Structure Evolution Of Sodium Aluminate Solution During The Seeded Precipitation Process

The precipitation of gibbsite from caustic aluminate solution is one of the most important operations during the alumina production process, which influences the precipitation efficiency and the physical and chemical properties of products directly. It has been the rate-limiting step in the Bayer process used for alumina refining since the innovation of Bayer process. The fundamental understanding of the structure of aluminate solution and the precipitation mechanism is essential for substantial advances in the precipitation process, although some improvements have been achieved over the years by incremental process modifications. In this thesis, the intrinsic relationship between the electric conductivity and the structure of sodium aluminate solutions was investigated by calculating the transference number of aluminate anions (t(Al)) in the solution of different alkali concentration and molar ratio of Na2O to Al2O3. Furthermore, combining with the characterization of FTIR analyses of the solution, the changes of aluminate ions in the Bayer seeded precipitation process were analyzed and thus the aluminate species favoring gibbsite precipitation were approximately confirmed. The gibbsite precipitation process was eventually enhanced by transferring other aluminate ions to those which were beneficial for gibbsite crystallization. The main results and progresses are listed as follows:(1) The electric conductivity of sodium aluminate solution firstly increases with increasing concentration of alkaline, and then reaches the maximum when the concentration of C(Na2O) is between100-120g/L, illustrating that the aluminate solution belongs to strong electrolyte solution. The relationship between electric conductivity and ion structure of sodium aluminate solution was investigated and the results show that in the solution of low concentration of C(Na2O)<175g/L, the aluminate anions dominantly exist in the form of Al(OH)4-and t(Al) is approximately0.6; that in the solution of medium concentration of C(Na2O) from175g/L to330g/L, the aluminate anions mainly exist in the form of Al(OH)4-and [A12(OH)8(H2O)2]2-,in which t(Al) is approximately0.2; and that in the solution of high concentration of C(Na2O)>330g/L, aluminate anions exist dominantly in the form of Al2O(OH)62-and [Al2(OH)8(H2O)2]2-with a little Al(OH)4-, where t(Al) is almost zero. This illustrates that the electric conduction ability of the aluminate anion in the sodium aluminate solution is closely related to its structure. It has been studied that the degree of polymerization of aluminate anions rises with the increasing concentration of the alkaline and the transference number of the aluminate anions declines with the increasing degree of polymerization of aluminate anions.(2) The aluminate species maily exist in the form of dimeric or polymeric species in the solution of high concentration of alkaline or high causitic ratio. The concentration of dimeric or polymeric species decreases with the diluting process and the concentration of Al(OH)4-increases gradually and eventually becomes the predominant species in alkaline aluminate solutions. The concentration of tetrahedral aluminate ions corresponding to the bands at720cm-1and880cm-1increases remarkably as the temperature rises, while the concentration of Al2O(OH)62-corresponding to the band at about550cm-1declines slightly, suggesting that the tetrahedral aluminate ions are transferred by Al2O(OH)62-.(3) Na2CO3and Na2SO4can change the ion structure of the aluminate solution, the concentration of Al(OH)4-declines remarkably, while the concentration of Al2O(OH)62-increases obviously according to the FTIR analyses when the concentration of Na2CO3is above10g/L and the concentration of Na2SO4is above5g/L. Besides, the viscosity of sodium aluminate solution is increased, the electric conductivity is reduced and the gibbsite precipitation is inhibited after adding Na2CO3and Na2SO4. Consequently, higher concentration of polymeric aluminate anions is harmful to the precipitation process of gibbsite from sodium aluminate solution. It is obtained that Al in Al-O-Al is replaced by Si and formed Al-O-Si, Al-O-Si-O-Al band, and larger molecular structures connected by hydrogen bonding are also formed, resulting in the increase of the viscosity and the decrease of the precipitation ratio.(4) The concentration of tetrahedral aluminate ions corresponding to the bands at720cm-1and880cm-1declines remarkably as the precipitation process proceeds, while the concentration of Al2O(OH)62-corresponding to the band at about550cm-1increases slightly, the content of Al(OH)4-in the attached-liquid on the aluminum trihydroxide surface has not diminished apparently, suggesting that Al-OH tetrahedral aluminate ions centered at about720and880cm-1are the most beneficial aluminate ions resulting in gibbsite precipitation from sodium aluminate solution, while the dimeric aluminate species centered at about550cm-1are disadvantageous to gibbsite precipitation. The gibbsite precipitation process can be enhanced by transferring other aluminate ions to ions which are beneficial for gibbsite crystallization.(5) The electrical conductivity of the sodium aluminate solution with low caustic molar ratio of Na2O to Al2O3varies with the intensity of the applied magnetic field and reaches a maximum in the presence of a magnetic field ranging from50to70kA/m and from150to200kA/m. IR spectra of sodium aluminate solutions suggest that magnetic field treatment modifies the structure of the aluminate solution. Moreover, the gibbsite precipitation from sodium aluminate solution would be promoted when the aluminate solution is magnetically treated with a magnetic field of52kA/m. The precipitation ratio increases about6%compared with the nonmagnetically treated aluminate solutions when the seeds include a fraction of finer particles, and the gibbsite products include more fine particles, indicating the secondary nucleation process is enhanced in the early stage of the precipitation process. The precipitation ratio increases about2～3%compared with the nonmagnetically treated aluminate solutions when the seeds with uniform size are added. And the products are made up of globular particles in uniform size. It can be concluded that the precipitation of gibbsite from sodium aluminate solution could be promoted by adjusting the structure and the physicochemical properties of aluminate solutions by introducing suitable magnetic field. Moreover, adding seed or impurities can also adjust the aluminate ions and eventually affect the gibbsite precipitation process.