The Electrical Conductivity of Molten Oxide-Fluoride Cryolite Mixtures

A new way to reduce the energy consumption during the operation of powerful aluminum reduction cells is suggested via reducing the resistance of the electrolyte, i.e., increasing its electrical conductivity. The electrical conductivity of molten cryolite mixtures NaF-AlF3-CaF2-Al2O3 with cryolite ratio (CR) of 2.1–3.0 and content of CaF2 and Al2O3, up to 8 wt%, was measured at the temperatures from liquidus to 1300 K. Based on the experimental results, a multifunctional equation for the electrical conductivity of oxide-fluoride cryolite melts was evaluated. The experimental and calculated values of the electrical conductivity agree within 1.5%. The activation energy of the electrical conductivity of the NaF-AlF3-CaF2-Al2O3 melts was estimated. The activation energy of electrical conductivity for molten NaF-AlF3 mixtures with CR 3.0 and 2.1, determined by the most mobile cations Na+, increased from 15.8 kJ/mol up to 18.5 kJ/mol. It was found that CR had a greater impact on the activation energy than the changes in the Al2O3 or CaF2 concentrations. Based on the ratio of the activation energies of the electrical conductivity and the viscous flow, the correlation between the electrical conductivity and viscosity of molten cryolite mixtures NaF-AlF3-CaF2-Al2O3 was illustrated.

CATASTROPHIC OXIDATION OF AISI M4 ALLOY

In the annealing of some pure metals and various alloys, the phenomenon of accelerated oxidation is known, which is due to the fact that the resulting surface oxides fail to inhibit or completely stop the oxidation process. Vanadium ferrous alloys are rapidly oxidized during high-temperature annealing in oxygen atmospheres. The rate of oxidation is so great that it is considered a catastrophic oxidation. In this work, the AISI M4 alloy was investigated. The mechanism and model of the oxidation of alloy AISI M4 were developed on the basis of an analysis of the oxidation in a temperature range of 700–1000 °C. The holding time was 6 h. The alloy was analysed with an optical microscope and scanning electron microscopy equipped with EDS, WDS and EPMA detectors. It was found that below 850 °C, molten oxide is formed, which is predominantly of V2O5 composition. Above 850 °C, the oxide layer is multi-layered. The oxide layer consists of complex oxides, which formed with other elements. The X-ray analysis confirmed the presence of the following oxides in the oxide layer: Fe2O3, Cr2O3, FeVO4, V2O3 and Fe2WO6; and the second oxide may additionally be: V2O5, Cr2O5 and FeWO4.

Bubble Nucleation in Core-Shell Structure Molten Oxide-Based Membranes with Combined Diffusion-Bubbling Oxygen Mass Transfer: Experiment and Theory

Oxygen-selective membranes are likely to play a leading part in the future separation processes relevant to the energy engineering. A newly developed molten copper and vanadium oxide-based diffusion-bubbling membrane with...

Oxidation and electrical properties of chromium–iron alloys in a corrosive molten electrolyte environment

Chromium–iron (CrFe) binary alloys have recently been proposed to serve as the “inert” anode for molten oxide electrolysis (MOE). Herein, the effects of anodic polarization on physical and functional properties of CrFe anodes in the corrosive environment of MOE are studied via empirical observations and theoretical calculations. The findings indicate that the alloys form an inner chromia–alumina solid-solution covered by an MgCr2O4 spinel layer. A survey into the electrical properties of the detected oxides suggests that the layered oxide scale function as an efficient conductor of electricity at elevated temperature. The formation mechanism of the oxides is also investigated.