Solar Thermal Electrolytic Process for the Production of Zn From ZnO: An Ionic Conductivity Study

Abstract

The ionic conductivities of mixtures of ZnO in $Na3AlF6$ and in $xCaF2–yNa3AlF6$ mixtures were established with a swept-sine measurement technique. A millivolt sinusoidal voltage at frequencies from 1000 Hz to 25,000 Hz was impressed on a system containing the electrolytes. The system’s frequency response was used to establish the conductivities. The influence of these conductivities on the potential of a solar thermal electrolytic process was evaluated using two process performance parameters: the back-work ratio and the fraction of minimum solar thermal energy required to drive the metal production reaction. We found the conductivity of mixtures of $ZnO–Na3AlF6$ to be independent of the concentration of ZnO for weight percentages of ZnO from 0.5% to 5%. For temperatures 1240–1325 K the conductivity is close to that of pure $Na3AlF6$, $3±0.5Ω−1cm−1$. At temperatures from 1350 K to 1425 K it jumps to $6±0.5Ω−1cm−1$ When $CaF2$ is added to the mixture, the electrolyte’s conductivity drops. We thus expect that calcium cations are not present to any important extent in the electrolyte. When $CaF2$ is part of the chemical system, the concentration of ZnO can have a measurable impact on the electrolyte’s conductivity. Combining the conductivity results with the two solar process performance parameters illustrates the importance of operating the solar process at low current densities when the temperature range is 1200–1500 K. The results further suggest that one should consider studying the electrolytic process at 1800 K.