The burnt remains of coal could be an untapped source of critical elements
As coal combusts at power plants across Canada, ash gets carried away with the resulting exhaust. While this fly ash contains several known pollutants detrimental to human and environmental health, there may be an untapped climate solution hidden in the fine particulate.
Researchers believe that fly ash, and potentially other coal byproducts, could be a source of rare earth elements (REEs), some of which are vital for the production of goods like batteries, solar panels, wind turbines and other green technologies. For instance, neodymium, commonly found in magnets, is essential for the creation of electric vehicles and wind farms. Coal byproducts have previously been studied as a source of REEs in the United States and China.
More recently, a team of Canadian researchers have published a paper digging into the REE content of coal ash from Saskatchewan and Alberta. Broadly speaking, they have a comparable amount of REEs compared to the U.S. and China, Brendan Bishop, one of the report’s authors and PhD candidate at the University of Regina’s geology department, told The Weather Network. He added that, in the short term, collecting REEs from coal ash could be a way for Canada to address the growing need for critical elements, which, by some counts, is expected to reach 150 million metric tons by 2050, compared to less than 10 million metric tons in 2020.
“We were just initially looking to see, like: What's the composition of these ashes? What's the REE element content? And then, like, how easy is it to just leech them out?” Bishop said.
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According to market research company IBISWorld, there are currently 6,941 Canadians employed in coal mining, and employment in the field has decreased by 1.6 per cent over the last five years. Canada produced 57 million tons of coal in 2019, ranking 13th out of every country that year. Coal for use in electricity is set to be phased out by 2030, but, as of 2019, 53 per cent of domestic coal found use in metallurgy, for the production of steel.
Bishop and his team studied five samples of fly and bottom ash — a heavier byproduct that sinks to the bottom of the combustion chamber after combustion — to check their REE content, and how easy it would be to extract them with an acid. In all, there were five samples taken from three power plants in Saskatchewan and one plant in Alberta.
At the University of Alberta, the researchers dissolved the samples in hydrochloric acid and tested the results using a mass spectrometry machine. The team also sent a sample away for a more advanced type of analysis at a private laboratory in British Columbia. Called bulk digestion, this process gave them the total REE concentration of their samples. By comparing the two results, the team could also see how easy it was to separate the REEs from the byproduct.
An auto-sampler used in research to find the rare earth element content in coal ash from western Canada, located at the University of Alberta. (Doug Johnson/The Weather Network)
According to the paper, the concentration of REEs in the Canadian samples ranged from 258.9 to 320.5 parts per million (ppm), which is a little lower than the global average of 368 pmm. The amount of these REEs that are also critical elements ranged from 32.5 to 38.7 per cent. How well acid took the REEs out of the samples also varied depending on where the sample came from. The acid extracted 100 per cent of the desired elements from the Poplar River plant sample.
Meanwhile, the samples from the other three sites in Saskatchewan and one in Alberta saw this number vary wildly between three and 65 per cent. Bishop said this is likely due to the structure of the coal itself. The fly ash that responded better to the acid was from coal that was high in iron, which breaks down easily. Meanwhile, the less reactive samples were from “glassy” coals, high in silicates or other structures that are harder to break down, he said.
Broadly speaking, collecting REEs likely involves using an acid to separate them from the coal waste. However, some researchers are exploring a similar approach using other methods and sources, like using extremophile bacteria to absorb REEs from mining wastewater.
Breaking it down
There’s still work to be done in the field, however. For one, the concentrations of the REEs in coal byproduct are low relative to traditional mining operations. Strong acids can also take a toll on the environment. However, Bishop noted that, in his research, they started with a particularly powerful acid, and that it’s possible weaker acids would also work. “We may be able to use some, you know, organic acids, like citric acid, or lactic acid, or acetic acid, something that's a lot more environmentally friendly and a lot cheaper and easier to produce,” he said.
Removing the individual elements from the solution, which contains many different compounds, also poses a technical problem, Bishop said. However, he noted that there has been some work in this field, including at the Saskatchewan Research Council’s Rare Earth Processing Facility. “So, you can break it down, and then you have all your different metals in a solution. And then, the challenge is to separate the REE elements from everything else,” he said.
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Omid Haeri Ardakani, a research scientist with the Geological Survey of Canada, another NRCAN organization, is a co-author on another paper looking into recovering REEs from coal and shale. He noted that the field would additionally benefit from more research characterizing the content of coal across Canada. He added that, while Canada produces a good amount of coal, most of it — 36.5 million tons in 2019 — is exported, meaning the byproducts end up in the country that imported it for energy.
However, Ardakani said that coal byproducts are potentially promising resources, that the process of taking REEs from them would likely be easier than traditional mining, and that the process could be an alternative to current disposal methods. “You don’t have the carbon footprint for the extraction of those resources,” he said.
A (potentially) moveable feast
According to Bishop, it would be much easier to set up an operation using acids to collect REEs than it is to get an actual mine operational, which, at least in Ontario, can take between 15 and 20 years, according to a story in the Financial Times, quoting the province’s government. There doesn’t need to be any construction involved, as such, and there’s no need to explore for a given element in the ground. Canada is also poised to phase out gas-powered vehicles by 2035, meaning that traditional REE mines that start operating today won’t be producing materials for electric vehicles in time, meaning the country may need to find other sources.
“It's gonna be really important in the next few years before we start to see these big mines start producing those rare earth elements we need for electric vehicles, especially,” Bishop said.
Currently, coal power plants dispose of byproducts either by putting them into ash ponds, which can potentially contaminate groundwater with harmful elements like lead and mercury. Other final destinations include the landfill, or, in some cases, being reused in cement or other applications. In cases where the ash is thrown away, however, there’s an opportunity to retroactively go back and snag the REEs from the leftovers of combustion, Bishop explained. Additionally, because the approach would only require large containers and quantities of an acid, an operation could move from plant to plant.
In theory, gathering critical REEs from coal byproducts could also create jobs during Canada’s transition towards net-zero energy, he added. This could include jobs in communities that have traditionally relied on coal mines or power plants, aiding them until new industries appear, he said.
According to Bishop, it’s still too early to say whether or not this is really a viable path forward, or if it could be scaled up to produce enough of the REEs. However, he believes that this could be a “near-term” solution to the growing need for these vital resources.
Thumbnail image: Coal bottom ash seen through a scanning electron microscope at 238x zoom. (Supplied/Brendan Bishop)
