With annual requirements of around one million metric tons, synthetic iron oxide pigments are the largest group of colored inorganic pigments. Their high weathering resistance, resistance to bleaching sunlight, and their warm, natural shades make iron oxide pigments very important raw materials for the coatings industry. Iron oxide red, also known as hematite, forms the most important subgroup. Applications for iron oxide red pigments include the coloring of building materials, plastics, and coatings.
Four processes in particular are used worldwide for the industrial production of iron oxide red pigments. The Laux process oxidizes iron with nitrobenzene. Yellow or black iron oxide pigments are formed in this process. The latter in turn can be converted to particularly high-quality red pigments through a calcination stage. Aniline is formed as a by-product of the iron oxide pigments and must be completely removed from the process wastewater. In addition to phase separations, this also requires effective biological cleaning to remove residual quantities efficiently and reliably. In the downstream calcination process necessary for producing iron oxide red pigments, small quantities of sulfur oxides are released and these need to be removed from the waste gas stream through a cleaning process. Starting with black iron oxide, also known as magnetite, an oxidation process takes place at temperatures of ~800 °C. Due to the exothermic oxidation reaction from magnetite to hematite, only very little energy in the form of heat is introduced in this step. Even when viewed overall, the Laux process (Figure 1) requires a particularly small amount of energy compared to all other iron oxide red processes due to the powerful oxidation effect of nitrobenzene and the release of reaction heat that can be used in the process.