PRODUCTION OF MAGNETITE NANOPOWDER BY HYDROGEN REDUCTION FROM α-FeOOH HYDROXIDE COMPOUND UNDER ISOTHERMAL CONDITIONS

Abstract

In this work, the process of obtaining magnetite nanopowder Fe3O4 by hydrogen reduction from α-FeOOH hydroxide compound under isothermal conditions was studied. The α-FeOOH nanopowder was prepared in advance by chemical precipitation from aqueous solutions of iron nitrate Fe(NO3)3 (10 wt. %) and alkali NaOH (10 wt. %) at room temperature, pH = 11, under the condition of continuous stirring. The hydrogen reduction process of α-FeOOH powder under isothermal conditions was carried out in a tube furnace in the temperature range from 310 to 375 °С. The study of the crystal structure and phase composition of the powder samples was performed by XRD phase analysis. The specific surface area S of the samples was measured using BET method by lowtemperature nitrogen adsorption. The average particle size D of powders was calculated using the data of measuring the specific surface area S. The size and morphology characteristics of the particles were investigated by scanning and transmission electron microscopy method. The calculation of the kinetic parameters of the process of obtaining the Fe3O4 nanopowder under isothermal conditions was carried out using the Gray–Weddington model and the Arrhenius equation. It was found that the reduction process of the α-FeOOH nanopowder begins to accelerate strongly in the temperature range from 340 to 375 °С. The reduction rate constant at 375 °С is about 2 times that of the reduction at 310 °С. The value of the activation energy for the process of obtaining Fe3O4 nanopowder under isothermal conditions was ~ 36 kJ/mol, which indicates a mixed mode of limiting the kinetics of the process. It has been shown that the time-temperature regime of the reduction of the α-FeOOH hydroxide compound with holding at 340 °С for 3 h makes it possible to obtain a nanosized magnetite powder Fe3O4 with high purity at an increased rate of the process. The obtained Fe3O4 nanoparticles have a crystalline structure and generally have a polygonal or irregular shape, the size of which ranges from 30–80 nm with an average value of 65 nm, each of them being connected to several neighboring particles by isthmuses.