Experimental Validation is Always Required for Molten Oxide Electrolysis Laboratory Crucibles

Abstract

Molten oxide electrolysis (MOE) is a promising electrochemical route to de-carbonize primary and secondary metal production. Developing MOE processes starts with laboratory experiments at temperatures > $$1000\,^\circ{\rm C}$$ 1000 ∘ C lasting ~10 hours and requiring long heating/cooling times to protect furnace hardware. Before investigating MOE processes, crucibles must be selected that tolerate the required temperatures while minimizing chemical interactions with the oxide to control melt contamination and contain the melt. Unfortunately no general procedure guiding MOE crucible selection is documented. Here we focus on laboratory crucibles in air for two MOE melts: titania-sodia at $$1100\,^\circ{\rm C}$$ 1100 ∘ C and neodymia-boria at $$1300\,^\circ{\rm C}$$ 1300 ∘ C . After shortlisting generic crucible materials using Ashby’s method, thermodynamic predictions were made for all-oxide titania-sodia charges using FactSage and cup test experiments were conducted on (i) all-oxide and carbonate charges for titania-sodia and (ii) neodymia-boria charges. While magnesia was predicted to be the best crucible for the titania-sodia melt, alumina was the best choice for both oxide and carbonate charges. The grain boundary networks of both magnesia and YSZ were infiltrated by the oxide and carbonate charges. Platinum was the best crucible for neodymia-boria melts. We show that compositional control during long, high-temperature MOE experiments requires experimental validation for specific chemistries every time.

Evaluating Phase Relations in Molten Oxides for Electrochemical Operating Windows for Selective Metal Reduction

“…[27] While our ultra-high temperature electrolytic cell vertical tube furnace has a maximum operating temperature of 1550 °C, operating at higher temperatures increases the difficulty of identifying a suitable crucible material that will not interact with the melt or break during experimentation. [19] In this work, a maximum temperature of 1400 °C was used where binary oxide pairs must be liquid over a suitable composition range for experimentation (�0.25 X M x O y ). The final condition for scoping binary oxide pairs with tantalum or neodymium oxides was that binary phase diagrams selected contained a congruently melting line compound with eutectics either side, like the TiO 2 À Na 2 O system, to investigate if the reduction potential of the metal of interest via MOE would become favorable at higher metal oxide compositions.…”

“…The next stage for testing electrolytes for MOE is to identify compatible crucible and electrodes materials. [19] This is also easier for simplified binary oxide system when compared to complex, multicomponent systems.…”