Mining and Refining: Titanium, Our Youngest Industrial Metal [Hackaday]

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Earlier in this series, we made the case for copper being “the metal that built technology.” Some readers took issue with that statement, noting correctly that meteoric iron and gold were worked long before our ancestors were able to locate and exploit natural copper outcroppings, therefore beating copper to the historical punch. That seems to miss the point, though; figuring out how to fashion gold decorations and iron trinkets doesn’t seem like building the foundations for industry. Learning to make tools from copper, either pure or alloyed with tin to make bronze? Now that’s how you build an industrial base.

So now comes the time for us to make the case for our most recent addition to humanity’s stable of industrial metals: titanium. Despite having been discovered in 1791, titanium remained locked away inside abundantly distributed ores until the 1940s, when the technological demands of a World War coupled with a growing chemical prowess and command of sufficient energy allowed us to finally wrest the “element of the gods” from its minerals. The suddenness of it all is breathtaking, too; in 1945, titanium was still a fantastically expensive laboratory oddity, but just a decade later, we were producing it by the (still very expensive) ton and building an entirely new aerospace industry around the metal.

In this installment of “Mining and Refining,” we’ll take a look at titanium and see why it took us over 11,000 years to figure out how to put it to work for us.

Starting With Sand

For something that has been commercially exploited for less than a century, titanium is surprisingly abundant. It’s the ninth most abundant element in Earth’s crust, making up more than half a percent by mass. Titanium is never found in the metallic state in nature, but rather as oxides like titanium dioxide (TiO₂), and as such is widely distributed around the world.

Once the titanium sponge slug is allowed to cool and removed from the Kroll reactor, surface scale formed by reaction products is removed. Different parts of the sponge have different grades of titanium — the low-grade waste from the bottom of the slug, mid-grade stuff at the top and around the outside, with the highest-grade metal at the core. A guillotine shear slices the slug into sections, with the mid-grade product going for blending and further refining while the high-grade metal is further chopped and crushed into small chunks. These pieces, along with any alloying metals that might be needed, get rammed into a press that forms them into an electrode.

Placed into a water-cooled vacuum arc remelting (VAR) furnace and heated to 1,700°C by an electric current, the electrode melts into a titanium ingot. Depending on the properties required from the finished metal, the ingot may go through another VAR purification. Pure ingots then go through a long series of forging, milling, grinding, and rolling processes, typically with a heat-treatment process in between each operation. The result is either plates, billets, or coiled sheets of titanium alloy, ready for manufacturing use.

White As Can Be

Metallic titanium isn’t the only desirable product from the mining and refining of titanium ore. Titanium dioxide itself has hundreds of industrial uses, most of which have to do with the brilliant whiteness of the powdered solid. Titanium dioxide is present in something like 60% of all pigments produced; look at the nearest painted wall or ceiling and it’s almost certain it contains at least some TiO₂. It’s also important in paper manufacturing, pharmaceuticals and cosmetics, sunscreen products, ceramic glazes, food additives, and thin-film optical products.

All TiO₂ starts with TiCl₄ feedstock; in fact, 90% of the “tickle-four” manufactured every year goes to the production of titanium dioxide, typically with a simple hydrolysis reaction:

Alternatively, the tickle can just be blasted with oxygen:

Either way, the oxidation of TiCl₄ results in fine white crystals of titanium dioxide, which are captured, dried, ground, graded, and packaged for shipping.

Thermite Makes It Better

Almost nothing about the mining and refining of titanium is environmentally friendly. Huge energy inputs are needed all along the way, from the fossil fuels needed to harvest and prepare the ore to the electricity needed to run the vacuum arc furnaces and recycle the magnesium chloride. Unfortunately, there aren’t many points in the process that are amenable to the green treatment, thanks largely to the chemical and thermal stability of titanium oxides.

That doesn’t mean the search isn’t on for ways to make titanium refining more sustainable, though. One Japanese company, Toho Titanium, is working on a promising pyrotechnic thermite method of producing titanium. The method combines powdered titanium ore with fluorite, the mineral form of calcium fluoride, along with some powdered aluminum. When the mixture is ignited it undergoes an extremely energetic thermite reaction, which heats the smelter without the need for external energy inputs. The reaction reduces the titanium oxides in the ore to a pool of molten titanium under a slag of aluminum oxide and calcium fluoride. The titanium is further purified by electrolytic refining, but even with that energy-intensive step, the company claims their process can reduce electricity consumption by 70% to 80%.