On the Electrochemical Reduction Mechanism of CaWO 4 to W Powder
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Recently, an electrochemical method to produce tungsten powder was reported.[1,2] Similar to oxide reductions of the Fray-Farthing-Chen (FFC) process,[3] it was based on the results of successful laboratory tests involving the reduction of CaWO4 powders in CaCl2 containing molten salt electrolytes. Metallic tungsten powder was produced with significant concentrations of CaCO3 and/or Ca(OH)2, which could then be removed by treating the reduced product with dilute HCl solutions.[1] The possible reactions, which may occur during electrochemical reduction, suggested based on constitutions of the reduced products and the lowest reversible decomposition potentials were[4,5] CaWO4 ðsÞ þ 3/2CðsÞ ¼ WðsÞ þ CaOðsÞ þ 3/2CO2 ðgÞ E o ¼ 0:31ðVÞ
½1
CaWO4 ðsÞ þ 2CðsÞ ¼ WðsÞ þ CaðsÞ þ 2CO2 ðgÞ E o ¼ 0:68ðVÞ
½2
The detection of chlorine gas during the experiments, especially with larger samples, cast some doubt on above equations as cell reactions. Since kinetic factors play an important role in the mechanisms of cell reactions during operation; results from constant voltage electrolysis complemented with cyclic voltammetry were used to gain an insight into the reduction mechanism of calcium tungstate in CaCl2 containing molten salt electrolytes. _ METEHAN ERDOGAN, Ph.D. Student, and ISHAK KARAKAYA, Professor, are with the Department of Metallurgical and Materials Engineering, Middle East Technical University, 06800 Ankara, Tu¨rkiye. Contact e-mail: [email protected] Manuscript submitted December 22, 2011. Article published online June 29, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
A detailed description of the experimental setup and procedure for constant voltage electrochemical reduction experiments can be found elsewhere.[1] The CaWO4 pellet (248665; Sigma-Aldrich, St. Louis, MO) was positioned onto a stainless steel spoon electrode, which was welded to the end of a stainless steel wire current collector. The pellets used in constant voltage electrochemical reduction experiments were prepared via the following route: First, the CaWO4 powder was impregnated with 2 pct by mass of a binder polyethylene glycol (PEG). Secondly, the components were mixed for 40 minutes and then left to dry in air at room temperature for 24 hours. Following the sieving stage, the pellets were formed by die pressing the powder and PEG was removed by holding the pellets at 873 K (600 °C) for 150 minutes. Because the melting point of pure CaWO4 is approximately 1873 K (1600 °C),[4,5] this temperature was too low for the CaWO4 powder to sinter. The pellets had 15.6 mm diameter and 3 mm height. A graphite rod (A10134; Alfa Aesar, Ward Hill, MA) having 13 mm diameter was used as the anode. The eutectic (48 pct mole NaCl) NaCl-CaCl2 salt solution, used as the electrolyte, was molten at the selected operation temperature of 873 K (600 °C).[6] The experiment durations varied between 200 to 500 minutes, depending on the applied potential. The schematic view of the experimental setup used in cyclic voltammetry is given in Figure 1. The CaWO4 pellet for
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