Particle Size Control Of Products From Precipitation Process With Seeds Prepared By Inducing Nucleation In Sodium Aluminate Solution

The seeded precipitation of sodium aluminate solution is the key process to control the chemical purities and physical properties of products in the Bayer process of alumina production. Though numerous investigations on seeded precipitation have been undertaken, the low precipitation rate, the big ratio of seeds, long precipitation time and low equipment productivity are still the severe problems in the gibbsite (Al(OH)3) precipitation process. In order to improve the utilization efficiency of equipment, this paper investigated the influence of inducing methods and species of initiators on the precipitation process with seeds prepared by inducing nucleation in sodium aluminate solution. Furthermore, the effect of temperature on the particle size of seeds prepared by slowly inducing nucleation in a small amount of solution was also discussed. Conclusions are as follows:1) The precipitation process of inducing nucleation by directly adding solid initiator occurred only under conditions of low precipitation temperature and low caustic ratio of the sodium aluminate solution, and super-fine products can be achieved. The coarse aluminum hydroxide cannot be prepared in precipitation process of rapid inducing nucleation in a small amount of solution due to the difficulty in controlling the conditions in the stage of preparing seeds. In contrast, coarse aluminum hydroxide particles were achieved under high temperature in the stage of preparing seeds during the precipitation process with seeds prepared by slowly inducing nucleation in a small amount of solution.2) Initiator species had great influence on the precipitation process, and initiator A was the best one among them.3) The precipitation rate was low and the particle size of seeds obtained was large under the high temperature in the process of preparing seeds by slowly induced nucleation from a small amount of solution. And then, in order to control the particle size of products, this paper has investigated the behavior of fine particles in the precipitation process. The influences on the growing of fine particles and the conditions of inhibiting the agglomeration have also been discussed in this paper. Conclusions are as follows:1) Coarse products with mean particle size of 50μm were obtained in the precipitation process with coarse seeds prepared by slowly inducing nucleation in a small amount of solution under high temperature (80℃).2) Fine products with mean particle size of 1-2μm were obtained in the precipitation process with seeds prepared by inducing nucleation under low temperature (45-50℃) while the agglomeration was inhibited. Moreover, organic additive B can facilitate the precipitation process, refine the particle size of products and improve the morphology of products.3) When the amount of initiator A was increased, the precipitation rate raised, but the particle size of products remained the same. Meanwhile, the slow inducing rate in the process of preparing seeds, the fractionated addition of sodium aluminate solution and organic additive E can increase the particle size of products. In addition, the sodium carbonate and sodium sulfate in sodium aluminate solution inhibited precipitation, but facilitated the growth of particles.4) Grow-up was the dominant behavior of particles in solution when a great amount of seeds recycled under low temperature or a large amount of active seeds were added into solution under high temperature. Agglomeration was the dominant behavior of particles in solution when a small amount of active seeds were added into solution under high temperature.5) The appropriate addition of coarse seeds during the precipitation can reduce the amount of fine particles in the products. The more the addition amount, the less fine particles in the products, but the fine particles still existed in products.