| Metals and alloys play an important role in the economy growth of each country. They are widely used in the chemical, metallurgical, marine, air and land transport, and aerospace and defense industries and also many other fields. However, due to the gradual exhaustion of fossil resources and the growing impact of CO2 emission on the global climate, the traditional technologies for the productions of pure metals and alloys are regarded to be complicated, energy intensive, and environment unfriendly. These disadvantages are also responsible for the high market prices of a number of special metals and alloys with desirable material properties, which limits serious their wide applications. Clearly, to address these issues of the traditional production for metals and alloys, it is necessary to develop new and low cost production technologies.Electrochemical extraction of metals and alloys from solid oxides in molten salts is a revolutionary technology for the material and metallurgical industries, which is developed in the University of Cambridge in recent years (FFC-Cambridge Process). This process works at low reaction temperature with simpler operations and lower production cost and is friendly to the environment, providing a new way for the preparation of high melting point metal and alloys. The FFC-Cambridge process has been the research focus for the preparation of a variety of pure metals and alloys. However, the process is still in the stage of development and many basic problems have to be overcome, such as the low current efficiency and the carbon contamination in molten CaCl2 electrolysis. All those factors give a hard restrict for the development of FFC-Cambridge process. In this process, the low current efficiency came from the limitation of the diffusion of the oxygen ion from the oxide to the electrolyte and the high background current, while the carbon contamination mainly came from the high solubility of CaO in molten CaCl2. So it could be concluded that all those problems came from the effect of free oxygen ions. In this thesis, based on the FFC-Cambridge process, using electrochemical methods (cyclic voltammetry and constant potential electrolysis) and material characterization tools (XRD, SEM, TEM, EDX., LECO), a series of studies on the optimisation of the cathode process and electrolyte were carried out in different chloride salts. The main topics and results of the research are summarized as follows:1. By studies on the electrochemical reduction process of the TiO2 in molten CaCl2, we found that the reactions of TixO (x≥1)-Ti to be the rate-determing step. Together with the reduction process of all kinds of metal oxide in molten CaCl2 and the current efficiency of metal extraction through the FFC-Cambridge process, we identified that the large metal-to-oxide molar volume ratio (Ti/TixO (x≥1)) as an intrinsic barrier to the solid state reduction of TiO2 to Ti. This barrier cause the decline of three phase interlines, equivalent to decrease of the active point for the reaction. To optimize the electrolysis of oxides with large molar volume ratio, a simple method is to improve the porosity of oxide pellets (precursors). Based on careful analyses of the pros and cons of various fugitive agents, we have chosen NH4HCO3 to change the porosity of the oxides pellets. As a green fugitive agent, NH4HCO3 is recyclable and clean. Research on the influence of the pellet porosity on the cathodic reductions of the FFC-Cambridge process was carried out by electrolyzing a series of TiO2 pellets with different porosities. This work identified the optimal porosity to be between 60-70% for 1 g TiO2. TiO2 pellets of this porosity could be electro-reduced in molten CaCl2 with current efficiencies ranging from 54.4% to 32.3%, yielding products with only 1.45 wt% to 0.19 wt% oxygen, respectively. Together with the concept of two-voltage electrolysis, this study achieved the highest process efficiency and lowest energy consumption to produce Ti metal with oxygen content reaching below 2000 ppm. Thus, this work represents not only an important advancement in electrochemistry and chemical metallurgy, but also technical innovations for the development of the solid-state electro-reduction of TiO2 to Ti into a more affordable and green commercial reality. In this work, we also adopted the "green" bypass to improve the electro-reduction process of Nb2O5 and identified the optimal porosity to be about 64.8% for 1 g Nb2O5. Meanwhile, we have now better understanding of the NbO-to-Nb conversion to be NbO->NbO0.7'Nb. The "green" bypass could also be employed to optimize the production of other metals, such as Zi, Si and so on.2. We made a comparison between alkaline-earth metal chlorides for the problems of free oxygen ions. Studies were carried out by choosing the molten MgCl2 as the electrolyte for its low solubility of MgO and simpler operation. Studies on the electrochemical behaviors of metallic cavity electrode filled with Ta2O5 or ZrO2 powder in molten MgCl2 suggest that solid metal oxides can also be reduced to the metals quickly. Focusing on the reduction of Ta2O5, we studied the reduction process of Ta2O5 in molten MgCl2 by cyclic voltammetry, potentiostatic electrolysis, together with spectroscopic, electron microscopic and elemental analyses. To decrease the evaporation of MgCl2 at elevated temperatures, the electrolysis of Ta2O5 was implemented in molten MgCl2-NaCl-KCl (700℃). By electrolysis at cell voltages from 1.4 V to 2.2 V, fine Ta powders (20-100 run) were produced with the oxygen content varying between 0.74 wt% and 1.54 wt%. Due to the low solubility of MgO in molten MgCl2, a high current efficiency (93%) was achieved by applying this method. Based on those studies, we proposed a new electrochemical deoxidation technology here. In this process, the oxygen produced in the cathode was seperated from the system as solid MgO and no anode discharge was needed, so the effect of free oxygen ion was avoided. The anode reaction was Cl" to Cl2, so the graphite rod could be used as an inert anode. As a result, CO2 emissions are eliminated, while the content of carbon in the product was also reduced sharply. Equally, metal Nb, Zr, Ti and etc were obtained in the mixed molten salts of MgCl2-NaCl-KCl at 700℃. However, the purity of the product was reduced because the by-product MgO was difficult to be removed completely by reaction with acid.3. Various alkai chloride molten salts were also studied according to alkai metal oxyde being much easier to dissolve in water compared with MgO. Nb2O5 was taken as an example in order to study the reduction of metal oxide in molten alkali chloride salts: 1.It was found that Nb2O5 could be reduced rapidly in molten LiCl with the reduction path being Nb1O5'NbO2、LiNbO3'LiNbO2'Nb.2. The reduction process of Nb2O5 in NaCl molten salt was also studied. Although Nb2O5 could be completely reduced in NaCl molten salt, the reduction is deadly slow because of limited polarization and oversize of reactant particles.3. According to cyclic voltammetry, the reduction of Nb2O5 could not reach at Nb metal in molten KCl at 900℃, before the formation potential of the K metal. So, the intermediate compound such as LiNbO2、Nao.66Nb02、NaNbO2、KNb3O5 might be produced in the molten alkali chloride salt for its poor conductivity to the O2- ions.4. Synthesis of complex oxide materials was studied in molten salts with poor solubility of metal oxide.1. Synthesis of MgTi2O4 was tried in molten MgCl2 via constant potential and constant voltage electrolysis, and also changing the precursor. However, although products with compositions matching closely to that of the target were obtained, the presence of MgO left in the product was evident, apparently due to its poor solubility in the molten salt.2. Synthesis of LiNbO2 in molten LiCl was tried. The result showed that it was promising to obtain single-phase LiNbO2 with relatively small particle size from partial electro-deoxidation of LiNbO3. In the same time, LiNbO2 was studied as electrode material for Li-ion battery, although the performance was inferior to expectation. |
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