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Research Papers

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

[+] Author and Article Information
L. Venstrom, K. Krueger, N. Leonard, B. Tomlinson, S. Duncan

Department of Mechanical Engineering, Valparaiso University, Valparaiso, IN 46383

R. D. Palumbo1

Department of Mechanical Engineering, Valparaiso University, Valparaiso, IN 46383robert.palumbo@valpo.edu

1

Corresponding author.

J. Sol. Energy Eng 131(3), 031005 (Jun 10, 2009) (9 pages) doi:10.1115/1.3142802 History: Received June 05, 2008; Revised December 17, 2008; Published June 10, 2009

The ionic conductivities of mixtures of ZnO in Na3AlF6 and in xCaF2yNa3AlF6 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 ZnONa3AlF6 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±0.5Ω1cm1. At temperatures from 1350 K to 1425 K it jumps to 6±0.5Ω1cm1 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.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 4

Conductivity of 2 wt % alumina in Na3AlF6 from 1260 K to 1300 K. The thin-line values were reported in Ref. 24. The rectangular values were determined by our measurements.

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Figure 5

Conductivity of 5 wt % alumina in Na3AlF6 from 1280 K to 1300 K. The thin-line values with their uncertainty intervals were reported in Ref. 24. The rectangular values and their uncertainty intervals were determined by our measurements.

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Figure 6

Conductivity versus temperature for various Na3AlF6, CaF2, and ZnO solutions. Uncertainty intervals are at a 95% confidence level.

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Figure 7

Conductivity versus temperature for 35–65 wt %Na3AlF6–CaF2 with ZnO solutions

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Figure 8

Conductivity versus temperature for 50–50 wt %Na3AlF6–CaF2 with ZnO solutions

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Figure 9

Back-work ratio versus temperature, the ideal values, and those based on this study’s conductivity results

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Figure 10

Fraction of energy required that is thermal versus temperature, the ideal values, and those based on this study’s conductivity results

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Figure 11

Schematic of the uncertainty analysis for the conductivity measurements

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Figure 12

Typical example of electrolytic resistance versus frequency obtained from the swept-sine method

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Figure 1

The minimum required energy input for the decomposition of ZnO into its elements each at 1 bar. ΔH is the minimum amount of energy required for the reaction and ΔGo is the minimum amount of energy that must be supplied as electric work.

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Figure 2

Schematic of the model of the electrolytic cell and the measurement system used to establish its ionic resistance

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Figure 3

Isometric view of our reaction chamber and the cell used to establish the ionic conductivity of various electrolytes. The crucible is made from hot pressed boron nitride. The electrodes are 1 mm diameter molybdenum wire.

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