Abstract

The effect of temperature, pH, the rate and time of oxidation, the concentration of ferrous ion in the starting suspensions as well as the amount of oxidant acting on the process of Fe₃O₄ synthesis by Fe(OH)₂ suspensions are investigated. After 2 h reaction at 90 °C under the oxidation of 10 g/L NH₄NO₃, solution containing 0.25–0.35 mol/L iron(II) ion initially would yield the greatest amount of Fe₃O₄, up to 95% Fe₃O₄ could be formed. pH of the solution should be controlled between 9.0 and 11.0. X-ray diffraction (XRD) analysis shows that the product has spinel structure, which indicated that the product is Fe₃O₄. Transmission electron microscopy (TEM) images show that the crystal size of ferrite is around 0.2 μm. The equilibrium composition of the synthesis reaction of Fe₃O₄ is optimized by the minimization of the free energy of thermodynamics. It was found that the optimal condition for the synthesis of Fe₃O₄ obtained through experiment is correspondent with that obtained through computer calculation. In the Fe₃O₄ formation area given by Kiyama [M. Kiyama, Conditions for the formation of Fe₃O₄ by the air oxidation of Fe(OH)₂ suspensions, Bull. Chem. Soc. Jpn. 47 (7) (1974) 1646–1650], the content of the product formed is not the same everywhere. The main factor that influences the content of the product is the amount of oxidant.

Keywords: Ferrite; Wet method; Pollution; Heavy metal; Wastewater treatment

Nomenclature

a_ij

atom number of the number j element in compound i

B_j

total mole number of the number j element

G

Gibbs free energy

m_i

molality of compound i

n_i

mole number of compound i

z_i

valency of compound i

Greek letters

γ_i

activity coefficient of compound i

λ_j

Lagrange undetermined constant

μ_i

chemical potential of compound i

standard chemical potential of compound i

Article Outline

Nomenclature

1. Introduction

2. Materials and methods

2.1. Chemicals and apparatus

2.2. Preparation of solutions

2.3. Reaction and sample handling

2.4. Orthogonal test design—synthesis of Fe₃O₄ with air as oxidant

2.5. Sample analysis

2.6. Thermodynamic assessment of synthesis of Fe₃O₄

2.6.1. Solution principles for Gibbs free energy minimization

2.6.2. Target function and its solving

3. Results and discussion

3.1. Synthesis of Fe₃O₄ with air as oxidant

3.2. Synthesis of Fe₃O₄ with NH₄NO₃ as oxidant

3.3. Thermodynamic assessment of Fe₃O₄ synthesis

4. Conclusions

Acknowledgements

References