Titanium dioxide — chemical composition and production

Titanium dioxide — chemical composition and production

Chemical composition of titanium dioxide

Titanium dioxide is a white substance used as a pigment (titanium whitewash) in the form of a colorless powder with a particle size of 0.2 microns, known as food additive E171. Titanium dioxide crystals exist in nature in the form of rutile (Latin rutilus — red), anatase (Greek anatasis — stretching) and brookite. The first one is the most stable and is more common in nature. Anatase got its name because the vertical axis of its crystal is longer than the vertical axis of rutile. Brookite is the least common modification of the substance, which is not of particular value.

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The elementary cells of rutile (A), anatase (B) and brookite (C), in which titanium atoms are depicted in pale red, oxygen in bright red.

The rutile form of titanium dioxide has a high refractive index and reflectivity in comparison with anatase, which is why it is more often used as a pigment. The anatase additive has similar properties, but this form is chemically unstable and turns into rutile when the temperature rises. This material is characterized by a lower refractive index than rutile, but is more sensitive to UV radiation, therefore this type of titanium dioxide is more often used for the manufacture of photocatalytic cleaning devices. But even a small presence of the anatase form in the composition of rutile causes the destruction of coatings under the influence of sunlight.

Comparison of forms of titanium dioxide

 Rutile Anatase 
 A more compact crystal shape bluer shade 
 Higher refractive index, better hiding power It diffuses ultraviolet light less
 Inertia, weather resistance It is used to produce paper, fibers and photocatalysts
It is used in the production of coatings, printing inks and plastics

Production of titanium dioxide

Titanium dioxide is extracted from titanium-containing ores: rutiles, which contain 93-96% titanium dioxide, ilmenites, which contain 44-70% titanium dioxide, leucoxenes with a titanium dioxide content of up to 90%. There are two main methods of obtaining substances named in accordance with the precursor of titanium dioxide: sulfate and chloride. Historically, the sulfate method was used earlier (1931), then a more economically profitable and less environmentally hazardous chloride method was introduced into industry (1948).

Sulfate production method

In this method of producing titanium dioxide, the feedstock is crushed and mixed with concentrated sulfuric acid. The resulting solution is cooled and purified from sulfate salts. The hydrolysis of the finished solution is carried out by introducing germs, after which caustic soda is added. To achieve a high degree of hydrolysis, close to 100%, heating, dilution and boiling cycles are carried out. The finished suspension of titanium dioxide bound to water molecules is purified from impurities and treated at high temperature in the presence of rutile or anatase nuclei, depending on which form of compound is required at the outlet. The disadvantage of the method is the difficulty of cleaning the resulting product from metal impurities and a considerable amount of waste.

Chloride production method

The chloride method for the synthesis of TiO2 material is based on one of the following technologies:

  1. Hydrolysis of an aqueous solution of titanium tetrachloride

  2. Steam phase hydrolysis of TiCl4

  3. Heat treatment of TiCl4 in an oxygen atmosphere

The method was invented for the production of a rutile form of titanium dioxide and consists in conducting high-temperature reactions in the gas phase. The ore containing titanium reacts with chlorine gas, and titanium tetrachloride and impurity compounds of chlorides of other metals are formed, which are removed. The purified titanium tetrachloride is then oxidized at high temperature, resulting in the formation of the desired product, titanium dioxide.

Sol-gel method for the production of titanium dioxide nanoparticles

In recent years, methods for manufacturing nanocrystalline titanium dioxide for photocatalytic purification of water and the environment from toxic organic compounds and harmful organisms have been actively studied. The sol-gel process is one of the main methods for obtaining materials in the form of nanoparticles, nanofilaments and thin films.

Advantages of the method:

  • Simplicity

  • Homogeneity

  • Purity of the final product

  • The ability to easily introduce alloying impurities

  • Synthesis of porous structures

This process is the transformation of a colloidal suspension - sol into a viscous gel with mechanical characteristics of a solid substance. Metal salts or alkoxides are usually used to form sol: aluminates, titanates. In the synthesis of titanium dioxide by this method, titanium tetrachloride is used as precursors. It is hydrolyzed and polymerized. Polymerization helps to increase the concentration of the dispersed sol phase and build contacts between the particles. After solvent extraction, a gel is formed, which is then dried and titanium dioxide nanoparticles with an average particle size of 22± 9 nm are obtained.

Hydrothermal method

Hydrothermal synthesis is the conduct of reactions in aqueous solutions at high temperatures (up to 500 °C) and pressures (10-300 MPa) in autoclaves. Titanium dioxide is formed from the same substances as in the sol-gel method. By changing the composition of the solution, temperature and pressure, it is possible to influence the size, crystal structure and composition of the product. The advantage of the method is the possibility of synthesizing large crystallites of titanium dioxide with high quality. The disadvantage is the high cost of the equipment. The hydrothermal method can be modified by exposing the reaction sphere to ultrasound or microwave radiation to increase the rate of synthesis. 

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TiO2 samples synthesized by the hydrothermal method

Titanium dioxide, or titanium whitewash, is a substance used as an additive in paints, food, pharmacological preparations, and polymer products. The material is contained in titanium-containing ores and is produced by sulfate and chloride methods, and more modern sol-gel and hydrothermal methods of its production are also being developed.


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