Mine water in the context of circular economy

When discussing mining waste, the general perception almost always focuses on waste rock and tailings because they are highly visible in mining operations. However, there is another waste stream that has received less attention from a circular economy point of view, and this is the influenced mine water. There are various sources and types of water in a mining operation. The effluent that has captured most of the attention and studies regarding its hydrogeology and chemistry is the water in the ore extraction area or commonly termed 'mine water'. The quality and volumes of this water depend on many conditions including the terrain morphology, the geology of the subsoil, the hydrology and connectivity with groundwater, the quality of surface soils, mining methods, volumes of operational water, etc. The chemistry of mine water is very complex and much depends on its interaction with the mineralized medium. During the ore extraction, the surface area of host rocks and mine waste is dramatically exposed to oxidative processes i.e., weathering. Tailings also have elevated surface area prone to weathering. For instance, weathering of sulphidic ores liberates minerals and acidity at increased concentrations into the mine water which we know as acid mine drainage (AMD).

The metal content in AMD depends on the geology and mineralization of the ore deposit being base metals iron, copper, zinc, aluminium the most representatives. But there are other metals too that are highly sought in the emerging battery industry or for the green energy transition. Many methods have formulated the selective recovery of dissolved metals in AMD. For example, copper cementation by hydrometallurgical processes has been the common industrial application for copper values close to or above 500 mg/L. Other techniques have focused on metals such as zinc, manganese, or iron, but their selective separation has always been challenging and never transitioned from research to industry.

A reference of a low acidic mine drainage (pH ~5.5) in a sulphidic ore mine. Callouts identify the average concentration of dissolved metals in the AMD and the concentration in sludge after alkaline precipitation.

AMD is also toxic for receiving environments, so almost all sulphidic ore mines have treatment plants with neutralisation technology, but the process is limited to producing large volumes of metal-laden sludges that are sent to disposal areas with risks of metal leaching again. This type of sludge can also be a source of secondary minerals such as gypsum, jarosite, and oxyhydroxides such as goethite, ferrihydrites, lepidocrocite, and schwertmannite.

In a way, weathering has created the in-situ leaching conditions with mine water dissolving the metals and minerals out of the host rocks and mining waste. The challenge for us is to tackle the selective separation of these valuable components so AMD becomes a potential secondary resource for metals and secondary minerals. This research at Oulu Mining School will integrate the management and treatment of AMD with opportunities for value creation and alignment with the circular economy concept. This approach allows a life cycle assessment of these low-economy streams and values not only the immediate utility but also the impact on the receiving user or environment throughout the life of the product.