Abstract
Cyclic and square-wave voltammetry methods are used to investigate the mechanism of the cathode process on molybdenum in a CaCl2-CaF2 melt at 750°C. It is shown that calcium reduces on an inert cathode in the form of its solution in catholyte at potentials more positive than are required for metallic calcium formation. The presence of Al2O3 in the catholyte increases currents of calcium reduction on the forward cathodic wave at potentials more positive than are required for metallic calcium formation. A decrease in the ohmic resistance between the anode and cathode is observed to have been caused by the appearance of electronic conduction during the calcium cathodic reduction in the studied melt. A mechanism for the reduction of Al2O3 in the catholyte during electrolysis of the CaCl2-CaF2 melt, including Ca+ subions and calcium cathode formation and the secondary reduction of Al2O3, is proposed. In order to demonstrate the supposed mechanism for the Al2O3 reduction, the electrolysis tests were performed in two laboratory-scale electrolyzers with different separation type of anolyte and catholyte. The necessity of separating the anolyte and catholyte for providing stable continual electrolysis in the CaCl2-based melt is demonstrated.
Highlights
Since Chen and Fray’s seminal paper published in 20001, over a thousand publications have been devoted to the further development and improvement of direct electroreduction or “electro-deoxidation” of metal oxides in CaCl2-based melts
Melt preparation.—The subject of testing was a 80CaCl220CaF2 melt with decreased liquidus temperature and lowered hydrolysis capability in comparison with pure CaCl2.22 The following individual reagents were used for its preparation: anhydrous CaCl2 and CaF2 – both chemically pure grade (99.9 wt%, CJSC “Vekton”)
Analysis of stationary polarization curves revealed that a number of electrons of the cathode process occurred at more positive potentials than calcium metal reduction is equal to one.[26]
Summary
Since Chen and Fray’s seminal paper published in 20001, over a thousand publications have been devoted to the further development and improvement of direct electroreduction or “electro-deoxidation” of metal oxides in CaCl2-based melts. Since the principal anode material used (as mentioned in most papers1–5) is graphite, the discharge of oxygen-containing ions on graphite is accompanied by the formation of anode gases (CO, CO2) which dissolves in the CaCl2-melts in the form of carbonate ions.[13,14,15,16] The latter interact with calcium dissolved in the melt or diffuse to the cathode with subsequent direct discharge to ultrafine carbon.[17,18] Analogously the evolution of CO and CO2 gases should have affected on the “electro-deoxidation” processes.[1,2,3,4,5] the presence of carbon in the melt changes its physical-chemical properties, significantly impacting the electrolysis parameters.[19,20] It seems that reactor with a separated anolyte and catholyte required for the electrochemical measurements as well as for electrolysis in the CaCl2-melts In this case, the mutual influence of the anode and cathode products on the results is minimized. This data supported by the elemental composition of the metals and alloys sintered by electrolysis of CaCl2-melts in alumina crucibles.[2,3,4,5,13,22]
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