Electrolysis of solid oxides in molten CaCl2 has been proven a straightforward process to extract various metals thanks to the wide electrochemical window of CaCl2 and CaO which is soluble in molten CaCl2. However, the CaO produced during the cathodic reduction of oxides not only shuttles O2−, but also captures CO2 to generate CO32− that can be reduced to carbon at the cathode, resulting in decreasing current efficiency and contaminating the cathodic products by carbon. To solve this problem, we employed CaSiO3 to replace CaO to suppress/avoid the formation of CO32− and thereafter prevent the carbon deposition. The suppression of CO32− formation is because SiO32− is thermodynamically more stable than CO32−, i.e., the CO2 generated on the graphite anode can not be captured by SiO32−. In molten CaCl2−8.0 wt% CaSiO3, Cr2O3 and SiO2 are reduced to Cr and Si with much less carbon deposition than in molten CaCl2. As a result, the current efficiency increases from 21.4% to 51.9% for the reduction of Cr2O3 and 37.1% to 72.2% for the reduction of SiO2. Although introducing SiO32− will slightly decrease the electrochemical window of CaCl2, it will be a promising method to avoid carbon contamination and increase current efficiency especially for Si production.
Kai Zheng,Fanghai Lu,Qiong Long,Cuilian Shi,Xingli Zou,Zhanling Zhang,Xiaoling Zuo,Chaoyi Chen,Xiangdong Su
The direct electrolytic production of TaC nanoparticles from a Ta 2 O 5 /C precursor in molten CaCl 2 by using solid oxide membrane (SOM) technology was investigated in this work. A Ta 2 O 5 /C pellet pressed under 6 MPa was used as the cathode, and a yttria-stabilized zirconia (YSZ) tube filled with carbon-saturated liquid tin served as the anode to control the electrolysis process. The phase composition and morphology characteristics of the electrolysis products were investigated. The results show that TaC nanoparticles could be obtained directly from a Ta 2 O 5 /C mixture in molten CaCl 2 at 1000 °C and 4.0 V within 4 h.
This study focuses on understanding the behavior of oxide ions (O 2– ) in molten CaCl 2 , which is important for applications in molten salt electrolysis. The dissolution mechanism of solid CaO and mass transfer characteristics of O 2– in molten CaCl 2 were investigated through comprehensive chemical analysis. The results revealed that the incongruently melting compound in the CaCl 2 –CaO system is Ca 4 OCl 6 (CaO·3CaCl 2 ), with a peritectic point observed at 19 mol pct of CaO and 1104 K. The previously reported compounds Ca 3 OCl 4 (CaO·2CaCl 2 ) and Ca 5 OCl 8 (CaO·4CaCl 2 ) were never detected, while only Ca 4 OCl 6 (CaO·3CaCl 2 ) was observed under air-tight conditions. The dissolution behavior of solid CaO in molten CaCl 2 is highly temperature dependent, with a critical threshold identified at 1104 K. Below this temperature, CaO reacts with CaCl 2 to form Ca 4 OCl 6 , which subsequently dissociates into simple ions (Ca 2+ , Cl – , and O 2– ). Above 1104 K, CaO directly dissociates to produce O 2– . Although temperature influences the formation and dissociation of Ca 4 OCl 6 , it has minimal impact on the dissociation of CaO and the mass transfer of O 2– in the melt.
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