Titanium dioxide
Titanium dioxide
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, or CI 77891. Generally it is sourced from ilminite, rutile and anatase. It has a wide range of applications, from paint to sunscreen to food colouring. When used as a food colouring, it has E number E171.
OCCURENCE
Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite, and additionally as two high pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form, both found recently at the Ries crater in Bavaria. It is mainly sourced from ilminite ore. This is the most wide spread form of titanium dioxide baring 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 to rutile upon heating.
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 leucoxene ores, 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 M stars, which are cool enough to allow molecules of this chemical to form.
PRODUCTION
The production method depends on the feedstock. The most common method for the production of titanium dioxide utilizes ilmenite. Ilmenite is mixed withsulfuric acid. This reacts to remove the iron oxide group in the ilmenite. The by-product iron(II) sulfate is crystallized and filtered-off to yield only the titanium salt in the digestion solution. This product is called synthetic rutile. This is further processed in a similar way to rutile to give the titanium dioxide product. The use of ilminite ore usually only produces pigment grade titanium dioxide. Another method for the production of synthetic rutile from ilminite utilizes the Becher Process.
Rutile is the second most abundant mineral sand. Rutile found in primary rock cannot be exstracted hence the deposits containing rutile sand can be mined meaning a reduced availability to the high concentration ore. Crude titanium dioxide (in the form of rutile or sunthetic rutile) is purified via converting to titanium tetrachloride in the chloride process. In this process, the crude ore (containing at least 70% TiO2) is reduced with carbon, oxidized with chlorine to give titanium tetrachloride; i.e., carbothermal chlorination. This titanium tetrachloride is distilled, and re-oxidized in a pure oxygen flame or plasma at 1500–2000 K to give pure titanium dioxide while also regenerating chlorine. Aluminium chloride is often added to the process as a rutile promotor; the product is mostly anatase in its absence.
