Can titanium get cheaper?
Dr Jeya Ephraim and Dr Raj Patel from the School of Engineering, University of Bradford, explain a possible new method for isolating titanium.
Titanium is a lightweight white lustrous metal with high strength, low density and excellent corrosion resistance. It is as strong as steel and 45% lighter. It is 60% heavier than aluminium, but its strength is almost double. Due to its impressive strength-to-weight ratio, titanium is alloyed with aluminium, molybdenum, manganese, iron and other metals for use in aviation, automotive and industries that require strong, lightweight materials that can withstand extreme temperatures.
Having such properties, titanium fits well with the new generation of aerospace design strategies, in which fuel efficiency is crucial. Hunter Dalton, Executive Vice President of ATI Long Products, says that titanium will remain a material of choice for engine components, even though engine temperatures continue to rise. High-performance titanium alloys are expected to be used for aircraft engine impellers, wing components and compressor blades. Recent advances in the development of compatible titanium composites with graphite fibres have seen use increase in aircraft construction. The compatibility between composites and titanium has also seen use increase – from 10 tonnes for the Boeing 717, to 41 tonnes for the Boeing 747, to 91 tonnes for the Boeing 787.
Currently, it is reported that around US$2.5bln per year is spent on corrosion prevention and repair of marine vessels. If titanium is used as an alternate metal for these vessels, the costs for corrosion and repair should be practically zero, but the price for titanium powder fluctuates between US$26–39/kg, which is expensive when compared with aluminium and stainless steel.
The stumbling blocks
It is already known that the production of titanium dioxide (TiO2) from its minerals such as ilmenite is challenging, and subsequent isolation of titanium from TiO2 is even more difficult because of the metal-oxygen bond strength – around 161±5kcal/mol. Currently, titanium sponge is mainly produced via the Kroll process, in which ilmenite or synthetic rutile is chlorinated at about 1,370K to produce gaseous titanium tetrachloride (TiCl4). The TiCl4 is then reduced with molten magnesium at 1,170K, which produces magnesium chloride (MgCl2) and titanium metal in a liquid form. The liquid titanium solidifies to form a sponge. The MgCl2, is removed by vacuum distillation and the magnesium separated by electrolysis for recycling.
However, titanium is not the only product here – partially reduced chlorides of titanium (TiCl2 and TiCl3) are also produced. The removal of these chlorides, produced from lower oxides of titanium, increases costs further. The process is tedious, making the production of titanium metal laborious, energy intensive and expensive. In the Kroll process, two weeks are needed for a batch of titanium sponge. This requires around 50kWh of energy per kg of titanium. It also generates large quantities of waste that damage the environment.
A number of international research groups have come up with technologies for the production of titanium metal. However, none of them have been commercialised, because of engineering or economic issues. To meet the growing demand, it is essential to develop less energy intensive, greener technologies for the metal.
Back to the drawing board
We attempted to isolate titanium by mixing TiO2 with calcium metal and reducing it at high temperatures, under suitable conditions, for five hours. The reduced product, prior to leaching, showed the presence of titanium nodules and calcium oxide (see page 38, top). On leaching the reduced product with very dilute hydrochloric acid (preferably 0.05 M), followed by drying, the morphology studies showed a highly porous spherical structure. Further analysis confirmed the presence of high-purity titanium (98.62%) without any oxygen impurity.
To confirm the significance of this finding, we carried out X-ray diffraction analysis for the reduced and leached sample. The diffraction pattern for the prepared powder clearly shows that all the peaks corresponding to α-titanium phase were present. All the planes corresponding to the α-titanium phase were also identified. There was a significant grain growth from nanoparticles (TiO2) to micron size (titanium) after the reduction. The particle size of the prepared titanium powder varied from 50–200 microns.
From the series of results obtained, it was confirmed that ultra-pure titanium powders can be separated from TiO2 powder through metallothermic reduction. This innovative method is quicker and cheaper than the current processes. The novelty of this process is that liquid titanium is not produced before the formation of titanium powder. Following the successful isolation of high purity titanium powder from TiO2 through the solid-state method at laboratory scale (3g), we carried out a mini scale up to around 50g, where we also obtained 98.62% pure titanium powder.
We are in the process of scaling up this technology with industrial support and studies are also in progress to recover the calcium used for reduction. The titanium powder we have produced is of high grade with no oxygen impurity. The isolation of α-titanium opens up new possibilities in the manufacturing sector. In the next phase, we will be investigating the production of master alloys and other novel alloys using the titanium powder produced from this route.
Based on the laboratory studies, the energy required for the production of 1kg was found to be approximately 3,500kJ/kg. This is substantially lower than the energy requirements of current production processes. The cost required to produce 1kg of titanium powder is estimated to be around US$5, which is also a substantial saving.
We are working with the Research Knowledge Transfer Support team at the University of Bradford to form a spinout company for scaling up the process to 1–5kg, prior to commercialisation.
The need for titanium:
- Increased demand for titanium sponge production in 2010–2012, for both aerospace and industrial application, resulted in a supply shortage for suitable titanium feedstock, particularly in 2012.
- There are 23 companies worldwide that can produce 333Kt per year of titanium sponge – 14 in China, three in USA, two each in Japan and Russia and one each in Kazakhstan and Ukraine.
- It is expected that there will be a steady growth in building around 29,200 new aircraft in the next 20 years, which will have a market value of around US$4.4tln. To meet this demand, it is expected that production of titanium tonnage will increase by 300% within a decade.
- Besides its use in aerospace and marine vessels, titanium is showing steady growth in medical applications, especially in the field of implants and joint replacements. Robert Daigle, Senior Vice President of Structure Medical, Florida, states that, ‘the orthopaedic market in the USA is valued at US$15bln’.
- Recent development of a new medical technology known as osseointegration, which uses porous titanium coatings, has further increased the demand for the material. This technology involves the application of spherical or asymmetric commercially pure titanium beads that are sintered on to the bone-contacting surface of implants, promoting osteoconductive bone ingrowth. Medical device manufacturers have been put under mounting pressure to reduce the cost of their products.