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Unit 3: Mining and Mining Impacts
- Contrast surface and underground mining.
- Identify a mining company's goals with each of the following: exploration, extraction, concentration, reclamation, and remediation.
- Describe how wastes are created during the different stages of product creation and use.
- Discuss how waste products are/can be managed.
- Summarize the effects of mining on land use and what can be done to minimize negative effects.
- Identify how air, water, and land can potentially be polluted by mining and associated activities.
- Give examples of how mining, beneficiation, etc. affects society and how mining processes/extent are influenced by societal factors (i.e., economics).
In this reading:
Cleaning Up Afterwards
Environmental and Societal Concerns
Other Information and Sources
Most of the mineral resources that we use in our daily lives are not easily found and do not come out of the ground in a useable form. Finding these resources, obtaining (mining) them, and processing them into something useable takes many varied and often technologically advanced steps. For this unit, we will focus on mining, particularly the mining of metal ores in the United States. An ore is a material that occurs naturally and that contains a mineral or minerals that can be extracted for a profit.
The typical steps in recovering a mineral resource and converting it to a useable state include:
- Locating it (Exploration)
- Obtaining it (Extraction)
- Concentrating it (Beneficiation/Smelting/Refining)
- Cleaning up during/afterward (Reclamation/Remediation/Mine Closure)
Every step of the mineral extraction process is much more complex than described here.
Estimates of the amounts of elements in the Earth's crust represent averages over the entire crust and seldom reflect the composition at a particular location. Rocks and minerals, and thus elements and compounds, are concentrated in certain locations due to rock-forming processes that occurred in the past and/or are occurring today.During the exploration process, a mining company seeks an area where the desired mineral resource is concentrated and attempts to determine the size of the ore body and the mineral resource's ore grade. Higher ore grades (higher concentrations) make the mining project more viable (see Table 1). However, there are many other factors that can influence the decision to extract ore from a specific area. These may include the shape and depth of the ore body, the available mining technology, the potential environmental impact, the need and availability of water, access to workers, proximity to transportation and consumers, state, federal, and other regulations, politics and/or political boundaries, social norms, and human health concerns.
Mining is actually a very expensive process, so mining companies invest time and money to make sure they have picked a good location. In the exploration part of the process, there are usually multiple locations explored, and it may take a number of years to determine which sites are viable. Some sites deemed unfit for development may become more appealing in the future if technologies change and/or the price of the ore rises.
Although not all ore bodies outcrop at the surface, some will. It is very important to determine not only the surface location (outcrop) of an ore body but also to figure out the size, depth, and orientation (trend) of the deposit. By just looking at the surface outcrop, it is impossible to tell the size and shape of the underground ore body.
During the exploration process, geoscientists will use several methods to find suitable locations and to determine the depth and shape of the ore body. These include:
- Creating and reviewing geological maps. Geologic maps show the locations of different types of bedrock (bedrock is the rock that is closest to the surface), give exploration geologists hints as to what geologic processes acted in a given area, and suggest how rocks are distributed at depth. Maps help geologists compare an area with other sites that have yielded highly concentrated ores in the past.
- Visiting a potential mine site and completing field studies, which might entail additional geological mapping, surface rock sampling, and/or chemical analyses of rock, soil, and water samples.
- Performing "noninvasive" studies to obtain underground information. These studies are similar to someone using a metal detector to find discarded coins on a beach. The larger-scale geophysical studies used by mining companies may include seismic, gravity, magnetic, or other surveys.
- Drilling down through the surface to obtain samples at depth. Hollow drills are used that bring cores (long cylinders of rock) to the surface.
Once an appropriate site is located, the mining company obtains any necessary permits, leases, etc. Then the extraction process can begin.
In general, mining techniques are divided into two primary types: surface mining (including pit, strip, and mountain-top removal) and underground mining (shaft). A single mine may employ both methods. Prior to 1900, underground mining was the most common method in the United States. Surface mining is now more common thanks to development of equipment that can easily move large amounts of rock at the earth surface. The large amount of rock broken up during mining that does not contain enough of the mineral resources to process the rock further is called waste rock.
The largest mines are usually surface mines. Heavy machinery and blasting procedures are used to remove large amounts of surface rock, which significantly disturbs the land. A typical surface mine can produce up to 150,000 tons of ore every day. Sometimes whole mountains (or tops of mountains) are removed via surface-mining processes.
Check out this 5-minute video of blasting at a surface mine:
"The World's Largest Recorded Mining Blast" from the Iron Ore Company of Canada. Credit: Jim Cole Productions. Courtesy of Australian Mining, Cirrus Media Pty Limited. http://www.miningglobal.com/video/361/World's-largest-mining-blast-ever-recorded.
Underground mining includes the use of tunnels or vertical shafts to obtain ore from below the Earth's surface. These shafts can penetrate down into the ground or sideways into a mountain side. Underground mines tend to be smaller operations than surface mines, generating a few hundred thousand to a million tons of ore over the lifetime of the mine. Generally, less land is disturbed in underground mining as compared to surface mining.
Potential Problem 1: Waste Rock
Waste rock can include the non-ore-containing rock on top of the ore body (overburden) and rock that contains ore that is not concentrated enough to mine. In surface mining, approximately 2--3 tons of waste rock is removed for each ton of ore. Underground mining generally creates less waste rock than surface mining; the waste rock is either moved to the surface or used to fill in areas of the mine no longer in use. Piles of waste rock are usually deposited close to the mine. These piles can cover hundreds to thousands of acres and be more than 100 feet high.
Mining increases rates of both weathering and erosion. Because digging and blasting break rock into smaller pieces (mechanical weathering), waste rock has more surface area exposed to chemical weathering. For some mining wastes, this is only a small problem. However, some waste rock creates hazardous conditions when chemical weathering mobilizes metals or other undesirable chemicals. These undesirable chemicals may make stream or groundwater more highly acidic. The acidic (low pH) water may be harmful to local organisms, and many of the mobilized metals are toxic to humans, plants, and animals.
In the United States, current mining operations carefully plan the placement and layering of waste rock, and monitor water flow through waste piles in order to minimize waste rock problems. However, this may not be the case in countries lacking government regulations and was not the case in the United States in the past. Several old waste piles in the United States are hazardous, although some have been remediated.
The extracted ore is usually a combination of the desired mineral resource and undesired rocks and minerals. During different concentration processes, the mineral resource is separated from other rocks and minerals and purified.
When taken from the mine, most ore looks like a bunch of rock chunks, with the desired mineral often visible only at the microscopic level. Therefore the desired mineral needs to be further concentrated in a process called beneficiation. The exact process varies, depending on the mineral resource and available technology, but usually requires a series of steps. Some examples of beneficiation processes are described below.
After the ore is transported from the mine site, it is crushed into smaller chunks and then may be milled. In milling, the crushed ore is placed into a rotating drum with steel rods or balls in order to break the ore down into individual mineral grains (something like a clothes dryer, but much louder!). The ore becomes a fine-grained powder. Afterward, water is added and the resultant rock slurry moves on to the next step in the process (flotation or leaching).
Flotation is one way to separate the grains of the desired mineral from the grains of other minerals. During flotation, the rock slurry is mixed with a specifically selected reagent that adds bubbles. Due to the particular chemistry, only the desired minerals will attach themselves to the bubbles and then float to the top (for example, pine oil is a reagent that can be used in copper flotation; copper will attach to bubbles of pine oil, but other minerals will not). The froth of bubbles and attached minerals is skimmed off the top. The extra water and/or reagent is filtered off, and the mineral may go on to be further concentrated, possibly with other processes including using activated carbon, electroplating, and/or leaching with other reagents such as sodium cyanide (discussed further below).
The slurry remaining at the bottom of the flotation tank is considered a waste product called tailings.
Potential Problem 2: Tailings
Tailings, the waste product from the flotation process, are usually pumped downhill into impoundments called tailings ponds. Tailings ponds can be thousands of acres in extent with thicknesses of a few hundred feet.
If one of the walls/dams of the impoundment breaks, then a lot of contamination can be released very quickly. Additionally, if a tailing pond dries out, the metals may be transported as dust on the wind and thus have the potential to be inhaled by nearby residents. Problems of a leaky tailings pile are especially problematic because sulfide minerals, often found in association with metal ores, may occur in high concentrations within the tailings. When exposed to oxygen, sulfide minerals can form sulfuric acid and lead to the development of acidic soils and waters. This may influence water quality in the area by making the waters highly acidic or by an increase in dissolved (and undesirable) metals that results from this acidity (sulfide minerals can be the ones to cause problems in waste rock as well).
Today, plastic liners can be put down to prevent drainage of these contaminated waters into the groundwater system, and the water from tailings ponds can be treated to neutralize the acids. Once filled, tailings ponds are covered (capped) with an impermeable liner or soil and water flow is managed, treating any water leaving the ponds. However, as with the waste rock, there is a legacy of issues surrounding older and unlined/untreated tailings piles.
Leaching is the use of chemicals (such as sulfuric acid or sodium cyanide) to dissolve only the desired metals. The liquid containing the desired metal is removed from the remaining solids and the desired metals are precipitated out of the solution.
Leaching can take place in a vat after milling or instead of milling (either before or after being crushed) or even within the ground itself as a form of "in-situ" extraction.
Potential Problem 3: Leach Piles
Heap leaching is one form of leaching where the leaching solution percolates down through a large pile of ore. Piles can be extensive, cover tens to hundreds of acres, and be a few hundred feet in height. After the metals are leached out, usually after successive treatments with recycled solution, the remaining rock piles have many of the same environmental concerns as tailings ponds. To limit problems, the leach piles can be rinsed once leaching is completed and built upon specially designed pads.
The process of smelting separates the metal from the mineral by heating the mineral in the presence of a material known as a flux. The desired mineral settles to the bottom of the melt and can be separated out. The (unwanted) material at the top of the melt can harden into slag. The smelter is often in a different location than the mine. After this, the desired mineral goes on to a refinery to be concentrated further.
Potential Problem 4: Smelter Emissions
Emissions from smelting can be a source of pollution, giving off sulfur dioxide and other gasses, heavy metals, and particulates. Increasing regulation and improved technologies in the United States have resulted in a reduction in dangerous emissions since the 1970s.
Cleaning Up Afterward
Preparing for Mine Closure
Once the ore material runs out, or becomes technologically or economically inaccessible, the mine will close. In places where water was continually pumped out to keep mining operations dry, the closed mine will fill with water.
The United States has many mines that were closed prior to the 1970s (when environmental regulations were passed) that have many environmental problems, such as acid mine drainage. Mines operating today in the United States must meet higher standards; they must provide some amount of environmental and human health protection while they are in operation and enact a plan to limit environmental and human health problems well after the mine closes. For example, while the mining is still happening, the company will plan how they will grade slopes to decrease erosion and make slopes more stable once they close the mine.
ReclamationReclamation, the restoration of land to either natural conditions or another useful purpose, often involves the process of stabilizing soils and slopes in an area through grading (creating a different, more-gentle slope) and planting trees and plants. Usually addition of new soil, or treatment of the existing soil, is necessary prior to revegetation. This step can be started before the mine fully closes; reclamation can occur as sections close (either parts of the surface mine or the waste rock piles). This can also help improve the aesthetics of the mine area while it is still open.
RemediationRemediation is the process of fixing, removing, or counteracting an environmental problem. In mining, the water leaving the mine area (waste rock, tailings ponds, leach piles, or from the mine itself) often must be treated (remediated) before being released back into the natural system. Like reclamation, treatment of acidic or otherwise contaminated water does not need to wait until the mine closes but should be part of the mining plan and be done as as mining happens.
Environmental and Societal Concerns
The mining industry provides raw materials for the products on which we rely, provides jobs (directly and indirectly tied to the mine), pushes technology forward, and plays a key role in local and global economic systems. In spite of these benefits, mining poses several environmental consequences.
Since the late 1960s, a series of U.S. federal regulations were enacted in the interest of protecting human health and the environment. A number of these acts directly or indirectly regulate the mining industry (see Table 2).
While regulations in the United States have become much more stringent since the 1970s, there are still environmental and health issues related to mining, including:
- The use of water, especially in arid environments where water is scarce.
- The impact on forests and ecosystems, including habitat destruction or alteration.
- Contamination of water through acid mine drainage, accidental spills, etc.
- Worker health and safety, although this has been improved through labor laws.
- Unsafe practices when mines do not meet the federal or state regulations.
- The use of public lands, such as National Forests, for mineral extraction.
- Ground subsidence above underground mines.
While some countries also have well-developed regulations similar to those in the United States, others do not, and some may not enforce the regulations that they do have in place.
Artisanal and small-scale mining (ASM) is common in some, often developing, countries. An estimated 13 million people work in the ASM industry globally, while another 80--100 million people are affected by the industry. ASM is a significant provider of mineral resources, making up about 15--20% of the nonfuel global mineral production.
ASM can offer rural and impoverished communities an improved local economy. However, ASM tends to lack mechanization, meaning that people carry out physically demanding labor. Workers tend to have low incomes and no established employer to provide safety and/or health protection. These small-scale mines are often not regulated and thus often employ children in harsh conditions and damage the environment.
Acid Mine Drainage (AMD): When sulfides (such as pyrite) oxidize and create acid. Not all mining activities create AMD, but it is often associated with metal mining because of the tendency of sulfides to be present in the rock. The acidity can mobilize metals (some toxic) into the water. Return to text
Beneficiation: Processes that separate the desired mineral from the rest of the rocks and minerals in the ore. Return to text
Electroplating: A process that uses an electrical current to encourage precipitation of the desired element. Return to text
Flotation: The beneficiation process in which bubbles of a reagent attract the desired mineral from the slurry and rise with it to the top of the mixture. This froth can then be removed for further concentration. Return to text
Leaching (in mining): The use of chemicals (such as sulfuric acid or sodium cyanide) to dissolve the desired metals and transport them in solution to a collection area. Return to text
Milling: The physical process of crushing and grinding the ore within the beneficiation process. Return to text
Mountain Top Mining/Removal: A type of surface mining in which an entire mountain or mountain top is removed to obtain the ore within or underneath. Return to text
Ore: A material that occurs naturally and that contains a mineral(s) that can be extracted for a profit. Return to text
Ore Grade: The concentration of the desired metal or element within the ore. Return to text
Reclamation: The restoration of land to either natural conditions or another useful purpose; this often involves the process of stabilizing soils and slopes in an area through the grading of slopes and use of vegetation. Return to text
Refining: The final process in purifying an ore to the desired concentration after previous beneficiation. A refinery is where this process happens. Return to text
Remediation: The process of fixing, removing, or counteracting an environmental problem. Return to text
Slurry: A mixture of water and fine particulate material. Return to text
Smelting: The process of melting the beneficiated ore (concentrate) to reduce the impurities and concentrate the desired element. Return to text
Superfund: The program established to address hazardous waste sites with no owners. It enables the Environmental Protection Agency (EPA) to fund and perform clean-ups as well as locate the responsible party if still in existence. Return to text
Tailings: Waste material created from the beneficiation process. Return to text
Waste Rock: Rock that must be moved in order to obtain the ore. This rock does not have a high enough concentration of the desired mineral to make it economically or technologically viable to extract. Return to text