The world runs on metals, and not (just) the kind produced by long-haired musicians with monochromatic wardrobes.
Our modern technologies, industries, and infrastructure would not be possible without 17 invaluable metals known as rare earth elements (REEs), which are used in computers, planes, cars, and more.
Unfaithful to their name, REEs aren't that rare, but expensive and difficult to extract in useful forms.
But as scientists are discovering, piles of discarded waste material left over from spent fossil fuels contain vast quantities of REEs that could be a homegrown source of these metals.
REEs are unevenly distributed across Earth, with Chinese mines producing around 70 percent of REEs and accounting for about 90 percent of global heavy rare earth processing, creating a magnet monopoly.
REEs are vital for the clean energy transition, needed in solar panels, batteries, and wind turbine motors, as well as for medical and defense technologies – one F-35 fighter jet contains around 400 kilograms (900 pounds) of REEs, for example.
Many countries see acquiring an independent REE supply chain as integral to national security, technological sustainability, economic stability, and energy independence. It would also help bring about a more circular materials economy.
Fortunately, there appear to be lots of REEs just 'sitting around', locked away in spent fossil fuel waste.
In fact, a 2024 study led by geoscientists at the University of Texas at Austin calculated that coal ash from coal-fired power stations in the US may contain nearly US$100 billion in feasibly extractable REEs.
Currently, imports from China comprise about 70 percent of the US' REE supply. The US has only one major operational REE mine in California, which produces 16 percent of the global rare earth supply, and it lacks national processing capabilities.
Intriguingly, the chalky coal ash is a potential powerhouse: a byproduct of burning coal, itself made possible only by hundreds of millions of years of extreme heat and pressure squeezing long-dead ancient plants within Earth.
Once most of coal's mass and combustible elements have been burned off, the REE concentration in the resulting ash is up to 10 times higher than in unburned coal, offering an already-unearthed source of critical materials.
The estimated 11 million US tons of REEs in accessible coal ash is almost eight times the amount that the US has in domestic reserves.
So a global race is on to develop practical extraction methods.
At Monash University in Australia, engineers are using environmentally benign acids to remove REEs from coal ash, reporting 90 percent recovery of all 17 elements in pilot demonstrations.
"The significance of this work lies in its dual impact: reducing environmental waste while securing domestic supply of critical minerals," explains Monash chemical engineer Sankar Bhattacharya.
This work is also versatile. The Monash researchers say their method, if scaled and commercialized, could be used on other common waste streams, including electronic waste as well as tailings, the material left over after valuable metals have been removed from mined ore.
"We don't have to dig up new mines. We can use something that's already processed and just sitting in landfill," Bennet Thomas, a sustainable resource recovery engineer at Monash, told AAP, adding that increased REE self-reliance can therefore address a "national risk".
A paper published in 2025 in the Journal of Environmental Management similarly highlights the untapped potential of coal ash.
Engineers Ruchi Agrawal and Arthur Ragauskas calculated that globally, coal ash could yield more than 300,000 US tons (272,000 metric tons) of REEs each year, "far exceeding global demand".
Yet conventional extraction methods have many drawbacks, including low yields and toxic post-processing waste.
Researchers at Northeastern University are working on the problem, boosting yields from conventional extraction methods three-fold by pretreating coal tailings.
Scientists are exploring other options, too.
These include 'green' leaching solutions using various substances like acids; electrochemical extraction methods; and nature-based solutions like microbe-made metabolites that precipitate the REEs from waste in an eco-friendly fashion.
Additionally, phytomining could use "hyperaccumulator plants" that collect REEs in their tissues, putting these natural wonders to work for us – because other than sequestering carbon, producing oxygen, cleaning the air, cooling cities, beautifying our world, and providing food and building materials, what have plants done for us lately?
However, each method has its pros and cons, primarily related to environmental sustainability, resource requirements, scalability, and complexity.
Extraction will need to hit a sweet spot to yield high-quality REEs in a cost-effective, energy-efficient way.
Related: Scientific Breakthrough Could Double Rare Earth Elements Extraction
Time will tell if any of these methods will be successful at scale.
The research was published in the Journal of Coal Science & Technology and the Journal of Environmental Management.
This article was fact-checked by Clare Watson and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.