Oxidation, titanium dioxide pigments

A high purity titanium dioxide of poorly defined crystal form (ca 80% anatase, 20% mtile) is made commercially by flame hydrolysis of titanium tetrachloride. This product is used extensively for academic photocatalytic studies (70). The gas-phase oxidation of titanium tetrachloride, the basis of the chloride process for the production of titanium dioxide pigments, can be used for the production of high purity titanium dioxide, but, as with flame hydrolysis, the product is of poorly defined crystalline form unless special dopants are added to the principal reactants (71). 

Iron Titanates. Ferrous metatitanate [12168-52-4] FeTiO, mp ca 1470°C, density 472(0), an opaque black soHd having a metallic luster, occurs in nature as the mineral ilmenite. This ore is used extensively as a feedstock for the manufacture of titanium dioxide pigments. Artificial ilmenite may be made by heating a mixture of ferrous oxide and titanium oxide for several hours at 1200°C or by reducing a titanium dioxide/ferric oxide mixture at 450°C. 

Two processes are used in the manufacture of titanium dioxide pigments the sulfate process and the chloride process. The chemistry of the sulfate process, the longer established of the two methods, is illustrated schematically in Scheme 9.1. In this process, crude ilmenite ore, which contains titanium dioxide together with substantial quantities of oxides of iron, is digested with concentrated sulfuric acid, giving a solution containing the sulfates of Ti(iv), Fe(m) and Fe(n). Treatment of this... 

The presence of mica in pearlescent pigments only partly accounts for the appearance of the pigment. A very thin layer of the inorganic oxide titanium dioxide (TiC>2) or iron oxide (Fe2C>3) or both is coated on the mica platelets. The various colors and pearlescent effects are created as light is both refracted and reflected from the titanium dioxide layers. The very thin platelets are highly reflective and transparent. With their plate-like shape, the platelets are easily oriented into parallel layers as the paint medium is applied. Some of the incident light is reflected... 

ICON [Integrated chlorination and oxidation] An improved version of the Chloride Process for making titanium dioxide pigment. It operates at above atmospheric pressure and is claimed to be cheaper to build. Chlorine from the oxidation section, under pressure, is introduced directly to the chlorinator. Developed by Tioxide Group, and first operated at its plant at Greatham, UK, in 1990. 

There are now two processes in widespread use for making titanium dioxide pigments. In the sulphate process, finely ground ilmenite is digested in sulphuric acid and the iron is reduced and separated as iron(II) sulphate. The titanium(IV) sulphate is hydrolysed by steam to a hydrous oxide, which is thoroughly washed to remove soluble impurities and finally calcined at a temperature of about 1000 °C to give the anatase form of titanium dioxide. 

The magnitude of n is related to the density of the substance and varies from 1.000 and 1.3333 for vacuum and water, to about 1.5 for many polymers and 2.5 for white pigment, titanium (IV) oxide (titanium dioxide). The value of n is often high for crystals and is dependent on the wavelength of the incident light and on the temperature. It is usually reported for the wavelength of the transparent sodium D line at 298 K. Typical refractive indices for polymers range from 1.35 for polytetrafluoroethylene to 1.67 for polyarylsulfone. 

Pigment Iron oxide Titanium dioxide Carbon black Carbon black —... 

Colorants can be divided into water-soluble dyes and water-insoluble pigments. Some of the insoluble colors or pigments can also provide opacity to tablet coatings or gelatin shells, which can promote stability of light-sensitive active materials. Pigments such as the iron oxides, titanium dioxide, and some of the aluminum lakes are especially useful for this purpose. 

In contrast to colorants, which stabilize by photon absorption, pigments are photon impermeable, for the most part, and stabilize by reflection and diffusion. Retinoic acid in Basis creme DAC is an example where the inclusion of titanium dioxide pigments improves the photostability of the product markedly. The drug content in the unprotected cream decreases by 28% with two hours of irradiation. Adding 2% of hydrophilic titanium dioxide (formulation E) reduced degradation to only 9% (Fig. 23) (45). The authors observed corresponding results with 2% zinc oxide and with 0.5% yellow iron oxide. 

Figure 25 Influence of zinc oxide and titanium dioxide pigments on the photostability of triamcinoloneacetonide in Basis creme DAC (Suntest CPS+, 415W/m, window glass filter, film 200 pm). ( ) 4% pigment, (A) o.5% pigment, (O) 2% pigment, ( ) without pigment. Source From Ref. 45.

There are two major processes for the manufacture of titanium dioxide pigments, namely (1) sulfate route and (2) chloride route. In the sulfate process, the ore limonite, Fe0Ti02, is dissolved in sulfuric acid and the resultant solution is hydrolyzed by boiling to produce a hydrated oxide, while the iron remains in solution. The precipitated titanium hydrate is washed and leached free of soluble impurities. Controlled calcinations at about 1000°C produce pigmentary titanium dioxide of the correct crystal size distribution this material is then subjected to a finishing coating treatment and milling. The process flow sheet is shown in Fig. 7.1 [4],... 

Liddell, P.V., The rheology of titanium dioxide pigment suspensions, Ph.D. Thesis, University of Melbourne, 1996, cited after Zhou, Z., Scales, P.J., and Boger, D.V., Chemical and physical control of the rheology of concentiated metal oxide suspensions, Chem. Eng. Sci., 56, 2901, 2001. 

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