Abstract
Molten salts play multiple important roles in the electrolysis of solid metal compounds, particularly oxides and sulfides, for the extraction of metals or alloys. Some of these roles are positive in assisting the extraction of metals, such as dissolving the oxide or sulfide anions, and transporting them to the anode for discharging, and offering the high temperature to lower the kinetic barrier to break the metal-oxygen or metal-sulfur bond. However, molten salts also have unfavorable effects, including electronic conductivity and significant capability of dissolving oxygen and carbon dioxide gases. In addition, although molten salts are relatively simple in terms of composition, physical properties, and decomposition reactions at inert electrodes, in comparison with aqueous electrolytes, the high temperatures of molten salts may promote unwanted electrode-electrolyte interactions. This article reviews briefly and selectively the research and development of the Fray-Farthing-Chen (FFC) Cambridge Process in the past two decades, focusing on observations, understanding, and solutions of various interactions between molten salts and cathodes at different reduction states, including perovskitization, non-wetting of molten salts on pure metals, carbon contamination of products, formation of oxychlorides and calcium intermetallic compounds, and oxygen transfer from the air to the cathode product mediated by oxide anions in the molten salt.
Highlights
The process is about electrolytic extraction of metals and alloys directly from their solid compounds in molten salts [1]
The main claim of the FFC Cambridge Process [1] is very general and states a method “for removing a substance (X) from a solid metal, a metal compound or semi-metal compound (M1X) by electrolysis in a fused salt of M2Y or mixture of salts, which comprises conducting the electrolysis under conditions such that reaction of X rather than M2 deposition occurs at an electrode surface, and that X dissolves in the electrolyte M2Y.”
When O2− is present in a CaCl2 or LiCl-based melt, which is inevitable in the FFC Cambridge Process, it can function as a phase transfer catalyst and transfer oxygen from the gas phase to the metal on cathode in the molten salt by the formation of peroxide (O23−) and superoxide (O−2) anions via reactions (41) and (44), respectively [77]
Summary
In December 1999, the University of Cambridge published an international patent on what is known widely as the Fray–Farthing–Chen (FFC) Cambridge Process. The process is about electrolytic extraction of metals and alloys directly from their solid compounds in molten salts [1]. Preliminary findings from the test of the FFC Cambridge Process were soon reported in a Letter to Nature for the extraction of titanium from titanium dioxide (TiO2) in molten calcium chloride (CaCl2) [2]. In the past two decades, world-wide research and development have confirmed the scientific principle and technical feasibility and flexibility of the process for the extraction of almost all metals listed in the periodic table and their alloys from their respective oxide or sulfide precursors [6–14]. The FFC Cambridge Process has versatile applications in other fundamental and industrial areas, such as near-net shape
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