Mining Science 101
There are two types of mining: open-pit and underground, depending on the location of the mineral deposits.
For open-pit mining, large pits are dug in the ground and explosives are used to release chunks of ore which are later refined. The refining process is the separation of the valuable minerals from the surrounding rock; the actual method used depends on the type of ore being mined.
The two types of underground mining are hard rock and soft rock. Hard rock mining refers to mining for minerals in hard ore, such as gems or metals. Soft rock mining is the mining done in softer sedimentary rocks, which includes mining for oil or coal. In underground mining, access shafts are dug into the ground where depending on the shape and size of the orebody, side tunnels can be dug. Depending on the specifics of the location of the mine project, the ore is taken out of the underground tunnels in a pre-determined way for refinement.
Ore Processing
Ore processing is done in slightly different ways depending on the mineral. All processes start with milling the ore to separate the valuable minerals from the surrounding rock (gangue). The ore is broken down into small, sand-sized pieces. The ore is then mixed in a liquid solution to ‘float’ the desired mineral and separate away as much of the gangue as possible. A leaching process is then undertaken. This is where the different mineral compositions become important, the additives to the slurry depend on what mineral is being refined.
More information on ore processing on the following common minerals can be found on these external webpages:
Gold: https://www.gold.org/about-gold/gold-supply/gold-refining
Copper: https://investingnews.com/daily/resource-investing/base-metals-investing/copper-investing/copper-refining-from-ore-to-market/
Molybdenum: https://www.imoa.info/molybdenum/molybdenum-processing.php
Silver: https://www.perthmint.com/refining-process.aspx
Zinc: http://metalpedia.asianmetal.com/metal/zinc/extraction.shtml
Iron: https://corporate.arcelormittal.com/who-we-are/from-ore-to-steel
After ore processing, the leftover waste cannot be easily disposed of. The most common disposal method is to store the waste, or tailings, into a pond where the water separates from the solids. This separation of water from the solid waste helps trap the small chemical particles from becoming airborne. Universally, tailings ponds are highly toxic pools where untreated dangerous minerals, such as cyanide, can be an environmental risk if not monitored. There are a multitude of other tailings disposal methods such as backfilling underground mines and dry stacking, but the most cost-effective and simple approach for a mining company is a tailings pond.
Environmental Risks and Monitoring Them
It is difficult to categorically list all the possible mining-related environmental risks due to the diversity of mining practices and their regulation, the unique geology of each mine, and unknown consequences related to climate change such as the increased rate and intensity of natural disasters.
Despite the vast number of hypothetical situations, there are a few common risks that local communities can monitor. Most dangerous mining pollutants impact communities due to negligence and lack of continuous monitoring from resource extraction companies. While mining harms the natural environment, proper state regulation and enforcement of environmental laws can help protect communities. Photo documentation and detailed notes of the environmental damage around a mine can be helpful in litigation.
Acid Mine Drainage
A large environmental risk resulting from mining is Acid Mine Drainage (AMD) which occurs when sulphurous rocks are disturbed and exposed to oxygen in or near bodies of water. Sulphurous rocks are highly acidic (have a low pH) making the water toxic to drink. Also known as acid and metalliferous drainage or acid rock drainage, AMD can naturally occur from erosion, but mining greatly increases its rate and frequency.
In an extraction context, AMD most often happens towards the end of an underground mining cycle. Since underground mining happens below the water table, water is constantly pumped out of the tunnels until a mine’s operations cease. The water then becomes contaminated and can cause pollution down-stream from the mine. Due to the chemical composition of the ore, copper mines are the most common culprit of AMD.
AMD is easy to recognize because it changes the appearance of water to the naked eye. Water becomes a rusty orange colour due to the pH level. The high concentration of potentially toxic minerals can decimate ecosystems by killing plant and animal life; many species of fish cannot survive the pollution. In addition, water can no longer be safely consumed or used for washing. AMD impacted water can cause severe irritation and burns to people’s bare skin, and if consumed, can burn the mouth, throat, and stomach. Sulphuric Acid is carcinogenic but there is no medical test to precisely determine amount of exposure. Water sampling is necessary to establish the exact toxicity of water impacted by AMD.
AMD can be treated by running the water through limestone which helps neutralize the water’s pH level. This is called “aeration-liming”. This, however, is a complex and potentially expensive process. Prevention of AMD is recommended.
Cyanide
Cyanide is the chemical used in the refining process for gold and silver. It separates the “valuable” mineral from the “unusable” surrounding rock. There are two ways to use cyanide in this way: heap leaching and vat leaching. In heap leaching, a solution of cyanide is sprayed onto piles of ore causing chemical reactions that make the valuable minerals (usually gold) trickle down and away for collection. In vat leaching, the ore is mixed with the cyanide solution in a large tub and the waste is stored in tailings ponds.
The danger of cyanide is the high toxicity of the chemical. If (and when) cyanide-filled tailings ponds spill, there are many environmental and human health risks. If cyanide spills encounter groundwater, aquifers and surrounding waterways can become contaminated. Cyanide readily interacts chemically with living organisms because it is a carbon-based organic compound; toxicity can be lethal and unpredictable.
Cyanide is highly toxic to humans and aquatic life when ingested. Decomposition (and dissipation) of spilled cyanide depends on environmental conditions. This means there is a lot of uncertainty in measuring the toxicity of cyanide after a spill.
Once cyanide has spilled, the current mitigation strategy is to wait until the chemical has dissipated enough to no longer be toxic. While the dissipation is happening, toxic gases can be released, such as HCN and cyanogen chloride. These gases can cause medical complications in a much wider area due to their extensive diffusion.
Physical Geography
Resource extraction can cause large disturbances to the physical geographical area surrounding the project. Aside from the disturbances caused in creating an open-pit mine, the environmental footprint of a mine can be excessive. Dangers such as erosion from exposed hillsides and sinkholes from collapsed tunnels are very real. Since mine projects have such large physical presences, animal migration paths can also be severely disrupted. Pollution from a site has the potential to modify the quality and diversity of local vegetation due to soil and water changes.