Anodic Dissolution of Tungsten from Super Hard Alloys in Molten Sodium Hydroxide

Abstract

Anodic dissolution of tungsten in a molten sodium hydroxide bath was investigated at 723K in order to simplify a tungsten dissolution step during tungsten recycling process from secondary resources like used super hard alloy tools. Cyclic voltammetry using tungsten, tungsten carbide and cobalt, which is used as binder in super hard alloys, electrodes suggested that tungsten is easily oxidized and dissolves into the melt. Cobalt was also oxidized and formed oxide or hydroxide film. When water vapor was introduced into the system, the obtained cyclic voltammograms changed with the partial pressure of water vapor. Based on the above mentioned results, anodic dissolution was carried out at constant voltage using throw-away tips as actual secondary resources. When the throw-away tip was anodically oxidized, both the tungsten carbide and binder metal of cobalt were dissolved at constant voltage of 1.0 V, and whole the tips were dissolved by a long-term electrolysis of 20 hours. Although the dissolved tungsten remained in the melt, the greater part of dissolved cobalt deposited as metal powder on the cathode. The current efficiencies of tungsten carbide dissolution were calculated to be 96%. When 0.7 atm of water vapor was introduced, the electrolysis was terminated within 5 hours, which indicates the effectiveness of water vapor pressure control in the present process.