Abstract
The molten salt-based direct reduction process for reactive solid metal outperforms traditional pyrometallurgical methods in energy efficiency. However, the simplity and rapidity of this process require a deeper understanding of the interfacial morphology in the vicinity of liquid metal deposited at the cathode. For the first time, here we report the time change of electrode surface on the sub-millisecond/micrometre scale in molten LiCl-CaCl2 at 823 K. When the potential was applied, liquid Li-Ca alloy droplets grew on the electrode, and the black colloidal metal moved on the electrode surface to form a network structure. The unit cell size of the network and the number density of droplets were found to depend on the applied potential. These results will provide important information about the microscale mixing action near the electrode, and accelerate the development of metallothermic reduction of oxides.
Highlights
Direct electrochemical reduction of solid oxides (XOx, X = Ti, Zr, Hf, V, Nb, Ta, U, and other rare dispersed metals) in molten chloride (e.g. CaCl2, LiCl, KCl, NaCl, and their mixtures) is a simple and straightforward electrolytic metallurgical method, which outperforms traditional pyrometallurgical methods such as carbothermic and metallothermic reductions in terms of energy efficiency
The LiCl-CaCl2 eutectic melt operates at a low temperature compared with simple salts, so it is attracting attention as a molten salt with high energy efficiency and high reducing properties[18,19,20,21]
In the case of metallothermic reduction of solid oxide XOx(s) using electrodeposited liquid metal (Me = Ca, Li, or their alloys), the morphology of Me near the cathode is crucial when the oxide is reduced in the following mechanism[8,9,12,22]: MeO → Me2+ + O2−
Summary
Among the present systems of titanium metal production[1], the direct electrochemical decomposition of TiO2 in molten CaCl2 or LiCl has received special attention because of its simplity and low energy cost. Because the Ti-O binary system contains many lower oxides than TiO2, oxygen in a higher oxide is removed to form a lower oxide TiOy(y < 2) upon receiving electrical charge from the cathode. For higher productivity, another promising method (“OS process”) has been proposed that has better utilisation of the oxide anion transfer in CaCl2, because as much as 20 mol% CaO can dissolve in molten CaCl2 at 1173 K6–12. Colloidal Me could be generated at the liquid Me-molten salt interface, its morphology on the electrochemically precipitated cathode surface (which is the root cause of such phenomenon) at the sub-millisecond/micrometre scale remains poorly understood for high-temperature molten salts
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