Titanium

This activity is aimed at students who have some knowledge of electrolysis and the extraction of metals. The reminder sheet Extracting metals – words could be used as an introductory activity for those who might need it.

Titanium

Titanium is a metal with incredible properties: it is lighter than steel; strong and tough enough to survive in space or at the bottom of the ocean; oxidation and corrosion resistant. And it looks good. What is more, it is very common – the ninth most common element in the Earth’s crust (found in the form of titanium oxide).

You might wonder why we do not use it more, but titanium has one real drawback: its cost. It is currently more than five times the price of stainless steel.

Metal	Cost per tonne
Titanium	£8000
Iron	£250
Stainless steel	£1500
Aluminium	£1500
Titanium alloys	£25000

Reactivity series of metals

Element	Symbol
Potassium	K
Sodium	Na
Calcium	Ca
Magnesium	Mg
Titanium	Ti
Aluminium	Al
Carbon	C
Zinc	Zn
Iron	Fe
Lead	Pb
Hydrogen	H
Copper	Cu
Silver	Ag
Gold	Au

Note: Carbon and hydrogen are not metals.

1. How is iron extracted from its ore?
2. Why is this method not suitable for extracting titanium?
3. What method is used to extract aluminium from its ore?
4. Why is aluminium more expensive than iron?

Scientists have been trying to extract titanium by electrolysis since the 1950s without success. Instead, it is extracted using a more reactive metal to displace it from its ore.

5. Suggest a metal that could be used to displace titanium from its ore.

Titanium extraction

The process used to extract titanium is called the Kroll process and is named after William J. Kroll, who invented the method in the 1930s. It is slow and has at least two steps. First the titanium oxide ore is reacted with chlorine to make titanium chloride.

The titanium chloride is reduced using either magnesium or sodium to form titanium metal. The magnesium is put into a steel reactor and titanium chloride is pumped in. The reactor is welded shut and then heated to 1200 °C. As titanium is very reactive, oxygen must be kept out of the reaction vessel so the reaction is done in an atmosphere of argon.

6. Why is an atmosphere of argon used for this reaction?
7. Suggest another gas which could be used instead.

After two or three days the vessel is broken open and the titanium removed. The whole process can take up to 17 days and can produce unpleasant waste gases. The largest reactors only produce about 1 tonne of titanium per day. (A Blast Furnace produces about 10,000 tonnes of iron in a day.)

8. Explain why titanium is an expensive metal.

William Kroll knew that his process was expensive and inefficient. In the 1950s he predicted that within 15 years an electrolytic process would replace his method. However, the Kroll process is still the main method used today and there have been many failed attempts at electrolysis. Scientists have tried using both titanium oxide and titanium chloride dissolved in other salts as the electrolyte. They have also experimented with various metals and carbon as the electrodes but none of the combinations they have tried have worked. Titanium has remained difficult to extract and expensive, although it offers properties that interest the designers of a wide variety of products.

21st century titanium

Titanium usually contains a small amount of dissolved oxygen near its surface, which can weaken the metal. Three scientists working in Cambridge – Derek Fray, Tom Farthing and George Chen – carried out some research on how to remove the impurity using electrolysis. They used the titanium they were trying to purify as the cathode, a block of carbon as the anode and molten calcium chloride as the electrolyte. They hoped that the current flowing through the titanium would drag the oxygen atoms to the surface of the metal, where they would gain electrons to form ions and dissolve in the salt. As they watched the experiment they noticed something they really had not expected: titanium oxide was being converted into pure titanium.

9. Why was this result so unexpected? (Hint: Would you expect this solid substance to conduct electricity?)

This discovery seemed to be too good to be true, but they decided to try and see if the same thing would work on a pellet of solid titanium dioxide (the stuff used to whiten paper and paint). They could hardly believe it when the electrolysis converted the oxide to titanium metal. “It was very surprising to see the little pellet of white titanium dioxide, which looks like an aspirin pill, being transformed into a piece of titanium,” Professor Fray recalls. “We sat around asking why no-one had done this before.”

10. Why do you think no-one had tried extracting titanium like this before?

The electrolytic cell

The process described above was named the FFC Cambridge process after the inventors and the university where they worked.

The formula of titanium dioxide is TiO₂. Although it is a compound of a metal and a non metal, it is a covalent compound. At the cathode, the oxygen atoms in the compound are reduced to form oxygen ions.

11. Write an equation for the reduction of the oxygen atoms to oxygen ions at the cathode.

The oxygen ions dissolve in the molten calcium chloride, leaving titanium metal behind. The oxygen ions then move through the calcium chloride to the anode where they are turned into oxygen gas.

12. Write an equation for what happens to the oxygen ions at the anode.
13. Why is some CO and CO₂ also produced?
14. How could this be prevented?

Titanium – from discovery to Mars

The FFC Cambridge process has been tested in successive experiments that have clearly shown it to work. However, across the world the majority of titanium is still produced by the Kroll process. The continuing story of the titanium discovery made by scientists in Cambridge shows that it is not always easy to introduce a new process, even if it has clear benefits. A year after the team at Cambridge had found their new process for extracting titanium, Professor Fray sent a report to Britain’s defence agency, DERA (Defence Evaluation and Research Agency). People at DERA were very excited and offered to develop the technology and scale it up.

15. Why would the defence agency be interested in titanium? What might they use it for?

The DERA and Cambridge scientists worked together on the idea for a while, but more money was needed to build a pilot plant (a small factory where the method for producing larger quantities of the product can be tested and improved). Funding was difficult to find as there was no guarantee that the technology would work on a larger scale, but eventually a pilot plant was built.

The pilot plant now produces kilogram quantities of titanium and there are plans to build a bigger plant and start producing commercial quantities. If the process scales up to an industrial level as expected, the price of titanium could fall substantially. British Titanium believes that this development could eventually increase the use of titanium from the current level of 60 000 tonnes per year to 1 million tonnes per year.

As titanium is the ninth most common element in the Earth’s crust there will be no shortage of raw material.

16. Think about the properties of titanium. Why might it be useful for? Hint: You could thank about aircraft, the motor industry and engineering/building.
17. What effect would using titanium in planes and cars have on fuel consumption?

There are still many difficulties to overcome to make this process suitable for commercial use and for production on a large scale. Chemical engineers have to work to develop process controls and to find a way of getting reactants in, and products out of the electrochemical cell. Efficient methods for these things will help lower costs. Whether this process is the one that is eventually used or not, it seems likely that titanium use will rise and its cost will fall over the next few years as its properties are so desirable for a wide range of applications. That is not the end of the story. In an exciting development, NASA (National Aeronautics and Space Administration) has now become interested in the process. Known for its shuttle programme and successful manned mission to the moon, NASA’s latest challenge is to enable man to live on the moon in order to pave the way for a trip to Mars. A large proportion of the moon’s surface is ilmenite (a titanium oxide ore). NASA scientists are interested in the FFC Cambridge process not for the production of titanium, but for its byproduct, oxygen.

18. Why would NASA be interested in producing oxygen?

 Answers

1. Iron is extracted from its ore by heating the ore with carbon.
2. Titanium cannot be extracted in this way because it is more reactive than carbon so would not be displaced by it. In addition, titanium carbide (TiC) might form.
3. Aluminium is extracted by electrolysis.
4. Aluminium is more expensive than iron because electrolysis of its ore uses a lot of electricity, which is more expensive than the carbon used in the blast furnace to extract iron.
5. Titanium could be extracted using magnesium/calcium/sodium/potassium/any more reactive metal.
6. Argon is used because it is a Noble gas and is extremely unreactive. It does not react with titanium so its presence does not affect the quality of the product.
7. Any other Noble gas could be used instead: helium, neon, krypton, xenon.
8. Titanium is expensive because the extraction process is slow, the chlorine and magnesium (or sodium) required are expensive, it costs a lot to heat the reactor and the process is labour intensive. Chlorine is also dangerous and difficult to handle.
9. The result was unexpected because titanium oxide is not a metal and would not be expected to conduct in the solid state. (It is a covalent compound – although as it is a compound of a metal and a non-metal, students may think that it would be ionic.) 
10. No-one had tried extracting titanium like this before as they did not expect a covalent/non-metallic solid to be able to conduct electricity and act as an electrode.
11. O + 2 e– → O^2–  
12. 2 O^2–→ O₂ + 4 e–
13. If a carbon anode is used, some of the oxygen produced reacts with the electrode to produce CO and CO₂.
14. This could be prevented by using an inert (unreactive) substance as the anode.
15. The defence agency might use titanium for producing lighter aircraft, tanks and other vehicles. A titanium ship would be lighter, sit higher in the water and might move faster than a steel one. Students may be able to think of a range of other benefits.
16. Titanium might be useful for: Aircraft – titanium is already used in some aircraft as it is very light and strong. The motor industry – as well as being light and strong, titanium is very corrosion resistant. If the engine had titanium parts it would be much lighter and use less fuel. Engineering/building – titanium looks good and has already been used on the surface of the Guggenheim Museum building in Bilbao, Spain. 
17. Fuel consumption would be cut as the metal is lighter than the commonly used material (steel).
18. One answer could be to allow humans on a mission to the moon to breathe. Oxygen is possibly even more crucial as an oxidiser to burn fuel.