[LUM#14] Precious gems

[LUM Magazine] - Published on March 30, 2021 in Research centers, Research structures

How can we achieve the cleanest possible ecological transition? Thanks to geologists. Three researchers from the Montpellier Geosciences Laboratory * aim to better understand the formation processes of rare metals used in so-called "green" technologies in order to relocate and facilitate their exploitation.

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Lithium, tungsten, germanium, gallium, indium... These metals all have one thing in common: they are essential to new technologies. Tungsten? Valuable in certain aeronautical alloys. Lithium? Essential to automotive and IT technologies. As for rare metals such as gallium, germanium, and indium, they are necessary for the manufacture of solar panels, making them essential ingredients in the development of renewable energies.

"To reduce our carbon emissions, we are promoting renewable energies, which are sometimes considered clean energies," explains Bénédicte Cenki-Tok." But the only clean energy is the energy we don't use , " says the geologist. And with good reason: the supply chain for these rare metals used in green technologies can hardly be described as "clean."

Clean energy

"Currently, the global market for rare metals is dominated by China," explains Alexandre Cugerone, who wrote his thesis on this subject. Europe is almost entirely dependent on Asia, the Americas, and Africa. Importing metal resources for our 21st-century technologies, some of which have strong 'green' or 'renewable' connotations , from distant countries with lax or non-existent environmental regulations is particularly paradoxical," laments the researcher.

This is where geologists come in. "These metals are essential for developing these technologies, so what we need to do is try to exploit them in a thoughtful and intelligent way, " explains Émilien Oliot. And to exploit them more effectively, we first need to understand how they form and concentrate in nature. "We have long been familiar with the processes that create metamorphic rocks and mountain ranges, and the temperature and pressure conditions inherent in their formation. This knowledge, which may seem outdated, is in fact precisely what enables us to understand the processes of formation and concentration of the critical metals involved in modern technologies," adds Bénédicte Cenki-Tok.

Mining slag heaps

The geologists' goal is to inspire new methods for the eco-friendly exploration and recovery of certain types of waste from past mining operations. These precious metals are not only found on the other side of the world, but sometimes right under our noses, or almost. "For certain rare metals, we know which base minerals contain them, so we know where to look for them," explains Emilien Oliot. Where? In mining spoil tips, for example, which are artificial hills created by the accumulation of mining by-products.

These critical metals are found either in minute quantities, scattered throughout base metals such as zinc and copper, or sometimes in hyperconcentrated minerals smaller than a tenth of a millimeter. To better understand the value of these slag heaps, researchers use a culinary analogy. "Consider a single cake, with chocolate powder evenly distributed throughout the cake batter, and chocolate chips. Once the cake is baked, which form of chocolate is easiest for chocolate lovers to retrieve? The chips, of course. The principle is the same in our study: it is easier to extract critical metals concentrated in small minerals—our chocolate chips—rather than scattered throughout the base ore—the chocolate powder in the cake batter."

Chocolate chips

For example, researchers have shown that the deformation of zinc sulfide ore, which occurred at the same time as the formation of mountain ranges, promotes the re-concentration of germanium in hyperconcentrated minerals, "our famous chocolate nuggets, which are found in particular in the heart of the Pyrenees." For geologists, it is therefore very interesting to search for mining sites where deformation by natural geological processes has acted as a "natural concentrator" of rare metals.

"Many mining sites were formerly exploited solely for their base metals, and the slag heaps resulting from this past exploitation could be reclaimed, particularly in the Pyrenees, the Massif Central, but also in the Alps and the Scandinavian mountains of northern Europe. They may constitute potential sources of rare metals," geologists point out.

Redistribute resources

With another significant advantage: these metals could be easier to extract. "When a rare metal, such as germanium, is scattered throughout the ore, extraction is complex and requires heavy processes. However, if these rare metals are concentrated—like chocolate chips—in small minerals, separating them could be simplified, " explains Alexandre Cugerone.

This is a promising avenue for European countries to break their near-total dependence on rare metals. "The social and environmental impacts of this mining are worrying; we cannot simply relocate the pollution generated by it," warn geologists. Above all, we must work inclusively with producing countries and involve all stakeholders in order to better redistribute resources. " Everyone should get their share of the pie.