| Smelting process, as the technology of extracting metal, semiconductor and compound materials from mineral, provides raw materials for the machining and forming post-treatment process and become the foundation process of metallurgical industry. With the development of science and technology for thousands of years, Smelting technology has continuously developed and improved to produce lower cost and better performance materials with cleaner and more energy efficient process. Thermometallurgy, hydrometallurgy and molten salt electrodeposition technology supply cheap quality steel, aluminium, copper and zinc et al for modern human life and industrial production. While the production of refractory metals such as titanium, zirconium, niobium and tungsten by those technologies presents complex, pollutional and costly process. The smelting process of solid electroreduction in molten salt developed in the early 21th century is aim to create a better technology for the production of refractory metals, which used solid mineral as the cathode and directly separated the metal and oxygen by solid electrolytic process, achieving one-step extraction of metal from mineral. The process performs some energy conservation and emissions reduction production advantages such as short procedure, cheap raw material and low pollution. So far dozens of metal, semiconductor, alloy and compound had been produced by solid electroreduction in molten salt. The efficiency of solid electroreduction in molten salt decides the effect of energy conservation and emissions reduction, efficient solid electroreduction would achieve lower costs and cleaner production process. In this work, the productions of super-refractory powder materials in molten salt were systematically studied to explore high efficiency and low consumption solid electroreduction process and new technology. The main conclusions of the work are as follow:(1) Review of the reasons for the low smelting process efficiency of the current solid electroreduction technology is used to present three main influential factors for the solid electrolytic process, which are electron transfer, molten salt diffusion and oxygen anion solid migration, respectively. Among the three influential factors, electron transfer is determined by the conductivity of the solid reactant, molten salt diffusion is determined by the void structure of the solid reactant and oxygen anion solid migration is determined by the void structure, oxygen concentration difference and physico-chemical property of the solid reactant.(2) The study of the production of nano tungsten powder from solid scheelite overcame the complex and difficult leaching process of scheelite, and provided a method for the improvement of the efficiency of the solid electroreduction. The determined solubility of scheelite in molten CaCl2-NaCl revealed that the value was below 0.2 wt% at 600-750 ? but more than 0.8 wt% at 800 ? or higher temperature. The temperature of the molten salt was controled below 750 ? to ensure the solid state of scheelite during the electrolytic process and over 600 ? to ensure electrodeoxidation dynamic process. The one-step eletroreduction process from scheelite to tungsten was determined by both the theoretical calculation and practical testing. The optimized voltage and temperature were 3.1 V and 750 ?, respectively. The transform of a conventional 2 g scheelite pellet to tungsten needed 24 h electrolytic process with 25% current effciency, which showed the usually low efficiency of solid electroreduction smelting process. The electrolyzed products for different duration and the comparison of inner and outer electrolyzed products indicated that the enhancement of electroreduction against electrolysis time is much more significant during the initial 10 h, while only minor enhancement occurs when further increasing the electrolysis duration from 10 to 24 h. According the solubility Characteristics of scheelite in molten salt, the pure nanosize tungsten powder was also obtained by the process of 850 ? molten salt washing for 1 h instead of the later electrolytic process for 14 h. The 10 h solid electroreduction and then 1 h molten salt washing two-step process decreased the electrolysis time to 10 h, which resulted in the current efficiency improved to 55%, the energy consumption of tungsten decreased from 10.63 kWh/kg to 4.97 kWh/kg and the yield slightly decreased from 97% to 95%.(3) The conductivity of the solid pellet cathode is difficult to significantly improve due to the low conductivity of mineral. The addition of powder with good conductivity could obviously enhance the conductivity of the solid pellet cathode. In this work the grahpite powder which behaved good thermal and chemical stability in molten salt was applied as the additional powder in the solid electroreduction process of transition metal oxides.5 kinds of nanosize transition metal carbide had been prepared by electrolyzed at suitable condition form the corresponding composite pellet which contained stoichiometric ratio oxides and graphite and formed by ball-milling and pressing processes. The partical sizes and morphologies of all the carbides were 20-30 nm and spherical particle, respectively. The lattice parameters were closed to the corresponding data of PDF card. The oxygen contents were about 5000 ppm and the M/C ratios were closed to 1.5 kinds of transition metal could divided to 2 types i.e. IVB and VB, while the two types of the carbides performed obvious differences in electrolytic efficiency, in which the electrolytic time and current efficiency of the VB type of carbide were about 6 h and 80%, respectively, but these of the the IVB type of carbide were about 20 h and 40%, respectively. The investigation for the smelting process of the two types of carbides indicated a similar solid electroreduction process which could be described as the two step process:MOn-C first transformed to CaMaOb and MCxOy, then transformed to MC with low oxygen content. The property of the MCxOy decided the formation rate of MC. M?CxOy behaved much stronger chemical stability than M?CxOy, which resulted in the eletrodeoxidation rate of M?CxOy was obviously slower than M?CxOy. The 3PIs interface electrochemical reaction was enhanced to all the pellet due to the existence of graphite, which increased the electrolysis current of oxides and accelerated the electrodeoxidation rate significantly. The solid transitions of the prepared nanosize carbides from the precursor were in similar process. CaMaOb and MCxOy were formed on the surface of graphite due to the 3PIs reaction of oxides and graphite, then becomed bulk phase with smooth surface. The smooth surface was destoried to rough surface with the prolonging of the elecrolysis and further becomed nanosize particle phase. The growth of the nanosize particle phase form outer to inner resulted in the complete formation of nanosize spherical particle of carbide. While long grain phase of CaM?aOb formed due to the fast growth could be observed in the solid transitions process of nanosize M?C.(4) The good stability of M?CxOy inhibited the electrodeoxidation of the solid pellet. According to the characteristics of M?CxOy, Ti2CO solid solution with uniform morphology was obtained by electrolyzed TiO2 and graphite composite with mole ratio 2:1. The voltage was 2.8 V, electrolysis time was 6 h and the temperature was 850 ?. The current efficiency reached to 68% and energy consumption was 5.3 kWh/kg. The pellt pressed by the Ti2CO powder showed good conductivity, the value of the electrical resistivity was 1.6×10-3 ?·mm, closed to some aluminium alloys. The applied voltage in the electrolysis should be controled at appropriate scope, the excessive voltage would result in the extraction of Ti metal, while the insufficient voltage would lead to a slow solid eletroreduction process. The oxygen content of Ti2CO powder continuously changed with the electrolysis time, the prepared product therefore present oxygen-rich state at 4-6 h, oxygen-poor state at 6-8 h and metal Ti impurity at 10 h or longer. The Ti2COn powder with oxygen-poor state showed the better conductivily than that with oxygen-rich state. The solid transition process of Ti2CO solid solution from TiO2 and graphite composite precursor was similar to the early solid transition process of nanosize TiC, the excessive electrolysis would destroy the structure of Ti2CO solid solution and lead to the extraction of metal Ti particle.(5) The ternary laryerd carbide Ti3AlC2 had been prepared by electrolyzed solid TiO2-Al2O3-C composite pellet in molten salt. The atomic stoichiometric ratio of Ti, Al and C was 3:2:1.8, the applied voltage was 3.0 V, the electrolysis time was 2 h and the temperature was 950 ?. The current efficiency and energy consumption were 83%and 8.1 kWh/kg, respectively. The excess Al2O3 was electroreduced to liquid Al which diffused in molten salt. The current efficiency and energy consumption regardless of the contribution of liquid Al formation were 69% and 9.3 kWh/kg, respectively. High temperature was beneficial to the assembly of ordered layered structure form unordered powder. Appropriate increase of the Al2O3 content could inhibit the formation of TiC impurity, but the excessive Al2O3 would result in the negative effect to the assembly of ordered layered structure. Appropriate decrease of the graphite could also improve the growth of ternary laryerd carbide, but further decrease would led to the formation of another ternary laryerd carbide Ti2AIC. The reasons for the high efficiency performance of Ti3AlC2 preparation mainly included three parts: conductivity effect of graphite, void effect of laryerd carbide and oxygen-resistant effect of Ti3AlC2. The solid transition processes of the laryerd structure form powder were including bluk phase formation due to the 3PIs reaction of oxides on the graphite surface, crevice fromation at the side of bulk and granulation on the surface of bulk. The increase of crevice and granulation resulted in the particle-assembly laryerd structure with multilayer overlay. |
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