Eefect of Electronic Current on the Solid Oxide Membrane (SOM) Process for Magnesium Production

"The solid oxide membrane (SOM) process has been used at 1150 'C to produce magnesium by direct electrolysis of its oxide. Magnesium oxide is dissolved in a molten CaF2-MgF2 ionic flux. An oxygen-ion-conducting yttria-stabilized zirconia (YSZ) SOM membrane separates the cathode and the flux from the anode. During electrolysis, magnesium ions are reduced at the cathode and Mg(g) bubbles out of the flux into a separate condenser. The flux has a small solubility for magnesium which imparts electronic conductivity to the flux. The electronic conductivity decreases current efficiency of the process and also degrades the YSZ membrane. This study investigates different methods for reducing the electronic conductivity of the flux due to magnesium solubility. One effective method shown here is inert gas flushing near the magnesium nucleation site. Potentiodynamic and potentiostatic experiments along with AC impedance spectroscopy are used to characterize the effectiveness of the approach.IntroductionMagnesium is the third most abundant metal in the earth's crust. Since magnesium is the lightest of all metals used for construction alloys, magnesium is the leading candidate to replace steel and aluminum in automobiles. Automobile manufacturers would like to increase magnesium content from 4-5kg per vehicle to 40-IOOkg per vehicle [I]. Using magnesium alloys instead of steel and aluminum can reduce automobile weight and improve fuel efficiency. However, in order for automobile manufactures to make this change, it is generally believed that new cost effective methods of magnesium production must emerge.One approach for magnesium production which has been shown to be effective on a laboratory scale is the direct electrolysis of magnesium oxide utilizing an oxygen-ion-conducting solid oxide membrane (SOM), this is known as the SOM process [2]. A recent study estimated that a commercial SOM plant could produce magnesium from magnesium oxide at a cost that is 40% less than current technologies [3]."