Solar Thermal Electrolytic Process for the Production of Zn from ZnO: The Electrolysis of ZnO from 1275-1500 K

The solar thermal electrolytic production of Zn from ZnO was studied in the temperature range of 1275-1500 K in a cavity-solar receiver located at the focal point of a concentrating solar furnace. This study establishes how cathode material, solvent, current levels, and operating temperature influence the electrolytic cell's performance. For a nominal current density of 0.1 A cm ^-2 at temperatures from 1275 to 1425 K, we found that our performance parameters, the back work ratio and substituted-solar fraction, are within 25 and 20 of the ideal values, respectively. This behavior was true whether the cathode was Mo or W and whether the electrolyte was pure cryolite or a 35 mol. cryolite-CaF₂ mixture. When the electrolytes were cryolite-CaF ₂ mixtures in the temperature range of 1275-1425 K, there was no measurable difference in the performance, but at 1500 K with a MgF₂ electrolyte, the performance dropped significantly. We have some evidence that the performance of the cell is better at current densities above 0.1 A cm ^-2 when the cathode is Mo as opposed to W. Furthermore, the difference in the performance values can be attributed to higher kinetic over voltages associated with W versus Mo as a cathode. Our data also suggest that kinetic over voltages increase as the operating temperature increases. The experimental evidence suggests the reaction mechanism at the cathode for ZnO in cryolite involves a reaction between Na and ZnF₂, and the anode reaction involves a reaction between the anions Al₂OF ₆^2- and ZnO₂^2-. Both Mo and W worked as cathode materials, but both the Mo and the W became brittle. Pt worked well as an anode without showing any evidence of degradation. Our SiC crucible may have suffered some carbothermic reaction with ZnO at temperatures exceeding 1275 K, with solvent mixtures of cryolite, CaF₂, and MgF₂.