Theoretical Research And Industrial Practice Of Sandy Alumina With High Strength From Diaspore

With the increasingly strict requirement of energy saving and environmental protection, sandy alumina is used widely in the industry of aluminum electrolysis, because of its particularly microstructure, suitable size distribution and high strength. In China, the available bauxite for alumina production is diaspore which requires rigorous digestion and as a result of that, sodium aluminate solution of high alkalinity and low supersaturation is formed. Therefore, it is difficult to adopt directly the mature precipitation technique of Bayer process to produce sandy alumina.Facing the current situation of aluminum industry in China, this paper is aimed at the production of sandy alumina with high strength from diaspore. By analyzing the stress state and the overlapping way of Al(OH)₃ crystals, the stress model and the relationships among the overlapping way, stress-state and strength were established for the first time. The finite element model of the actual Al(OH)₃ grains was first set up to study the strength of Al(OH)₃ (alumina) with different microstructure in static. The relationship between the strength of Al(OH)₃ (alumina) and the characters of agglomeration was obtained by analyzing the information of size distribution in the process of seeded precipitation. Based on the above theoretical results, the technique of liquor distribution was created and put into effect in the practical process of alumina production. Sandy alumina with high strength was produced from diaspore. The main argumentations and original conclusions were made as follows:1. Using stress-state analysis on the overlapping face of Al(OH)₃ crystals, it was found that the main reason for the breakage of the overlapping face is direct stress. The direct stress is greater on the loaded side which is the decisive position to the breakage of Al(OH)₃ grains. The strength of Al(OH)₃ grains is closely related to the stress-state of the overlapped Al(OH)₃ crystals. Of all the different ways of overlapping, the strength of two overlapped Al(OH)₃ crystals facing to facing is the highest and is reduced as the overlapping area decreased.2. By summarizing and simplifying the structure of the actual A1(OH)3 grains, the finite element model was set up to analyze the strength of A1(OH)3 (alumina) in static. It was found that the strength of A1(OH)3 grains will be improved if there are small filling particles in the gap of the grains at suitable sites. A1(OH)3 grains with more filling particles which is closer to the optimum site of the gap will have larger strength. When the overlapping angle between different A1(OH)3 crystals is among 60°⁸0 °, the strength is worst. The strength reduces as the dimension of the overlapped A1(OH)3 crystals (20um⁴0|nm) increases. When small A1(OH)3 crystals are inlayed among large ones, the strength of A1(OH)3 (alumina) product will be improved.3. With the methods of analogous pattern recognition, the relationship between the strength of A1(OH)3 (alumina) and the characters of agglomeration was obtained by analyzing the information of size distribution in the process of seeded precipitation and the microstructure of A1(OH)3 grains. Researches showed that the agglomeration process exists at high temperature and high aK if there are small A1(OH)3 crystals. Smaller A1(OH)3 crystals are agglomerated together or pasted to larger ones with smaller overlapping angles. The size range of agglomerated A1(OH)3 crystals can be comparatively wide and smaller crystals may be pasted to larger ones, especially in the subsequent process. Therefore, it is important for the production of sandy alumina with high strength to control the agglomeration process.4. The technique of liquor distribution was applied to the sodium aluminate solution of high alkalinity and low supersaturation from the digestion of diaspore. Sandy alumina with high strength was produced in laboratory. The percentage of-45um A1(OH)3 grains was in the range of 10¹6%. After calcination in the kiln, the percentages of alumina particles at -45um and -20um were 8¹1% and 0-2% respectively. The attrition index of alumina product was 20%.5. The technique of liquor distribution was also put into industrial practice for the production of sandy alumina. The average decomposition rate was 50.3%. The average yield was 82.9 kg/m3. The percentage of-45um A1(OH)3 grains was in the range of 13— 15%. After calcination in the kiln, the percentages of alumina particles at -45urn and -20um were 8¹2% and 0²% respectively. The attrition index of alumina product was 35.3%.