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Interaction of Iron with Alumina Refractory Under Flash Ironmaking Conditions

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Abstract

A novel flash ironmaking technology (FIT) based on the direct reduction of iron ore concentrate by a reducing gas such as hydrogen, natural gas, or coal gas has been developed at the University of Utah. A proper choice of refractories is expected to play an important role in the development of FIT and its scale-up. In this work, the interaction of iron with alumina refractory under flash ironmaking conditions has been investigated. A thermodynamic basis for the interaction phenomena has been developed by considering the Fe-Al-O system, and a kinetic model based on solid-state diffusion is proposed to describe the growth of the hercynite (FeAl2O4) spinel formed as a result of the interaction between Fe, O (from H2O and CO2), and Al2O3. Experiments were conducted with Fe powders and pure Al2O3 in the temperature range 1200 to 1500 °C under gas atmospheres relevant to flash ironmaking, and the reacted samples were analyzed using XRD, SEM-EDX, and EPMA techniques. The analyses of reacted samples confirmed the formation of the hercynite (FeAl2O4) spinel phase and the thickness of the FeAl2O4 spinel as measured using elemental line scans in SEM-EDX and EPMA varied in the range of 0.5 to 1.3 mm. Results showed that the proposed model suitably describes the growth of the hercynite spinel layer, which obeyed the parabolic rate law at all temperatures. The parabolic rate constants were obtained and the diffusion-controlled growth of the FeAl2O4 layer strongly depended on temperature. Furthermore, using the kinetic model, an expression for the effective diffusivity (\( \overline{{D_{\text{eff}} }} \)) was obtained and its values at the experimental temperatures were determined. The solid-state diffusion was an activated process, as expected, with an activation energy value of 231 kJ/mol.

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Abbreviations

\( a_{i} \) :

Activity of component i (–)

\( \left. {a_{\text{FeO}} } \right|_{z = 0} \) :

Activity of component i at z = 0, i.e., at the iron (Fe)-hercynite (FeAl2O4) phase boundary (–)

\( \left. {a_{\text{FeO}} } \right|_{z = \delta } \) :

Activity of component i at \( z = \delta,\)i.e., at the hercynite (FeAl2O4)-alumina (Al2O3) phase boundary (–)

c i :

Molar concentration of cation i in hercynite (FeAl2O4) (mol/m3)

k 1 :

Parabolic rate constant for the diffusion-controlled growth of the hercynite (FeAl2O4) layer (m2/s)

\( n_{i} \) :

Number of moles of component i (–)

\( p_{i} \) :

Partial pressure of component i (atm)

t :

Time (s)

z :

Position (m)

\( D_{i} \) :

Diffusivity of cation i any point in hercynite (FeAl2O4) (m2/s)

\( \overline{{D_{i} }} \) :

Average diffusivity of cation i in hercynite (FeAl2O4) layer (m2/s)

\( \overline{{D_{\text{eff}} }} \) :

Effective diffusivity for the diffusion-controlled growth of the hercynite (FeAl2O4) spinel as defined in Eq. [31], averaged over hercynite layer (m2/s)

E :

Young’s modulus (GPa)

E a :

Activation energy for \( \overline{{D_{\text{eff}} }} \) (kJ/mol)

F :

Faraday constant (Coulomb/mol)

H :

Hardness (GPa)

\( J_{i} \) :

Molar flux of cations of i any point in hercynite (FeAl2O4) (mol/m2.s)

K 1 :

Equilibrium constant for the equilibrium between \( \text{Fe (s)}, {\text{O}_{2}} (\text{g}),\)\( \text{Al}_{2} \text{O}_{3} \text{(s)}, \) and \( \text{FeAl}_{2} \text{O}_{4} \text{(s)} \) per Reaction [1] (–)

\( K_{2} \) :

Equilibrium constant for the equilibrium among \( \text{FeO (s,l)}, \)\( \text{Al}_{2} \text{O}_{3} \text{(s)}, \) and \( \text{FeAl}_{2} \text{O}_{4} \text{(s)} \) per Reaction [35] (–)

K 3 :

Equilibrium constant for equilibrium among \( \text{Fe (s)}, {\text{O}_{2}} (\text{g}) \) and \( \text{FeO (s,l)} \) as per Reaction [37] (–)

R :

Universal gas constant (J/mol·K)

T :

Temperature (K)

V m :

Molar volume of hercynite (FeAl2O4) (m3/mol)

\( X_{i} \) :

Mole fraction of component i (–)

\( \alpha \) :

expansion coefficient (K−1)

\( \beta \) :

Ratio of diffusivities of the cations in hercynite (Fes defined in Eq. [23] (–)

δ :

Thickness of hercynite (FeAl2O4) layer (m)

ζ i :

number of coulombs of charge associated with 1 mole of cation i (–)

\( \eta_{i} \) :

Electrochemical potential of cation i in hercynite (FeAl2O4) (J/mol)

\( \nabla \eta_{i} \) :

Gradient in the electrochemical potential of cation i in hercynite (FeAl2O4) (J/mol·m)

\( \mu_{i} \) :

Chemical potential of component i in hercynite (FeAl2O4) (J/mol)

\( \left. {\mu_{i} } \right|_{z = 0} \) :

Chemical potential of component i at \( z = 0,\)i.e., boundary (–) (J/mol)

\( \left. {\mu_{i} } \right|_{z = \delta } \) :

Chemical potential of component i at \( z = \delta,\)i.e., at the hercynite (FeAl2O4)-alumina (Al2O3) phase boundary (–) (J/mol)

\( \mu_{i}^{0} \) :

Chemical potential of component i at standard state (J/mol)

\( \nabla \mu_{i} \) :

Gradient in the chemical potential of component i in hercynite (FeAl2O4) (J/mol·m)

\( \Updelta \mu_{i} \) :

Difference between the chemical potentials of component i at the iron (Fe)-hercynite (FeAl2O4) and hercynite (FeAl2O4)-alumina (Al2O3) phase boundaries : \( \left( {\Updelta \mu_{i} = \left. {\mu_{i} } \right|_{z = \delta } - \left. {\mu_{i} } \right|_{z = 0} } \right) \) (J/mol)

\( \varphi \) :

Electrical potential (J/Coulomb)

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Acknowledgments

The help received from Dr. Barbara Nash, Department of Geology and Geophysics, University of Utah, in obtaining the elemental line scans using the Electron Probe Microanalyzer (EPMA) is gratefully acknowledged. The authors acknowledge the financial support from the U.S. Department of Energy under Award Number DE-EE0005751 with cost share by the American Iron and Steel Institute (AISI) and the University of Utah.

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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  1. Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT, 84112, USA

    Rahul Sarkar & H. Y. Sohn

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  1. Rahul Sarkar
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Manuscript submitted December 17, 2018.

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Sarkar, R., Sohn, H.Y. Interaction of Iron with Alumina Refractory Under Flash Ironmaking Conditions. Metall Mater Trans B 50, 2063–2076 (2019). https://doi.org/10.1007/s11663-019-01610-3

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