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. 2018 Nov 26;8(1):17391.
doi: 10.1038/s41598-018-35739-z.

Minimising Oxygen Contamination Through a Liquid Copper-Aided Group IV Metal Production Process

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Free PMC article

Minimising Oxygen Contamination Through a Liquid Copper-Aided Group IV Metal Production Process

Bung Uk Yoo et al. Sci Rep. .
Free PMC article

Abstract

This paper demonstrates for the first time the fabrication of Zr-Cu alloy ingots from a Hf- free ZrO2 precursor in a molten CaCl2 medium to recover nuclear-grade Zr. The reduction of ZrO2 in the presence of CaO was accelerated by the formation of Ca metal in the intermediate stage of the process. Tests conducted with various amounts of ZrO2 indicate that the ZrO2 was reduced to the metallic form at low potentials applied at the cathode, and the main part of the zirconium was converted to a CuZr alloy with a different composition. The maximum oxygen content values in the CuZr alloy and Zr samples upon using liquid Cu were less than 300 and 891 ppm, respectively. However, Al contamination was observed in the CuZr during the electroreduction process. In order to solve the Al contamination problem, the fabrication process of CuZr was performed using the metallothermic reduction process, and the produced CuZr was used for electrorefining. The CuZr alloy was further purified by a molten salt electrorefining process to recover pure nuclear-grade Zr in a LiF-Ba2ZrF8-based molten salt, the latter of which was fabricated from a waste pickling acid of a Zr clad tube. After the electrorefining process, the recovered Zr metal was fabricated into nuclear-grade Zr buttons through arc melting following a salt distillation process. The results suggest that the removal of oxygen from the reduction product is a key reason for the use of a liquid CaCu reduction agent.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Electroreduction of ZrO2 in a CaCl2–CaO molten salt. (a) Solid copper cathode system at 1080 K. (b) CVs of the solid cathode system at various scan rates. (c) CV results of the liquid cathode system at various scan rates. (d) Liquid copper cathode system at 1380 K. (e) Chronopotentiometry during electroreduction at E4 experimental conditions in CaCl2 with 5 wt% CaO at 1380 K. The E4 condition means that 8.47 g of ZrO2 is charged on the 10 g of liquid copper and a current of 1A is applied for 11.054 hours. The weight ratio of ZrO2/Cu is 84.7.
Figure 2
Figure 2
Reduction behaviour of ZrO2 according to the process temperature. Schematic representations of the electroreduction system at (a) the initial state, (b) below the melting point of Cu, and (c) above the melting point of Cu. DFT-MD simulation snapshots after 10 ps of reaction time at (d) 400 K, (e) 1000 K, and (f) 1380 K. (g) The simulations indicated that ZrO2 on the Cu(111) surface at 1380 K did not penetrate into liquid Cu phase because of repulsive forces. Copper, cyan, and red spheres represent Cu, Zr, and O atoms, respectively.
Figure 3
Figure 3
The microstructures displayed CuZr (white, ad) as well as CuAl intermetallic compounds (dark gray, ad). The CuAl portion decreased with increasing ZrO2 concentration (d) for the benefit of the CuZr alloy. (a) E1;6.2, (b) E2;15.2, (c) E3;40.1, and (d) E4;84.7 in ZrO2/Cu mass ratio. Under each condition, a current of 1A was applied to 10 g of liquid copper. (e) Zr and O concentrations in the cathode ingots. (f) Comparison between XRD patterns for different CuZr samples.
Figure 4
Figure 4
Electrorefining of Zr using a LiF–Ba2ZrF8 molten salt system. (a) Cyclic voltammograms obtained under various scan range conditions at 1053 K. (b) Chronopotentiometry during the electrorefining process at 1053 K. Photographs of the (c) anode and (d) cathode after 10 h of electrorefining. (e) Deposition of Zr on the cathode after vacuum distillation. Photographs of the nuclear grade Zr powder after vacuum distillation process of cathode electrodeposits. (f) Nuclear-grade Zr button.
Figure 5
Figure 5
A graphical abstract showing the steps of each process to recover group IV transition metals, such as Zr, Hf and Ti.

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