Mine tailings are one of the biggest environmental concerns in the mining industry. They are the materials left over after the desired product, typically minerals are extracted from ore through different mining processes. The type of mineral and the physical and chemical processes used for mining can cause the characteristics of mine tailings to vary, but the tailings are usually made up of the following properties: crushed rock, water, traces of metals, and additives left after the processing. Due to this combination of substances, mine tailings are usually mud-like.
The method used for mining can affect the materials leftover in the tailings; for example, older grinding mills leave behind valuable material in the tailings, such as small bits of the desired metals. Tailings result from a process called concentration, which is when the desired product is extracted from the ore that has been ground and leaves behind waste.
What Are the Dangers of Tailings?
Because tailings are made up of some metallic and chemical properties, they are considered an environmental hazard and need to be stored or reused properly. Some mining, such as uranium mining, can even involve radioactive substances, which are especially harmful to the world. In the past, the smaller scale of mining meant that disposing of the mine tailings was the recommended solution for the substance. However, mine tailings are only increasing in size due to the ever-growing mining industry, making the disposal of tailings more dangerous. Often, the number of tailings even exceeds the amount of valuable metals mined.
One reason why it is important to dispose of tailings correctly is because of acid rock drainage (ARD), also referred to as acid mine drainage (AMD). ARD occurs when the tailings are oxidized after being exposed to air and water, making them spread to other areas through runoff. For example, when mining copper, there are sulphidic tailings that remain after the copper is extracted. These tailings can contain sulfuric acid and even some dissolved copper that can leak into the water after wind or rain has interfered. Or, when gold is mined, the cyanide that is used in the mining process can contaminate water and other living creatures after the same experience.
Another reason why it is important to dispose of tailings correctly is aqueous dumping. For a long time, large amounts of mine tailings have been dumped into different bodies of water and have had toxic effects on aquatic life. But tailings are toxic because of the trace metals or chemicals left over from the mining processes, which can contaminate aquatic life or reduce oxygen levels underwater. These toxic materials can even make their way to humans if any of the contaminated creatures are eaten.
Instead of this method of disposing mine tailings, it is now recommended to try and store them, reuse them, or reduce them in the safest possible ways. Improvements in the management of mine tailings have been made, and workers have found beneficial storage methods to both increases the safety of tailings storage facilities and make the tailing waste more useful.
How Are Mine Tailings Stored?
Several options have been created for storing or reusing mine tailings, such as:
- High density thickened tailings
- Dry stacking or filtered tailings
- Backfilling
- Tailing dams
The first method mentioned, high density thickened tailings (HDTT), is a process where the water content of tailings is decreased, which is beneficial in three ways: 1) it increases the amount of recoverable water, lowering the costs that come from evaporation and seepage during mining, 2) the lower amounts of liquid reduces oxidation, and 3) it makes the storage of the tailings more convenient, for more of it can be compacted and stored together, while less effort needs to be made creating the storage facility.
Dry stacking or filtered tailings is similar to the previous method in that water is reduced in the tailings, making it easier to “stack” the tailings together. This drier form of tailings is placed in covered pits and takes up less space. However, it is costly because of the time it takes for it to dry and the methods needed to transport it to the storage facility.
The third method, backfilling, combines the tailings with cement or a substance similar to cement, binding the tailings together. Afterward, these tailings are put into pillars or other structures underground that help support the mine, and where the chemicals can’t be exposed to the air.
Finally, there are tailing dams—one of the most common storage methods for tailings. In a tailing dam, the tailings are usually made into a wet slurry and then put behind an above-ground dam. These tailing dams can be built upstream, downstream, or centerline. Although downstream is most likely the safest choice, it is also the most expensive, so it is up to mining companies to decide the type of tailing dam they will build. However, experts highly recommend mining companies favor the safety and reliability of the storage method over everything else.
Conclusion
These storage methods don’t account for all of the ways tailings can be safely kept, and none of these methods work perfectly every time. Additionally, these storage facilities need to be as fail-proof as possible, or else the mine tailings can cause damage to the environment in surrounding areas. Recently, more and more tailing storage facilities have failed because of the sheer amount of mining production, which increases the number of tailings that need to be stored. These failures have caused environmentally catastrophic and sometimes even fatal accidents.
Therefore, it is best if careful planning occurs before the storage method is created, taking into account the characteristics of the mine tailings in addition to the regular design planning for the storage facility site. All mining companies should ensure that their tailings are stored in a manner that ensures no harmful chemicals can be spread to living things.
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