Zero-direct-carbon-emission aluminum production by solid oxide membrane-based electrolysis process
The traditional aluminum production process (Hall-Héroult process) involves electrolyzing the alumina dissolved in the molten cryolite salt. This process is energy intensive and emits massive amounts of CO2 and other greenhouse gases. The market demand of aluminum and the environmental impact of the current aluminum production process justify research and development of alternative electrolytic processes for aluminum production that can both reduce the cost and eliminate adverse environment impacts. Solid oxide membrane (SOM) based electrolysis process is an innovative technology that has been demonstrated to successfully produce many energy-intensive metals directly from their oxides in an efficient, economical and environmentally sound way. During the SOM electrolysis process, an oxygen-ion-conducting SOM tube made of ytteria-stabilized zirconia (YSZ) separates the pre-selected molten flux with dissolved metal oxide from the inert anode assembly inside the YSZ tube. When the applied DC potential between the cathode and the anode exceeds the dissociation potential of desired metal oxide, the metal is reduced at the cathode while oxygen ions migrate through the YSZ membrane and are oxidized at the anode. Employing the inert anode allows the oxygen to be collected at the anode as a value added byproduct. In this work, a zero-direct-carbon-emission aluminum production process utilizing SOM electrolysis is presented. The molten flux used in the electrolysis process is optimized through careful measurements of its physio-chemical properties. The liquidus temperature, volatilization rate, alumina solubility, aluminum solubility, YSZ membrane degradation rate and electrical conductivity of various flux compositions were measured, and the flux chosen for SOM electrolysis was a eutectic MgF2-CaF2 system containing optimized amounts of YF3, CaO and Al2O3. Laboratory scale SOM electrolysis employing the inert anode were performed at 1100 ~ 1200oC to demonstrate the feasibility of producing and collecting aluminum while producing pure oxygen as a byproduct. The aluminum product was characterized by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). An equivalent circuit model for the electrolysis process was developed in order to identify the polarization losses in the SOM electrolysis cell.