Soaking up, breaking down: what sponges and microbes make of mine waste

Tailings at a Northern Territories gold mine. Credit: Steve Lovegrove via Shutterstock

As the ore grades of metals deposits decline worldwide, what once was waste now seems like a wasted opportunity.

Australia has more than 50,000 old mine sites that could potentially become sources of metals extracted from mine waste. In May this year, the Government of Australia released a map of 1,050 tailings deposits, each of which could become valuable as they are cleared.

As the global uptake of green energy technology increases, the demand for rare earths is also expected to increase by 600% by 2040. Short- and medium-term supply of these minerals proves difficult as mining facilities can take upwards of three years to develop. Moreover, the Idaho National Laboratory estimates that 100,000 tonnes of rare earths end up in waste just from the production of phosphoric acid, a common fertiliser.

Six global projects are targeting output of over 10,000 tonnes of key elements neodymium and praseodymium (NdPr) oxide by 2027, recovered from tailings and byproducts. Countries like Australia, Sweden and South Africa are already at the forefront of a push to use mine waste and by-products from coal discards to extract rare earths.

According to United States Geological Survey data from 2022, China accounts for 70% of the world’s mine production of rare earths, followed by Australia, Myanmar and Thailand. China also hosts 85% of the world’s capacity to process rare earths into materials that manufacturers can use.

However, the US and the EU- two biggest markets for China’s exports have been long trying to wean off dependence on the country. In the US’ case, this can be seen in its latest move to halt shipment of semiconductor chipmaking equipment to China. In retaliation, China curbed its export of critical metals widely used in the semiconductor industry.

While governments plan policy frameworks for reaching climate goals, achieving net zero and switching to a renewable energy mix, a geopolitically-stable supply of critical minerals remains a rising priority. Not only is China’s supply chain ridden with allegations of human rights abuses, but the lack of regional diversity in supply leaves little bargaining power for economically weaker countries in the renewables order.

In this scenario, two new recent breakthroughs in research and technology offer solutions for a sustainable mining landscape in the supply chain of critical metals and minerals: 1) the use of microbes for extracting critical metals and 2) the utilization of nanoparticle sponges for extracting critical minerals.

In recent research by the University of Waterloo, researchers used microbes to extract metals and store carbon in the waste products generated by mining activities. This approach, known as bioleaching, is gaining traction as a sustainable alternative to conventional extraction methods.

Bioleaching involves cultivating metal-loving microbes in tanks containing mineral-rich ores. By taking advantage of the metabolic activity of these microorganisms, valuable metals can be extracted efficiently from waste while reducing the environmental impact of a mine.

Microorganisms, such as bacteria and fungi, possess abilities to break down complex minerals and extract valuable metals from ores. Just as rocks weather with air and water over time, microbes also “contribute to weathering by attaching to mineral surfaces, generating rock-dissolving acids through fermentation and respiration and releasing chelating compounds,” the research observes.

“We can take tailings that were produced in the past and recover more resources from those waste materials and, in doing so, also reduce the risk of residual metals entering into local waterways or groundwater,” says researcher and author of the report Jenine McCutcheon, an assistant professor in the university’s Department of Earth and Environmental Sciences.

While the natural process of microbial weathering is slow, it can be accelerated when applied to fine-grained rock, or by-products such as mineral waste generated from ore processing. Nickel and cobalt are among the best suited metals for this purpose. Recent studies have also demonstrated the effectiveness of bioleaching in extracting critical metals, such as copper, gold, and uranium.

Apart from aiding metal recovery, microbes also capture carbon dioxide from the atmosphere and store it in the mine waste as new minerals, forming a carbon sink in itself. This process, called mineral carbonation, can offset the emissions released in a mine’s daily activities, while also stabilising dry tailings.

When applied to an entire mine, carbonation could offset up to 34% of annual greenhouse gas emissions. “Emission offsets could be increased through further process optimization to achieve carbon-neutral or potentially carbon-negative mining,” the research notes.  

“This technique makes better use of current and past mine sites. Rethinking how future mine sites are designed in order to integrate this process could result in mines that are carbon neutral from the get-go rather than thinking about carbon storage as an add-on at the end,” says McCutcheon.

While microbes provide a natural method of metal extraction, another innovation comes in the form of nanoparticle sponges. Engineers at the Northwestern University in the US have developed tiny particles, known as nanoparticle sponges, which possess an exceptional ability to selectively bind with specific minerals from ore solutions.

Using nanoparticle sponges involves dispersing them in solution with the ore. The particles, with their high surface areas and affinity for specific minerals, selectively bind to the target elements and separate them from the remaining solution. This ability to extract heavy metals is particularly valuable for removing critical metals such as cobalt, from water sources.

Moreover, these sponges can also remove toxic heavy metals, like lead, and critical metals such as cobalt from contaminated water, leaving it safe to drink. In their paper, the researchers demonstrate their findings by using the “sponge” on a sample of water containing more than 1 part per million of lead. With one dip, the sponge filters the lead to undetectable levels.

A notable example of nanoparticle sponge extraction is the retrieval of lithium, a vital component in the production of batteries for electric vehicles. Traditional lithium extraction methods often rely on environmentally harmful processes, such as evaporation ponds, which consume large amounts of water and pose a risk of contamination. In contrast, nanoparticle sponges offer an environmentally friendly alternative by efficiently capturing lithium ions from brine solutions, paving the way for sustainable lithium extraction.

As a result, this method offers reduced water usage, faster extraction rates and minimal environmental impact. 

Those who point to energy as “an exciting sector” often use the same breath to point to the expansion of life sciences and material science. This crossover between the two sectors holds immense potential for transforming the mining industry.

These approaches not only address the pressing need for sustainable metal extraction, but also mitigate the environmental impacts associated with traditional mining practices. By adopting these technologies, mining companies can reduce energy consumption, water usage and the release of harmful chemicals into the environment.

However, this technology remains at an early stage. Further research and development are necessary to optimize the extraction efficiency, cost-effectiveness, and scalability of these approaches.