Titanium Dioxide Manufacturing Processes
Manufacture of titanium dioxide pigment is a combination of two distinct processes: base pigment particle production and surface treatment, drying and milling (collectively known as 'finishing'). There are two different process routes used to extract and purify TiO2 from ore to produce core pigment particles, both followed by similar surface treatment procedures.
The Sulfate Process
The Sulfate Process was the first commercial process for the manufacture of TiO2. Originally this used ilmenite as a raw material, but beneficiated ores with a much higher TiO2 assay have been used more recently.
The ore is first dried, ground, and classified to ensure efficient sulfation by agitation with concentrated sulfuric acid in a batch or continuous exothermic digestion reaction. Controlled conditions maximize conversion of TiO2 to water-soluble titanyl sulfate using the minimum acid. The resultant dry, green-brown cake of metal sulfates is dissolved in water or weak acid, and the solution treated to ensure that only ferrous-state iron is present. The solution temperature is reduced to avoid premature hydrolysis and clarified by settling and chemical flocculation. The clear solution is then further cooled to crystallize coarse ferrous sulfate heptahydrate (known as "copperas", FeS04.7H20) which is separated from the process and sold as a by-product.
The insoluble "mud" is washed to recover the titanyl sulfate liquor which is filtered to remove final insoluble impurities. The solution is evaporated to a precise composition and hydrolyzed to produce a suspension ("pulp") consisting predominantly of clusters of colloidal hydrous titanium oxide.
Precipitation is carefully controlled to achieve the necessary particle size, usually employing a seeding or nucleating technique. The pulp is then separated from the mother liquor and extensively washed to remove residual traces of metallic impurities, using chelating agents if necessary. The washed pulp is treated with chemicals which adjust the physical texture and act as catalysts in the calcination step. This process can produce either anatase or rutile crystal forms depending on additives used prior to calcination.
The Chloride Process
The feedstock for the chloride process is a mineral rutile or synthetic beneficiates containing over 90 percent TiO2. A suitable ore blend is mixed with a source of carbon and the two are reacted in a fluidized bed with chlorine at approximately 900°C. The reaction yields titanium tetrachloride, TiCl4, and the chlorides of all the impurities present. The reaction is exothermic, and accurate temperature control is essential.
The mixed chlorides are cooled and the low-volatile chloride impurities (e.g. iron, manganese and chromium) are separated by condensation and removed from the gas stream with any un-reacted solid starting materials.
The TiCl4 vapor is condensed to a liquid, followed by fractional distillation to produce an extremely pure, colorless, mobile liquid TiCl4 intermediate product, freezing at -24°C and boiling at 136°C. Much of the success of the chloride process lies in this stable intermediate which can be purified, tested, stored, reprocessed as necessary; and handled as a liquid or vapor. Being a vapor-phase distillation process, potentially discoloring trace contaminants can be virtually eliminated, with subsequent benefits to pigment color.
The second critical stage in the chloride process is oxidation of the TiCl4 to TiO2 pigment particles. Pure titanium tetrachloride is reacted with oxygen in an exothermic reaction to form titanium dioxide and liberate chlorine, which is recycled to the chlorination stage. The high temperature ensures that only the rutile crystal form is produced. After cooling, the gas stream passes through a separator to collect the pigment particles, and treated to remove adsorbed chlorine from the pigment. Because the reactor controls the efficiency of the conversion of the TiCl4 to TiO2 and the particle-size mean and distribution, its design is critical to efficient, high-quality pigment production.
Figure 10.1 compares the essential process steps in each process
Finishing, Surface Treatment
In both processes, the raw pigment may either be dried, milled, packed and sold or more likely, especially for rutile pigments, surface-treated to produce a range of special products for various applications. The end-product is either sold as a dry pigment or, especially in North America, converted to a slurry for the manufacture of water-based paints.