| High purity titanium powder is widely used in various walks of life because of its excellent properties such as excellent anticorrosion, gas absorption, non-magnetism, and biocompatibility, etc. However, the high purity titanium powder industry is facing with two neck bottles:the first is how to reduce the impurity concentration effectively; the second is how to manipulate the particle shape and size.In this paper, high purity titanium powder is prepared by molten salt electrorefining with sponge titanium as the anode, pure titanium slab as the cathode, and NaCl-KCl as the electrolyte. The preparation process was conducted as follows. First, the mixed molten salt(NaCl-KCl-TiClx) was obtained at550癈by the reaction of the added TiCl4and the sponge titanium at anode without electrolysis; then titanium powder was produced on cathode by direct current electrolysis with protection of argon and temperature ranging from700～950℃. This paper optimized the process, and studied the electrolysis mechanism based on the valence transition of titanium ions, impurity concentration and particle shape and size of titanium powder. High purity titanium powder with oxygen concentration lower than200ppm, and mean particle size of60.70μm was obtained. The main conclusions are as follows:1. Valence transition of titanium ions:(1) The valence of titanium ions varies along the vertical direction of the melt because of loading in the vapor phase chamber. The titanium ions valence decreases from top to bottom in the electrolyzer.(2) The valence of titanium ion decreases as the electrolysis conducting. Ti4+dominates during the prophase while Ti2+dominates during the later stage.2. Transition of impurities:(1) The contents of impurities in the melt which have similar electrode potential with titanium, such as Mn, V, Cr, Al, will not change significantly before and after the electrolysis; with the electrolysis conducting, the elements content such as O, N, and C,etc. will increase;(2) The impurities, as represented by Fe, will precipitate previously, which can be eliminated by pre-electrolysis;(3) The impurities of electrolyzed titanium mainly exist in a "tunnel" structure of the dendrite boundary or the interior crystal. The impurity concentration decreases when it is farther away from the boundary or the internal "tunnel". The main impurities appear on the dendrite boundaries are O, Fe, V, and Cl. The impurities in the internal crystal include not only O, Fe, and V, but also molten salt such as KCl and NaCl.3. Particle shape and size:(1) the titanium powder near the inner layer of cathode has lump and dense morphology, accompanied with sintering phenomenon, and the impurity concentration is relatively high, while the titanium of outer layer is dendrite with higher purity and obvious metallic luster. This structure will affect the yield and quality of every single batch;(2) fine particle can be obtained by increasing the current density and reducing the concentration of soluble titanium. The soluble titanium concentration has a more significant effect on particle size than the current density. |
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