A tantalising prospect (The Economist – February 16, 2013)


Exotic but useful metals such as tantalum and titanium are about to become cheap and plentiful

ALUMINIUM was once more costly than gold. Napoleon III, emperor of France, reserved cutlery made from it for his most favoured guests, and the Washington monument, in America’s capital, was capped with it not because the builders were cheapskates but because they wanted to show off.

How times change. And in aluminium’s case they changed because, in the late 1880s, Charles Hall and Paul Héroult worked out how to separate the stuff from its oxide using electricity rather than chemical reducing agents. Now, the founders of Metalysis, a small British firm, hope to do much the same with tantalum, titanium and a host of other recherché and expensive metallic elements including neodymium, tungsten and vanadium.

The effect could be profound. Tantalum is an ingredient of the best electronic capacitors. At the moment it is so expensive ($500-2,000 a kilogram) that it is worth using only in things where size and weight matter a lot, such as mobile phones. Drop that price and it could be deployed more widely. Neodymium is used in the magnets of motors in electric cars.

Vanadium and tungsten give strength to steel, but at great expense. And the strength, lightness, high melting point and ability to resist corrosion of titanium make it an ideal material for building aircraft parts, supercars and medical implants—but it can cost 50 times as much as steel. Guppy Dhariwal, Metalysis’s boss, thinks however that the company can make titanium powder (the product of its new process) for less than a tenth of such powder’s current price.

At the moment, titanium is usually produced by the Kroll process, which William Kroll, a metallurgist from Luxembourg, developed in the 1940s. The Kroll process starts with titanium oxide, which is derived from ores like rutile and is cheaply available (artists use it as a brilliant-white pigment). First, the oxide is reacted with chlorine, to get rid of the oxygen.

The resulting chloride is then reacted with liquid magnesium or sodium, to get rid of the chlorine. This creates a porous material, titanium sponge, which is crushed and melted in a vacuum furnace to yield titanium ingots. Making tantalum is similarly onerous. The oxide (which comes from an ore called coltan) is converted to a fluoride using hydrofluoric acid, and the fluoride is then reduced with liquid sodium. Both processes are similar to the way aluminium was prepared before the days of Hall and Héroult.

Their insight was that electricity, which was starting to be generated in industrial quantities in the 1880s, could be used instead of chemicals to split the metal from the oxygen in aluminium oxide. And that is what Metalysis is doing in its new tantalum factory, and what it hopes to do for titanium and the rest.

The difference between its process and that of Hall and Héroult (and why electrolysis has not previously been used to make metals such as tantalum and titanium) is that the Hall-Héroult method requires both input oxide and output metal to be in liquid form. That demands heat. But aluminium has a fairly low melting point and its oxide can be dissolved in a substance called cryolite that also has a low melting point, so the amount of heat needed is manageable. Titanium and tantalum are not so obliging. The Metalysis trick is to do the electrolysis on powdered oxides directly, without melting them.

For the rest of this article, please go to The Economist website: http://www.economist.com/news/science-and-technology/21571847-exotic-useful-metals-such-tantalum-and-titanium-are-about-become-cheap


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