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A variety of modern technologies, including permanent magnets that have been used in everything from earbuds to wind turbines, rely on rare earth elements. While the metals aren't actually especially rare, they don't occur at high concentrations in the Earth's crust. As such, extracting them is expensive and tends to produce a lot of environmental damage, meaning that most of the supply comes from a small number of countries (see the chart here), leaving the supply at risk of political fights.
So the potential to get much more out of existing rare earth mines is obviously very appealing. And the method described in a paper released on Monday seems to offer it all: more metal per ore, much lower cost, and far less worry about mining waste.
Less leaching
Many of the best rare earth deposits occur in places where nature has concentrated the elements for us. These tend to be sediments formed from materials where the rare earth elements will react or interact with the sediment, coming out of solution and gradually building up the concentration in the ore. The usual method of extracting the elements from these ores essentially involves reversing that process. An ion-rich solution is pumped through the ore, and these ions displace the rare earths, allowing them to leach out of the ore. Typically, the solution used is ammonium sulfate.
The production of ammonium sulfate has its own energy and materials costs, and it leaves the material behind in the ore, which may require an environmental cleanup afterward. And the process isn't very selective; lots of other, cheaper metals, like aluminum and calcium, also come out of the ore and need to be separated from the desired products.
The idea behind the new work was to use an electrical current to simplify the process. The standard leaching relies on the flow of an ion-rich solution through the ore to move the rare earth elements out of it. But once that solution displaces these elements from the ore, they return to being ions in a solution. In that state, an electrical current should drive them to the oppositely charged electrode. In theory, this should mean that less of the leaching solution is needed to get material out of the ore, and thus there should be fewer environmental issues afterward.
This sort of electricity-driven purification has been used to decontaminate soils with high levels of metals. But it's not been tried on this sort of mining before. The idea worked even better than the researchers expected.
Hyper efficient
The basic procedure was pretty straightforward. Samples of rare earth ore were saturated with the same leaching solution that would normally be used to extract the metals. At this point, instead of just adding more solution, an electric current was applied. Over time, some of the metals migrated to the negative electrode; the amounts collected there were compared to the known contents of the ore.
In the first run, done on a sample of ore that could fit on a lab bench, the results were promising. The efficiency of metal extraction was 84 percent, or more than double what could be obtained by leaching. Because the materials were being actively pushed to the collection point, it only took about a third of the time to reach that level of purification, and only a small fraction of the leaching solution was needed.
Surprisingly, the contamination by other metals was also a third of the normal volume. The researchers figured out what happened to these contaminants, and it turned out to vary. Some ions, like potassium, only carry a single positive charge while in solution, so they move more slowly than rare earth elements, which have multiple positive charges. The fact that water was split at the electrodes also had an effect. Some metal ions reacted with oxygen to form negatively charged ions that migrated in the opposite direction. Others reacted with hydroxide ions and precipitated out of solution.
The net result was simply lower contamination levels, meaning the rare earth metals were of much higher purity.
So the researchers scaled up the test, working on a 20-kilogram sample of ore. That actually worked even better, requiring less leaching solution and lower currents to work. Here, rare earth extraction cleared 90 percent efficiency, taking 67 hours to get there. By contrast, leaching didn't max out until 130 hours after extraction started, and its maximum efficiency was only 60 percent. Again, the extraction that used electricity had fewer contaminating metals.
Going big
At that point, the researchers went to an actual mine and got a 14-ton sample of ore. To power their experiment, they simply threw a handful of solar panels on top of it. Leaching solution was added to the ore until the electrical resistance dropped and remained stable. While they didn't collect the extracted metals, the researchers sampled through the pile of ore afterward and found both that the rare earth elements had been depleted and that the contamination from the leaching solution was low enough that no environmental cleanup would be needed.
That plays a role in their estimations of the cost of the procedure. For normal leaching, the single biggest cost is the environmental cleanup, which accounts for nearly two-thirds of the expense. When electrical-driven purification is used, that cost would go away. As a result, the predicted cost of producing 2,000 tonnes of rare earth oxides from ore drops from $52 million to under $19 million.
Obviously, electrical use goes up, but not by a huge amount. The researchers estimate that the extraction process requires less than a third of a kilowatt-hour for each cubic meter of ore—within a range they expect can easily be provided by some solar panels.
The cost estimates, however, don't really get into the details of what is needed to scale this process up to industrial production levels, which may require a significant amount of additional or modified hardware—not to mention batteries if this is going to run off of solar power. Still, the approach's savings are pretty substantial, and its use would mean far less mining per rare earth materials produced. So if it does work anywhere near as well as this paper suggests, it's hard to imagine that it wouldn't pay off given enough time.
Nature Sustainability, 2022. DOI: 10.1038/s41893-022-00989-3 (About DOIs).
Listing image by NASA
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