Thermal Decomposition of Compounds

Red iron oxide pigments are mainly obtained by roasting and calcining processes. α-Fe₂O₃ is obtained by oxidative calcination of all decomposable iron compounds. Decomposition of iron sulfate and α-FeOOH and the oxidation of Fe₃O₄ are industrially important.

After prior dehydration to the monohydrate, ferrous sulfate heptahydrate is oxidatively roasted at temperatures above 650°C, producing clear red-colored α-Fe₂O₃ pigments (copperas reds):

The sulfur dioxide formed can be processed to sulfuric acid. A disadvantage of this process is the contamination of the effluent with the soluble unroasted sulfate when the roasted charge is washed, due to the incomplete roasting of the sulfate.
Ferrous chloride, the salt available in the largest quantities from pickling plants, can also be thermo-hydrolytically decomposed. The iron oxide formed does not, however, possess the usual pigment qualities and is normally pelletized and reused in steel manufacture. FeCl₂ solutions from the Ruthner process are an exception. These are utilized in the hard- and soft-ferrite industries.

In principle, all the α-FeOOH or Fe₃O₄  pigments produced by the precipitation process, the Penniman-Zoph process and the Laux process can be calcined to red α-Fe₂O₃ pigments. The oxidative calcination of Fe₃O₄ from the Laux process is the most important process. A complete range of red tones can be obtained by rotary kiln calcination, by varying the particle size through the choice of material and temperature conditions. Working up only requires a grinding step, a washing step being unnecessary due to the purity of the starting materials. The calcination of yellow pigments also leads to needle-shaped red pigments. Transparent α-Fe₂O₃ pigments result from the calcination of very fine α-FeOOH particles. These can also be obtained by oxidative thermal decomposition of Fe(CO)₅.

Oxidation Processes in Aqueous Media

FeOOH-yeIlow,Fe₃O₄-black and Fe₂O₃-red pigments are obtainable by an air oxidation process from iron(Ⅱ) sulfate solutions with appropriate choice of reaction conditions and the use of certain nucleating seeds. The manufacture of α-FeOOH-yellow pigments is very important. The first reaction step is the production of the nucleating seeds, so important to the properties of the final pigment, by adding sodium hydroxide to a FeSO₄ solution and converting the resulting basic ferrous sulfate precipitate into a suspension of α-FeOOH-nuclei by air oxidation:

The crystalline α-FeOOH-nuclei are very small and can be used as transparent pigments. Pigments with covering properties are obtained by crystal growth on the α-FeOOH nuclei, by adding iron sulfate solution, sodium hydroxide and air, to obtain the particle size corresponding to the hue required (precipitation process).

Particles can also be grown by adding scrap iron and oxidizing it with air without the consumption of any other chemicals (Penniman-Zoph process).

The effluent from this process does not contain any additional salt, because the sulfuric acid formed reacts with the scrap to FeSO₄. The reaction is accelerated by maintaining the temperature at 70 to 90°C with steam.

In the manufacture of black pigments by the precipitation process, iron(Ⅱ) salt solutions are neutralized and oxidized with air at 90 to 100°C to a Fe(Ⅱ)/Fe(Ⅲ) ratio of 0.5 (Fe₃O₄). Using a similar process and appropriate nuclei formation conditions, small α-Fe₂O₃ nuclei are formed upon complete oxidation, which after growth yield easily dispersible red α-Fe₂O₃ pigments with high tinting strength (direct red process).

The long known reaction of nitrobenzene with metallic iron, previously used exclusively for the manufacture of aniline, has been so adapted by Laux (Bayer AG) that high
tinting strength Fe₃O₄ pigment is formed instead of the previously unstable iron oxide-containing residue. This pigment can either be used directly or calcined to strongly
tinted red α-Fe₂O₃ pigments. The addition of aluminum chloride to the reaction mixture yields yellow α-FeOOH pigments.

The raw materials for this “Laux process” are ground and sieved largely degreased cast iron or wrought iron chips. Particle size, metallurgical state and addition rate determine the quality of the resulting pigments. The reaction is started by feeding in steam and is exothermic:

Pigment is formed in the absence of bases and nitrobenzene is used as an oxidation agent. At the end of the reaction most of the aniline is separated off, the residue being distilled off by steam distillation. The pigment suspension is freed from excess iron chips by sieving and is then washed free of salt in a series of sludge thickeners and vacuum rotary filters. The pigment paste is dried in belt driers, drum driers, spray driers or fluidized bed driers and is then ground or micronized (pulverized in a jet mill).