(Invited) Solid Oxide Membrane (SOM)-Based Electrolytic Production of Metals

Abstract

Metal producers, such as primary aluminum, ferroalloy, iron and steel, lead, magnesium, and zinc, employ carbothermic, metallothermic, halide reduction, and electrometallurgical methods, to produce the metals and in the process generate significant amounts of pollutants including greenhouse gases (GHG). In the US alone, these industries produce more than 100 million metric tons of CO2e. Because of its impact on the environment there is an urgent need to decarbonize this industrial sector. Electrolysis offers the opportunity to substitute clean electrical energy for fossil fuel energy sources. But electrolytic oxide reduction typically requires carbon anodes and emits CO2 and perfluorocarbons with very high global warming potential. As of this writing, no oxygen-producing inert anode technology has yet scaled commercially beyond a single pilot electrolysis cell.For metal oxide reduction a cost-effective, energy-efficient and low-emissions approach to the inert anode uses a solid oxide membrane (SOM) as a selective electrolyte between the molten salt and anode for the generic production of metals and alloys. The SOM-based electrolysis technology produces metals (Me) and/or their alloys from their respective oxides (MeOx) through direct electrolysis of the oxides: MeOx → Me (cathode) + x/2 O₂ (anode), where x is the stoichiometric amount of oxygen in the metal oxide.The SOM-based electrolytic process brings various advantages such as simplified design, lower cost, lower energy use, lower use of consumables, higher efficiency, zero emissions, and higher purity of metal and oxygen produced. The SOM process features the utilization of an oxygen-ion-conducting membrane-based anode, typically made of stabilized zirconia such as yttria-stabilized zirconia (YSZ) with an oxygen producing electrode on one side of the membrane, a supporting ionic molten salt electrolyte bath, typically an alkali fluoride salt mixture, and a cathode to collect the reduced metal. The oxygen-ion conducting membrane is placed inside the salt and it separates the cathode, also placed in the salt, from the oxygen electrode. The metal oxide is electrolyzed inside the salt medium by applying an electric potential between the cathode and the anode. The metal ions are reduced at the cathode and the oxygen ions migrate through the salt and the membrane and are oxidized at the oxygen electrode. The oxygen produced is pure due to the membrane selectivity, a valuable byproduct, and the membrane completely isolates the anode from the salt bath and metal produced at the cathode. This isolation simplifies anode materials selection, as the anode does not need to be robust to molten salt corrosion. Also, because of the oxygen-ion conducting membrane, the applied potential does not oxidize the halide ions nor the multivalent cations.Since the late 1960s, researchers have recognized and investigated the use of zirconia solid electrolyte for metals production and refining; however, limited progress was achieved due to the poor stability of zirconia membrane in the salt medium. Our work on YSZ membrane stability during SOM electrolysis using eutectic molten salt composition in CaF2 –MgF2 system demonstrated that membrane stability can be greatly enhanced by matching component activity and optical basicity in the salt with that of the membrane. It was shown that the oxide solubility in the salt can be controlled through tailored additions of more stable alkali oxides and monitoring the heat of mixing; higher heat of mixing promotes oxide solubility and vice-versa. Following our initial work, the SOM process has been applied to reduce various metal oxides or oxide compounds to the respective metals or alloys.This talk will provide generic guidelines with examples for the selection of the cell components, including salt electrolyte, cathode, anode configuration, and current collectors. Examples will include both lab and industrial scale applications.