TiO2 is produced as anatase TiO2 and rutile TiO2. The forms of TiO2 differ in terms of the crystalline structure and purity of the final product. Anatase TiO2 may be produced by digesting ilmenite (essentially ferrous titanate (FeO.TiO2)) with sulphuric acid, the sulphate process, or from titanium slag. Basic reaction equations for the acid digestion route are (Lowenheim and Moran, 1975; p. 814):
FeTiüs + 2H2SO4 ^ FeSÜ4 + TiO.SO4 + 2H2O TiO.SO4 + 2H2O ^ TiO2.H2O + H2SO4 TO2.H2O + heat ^ TiO2 + H2O
The sulphate route process does not give rise to process greenhouse gas emissions that are of significance.
There are three processes that are used in the production of TiO2 that lead to process greenhouse gas emissions: titanium slag production in electric furnaces, synthetic rutile production using the Becher process, and rutile TiO2 production via the chloride route.
Titanium slag used for production of anatase TiO2 is produced from electric furnace smelting of ilmenite. Where titanium slag is used the acid reduction step is not required as the electric furnace smelting reduces the ferric iron contained as an impurity in ilmenite. Rutile TiO2 may be produced by further processing of the anatase TiO2. Process emissions arise from the reductant used in the process.
Production of synthetic rutile can give rise to CO2 emissions where the Becher process is used. This process reduces the iron oxide in ilmenite to metallic iron and then reoxidises it to iron oxide, and in the process separates out the titanium dioxide as synthetic rutile of about 91 to 93 percent purity (Chemlink, 1997). Black coal is used as the reductant and the CO2 emissions arising should be treated as industrial process emissions. The main route for the production of rutile TiO2 is the chloride route. Rutile TiO2 is produced through the carbothermal chlorination of rutile ore or synthetic rutile to produce titanium tetrachloride (TiCl4) and oxidation of the TiCl4 vapours to TiO2 according to the following reactions (Kirk-Othmer, 1999; p.2017):
Based on stoichiometry and assuming complete conversion of the input C to CO2 through further conversion of CO in excess air, the CO2 emission factor cannot be less than 0.826 tonnes of CO2 per tonne of TiO2 (based on 1.5 moles of CO2 per mole of TiO2).
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