TiO2 is produced from either ilmenite, rutile or titanium slag. Titanium pigment is extracted by using either sulphuric acid (sulphate process) or chlorine (chloride route). The sulphate process employs simpler technology than the chloride route and can use lower grade, cheaper ores. However, it generally has higher production costs and with acid treatment is more expensive to build than a chloride plant. But the latter may require the construction of a chloralkali unit.
The chloride route produces a purer product with a tighter range of particle size, but anatase pigments can only be produced by the sulphate route. The sulphate route is perceived to be less environmentally friendly but acid recycling or neutralisation, combined with other byproduct developments, can make it as clean as the chloride route.
In the sulphate route, there are three main stages. The ore, usually an ilmenite, is dissolved in sulphuric acid to form a mixture of sulphates. Any iron is removed from the solution so the colour of the final product is not spoiled. The titanyl sulphate is then hydrolysed in solution to give insoluble, hydrated titanium dioxide.
The final stage involves heating the solid in a calciner to evaporate the water and decompose the sulphuric acid in the solid. It also turns the solid into seed crystals which can be milled to the size needed. These crystals can be coated with another substance, such as aluminium oxide, to make the titanium dioxide mix more easily with liquids or extend the life the paint manufactured from them.
There are two main stages to the chloride process. First, the dry ore is fed into a chlorinator together with coke and chlorine to make titanium chloride. Once the fluid bed has been preheated, the heat of reaction with the chlorine is sufficient to maintain the temperature and recycled liquid titanium chloride may be used to control the temperature.
The next step involves the oxidation of titanium chloride by burning it in oxygen together with another combustible gas (often carbon monoxide). By adding seed crystals, the titanium dioxide is formed as a fine solid in a gas stream and is filtered out of the waste gases. Crystal growth is controlled by adding nucleating agents to the gas stream and the products are cooled by mixing with chlorine gas. The product is then washed and dried before milling and surface treatment.
The surface treatment of the base pigment is very important and the surface finishing unit can account for up to one-third the cost of a titanium dioxide plant. The treatment is needed to maximise optical properties, improve durability and reduce yellowing, and improve dispersibility.