To this day, rhodium is the most valuable natural metal in the world. With a market price of 11,000 dollars for a single ounce (almost 50 million pesos). And, at its best, it has been worth 10 times more than gold and fourteen times more than platinum.

The increase in demand for rhodium is closely related to the search for efficient and non-polluting technologies: it is the basis, for example, of artificial photosynthesis. However, the Earth does not understand markets or remedies, and does not make it easy. Rhodium only exists in very few places on the planet, geologically unique regions, where billions of years before humans traveled in polluting vehicles the great continental masses cracked. Through those cracks the rhodium ascended.

The price of rhodium has multiplied more than 3,000% in this five-year period. If at the beginning of 2017 a kilogram of rhodium was quoted at 21,500 euros -then gold surpassed it with a price of 36,000 euros-, a historical record of 822,000 euros was achieved in April 2021. The most recent studies predict that this escalation Prices will continue in the near future due to the increase in demand for this metal for new technological applications, mainly related to the search for sustainable solutions.

The world consumes around 32 tons of rhodium per year. Industrial recycling of rhodium is possible, and economically viable, but it occupies only 9.5 tons. In such a way that the extraction of primary natural sources is still necessary to cover the rest of the demand.

In the European Union, the dependence on rhodium imports exceeds 95 percent per year, since it does not have its own production. This makes rhodium a critical raw material of strategic interest, priority objective for scientific-technological research and mining exploration, as has been reflected in different European plans and the recent roadmap for the sustainable management of raw materials of the Government of Spain.

Approximately 80 percent of the world’s annual rhodium production goes into the manufacture of catalytic converters and catalysts for low-emission vehicles. In these exhaust system components, rhodium – often together with platinum and palladium – reacts with polluting gases to produce less harmful noble or inert gases. For example, nitrogen oxides are transformed into molecular nitrogen (N2) and oxygen (O2); these are harmless gases that are found naturally in the air we usually breathe.

The new legislations implemented in the climate change laws clearly encourage greater use of pure electric vehicles. However, this type of vehicle will not dominate the scene until at least 2050. In this context of transition towards climate neutrality, rhodium is, and will continue to be, a fundamental player, especially in the field of manufacturing of hybrid vehicles.

In the arena of energy efficiency, rhodium is also a great ally. It is the essential catalyst for the manufacture of incomparable thermal waterproofs such as refractory glass fibers. In addition, the electrical and electronic components of wind turbines contain rhodium.

Rhodium is the basis of the revolutionary technology of artificial photosynthesis. Its ability to catalyze artificial photosynthesis reactions in micro solar panels is much greater than that of other components and allows the transformation of sunlight, water and carbon from the air into hydrogen that can be used as fuel.

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A rare and expensive metal

The high price of rhodium is not only due to its irreplaceable nature for technology and our limited capacity for recycling, but also to its scarcity in the earth’s crust.where it is found in extremely low concentrations, rarely exceeding 0.001 grams per ton of rock.

The Bushveld igneous complex in South Africa accounts for 90 percent of the world’s known rhodium resources. and between 80 and 90 percent of global annual production. In this restricted area of ​​the earth’s crust, rhodium is concentrated in economically viable quantities, by a factor of up to 10,000, which is equivalent to 0.5-10 grams per ton of rock.

Here the rhodium forms a wide range of its own minerals that could eventually be exploitable, although these do not give rise to specific mines of this mineral. Rhodium is a subsidiary product of the exploitation of platinum and palladium.

In South Africa, rhodium is mined together with platinum and palladium from masses of nickel and copper sulfides (Merensky Reef and Platreef deposits) and chromium oxides (UG-2 deposits) hosted in basic natural rocks. and ultra basic.

Other smaller deposits that also produce rhodium as a by-product of the extraction of platinum, nickel or copper are located in Zimbabwe. (Hartley, Mimosa, Ngezi and Unki mines in the Great Dam) and the United States (JM Reef mines in the Stillwater Complex). Russia also produces notable amounts of rhodium from platinum nuggets present in river sediments that washed away ancient basic and ultrabasic rock complexes in the Ural Mountains.

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Cracks in the magma

The origin of the mineral deposits that contain rhodium is related to extensive phenomena that cause the rupture of large continental masses., allowing the rise of deep magmas from the roots of the continents, more than 250 km from the upper mantle. Once these magmas reach the crust, they become contaminated by saturating themselves with chromium oxides and sulfides that concentrate rhodium along with other valuable metals such as platinum, palladium, osmium, iridium, and ruthenium.

Most of the planet is poor in rhodium. Neither in Europe nor in Spain are there mines. However, we have great specialists in the study of the metallogeny of this metal. In the Serranía de Ronda in the province of Malaga and in the Cabo Ortegal Complex, in Galicia, there are rocks with this element, but they are not exploitable. Rhodium is on the list of fundamental metals for the energy transition, but the Earth offers very few opportunities for its extraction. Geology does not understand markets nor does it know about human needs.

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JOSE MARIA GONZALEZ JIMENEZ
AND FERNANDO GERVILLA LINARES (**)

The conversation (***)

Senior Scientist, Professor of Geology of Mineral Deposits at the Andalusian Institute of Earth Sciences (IACT – CSIC).

(**) Professor of Crystallography and Mineralogy at the University of Granada. (***) The Conversation is a non-profit organization that seeks to share ideas and academic knowledge with the public. This article is reproduced here under a Creative Commons license.