Preparation Of Primary Al-Si Alloy From Bauxite Floated Tailings By Carbothermal Reduction Process

The development of ore dressing Bayer process contributes to improving the alumina market competitiveness and to using soundly the middle-low grade resources in China. In the process of ore dressing, however, has to produce plenty of floated tailings occupied about 25% of raw ore.It would not only consume resources,but also occupy land and pollute environments if these tailings cannot be used. At present time, the consumption quantity of Al-base alloys accounts for 60% in Al consumption field, and of all the Al-base alloys, the largest consumption quantity is Al-Si alloy. Compared with fusion method, preparation of primary Al-Si alloy by carbothermal reduction owning lots of merits such as lower requirement of materials, better usage of the secondary resources, lower price equipments and higher energy ratio, is the future development trends of aluminum industry.The ratio of raw materials, consists of bauxite floated tailings, alumina, bituminite and sulphite pulp liquor, was calculated exactly in this paper. The results shown that the theoretical ratio were tailings: alumina: bituminite: sulphite pulp liquor: water=100: 25.1:74.0:19.7:7.91.Based on these, the influences of pressing pressure, dry agglomerant content, moisture content and particle size on the properties of green pellets were investigated by single factor and orthogonal experiments using the compressing strength and pore ratio as indexes. The optimum conditions for the properties of green pellets are as follows: the pressing pressure 22.5 MPa, dry agglomerant content 9%, moisture content 13% and particle size 0.420～0.178 mm. Under the optimum conditions, the compressing strength and pore ratio of green pellets are 27.8 MPa and 18.4%, respectively. Both of them are better than industry applied indexes.The properities of roasted pellets were also be tested in the study.The results indicated that with the roasting temperature rising and the roasting time prolonging,the pore ratio of pellets increased,and the increasing amplitude was great.Meanwhile,the compressing strength decreased continuously,the decreasing amplitude,however,became small.When the roasting temperature bellowed 500℃,the pellets hardly conducted.Nevertheless, when the roasting temperature rised to 700℃,the resistivity decreased apparently.After 1000℃, the decreasing amplitude of resistivity was small,and the increasing tendency emerged inversely.Preparation of primary Al-Si alloy from bauxite floated tailings by carbothermal reduction process was studied in vacuum graphite furnace with argon as the protective atmosphere. It was found that when the atmospheric pressure approach to 0.1 MPa, heating temperature 1900℃wintering time 1 h and bituminite content 95 wt.% of theoretic bituminite content, the composition of the products was preferable.The primary alloy obtained consisted of 6 phases, besides primary Al-Si alloy phases, there were oxides and carbides phases existed.In the process of smelting operated in arc furnace, when the output current was 1100 A, the suitable voltage was 25～30 V. Meanwhile, when the bituminite content was 93 wt.% of the theoretic amount, the content of Al₂O₃ and SiC in the slag was relatively small. The alloy obtained composed of 6 phases including mainly primary Al-Si alloy phases, and minute impurities such as Ti,Ca,oxidants and carbides.The Al content was 52.5%, Si content was 23.2% in the obtained alloy.The producted fume was mainly composed of Al₂O₃ and SiO₂.The thermodynamic analysis on the carbothermal reduction of Al₂O₃ and SiO₂ was carried out, the results shown that the reduction temperature was high to 2000～2100℃.The composition of products obtained under different temperature in vacuum graphite furnace and different reaction zone in arc furnace was studied by XRD, SEM coupled with EDS methods and the process of carbothermal reduction was tested by TG/DTA technology. It was found that the process of carbothermal reduction of bauxite floated tailings is similar to that of Al₂O₃ and SiO₂, which can be divided into 4 stages: bituminite pyrolysis stage, the crystal phase transformation of Al₂O₃ and SiO₂ stage, the formation and decomposition of carbides stage and products loss stage.Of the 4 mechanisms of carbothermal reduction of Al₂O₃ and SiO₂, the formation and decomposition of carbides theory might be the best one to interpret the reaction phenomena, and this theory was amended according to test phenomena as well,the concrete reaction equations were as follows: SiO₂+3C=SiC+2CO, 2Al₂O₃ + 9C = Al₄C₃ + 6CO,3SiO₂+2Al₄C₃=8Al+3Si+6CO,3SiC+Al₂O₃=2Al+3Si+3CO.The kinetics of preparation of Al-Si alloy by carbothermal reduction of Al₂O₃ and SiO₂ was studied by means of differential temperature analysis (DTA) at different temperature rising rates of 10,15,20,25 K/min. It was shown that there were 6 endothermic peaks in each different heating-up velocity,and the temperature ranges of these 6 endothermic peaks were in 1220～1240 K, 1340～1360 K, 1680～1730 K, 1850～1950 K,1970～2050 K, 2100～2160 K,respectively.The apparent activation energy of each endothermic peak was obtained by Flynn-Wall-Ozawa and Kissinger methods as follows: 848.9, 945.4, 569.7, 325.7,431.9 and 723.1 kJ/mol. Reaction orders, frequency factors and kinetic equations were also determined: Furthermore, the feasibility was verified by theory and test.