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Electrorefining for direct decarburization of molten iron

Abstract

Recycling iron and steel is critical for environmental sustainability and essential to close material loops in circular economics. A major challenge is to produce high-value products and to control impurities like carbon in the face of stringent consumer requirements and volatile markets. Here, we develop an electrorefining process that directly decarburizes molten iron by imposing an electromotive force between it and a slag electrolyte. Upon anodic polarization, oxide anions from the slag discharge directly on carbon dissolved in molten iron, evolving gaseous carbon monoxide. In a striking departure from conventional practice that highly relies on reaction with solubilized oxygen, here electrorefining achieves decarburization by direct interfacial reaction. We demonstrate that this technique produces ultra-low-carbon steels and recovers silicon as a by-product at the cathode, requiring a low energy input and no reagents. We expect this process to be scalable and integrable with secondary steel mills.

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Fig. 1: Experiments demonstrate electrorefining for direct decarburization of molten iron containing 3.78 wt% carbon at 1,600 °C.
Fig. 2: Electrochemical techniques elucidate the decarburization pathway of molten iron at 1,600 °C.
Fig. 3: Schematic showing the direct decarburization mechanism during electrorefining of molten iron.
Fig. 4: Performance and benchmarking of the electrorefining process across different ranges of carbon concentration.

Data availability

The data supporting the findings of this study are available within the paper and its Supplementary Information files and from the corresponding author upon reasonable request. Charge-transfer resistances, interfacial capacitances, exchange currents, Tafel slopes, rate constants and data from electrorefining trials are given in the Supplementary Information files.

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Acknowledgements

G.A. gratefully acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (grant no. 503603), Ontario Centres of Excellence (grant no. 503616) and Tenova Goodfellow (grant no. 503604). We sincerely thank V. Scipolo and E. Malfa (Tenova Goodfellow) for their collaboration and invaluable feedback throughout the project.

Author information

Authors and Affiliations

Authors

Contributions

G.A. conceived and supervised the research. W.D.J. and G.A. designed the electrochemical cell and electrochemical testing approach. W.D.J. and J.P. performed the experiments and characterization. W.D.J. analysed the results and draughted the manuscript. All authors contributed to discussing and revising the manuscript.

Corresponding author

Correspondence to Gisele Azimi.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Materials thanks Geoffrey Brooks, Uday Pal, Umapada Pal and the other, anonymous, reviewers for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Plot of the interfacial capacitance determined by electrochemical impedance spectroscopy against carbon concentration in the alloy.

The strong correlation suggests carbon is associated with adsorbed species on the surface of molten iron.

Extended Data Fig. 2 Anodic polarization curves and exchange currents for Fe–0.45 wt% C at 1600, 1650, and 1700 °C.

Results show a the slope of the fitted lines become steeper with temperature as predicted by Tafel’s equation and b the activation energy for the Tafel exchange current is high (273 kJ mol–1) and consistent with activation control.

Extended Data Fig. 3 Interfacial capacitance of Fe–0.45 wt% C measured by electrochemical impedance spectroscopy as a function of temperature and anodic polarization.

a, Interfacial capacitance at the rest potential strongly increases with temperature which is consistent with pseudo-capacitance caused by adsorbed species on the surface of the electrodes. Double-layer capacitance should hardly change over a small interval of 100 °C. On the other hand, specific adsorption, which causes pseudo-capacitance, requires activation energy and explains the strong effect of temperature. b, Differential capacitance curves of Fe–0.45 wt% C at different temperatures show capacitance values diminish to low values on anodic polarization. As pseudo-capacitance caused by adsorbed intermediates depends on their surface coverage, the results are consistent with the proposed mechanism involving surface coverage of an intermediate approaching zero.

Extended Data Fig. 4 Electrochemical investigation of the cathodic half-cell corresponding to silicon reduction.

a, Schematic illustration of the electrochemical setup (to scale) for investigation of the silicon reduction reaction on a molybdenum electrode in the same slag used for electrorefining. b, Cyclic voltammogram at 1600 °C showing redox reactions for silicon and molybdenum (20 mV s–1, 95% iR compensation, 4th cycle, net charge passed was –0.71 C). c, Steady-state cathodic polarization curves reveal Tafel behaviour for silicon reduction at 1600, 1650, and 1700 °C (curves fully corrected for ohmic drop). d, Arrhenius plot of the Tafel exchange current reveals the activation energy (415 kJ mol–1) for the silicon reduction reaction. e, Diagnostic criteria derived for possible rate-determining steps and surface coverages (θ) of silicon reduction. See Supplementary Note 8 for more discussion.

Supplementary information

Supplementary Information

Supplementary Figs. 1–34, Tables 1–22, Notes 1–14 and video description.

Supplementary Video

Compilation of three parts showing submersion of counter electrodes in slag, gas evolution during operation of the electrorefining cell and raising of the electrodes after electrorefining. The video was filmed from a bird’s-eye view and is presented in real time.

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Judge, W.D., Paeng, J. & Azimi, G. Electrorefining for direct decarburization of molten iron. Nat. Mater. 21, 1130–1136 (2022). https://doi.org/10.1038/s41563-021-01106-z

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