Study On Pretreatment Of Bauxite During The Production Of Al-Si Alloy

Around the world, about90%of bauxite was used to produce alumina and25%of the aluminum was used to produce casting alloy. Among the aluminum alloys, it is mainly of Al-Si alloy. Al-Si alloys are most widely used in aerospace, automobile cylinders, welding wire, air conditioning rotor, etc. due to very attractive characteristics such as good mechanical properties, hardness, low proportion, corrosion resistance and so on.At present, most Al-Si alloys are produced by melting pure aluminum and silicon. The technics suffer from high energy consumption, long cycle of production, high transportation costs and serious environmental pollution. Low grade bauxite and other resource can be used by electrothermal reduction. It is beneficial to alleviate the resource exhaustion and satisfy the characteristics of aluminum resources in China. However, this method also suffers its defect that there are so many impurities in coarse Al-Si alloy. The main impurities are metal and non-metal slag such as iron and titanium and so on. Iron and aluminum in Al-Si alloy can produce α-Fe (Al8SiFe2) and β-Fe (Al5SiFe). The β-Fe is hard and brittle and it can destroy the metal base of the connection strength, reduce the mechanical properties of the alloy. When iron exists in the Al-Si alloy, the alloy surface would not form a continuous oxidation film. Consequently, the β-Fe which separate out from grain boundaries can promote electrochemical corrosion occurred and make alloy corrosion resistance down. Excessive amounts of titanium can reduce the performance of the alloy, for the titanium can produce TiAl3with aluminum which can increase the viscosity of the alloy. Therefore, it is necessary to remove iron and titanium from the raw material when low grade bauxite is used as raw material to produce Al-Si alloy by electrothermal reduction.The raw material was analyzed in thesis, and the results indicate that the main object of the Sichuan bauxite (A ores) is kaolinite, boehmite, cronstedtite and rutile, while the Yunnan (B ores) was consisted of diaspore and rutile. The titanium in bauxite is very dispersive which may even replace part of the grid of the aluminum, so the removal rate of titanium can be increased by fine grind. The theory of removal of iron and titanium was researched as well, and the results indicate that the titanium cannot be removed effectively by using conventional acid leaching while the titanium can be removed the titanium in bauxite effectively with high concentration acid at high temperature.The Sichuan bauxite (A ores) was used in this experiment. Leaching by hydrochloric acid and sulfuric acid, high pressure hydrochloric acid leaching, roasting by ammonium-leaching by acid and magnetic separation were researched. Finally the method of roasting by ammonium-leaching by acid was took for the process route. Under this process route, the effects of removal rate of iron and titanium such as the addition of ammonium, roasting time and temperature, leaching temperature and time and the concentrate of acid have been studied. The results indicate that the optimum conditions are ammonium addition of1:1.0, roasting time of150min, roasting temperature of400℃, acid concentrate of lmol/L, leaching time of60min, leaching temperature of70℃. Under the optimum congditions, the average removal rate of iron and titanium can reach up to89.88%and61%, respectively. However, the loss rate of aluminum is19.21%.Based on the characters of Yunnan bauxite, the raw materials have been cured by sulfuric acid in order to remove iron and titanium from bauxite. The results indicate that the optimum conditions are sulfuric acid addition of1:1.0, cure time of150min and cure temperature of200℃. Cured by sulfuric acid after roasting have been researched as well. The results show that the roasting have little influence on the removal rate of iron and titanium in bauxite.