Titanium – The element
Titanium is one of the transition elements of Group IVB in the periodic table. Titanium appears as a silver-white metallic element with a low density, good strength, excellent corrosion resistance, very low electrical and thermal conductivity, and is paramagnetic. Titanium is as strong as steel but 45% lighter. It is 60% heavier than aluminum, but twice as strong. The natural titanium consists of five stable isotopes, 46Ti (8%), 47Ti (7.3%), 48Ti (73.8%), 49Ti (5.5%) and 50Ti (5.4%), but several unstable isotopes (radioactive) are also known, such as 51Ti, the radioactive isotope with highest half-live (5.76 min). In nature titanium can not be found as a pure due to its strong affinity for oxygen, carbon and nitrogen. 
The brief history of titanium dioxide
Titanium was discovered by the British clergyman and mineralogist William Gregor in 1791 who produced a white metallic oxide from the mineral menachanite and named the new element menachite. A few years later M.H. Klaproth, a German chemist, separated TiO2 from the mineral rutile and named the new element titanium after the giants of Greek mythology. In 1910 an American chemist, M.A. Hunter produced pure titanium from the principal titanium ore mineral ilmenite, which he named after the Ilmen Mountains in the south Urals.  The commercial production of titanium dioxide started on the 1920 decade, and the Ti metal started on the 1950 decade due to the increase of demand generated by the aircraft industry, reaching its metal production 166,000 metric tons in 2008 [1, 2].
Resources of titanium dioxide
Titanium is the ninth most abundant element in the Earth’s crust and the seventh metal most abundant in the whole Earth found in all rocks and sediments. Titanium ores are basically found in nature as mainly ilmenite (FeTiO3, the most important economic mineral), rutile, anatase and brookite different in their crystalline structure, however sharing the same chemical formula. [1, 2, 3, 4]
The term “titanium ore” refers to a wide variety of natural or industrial titanium resources. The most important titanium ores can be found in nature in the form of mainly ilmenite (TiFeO3) with approximately 40% - 65% TiO2 content, with the rest being mainly ferrous and ferric iron oxides with some amounts of other oxide impurities of chromium, manganese, vanadium, magnesium, aluminum, calcium, silicon and others [1, 5]. The TiO2 may exceed 60% when the ore is altered to leucoxene, which is a mixture of rutile or anatase amorphous TiO2 and iron oxides. In nature titanium can be found as well in the form of rutile with higher (93% - 96%) TiO2 content [1, 5, 6].
Production and purification
There are three types of titanium dioxide production processes, namely the sulfate, the chloride and the chloride-ilmenite process. The oldest production process for titanium dioxide is the sulfate process. The major difference between the chloride and the chloride-ilmenite process is the process feed stock. The chloride process produces a smaller quantity of waste materials than the sulfate process, but the chloride process is difficult to operate. The extreme corrosiveness of the high temperature chlorine employed in the process contributes to the difficulty. The oxidation step in the process is also extremely difficult to control due to burner configuration and product recovery. 
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 FINAL TITANIUM DIOXIDE LISTING BACKGROUND DOCUMENT FOR THE INORGANIC CHEMICAL LISTING DETERMINATION. US EPA Archive Document. 2001.