| The production of aluminum in China is more than 13 million tons in 2008, which is about one-third of the world’s total output. But the bauxite self-sufficiency rate has been below 50% and the lack of bauxite in China has become the the primary issue to constrain the development of aluminum industry. The method producing Al2O3 by ore-dressing Bayer process with low-grade bauxite is one of the main directions of aluminum industrial development. It is about 25% of the low grade bauxite becomes tailings in the process. If the tailings is not be used, it will not only waste the resources, but also pollute the environment. At present more then 25% of the industrial aluminum was used in the production of Al-Si alloys. Producing the coarse Al-Si alloy by using bauxite tailings as raw material and then the cast Al-Si alloys can be obtained after the coarse Al-Si alloy diluted by pure aluminum and refined by purificant. This method can save industrial aluminum, reduce power consumption and shorten the process.In this paper, the process was studied that is producing coarse Al-Si alloys by using bauxite tailings as raw material, soft coal as reductant in arc furnace and the process of producing cast eutectic Al-Si alloys by carbothemal reduction in industry were studied. The results show that the content of iron in the coarse Al-Si alloys is too high to producing cast Al-Si alloys by using the production technique at the present stage.The removing iron process of bauxite tailings by leaching with hydrochloric acid to produce low iron bauxite tailings and the process of producing coarse Al-Si alloy used the bauxite tailings which removed iron as raw material were studied.In the removing iron process of bauxite tailings by leaching with hydrochloric acid, the most important influencing factor is leaching temperature, followed by the concentration of hydrochloric acid, and then the leaching time, the minimal is liquid-solid ratio. The leaching rate of iron oxide in tailings rise with the rising of leaching temperature, the accreting of the concentration of hydrochloric acid, the growing of leaching time and the increasing of liquid-solid ratio. The optimal leaching factors are leaching temperature 80℃, concentration of hydrochloric acid 22%, leaching time 120min and liquid-solid ratio 5:1. On this conditions, the leaching rate of iron oxide is more than 95%, the leaching rate of aluminum oxide is less than 4.3%, and the iron oxide content can be reduced from 10.52% to 0.6% after leaching process.The result of kinetics analysis of leaching of bauxite tailings with hydrochloric acid shows that there are two phases in the process:the rate of iron dissolution is controlled by the interface chemical reaction when the leaching rate of Fe2O3 is lower than 55%. Its apparent activation energy is 86715J/mol. The rate of iron dissolution is controlled by the non-steady-state diffusion in porous solids of the fluid reactant HCl when the leaching rate of Fe2O3 is higher than 60%. Its apparent activation energy is 119647J/mol.The flocculants were used for solid-liquid separation in the slurry of leaching tailings. The tailings and leaching solution can rapid separation used as polyacrylamide flocculant and the solid content in the supernatant is below 0.2g/L.Because of the low utilization of HCl (only 14%), the method of repeated distillation and leaching was investigated to increase the utilization of hydrogen chloride. The leaching solution can leach five times by distillation-leaching- distillation-leaching, the better distillation rate is 10% every time. After fifth leaching, the utilization of HCl is more than 50%, the Ferric chloride concentration is about 18.5%, and it can deal with 0.8 kg bauxite tailings every liter hydrochloric acid.The solution obtained by leaching of bauxite tailings was concentrated by distillation firstly and then baked on high temperature. Hydrogen chloride vapor and ferric oxide which concluded some aluminum oxide can be obtained after the leaching solution treated, the hydrogen chloride vapor can be absorbed by water to produce hydrochloric acid and recycle, the ferric oxide can be used as raw materials of iron. The cost is below three hundred yuan per ton bauxite tailings.Mixture and briquetting by using bauxite tailings which removed ferric oxide as raw materials, soft coal as reductant, sulfite pulp waste liquor as agglomerant. Based on the mixture calculating and the testing of conglomeration property, the optimum condition of mixture and briquetting are as following:adding 52.34 kilograms soft coal, nine kilograms agglomerant power and nine kilograms water every one hundred kilograms tailings. The pressure of briquetting is 20-25MPa and the grain of raw materials is 0.087-0.124mm.The coarse Al-Si alloy produced in arc furnace which used the tailings removed ferric oxide as raw material contained more than 55% aluminum,30.87% silicon and lower than 1.7% Fe. The main impurities were calcium, titanium, potassium and sodium. The aluminum recovery was 91.78%, the silicon recovery was 87.89%. the coarse can be used to produce cast Al-Si alloys or new type of Al-Si-Ti complex alloys.The reduction mechanism of bauxite tailings was studied in vacuum carbon tube furnace. The phases of products in different temperatures and carbon additions were analyzed by XRD. The reaction process of producing coarse Al-Si alloy was studied. It showed that the carborundum was gained in low temperature (below 1600℃) when the raw materials deoxidized in the vacuum carbon tube furnace. When temperature rised to 1700-1800℃, alumina began to react with carbon to form aluminum oxygen carbide. When the temperature was higher than 1900℃, the carborundum decomposed aluminum oxygen carbide into aluminum and silicon, and the aluminum and silicon reacted to form coarse Al-Si alloy.The results show that the coarse Al-Si alloy produced by carbothermal reduction used the bauxite tailings which removed iron by leaching with hydrochloric acid as raw material can be to produce cast Al-Si alloys or new type of Al-Si-Ti complex alloys. The process not only save save resources but also protect the environment, and It has significance for practice reference. |
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