Solid oxide membrane (SOM) stability in molten ionic flux for the direct electrolysis of magnesium oxide
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Direct electrolysis of magnesmm from its oxide is less expensive and more environmentally friendly than current methods of magnesium production. The solid oxide membrane (SOM) process is a viable method for production of magnesium via direct electrolysis. In the SOM process magnesium oxide is dissolved in a molten flux, which acts as a supporting electrolyte. A yttria stabilized zirconia (YSZ) membrane is immersed in the flux and separates the anode from the cathode. When an electrical potential is applied between electrodes, magnesium cations travel through the flux to a steel cathode where they are reduced. Simultaneously, oxygen anions travel through the YSZ to a liquid metal anode where they are oxidized. However, in order for the SOM process to be commercially successful it must run for thousands of hours at high current efficiencies. It is believed the degradation of the YSZ membrane determines the lifetime and operating costs of the SOM process. This study investigates the mechanisms of YSZ membrane degradation. There are two main pathways of YSZ degradation: 1) yttria (yttrium oxide) diffusion out of the membrane, and 2) electronic conductivity in the flux providing a pathway for the applied potential to reduce the YSZ membrane. It is shown through diffusion experiments that the loss of yttria from the membrane into the oxy-fluoride flux can be prevented by adding yttrium fluoride to the flux, so that the activity of yttria in the flux is equal to the activity of yttria in the membrane. The electronic conductivity then becomes the primary source of membrane degradation in the SOM process. Electronic conductivity lowers the current efficiency of the SOM process. It is shown through measurements that the electronic conductivity is reduced by lowering the magnesium solubility in the flux. This is accomplished by performing SOM electrolysis at a reduced pressure (<0.1 atm). When SOM electrolysis of magnesium oxide is carried out at reduced pressure, the membrane is not degraded and the current efficiency is high (>70%). Thus tlus process provides a basis for a successful commercial operation for the direct electrolysis of magnesium oxide.
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