Mine Water Treatment

There are three main elements of a mine water management program: water quantity, water quality and closure plans.

Water quantity
The quantity of water available at a mine site varies depending on the region. For example, in the western United States, water scarcity can play a role where the mine is reusing its pit water, but in parts of Canada, we are seeing the opposite where some mines have a positive water balance and have too much. Another thing to keep in mind is how climate change is going to impact the site’s water balance. This must be taken into account when determining the program. And finally, it is important to find the right balance between treatment capacity and storage needs: increased active treatment capacity reduces storage capacity and vice versa.

Water quality
What is the quality of the source water and how does the waste rock and tailings management impact the water quality? This along with the environmental regulations for effluent discharge are key in selecting the right technology for water treatment. It’s important to anticipate potential future regulations. For example, in British Columbia, Canada, regulations for selenium are becoming more and more stringent; mining companies are anticipating future regulations that may come into play in the near future, so they are already planning for these tighter regulations when developing their mine water treatment programs.

Closure plans
An often forgotten area when developing a mine water management program is to also keep in mind its closure plan. What is needed on a day-to-day basis for operations during the actual life of the mine is different from the actual post-closure operations.

Acid Rock Drainage (ARD) is a naturally occurring process that generates acidic (low pH) and metal-bearing solutions from reactions of certain minerals in rock material in the presence of oxygen (i.e. air) and water. While ARD is a natural process, its occurrence may be intensified by human activities such as road construction which disturb the rock. It is a common problem at mine sites that have sulfide material in the ore and waste streams (e.g. copper, lead/zinc and nickel sulfide operations, some gold operations, and some coal operations). Acidic solutions generated by ARD dissolve metals from the wastes, including copper, lead, zinc, arsenic, cadmium and iron. Such metals can be extremely toxic to fish and other aquatic life downstream and must be treated.
 

Operating mines, as well as abandoned mine sites, generate acid mine drainage which requires water treatment to prevent adverse impacts to rivers and streams. Veolia has developed a proprietary process known as DenseSludge™ to reduce sludge management costs and improve the operating efficiency of treatment plants that have acid mine drainage. 

The DenseSludge™ recycle process forms particles that settle quickly, dewater readily, and hold little water. The result is a reduction in sludge generation by nearly 90% and production of a drier sludge, which reduces the volume of water that returns to the mine pool for re-treatment. The technology has enabled mining companies to extend the life of existing sludge storage facilities and reduce requirements for the building of new storage.

The process of dealing with — or remediating — a closed mine site can come with several key issues and challenges, especially from a water treatment perspective. At the end of their productive working lives, it is vitally important to ensure that mines are managed in a manner that mitigates or eliminates any potentially adverse effects. In practice, this means that the owners of individual abandoned or closed mine sites often have a duty of care to protect the natural environment — both in and around the area. They need to make every effort to eliminate the possibility of any negative impacts, which can include contamination of the surrounding land and surface watercourses, as well as the pollution of local groundwater supplies and drinking water aquifers.

In doing so, the deployment of an effective water treatment strategy is likely to form a key component for effective mine closure and reclamation. Contaminated water at closed mine sites can contain a range of harmful and toxic substances and dangerous chemicals, as well as metals such as cadmium, nickel, iron, copper and zinc. Such materials will tend to vary depending on the nature and location of the mine site. Furthermore, water at abandoned mine sites can be highly acidic, which can pose a dangerous threat to human, animal and plant life.

Various processes throughout the mining value chain – ore extraction, mineral processing and refining operations – generate substantial quantities of wastewater, some of it in the form of concentrated and highly polluted streams. Stringent regulations require these different types of wastewater to be treated before discharge to the environment. The treatment complexity of these wastewaters varies widely depending on their origin and the nature and quantity of contaminants they contain, either from the original ore, the mine environment, or the treatment processes employed.

Some highly toxic contaminants, such as cyanide, selenium, arsenic, cadmium, mercury and radioactive elements, may be found in wastewater from certain mining operations. Water from some hydrometallurgical processes such as leaching may be particularly heavily contaminated and need intensive treatment to filter the solution. In some cases such as gold cyanidation, appropriate filtration can clean the solution sufficiently to permit in-process reuse and minimize the impact on the WWTP. Pollution abatement processes to control gas or fume emissions in pyrometallurgy also result in water containing high concentrations of contaminants. However, it should be noted that some contaminated streams may contain substances that have economic value when recovered as by-products during treatment.

Some wastewater streams may be treated at source point for direct reuse onsite in another process, while others have to be routed to the wastewater treatment plant or to tailings ponds. In many cases, the largest volumes of wastewater end up in tailings ponds