Preparation Of Mullite-based Complex Refractory From Bauxite Tailings

Bauxite tailings are the main raw material to produce Aluminum in industrial. In China, Flotation-Bayer process is proceed to utilize the low grade bauxite ore as raw materials to produce alumina, however, one major type of solid wastes equal to20%-30%mass percent of raw bauxite are generated in the flotation process, which is named as bauxite tailings. They are deposited on the land surface without any utilization, which causes significant environmental deterioration due to its random deposition. Besides, there is a large amount of Al2O3and SiO2in bauxite tailings, some attention has been paid to re-utilize bauxite tailings, such as bauxite cement, artificial stone block, Al-Si alloy and water absorbing material. However, the low valuable of these products prominently hinder the large scale re-utilization of bauxite tailings.It is critical to point out that the major chemical components in bauxite tailings are Al2O3and SiO2, respectively, they are accounting for over70%, and there are few Fe2O3, TiO2, K2O and other trace components in bauxite tailings. The major components (Al2O3and SiO2) in bauxite tailings are the same major component in mullite-based complex refractory. Hence, in order to high valuable utilization of bauxite tailings, a feasible and cost-effective route was conceived and implemented in this thesis. Bauxite tailings were used as raw material to prepare mullite-based complex refractory. Three steps were involved in the bauxite conversion, namely, active roasting, acid leaching and sintering, mullite-based complex refractory were successful synthesized. According to performance measurement, the mullite-based complex production showed better high temperature property compared to the commercial products. Moreover, there are no other addictions in the whole process, the bauxite tailings were re-utilized completely.In this thesis, the chemical compositions, mineral constitutions, element contributions of bauxite tailings were systematic studied, the accurate mineral constitution of bauxite tailings were calculated. The results indicated that, the major chemical compositions of bauxite tailings are Al2O3, SiO2, K2O, TiO2and Fe2O3respectively. The mass percent of major mineral constitution in bauxite tailings, that was, diaspore of46.22%, illite of30.00%, kaolin of6.05%, quartz of1.05%, hematite of10.80%, anatise of2.57%, respectively.According to the direct acid leaching for bauxite tailings, the optimal acid leaching parameter were proposed, that was, acid leaching temperature of100℃,the concentration of hydrochloric acid of3mol/L, the mass ratio of hydrochloric acid to bauxite tailings of10:1and the leaching time of2hours. After the acid leaching, the Fe2O3content in concentrate was deceased to0.7%, but the K2O in bauxite tailings could hardly be decreased, due to the existence of K in illite. The K2O was located illite, it has a strong binding force with illite network, and hardly to react with the low concentration HCl.And then, active roasting-acid leaching innovation process was proposed. The effect of varied roasting temperature to the mineral constitution, average grain diameter and the specific surface area of bauxite tailings were detailed investigated. It indicated that, after the active roasting, the hydroxyl radical of illite was removed, the chemical activation of K2O at inter laminar illite was increased effectively. So the K2O content was successfully decreased in the concentrated bauxite tailings after3mol/L HC1acid leaching process. Both Fe2O3and K2O content were decreased to the minimum after700℃active roasting, and700℃was the optimal roasting temperature. However, liquid phase would be generated at the contact surface between illite and hematite if the roasting temperature was above700℃, which would prevent reaction between Fe2O3and HCl.The concentrates after active roasting-acid leaching process of bauxite tailings were used as material to synthesis mullite-based complex by the sintering process, the sintering mechanisms were illustrated. It was indicated that mullite-based complex has been prepared successfully at1500℃for4h. The mullite and corundum consisted of the major phases, no other impurity appeared. It was shown the typical pillared shape crystal. The kinetic was described as follow,5steps were involved during the mullite preparation, it was,the migrating of liquid phase to the corundum surface, the mullite nucleation at the contact surface between corundum and the liquid phase, the migrating of Al, Si, O atom to the surface of mullite crystalline, and the growth of mullite crystal grains, the migrating of liquid phase to the interior of corundum. The reaction was stopped at the moment when the SiO2concentration was no longer sufficient to support the mullite nucleation and growth.Varied Fe2O3and K2O contents in concentrate were firstly obtained by controlled HCl concentration and mass ratio of HCl liquid to bauxite tailings solid, and then the concentrates were sintered to synthesize mullite-based complex. The effect of the Fe2O3and K2O contents on the room temperature and high temperature properties were investigated. The results showed that, as the Fe2O3and K2O content was less than2.0%, the crystalline phase content and the refractoriness under load in the final products was relatively high. As the Fe2O3content was above2.0%, the amorphous phase content in the final products and room temperature properties were increased with Fe2O3content increasing, the refractoriness under load decreased therewith. The rank of the effect to the high temperature properties were the relative content of crystalline and amorphous phase, the compositions of the the amorphous phase and the sintering degree of the material. As the Fe2O3and K2O content was less than2.0%, the refractoriness under load was higher than1400℃, it reached the standard of the commercial mullite, which indicated that, HCl dose could be decreased in the practical Application.