Study On The Preparation Of Titanium-iron Alloy Powder By Reduction Of Ilmenite By Molten Salt Electrolysis

Due to the fact that ferrotitanium alloys are used as the well-known hydrogen-storage materials, deoxidizers and alloying elemental additives, they have been applied widely in the hydrogen-storage business and steelmaking industries where ferrotitanium alloys are served as the alloying elementals in liquid steel and play an important role of refining the grains and improving the intensity of the steel. Traditionally, ferrotitanium is prepared by aluminothermic reduction method or by re-melting iron and titanium scraps together at high temperatures. However, these two methods have many disadvantages such as complex extraction process, high commercial cost, low quality of products and thus restrict the applications of FeTi products. The direct deoxidization method, high-temperature molten salt electrolysis that is well known as the modern metallurgical method, combines the electrochemistry and pyrometallurgies with the advantages of simple smelting process, high quality of products, lower energy consumption and so on. Thus, in order to solve the problems existing in traditional techniques and improve product qualities, the application of molten salt electrolysis into the preparation of ferrotitanium alloys could be a potential alternative method that embodies importantly scientific significance and wide application foreground.In this paper, ferrotitanium alloys have been successfully prepared from solid ilmenite by methods of the direct molten salt electrolysis (the FFC-Cambridge Process) as well as the indirect calcium electrodeposition-calciothermic reduction (the OS Process), respectively. The reducing procedures together with the ilmenite deoxidization mechanisms during the direct electrolysis have been studied systematically by use of the electrochemical measurements such as cyclic voltammetry, cathodic potentiostatic electrolysis, and linear sweep voltammetry. The effects of electrolytic technologies such as the cell voltage, temperature, bulk porosity and the concentration of CaO on the product compositions and microscopic morphologies of products have also been examined. The calcium electrodeposition-calciothermic reduction process is introduced for the first time to investigate the indirect reducing path as well as the corresponded mechanism. The major research results are drawn as follows:(1) According to the thermodynamic calculation, the reduction can be carried out easily to produce iron and perovskite, but the further reduction of perovskite is difficult. The increase of the activity of CaO could decrease the decomposition voltage of FeTiO3 but increase that of CaTiO3. The effect of the activity of CaO on the decomposing reaction of CaO is more obovious, which could lead to the indirect electrolytic reduction of ilmenite.(2) The direct electrolytic reduction of ilmenite in equimolar CaCl2-NaCl melt is a complicated stepwise and heterogeneous process. The main reducing process is summarized as follows:the ferrous oxide with more positive reduction potential is first reduced to form metallic iron and the titanium oxide in ilmenite spontaneously incorporates with Ca2+ from electrolytes to form CaTiO3; CaTiO3 is further deoxidized directly in the presence of metallic iron to produce Fe2Ti, FeTi. The preferentially produced iron metals are embedded in the perovskite particles to form network structures with the better conductivity performance, leading to the increase of electron transport inside the pellet and promoting the deoxidization of perovskite in the presence of iron. In the early electrolytic period, the reduction rate is fast due to the larger reaction interface and the positive reduction potential of ilmenite, but in the later period, as electrolysis penetrates inside the pellet, the reaction interface areas are decreased. Moreover, compared to that of ilmenite, the decomposition potential of perovskite is much more negative. At last, the reduction kinetics is decreased. The O2- concentration in electrolyte affects the reduction process. When 0～1.0mol% CaO was manually added into equimolar CaCl2-NaCl molten salt, the primary reduction kinetics of ilmenite to iron and perovskite is accelerated with the increase of CaO amount; but as controlling the CaO concentration in 1.5～2.0mol%, increasing CaO concentration restricts the positive deoxidization of CaTiO3, resulting into the decrease of the reduction kinetics. Therefore, about 1.0mol% CaO added into the equimolar CaCl2-NaCl molten salt could accelerate the reduction process and save electrolytic time.(3) The cell voltage, the working temperature and the porosity of the pellet affect the direct electrolytic reduction of ilmenite significantly. During electrolysis, the applied cell voltages above 3.0V are favored to the deoxidization of ilmenite; the porous FeTi alloys with homogenous grains are more easily produced as increasing the cell voltages. When temperature is carried out at 873～1173K, increasing the working temperature helps to increase the reduction kinetics of the reactant ilmenite and greatly influences the grain growth of the product ferrotitanium, resulting in the increase of the average particle size from 2.1μm to 5.2μm. The better electrolytic temperature is 973K to produce the homogeneous ferrotitanium alloy grains. The porosity of the pellet plays the critical effect on the reduction of ilmenite. The elevation of the porosity helps increase the interface areas between the solid phases and the electrolyte, and also helps accelerate the diffusion process of O2-. In order to prepare the multi-porous ilmenite pellet with moderate mechanical strength, the optimal experimental arrangement is mixing 25% of NH4HCO3 by mass with ilmenite powders, resulting in producing the largest pellet porosity that is up to 39.08%. After electrolysis, porous ferrotitanium are produced, the structure of which is body-centered cubic structure and the preferred orientation of which is (100) plane. Moreover, as the FeTi alloy grains grow up, the preferred orientation of FeTi is not affected by the changes of temperature, electrolytic time and cell voltage.(4) The indirect electrolytic reduction of ilmenite is also evidenced to be stepwise: ilmenite is first reduced by metallic calcium to iron and perovskite at the ilmenite/CaCl2-NaCl molten salt interfaces; the intermediate CaTiO3 is further reduced by calcium in the presence of metallic iron to form ferrotitanium alloys. Thus, pores presented inside the ilmenite pellets play an important role during the calciothermic reduction. Increasing the cell voltage increases the deposition kinetics of metallic calcium at the cathode and further increase the reduction process. The technical performance of calcium electrodeposition-calciothermic reduction process has been improved by applying ilmenite powders as the raw materials. After 14h of reduction at 973K, ilmenite is thoroughly reduced to ferrotitanium alloys.