It’s no secret that mining can produce a lot of waste. This is evident by catching a glimpse of any mining operation and the scale at which they operate. In fact, in most cases, you’ll likely only notice the actual mine waste because the majority of the actual operation is occurring underground. Mine wastes can be problematic due to the fact they contain hazardous material that can be released into the environment if not properly handled. Some of these hazardous materials include heavy metals, metalloids, radioactive waste, acidic water, and process chemicals.
Water Contaminated by Acid Rock Drainage
Mine Tailings Storage Site
Rock or Solid Mine Waste
Open pit mining has several stages of waste. First, to access the actual seams or veins of ore, the top layers of rock, or overburden, must be removed. Once the overburden has been removed, the seams can be extracted. When extracting the seams, the additional waste rock will be removed to get to the actual ore, called gangue. Then, as the material is processed and finely ground, even more, waste, called mine tailings, will need to be removed from the site.
Overburden -> Gangue -> Tailings
Waste rock, or overburden, refers to the often large mass of initial soil and rock that is removed in order to get to the valuable mineral deposits in mining operations. Typically, for every ton of ore that is mined, 5 tons of overburden must be displaced. Overburden is not subjected to any chemical processes, but must still be removed in order to access ores. Overburden is usually managed by piling it on the nearby surface of mining sites in an area that will not interfere with ongoing operations. Transporting such large volumes of material is costly, so overburden is stored nearby and often used for revegetating the land upon closure of the mine.
Gangue is the worthless rock or material that is closely mixed with the valuable material to be processed. The separation of mineral from gangue is called mineral processing. Often times, inefficient processing methods can produce gangue that still holds an ample amount of valuable minerals. As values of minerals increase, it can even be profitable to reprocess gangue to extract additional minerals that may have been missed during the first processing.
Tailings are finely ground rocks and other mineral waste as a result of mineral processing. Due to the way minerals are processed, tailings can contain concentrations of processing chemicals. This can make mine tailings an environmental concern, so proper transportation and disposal are crucial. Consequently, the next step is to pump mine tailings away with slurry pumps into tailings ponds. Tailings ponds are sedimentation holding ponds enclosed by dams and liners to capture and store the waste.
Liquid Mine Waste
Mine water is produced in a few different ways at mine sites and can vary in levels of contamination. Water exposed to mining processes is also often acidic and can contaminate local water sources in a process called acid mine drainage (AMD) or acid rock drainage (ARD). Acid mine drainage is a heavy contributor to pollution of surface water across the globe. AMD is primarily caused when water flows over the sulfide-heavy material, forming an acidic solution. Water at mine sites is usually heavily monitored and management strategies are used to not only reduce the amount of mine water produced but also to treat the water before it is released back into the environment.
Water Treatment Sludge
Sludge is produced at some mine sites and is similar to mine wastewater, but has the additions of solids and processing chemicals. These additions turn the water into a more viscous sludge which can then be pumped away from the site. Since the majority of sludge has little economic value, it’s essentially handled as waste. In extreme cases where the sludge is rich in harmful or radioactive material, it may be classified as hazardous waste and require special handling and disposal methods.
Mine Waste Management
Waste management techniques employed by mining companies are routinely under intense scrutiny from local governments and the general public. Improper disposal methods and the resulting environmental damages have plagued the history of the mining industry. These actions have left a negative stigma associated with mining and associated waste materials. For this reason, many countries now require miners to prepare a complete mine waste storage proposal before a mining permit will even be granted. To ensure long-term storage stability and prevent breaking any regulations, mine waste is carefully managed at every step of the process.
The volume of waste from mining operations is high, and due to the large volumes of waste, environmental concerns will inevitably arise. In response, mining engineers have developed clever ways of waste management, making the mining industry one of the few that actively recycle their own waste. Overburden is used for reprocessing, contouring land, and as a construction aggregate for buildings and roads. Mine tailings are reused for producing clay, tiles, glass, and concrete. Mine water is treated and then used for dust and particulate suppression, agricultural/industrial use, and as a coolant.
Despite numerous recycling methods, the majority of mine waste is still stored in facilities or waste sites. The long-term storage of these facilities has become an important topic in modern mine closures. Various regulations have come to pass that require the waste to be stable for years, sometimes centuries. This requires engineers to develop storage methods to withstand catastrophic events like floods, heavy storms, and earthquakes.
Lasting Environmental Impact
The main environmental impacts of mining waste include the loss of land following its conversion to a tailings pond or waste storage area and the introduction of acidic runoff or other contaminated sediments into the local environment. The specific environmental impact of waste depends heavily on the materials composition, type of ore mined, and the way the ore is processed. For example, gangue and tailings from mining heavy metals could have a high concentration of sulfides which could cause acid rock drainage to occur. Due to the many variables, mining operations will need to develop their own methods of waste disposal in accordance with regulations pertaining to different types of waste they produce. However, it is worth noting that a large portion of mining waste is benign to the environment and is routinely used to revegetate or contour the land when the mining operation has been completed.
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