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.
This was shown to be possible in 1997 when researchers at Cambridge University found that immersing small samples of certain oxides in baths of molten salt and passing a current through them transformed the material directly into metal. Metalysis was set up in 2001 to commercialise the idea and in 2005 the firm moved to Wath-upon-Dearne, a village near Sheffield, in South Yorkshire, in order to tap the local engineering skills that once made the county (home of silver plate, stainless steel and the Bessemer converter) the California of metallurgy. These skills turned a technique that could produce a few grams of metal in a laboratory into a process that operates on an industrial scale.
The full Monty
The process starts with powdered metal oxide, which serves as the cathode. The anode is made of carbon, and the molten salt (which has a temperature of 1,000°C) acts as an electrolyte, permitting current, in the form of oxygen ions, to pass from cathode to anode. There, the ions react to form carbon dioxide, while the cathode is gradually transformed from oxide to metal.
The company’s first product is tantalum. Its factory is not much bigger than a house, but has enough capacity to supply 3-4% of the 2,500 tonnes of this metal that are used around the world each year. The resulting income, the firm hopes, will provide it with the grubstake it needs to move on to the big prize: titanium.
Even at its current price, some 140,000 tonnes of titanium are sold every year, for a total of around $4 billion. As the price falls, that tonnage is likely to rise, for cheap titanium has many potential uses, such as making car components that would be lighter than steel ones, thus saving fuel. This would require the powdered metal produced by Metalysis’s process to be made into ingots or sheets of the sort currently used in factories. The powder itself, however, could be employed directly in what is known as additive manufacturing, which uses 3D printers to build up objects a layer at a time. Cheaper metal powders would make 3D printing much easier.
This will require a far bigger factory, so Metalysis is on the lookout for a redundant aluminium smelter to convert. And if things go well with titanium, the other metals will gradually be introduced—possibly as mixtures, for Mr Dhariwal says the process will allow mixed oxides to be converted into alloys that were hitherto hard or impossible to make because of the different melting points of the components: titanium and tungsten, for example.
Modern economics have not been kind to South Yorkshire. A place that was once a centre of innovation has been sidelined by sunnier, sexier climes. But if Metalysis’s technology does work at scale it may prove every bit as important as Henry Bessemer’s converter. That was the device which made steel the material of the industrial revolution, displacing wrought iron. The idea that steel itself might one day be displaced by titanium is a long shot, but it is no longer inconceivable.
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