Study On Agglomeration Process During Gibbsite Precipitation From Sodium Aluminate Solution And Its Enhancement

With the increasingly strict requirement of energy saving and environmental protection, sandy alumina is used widely in the industry of aluminum electrolysis, because of its coarse particle size and high strength. Agglomeration and crystal growth must be promoted for producing sandy alumina. Agglomeration is the important precondition for producing coarse aluminium hydroxide, this paper especially focuses on agglomeration process.In this paper, infrared and raman spectrum are applied in semi-quantitative analysis of sodium aluminate solution. It is investigated the effect of crystal growth modifier (CGM) on structure of sodium aluminate solution and the relationship between microscopic properties of sodium aluminate solution and macroscopic indices of precipitation process. Effect of operation factors on the agglomeration process is investigated and favorable conditions are attained. Effect of surfactant and CGM on the agglomeration process is investigated and coarse aluminum hydroxide products with high strength are attained by improvement of agglomeration process. The results in the paper may be a technical support and theoretical guidance for alumina production. Main conclusions are drawn as follows:1. The determination method for structure of sodium aluminate solution is developed and suitable operation conditions are obtained. Using cover-glass as internal standard, a semi-quantitative method of raman spectrum for the analysis of sodium aluminate solution is invented, the type of aluminate ion can be determined according to the position of characteristic peak of sodium aluminate solution in raman spectrum and the content of aluminate ion can be analyzed by semi-quantitative method according to the ratio of intensity and area of characteristic peak of sodium aluminate solution to that (920cm-1) of the cover-glass.2. Through the analysis of structure of sodium aluminate solution, it is found that main aluminium-containing ion is [Al(OH)4]- when molecular k) of liquor is 1.4-3.2 and caustic alkali concentration (Nk) of liquor is 120-260g/L. At the same Nk, the concentration of [Al(OH)4]- and [Al2O(OH)6]2- decreases gradually with increasing ak of the liquor. When Nkis 120-220g/L, the R value (Ratio of intensity of characteristic peak of 620cm-1 to that of 535cm-1) is high and the tendency of precipitation is strong at the same Nk when ak of the liquor is low. The change of R is small with the change of ak when the Nk of sodium aluminate solution is very high (260g/L), but the R is high relative to that of the liquor of lower Nk (220g/L), the liquor of high R has stronger precipitation tendency. At the same ak of the liquor, the R is high and the precipitation tendency is strong when the Nk of sodium aluminate solution is low, but the R of liquor of moderate Nk(157-174g/L) and very high Nk (260g/L) is higher and the precipitation tendency is relatively strong. Through calculating contrast function, it is found that additive has limited effect on the structure of sodium aluminate solution by changing the concentration of [Al(OH)4]-.3. The precipitation yield of sodium aluminate solution decreases and the agglomeration efficiency of gibbsite particles increases with the increasing liquor temperature, high temperature is advantageous for agglomeration of particles less than 25μm. The precipitation yield decreases and the agglomeration efficiency at 8h increases firstly and then decreases with the increasing ak of liquor, the agglomerates contain much more particles less than 45μm when ak is too high or low. The precipitation yield, the particle size of agglomerates and the agglomeration efficiency all decrease with the increasing Nk of the liquor. The precipitation yield increases and the agglomeration efficiency increases first and then decreases with the increasing mass ratio Ks of crystal seeds, the agglomerates contain much more median and small particles when Ks is too high or low. The precipitation yield and the agglomeration efficiency all decrease with the increasing particle size of initial seeds. The precipitation yield increases with the increasing agitation rate, the agglomeration efficiency of gibbsite products is highest and particle size distribution of agglomerates is best at a moderate agitation rate. High temperature (78℃), lower initialαk(1.45), low Nk, Ks 0.25 and agitation rate 100r/min are favorable for the agglomeration process.4. By comparing with morphology and particle size distribution of products from one-stage process and two-stage process, it is found that alumina products with bigger particle size and higher strength can be attained by enhancing agglomeration in seeded precipitation process of sodium aluminate solution. The agglomeration mechanism is also discussed, the agglomeration begins from the smallest particles.5. The precipitation yield of sodium alumiante solution decreases by adding saturated stearic acid without C=C bond when the dosage of stearic acid, oleic acid and linoleic acid surfactants is less(10ppm). The precipitation yield increase and they are close by adding three different surfactants when the dosage is above 20ppm. The secondary nucleation can be controlled by adding surfactants when temperature is lower(65℃) and degree of supersaturation is higher. The agglomeration can be promoted by adding surfactants when temperature is higher(85℃) and supersaturation degree is lower. The strengthening effect of agglomeration is not remarkable by adding surfactants when Ks is too high or low. The low agitation rate and fine initial seed are advantageous for strengthening effect of surfactants on agglomeration. The strengthening effect of surfactants on agglomeration is mainly reflected on fine particles. Whether the secondary nucleation is controlled or agglomeration is promoted, the best effect can be attained by adding linoleic acid, oleic acid and stearic acid. Among linoleic acid, oleic acid and stearic acid, stearic acid is the worst one. It has a close relation with C=C bond of surfactants. The surface tension decrease extent of sodium alumiante solution is the greatest by adding linoleic acid with C=C bond and it is the smallest by adding stearic acid without C=C bond.6. When CGM is added into sodium aluminate solution, both agglomeration and coarsening of particles are markedly enhanced and also allow for secondary nucleation control, but the precipitation yield of the liquor decreases. The precipitation yield of liquor can be increased, but the promotion of agglomeration is weak when surfactant is added into sodium aluminate solution. However, both precipitation yield of liquor and particle size of aluminum hydroxide product increase at the same time when adding CGM with anionic surfactant. It is possible for powdery aluminum hydroxide to become sandy aluminum hydroxide by improvement of agglomeration process.