Reduction of Solid Al₂O₃with Electrolysis of CaCl₂-Based Melt

Abstract

Cyclic and square-wave voltammetry methods are used to investigate the mechanism of the cathode process on molybdenum in a CaCl 2 -CaF 2 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 Al 2 O 3 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 Al 2 O 3 in the catholyte during electrolysis of the CaCl 2 -CaF 2 melt, including Ca + subions and calcium cathode formation and the secondary reduction of Al 2 O 3 , is proposed. In order to demonstrate the supposed mechanism for the Al 2 O 3 reduction, the electrolysis tests were performed in two laboratory-scale electrolyzers with different separation type of anolyte and catholyte. Since Chen and Fray's seminal paper published in 2000 1 , over a thousand publications have been devoted to the further development and improvement of direct electroreduction or "electro-deoxidation" of metal oxides in CaCl 2 -based melts. However, despite this significant attention from researchers from all over the world, 2-5 this promising method is yet to be meaningfully applied at an industrial scale. This is likely to be due to several important mechanisms in the metal oxide reductions remaining to be adequately explained. Thus, in our previous theoretical research, 6 we have shown that non-conductive crystal oxides can be reduced by calcium solution (in a form of Ca, Ca + or CaCl) in its chloride. 7-10 The latter can be formed at the inert cathode during electrolysis or due to introduction of calcium metal into the melt. Dissolution of alkali or alkaline earth metal in their halides reveals the nature of such melts, which are representative of ion-electron liquids. 7,8 Therefore, the nature of the melts under study enables using the innovative method only in electrolysis lab tests. Moreover, the presence of calcium (in a form of solution or metal phase) in the molten calcium chloride leads to the appearance of electronic conduction that causes a side reaction of calcium oxidation in the anolyte and at the anode. 11,12Since the principal anode material used (as mentioned in most papers [1][2][3][4][5] ) is graphite, the discharge of oxygen-containing ions on graphite is accompanied by the formation of anode gases (CO, CO 2 ) which dissolves in the CaCl 2 -melts in the form of carbonate ions. 13-16The 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 CO 2 gases should have affected on the "electro-deoxidation" processes.1-5 Finally, the presence of carbon in the melt chang...

Electrowinning of Aluminum and Scandium from KF-AlF3-Sc2O3 Melts for the Synthesis of Al-Sc Master Alloys

“…Similar polarization dependencies with the rapid consumption of reduced forms were obtained earlier. 33,34 The potassium electrowinning was not achieved at low sweep rates, whereas the typical slow discharge wave K of the potassium electrowinning at potential negatively than −1.3 V was obtained at high sweep rates.…”

Electrochemical Reduction of TiO2/Al2O3/C to Ti3AlC2and Its Derived Two-Dimensional (2D) Carbides

“…[22][23][24][25][26][27] In addition, the direct electroreduction of solid Al 2 O 3 has also been investigated in different molten salts, such as CaCl 2 -NaCl, CaCl 2 -LiCl, and CaCl 2 -CaF 2 . [28][29][30] Noteworthily, Al has a low melting point of 660 • C, which means that formed Al is liquid when the electrolysis was carried out at a higher temperature (such as 900 • C). In order to overcome this z E-mail: xzou@utexas.edu; luxg@shu.edu.cn drawback, the mixed oxides can be used as the precursor to directly synthesize refractory alloys, such as Ti-6Al-4Al.…”

Electrolytic Reduction of Solid Al2O3to Liquid Al in Molten CaCl2

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Fray et al reported the direct electrolytic reduction of TiO 2 pellets in a CaCl 2 -based melt, known as the FFC Cambridge process. [1][2][3][4][5][6] This process has the potential to replace the conventional Kroll process for producing Ti metal, owing to its simple system and relatively low temperature.…”

“…Meanwhile, electrolytic reductions of various kinds of solid oxides in CaCl 2 and/or CaCl 2 -based molten salts have been investigated by many researchers in the last two decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Fray et al reported the direct electrolytic reduction of TiO 2 pellets in a CaCl 2based melt, known as the FFC Cambridge process. [1][2][3][4][5][6] This process has the potential to replace the conventional Kroll process for producing Ti metal, owing to its simple system and relatively low temperature.…”

“…For the electrochemical reduction of Al 2 O 3 , there have been several studies conducted in CaCl 2 -based melts. [13][14][15][16][17] Yan investigated the reduction of Al 2 O 3 tube using a Nb box-type cathode in molten CaCl 2 -NaCl at 1173 K. 13 In this study, Al-rich Al-Ca alloy droplets with 84.9-98.4 at%Al was obtained. Yan and Fray also reported the electrochemical reduction of Al 2 O 3 in CaCl 2 -LiCl at 973 K and CaCl 2 -NaCl at 1173 K. 15 They reported that calcium aluminate was formed as an intermediate product during the electrolysis.…”