The environmentally friendly goals of renewable energy have driven demand for the extraction and processing of minerals for lithium-ion batteries, wind turbines, motors and solar panels. Fortunately, technology is available to minimise any adverse environmental and social consequences of the mining industry.
By Kirsten Kelly
“Veolia Services Southern Africa addresses this irony via our unique water-waste-energy perspective for mining operations. We aim to create an environmental circular economy through the complete management of a mine’s water cycle,” says Reinier Delport, business development manager: Heavy Industry, Veolia Services Southern Africa.
Veolia’s water cycle management solutions for underground mines focuses on organisational performance, driving operational efficiencies and quality management systems. “We then develop continuous improvement strategies that work to optimise a mine’s water systems,” he adds.
This operational performance is based on five elements: environmental, economic, commercial, human resources and social. A phased methodology is used whereby Veolia assesses a mine’s needs, breaks them down into smaller projects, and then aligns and allocates these projects with their in-house expertise, divisions and technologies to make available the best possible solutions and services.
“Veolia’s solutions for water, waste and energy management offer customers so much more than just a product or technical expertise – it helps them achieve their sustainability goals,” explains Delport.
Mine water reticulation
- Water collection: Water is collected from various sources within the mine, such as underground springs, surface run-off or dewatering processes.
- Water treatment: The collected water undergoes treatment processes at the settlers to remove impurities and ensure it meets the required quality standards. Treatment methods may include filtration, sedimentation, chemical dosing and disinfection.
- Water distribution: The treated water is then distributed throughout the mine to different areas and processes where it is needed. This can involve the use of pipes, pumps and storage tanks to transport and store water. Water is pumped to cooling towers on the surface of the mine and then underground for mining activities and for cool air for operations.
- Water recycling: After being used in mining operations, the water is collected, treated and recycled back into the reticulation system. This helps reduce the demand for freshwater intake and minimises the overall water footprint of the mine.
- Water discharge and environmental management: In some cases, excess or wastewater from the mine may need to be discharged. Before discharge, it normally undergoes additional treatment to ensure compliance with environmental regulations and to reduce any potential impacts on surrounding ecosystems.
Water is a crucial component that is needed at different phases of mining process. The process where water is collected underground, treated, cooled and reused in various process units is called the underground mine water reticulation cycle.
Delport explains that an underground mine has a surplus balance of water; there is always more water that needs to be treated or discharged than water that enters the mine. “This underground water has significant concentrations of heavy metals, toxins and impurities that can be harmful to the environment, human health and mining processes. Furthermore, it would be excessively expensive (and unsustainable) to exclusively use potable water.”
Different types of water in an underground mine includes:
- Fissure water: groundwater that is found within fissures or fractures in rock formations underground and normally has high acidity levels.
- Potable water: drinking water supplied by municipalities that is pumped underground for human consumption and certain processes.
- Mining activity water (service water): water released during mining operations, which can be present in mine shafts, tunnels, or underground chambers and usually has a concentration of salts, toxins and heavy metals.
pH neutralisation and softeners
Mine water will enter the settler through an inlet and will be treated for low pH levels. Mine water often contains high concentrations of acidic compounds, such as sulfuric acid, which are generated during the mining process. Called acid mine drainage (AMD), this water can be highly corrosive and toxic to humans and the environment, and can corrode water-related structures and process equipment. It is generally formed when water encounters pyrite.
AMD is formed in three stages:
- The oxidation of iron sulfide, forming iron hydroxide and sulfuric acid.
- The iron hydroxide reacts with the sulfuric acid, forming ferrous sulfate.
- The ferrous sulfate reacts with the sulfuric acid, forming ferric sulfate.
The two processes of neutralisation and salt softening play a critical role in ensuring minor mechanical and hydrodynamic stresses to the water reticulation process units and enhanced outputs from the plant. These processes go hand in hand and characteristically occur concurrently.
pH neutralisation adjusts the pH of the water to a more neutral range, typically around pH 7-9. The neutralisation process involves the addition of alkaline substances, such as lime (calcium hydroxide) or caustic soda (sodium hydroxide), to the acidic mine water.
These alkaline and pH buffering compounds react with the acidic components, such as hydrogen ions, present in the water, resulting in the formation of water and a salt. The reaction helps to increase the pH of the water, making it less acidic, and contributes to the precipitation of salts.
In addition to salt, mine water also contains high levels of dissolved minerals – such as chloride, calcium, sulfate, magnesium and iron – which can cause scaling and other operational issues in equipment and pipes. The underground settlers are used to remove suspended solids and contaminants from the water through clarification otherwise referred to as salt softening. Salt softening is a treatment process that aims to reduce the concentration of these minerals in the water, making it less prone to scaling and corrosion, while also improving the water body’s overall conditioning characteristics.
During the neutralisation treatment process, the turbidity of the water increases as finely divided particles agglomerate together. Flocculants and coagulants are then added to the process to assist large particles to settle under gravity. A mud bed is formed and, after a certain retention time, the mud is removed from the settler and reworked in extraction plants to remove precious minerals. As the solid particles settle, the clear or clarified liquid rises to the top, known as the overflow.
“Total suspended solids (TSS) in the overflow must be reduced and kept at a process-specified operating range to prevent particles from damaging multistage pumps. From there, water is cooled, chilled and sent to chiller dams,” he says.
According to Delport, lime is preferred over caustic soda for pH neutralisation. “This is because lime adds buffering to the water body where both iron and calcium are precipitated. Buffering capacity is the ability of the water body to maintain its pH level for a period of time. Furthermore, lime aids with softening and increases the activity of coagulants and flocculants. This is due to the pH level achieved by using lime. Veolia is able to deploy a wide range of chemical technologies to treat and condition different types of mine water.
“Anyone can add lime to a system; it’s about having an in-depth understanding of the hydrodynamics of the systems and how to best mitigate the process risks. We have a proven track record, and can assist a mine with dynamic solutions for their operational challenges, including specific applications like pH neutralisation and softening. Veolia has ample experience and expertise by working with mining companies from all over the world. It is through this expertise and experience that Veolia can optimise mine water chemistry to achieve the best possible commercial and environmental results,” he concludes.
