When carbonates are key to the concentration of Rare earth elements in the Earth’s crust

Rare earth elements (REE) are a group of 17 precious metals used in many electronic applications of our everyday life. They are vital components of alloys and magnets used in wind turbines or electric cars and have become key resources to developing a carbon-free alternative.

However, they are only present at very few ore mines in the world, with China accounting for over 85 percent of the world's production of REE, and the extraction process is far from being environmentally friendly. Today, most of the extraction is carried out using the technique of acid leaching, which consists of using acid on the ore to convert the heavy metals from the soil into soluble salts. This damages the fertility of the soil and it has become a concern in farming.

In the quest to understand the origin of these REE deposits and to design improved extraction methods of these mineral deposits, scientists came to the ESRF to reproduce the conditions where REE are embedded.

“REE originate in carbon-rich magmatic rocks called carbonatites, which represents only 5% of all the magmatic rocks of the world. When the magma crystallises, the REE originally present as trace elements in the magma are extracted by high temperature fluids and transported to a new location where they precipitate as REE ores, which can be more easily mined”, explains Marion Louvel, scientist at the University of Münster and leading author of the study.

In order to be transported in these fluids, REEs need to be bonded to the so-called ligands, otherwise they precipitate and become solid. “We need to understand how they dissolve, and don’t become solid, so we studied different possible ligands, to see to which one REE best attaches to”, says Denis Testemale, CNRS scientist at FAME beamline and co-author of the study.  “This is important because it will help us understand how REE-rich minerals formed in carbonatite rocks, but also how we can best recover metals from these ores in sustainable ways”, he adds.

With the aim of shedding new light on REEs solubility and speciation in hydrothermal fluids, the team conducted in-situ X-ray absorption (XAS) experiments at the beamline BM30 FAME at the ESRF. They used a hydrothermal autoclave, where they reproduced the process the REEs go through, from rocks to fluids, in samples of half a millilitre exposed to the x-ray beam. “This beamline is the only one in the world where we can use an autoclave that reproduces the pressure and temperature conditions of the Earth’s crust with such stability that the data quality for samples with realistic concentrations is very good”, explains Louvel.

The results of experiments carried out over several weeks surprised the team: Metals often associate with chlorine or sulphur, but not carbonates. However, because the magmatic rock contains a lot of carbon, the scientists decided to evaluate carbonates as ligands and found that the REEs really bound to them.

“Our experiments reveal that carbonate-rich fluids (pH >10) can concentrate high amounts of REEs, even in the presence of high amounts of fluorine, which typically precipitates REEs under acidic conditions”, says Louvel.

The results can also lead to a new avenue for REE processing, where REE-rich minerals are leached in hydrothermal fluids with basic pH, using environmentally benign carbonate-based solutions. “This new type of chemistry provides some hints about how we can reverse-engineer natural ore-forming process to extract rare metals in a more sustainable manner”, concludes Testemale.

Reference:

Louvel, M., et al. Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids. Nat Commun 13, 1456 (2022). https://doi.org/10.1038/s41467-022-28943-z

Text by Montserrat Capellas Espuny

Top image: An aerial picture of a mine.