UC professor extracts rare minerals from coal waste

Our relationship with coal is complex. We rely on the cheap energy coal provides to cool our homes, charge our phones and power the factories that produce everything from steel to plastic – but we tend to ignore its profound environmental costs. Though coal does provide cheap energy, cleaning up its industrial waste is not cheap at all. 

What if, though, the US coal industry could make money off coal waste and invest the profits into reducing environmental impacts, all while decreasing American dependence on foreign imports?

University of Cincinnati chemical engineering professor Vadim Guliants, PhD, is exploring ways to make the most out of this coal-mining waste by extracting rare-earth minerals found in this black rock. The Ohio Development Services Agency and UC recently awarded him a two-year $375,000 grant for his project, “Techno-Economic Feasibility Study of a Novel Process to Recover Rare Earths from Coal Preparation Plant Refuse.”

Rare-earth elements like neodymium, europium and terbium are found in everything from cell phones to car batteries to televisions. Though coal typically has very dilute concentrations of rare-earth minerals, if researchers can find a way to extract these minerals, the new technology could have profound domestic impacts.

“There is going to be a huge rise in demand for these rare-earth metals in the next few years,” says Guliants. “New widespread technology will require many more of these elements.”

Though coal may still remind us of a past era of industrial America – a simpler time of steam-engine trains and coal-gas lamps – coal still accounts for over 30 percent of our nation’s electricity generation (US Energy Information Administration). That’s a lot of coal and a lot of potential rare-earth materials that may be recovered from coal refuse.

Rather than rely on countries like China to import these rare-earth minerals, as we currently do, the US could use its own, strengthening national security and defense by reducing its dependence on foreign sources. This strategy also makes sense geographically: With 28 percent of global recoverable coal reserves, the US has more coal reserves in its own backyard than any other country in the world (US Energy Information Administration).

When companies mine a coalfield, they extract coal from a seam, a layer of coal typically beneath the surface. Since the chemical composition of coal varies, it is impossible to get all the “good” stuff – the coal with the highest concentrations of carbon – in one fell swoop. (Coal isn’t exclusively composed of carbon, the element that allows coal to burn, which heats the water that provides steam to power turbines. In fact, anthracite – a type of coal that has carbon content between 92 and 98 percent – only accounts for one percent of coal reserves. Most coal used in power plants have carbon content between 60 and 80 percent.)

Because coal companies cannot determine where all this “good” coal within a seam is, they typically mine slightly more than they need. Once they bring the coal up to the surface, they start sorting it. Operators use heavy mechanical equipment called cyclones to grind the coal into evenly sized chunks. The machines then use gravity to separate the higher density material from the lower density material. Carbon is much lighter than other elements, so the coal these companies want settles at the top of this pile.

Everything left behind is considered coal refuse, or unwanted rock material – that’s the stuff Guliants is after. “All these rare-earth minerals are associated with waste rock,” says Guliants. “There are fewer elements like carbon and more elements like silicon, aluminum and iron oxide in this refuse. That’s where you find the rare-earth elements.”

The biggest challenge with extracting rare-earth minerals from coal refuse is processing the high volumes of waste for the incredibly low concentrations of actual minerals within it. Certain coal refuse may only contain a couple hundred parts per million of rare-earth minerals, which is so dilute it’s unusable.

“So far, the only economic or feasible way for recovering something this small is to transfer it into soluble form,” says Guliants.

To begin this process of extracting rare-earth materials from coal refuse, Guliants explains, you first use different acids to isolate the minerals through chemical leaching.

“Imagine you have a tiny rare-earth element in a pile of garbage,” he says. “You hit it with nitric acid, and that garbage will not react with nitric acid because it’s resistant, but the mineral you’re after will.”

Once isolated, the rare-earth mineral can be further concentrated through a solution that dissolves parts of the waste rock. This increases the element concentration to several thousand parts per million, but it is still too dilute for conventional technological uses, says Guliants. So, the final process employed is solvent extraction.

Solvent extraction is a process of separating compounds using water and an organic solvent. Guliants uses the example of neodymium, a soft silvery rare-earth metal that is hydrophobic.

“You have your neodymium in water on one end,” says Guliants, “and on the other end, you have a solvent, which is organic and doesn’t mix with water. When the solvent and water are mixed, the organic molecule binds to the neodymium, but not the water, pulling the neodymium into the solvent.” 

So, if you start with 10 volumes of solvent and 1,000 volumes of neodymium in water, soon the neodymium will concentrate more and more until eventually it’s at 20,000 parts per million, the necessary concentration for conventional technological use. 

Guliants isn’t the only person exploring the extraction of rare-earth minerals from coal waste. Researchers across the country, especially in the Appalachian states, are researching the value of rare-mineral extraction from coal ash, acidic water, and lignite, the youngest type of coal. Guliants, however, may be one of the few pursuing this research in Ohio.

Ohio’s coal reserves aren’t as expansive as traditional coal powerhouses like Pennsylvania or West Virginia that have massive coal reserves along the Appalachian Basin. Ohio also doesn’t have the advantages of federal funding into coal composition research and analysis (the Department of Energy’s National Energy Technology Lab has locations in Pittsburgh and Morgantown). Guliants’s research, therefore, could be some of the first of its kind in the region.

With investments into “clean coal” technology, like scrubbers and carbon capture-and-store, the coal industry is trying to catapult forward through the 21st century. But the environmental and societal impacts involved in coal mining and production are still obvious – coal accounts for massive air pollution and environmental damages, making cleaner energy sources more appealing.  

Guliants’s research can help utilize this black rock for more than just its traditional combustion, providing the rare material we need to create valuable products while reducing our dependence on foreign imports. In an era with increasing investments into energy powered by renewable resources and natural gas, coal may no longer be the king it once was, but the rare-earth minerals in its composition can add several sparkling jewels to its fading crown.

Featured image at top: Coal refuse is unwanted rock material left behind from coal-mining operations. This waste may contain rare-earth minerals that can be used in the production of cell phones, car batteries and televisions. Photo/Dapur Melodi/Pexels.