Kinetics and mechanisms of precipitation of nickel ferrite by hydrolytic stripping of iron (III)-nickel carboxylate solutions

Abstract

Magnetic nickel ferrite particles were precipitated from a tertiary C 10 monocarboxylic acid solvent extractant loaded with Fe(III) and Ni(II), in an aliphatic hydrocarbon diluent, by treating with water at 140–180 °C. The reaction, an example of hydrolytic stripping, can be represented as: x Ni(RCOO) 2 + (8−2x) 3 Fe(RCOO) 3 + 4H 2O = Ni x Fe (8−2x) 3 O 4+8 RCOOH x⩽1 Homogeneous precipitation occurred in the absence of any seed. Active alumina, colloidal graphite and magnetite had no effect on the precipitation kinetics, whereas activated carbon, silica and sintered nickel ferrite slowed precipitation. Hematite and nickel hydroxide were ineffective as seeds for nickel ferrite, hematite or nickel hydroxide. Heterogeneous precipitation occurred only in the presence of precipitated ferrite; the rate was linearly proportional to the seed area and independent of stirring speed, or water concentration above the stoichiometric requirement. At Fe:Ni concentration ratios of 2:1, the rate was inversely proportional to the concentration of free carboxylic acid, and independent of the iron and nickel concentrations. The rate increased for Fe:Ni ratios of 4:1, which gave an iron-rich ferrite, and decreased at ratios of 1:1. The apparent activation energy for the process was 124 kJmol −1. The behavior is discussed in terms of the species present in the metal-loaded carboxylate solutions, particularly the dominant mixed complex, thought to be NiFe 2(OH) 4(RCOO) 4(RCOOH) 2.