Study On Technologies And Theories Of Bayer Seeds Precipitation From Caustic Aluminate Solutions Added With Additives

Study on the promotion of Gibbsite precipitation from caustic aluminate solutions is always a hotspot in aluminum industry. As a method with good application foreground, Al(OH)3 precipitation from caustic aluminate solutions with the addition of additives has important advantages such as simple operation and low cost, and is worthy of further investigation.The precipitation mechanics of Gibbsite particles precipitated from caustic aluminate solutions added with additives was summed up. The nucleation, agglomeration and kinetics added with additives, the basic rules of how the additives react to the surface of solid and liquid, the increase of decomposition ratio and the improvement of particle size of product with additives, and the principles about how to search suitable additives are all investigated in this work, and the technique parameters are further decided experimentally. The main conclusions made are as follows:1)Hydroxylamine functional groups were added in polyacry lamide through organic synthesis. For example, on the condition of pH=11, the ratio of hydroxylamine and amide 1:2, strong electrolyte added and 80 C, the degree of hydroxylamine functional groups added in the non-ionic polyacry lamide can be attached to the maximum.2)The decomposition ratio of Al(OH)3 precipitated from caustic aluminate solutions with seeds promoted by additives was investigated. It was found that two mixed additives are better than one to increase the decomposition ratio, in the sequence of positive ion + negative ion, non ion + negative ion , negative ion , negative ion. Positive ion polyacry lamide and sodium polyacrylate with the ratio of 5:1 was the best additives. After factor experiments, the decomposition ratio was promoted by 4.3%. The decomposition ratio can be increased by 1.9% with modified positive polyacry lamide as additive than with unmodified positive polyacry lamide as additive.3)Fractal kinetics is applied to study the precipitation of causticaluminate with additives. The irregular crystal surface of seeds in the precipitation can be characterized by spectral dimension ds of fractal. Based on the fractal characteristic of gibbsites and the influence of seed content, the kinetic equation for the precipitation of Gibbsite from caustic aluminate solution with seeds can be modified as:where ds implies the spectral dimension4)Nucleation kinetics with additives was studied. Crystal growth and agglomeration were eliminated or weaken, to study the secondary nucleation on condition of low temperature, moderate degree of supersaturation, short reaction time and coarse seeds. It was found that secondary nucleation can be influenced by temperature, ak, the amount of seeds, particles sizes and intensified by additives, low temperature, moderate ak, large amount of seeds and small particles.5)Agglomeration influenced by additives was investigated. It was found that on the condition of our experiments, agglomeration can be almost finished in 8h, where the particle sizes was mainly under 5um especially under 2um. High temperature, low stirring speed, moderate ak, small particles and a small amount of seeds were in favor of the agglomeration. Stearic acid and paraffin oil were the best additives to enhance the agglomeration.6) Technique experiments with additives to increase the particle sizes of A1(OH)3 precipitated from caustic aluminate solutions were made. It was found that stearic acid and paraffin oil can improve the particle sizes ' with the same processing parameters and decomposition ratio. As a result of that, it is possible for powdery aluminum hydroxide to become sandy aluminum hydroxide with the addition of additives.