The Influence Of Molten Salts On The Kinetics Of The Solid Compound Cathod

Preparing metals and alloys by direct electro-reduction of solid oxides in molten salts (FFC Cambridge Process) has been regarded as a new metallurgical method. This new process works at relatively low temperatures and is easy in operation, low in energy consumption and friendly to the environment. By this method, lots of metals and alloys such as titanium, tantalum, chromium, niobium, tungsten, silicon as well as some heavy rare metals-Tb have already been successfully prepared. But the studies concerning the influence of molten salt composition to the reduction mechanism of solid compound have scarcely been reported up to now.In this work, MoS2and WS2were used to investigate the influence of molten salt composition to the electro-reduction mechanism. The studied electrolyte included CaCl2, CaCl2-NaCl, NaCl-KCl, LiCl, NaCl-LiCl and KCl-LiCl melts. It should be pointed out that MoS2is the main natural sources of Mo, hence study the direct electrolysis of MoS2to metal and elemental S has great significance not only in low cost metallurgy of sulfide, but also in environmental protection.On the other hand, mass transfer could be a major factor affecting the rate of electrolysis during the electroreduction of solid compound. Presently there are no suitable theoretical and experimental methods for the study of ion diffusion in a porous cathode. In this work, an instantaneous ion release diffusion (IIRD) model has been developed to study the diffusion of O2-ions in the liquid electrolyte contained in cathodically generated porous Fe from solid Fe2O3in molten CaCl2and the diffusion coefficient was obtained at various temperatures. In addition, the diffusion of sulfide ion the electrolytic porous W in NaCl-KCl mixture and LiCl was also studied.The main topics and results of the research are summarized as follows:1. The electro-reduction of MoS2was investigated in CaCl2based molten salts. The experiments demonstrated that the optimizing electrode potential is-0.4～-0.6V (vs.Ag/AgCl) in the850℃CaCl2. Large overpotentials are favourable for a fast reduction of the sulfide, but excessive overpotential may lead to S2-saturation, and then the in-situ precipitation of CaS at the three phase electrochemical interline, impeding the reduction of the remained MoS2.The electro-reduction of MoS2and WS2were investigated in700℃CaCl2-NaCl and it was found that the electrolysis speed in CaCl2-NaCl molten salt was much slower than that in850℃CaCl2, possibly due to the lower solubility of CaS in the low temperature CaCl2-NaCl melt. In addition, the Na+would have changed the electro-reduction of the sulfides to a slower mechanism. In a word, the CaCl2based molten salts with low solubility of sulfur ions or CaS are not suitable electrolyte for the metal sulfide electrolysis.2. Nano-molybdenum and nano-tungsten powder have been successfully obtained through electro-desufidation of solid MoS2and WS2in NaCl-KCl molten salt at700℃. In addition, the details of the electro-desulfidation mechanism of solid MoS2and WS2were also investigated by cyclic voltammetry, potentiostatic and constant voltage electrolysis. The NaCl-KCl has high capacity in dissolving and transferring the S2’ions. The electro-reduction of WS2was simple in mechanism, undergoing mainly two steps:the intercalation compound containing Na+/K+was first generated and then was further reduced to metal W, both were very fast. In the electro-reduction of MoS2, intercalation compounds containing Na+/K+and M03S4were also found as intermediates first, however, the whole reduction of MoS2in the NaCl-KCl mixture was much slower, mainly due to the tardy reduction of the intermediates. Anyway, NaCl-KCl should be ideal electrolyte for fast electroreduction of WS2, and also applicable for the electrolysis of MoS2.3. The reduction mechanism of MoS2in LiCl molten salt was investigated by cyclic voltammetry, three-electrode potentiostatic electrolysis and constant voltage. The product was characterized by XRD and SEM. The electroreduction mechanism may be speculated as followings:intercalation compound containing Li+(LixMoS2) was first generated and then LixMoS2was reduced to nano-Mo. It can be found that the rate of electroreduction MoS2in LiCl salt is much faster than that in NaCl-KCl molten salt. The participation of Li+led to a reduction mechanism with low activation energy. It can be concluded that, LiCl melt is suitable for the fast electrolysis of MoS2to nanometer Mo powder.4. MoS2has also been studied in LiCl-NaCl and LiCl-KCl molten salts, aiming to find a more desirable system. The experiments demonstrated that the Li+, Na+and K+would be all involved in the reaction of MoS2but showing competitive relationships. The intercalation compound containing Li+and Na+can be formed during the electrolysis of MoS2in NaCl-LiCl. The former can be reduced quickly and the latter would suffer from the dynamic barrier. However, the electroreduction speed of MoS2in NaCl-LiCl is much faster than that in NaCl-KCl, which might be due to that the Mo generated fast from the intermediate containing Li+enhanced the conductivity. The electrolysis of MoS2in KCl-LiCl molten salt was faster than that in NaCl-LiCl, probably due to the ionic radius of K+is larger and intercalation intermediate containing K+was fewer. Owing to its easy operation, lower cost and high current efficiency, LiCl-KCl molten salt would be better choice for electrolysis preparation of nanometer Mo powder from solid MoS2.5. Diffusion theory and experimental method have been established for investigation the ion in a porous metal layer. By solving the Fick’s second law, an instantaneous ion release diffusion model-IIRD was set up. The former designed powder metal cavity electrode was used as the mini-porous electrode for the double-potential step experiments. The experimental results of O2-and S2-in porous metal were in well agreement with the theory. The diffusion coefficients and diffusion activation energy of O2-was9.1×10-6cm2/s (850℃) and67.8kJ/mol in the electrolytic Fe from Fe2O3in molten CaCl2, while the diffusion coefficients and diffusion activation energy of S2-was0.92×10-5cm2/s and53.375kJ/mol in NaCl-KCl,1.0×10-5cm2/s and54.023kJ/mol in LiCl in the electrolytic W from WS2at700℃. The IIRD model and the mini porous electrode design provide a convenient approach for the study of liquid diffusion in a porous electrode. These findings may be helpful to understand and optimize the solid cathode electrolysis process from the view of mass transfer.