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

Seeded precipitation of sodium aluminate solution is one of the key processes in alumina production. The low precipitation rate and contradiction between increaseing precipitation rate and improving product quality make precipitation a big problem for alumina production. To intensify seeded precipitation of sodium aluminate solution, this paper studied the influence of thermal history on structure of aluminate species, and explored the transformation law of aluminate species. Then coordinate adjustment of solution structure and particles behavior was employed to increase precipitation rate and improve product simultaneously. At last, the behavior of particles in the process of precipitation was researched, and laws of nucleation, agglomeration and growth of aluminium hydroxide were made clear. The main results are listed as follows:1) Thermal history of sodium aluminate solution has great effect on the transformation of aluminate species. Raising temperature and prolonging heat preservation time is conducive to transform Al2O(OH)62-to [Al2(OH)8(H2O)2]2-and Al(OH)4-especially for the species in the sodium aliuminate solution with low caustic ratio and high concentration. Al2O(OH)62-ions rapidly convert to [Al2(OH)8(H2O)2]2-and Al(OH)4-during dilution process, however, after sodium aluminate solution being diluted, the restoration transformation of Al(OH)4-into [Al2(OH)8(H2O)2]2-and Al2O(OH)62-can occur. In concentration process, Al(OH)4-ions are easy to transfer to [Al2(OH)8(H2O)2]2-but hard to transfer to Al2O(OH)62--2) Coordinate adjustment technology is proposed to enhance seeded precipitation of sodium aluminate solution. The technology contains adjustment of solution structure and warming-insulation ripening in the late period. Adjustment of solution structure is aimed to promote the trans formation from other aluminate species to the simple tetrahedral aluminate anions Al(OH)4-which are beneficial to gibbsite crystallization. Compaired with traditional precipitation process, the technology can increase precipitation rate by6.65%and improve the quality of product.3) The laws of nucleation, agglomeration and growth of A1(OH)3particles are studied. Secondary nucleation occurs before22h of precipitation, especially between10-22h. Particles agglomeration exists before10h and after22h during precipitation. Crystal growth happens throughout the entire process of precipitation. Breakage and abrasion of particles become notable after34h. The warming-insulation ripening can promote dissolution of fine particles, growth of coarse particles and agglomeration of fine particles. By matching the coarse and fine particles as well as the curing temperature can enhance ripening effect. Adjustment of solution structure to enhance precipitation can promote secondary nucleation. The existence of a large number of fine particles restrains the formation of secondary crystal nucleus in the solution. Precipitation rate is closely related to the behavior of fine particles during precipitation.