New chemistry provides fresh clues on how to extract rare metals more sustainably

The economic and environmental costs of rare earth element mining are strongly affected by the difficulty in separating the different elements.

A new international study has revealed how nature can inspire the reverse engineering of natural ore forming processes and support the transition to a carbon neutral economy.

Rare earth elements (REEs) are relatively widespread in the Earth’s crust compared to gold and platinum, however large economic deposits are much scarcer, suggesting that exceptional circumstances must be at play. The Bayan Obo mine in Inner Mongolia, China, is the largest REE deposit in the world.

Research led by the Institute for Mineralogy in Germany and involving geologists from the Monash University School of Earth, Atmosphere and Environment suggests that binding REEs to carbonate ions could play a critical role in the formation of such giant deposits.  The study is published today in Nature Communications.

“This new type of chemistry also provides some hints about how we can reverse-engineer natural ore-forming processes to extract rare metals in a more sustainable manner,” said study author Prof. Joel Brugger.

“Our experiments show that carbonate-rich fluids will concentrate more light (La) or heavy (Gd and Yb) rare earths at different temperatures.

“As not all rare earth elements have the same price (Dy, Eu, Lu being the most valuable), fractionation is important for defining the economic value of the ores.”

Most importantly, the economic and environmental costs of rare earth element mining are strongly affected by the difficulty in separating the different elements, and also by the fact that the ores also are enriched in radioactive elements such as uranium and thorium, that need to be suppressed or dealt with as part of the waste stream.

The geologists recreated the conditions that reigned during ore formation within a specially designed autoclave (a geological cooking pot): temperatures up to 600˚C and 2 kbar, a pressure that corresponds to depths of about 7 km in the Earth’s Crust.

At the ESRF synchrotron, which is effectively a giant X-ray gun, 100 billion times more powerful than hospital radiography devices, they used a technique called “X-ray absorption spectroscopy” to probe the composition and molecular structure of fluids and solutes in-situ.

Specifically, they probed the atomic interactions between different dissolved atoms – in this case the bonding between rare earth elements and chlorine, fluorine, hydroxide or carbonate present in solution at high pressures and temperatures between the rare earth elements and these so-called ‘ligands’ are responsible for the solubility of rare earth minerals.

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