Edta Sodium Aluminate Solution Agglomeration Effects

The crystallization of Al(OH)3 from seeded supersaturated sodium aluminate solutions is one of the most important processes in alumina production. But the development of alumina industry is seriously restricted by many problems such as the slow seeded precipitation rate and too broad distribution of the particle size of Al(OH)3 product. Among all the enhancing methods,the advantage of additive is obvious.But the effect of additives in sodium aluminum solution on the agglomeration kinetics and particle size distribution is less impact of information.EDTA with four carboxyls, two aminos was used as additive in the paper. The changes of decomposition rate and particle size distribution during the pre-agglomeration of sodium aluminate solution at different temperatures are studied by using dynamics and fractal analysis.The effect of EDTA on the structural changes of sodium aluminum solution was also investigated and the enhancement mechanism was derviced. Conclusions were made as follows:1.During the initial stage of agglomeration, EDTA can promote the agglomeration of small particles and contribute to secondary nucleation. The size of nucleation particles is about 2.5～10μm.2.The decomposition rate of sodium aluminate solution at different temperature is vibrated. 0.01mol·L-1 of EDTA can enhance the decomposition of sodium aluminate solution during the agglomeration stage.The activation energy of the seeded precipitation is decreased from 42.97kJ·mol-1 to 32.97kJ·mol-1 with the addition of EDTA.3.Fractal theory was applied to study the particle size distribution of aluminum hydroxide.The function of particle size distribution at different temperature with or without additives was obtained as follows:72℃without additives, N(r,t)～r0.42552t-1.99978;72℃with additives, N(r,t)～r0.24087t-2.01675;74℃without additives,N(r,t)-0.35849t-2.01833; 74℃with additives,N(r.t)～r0.26696t-1.97089;76℃without additives,N(r,t)～0.51111t-1.99344;76℃with additives,N(r,t)～0.34099t-2.0572The rate of grain growth was calculated by the introduction of the differentiation of distribution function.4.By adding EDTA, the structure of sodium aluminate solution is changed with OH bond strengthened. It is deduced that EDTA anion and sodium aluminate ions are combined to form hydrogen bonds, which are beneficial to the favorable growth unit for the precipitation of Al(OH)3.