Inorganic Salt Impurities On The Seed Crystal Of Sodium Aluminate Solution Decomposition Process

The seeded precipitation of sodium aluminate solution is a key step in the Bayer process of alumina production. At present, there wildly exists a series of problems, such as long time of the precipitation process, low rates of precipitation and the large ratio of seed. The following study of the seeded precipitation of synthetic sodium aluminate solution focuses on the aspects of strengthening precipitation. There are a wide range of impurity components in industrial sodium aluminate solution; these impurities include silicon dioxide, sodium sulfate, sodium carbonate, sodium chloride, kalium ion, organic compound and other trace impurities. The existence of impurities in solution affects not only the precipitation rate, but also the quality of the precipitation product. This paper studied the influences of the major impurities in the sodium aluminate solution on the seeded precipitation rate and the granularity of precipitation products. What is more, this paper also studied the influences of these impurities on the surface tension of sodium aluminate and the surface Zeta potential of aluminum hydroxide, and this paper also explore the mechanism of these impurities in the process of the seeded precipitation of synthetic sodium aluminate solution, and the results were as follows:(1) When the concentration of sodium chloride and Na₂O_c is more than 10g/L, or the concentration of Na₂O_s is more than 5g/L, both of their restraints on seeded precipitation are remarkable. Potassium hydroxide and sodium phosphate can promote seeded precipitation; Sodium zincate, kalium sulfate, sodium carbonate and sodium sulfate existed at the same time can restrain the seeded precipitation of sodium aluminate solution, and when the concentrations increase, the inhibition will be enhanced; Sodium metavanadate has in significant effect on seeded precipitation.(2) when the concentration of sodium chloride is less than 10g/L, and the concentration of kalium sulfate is less than 20g/L, they can not thin the particle size of precipitation products, but sodium phosphate can increase the particle size of precipitation products; Sodium carbonate, sodium sulfate has no effect on average particle size, but potassium hydroxide, sodium metavanadate and sodium zincate can reduce the particle size of precipitation products.(3) The other inorganic salt impurities except sodium zincate can increase the surface tension of the sodium aluminate solution in different degree, sodium zincate has no significant effect on the surface tension. The surface tension is one of the factors of the impact on interfacial tension of the sodium aluminate solution, it also has effect on the precipitation rate.(4) The sodium sulfate, sodium carbonatea and kalium sulfate can make Zeta potential of aluminum hydroxide grain more negative in sodium aluminate solution. However, sodium chloride has faint impact on Zeta potential. Sodium phosphate has no significant effect on Zeta potential. The size of Zeta potential is decided by the two factors that the exchange of hydronium and hydronium is absorbed by the crystal surface. Zeta potential directly affects the precipitation of ratio the solution. If Zeta potential is more negative, it is not conducive for aluminum hydroxide grain to absorb aluminate anions, so that will be restrained the seeded precipitation of sodium aluminate solution.