Sulphate process, titanium dioxide

Titanium dioxide is commercially produced by two different processes. In the sulphate process, titanium dioxide is prepared by reacting titanium ores with sulphuric acid. In the chloride process, titanium dioxide is produced by reacting titanium ores with chlorine gas. Compounds with anatas Ti02 show an outstanding bluish white color. Rutile types exhibit a creamy white [26]. 

Sulphate process. The ilmenite is reacted with sulphuric acid giving titanium sulphate and ferric oxide. After separation of ferric oxide, addition of alkali allows precipitation of hydrous titanium dioxide. The washed precipitate is calcined in a rotary kiln to render titanium dioxide. The nucleation and calcination conditions determine the crystalline structure of titanium dioxide (e.g. rutile or anatase). 

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. 

Iron(II) sulphate is a by-product in many industrial processes, such as the manufacture of titanium dioxide, the pickling of steel sheet before galvanising and the reduction of aromatic nitro compounds to amines using iron catalysts. Conversion of waste iron (II) salts to usable iron oxide pigments, where the quality requirements are not too stringent, is therefore a useful proposition, since it uses up chemicals that would otherwise be regarded as waste products. 

There are two crystalline forms widely used in papermaking applications, anatase and rutile. The major difference between the two is crystal uniformity and size, which yields a slightly higher index of refraction in the rutile crystal (Fig. 6.8). There are two chemical processing manufacturing routes commercially viable for titanium dioxide production, the sulphate and the chlorine process. The older sulphate method may be used to produce both anatase and rutile, while the more recent chlorine process is utilised only for rutile crystal production. 

Sulphate (46% of production, and declining) a multi-stage wet chemical process, batch or continuous. Titanium dioxide is dissolved in the raw materials by concentrated sulphuric acid, precipitating the hydrous salt. This is then calcined to produce one of the two crystalline forms, anatase or rutile. 

There is a paradigmatic NORM (Naturally Occuiring Radioactive Material) indushy, in south-western Spain, which produces titanium dioxide pigments by applying the so-called sulphate method. In the production process the red gypsum (mostly calcium sulphate) waste is generated which, until now, has not had any use and has been disposed of in an authorized area. This policy has been costly, so the company decided to re-evaluate its options on this waste. 

In the Huelva factoiy, the oldest and most common process for titanium dioxide production is used the sulphate process. This process uses concentrated sulphuric acid (H2SO4) to dissolve the titaniferous feedstocks which are milled and dried beforehand to aid the digestion process. 

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