Intrinsic Kinetics of Reduction of Hematite Concentrates by H2 at Moderate Temperatures

In this work, the intrinsic kinetics of reduction of hematite concentrates was determined in the temperature range from 700 degrees C to 900 degrees C, and an integrated rate law was formulated for the same. Hematite concentrates were isothermally reduced in hydrogen atmosphere using a vertical tubular furnace equipped with a micro-balance. The influence of key process variables such as hydrogen (H2) flow rate, particle size of concentrates, reduction temperature, and partial pressure of hydrogen gas on the reduction rate was ascertained. Characterization of different phases, morphological changes, and chemical compositions were analysed using X-ray diffraction (XRD), Scanning electron microscopy (SEM), and X-ray fluorescence (XRF), respectively. The reduction rate became independent of the gas flow rate once a critical flow rate was achieved, but it was found that an increase in the reduction temperature monotonically increased the rate of reaction in the range studied. Furthermore, the intrinsic kinetics of hematite reduction by H2 were best described by a nucleation and growth mechanism with an average Avrami parameter n\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$n$$\end{document} = 2, while the partial pressure dependency of H2 gas on the reduction rate was found to be first order. The reduction rate also depended on the average particle size of the concentrates, but the dependency was found to be relatively weak, in the particle size range (56-181 mu m) tested in this work. The activation energy for the overall reduction of hematite to iron was found to be 32.7 kJ/mole. Based on the above, the intrinsic kinetics of hematite reduction by H2 can be described by the following integrated form of the rate law: -ln1-X12=40.80xexp-32700RTxd-0.23xpH2-pH2OK<middle dot>t,\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\left[-\text{ln}\left(1-X\right)\right]}<^>\frac{1}{2}=40.80\times \text{exp}\left(\frac{-32700}{\text{RT}}\right)\times {d}<^>{-0.23}\times \left({p}_{{\text{H}}_{2}}-\frac{{p}_{{\text{H}}_{2}\text{O}}}{K}\right)\cdot t,$$\end{document}where X is a degree of reduction, R is the gas constant, T is the reduction temperature in Kelvin, d is the particle size in mu m, t\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$t$$\end{document} is time in minutes, and K is the equilibrium constant for wustite to iron reduction by H2. T\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document} in the above expression lies within the range 973-1173 K and d lies within 56-181 mu m.