the metal. However, milling also allows a more complete release of contaminants when these particles become tailings. Tailings are what remains following milling of the ore to fine particles and extraction of the valuable metal(s).
Tailings, also called mine dumps, culm dumps, slimes, tails, refuse, leach residue or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct from overburden.
Beneficiation includes physical and/or chemical separation techniques (sometimes involving the use of mercury). Wastes from these processes include waste rock dumps, tailings, heap leach materials (for gold and silver operations), and dump leach materials (for copper leach operations).The chemical separation process involves the usage of sulfuric acid for treating copper ores and cyanide for gold ores. Metallic ores generate large quantities of waste with toxic chemicals left from this separation.
- The copper content of a good grade copper ore may be only one quarter of one percent (0.25%).
- The gold content of a good grade gold ore may be only a few one-hundredths of a percent.
7. Tailings disposal
Even high-grade mineral ores consist almost entirely of non-metallic materials and often contain undesired toxic metals (such as cadmium, lead, and arsenic). The beneficiation process generates high-volume waste called ‘tailings’, the residue of an ore that remains afterit has been milled and the desired metals have been extracted. If a mining project involves the extraction of a few hundred million metric tons of mineral ore, then the mine project will generate a similar quantity of tailings.
8. Site reclamation and closure
When active mining ceases, mine facilities and the site are reclaimed and closed. Mines that are notorious for their immense impact on the environment often made impacts only during the closure phase, when active mining operations ceased. These impacts can persist for decades and even centuries.
The goal of mine site reclamation and closure should always be to return the site to a condition that most resembles the pre-mining condition.
Therefore, the EIA for every proposed mining project must include a detailed discussion of the mine Reclamation and Closure Plan offered by the mining proponent.
Mine reclamation and closure plans must describe in sufficient detail how the mining company will restore the site to a condition that most resembles pre-mining environmental quality; how it will prevent – in perpetuity – the release of toxic contaminants from various mine facilities (such as abandoned open pits and tailings impoundments); and how funds will be set aside to insure that the costs of reclamation and closure will be paid for.
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BC Wild, Environmental Mining Council of BC (2006). “Acid Mine Drainage: Mining and Water Pollution Issues“