Kinetics of Ferrite Formation in a Three-Phase System

Abstract

We have measured the change in concentrations of the ferrous and divalent metal ions over time in a three-phase system consisting of air, a pH-controlled solution of ferrous and various divalent ions (Co, Zn, Ni and Cd), and precipitated ferrite. To quantitatively interpret the kinetics of the ferrite formation process, we proposed a reaction model which takes into account the absorption rate and the chemical reaction rates in the aqueous phase. The concentration changes early in the reaction can be expressed by a model with two fitting parameters. The value of the fitting parameter k₁, which expresses the oxidation rate of the ferrous ion, greatly depends on the specific divalent metal ion used and its initial concentration. The decrease in the relative concentration of the ferrous ion (Y = C_A/C_A0) correlates with the decrease in the concentration of the divalent metal ions, (X = C_M/C_M0). To quantitatively express the relationship between X and Y, using the model we derive a theoretical equation with a fitting parameter, (k₂/k_M), which is the ratio of rate constants for magnetite and ferrite formations. The value of this parameter decreases for a lower initial concentration ratio of ferrous ion to divalent ion in the following order: cobalt, zinc, nickel and cadmium. The order corresponds to the fixation of the divalent metal ions into the ferrite at equilibrium.