Studies On Enhancing Seeded Precipitation Of Concentrated Sodium Aluminate Solution And Particles Behavior In Precipitation Process

The seeded precipitation of sodium aluminate solution is a key process in alumina production. The bottleneck of the process is the contradiction between the precipitate rate and the product size. To strengthen the process, this paper have studied the type and mutual transformation of aluminate ion in sodium aluminate solution, precipitation kinetics of the process, the effect of nonionic organic additive, behavior of particles, evolution of crystal morphology, etc. The main results are as follows.(1) The electronic conductivity of sodium aluminate solution is related to interspecific variation among ions in the process. According to the three-factor quadratic equations, a mathematical model to calculate the conductivity of the sodium aluminate solution was built.A=176.95935-3.66763Nk+249.20679ak+0.0096Nk2-31.38902ak2The molar conductivity of aluminum ions under different caustic concentration were calculated according to the model.Based on Fourier Transform Infrared Spectrometer, the aluminate species mainly exist in the form of A1(OH)4". Lots of [Al2(OH)8(OH)n]2-dissociate into Al(OH)4-in the early stage of the process(<5h). Thus, the electronic conductivity of the sodium aluminate solution increased.(2) A kinetic equation on seeded precipitation of concentrated sodium aluminate solution was built. The apparent activation energy for the process is67.226kJ/mol, which indicates that the precipitation reaction is controlled by the surface chemical reaction step.(3) Strengthening effect of additive P varies with the character of the seed and the structure of the solution. Adjusting solution structure is in favor of the precipitate rate, but leads to a large number of secondary nucleation. Additive can effectively suppress generation of fine particles. Products size is larger when seed coefficient is relatively high (Kr=2.62～3.4). Particles behavior is related to the size, and influenced by the behavior with similar size. XRD patterns indicate that the seed with a low intensity of (002) plane is more easily to be broken, and lead to secondary nucleation.(4) The structure of large seeds grows more regular after precipitation, and the shape will gradually transform from ball to square stacked column through analyzing the morphology of large seeds. Besides, defects on the crystal surface decrease in the process. These changes in morphology result in a crystal with decreased strength and activity, which is no longer suitable to be seed.