Mines can have a major effect on the environment, both the appearance of the landscape and the chemical environment. How long is a mine operational, and how is its environmental impact during and after operations?
THE LIFE OF THE MINE
Mining operations are generally continued as long as extraction is profitable and there are deposits left to extract. Profitability is largely steered by raw material prices. It is common for the mining company to prospect while the mine is operational. During operations, knowledge is gathered about the geology of the area and around the deposits. It is not unusual for further information about the deposits and the immediate area to bring the discovery of more ore near the mine. The number of years the mine will be in operation is usually decided by the mine’s ore base. The ore base is the known deposits of ore, often expressed in millions of tonnes. The ore base divided by ore extracted per year gives the mine’s life at the time of calculation.
As an example, a mine with 100 M t of ore and extraction rate of 5 M t per year has a life of 20 years. If prospecting near the mine leads to the discovery of a further 50 M t, the life of the mine will increase by 10 years if the production rate stays the same. For a newly started mine it is often stated how long it will be in operation, but that is not necessarily correct, since it only reflects the ore base the company has at the time. In several places in Sweden, there have been mining operations for a very long time even though it was previously thought that there was no ore left. With changed raw material prices, also rock previously considered an interesting becomes interesting to extract. New technology also contributes to this. Several open pits have gradually developed into underground mines as more ore was found at depth.
Ores are finite resources and eventually even the largest deposits become exhausted. When a new mine becomes operational, through drilling and analysing, an ore body will have been defined that is sufficiently well-known to present an ore base with a given content and volume. The ore base is then used to make a production plan for the best economic outcome, where the life of the mine becomes a function of the total known number of tonnes and the annual extraction rate.
In many cases at the time of starting up the ore body has not been completely defined – but a sufficient large quantity of ore has been found to ensure a sufficiently positive economic outcome and lifespan to begin operations. In most cases the life of the mine changes, which usually means extending operations. The reason is that new ore is added through further investigations in the known extensions of the ore body. Through prospecting near the mine, there can also be discoveries of previously unknown but nearby ore bodies, or ore bodies at greater depth which extend the life of the mine. Other factors can be increased productivity which reduces extraction costs and thereby makes deposits previously too low-grade profitable to extract. There is a similar effect from increased metal prices.
It is less common for the life of a mine to become shorter than planned. The reason is then often falling metal prices which mean that certain lower grade parts of the ore body become unprofitable to extract, thereby shortening the life. Other negative factors which can reduce the life are incorrect economic calculations, unsuitable mining methods which create ore losses or high waste rock dilution, problems with rock strength, difficulties to produce concentrate of sufficiently high quality, increased operational costs, etc.
Restoration at the Svärtträsk mine. Photo: SGU
TERMINATED OPERATIONS AND RESTORATION
When the mine is to be closed for different reasons, often because the ore base is exhausted or the operations can no longer be profitable, then by law* it must be restored and remediated. Remediation of a mine is often carried out to a so-called remediation plan. It describes how different types of waste are to be deposited, covered or in some other way handled to ensure optimum restoration. It also describes how the mine is to be remediated. Waste management differs depending on the hazard level. This is often governed by the content of sulphide mineral in the waste. Sulphide mineral weathers easily in contact with oxygen and must therefore be covered with other material to create oxygen free conditions. Read more about sulphide mineral in chapter 6.
Covering waste is often done using one of two main methods: dry covering ore water covering. Both methods aim to reduce the supply of oxygen to the waste.
Dry covering is done both with hazardous and less hazardous waste. In the case of less hazardous waste a so-called simple dry covering is done, and comprises a layer of till. In the case of more hazardous waste, among other things waste with a higher concentration of sulphide mineral, qualified dry covering is required. This contains different layers to ensure that conditions are oxygen free. A surface containment layer is dumped closest to the waste that comprises clay or clayey till, but different kinds of ash, slurry or liner products have also proven effective. Above the surface containment layer there are drainage layers to lead water away from the waste. This material is often more coarse grained. To secure the surface containment layer, a protective layer is also placed above the drainage layer. This is designed to protect from frost action and mechanical effects from among other things root penetration. The protective layer often comprises a thicker layer of till. Finally, vegetation is added to prevent erosion of the layers.
Water covering means that an artificial lake is constructed above the mining waste. The water should be stagnant to avoid oxydation. This method is common when covering tailings or filling an open pit where the waste can be deposited. The waste is often also covered by a layer of till and other material before water covering.
* 10 kap 5 § miljöbalken, 71 § utvinningsavfallsförordningen. ** Among other things bentonite mats or geomembranes of dense rubber sheeting.
Facts: Types of mining waste
Waste rock
Waste rock is the rock that is removed to access the ore. Waste rock is produced in both open pit and underground mines.
The environmental hazard from waste rock varies. In some cases, waste rock can contain sulphide mineral (Read more on [sida ]. It is then called acid producing waste rock and must be covered in order not to weather. Waste rock also be non-hazardous and contain only small quantities of harmful substances. In many cases, waste rock is used as material for building roads and dams in mine areas. In certain cases, the waste rock can also be used outside the mine area as ballast. Waste rock is also used to backfill mines and cavities.
Tailings
Tailings are produced after the ore is crushed and processed into mineral concentrate. What remains is called tailings. This material, like mineral concentrate, is very fine-grained. Since all minerals and metals cannot be extracted from the ore, tailings often contain small amounts of valuable minerals. Tailings from sulphide ore contain sulphide and must be covered to prevent weathering. In certain cases, tailings can be used to backfill cavities. The amount of tailings that occurs is related to the contents of the ore. Higher content, like in iron ore mines (often over 60 per cent), results in less tailings.
Illustration of the different mine wastes.
Source: SGU.
THE ENVIRONMENTAL IMPACT OF MINES
Mines are environmentally hazardous operations according to Swedish law. Mines can affect our environment and health in different ways, primarily through the discharge of metals and other substances into water, but also through noise, vibrations and dust, which can disturb people living nearby. A mining operation also takes up land and changes the landscape. There are several factors that steer how much the impact is from a mining operation, where the most significant factor is how waste is handled. Other factors can be topography, climate, geology and the sensitivity of lakes, watercourses and the water table. How much may be emitted by the operation, what noise levels, vibrations or other things the operation may and may not do are regulated by the conditions set in the environmental permit (see Chapter 3). The company and the supervisory authority are responsible for ensuring that the conditions are complied with and that there is no harm to the environment. The most significant environmental impact mines have is the discharge of metals and other substances into water and the ground. The impact from the emission of metals and substances varies from substance to substance. Certain substances are needed for example for the existence of life, but at excessive content levels the substances can be toxic in the short-term or long-term. Acute* and chronic* effects vary by substance, content and time of exposure. Metals are elements and do not weather, but stay in the environment for a long time. Their mobility and capacity for dissemination can however often be counteracted, among other things by raising the pH level. At a high pH level, most metals are bonded in fairly insoluble compounds or adsorbed into other minerals. It is the redox potential (reducing v. oxidising environment) and pH (concentration of hydrogen ions) that largely governs the solubility of metals and their dissemination in the environment. This is the reason why mining waste is often treated with lime, which raises the pH level. Weathering can also be counteracted by creating oxygen deficient environments that prevent oxidation. Modern mine operations work ongoing with decontamination of waste during operations. This is to secure the environment, but also for improved profitability, since weathered material is more difficult and expensive to handle. If an operation breaches the regulations of the Environmental Code the supervisory authority is obliged to file a complaint.
* Acute effects concern short exposure, often a few days, depending on the organism. It is often a matter of direct mortality, but other effects can also arise.
** Chronic effects concern more long-term exposure. In these cases it is often a question of impact upon behaviour, growth or reproduction.
METALS, ENVIRONMENT AND HEALTH
Metals are elements that occur naturally in all bedrock, soil and water. Weathering of rock occurs all the time, and metals are constantly emitted into lakes and watercourses. When rock is extracted and crushed, a greater area per grain is exposed to air and water, weather and wind. This means that the weathering processes continue much faster and that metals are more easily emitted into nature. Since metals are elements, they do not decompose, but stay in our environment or in our bodies long-term. Most substances have a negative effect on health in high concentrations. What concentrations are dangerous to humans depends on the substances and are often governed by limit values. Exposure to certain metals can be carcinogenic, cause damage to the nervous system or affect kidney functions. You can read more about metals and our health on the National Food Agency website*.
The impact on our ecosystem and our environment from metals occurs both through natural emissions and emissions from the general public and industry. Mining operations are often regulated according to emission conditions set in the environmental permit. The handling of mining waste brings an impact on nearby land and water. How great the impact is, and whether it can be considered harmful, is often assessed on the basis of environmental quality norms. Environmental quality norms for surface water can be read about on HaV**. Metals and their different effects on our environment can be read about on the Environmental Protection Agency website***.
* https://www.livsmedelsverket.se/livsmedel-och-innehall/oonskade-amnen/metaller1
*** http://www.naturvardsverket.se/Sa-mar-miljon/Manniska/Miljogifter/Metaller/
