TiO₂-pigments are either manufactured using the older sulfate process or newer chloride process. The economics of the two processes are very much dependent upon the raw materials available.

Sulfate Process

The sulfate process (see Figure 5.9-1) utilizes ilmenite or titanium slag.

The finely ground raw materials are digested with concentrated sulfuric acid in an exothermic reaction, the digested cake dissolved in cold water and the residue separated off. To prevent coprecipitation of (Ⅲ) ions during the subsequent hydrolysis, they are reduced to iron(Ⅱ) by adding a Ti(Ⅲ) solution or scrap iron. Upon evaporating the solution, the large quantities of iron(Ⅱ) sulfate heptahydrate produced when ilmenite is used, crystallize out. The titanium sulfate is then hydrolyzed by feeding in steam at 95 to 110°C. TiO₂ seed-crystals are added or are formed before hydrolysis to ensure yields of 93 to 96% TiO₂ and to obtain a hydrolysis product which yields the optimum particle size of ca. 0.2 μm upon firing. During hydrolysis sulfuric acid is produced as a dilute aqueous solution, so-called "waste acid".

The hydrolysis product is washed, treated with a Ti(Ⅲ) solution to remove adsorbed heavy metal ions (Fe, Cr, Mn, V) or bleached with aluminum and acids and calcined at temperatures of ca. 1000°C. With doping or an appropriate choice of additive, latterly rutilization nucleating seeds, before calcination, anatase or rutile pigments can be produced in the calcination process.

The byproducts produced, dilute sulfuric acid (waste acid) and ferrous sulfate heptahydrate, if ilmenite is used, are to an increasing extent being processed further (see Fig. 5.9-2). In a process operated by Bayer AG since 1958, the waste acid is concentrated to 65% in lined vessels with the aid of a submerged burner. After separation the 65% sulfuric acid can be used as such or is evaporated in a distillation step to 96% sulfuric acid, which can be reused for the digestion of ilmenite or titanium slag.

Iron sulfate is cracked to sulfur dioxide and iron oxide in a fluidized bed reactor at 800 to 1000°C. The energy is provided by the combustion of sulfur or other fuels, the sulfur dioxide formed being, after purification, processed to sulfuric acid. The iron oxide residue can be employed in the cement industry.

An alternative to the concentration of waste acid and reuse thereof, is neutralization of waste acid with calcium carbonate and using the gypsum formed either in the construction industry or for land infill, depending upon purity.

Chloride Process

In the chloride process natural rutile ore or synthetic rutile with a TiO₂-content of up to 96% is chlorinated in a fluidized bed reactor with oil-coke and chlorine. The raw TiCl₄ produced is mixed with reducing agents to convert impurities, such as vanadium oxychloride, to lower valency state vanadium compounds. The titanium tetrachloride formed is then distilled yielding titanium tetrachloride in almost any required purity. Finally it is burnt with pure oxygen to TiO₂ and chlorine, which is reused in the chlorination process (see Fig. 5.9-3).

The formation of impurity metal chlorides is dependent upon the raw material. If there were sufficient rutile available, this process would be particularly ecologically favorable. However, in view of its scarcity and hence its high price, raw materials with low titanium contents, such as 85% RBM slag, are currently being used in the chloride process. Allowance has then to be made for the resulting increased formation of impurity metal chlorides. TiO₂-pigment manufacture without byproducts is not possible. These byproducts are disposed of by the "deep well process" (USA), dumped or, after processing, used for water processing.

Posttreatment of TiO₂

Most TiO₂ pigments undergo an inorganic and organic posttreatment to increase their weathering resistance and to improve their dispersibility in paints and plastics.

Untreated anatase pigments, in particular, exhibit poor weathering resistance due to absorption of near UV-light not only by TiO₂, but also by paint binders and plastics. This leads to UV-induced degradation of the organic materials and to the formation of OH^- and HO₂-radicals on the surface of the TiO₂-pigments. These accelerate the photochemical degradation of tlie binder resulting in the exposure of pigment particles, which are washed out (chalking).

These effects can be suppressed or even reversed by using the more stable rutile pigments, by doping the hydrolysis product with Zn²⁺ , Al³⁺, Zr⁺ or Si⁴⁺ prior to calcination and by precipitation of poorly soluble colorless inorganic compounds such as hydrated SiO₂, Al(OH)₃, hydrated ZrO₂, water-containing aluminum silicates, aluminum phosphates etc. onto the TiO₂-surface. The so-treated stabilized rutile pigment still absorbs UV-light, but without secondary photochemical reactions and hence protects the organic binder by screening it from UV-light.