Electrochemical Kinetics during Production of Liquid Iron at 1550 C Via Molten Oxide Electrolysis
Authors: Wiencke Jan-Christian , Lavelaine Hervé , Rapin Christophe , Panteix Pierre-Jean , Petitjean Carine 
Published: February 2020

Molten oxide electrolysis is a potentially new route to lower CO2 emissions in steel industry. The concept, pioneered by researchers of the M.I.T.[1], enables the use of renewable energies for electrolytic production of molten iron. The challenges of this procedure are elevated temperatures above 1550°C, the choice of suitable electrode materials and the up-scaling of the laboratory cells to pre-industrial dimensions. The general electrochemical reactions at 1600°C at the electrodes are:
2Fe2+ +4e-→ 2Fe (Cathode)
2O2- → O2 + 4e-(Anode)
Choice of electrolyte composition has to be done following the premise of a maximum in faradaic efficiency, which is highly dependent on the ratio between the ferrous and ferric iron in the slag. If the inherent ferric proportion is high, the production of liquid iron can be disturbed by two different processes. At low iron concentration in the melt, ferric iron is reduced at the cathode and subsequently migrates towards the anode, where it is reoxidized. At iron concentrations above 20 wt%, onset of electron charge hopping between the two species is likely to occur [2]. One possibility to avoid both scenarios is the utilization of a silica based composition, in which ferric iron is unstable.
According to these requirements, a device to conduct molten oxide electrolysis in laboratory scale was built, comprising an electrolyte based on an alumina-silicate composition on which current is applied through 2 Pt wire-electrodes at 1550°C. After slow cooling of the cell, the produced metal is obtained as a small sphere at the cathode at the bottom of the cell (Figure 1). The maximum weight of the deposit produced so far is 500 mg with an experimental duration exceeding 11 hrs. Recovered metal shows a high purity, suggesting a high selectivity of the electrochemical process. Depending on the imposed potential, stable current signals of up to 150 mA were achieved. Faradaic current efficiency lies between 70-80 % for considering the Mol O2 produced at the anode during electrolysis.
Ongoing research in ArcelorMittal is dedicated to the characterization of the electrolyte voltammetric assessments (Figure 2). Reduction potentials of ferrous iron and silica in the experimental setup were determined to 0.98 and 1.6 V respectively. Furthermore the currently used electrolyte exhibits a current limitation in correlation with the total content of iron oxide in the melt. Behavior of this limitation implies that the addition of iron oxide has a strong impact on transport properties of the melt. To validate this hypothesis additional 2-electrode configurations have been designed (Figure 3). In these designs a large surface of one of the electrodes allows the study of the kinetic overpotential of the other. Experiments performed so far indicate the presence of a process rate limitation at the cathode for low iron concentrations. Electrochemical results are complemented with thermodynamic calculations assisted by thermochemical software [3]
[1] Sadoway D, Kheptal D, Ducret A. (2004) TMS Charlotte
[2] Barati M. and Coley K.S. (2007) Metallurgical and Materials Transactions B, pp. 51-60
[3] Gatellier C., Gaye H., Lehmann J., Zbaczyniak Y. (1992) CIT Revue de Métallurgie, pp. 887-888

Figure 1