TYTANPOL® titanium dioxide pigments are manufactured by the Grupa Azoty Zakłady Chemiczne “Police” SA on the basis of the sulphate method based on the licence of the German company KRONOS INTERNATIONAL LTD.
The sulphate method of manufacturing of titanium white (also that used in Police) is very complex. It is basically divided into two parts, conventionally called as black part (from the colour of raw material) and as white part (from the colour of pigment). Each part includes several subsequently following unit operations. The time that lapses from preparing of raw material up to packing of ready product is from 10 up to 14 days depending on the brand. Keeping of technological regime in such complex and long production cycle requires high competence, experience and commitment of personnel as well as technical efficiency of the installation.
The titanium-bearing raw materials in the sulphate technology are ilmenites (enriched titanium ores) or titanium slag being a product of metallurgical improvement of ilmenite. Apart from pigment being the main product, also semi-finished products are manufactured in the process: copperas, titanyl sulphate, suspension of titanium dioxide and wastes: post-hydrolytic acid and post-digestion sludge.
Copperas is used for e.g. manufacturing of coagulants for sewage and water treatment, as a raw material for manufacturing of ferric pigments and recently also as agent reducing hexavalent chromium (Cr6+ ions) in the cement industry. Most often, the waste post-hydrolytic acid is concentrated and recycled to the process. At Police, due to the neighbourhood of fertilizer installations, it is used for manufacturing of phosphoric acid, used in turn for manufacturing of phosphoric fertilizers.
The post-digestion sludge is sent to storage yard but efforts aiming at its utilization are advanced.
For presentation of the manufacturing process of titanium dioxide, its description has been divided into two parts: a black part and a white part, to make it simple and easier to understand.
The titanium-bearing raw material (ilmenite or titanium slag), after drying and grinding, is mixed in strictly specified proportions with concentrated sulphuric acid. Digestion of raw material taking place as the result of its reaction with the sulphuric acid is carried out by periodical method in special reactors. The product of exothermic reaction (porous cake) is leached with water, and the obtained solution is subjected to the reduction process. Then, the obtained titanium liquor is subject to clarifying in order to separate settling insoluble particles, mainly silica and remainders of non-digested ore. In the next operation, crystallisation and separation of copperas is carried out. The copperas is washed and sent for further use. In case of use of titanium slag as a raw material, where the iron content in relation to titanium dioxide is significantly lower than in case of ilmenite, there is no need to carry out crystallisation of copperas. Titanium liquor separated from copperas is subject to filtration allowing for precise separation of fine solids, and then to concentration in vacuum evaporators. At the further stage, liquor purified of remainders and concentrated up is subject to hydrolysis, as a result of which suspension of colloidal titanium dioxide is precipitated. The progress of hydrolysis process has got great influence on the final quality of pigment as at this stage a basic pigment particle is created, characterised by specific size and level of chemical purity. During filtration of the obtained titanium dioxide suspension, waste post-hydrolytic acid is separated (approximately 20% solution of sulphuric acid contaminated with metal sulphates, mainly of iron origin). At the next stage, preliminarily washed titanium dioxide suspension is subject to bleaching process, where trace ions of transient metals (especially of iron) are reduced to the lower level of oxidation, which enables their efficient leaching at the next stage of filtration and washing. The obtained titanium dioxide suspension of high purity is a raw material for further processing.
Please see sulphate method of manufacturing of titanium white.
At the next stage, substances controlling growth of crystals and progress of phase change of anatase into rutile taking place in calcination furnace are added to the obtained titanium dioxide paste. The quantity of these substances and parameters of execution of calcination process depend on the type of manufactured clinker sort. Such prepared titanium dioxide paste is subjected to calcination process. This process requires extremely careful controlling as it is extremely important for the product quality. After the hydrolysis process, it is the next and the final stage of shaping of crystallographic structure of pigment. During calcination the processes of drying, dehydration, desulphurisation and finally forming of appropriate crystallographic structure of pigment (rutile or anatase) take place one by one. Flue gases containing sulphur oxides forming during calcination are purified in chimney towers, electro-filters and modern desulphurisation system. Depending on the method of execution of calcination, the obtained calcinates have got either rutile or anatase structure and are an initial raw material for production of particular pigment brands.
The obtained calcinate after cooling is subjected to dry grinding in roller mills or Raymond mills. Then, pigment slurry is prepared in water by means of high-speed dissolvers and is subject to precise grinding in bead mills. At the next stage pigment is subjected to a special inorganic treatment attributed to a particular brand, based on surface coating of pigment particles with very thin coats of inorganic oxides and hydroxides, the amount and type of which depend on the pigment application. The most often used are: aluminium hydroxide, silicon dioxide and recently zirconium dioxide. The principal purpose of inorganic treatment is to improve the pigment’s resistance to atmospheric conditions and to improve its dispersibility.
Treatment is carried out in water phase, adding solutions of treatment substances in adequate sequence or simultaneously. By regulation of pH of the suspension, precipitation of hydrated oxides on the surface of pigment particles is carried out. The form of these oxides very strongly depends on the precipitation conditions, an so the aluminium hydroxide, silicon dioxide and zirconium dioxide and other are present in the pigment not on the basis of a simple admixture but as molecular layer bounded with it. After surface treatment, the suspension is subjected to careful washing and rinsing, and then drying in a spray dryer. The dried pigment is subjected to jet milling in steam jet mills, which allows for obtaining of adequate degree of fineness of pigment. Whereas the progress of hydrolysis and calcination decides on the crystal size, then grinding and jet milling of calcinate decide on the final distribution of pigment particles, and thus on the quality of the obtained pigment. During jet milling, pigment normally is carried out surface treatment with organic substances. The organic treatment facilitates jet milling and improves dispersibility of pigment at the final application. Usually multi-hydroxide alcohols (polyols), silicone oil and recently oxosilanes are used as treatment substances.
The final operation is the packaging of pigments. There are three principal loading forms: into 25 kg bags, 500 or 1000 kg bags (so called big-bag) or in a bulk form to truck tankers.
To sum up, commencing from different crystallographic structures (rutile, anatase) up to different variants of inorganic and organic treatment of pigment surface, there are possibilities of production of many brands of titanium dioxide, each of strictly specified application.