| Seeded precipitation of synthetic gibbsite from supersaturated sodium aluminate solution is a crucial step in the Bayer process for the production of alumina from digested bauxite. It's a complicated crystallization reaction involving a variety of physico-chemistry sub-processes contributing to the formation of the particle size distribution such as secondary nucleation, agglomeration and crystal growth etc. The early researchs are emphasised on the prophase process of seeded precipitation. With the resource decreased and the environment protected,the prodution of sandy alumina become more and more important.The factors affected on the late process of seeded precipitation have been studied on the research, and the following results are made:(1) The temperature affects on the late process mainly. During the late process of decipitation, with the temperature decreased,the corresponding ratio of precipitation is increased. While the temperature is too low, the ratio of precipitation is decreased. With the temperature reduced,the averaged particle diameters of trihydrate alumina are increased, while the temperature is decreased deeply, the averaged diameters of particles are decreased.(2) Seeds are influenced on the process of precipitation, and the influence of different seeds, the particle size and the seed ratio are studied on the research. From the result, it is indicated that the influences of seeds of different types on the precipitation are different. With the change of particle size and coefficient of seeds,the precipitation ratioes and the particle size of production corresponding are different.(3) Form the research of the hydrate sodium concentration on the process, it can be indicated that the precipitation ratio is debased significantly with the augment on the concentration of hydrate sodium, but it exert little influence on the particle size and quality of production.(4) When the strength of agitation at the range of right rule, the decipitation ratio and particle size of production reduced with the changes in the value of agitation. Though research on the production's appearance of decipitationprocess and kinetics of agglomeration and growth, it can be made thefollowing results:(l)From the change on the appearance of production during the decipitation process, it can be indicated that the crystal growth and nucleation are mainly happened on the surface of large particle, while the agglomeration is going on the surface of small particle.(2) The regularity of crystal growth is studied during the late period of precipitation, the growth rate is simulated by the computer. It can be reduced that the apparent growth rates of crystals are slowly when the concentration of hydrate sodium is higher. Generally speaking, there is an obvious increase in the apparent crystal growth rate with the augment in particle size;The apparent crystal growth rates corresponding to particles of all level sizes decrease gradually with the proceeding of precipitation due to the decrease in driving force—the supersaturation degree of the solution.(3)Thongh the research, the kinetics equation of agglomeration during late period process of seeded precipitation can be expressed as:— = 2.399 xl04xexp(-50.02xl03/i?r)x(C-CM)2 dtFrom the equation, it can be conclued that the agglomeration are controlled by the chemical raaction during the late period of precipitation(4)Thongh the research, the macroscopic kinetics equation of crystal growth during late period process of seeded precipitation can be, 5.745x104 / , j \ 2-0121expressed as: 4, ^ = 5.9054 xlO4 xfrOime~ RT ------e-\ . Fromthe eqution,it can concluded that the supersaturation can influence onthe pecipitation activity that the seeds. It can be calculated that the activation energy of reaction is 57.45 kJ/mol, which indicated that thecrystal growth is controlled by the surface chemical reaction.(5)The change of precipitation ratio and particle size of production are studied primarily during the ageing process of decipitation . |
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