CHAPTER 4: MINING WASTE
This chapter covers the different types of mining waste, how they are formed and how they are disposed of or used.
Mining operations can generate enormous amounts of waste (i.e. waste rock and tailings) that must be managed and handled in a way that avoids negative environmental impact. In Sweden, the mining industry accounts for over 80 percent of the total waste production each year. However, not all waste is to be considered harmful to the environment. Some of the waste might even be safe to use in other applications, such as in building materials. However, when the rocks contain certain minerals in enough quantity combined with poor management of the waste, it can become a large risk to the environment and health of local communities.
Waste occurs at several stages of the mining process and throughout all the life of the mine, from exploration (drilling) to mine closure. Several types of waste are generated in a mine, but three types stand out with the largest volume: waste rock, tailings and mine water. In some cases, the mine uses certain chemicals such as cyanide in the processing stage. These chemicals often account for a small volume of the total waste but can pose a large risk due to their high toxicity. Process water is retrieved from a nearby water shed (river or lake). To minimize the use of freshwater, process water is re-circulated into the process. Tailings are stored in TSFs, whereas solids separate from the water phase. Water is then pumped into clarification ponds, for further treatment and is then re-used in the process. Below follows a summary of the most important types of waste emanating from mining processes.
Waste rock is a heterogeneous (coarse materials mixed with fines) material that must be removed to reach the ore. The ore is then removed, crushed and refined using various enrichment methods that extract the desired metal and/or mineral. The amount of waste rock that needs to be removed depends on the geometry and location of the ore body, mining method and the composition and stability of the bedrock. The ratio of waste rock/ore is called stripping ratio. For example, a stripping ratio of 2:1 means that 2 tonnes of waste rock needs to be removed to mine 1 tonne of ore. Generally, an open pit mine has a higher stripping ratio than an underground mine.
Waste rock is often deposited in piles or heaps close to the mine to minimize transports.
As waste rock is exposed to air and water, a weathering process will proceed. Weathering of waste rock could contribute to the generation of an acid drainage (called acid rock drainage (ARD) or acid mine drainage (AMD) when in association with mines) with high contents of metals, especially if sulfide minerals are present. This topic will be further discussed in chapter 5.
A large block of rock weathers slowly as it has a low surface area while smaller pieces have higher surface areas, making them more susceptible to weathering. Some elements are highly toxic even in small concentrations, for example mercury, while other elements are less toxic but pose a threat to aquatic wildlife in higher concentrations, for example zinc and copper. There are often different types of waste rock within a mine, containing more or less sulfide minerals. One effective way to reduce the environmental impact is to perform separation of different types of waste rock.
When separating waste rock, terms like potentially acid generating rock (PAG, or acid forming rock, PAF) and non-acid generating rock (NAG, or non-acid forming rock, NAF) are often used. This classification is further explained in chapter 7.
A collection of rocks taken from a waste rock pile. There are several types of rocks, and the weathering differs between them. The red rocks are the most weathered rocks in the collection. Photo: Pontus Westrin.
Tailings is the residual material from the enrichment process and consist of a fine grained, silty slurry with a high-water content. The amount of tailings generated is dependent on the ore grade, (proportion of valuable minerals in the ore). For example, if the cut-off grade in the ore is 1 % Copper, 99 percent of the total ore would be deposited as tailings. Iron ores generally have higher grades than sulfide or gold ores, often going over 50 per cent or more. Less tailings are therefore generally produced in iron ore projects. In gold and silver mines, ore grades are generally in the range of grams per tonne.
Tailings are often pumped through pipes to be deposited in a tailings storage facility (TSF) (also called tailings management facility, TMF). The pipes are equipped with spigots in several different places around the TSF to evenly distribute the tailings. The TSF is built especially for storage of the tailings and is designed to fit the location. There are different ways to build a TSF. The most common type of TSFs are placed on the surface and built with high embankments engineered to retain the tailings and water. This type is called conventional impoundment storage.
TSF-embankments are often built with waste rock that needs to be non-acid generating rock (NAG) to ensure long-term stability. Sometimes, the embankments include a layer of geomembrane, a type of synthetic liner or barrier with low permeability, which is mainly used to stop the fine-grained tailings from seeping into the coarse waste rock.
Tailings composition is like that in the ore. Characterization of tailings is important to identify minerals present and their abundance. Tailings is a fine-grained material, having a large surface area, which in turn increases the weathering rate. This is especially important when there are sulfide minerals present. To obstruct the oxidation of sulfide minerals, tailings can be kept under water (lower oxygen levels in water). The role of sulfide minerals in tailings will be further discussed in chapter 5.
When tailings are stored under water it is called a subaqueous deposition. The technique helps to minimize the oxidation of mainly sulfide minerals, but also influences the way the tailings settle in the TSF which in turn impacts the long-term stability. Subaerial deposition, deposition above water, is more common than subaqueous.
A tailings storage facility at a gold mine. Spigots are seen going out from a pipe which is transporting the tailings. Photo: Pontus Westrin.
Water is used in many mining processes. Large volumes of water are used in some processes, i.e. milling. In a mine process, water is a valuable resource, and is often recycled and re-used. Eventually the water is collected and released to the environment. Sometimes the water must be treated in different ways before it can be released. Water treatment can be categorized in active or passive treatment which is discussed further in chapter 7.
Chemicals used in processing
The amount of chemical wastes originating from mining processes are much lower in volume compared to that of waste rock and tailings but can be of a high risk due to high toxicity. A common chemical used in gold extraction is cyanide. The cyanide dissolves the gold to an aqueous solution.
Cyanide is highly toxic for people and the environment. Even low doses of cyanide can cause permanent damage in humans or animals. However, aqueous cyanide is easily hydrolyzed, especially in contact with UV-rays from the sun. When cyanide hydrolyses, less toxic forms of cyanide are formed. The cyanide solution is kept in a closed system during the extraction phase and is then hydrolyzed and oxidized using a cyanide destruction plant. Other chemicals are i.e. xanthates (used in flotation), lime and different kind of flocculation chemicals.