«Titanium dioxide, also known as titanium(IV) oxide or titania, is the naturally occurring oxide of titanium, chemical formula TiO2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. Generally, it is sourced from ilmenite, rutile and anatase. It has a wide range of applications, including paint, sunscreen and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million metric tons. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and the oxide has been valued at $13.2 billion.
Titanium dioxide occurs in nature as the well-known minerals rutile, anatase and brookite, and additionally as two high pressure forms, a monoclinicbaddeleyite-like form and an orthorhombicα-PbO2-like form, both found recently at the Ries crater in Bavaria. One of these is known as akaogiite and should be considered as an extremely rare mineral. It is mainly sourced from ilmenite ore. This is the most widespread form of titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The metastable anatase and brookite phases convert irreversibly to the equilibrium rutile phase upon heating above temperatures in the range 600–800 °C (1,112–1,472 °F).
Titanium dioxide has eight modifications – in addition to rutile, anatase, and brookite, three metastable phases can be produced synthetically (monoclinic, tetragonal and orthorombic), and five high-pressure forms (α-PbO2-like, baddeleyite-like, cotunnite-like, orthorhombic OI, and cubic phases) also exist:
The cotunnite-type phase was claimed by L. Dubrovinsky and co-authors to be the hardest known oxide with the Vickers hardness of 38 GPa and the bulk modulus of 431 GPa (i.e. close to diamond's value of 446 GPa) at atmospheric pressure. However, later studies came to different conclusions with much lower values for both the hardness (7–20 GPa, which makes it softer than common oxides like corundum Al2O3 and rutile TiO2) and bulk modulus (~300 GPa).
The oxides are commercially important ores of titanium. The metal can also be mined from other minerals such as ilmenite or leucoxeneores, or one of the purest forms, rutile beach sand. Star sapphires and rubies get their asterism from rutile impurities present in them.
Titanium dioxide (B) is found as a mineral in magmatic rocks and hydrothermal veins, as well as weathering rims on perovskite. TiO2 also forms lamellae in other minerals.
Spectral lines from titanium oxide are prominent in class Mstars, which are cool enough to allow molecules of this chemical to form.
The production method depends on the feedstock. The most common mineral source is ilmenite. The abundant Rutile mineral sand can also be purified with the chloride process or other processes. Ilmenite is converted into pigment grade titanium dioxide via either the sulfate process or the chloride process. Both Sulfate and chloride process produces the oxide in the rutile crystal form but the sulfate process can also produce another form of the oxide, anatase, which is softer and which is used for a small number of specialist applications. The Sulfate Process is run as a batch process; the Chloride Process is run as a continuous process.
Sulfate process plants must utilise ilmenite concentrate (45-60% TiO2) or pretreated feedstocks as suitable source of titanium. In the sulfate process Ilmenite is treated with sulfuric acid to extract iron(II) sulfate. The resulting synthetic rutile is further processed according to the specifications of the end user, i.e. pigment grade or otherwise. In another method for the production of synthetic rutile from ilmenite the Becher Process first oxidizes the ilmenite as a means to separate the iron component.
An alternative process, known as the Chloride process converts ilmenite or other titanium sources to Titanium tetrachloride via reaction with elemental chlorine, which is then purified by distillation, and reacted with oxygen to regenerate chlorine and produce the Titanium dioxide. Titanium dioxide pigment can also be produced from higher titanium content feedstocks such as upgraded slag, rutile and leucoxene via a chloride acid process.
The five largest TiO2 pigment processors are Dupont, Cristal Global, Huntsman, Kronos and Tronox, Dupont having pioneered the chloride process in the 1960s and having converted to the use of the chloride process for all its applications. Major paint and coating company end users for pigment grade titanium dioxide include Akzo Nobel, PPG Industries, Sherwin Williams, BASF, Kansai Paints and Valspar. Global TiO2 pigment demand for 2010 was 5.3 Mt with annual growth expected to be about 3-4%.
For specialty applications, TiO2 films are prepared by various specialized chemistries. Sol-gel routes involve the hydrolysis of titanium alkoxides, such as titanium ethoxide:» (wikipedia)
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