Curiosity

"Today's electric cars and wind turbines rely on a few elements that are mined almost entirely in China. Demand for these materials may soon exceed supply. Will this be China's next great economic advantage?"

C'est par cette question que démarre l'article The Rare-Earth Crisis de Katherine Bourzac sur Technology Review (06/2011).

"Worldwide supplies of some rare earths could soon fall short of demand. Of particular concern are neodymium and dysprosium, which are used to make magnets that help generate torque in the motors of electric and hybrid cars and convert torque into electricity in large wind turbines. In a report released last December, the U.S. Department of Energy estimated that widespread use of electric-drive vehicles and offshore wind farms could cause shortages of these metals by 2015.

What would happen then is anyone's guess. There are no practical alternatives to these metals in many critical applications requiring strong permanent magnets—materials that retain a magnetic field without the need for a power source to induce magnetism by passing an electric current through them. Most everyday magnets, including those that hold notes on the fridge, are permanent magnets. But they aren't very strong, while those made from rare earths are tremendously so. Alloys of neodymium with iron and boron are four to five times as strong by weight as permanent magnets made from any other material. That's one reason rare-earth magnets are found in nearly every hybrid and electric car on the road. The motor of Toyota's Prius, for example, uses about a kilogram of rare earths. Offshore wind turbines can require hundreds of kilograms each."

Les données fournies sont assez vagues. En creusant ailleurs sur le net, on trouve par exemple les infos suivantes :

"la fabrication du véhicule Prius, du constructeur automobile Toyota, nécessite de 10 à 15 kg de lanthanum pour la batterie, de 1 à 2 kg de néodymium^[1] pour le moteur électrique et de petites quantités de dysprosium et de terbium pour diverses composantes.
(...)
Environ 0,6 à 0,9 t d’aimants aux néodymium (30 % ETR) par mégawatt sont utilisées dans les éoliennes de nouvelle génération."

L'article parle ensuite du coût environnemental intrinsèque au processus d'extraction des terres rares, tout en restant assez évasif sur les ordres de grandeur pertinents :

"The first step in extracting rare-earth oxides from the surrounding rock is to crush the rocks and grind them into a fine powder. This is passed through a series of tanks, where the rare-earth elements float to the top. Unwanted minerals sink to the bottom, and this hazardous waste material, called tailings, is sent to ponds for storage. Meanwhile, the resulting concentrate of rare-earth metals is roasted in kilns and then dissolved in acid.
(...)
The reaction generates a lot of salt: when the Mountain Pass mine was running at full capacity in the 1990s, it produced as much as 850 gallons of salty wastewater every minute, every day of the year. This waste also contained radioactive thorium and uranium, which collected as scale inside the pipe that delivered the wastewater to evaporation ponds 11 miles away."

Si les aimants permanents à terres rares venaient à manquer, les constructeurs de véhicules électriques et de turbines se tourneraient sans doute vers le magnétisme induit, en dépit de leur moindre performance :

"the reason permanent- magnet motors are used is that their efficiency is almost always higher in the range where you use it a lot—typically you can get more torque for a given supply of current."