Mining is defined as the extraction, from the Earth, of valuable material for societal use. Usually, this includes solid materials (e.g. gold, iron, coal, diamond, sand, and gravel), but can also include fluid resources such as oil and natural gas. Modern mining has a long relationship with modern society. The oldest evidence of mining, with a concentrated area of digging into the Earth for materials, has a history that may go back 40,000 years to the hematite (used as red dye) of the Lion Cave in Swaziland . Resources extracted by mining are generally considered to be nonrenewable.
16.1.1: Renewable vs. Nonrenewable Resources
Resources generally come in two major categories: renewable, which can be reused over and over, or replicate over the course of a short (less than a human life span) time, and nonrenewable, which cannot.
Renewable resources are items that are present in our environment which can be exploited and replenished. Some of the more common energy sources in this category are linked with green energy resources because they are associated with environmental impacts that are relatively small or easily remediated. Solar energy is the energy that comes from fusion within the Sun, which radiates electromagnetic energy. This energy reaches the Earth both constantly and consistently and should continue to do so for about 5 billion more years . Wind energy is maybe the oldest form of renewable energy, used in sailing ships and windmills. Both solar and wind-generated energy are variable on the Earth’s surface. These limitations may be offset through the use of energy storing devices such as batteries or electricity exchanges between producing sites. The heat of the Earth, known as geothermal, can be viable anywhere if drilling goes deep enough. In practice, it is more useful where heat flow is great, such as volcanic zones or regions with thinner crust . Hydroelectric dams provide energy by allowing water to fall through the dam activating turbines that produce energy. Ocean tides can also be a reliable source of energy. All of these types of renewable resources can provide the energy that powers society. Other renewable resources that are not directly energy-related are plant and animal matter, which are used for food, clothing, and other necessities.
Nonrenewable resources cannot be replenished at a sustainable rate. They are finite within a human lifetime. Many nonrenewable resources are chiefly a result of planetary, tectonic, or long-term biologic processes, and include items such as gold, lead, copper, diamonds, marble, sand, natural gas, oil, and coal. Most nonrenewable resources are utilized for their concentration of specific elements on the periodic table. For example, if society needs sources of iron (Fe), then it is the exploration geologist who will search for iron-rich deposits that can be economically extracted. Non-renewable resources may be abandoned when other materials become cheaper or serve their purpose better. For example, abundant coal is available in England, but the availability of North Sea oil and natural gas (at a lower cost and lower environmental impact) led to the decrease in coal usage.
The elements of the periodic table are found within the materials that make up the Earth. However, it is rare for the amount of the element to be concentrated to the point where the extraction and processing of the material into usable product becomes profitable. Any place where the amount of valuable material is concentrated is a geologic and geochemical anomaly. If the material can be mined at a profit, the body constitutes an ore deposit. Typically, the term ore is used for only metal-bearing minerals, though the concept of ore as a non-renewable resource can be applied to valuable concentrations of fossil fuels, building stones, and other non-metal deposits, even groundwater. The term “natural resource” is more common than ore for these types of materials.
It is implicit that the technology to mine is available, economic conditions are suitable, and political, social and environmental considerations are satisfied in order to classify a natural resource deposit as ore. Depending on the substance, it can be concentrated in a narrow vein or distributed over a large area as a low-concentration ore. Some materials are mined directly from bodies of water (e.g. sylvite for potassium; water through desalination) and the atmosphere (e.g. nitrogen for fertilizers). These differences lead to various methods of mining, and differences in terminology depending on the certainty. Ore mineral resource is used for an indication of ore that is potentially extractable, and the term ore mineral reserve is used for a well defined (proven), profitable amount of extractable ore.
16.1.3: Mining Techniques
The style of mining is a function of technology, social license, and economics. It is in the best interest of the company extracting the resources to do so in a cost-effective way. Fluid resources, such as oil and gas, are extracted by drilling wells. Over the years, drilling has evolved into a complex discipline in which directional drilling can produce multiple bifurcations and curves originating from a single drill collar at the surface. Using geophysical tools like seismic imaging, resources can be pinpointed and extracted efficiently.
Solid resources are extracted by two principal methods, of which there are many variants. Surface mining is the practice of removing material from the outermost part of the Earth. Open-pit mining is used to target shallow, broadly disseminated resources. Typically, the pit progressively deepens through additional mining cuts to extract the ore, and the walls of the pit are as steep as can safely be managed. A steep wall means there is less waste (non-valuable) rock or overburden to remove and is an engineering balance between efficient mining and mass wasting. Occasionally landslides do occur, including a very large landslide that occurred in the Bingham Canyon mine in 2013. These events are costly and dangerous, though careful monitoring gave the Bingham Canyon mine ample warning time. Strip mining and mountaintop mining are surface mining techniques also used for resources that cover large areas, especially layered resources like coal. In this case, an entire mountaintop or rock layer is removed to gain access to the ore below. The environmental impacts of surface mining are usually greater due to the larger surface disturbance footprint .
Underground mining is often used for higher-grade, more localized, or very concentrated resources. Some ore minerals are mined underground by introducing chemical agents that dissolve the target mineral followed by solution extraction and subsequent precipitation in a surface operation, but more often a mining shaft/tunnel (or a large network of these shafts and tunnels) is dug to access the material. Whether mining occurs underground or from Earth’s surface is dictated by ore deposit depth, geometry, land-use policies, economics, strength of the surrounding rock, and physical access to the ore to be mined. For example, deeper deposits might require removal of too much material, it may be too dangerous or impractical to remove, or it may be too expensive to remove the entire overburden. These factors may prevent materials from being mined from the surface, and cause a project to be mined underground. Also, places where the mining footprint can not be large may force underground mining to occur. The method of mining and whether mining is feasible depends on the price of the commodity and the cost of available technology to remove it and deliver it to market. Thus mines and the towns that support them come and go as the price of the commodity varies. Technological advances and market demands may reopen mines and revive ghost towns.
16.1.4: Concentrating and Refining
All ore minerals are mixed with less desirable components called gangue. The process of physically separating gangue minerals from ore-bearing minerals is called concentrating. Separating the desired element from a host mineral by chemical means (including heating in the presence of other minerals) is called smelting. Finally, taking a metal such as copper and removing other trace metals such as gold or silver is done through the process of refining. Typically, this is done one of three ways:
- Items can either be mechanically separated and processed based on the unique physical properties of the ore mineral, like recovering placer gold based on its high density
- Items can also be heated to chemically separate desired components, like refining crude oil into gasoline
- Items can be smelted, in which controlled chemical reactions unbind metals from the minerals they are contained in, such as when copper is taken out of chalcopyrite (CuFeS2).
Mining, concentrating, smelting and refining processes require enormous amounts of energy. Continual advances in metallurgy and mining practices aim to develop ever more energy-efficient and environmentally benign processes and practices.