SIPS2022 Volume 4 Kipouros Intl. Symp. Molten Salt, Ionic & Glass-forming Liquids & Powdered Materials

Editors: | F. Kongoli, R. Fehrmann, V. Papangelakis, I.Paspaliaris, G. Saevarsdottir. |

Publisher: | Flogen Star OUTREACH |

Publication Year: | 2022 |

Pages: | 100 pages |

ISBN: | 978-1-989820-40-7(CD) |

ISSN: | 2291-1227 (Metals and Materials Processing in a Clean Environment Series) |

We examined the kinetic and transport processes involved in Mg production from MgO via electrolysis at circa 1250 K with in a eutectic mixture of MgF2-CaF2 , a Mo cathode, and carbon anode. Exchange current densities, transfer coefficients, and diffusion coefficients of the electroactive species were established with a combination of cyclic and linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The cathode kinetics are described by a concentration dependent Butler Volmer equation. The exchange current density and cathodic transfer coefficient are 11 ± 4 A-cm-2 and 0.5 ± 0.12 respectively. The kinetics of the anode are described by two Tafel equations: at an overvoltage below 0.4 V, the exchange current density is 0.81 ± 0.2 mA-cm-2 with an anodic transfer coefficient of 0.5±0.1; above 0.4 V overvoltage the values are 0.14 ± 0.05 mA-cm-2 and 0.7±0.2 respectively. The diffusion coefficients of the electroactive species are DMg2+ = 5.2 e -5 ± cm2 s-1 and D_(〖Mg〗_2 ) OF_4^(2-) 7.2 ± 0.2 e-6 cm2-s-1. The ionic conductivity of the electrolyte is circa 2.6 S-cm-1. A 3D finite element model of a simple cell geometry incorporating the these kinetic and transport parameters suggest that 30% of the energy required to drive the electrolysis reaction can be supplied thermally for a current density of 0.5 A-cm-2.

Although the work has broad relevance to the vast number of current and developing industrial processes that use or may use molten halide systems, our motivation for doing this work was narrow: the development of a new process for producing Mg from MgO. Phase diagrams (solubility), density, electrical conductivity and viscosity of molten system (MgF2 – CaF2)eut – MgO have been investigated. The phase diagram of (MgF2 – CaF2 – LiF)eut – MgO and (MgF2 – BaF2)eut– MgO and the density of (MgF2 – CaF2 – LiF)eut – MgO have been also investigated. The solubility of MgO was measured by means of thermal analysis, the density by means of a computerized Archimedean method, electrical conductivity by means of a tube–cell (pyrolytic boron nitride) with stationary electrodes and the viscosity of the melt by computerized torsion pendulum method. It was found that the all investigated properties varied linearly with temperature in all investigated mixtures. On the basis of density values, the molar volume of the melts and partial molar volume have been calculated. The coordinates of the eutectic systems has been established as follows: (CaF2 – MgF2)eut – MgO as 0.30 mole % at 972 °C; (CaF2 – MgF2 – LiF)eut – MgO as 0.20 mole % at 941 °C; and (MgF2 – BaF2)eut. – MgO as 0.25 mole % at 883 °C. The density of the molten system of (CaF2 – MgF2)eut – MgO was found to be at 1000 °C: 2.687 g.cm-3 for the system with 0 mole % of MgO; 2.700 g. cm-3 for the system with 0.30 mole % of MgO and 2.728 g.cm-3 for the system with 0.50 mole % of MgO. The density of the molten system of (CaF2 – MgF2 – LiF)eut – MgO was found to be at 1000 °C: 2.875 g cm-3 for the system with 0 mole % of MgO; 2.690 g.cm-3 for the system with 0.20 mole % of MgO and 2.650 g.cm-3 for the system with 0.30 mole % of MgO. The viscosity of basic eutectic mixture of (CaF2 – MgF2)eut at 1000 °C was found to be 7.806 mPa.s.