Fundamental Aspects of a New Process for the Production of Pure Zirconium

Fundamental Aspects of a New Process for the Production of Pure Zirconium

Laging, V.J.W.

Yang, Y. (mentor)
Xiao, Y. (mentor)
Van Sandwijk, A. (mentor)

Civil Engineering and Geosciences

Geotechnology

Section Resource Engineering

2010-10-07

The metal zirconium (Zr) and its related alloys have high melting points and an excellent resistance to corrosion. Consequently, zirconium is applied in chemical technology and particularly in nuclear reactors. The low neutron-capture cross-section of zirconium, combined with its corrosive resistance, make the metal pre-eminently suitable as cladding for nuclear fuel rods. In nature, zirconium is nearly always found in combination with hafnium (Hf). Chemically the metals zirconium and hafnium strongly resemble each other. However, contrary to zirconium, hafnium has a high neutron-capture cross-section. As a low absorption coefficient for neutrons is essential for fuel rod cladding, the hafnium content of nuclear-grade zirconium should be as low as possible. Currently, most of the world’s zirconium is produced by the Kroll method. This process is over sixty years old, and due to its inefficient batch production the production costs are high. A (semi-)continuous molten salt electro-refining process could result in a cheaper, purer product that still meets the high requirements of the industry. A new process for the production of pure zirconium has been recently developed and patented at Delft University of Technology. In the current work, fundamental operating aspects of the patented process have been investigated. The patented process consists of three steps. The first step is the reduction of ZrO2 and HfO2 to Zr and Hf metal. For this step, the solubility of ZrO2 in molten salt has been investigated. Different salt compositions, additives and temperatures have been used. The best results were found using an equimolar NaCl-CaCl2 mixture with 5 mol% CaO and 5 mol% NaF added, at a temperature of 750 °C. This resulted in a ZrO2 solubility of 2.86%. The second step is the separation of hafnium from zirconium. By contacting a molten Cu-Sn-Zr-Hf alloy with a CuCl2-containing salt, the hafnium is transported to the salt phase. The effects of different salt compositions at different stoichiometric ratios have been investigated. The best results were obtained with a NaCl-CaCl2-CuCl2 salt mixture at a stoichiometric ratio of 1.5, reaching single-step removal efficiencies of 95%. Higher CuCl2 concentrations in the molten salt have a positive effect on the removal efficiency. The final step is the electro-refining of zirconium from the Cu-Sn-Zr alloy, which has been tested on a laboratory scale. The best results were obtained using vitreous carbon anode lead, a zirconium block cathode and a molten salt electrolyte consisting of equimolar NaCl-KCl with CuCl2. It is essential that the molten salt electrolyte is purified with hydrochloric gas to remove oxide and hydroxide ions. A dense metallic deposit was formed, which consisted of ~25 wt% Cu, ~28 wt% Zr and ~ 47 wt% Sn. The applied potential was too high for selective refining of zirconium. Future experiments should use a lower potential, and have a sufficiently high concentration of zirconium in the molten alloy in order to prevent the deposition of tin and copper.

Zirconium
Hafnium
Separation
Molten salt
Electro-refining
Electrolysis
Solubility

http://resolver.tudelft.nl/uuid:f793b2ad-2e31-4f68-96d6-5a7958ca1f70

2012-10-01

Student theses

master thesis

(c) 2010 Laging, V.J.W.