Anodic Dissolution of Tungsten in a Molten Sodium Hydroxide

Abstract

Anodic dissolution of tungsten or tungsten carbide 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 cemented carbide (super hard material) tools. Cyclic voltammetry using tungsten and other metal wire electrodes suggested that tungsten is easily oxidized and dissolves into the melt, while nickel, iron and copper are hardly dissolved because of stable film formation on them as a result of oxidation. Cobalt was also oxidized, but the resultant oxide or hydroxide film seemed unstable. Anodic dissolution was carried out at constant voltage using throw-away tips and rotators of vibration motors. When the rotator was anodically oxidized at constant voltage of 1.0 V using a nickel plate as the cathode, tungsten was selectively dissolved at a high current efficiency of 97% and binder metals (nickel and copper) remained as residue. The amount of tungsten remained in the residue was less than 1% of initial value after the electrolysis for 9 hours. In the case of throw-away tips, both the tungsten carbide and binder metal of cobalt were anodically 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 86% and 96% in the cases of ceramic coated and uncoated tips, respectively. These results suggest that the proposed electrolysis is effective to dissolve tungsten from secondary resources, and indicate the possibility of selective recovery of both the tungsten and cobalt in the case of cemented carbide tools.

Solubility of Sodium Tungstate in Molten Sodium Hydroxide

“…The experimental setup for anodic dissolution of a throw-away tip was essentially the same as that reported, 14,15 that is, a commercially available throw-away tip with no coating was placed on a nickel (Ni) plate and used as the anode. The cathode was an Ni plate of dimensions of 10 mm © 20 mm.…”

“…Figure 7 shows the time dependence of the current during the anodic dissolution of a throw-away tip in an NaOH melt containing Na 2 WO 4 at P H2O of 0 or 0.7 atm. These P H2O values were selected in order to compare with the data obtained previously, 14,15 although these are outside of the measured range in the present study. The molar fraction of Na 2 WO 4 corresponding to the mass ratio of used NaOH and WO 3 is 0.04, which is higher than the solubility at P H2O of 0 atm and much lower than that at P H2O of 0.7 atm.…”

“…In this process, a super-hard alloy tool consists of WC and binder metal of cobalt (Co) is anodically dissolved in a molten NaOH, and tungstate ions are accumulated in the melt. At the same time, dissolved Co is reduced on the cathode and is separately recovered as Co powder, 14 although Kamimoto et al claimed that Co remained in the super-hard alloy under a similar condition. 17 Then, a solution of sodium tungstate is obtained by dissolving the solidified NaOH melt and W could be recovered via the conventional hydrometallurgical steps.…”

“…[11][12][13] In recent research, anodic dissolution of WC in a NaOH melt in which WC is anodically oxidized and dissolved into the melt with high current efficiency and a relatively low applied voltage of 0.4 V has been proposed. 14,15 One of the common points of the abovementioned processes is that the reaction product is sodium tungstate (Na 2 WO 4 ) dissolved in the melt. Therefore, the solubility of Na 2 WO 4 is regarded as one of the most important parameters to determine the performance of these processes and optimize the operating conditions.…”

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

“…From this background, some attempts have been done in order to simplify the dissolution step using chemical and electrochemical reactions (1)(2)(3)(4). Recently, the author has proposed another process using anodic dissolution in molten sodium hydroxide (5). In this process, the super hard alloy tool is used as the anode and all the tungsten carbide and cobalt are dissolved into the melt.…”