Study Of The Effect Of Potassium Ions On The Seeded Precipitation Of Sodium Aluminate Solutions

Seeded precipitation of sodium aluminate solutions is an important process in alumina production. The yield and the quality of the product in the process are often affected by the impurities exist in the solutions. Potassium ion is one of the impurities in sodium aluminate liquor. Potassium and sodium elements are in the same main group and their chemical properties are very similar. Potassium ions, as an impurity, are not attracted enough attention. However, researchers have already found that there are some differences between potassium aluminate and sodium aluminate solutions in the kinetics of homogeneous nucleation, secondary nucleation and crystal growth during the seeded precipitation. Since the accumulation of potassium ions has come to a considerable high level in some alumina plant, study of the influence of potassium ions on the seeded precipitation process is of great practical significance.In this thesis, the influences of potassium ions on the seeded precipitation process were studied systematically, and conclusions were made as follows:1. During the seeded precipitation process of sodium aluminate solutions, the presence of potassium ions can promote the precipitation process. The decomposition ratio is obviously increased at relatively low temperature and α_k.2. The presence of potassium ions can affect the particle size distribution (PSD) of the product, especially the number of fine particles. The influence is weakened as the temperature, α_k and seed amount are increased. The main reason is that potassium ions could promote the secondary nucleation during the precipitation process of sodium aluminate solutions.3. The presence of potassium ions leads the content of Na₂O in the products decreased. It may be caused by the radius diversity of K⁺ and Na⁺.4. The presence of potassium does not change the phase of crystal, thethermal stability, and the micrographic structure of the product. 5. By the kinetics studies on secondary nucleation, agglomeration and crystal growth, it was found that the presence of potassium ions can prolong the induction period of secondary nucleation process of sodium aluminate solutions; and promote the secondary nucleation process. The effect is decreased as the temperature, a k and seed amount increased. The promotion on the agglomeration of the crystals with the particle size in the range of 0¹0 u m is small, but obvious in the range of 10³0 u m, and will be weakened as the temperature, a k and seed amount increased. As potassium ions can promote the formation of fine particles, it has different effect on the agglomeration of crystals with difference particle sizes. The kinetics of crystal growth was studied with a self-designed differential reactor that can be performed on a relatively single condition. It was found that the growth of crystals obeys the rules that the rate of crystal growth is not related to the crystal size under constant impulsion. The presence of potassium does not affect the kinetics of crystal growth and the crystal growth of sodium aluminate obeys the law of diffusion theory.