TiO2 pigment is made by two processes – the chloride process and the sulphate process.
The former uses chlorine gas and petroleum coke at high temperature (circa 900 degrees Celsius) to convert the titanium ore into metal chlorides in fluid bed chlorinators. The resulting TiCl4 is then separated and purified from other metal chlorides by selective distillation and cooling. The purified TiCl4 is converted to TiO2 by flash burning the chloride in oxygen at high temperature. The TiO2 undergoes some further processing to make suitable grades of pigment for various end market applications. The chlorine gas, generated during the oxygenation step, is recycled back to the chlorinators.
The sulphate process uses concentrated sulphuric acid to dissolve titanium feedstocks into metal sulphates. Iron and other impurities are removed from the solution by precipitation before the titanium is precipitated via hydrolysis. The resulting titanium hydroxide is then calcined to form TiO2 crystals, which undergo similar finishing steps to the chloride process to make a variety of pigment products.
These two pigment processes require quite different quality specifications and are a key determinant in the resource assessment of mineral sand deposits containing ilmenite. For example, the chloride process recycles the chlorine gas from TiCl4 but any chlorine within other metal chlorides is lost to waste. After feedstock costs, chlorine is one of the key cost drivers within the chloride process, hence chloride pigment producers have a preference for higher grade TiO2 feedstocks.
Historically chloride pigment producers used natural rutile as their preferred feedstock, but when rutile reserves were unable to match the growth demand, various upgraded titania products, such as titania slag and synthetic rutile, were made from ilmenite to match demand. Other feedstock impurities which are important to the chloride process include manganese, magnesium, vanadium, alumina, silica and radionuclides such as thorium and uranium (plus their respective daughter elements). Lastly, the grain-size of the titanium feedstock is important in the chloride process as fine-grained ore (generally less than 75 microns) tends to escape from the fluid bed chlorinator before reacting with the chlorine gas, thus resulting in ore losses.
- In the sulphate pigment process, the most important quality consideration is the ability of the feedstock to readily dissolve in sulphuric acid. Ilmenites with high FeO:Fe2O3 ratios are most suitable to sulphuric acid digestion and therefore unaltered ilmenite is most widely used as a feedstock for this process. More altered ilmenites do not effectively digest in sulphuric acid and are therefore considered unsuitable for the sulphate process. Instead, these altered ilmenites can be used in the chloride process. Hence the terms “sulphate ilmenite” and “chloride ilmenite” to the types of ilmenite commonly used in these respective pigment processes.
Natural rutile does not dissolve in sulphuric acid and is therefore avoided by sulphate pigment producers. Titanium slag containing high percentages of pseudobrookite is soluble in concentrated sulphuric acid and is preferred by European pigment producers, as this feedstock produces far less iron waste compared to ilmenite. The sulphate process is more tolerant to impurities in the feedstock, with the exception of chromium. This impurity is strong colourant and is difficult and expensive to remove from the TiO2 pigment. Traces of chromium impart a yellow tint to the final pigment and hence producers place strict specifications on the level of Cr2O3 acceptable within sulphate feedstocks.