Electrochemical separation technology and mechanism of tungsten and cobalt in Na2WO4-WO3-CoO molten salts

Abstract

The separation and recovery of tungsten and cobalt from cemented tungsten carbide (WC-Co) scraps can be carried out by adding WO3 and CoO into the Na2WO4 molten salt to simulate the calcination of WC-Co scraps. The Raman spectra and the XRD patterns of the molten salts cooled naturally to room temperature, and the predominance diagram of Na-W-Co-O system, show that the addition of CoO reduced the concentration of Na2W2O7 in the molten salt. Molecular dynamics simulations based on first principles indicated the following reaction mechanism: WO42− + WO3 → W2O72−, W2O72− + O2– → 2WO42−. Electrochemical cyclic voltammetry (CV) and square wave voltammetry (SWV) were used to analyze the redox behaviors of tungsten and cobalt with different molar ratios of WO3 and CoO in the molten salts. The reduction of the tungsten ion was a two-step reaction, W6+ → W2+ → W, and the reduction of the cobalt ion was a one-step reaction, Co2+ →Co. On increasing the amount of added CoO, the deposition potential of tungsten ions shifted towards a more negative potential, while the deposition potential of cobalt ions remained unshifted. Therefore, W coatings, Co3W alloys and Co powders were obtained by selective electrodeposition.