Influence of partial pressure of water vapor on anodic dissolution of tungsten from super hard alloy tools in molten sodium hydroxide

As part of an ongoing study of a new tungsten recycling process using a molten sodium hydroxide bath, in which tungsten is anodically dissolved in the melt from tungsten-containing secondary resources, the influence of the partial pressure of water vapor on the electrochemical behavior of tungsten, cobalt, nickel, and tungsten carbide was investigated. Cyclic voltammograms of cobalt suggested that the solubility of cobalt compounds increased with the partial pressure of water vapor, whereas tungsten and tungsten carbide underwent no significant changes during their oxidation step. In addition, the current attributable to the evolution of hydrogen gas was measured when the partial pressure of water vapor became higher than 0.5 atm. On the basis of the abovementioned results, anodic dissolution was carried out at a constant cell voltage using a super hard alloy tool. When water vapor was introduced into the system, the cell voltage and period required to dissolve the entire tip were reduced, even though the applied cell voltage was less than half that required under a pure argon atmosphere. These results indicate that the introduction of water vapor has the potential to improve the anodic dissolution step in the tungsten recycling process using a molten sodium hydroxide bath.