How to close a mine sustainably
Even the most productive and longest-running mines must one day retire. At best, it could mean the start of a new life for the mine in some other purpose – but the initial focus must be on minimizing the environmental changes caused by the mine’s operations.
- Take these into account as early as possible:
- All possible fractions should be recovered. The less waste there is during production, the less there is to worry about during the closure phase.
- Water consumption should be minimized already in the operational phase. Less water means less water-borne emissions.
- Sort all reactive and hazardous substances separately – this will also mean less hazardous waste is generated.
- Take the needs of the closure phase into consideration when planning the foundation structures of waste areas.
- When soil mass is sorted properly, it can be used in the earthworks of the closing phase. Purchasing and transporting soil mass can become surprisingly costly.
- While it might be difficult to predict exactly when a mine’s production will end, planning of the closure should always be started as early as possible – preferably even before the mine opens.
Although planning the closure of a mine at the foundation stage would bring extra costs, it should not be looked at as merely an expense. In the best-case scenario, it could end up turning a profit. For example, with market prices and technology changing and evolving over the years, the precise sorting of tailing piles and the separation of products other than the main product from mining fraction can prove to be a profitable solution.
“In an ideal situation, for example, waste minimization would be taken into account in the planning phase of the process. If everything is recovered from the fraction already in the production phase, there is much less to worry about during the closure phase. In addition, minimizing water consumption during the operational phase, for example in waste material stockpiling, reduces the need for post treatment,” explains Senior Scientist Päivi Kauppila, of the Geological Survey of Finland.
Different mines, different needs
It is difficult to set universal guidelines for closing down a mine. Every mine has its own special characteristics, related to, for example, the mineralogy and chemical composition of the ore, the method of extraction, the conditions of the mine’s area and land use, the size of the mine and the ore utilization methods.
In an industrial mineral mine, for instance, a good end result can be achieved by shaping the waste rock piles and tailing areas and covering them with topsoil, and through revegetation. At the other end of the spectrum are sulfidic ore mines, which may require long-term measures in terms of, for instance, monitoring and water treatment.
The type of mining operation also affects the requirements set for the mine’s closure. The calculations influencing the choice of mine type should absolutely take into account also the costs of its eventual closure. Even though establishing an underground mine generally comes with a higher price tag, they are often easier to close down than open-pit mines, because the waste rock can be used as backfill. Underground mines also suffer less oxidization of the walls, as the sulfide minerals are not exposed to air and oxygen to the same extent as in open-pit mines. Generally, however, the type of ore is what ultimately determines the type of mine.
Because mines usually operate on a very long time frame and over the years the mine’s operations are often reconfigured, it is wise to update the closure plan at regular intervals. The final measures are naturally decided on only towards the end of the mine’s operations.
From landscaping to overall impacts
Even as recently as the last century the focus of mine closures was mostly on fencing off hazardous areas and on landscaping. Nowadays when mines are closed down, the overall impacts of the mine are considered to a greater extent.
“Water management is a good example of this. Whereas building wetlands used to be considered a sufficient measure, nowadays a more holistic view of the situation is taken. If a passive measure like building wetlands is not enough, it might be necessary to consider, for example, chemical treatment,” says Kauppila.
Areas that have been utilized generally cannot be remediated completely to their original state, but with the help of landscaping and physical and chemical stabilization, the ecosystem can be restored to one that is as diverse as possible, and some form of safe, new land use can even be planned for the site.
Safe now and in the future
The biggest challenges involved in closing down a mine most often involve, in addition to water management, also extractive waste and acidic leachates. Managing these challenges is considerably easier if a mine-closure plan is drawn up at an early stage.
Power of bacteria
Sulfate-reducing bacteria can be used in the water treatment process of open-pit mines. The bacterial process reduces the sulfates to sulfides, which react with the metals and causes them to precipitate. These bacteria appear in the natural environment, but their action can be accelerated by creating favorable conditions. Depending on the situation, slurry manure or wood materials, for example, can be used to accelerate the process. “Sulfate-reducing bacteria is a rather effective passive method. And as it does not require continuous chemical dosing, it also means it is relatively low cost,” says Kauppila.
“The biggest risks in closing a mine are related to waste areas, so that is something that should be given the most attention. It is a good idea, for example, to sort waste materials systematically right from the start according to their environmental properties,” stresses Kauppila.
When a mine is closed down, in addition to landscaping and current emissions, another thing to consider is how natural conditions could change the situation over the years: When the mine fills with water, will any hazardous substances leach from the materials contained therein? Will the structures used to cover waste areas endure the force of floods and storms in the coming decades, for example?
Over the past few decades, mine closures and the related responsibility issues have become a topic of discussion in the industry. The matter has been influenced by the rise in the broader theme of sustainability and especially by the development of environmental studies and subsequent legislation. In many emerging countries, however, legislation is just now waking up to the issues surrounding mine closures.
“The US and Canada are at the forefront in terms of legislation. Large, socially responsible mining companies do take care of their environments everywhere in the world, whether the law demands it or not,” says Kauppila.
“Overall, the problems and various possibilities related to closing down mines have only begun to be understood in recent years. We still need a lot of research, and best practices in the industry are still quite new,” Kauppila points out.
”Prepare for the unknown”
Päivi Kauppila urges those involved in the closure process also to prepare for the unknown.
“While obvious hazardous substances, such as cyanide, are given due attention, the impacts of, for example, nitrogen emissions have only been charted in the past decade. Likewise, the detrimental effects of sulfates on waterways have only begun to be understood in recent years. Only fairly recently has it been discovered that uranium can also pose a considerable hazard when exploiting other than uranium ore deposits,” says Kauppila.
“As early as during process planning, it is recommended that extremely thorough chemical analyses be carried out – for example, on ore stones, process chemicals, various waste fractions and the area’s environment (surface and groundwater, soil), to find out the starting levels of the substances both in the area and in the process materials. Later, similar comparison measurements are carried out to determine whether the operations have generated the kind of emissions that went unnoticed during planning. This is bearing responsibility for the future,” Kauppila sums up.