Direct iron production at molten metal state from iron oxides by the sole application of electrical energy represents a possible route to decarbonize steel industry. Although chemically simple, this reaction is difficult to implement due to the problem of the multiple valence states of iron and to an operating temperature above 1811 K. Thermal, chemical, and electrical conditions have been identified based on thermodynamic considerations to carry out this reaction in a laboratory device. Experiments were undertaken to determine the contribution of the thermal level to the decomposition of iron oxide and to estimate the electronic current resulting from iron multiple valence states. The production of liquid iron was obtained resulting in recoverable samples produced at liquid state and from a faradaic process checked in real time by its accompanying anodic oxygen evolution.
Graphical AbstractKeywords Molten oxide electrolysis · Metal extraction · Oxide melts · Electrochemical engineering · High-temperature processing
The anodic charge transfer during the production of iron by Molten Oxide Electrolysis in an Al 2 O 3 -MgO-SiO 2 electrolyte has been investigated at 1793 K in dependence of the iron oxide concentration. Experiments were performed at laboratory scale using an asymmetric electrode configuration. The kinetic relation to the cell voltage was analyzed by a stepped linear scan voltammetry at various iron oxide concentrations up to 15 wt%. Complementary gas analysis allowed the derivation of the oxygen production yield. The obtained results show an electronic contribution to the overall conduction. This contribution diminishes in proportion with increasing iron oxide concentration. Charge transfer at the anode is accomplished by the oxidation of ferrous iron ions and of oxide anions. Conditions for the electrochemical charge transfer to occur solely by the oxidation of oxide anions exist for a limited cell voltage range at iron oxide concentrations of less than 10 wt%. For these concentrations a mass transfer limitation of oxide anions was detected with increasing cell voltage. However, a limit of the total current is absent as ferrous iron participates to the anode charge transfer at cell voltages above the mass transfer limitation of oxide ions.
The effect of iron oxide concentration on the conductive behavior of a molten oxide electrolyte has been investigated at 1823 K using stepped linear scan voltammetry. To maximize the current flow through the electrolyte the ohmic drop in the cell was minimized by shortening the electrode distance. The acquired current was then interpreted by means of an ohmic drop correction, taking into account the conductivity of the alumina-silicate electrolyte and the geometrical form factor of the cell. Via this methodology, a mass transfer limitation in dependence of the iron oxide concentration was identified. This mass transfer limitation vanishes above 7 wt pct of iron oxide where charge transfer starts to be limited solely by electrochemical reaction kinetics. In the analyzed range of concentration, an impact of iron oxide on electronic conduction was not measurable. In addition to these findings, the faradaic yield of the anode half-reaction was determined by the life-measure of O 2 -production. Hereby, a domain of an anodic yield close to 100 pct for various iron oxide concentrations was identified. Based on these findings, suitable conditions for the electrochemical production of liquid iron were determined.
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