Lifecycle of a Mine

Almost all the metals we need for everyday life are mined from the earth's crust.


In contrast to other industries, mining cannot freely choose where to operate: it can only take place in areas where minerals are concentrated on the surface or underground. This can mean operating in sensitive or challenging environments; a trend that looks set to continue as older, easier to reach deposits run out.


Mining and metal production has evolved from being a tough and dirty activity to a high-technology operation, using computerised remote-control equipment and complex machinery. Unlike their predecessors, today's workers are highly trained personnel performing skilled work according to the highest standards of safety and efficiency.

Pictorial illustration of the lifecycle of a mine

(Image courtesy of The Extractives Baraza 2017)

Preliminary Investigation: This involves advanced scientific techniques, including geochemical analysis of soils, or airborne surveys to measure magnetic, gravitational and electromagnetic fields, all of which help to determine if there's a sufficient mineral deposit in that location to warrant mining.


Rock Sampling: Once this preliminary investigation is carried out, rock samples are drilled and sent to a laboratory for testing. In order to carry out these drilling activities, companies apply for exploration licenses. Rock sampling raises the degree of confidence in the measurement of the shape, size (quantity) and grade (quality) of the mineralisation held in the earth's crust.


Reserves and Resources: The results of this process are measured in terms of mineral resources and ore reserves.

Mineral resources refer to the concentration of materials in or on the earth's crust that have reasonable prospects for economic extraction.
Ore reserves are those parts of a mineral resource that can be economically mined.


Categories of Mineral Finds: (Courtesy of the Natural Resource Governance Institute)
Mineral finds are often classified using three categories:


Inferred mineral resources, when it can be inferred that there are minerals but there is insufficient evidence to be certain;
Indicated resources, when there is reasonable confidence, also called a probable reserve; and
Measured resources, when there is a high degree of confidence, also called a proven reserve.

Feasibility Studies: Once a reserve is proven, a company must consider a variety of factors before deciding whether extracting the minerals will generate enough profit to justify the effort.
The following are examined during the feasibility study:


• Mine Design;
• Treatment Plant: Separation of waste from the desired minerals, and
• Waste disposal
• Minimizing the social, environmental and economic impact of extraction; and
• Transportation: how to get the mineral from the mine site to market.
• Budget and capital costs

Environmental Impact Assessment: If a company decides that a mining operation is feasible, a social and environmental impact assessment is undertaken and submitted to environmental regulatory authorities for approval. This study is usually a major undertaking and should include provisions for public hearings and submissions.


In Kenya the Environmental Management and Coordination Act (EMCA) 1999 requires for an Environmental Assessment to take place.

Construction: Here once all the permits have been received the company put up the required infrastructure to pave way for the mining project.

At this stage the actual extraction operations take place.


1. Recovery


There are various types of mines depending on the type of mineral and the pre-determined plan. Types of mines are generally divided into 4 categories as explained: There are four major types of mining operations:


Surface. Surface mining includes many types of mining during which the ore is all removed from the ground. In this process, the minerals are often separated from the other rocks after they are removed from the mining pit. This form of mining often has a large impact on the surface environment both from the extraction site and the nearby waste deposits. Below are common methods of surface mining:


Open Pit: is used when there are hard rocks, such as coal and diamonds; It involves blasting layers of rock from the surface and then loading the ore onto trucks to transport to a plant for processing.
Open Cast Mining is used for soft rocks, such as limestones.
Strip Mining: this is the targeted removal of surface layers to reveal the useful ore/seams underneath.
Mountaintop removal mining: involves taking off the top of the mountain to reach the ore underneath and is usually associated with coal mining.

Underground. With underground mining, the surface remains intact and workers and machines remove the minerals through tunnels or shafts. Underground mining begins with a phase of development mining whereby rocks are extracted so that miners can get closer to the ore. Production mining is when the ore with the desired mineral is extracted. Health and safety of workers is particularly important to successful underground mining, including ensuring a proper ventilation system and stable tunnels. Below are some common underground extraction methods:


Room and pillar: it involves excavating rooms with pillars that hold up the roof and is used for shallow mines.
Block Caving: miners drill tunnels underneath the ore deposits and then draw the material down.
In situ mining uses water that is injected into the soil and then pumped back up to the surface where the ore is then removed. This is used to in extracting rare earth elements and uranium.
Cut-and-fill method, miners work in horizontal slices underneath the surface, and the slices are then backfilled after the mining is finished.

Dredge. This process occurs when rocks and sediments are removed from the floor of a body of water. The sediment and ores are sorted and the undesired minerals are returned to the water or deposited elsewhere.
Traditionally this type of extraction has taken place in shallow areas, but new technology is taking it to deeper sea locations where there are expectations for high yields. This form of extraction often has a large impact on aquatic life that is very difficult to restore after mining operations are complete.
The Tata Chemical Company, Magadi uses this method to extract Soda Ash from the lake.

Artisanal. Artisanal mines can be either on the surface or below the ground but are distinct in that there is no large company overseeing the extraction.

Informal: Artisanal mines can be completely informal where the miners do not possess mining rights and are self-organized,
Small Companies: in other cases, there is a small company that owns a license and hires day labourers to extract the minerals. Often those who work at artisanal mines do so on a subsistence basis.

Artisanal mining is notable for having fewer protections for health and safety of workers and environmental impact. It is also an area of mining that involves a comparatively large percentage of women and children.

Milling

After ore is extracted from the earth's crust, the metal bearing minerals are separated from waste material to form a concentrate using a process known as milling.
Milling can either be:

Chemical: done by adding chemicals to the ore materials and running them through a series of processes. The waste from the mill, or tailings, is then pumped to a tailings storage facility.
Gravity Separation, where two or more minerals are separated by movement and the force of gravity and one or more other forces (such as centrifugal forces, magnetic forces or buoyant forces). This is an economic and more environmentally friendly alternative to chemical separation but isn't commonly used due to limitations in its application.

2. Processing and Refining


Once the metal-bearing minerals are separated from waste materials to form a concentrate, the metal content must be removed and refined. This can be done via a number of methods:


Smelting: The process involves the chemical breakdown of the minerals through heating and melting. Many smelters are uniquely designed to suit a specific concentrate rather than a variety of concentrates – for example, copper smelters are distinctly different from aluminium smelters and are not interchangeable.
The outputs of smelting are impure metal shapes that are shipped to refineries for purification. Here, the metal products are purified to standards set for world metal markets. While there are numerous grades of metal, the bulk of trading is done at about 99.9 per cent purity.


Hydrometallurgical –metals are separated from the minerals by dissolving them. This usually entails a chemical process using acids or cyanide. The metals are finally removed from solution using processes such as solvent extraction and electro-winning – resulting in high purity metals.
The environmental challenges of these refining processes include the handling of solid, liquid and gaseous wastes. The most wide-ranging issue is usually the airborne gases and dusts that can be emitted from the smelting process – including mercury, and the use of cyanide in heap leaching processes.

The company often has to make the area around the mine safe, including securing the waste piles produced by the mine. This is a very important phase for the community surrounding the mine, however it is not a very profitable phase for the company. The government must be careful to put in place legal requirements (in law and in contracts) for the company to ensure that the land is restored to its state before the mining activities began, and if this is not possible the land and community should be compensated in a different manner.


A comprehensive rehabilitation program has many clearly stated objectives which may include:


• ensuring public health and safety
• minimizing environmental effects
• removing waste and hazardous material
• preserving water quality
• stabilizing land to protect against erosion
• establishing new landforms and vegetation

Seabed Mining (SBM) is an experimental industrial field which involves extracting submerged minerals and deposits from the sea floor.

Shallow Water Mining Vs Deep Sea Mining

An emerging consensus says that DSM is the removal of minerals from sea beds deeper than 500 meters.
Under that definition, DSM does not yet exist. Its first incarnation will most likely occur off the coast of Papua New Guinea in late 2017 / early 2018 when the Nautilus Minerals Inc. seeks to remove gold and copper from inactive hydrothermal vent zones at depths between 1000 and 1500 meters.
Extensive deep sea mineral exploration is currently underway in international waters governed by the International Seabed Authority (ISA), under the U.N. Convention on Law of the Sea (UNCLOS), and within Exclusive Economic Zones (EEZs) of many coastal nations.

There are currently three main types of deep sea mineral deposits of interest to industry and governments:

i. Polymetallic nodules (also called "manganese nodules") are potato-sized metal nodules found on the abyssal plain from 4,000 m - 6,000 m deep. These nodules are rich in manganese, nickel, cobalt, copper, lithium, molybdenum, iron, and Rare Earth Elements.
ii. Seafloor Massive Sulphide (SMS) deposits are found beneath deep sea hydrothermal vents along the 67,000 km of volcanically active mid-ocean ridges and back arc basins, between 1,500 m - 5,000 m deep. These contain high-grade copper, gold, silver, zinc, and other trace metals. China and Korea hold contracts to explore SMS deposits in international waters of the Indian Ocean, and Russia and France hold exploration leases on the Mid-Atlantic Ridge.

The Nautilus Minerals "Solwara 1" project in PNG waters is fully permitted, the mining ship and equipment are being built, and mining is scheduled to begin in 2018. This would be the first commercial deep sea mining project in history.

iii. Cobalt-rich ferromanganese crusts are found on summits and flanks of seamounts at 400 m - 4,000 m depth. There are some 10,000 seamounts in oceans rising at least 1,000 m above the seabed (and perhaps another 90,000 smaller seamounts). Metal crusts form on shoulders of seamounts, rich in cobalt, nickel, copper, iron, manganese; rare metals such as tungsten, platinum, bismuth, tellurium, etc.; and Rare Earth Elements.

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