The Role of Intermediates in the Process of Red Ferric Pigment Manufacture from FeSO4.7H2O

One method of industrial manufacture of red ferric pigments is based on the thermal decomposition of FeSO4.7H2O into α-Fe2O3(copperas red). The difficult reproducibility of the color quality of the final pigment is the main problem of this process. One of the factors that can influence the pigment color is contamination by some of the intermediates formed during the transformation process. The identification of two groups of intermediates is the basic result of an extensive laboratory investigation carried out using 57Fe Moessbauer spectroscopy and X-ray powder diffraction (XRD). The first group of intermediates includes sulfato-phases as FeSO4.H2O, FeSO4, Fe(OH)SO4, Fe2O(SO4)2, Fe2O(SO4)2.xH2O x element of (0,1), and Fe2(SO4)3. Thermally metastable polymorphs of iron(III) oxide, β-Fe2O3, γ-Fe2O3 and ε-Fe2O3, represent the other group. Moessbauer characterization of all intermediate products is given. A significant influence of β-Fe2O3 on the pigment color was found.