| The energy consumption and major pollutant emission per unit of GDP dropped by nearly 20 percent and 10 percent respectively is the binding targets of energy conservation and emission reduction set in the 11st 5-year Plan as scheduled. While energy conservation and emission reduction is a long-term goal for Chinese metallurgical industry industry. There is a high energy consumption and high pollution for the traditional pyrometallurgical process in our country, especially for the metallurgical process of sulfides minerals. They are firstly calcined in air to metal oxides and then the oxides are thermal reduced to metals. This complicated process is energy intensive and suffers from double emissions of SO2 and CO2. Therefore, desulfurization of sulfide minerals effectively is the key point for metallurgical industry with efficient recycling economy and green sustainable development.Molten salts electrolysis of solid oxides have been investigated thoroughly since the FFC Cambridge process be reported at the end of 20th century. However, the research that develops in directly electrolysis of solid sulfides in molten salts are relatively rare. Electrochemical split of various metal sulfides to respective metals and sulfur could avoid the handling of the hazard SO2, therefore, it has important significance in theory and practice. Currently,85-90% of copper was extracted through pyrometallurgy technology, and the main copper mineral for Cu extraction is chalcopyrite (CuFeS2). Directly electrolysis of molten Cu2S has been reported for decades. However, pure Cu2S melt is not a suitable electrolyte due to its high electrical conductivity. Adding electrolytes have been tried to decrease the electrical conductivity of Cu2S, but the effects were unsatisfied. Alternately, this problem could be effectively solved by directly electrolysis of solid copper sulfides in molten salts. Unlike the molten salt electrolysis discussed above which required the fusion or dissolution of metal sulfide in the electrolyte, in the solid metal sulfide cathode electrolysis, it is ideally that the dissolution of cathode is insignificant. During the electrolysis, both the metal sulfide cathode and the resulting metal stay in solid, and no occurrences of dendrite at the cathode and no dissolution of metal ions into electrolyte. It is the S2- ions that leave the cathode, diffuse through the electrolyte, and discharge to S2 gas at the graphite anode. Therefore, it is an environmental friendly electrolytic process for the metallurgy of chalcopyrite and other sulfur-copper mines.Titanium is an important lightweight structural material of high quality and new functions. The development of advanced titanium industry has great strategic significance to the national defense, economic construction and social development, and it is also the symbol of national comprehensive in international competence. However, titanium is a gas-sensitive metal, especially strong affinity for oxygen and nitrogen as well as hydrogen, so it is very difficlt to extract pure titanium metal. Large-scale pure titanium has already can be produced by Kroll process, and up to now, this process dominant the current industrial pyrometallurgy process to produce sponge titanium. Nevertheless, the challenge in this technology is how to realize the energy conservation and emissions reduction. The difficulty of titanium extraction since oxygen has a great solubility in low-temperature cubic phase titanium (a-Ti), with the lowest solubility of 10 at.% (400?). However, the highest solubility of sulfur in a-Ti is approximately 0.02 at.% , more importantly, compared with oxygen, sulfur has smaller influence for the performance of titanium. As a consequence, extraction of titanium from titanium sulfide has a great potential application in the titanium metallurgy. The main topics and results of the research are summarized as following: 1. The electrochemical reduction of FeS and Cu2S were firstly investigated in the NaCl-KC1 molten salts respectively to evaluate the electro-reduction process of CuFeS2. The reduction mechanism of FeS and Cu2S were studied by cyclic voltammetry, three electrode potentiostatic and constant voltage electrolysis. The reduction mechanism of FeS contains two major process:It firstly intercalat Na+ and/or K+ ions and then followed with reduction of the intercalated compounds. The iron metal could be electrochemical obtained after the voltage higher than 1.5 V. Compared with FeS powder, the cyclic voltammetry curve shows that the reduction of Cu2S looked much simpler. Theoretically, the reduction of FeS should be slightly easier than copper sulfide, considering their thermodynamic decomposition voltages, which are 516 mV and 532 mV for FeS and Cu2S respectively. The analysis of obtained products indicated that the electro-reduction of Cu2S to Cu could be finished in a short time. During electrolysis, only one electron transferred in the reduction process of Cu2S to Cu. Meanwhile, with the same quality of sulfide, there exist less S2- concentration in the porous copper than iron, and the intercalations compounds of FeS might change the reduction mechanism of FeS, therefore, the S2- ion migrated from porous copper needs much less time. The current efficiency and energy consumption for electrolysis of Cu2S were 92% and 1 kWh/kg-Cu, respectively. Based on the experiments, solid FeS and Cu2S can be electrolyzed to pure Fe and Cu and elemental S respectively at high speed and current efficiency against the inert graphite anode.2. The electrochemical behaviors of chalcopyrite (CuFeS2) was studied in molten NaCl-KCl salt at 700 ?. The multi-cyclic voltammetry at different potential range shows that the Na+and/or K+ions could intercalate into CuFeS2 reversibly. XRD and EDX analysis indicated that the mainly ions are K+ in the intercalation compounds LxCuFeS2 (L=Na or K, x?1). The experiments suggest that iron could be electrochemical reduced priority, and the reduction of LxCuFeS2 determinates the rate of whole reduction process. The electro-reduction mechanism of CuFeS2 undergoing mainly three step:CuFeS2 ? LxCuFeS2 ? L(x-w)CuFe(1-y)S(2-z)+Fe ? Cu+Fe. Through constant cell voltage electrolysis, the current for reduction of CuFeS2 decreased to background in one hour at 2.4 V. The current efficiency is over to 80%, and the energy consumption about 2 kWh/kg (CuFeS2) even after two hours electrolysis at this voltage. In addition, the mixture of Cu and Fe from complete reduction, can be magnetically separated. After the separation, pure Cu can be obtained by leaching out the residual Fe (10 wt.%) with acid. These findings can form the base for the development of a new, fast, and environmental friendly electrolytic process for the metallurgy of chalcopyrite and other sulfur-copper mines3. In order to evaluate electrolysis of titanium sulfide to prepare titanium metal, the synthesis of titanium sulfide in a large scale was firstly investigated. Highly crystalline hexagonal titanium disulfide (TiS2) were synthesized at the gas/liquid interface between TiCl4 vapor and the dissolved Na2S in equimolar NaCl-KCl melts at temperatures between 700? and 850?. TiS2 sheets with sizes of about 10?20 ?m and thicknesses of about 50?200 nm have been collected at 700??750?, and much bigger (50?100 ?m in sizes) and thicker (1?5 ?m in thickness) TiS2 sheets were obtained at 85O?. Meanwhile,10 mm × 5 mm in area and 30?50 ?m in thickness have been collected without special protection to the surface layer of the molten salts during washing. Upon ultrasonic, these assembled TiS2 could be exfoliated to few-layer TiS2 sheets with sizes of hundred to thousand nanometers and thicknesses of 3-5 nm. The mass transfer preferential growth of an interphase layer and the thermodynamically preferential enlargement of the ab-axis of the hexagonal TiS2 crystal were thought to have led the formation of large and thin TiS2 sheets in this proposed gas/liquid interface process. Due to the density difference, the ripe TiS2 sheets sank into the melt allowing the continuing preparation of TiS2 sheets. However, the formation of more regular, larger and thicker TiS2 sheets accompanying such kind of fine prisms at higher temperature could also follow the Ostwald ripening mechanism. The presented approach should be potentially extendable to the synthesis of large and thin sheets of other layered metal dichalcogenides in large scale. The preparation of TiS2 from K2TiCl6and Na2S was tried and in-situ import S2- from iron sulfide in molten NaCl-KCl was also tried.On the other hand, Ti5S8 was prepared via carbothermic sulfidation of TiO2. The different carbon materials determinates the degree of the reaction. Graphite, coke, active carbon, and acetylene black were selected to compare the activity of this reaction, we found that the active carbon exhibits highest activity and graphite hardly participate in the reaction. Moreover, nano titanium dioxide powders will contribute to the carbothermic sulfidation. To obtain pure Ti2S8, the temperature must be higher than 1000 ? and should maintain more than 20 h, while the holding time could decrease to 10 h after temperature increased to 1100 ?, and the more sulfur addition, the higher mole ratio of S/Ti.4. The extraction of titanium by directly electrochemical reduction of titanium sulfide intermediates have been investigated in molten LiCl salt at 700 ?. The electro-reduction mechanism of titanium sulfide was studied via cyclic voltammetry, constant cell voltage and potentiostatic electrolysis together with XRD. The feasibility of the electrochemical reduction titanium sulfide intermediates to Ti was confirmed by the reliable experimental results from this work. Based on the experiments, the TiS2 firstly formed LixTiS2 (0?x?1), and then with the cathodic polarization, TiS can be detected easily. In addition, compared to other steps, the reduction of TiS was more difficult in kinetics, however, a litter over potential would contribute to quickly reduce and obtain pure Ti. Upon applying constant cell voltage and potentiostatic electrolysis, pure titanium obtained successfully in the molten LiCl salt, in the form of either titanium powder or sponge. Also the electrochemical exfoliation of TiS2 has tried in the molten LiCl, and the best exfoliation of titanium sulfide was obtained from-1.0V for 1h. The highest current efficiency reached up to 99.6% and energy consumption lower than 4.5 kWh/kg-Ti via potentiostatic electrolysis of TiS2 pellets at -2.1 V (vs. Ag/AgCl). And the current efficiency of 78% together with energy consumption of 7.15 kWh/kg-Ti was obtained from constant cell voltage electrolysis at 2.5 V for 4 hours. The total energy consumption from TiO2 to Ti5S8 to Ti calculated approximately to be 16 kWh/kg-Ti, which is about one third of the Kroll process (45-55 kWh/kg-Ti). |
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