Image courtesy of ujjwal at Adobe Stock

Expert Opinion Article by Anne Thatcher, Senior Vice President and
Jeffrey Gillow, Vice President, Technical Expert – Geochemistry, Arcadis

The US mining industry plays an increasingly critical role in supplying domestically sourced raw materials necessary for low-carbon energy production. Forward-thinking mining companies are pursuing additional ways to reduce environmental impact and add revenue streams after mining ends.

During open-pit surface mining operations for metals like copper and zinc, groundwater is continuously pumped from the pit so that mining can continue. Once the ore has been extracted and mining operations end, the pumping stops and the open pit fills with water, creating a ‘pit lake’.

Pit lakes are usually filled with acid mine drainage and dissolved metals, making the water unusable in that state. As a result, abandoned mine sites have developed a reputation of becoming largely redundant, dirty and unsightly properties.

Fortunately, the technology to make these practices possible and practical is already in place for ESG-minded mining companies to implement. With many potential assets sitting unused, it just requires a commitment to change the approach to these properties and an understanding of what is possible.

Unconventional Water Source

Integrating reverse osmosis into the treatment approach makes it possible to remove dissolved salts, sulfate and other industrial effluents, so that otherwise unusable water can be safely reused or discharged.

WSP combines multiple pieces of water treatment (such as high-density sludge (HDS) water treatment plants) and applies it to reclaim contaminated pit water. HDS mixes sludge with lime, a neutralizing agent, to remove heavy metals from wastewater.

A responsible attitude toward the need to find sustainable uses for mining water will benefit all. Forward-thinking companies are using advanced treatment techniques to clean underground and open pit water, which in turn is successfully being repurposed to support irrigation districts for crops, particularly in drought-prone regions of the western US. Also, with more hydrogen power facilities being constructed in the west, owners are seeking unconventional water sources, since most water rights in these areas are claimed. Treated mine effluent water is viewed as an ideal alternative for these facilities.

Finding Hidden Critical Minerals

Today, many mining companies are implementing processes for the extraction of energy transition elements like lithium, copper and cobalt from traditional acid mine drainage sources and from mine waste. In addition, REEs, such as neodymium, a common metal used in mobile phones and wind turbines; lanthanum, used in lithium-ion batteries; praseodymium, used in EV magnets; and other critical minerals, are also being evaluated for removal from mine water and mine waste. This creates a domestic supply of these raw materials as well as a valuable revenue source.

WSP has been involved with plans for proposed metal recovery operations at two legacy pit lakes in the Western US containing acid mine drainage from former copper mining operations. Both of these pit lakes store water with high concentrations of dissolved metals and are actively managed as hydrogeologic sinks, with water inflow but no outflow. In our experience, technologies such as ion exchange, solvent extraction, electrowinning and sulfide precipitation can be used for both copper and zinc recovery and can have positive returns on investments for our mining clients.

That additional revenue is being used to offset maintenance costs while advancing corporate environmental-social-governance objectives for mining companies.

A Critical Need

Nearly all REEs are currently mined and processed by the Chinese government, which puts the US at a disadvantage in the event of a supply chain disruption. This risk came to the forefront during the Covid-19 pandemic, when supply lines were halted for an extended time and the US stopped receiving many raw materials required to produce vital things.

The use of previously unusable water and the recovery of critical minerals from pit lakes has the potential to significantly improve perceptions of abandoned mines and how they can be transformed from post-closure liabilities to future resource assets.

Fortunately, the technology to make these practices possible and practical is already in place for ESG-minded mining companies to implement. With many potential assets sitting unused, it just requires a commitment to change the approach to these properties and an understanding of what is possible.