Recovery of rare earths from coal ash for clean energy technologies

As the world transitions away from fossil fuels, the demand for rare earths will only increase. These elements are essential to producing the technologies that will make the transition to green energy possible. While rare earths are not technically rare, they are found in large deposits in only a few places around the world, primarily in China, and are difficult to extract.

“If we want to transition to electric vehicles by 2035 and achieve carbon neutrality by 2050, we will need new sources of these metals,” says Brendan Bishop, a PhD student studying rare earths at the University of Regina.

Bishop and his colleagues investigated a potential new source of these precious elements: ash produced by coal-fired power plants. Researchers have studied rare earths in coal waste in the United States and China, but little work has been done on Canadian coal ash.

The team analyzed ash samples from coal-fired power plants in Alberta and Saskatchewan to determine how much rare earths were in the ash and how to extract them. While the concentration of rare earths in Canadian coal ash is comparable to that found in ash from other parts of the world, questions remain about whether the rare earths are uniformly dispersed throughout the ash particles or concentrated in specific minerals in the ash.

Using the powerful X-ray beamlines at the Canadian Light Source (CLS) at the University of Saskatchewan (USask), Bishop probed the ash for a rare earth element called yttrium. They found that it was distributed in specific mineral phases within the ash particles, most often as silicates or phosphates such as xenotime, which remain unchanged when coal is burned. The work was published in Environmental science and technology.

Bishop says the data could help develop an efficient and environmentally friendly process for recovering rare earths from the ash. “This will be important when we develop a recovery process, because extracting rare earths is technologically challenging,” he says. “In this case, because it’s xenotime, an ore, we can perhaps use an existing process and modify it for coal ash.”

How much rare earths could be extracted from coal ash will depend on the recovery process, Bishop says. But he thinks it could be a good source of metals in the short to medium term. The concentration is not particularly high, but that is offset by the fact that residual coal ash is abundant. The concentration in the ash is also fairly uniform, so there is no need for complicated classification as with mined ores. Once the extraction process is perfected, it will also be much faster than opening new mines, which often have gaps of up to 17 years between exploration and production.

Recovering rare earths from ash is also an important step towards a circular economy. Some ash is used in the manufacture of concrete, but most remains in landfills or tailings ponds near power plants.

“Not only does this allow us to eliminate an environmental problem, it also provides us with the metals we need for clean energy technologies,” Bishop says.