Influence of the precipitation pH of magnetite in the oxidation process to maghemite

The aim of this work is to study about the synthesis of maghemite starting from cubic magnetite, using a mathematical model, which not only includes factors associated to the oxidation process of magnetite, but also factors related with precursor characteristics, such as the precipitation pH for the Fe(OH)2. Two samples of magnetite, obtained by oxidation of Fe(OH)2 to pH 8–9 and pH>11, respectively, were air-heated under different conditions, planned according to the fractional factorial design 26−3. The experiment was done in two zones, one (zone 1) that considered the values reported in the literature and tentative experiments and another (zone 2), whose values are far removed from the first set. Based on the results of the statistical analysis, it was concluded that the model was adequate to zone 1 (M and N samples) where the relevant parameters are those corresponding to: the independent term, the air flow, the time, the mass, the pH (of the magnetite precipitation) and the interaction of the time and pH variables. In zone 2 (O and P samples) it was impossible to write a corresponding equation due to parameter interactions; also, the model proved inadequate. The oxidation process of Fe3O4 to γ-Fe2O3 also depends on the precursor characteristics. The magnetite obtained at pH between 8 and 9 is oxidized to maghemite more completely than magnetite synthesized at pH>11.