Behavior And Adjustment Of Particles In Seeded Precipitation Process Of Sodium Aluminate Solution

The precipitation of gibbsite from sodium aluminate solution is a key process of the Bayer alumina production. The contradiction between the increase of the precipitate rate and the improvement of product quality has been the technical bottleneck of alumina production that urgently needs to break through.In this dissertation, the thermodynamics and solid-liquid interfacial properties of the seeded precipitation system were investigated, the laws of nucleation, agglomeration and growth of aluminium hydroxide were determined and the coordinated adjustment technology was proposed. The seeded precipitation process was intensified and the quality of product was improved. The main results are listed as follows:1) Based on the solubility data of Al(OH)3in the sodium aluminate solution, the model of the thermodynamic properties for the NaOH-NaAl(OH)4-H2O system was improved, providing the basis for obtaining the thermodynamic properties of precipitation process of the sodium aluminate solution.The theoretical model for calculation of the equilibrium constant was proposed based on the correlation between the modified Debye-Hiickel model and Pitzer equation:logK=-161.1495+4629.7868/T+26.6959ln(T)-0.0256TAccording to the relation between the equilibrium constant and the solubility, the Pitzer model parameters of β(0)NaAl(OH)4,β(1) NaAl(OH)4and CΦNaAl(OH)4for NaAl(OH)4, the binary mixing parameter of θOH-Al(OH)4-for Al(OH)4-with OH-, and the ternary mixing parameter of θOH-Al(OH)4-for Al(OH)4-with OH-and Na+, were obtained, allowing the calculation of the activity coefficient for NaOH-NaAl(OH)4-H2O system. The applicability analysis shows that the proposed model and parameters for calculation of the equilibrium constant of gibbsite dissolution and the equilibrium solubility are feasible.2) The relationship between the gibbsite solubility and particle size in the synthetic Bayer liquors was studied, providing the theoretical basis for the increase of the precipitate rate by controlling the content of small particles at different precipitation periods of the sodium aluminate solution.Based on the measurement of the liquid surface tension, the solid surface energy and the solid-liquid contact angles, the solid-liquid interfacial tension between sodium aluminate solution and aluminium hydroxide was calculated. Based on the above analysis, the relationship between the gibbsite solubility and particle size in the synthetic Bayer liquors and the variation of the critical nucleus sizes were determined through Ostwald ripening formula, the results show that the solubility of small particles is much higher than that of large particles, and the critical nucleus sizes increased obviously with the increase alkali concentrations, caustic ratio and temperatures, and the presence of small particles in seeded precipitation system is an important factor to limit the depth of precipitation.3) The laws of nucleation, agglomeration and growth of Al(OH)3particles were determined, providing the theoretical supports for adjusting the balance of number of particles in seeded precipitation process.Through the calculation of particle number per unit mass of Al(OH)3products at different size intervals, the results show that the secondary nucleation occurs mainly in the initial period (<10h) of gibbsite precipitation from sodium aluminate solution. In the middle period (10-34h), the agglomeration of A1(OH)3particles plays a dominant role with small seed coefficient of0.7-1.4, while the crystal growth becomes the major step with large seed coefficient of2.0-3.8. In the late period (>34h), there is little change in the number of particles and the crystal grow slowly.4) The mechanism that the additives intensify the seeded precipitation process was studied, providing the theoretical basis for selection and development of the efficient surfactant to improve the solid-liquid interfacial properties.The influences of the selected two types of additives on the seeded precipitation process were researched:the precipitate rate can be raised by4%and the average particle size of products increases by～10μm when the dosage of ethers additive A is500mg/L, while the precipitate rate can be raised by3.8%and the average particle size of products increases by～20μm when the dosage of anionic surfactant B is150mg/L. The mechanistic studies indicate that the additives reduce the surface tension of sodium aluminate solution and change the Zeta potential of solid surface, thus the solid-liquid interfacial properties can be improved and the seeded precipitation process can be intensified.5) The coordinated adjustment technology of the behavior of Al(OH)3particles was proposed, providing technical supports for intensifying the seeded precipitation process of sodium aluminate solution.Based on the above theoretical researches, the measures for intensifying the seeded precipitation process of sodium aluminate solution were proposed:(1) accelerating other aluminium species transfer to the simple tetrahedral aluminate anions of Al(OH)4-which are beneficial for gibbsite crystallization in the initial period,(2) modifing the solid-liquid interfacial properties by additives, and (3) controlling the Ostwald ripening process in the late period. The precipitate rate can be raised by6-8%with no remarkable refinement of Al(OH)3particles compared with the traditional seeded precipitation process.