Vacuum Carbothermic Reduction Chlorination Aluminum

In this dissertation, the usage and all kinds of production method of aluminum have been discussed, and, the bauxite resource of all over the world and the development situation of our country's aluminum industry have been discussed. Through these discussions, all the production methods of alumina which include Bayer process, sintering process, Bayer and sintering combined process will face the crisis that the bauxite resource which suits present production methods is getting less and less, and all above-mentioned methods produce a lot of waste water and waster residue which pollute the environment. On the other hand, the HEROULT-HALL reduction process which is used to produce aluminum from alumina is a process which is high energy consumption and big investment and high pollution. In order to find a new method for producing aluminum, the process of producing aluminum by carbothermic reduction-chlorination under vacuum has been studied in this dissertationThe aluminum in bauxite exits in the form of alumina, but it can not be produced directly through reducing bauxite with reductant such as carbon, because the aluminum from bauxite mixes with other impurities, and the separation of the aluminum is very difficult. Now the author will produce aluminum from bauxite directly by carbothermic reduction-chlorination. The main principle is that with the aid of the property of aluminum subchloride (AlCl) which is stable at high temperature, but disproportionate into aluminum and aluminum trichloride (AlCl₃) at low temperature. Aluminum trichloride vapor is vented in the reaction system of bauxite and carbon at high temperature, thereby initiating the reaction among bauxite and carbon and AlCl₃ to produce aluminum subchloride (AlCl), venting the said aluminum subchloride into a cooling zone in the reaction system where the said aluminum subchloride disproportionate into aluminum and aluminum trichloride, taking advantage of the difference in boiling point of the two substance, the aluminum is condensed in a cooling zone and the aluminum trichloride can be collected in a more cooling zone, thus aluminum is produced, and the collected aluminum trichloride can be recycled back into the process. In a word, in the whole reaction system the aluminum trichloride only acts as a medium, through which aluminum can be produced by reducing the alumina in the bauxite directly with carbon, but because of the existence of the said aluminum trichloride, the reaction temperature of the main reaction which produce aluminum subchloride from bauxite and carbon andaluminum trichloride is dropped greatly, and the purity of the metallic aluminum is high. In other words, because of the existence of the aluminum trichloride, it is possible to produce aluminum directly by reducing bauxite with carbon. Because the gas volume of the main reaction which produce aluminum subchloride is increased from one to six (Al2O3(SOiid) + AlCl3(gaS) +3C(soiid) = 3AlCl(gas) + 3CO(gas)), under vacuum, the reaction will become more easily.By the research of equipment, a vacuum kiln which suit to the said reaction system;is obtained. The vacuum kiln is heated by electricity. There are for temperature zones in the vacuum kiln, i.e. high temperature zone, moderate temperature zone, loWj temperature zone and the zone for AICI3 sublimating. After the vacuum kiln being used in the experiment, it is verified that the main reaction which produces aluminum subchloride, the disproportion reaction of the said aluminum subchloride to product;liquid aluminum and vapor aluminum trichloride, the condensation or collection of the said aluminum trichloride and the independent sublimation of AICI3 can take place in the same vacuum kiln at the same time.In this research, the experiment is divided into two steps. In the first step, the alumina-bearing material is commercial alumina, the reductant is coal, and chloriding agent is aluminum trichloride. From the condition experiment, the law how reaction temperature, the kind of the reductant coal, the ratio of the reductant and alumina, and the reaction time affect the process of producing aluminum from commercial alumina is concluded. In this step, the purity of the metallic aluminum can arrive at 84%⁸8%. After analysis, the reason why the quantity of the metallic aluminum is not regular, the reason why the recovery per cent of the said aluminum trichloride is small, and why the purity of the metallic aluminum only arrived at 88% have been found. The experiment laid a foundation for the second step where aluminum is produced directly from bauxite. In the second step, the alumina -bearing material is bauxite, in this step, the factors such as reaction temperature, the kind of reductant, the ratio of the reductant and bauxite, anjd reaction time which affect the main reaction were observed. From the experimental result ,we know: ?Among coke, charcoal and coal, coke is more suitable than charcoal and coal. ?The most suitable reaction temperature is about 1370°C. ?Along with thje reaction time prolonged, the extraction rate of the aluminum increase, but when the time is longer than 120 minutes, the increase speed of the extraction rate will slow down. Soit is advantageous to prolong reaction time properly, but there is no point in prolonging too much. ?The most suitable ratio between bauxite and coke is that the quantity of the carbon in the coke is 1.5 times of the theoretical quantity of carbon which just meet the reduction of all the alumina in the bauxite.After analyzing the composition of the metal pellet, we know that the purity of the metallic aluminum abstained directly from bauxite can arrive at 90%. After analyzing the composition of the reaction residues, we know that besides the main reaction among the alumina in bauxite, the carbon in coke and aluminum trichloride, other component in the bauxite such as Fe2C>3, T1O2 and SiC>2 also can react with carbon to produce carbide or monoxide respectively such as Fe3C, FeO, TiO, TiC, SiC, but not to produce their elemental substances, these carbides and monoxides mainly remain in the residue, only a little is taken to the condensing plate by the arising gas and adsorbed by the condensed metallic aluminum, which can influence the purity of the metal.After thermodynamic analysis of the main and side reaction in the reaction system, we know: (A) it is feasible that producing metallic aluminum from bauxite directly by carbothermic reduction-chlorination under vacuum. Through calculation, under normal pressure the initial temperature of the main reaction among bauxite, carbon and AICI3 which produce AlCl is 1848.21 °C(2121.21K). (B) if the system pressure can be reduced, the initial temperature of the above -mentioned main reaction among bauxite, carbon and AICI3 to produce AlCl will descend. (C) when the above-mentioned main reaction takes place to produce AlCl, the side reaction between alumina and carbon will take place to produce AI4C3, but because AlCl is gas and AI4C3 is solid, it is avoidable that the metallic aluminum coming from the disproportionation reaction of AlCl mixes with the AI4C3, which is the main problem that could not be solved when aluminum is produced directly from alumina by carbothemic reduction. At the same time, the impurities in the bauxite such as Fe2C>3, TiC>2 and SiC>2 will react with carbon to produce SiC or Si, TiO or TiC, Fe3C or FeO, under the reaction condition, all the impurities and their product is not volatile, so all the above substances can separate from the gas AlCl, thus, theoretically, the gas AlCl will be pure enough and the metallic aluminum from the disproportionation of the gas AlCl will be pure enough too. (D) the decomposition temperature of AlCl is influenced heavily by the system pressure, the higher system pressure, the higher decomposition temperature. At the same pressure, when thetemperature drops, the decomposition of A1C1 will become easily.After calculating the theoretical energy consuming, and comparing with present production methods of aluminum, we know that the process of producing aluminum by carbothermic reduction-chlorination under vacuum will be more superior. There are five points of creation in this research:(1) There is creation in technology. The chloriding agent is aluminum trichloride, it can be recycled.(2) There is creation in the material for producing aluminum. Aluminum is produced from diasporic bauxite which is peculiar to our country.(3) The thermodynamic analysis of the process which produce aluminum from bauxite by carbothermic reduction-chlorination is firstly carried out systematically.(4) The process of producing aluminum from alumina and bauxite by carbothermic reduction-chlorination under vacuum is firstly researched systematically.(5) There are creations in reaction equipment. The reaction equipment -vacuum kiln is designed by the author herself, and the vacuum kiln has been used in th experiment successfully.