Retro-Technics • author: a3310i • last modified: 2022.01.21 •

Titanium is a metal which in recent years has made a worldwide career as a construction material. Its low density (4.43 g/cm³) makes it possible to obtain light alloys with good mechanical properties and resistant to corrosion. Apart from titanium dioxide TiO₂, there are two other oxides of this metal: Ti₂O₃ and TiO. Titanium white is the compound in which titanium occurs at the highest oxidation state. This is what determines the durability of titanium white as a pigment. In nature titanium dioxide occurs as a mineral called rutile, which is mined mainly in Australia, India, Sri Lanka, the Republic of South Africa. Natural rutile contains about 95% TiO₂ and admixtures of iron, tin (up to 1.5%), chromium vanadium and other metals. Depending on the type of additions rutile can be dark yellow, brown, red, black, and there are also colourless crystals. Titanium dioxide exists in the form of three crystallographic varieties: anatase, brucite and rutile. Heating TiO₂₂ to 2200 °C causes a partial separation of oxygen and thus a change in colour. Titanium dioxide is a white powder insoluble in water, resistant to bases and most acids. It reacts only with concentrated sulphuric acid. The basic raw material for the production of titanium white is ilmenite, a mineral with the formula FeTiO₃ (FeO*TiO₂). Its content of titanium dioxide varies within the range of 45-55%. It is the cheapest raw material for production of titanium white but at the same time it is also the most troublesome in production due to a large amount of iron compounds which constitute waste. The TiO₂ content in ilmenite is also sometimes significantly lower in the case of strong silica contamination of the ore. The largest ilmenite producers are Canada, Australia, Norway and the United States. Of increasing importance are the anatase deposits discovered in Brazil, estimated at 1 billion tonnes.

[001] Ilmenite. The mineral is named after the Ilmen Mountains in the Southern Urals in Russia, where it occurs.

Titanium white is obtained using two methods: the traditional sulphate method and the newer chloride method, first used by the American company Du Pont in 1959. The starting material in the sulphate method is ilmenite, which contains relatively little TiO₂, and in the chloride method the much more expensive rutile, in which 95% of the composition is TiO₂. The chlorine gas used to chlorinate the rutile remains in a closed circuit, and there is little waste. In the sulphate method, in which ilmenite is treated with sulphuric acid, for every tonne of white produced there are 4 tonnes of ferrous sulphate and 11 tonnes of dilute sulphuric acid; these are difficult wastes to dispose of. The first stage is to dry and grind the ore. It is then chemically decomposed in reactors under the action of concentrated sulphuric acid at 250 °C:
FeTiO₃ + 2H₂SO₄ → (TiO)SO₄ + FeSO₄+2H₂O
Iron reacts with sulphuric acid to form ferric sulphate:
Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O
The resulting sulphate suspension is then leached with acidified water and the trivalent iron Fe³⁺ is reduced to divalent Fe²⁺ using iron shavings (iron scrap). Flocculants are used to clarify the solution and separate the resulting sludge. After these operations there is still 0,005 g/l of ferrous sulphate in the titanyl sulphate solution. The reduction of iron and clarification of the solutions are carried out one after the other in order to achieve the lowest possible content of ferrous sulphate in the solution, which is one of the conditions for good pigment quality.
After separation of FeSO₄, the solution is hydrolysed to produce titanic acid:
(TiO)SO₄ + 2H₂O -> H₂TiO₃ + H₂SO₄
Ferrous sulphate is stored in heaps as a waste product, and the sulphuric acid formed as a result of hydrolysis is separated from the titanic acid using Moore filters (humic sheets covered with filter cloth), and then the H₂TiO₃ solution is concentrated in rotary filters. After the second filtration, 550 g of TiO₂ is contained in 1 litre of white titanic acid suspension. Now, after the introduction of additives to it, the next operation takes place, which is thermal decomposition, i.e. calcination of titanic acid resulting in titanium oxide - a white powder which is the basic component of the pigment. Calcination takes place in 40-metre-long calcination furnaces at a temperature of about 900 °C. The calcine is then subjected to surface treatment, i.e. the appropriate shaping of the grains, during grinding in ball mills.

[002] Titanium dioxide.

One of the major disadvantages of the sulphate method of producing titanium white is the huge mass of waste, which is difficult to dispose of. Sulphuric acid after hydrolysis with a concentration of about 18% is supplied to the concentrating plant. Such diluted acid causes very fast corrosion. The acid-resistant steel installation is corroded within 10 days. An effective method is to reduce the pressure. Currently, the process is carried out at a temperature of 60-80 °C under reduced pressure; corrosion is slowest under these conditions. After passing through an evaporator, acid of 50-55% is obtained, which, after mixing with concentrated 96% sulphuric acid, is discharged into storage tanks as 65% acid. Titanium white significantly exceeds the properties of other white inorganic pigments (zinc white, barite white, lithopone) both in terms of colour durability - which is due to the stability of the titanium atom on the 4th oxidation level, as well as in terms of colour intensity and ability to cover painted surfaces (on 1 m2 of painted surface several times less titanium white is needed than other pigments). Thanks to these advantages titanium white is used in many branches of industry. Titanium white is used for obtaining top quality white enamels used in the automotive industry for coating car bodies. The addition of white to other coloured enamels increases their colour intensity and increases the durability of the coating. The paper industry uses white to produce top-quality white thin papers. In many tablets the filling is titanium white.

[003] Schematic of the post-hydrolytic acid concentration plant. 1 - mixer, 2 - heat exchanger, 3 - crystalliser, 4 - steam injectors, 5 - salt separator, 6 - centrifuge, 7 - tank, 8 - buffer tank, 9 - heater, 10 - evaporator, 11 - heat exchanger, 12 - carboniser, 13 - cooler, 14 - acid cooler, 15 - vacuum drum filter, 16 - post-hydrolytic acid, 17 - FeSO₄ x H₂O, 18 - steam, 19 - condensate, 20 - vapour, 21 - sulphuric acid, 22 - 55% sulphuric acid solution, 23 - 96% sulphuric acid solution, 24 - water, 25 - 65% sulphuric acid solution for storage tanks

The properties of TiO₂ such as temperature resistance, low chemical reactivity, non-toxicity and the ability to grind into very small grains are important for the plastics industry. In the chemical fibres industry also anatase white grades containing at least 94% of grains of diameter less than 1 µm are used. In the process of manufacturing artificial leather and in the rubber industry, titanium white is valued above all for its ability to provide adequate white cut-off of products. Titanium dioxide has found many important applications in the metallurgical industry, where significant amounts are used to produce electrodes, alloys, nickel-titanium ligatures and ceramic radioelements. Titanium dioxide is not always used as a pigment, its high mechanical properties also play an important role.