The Laux process
Reaction of black: 9 Fe + 4 C6H5NO2 + 4 H2O 3 Fe3O4 + 4 C6H5NH2
Reaction of yellow: 2 Fe + C6H5NO2 + 2 H2O 2 FeO(OH) + C6H5NH2
Reaction of red: 2 Fe3O4 + ½ O2 3 Fe2O3
Initially, the Laux process was exclusively used to manufacture aniline (C6H5NH2) from nitrobenzene (C6H5NO2). Only when in 1925 the chemist, Dr Laux, found out that the iron oxide, being a by-product of this reaction, could be used, subject to certain chemical conditions, as an iron oxide pigment with extremely high colour strength. This process is primarily used for black iron oxide (e.g. Black 330), however, the reaction to get iron oxide yellow (e.g. Yellow 420) is also possible. From black iron oxide, it is also possible to produce red iron oxide pigments (e.g. Red 110) in an additional reaction stage.
The Precipitation process
2 FeSO4 + 4 NaOH + ½ O2 2 FeO(OH) + 2 Na2SO4 + H2O
In the precipitation process, iron sulphate is oxidized to yellow iron oxide pigment (Bayferrox® Yellow 920) in an alkaline environment with atmospheric oxygen. In this case, similar to the Penniman process, a yellow nucleus pigment is necessary in order to obtain yellow pigments of a high quality.
The Penniman process
2 Fe + ½ O2 + 3 H2O 2 FeO(OH) + 2 H2
In the Penniman process, iron scrap is oxidized to yellow iron oxide in a sulphate acid medium in the presence of atmospheric oxygen. In this case, iron sulphate acts as a catalyst. As to obtain yellow pigments of a high quality, yellow nuclei are furthermore necessary.