The Study Of Direct Preparation Tungsten Powder By Molten Salt Electrolysis From Tungstate

In this paper, the theoretical basis and the feasibility of the direct preparation tungsten powder by molten salt electrolysis from tungstate was proposed,and have studied using NaCl-KCl-Na₂WO₄ system,NaCl-CaCl₂-Na₂WO₄ system and the NaCl-CaCl₂-CaWO₄ system directly prepared tungsten powder by molten salt electrolysis experiments.And the principle of electrolysis, the liquidus temperature of molten salt, electrolyticconditions, some phenomena in the electrolysis process and the characterization results of the cathode products were analyzed.For the system of NaCl-KCl-Na₂WO₄, Na₂WO₄ dissociation for the Na⁺ and WO₄^2- in molten salt. In electrolysis process, The WO₄^2- will be broken down into the O^2- and WO₃ on surface of the cathode. When voltage is low, W6⁺ discharge on the cathode,WO₃ is directly reduced to W. When the voltage is high, the cathode of the electrolytic reaction in two steps, The first step is Na⁺ or K⁺ on the cathode surface discharge generated Na or K,The second step is the reduction of Na or K from WO₃ to tungsten powder. For the system of NaCl-CaCl₂-Na₂WO₄, pre-electrolysis reaction occurs before reaction: Na₂WO₄+CaCl₂=CaWO₄+2NaCl. CaWO₄ will not melt into the molten salt at the first, but sink to the bottom of the crucible.The active substances of the actual electrolysis is CaWO₄. In not sure CaWO₄ solubility in molten salt the size of the two cases suggested by electrolysis in the form CaWO₄. The first is direct contact with the cathode by electrolysis CaWO₄ tungsten powder; The second is part of the CaWO₄ dissociation for the Ca²⁺ and WO₄^2- and in the form of WO₄^2- preparation tungsten powder by electrolysis in molten salt. For the system of the NaCl-CaCl₂-CaWO₄, the two comparative experiments of CaWO₄ made direct contact with the cathode and the cathode is not suspended contacts with CaWO₄ was implemented. Experiments show that CaWO₄ has a degree of solubility in molten salt.That indicate some CaWO₄ dissociate to the Ca²⁺ and WO₄^2- in molten salt.In this study, differential thermal analysis method using different proportions of NaCl-CaCl₂-Na₂WO₄ system and the NaCl-CaCl₂-CaWO₄ system for the determination of initial crystallization temperature. Conclude the temperature of the liquidus molten salt system with improved content Na₂WO₄ or CaWO₄ linearly increasing.The electrode material of experiments are mainly graphite rod. Electrolysis temperature is 750℃, the cell voltage is 1.5-3V, the initial voltage keep on 1.5V, to be pre-molten salt electrolysis system is stable 1-3h after the cell voltage stabilized at 2.5V, the total duration of electrolysis for 6h. The cathode discharge both phenomena in the electrolysis process especially in the system of NaCl-KCl-Na₂WO₄ cathode discharge phenomenon is most frequent. Because the corrosive effect of KCl on the cathode, electrolytic cathode almost entirely after the end of fragmentation, the graphite particles droped into molten salt, a number of the C,WC and W2C appear in the Cathode product. For the NaCl-CaCl₂-Na₂WO₄ system, the insolubled solid material was discoveried at the bottom of the crucible before the molten salt electrolysis. After the electrolysis, molten salt was divided into three levels: the first layer is very thick and gray above the electrolyte layer; the second layer is solid dense black material, mainly for the deposition of tungsten powder; CaWO₄ is the white bottom layer. Relative to the NaCl-CaCl₂-CaWO₄ system,the electrode materials of NaCl-CaCl₂-CaWO₄ system are not in direct contact with the electrolysis,and after the end of the electrolysis ,there is a new layer what is not very dense and have many small spherical voids within the black solid layer at the top of the solid salt. After testing the substance is mainly C, also mixed with a small amount of W.The tungsten powder obtained after experiment was tested by X-ray diffraction analysis, spectrum analysis, purity analysis, particle size analysis and scanning electron microscope photograph of the morphology. Without considering the tungsten powder that preparation from NaCl-KCl-Na₂WO₄ system what was adopted the graphite cathode,the purity of the tungsten powder from other experiments is in the range of 95-97.5%, and the average particle size is in the range of 0.5-1.2μm.