Study On The Seeded Agglomeration Process Of Supersaturated Sodium Aluminate Liquors And Its Enhancement

The seeded agglomeration process of supersaturated sodium aluminate liquors is not only an important method for rapid enlargement of gibbsite particles, but also an important process to attain sandy alumina. Research object of the previous work mainly involves the overall precipitation process, of which many basic processes affectes with each other greatly. In this paper, the basic process of seeded agglomeration has been investigated in order to reveal the whole precipitation process by means of reductionism.Two criteria to judge the agglomeration process were proposed firstly. Effect of operational factors on the agglomeration process was investigated and favorable conditions were attained. Oscillation phenomena of particle size distribution (PSD) of gibbsite particles was found and chaotic and fractal theories were firstly used to study the particle size information. Agglomeration kinetics was studied according to the particle size information. Effect of polyacrylamide (PAM) on the agglomeration process was investigated and the reaction mechanism between additives and sodium aluminate liquors was firstly studied by semiquantitative analysis of infrared spectrometry. The results in the paper may be a technical support and theoretical guidance for alumina production. Main conclusions are drawn as follows:1 Two criteria to judge whether a precipitation process is controlled by agglomeration and when it is terminated were proposed. One is according to the change of PSD curves' site and the values of the main peaks, the other is according to the change of the content of the smallest particles. The criteria based on PSD data is simpler and visual.2 The initial degree of supersaturation and the precipitation ratio of sodium alumiante liquors decrease with the increasing temperature, the initial molecular ratioα_k and the caustic alkali concentration C_Na2O of the liquors. The agglomeration ratio of gibbsite particles increases with the increasing temperature. The agglomeration ratio increases firstly and then decreases with the increasing initialα_k. With the increasing C_Na2O of the liquors, the supersaturation degree, the precipitation ratio of the liquors and the agglomeration ratio of gibbsite particles all decrease, while the content of particles less than 45μm increases for gibbsite products. High temperature (78℃), lower initialα_k (1.45) and low C_Na2O are advantageous for the seeded agglomeration process.3 Mechanically activated seeds can enhance the seeded agglomeration process. Investigation on activation mechanism indicates that mechanically activated seeds own larger specific surface area and solid-liquid interface with sodium aluminate liquors because of their smaller particle size, and they have more new fracture planes and more lattice distortion of high activity. With the increasing mass ratio Ks of crystal seeds, the precipitation ratio increases and the agglomeration ratio increases first and then decreases. The agglomerated products contain much more median and small particles when Ks is too high or low. Agitation accelerates ionic diffusion of the liquors, especially of concentrated liquors, and makes solid-liquid interface between seeds and the liquors larger. The precipitation ratio increases with the increasing agitation rate. The agglomeration ratio of gibbsite products is highest at a moderate agitation rate, of which agglomerated products contain least particles less than 45μm. Mechanically activated seeds, Ks 0.25 and agitation rate 100 r.min^-1 are favorable for the seeded agglomeration process.4 PSD of gibbsite particles is in macroscopic disorder in the whole size range, while it is regular in certain size range. PSD curves oscillate and become unstable sometimes, while it has a trend in the whole process. Minor changes of initial conditions will be amplified in the process, and butterfly effect appears for the change of PSD. Each curve of PSD owns similar space fractal character in a certain range of particle size. Preciser structures of the curves of PSD can be identified if smaller divided rule of particle size is adopted. Oscillation phenomena of the particle size information may appear both on laboratory and industry scales.5 Based on a binary collision model of the agglomeration process, a model was proposed, where particles of different size of r_i-1, r_i and r_i+1 were used as reactants and exhaustion method was adopted. Equations of the agglomeration rate were attained according to the model and the total agglomeration rate was worked out according to the experimental data by a difference method. Equation groups were solved by a software of 1 stOpt of data analysis. Kinetics results shows equal size particles have larger agglomeration rate constants than different size particles'.6 all three additives of 1-Octanol, 1-Dodecanol, and 1-Octadecanol can increase the precipitation ratio of sodium aluminate liquors at the adopted dosages. They can accelerate the precipitation process obviously at lower dosage at the beginning and the effect lasts longer at higher dosage.Additives with shorter carbon chain are more effective at the beginning,while additives with long chain have better continuous improvement of the precipitation ratio. Particle size of gibbsite products is enlarged when 1-Octadecanol is added at low dosage. With the increase of the dosage of carboxylic acids, the precipitation ratio of the liquors rises. Carboxylic acids of high dosage can enhance the precipitation ratio. Carboxylic acids of shorter carbon chain are more effective at the beginning. Yet additives with long chain have better continuous improvement of the precipitation ratio. For carboxylic acids, high dosage or long carbonic chain is favorable for PSD of gibbsite particles. Gibbsite products will be a little larger when stearic acid is added. Combination of octadecanol with stearic acid or combination of octadecanol with oil carrier has no synergetic effect on the agglomeration process.8 Ionic PAM can improve the precipitation ratio obviously, but non-ionic PAM can not. The precipitation ratio is 2.5% more at the dosage of 2.5×10^-6 (W/W) cationic PAM. PAM of proper dosage enhances the agglomeration of particles less than 45μm and enlarges PSD of gibbsite products. Compared with the blank, d50 of the products is 4.9μm larger and particles less than 45μm is 23% less at the dosage of 1.5×10^-6 (W/W) anionic PAM. Gibbsite particles with the addition of PAM are made up of median size crystallines which are filled with many crystallines of 5μm or so, and there fills with even less ones among 5μm crystallines. Crystallines are connected in a mosaic form tightly. Intensity of gibbsite products with the addition of PAM is higher than that of the seeds and the blank. The products with the addition of cationic PAM or anionic PAM have higher intensity.7 It is feasible to determinate aluminate ions semiquantitatively with internal standard method of NaNO₃. Cationic PAM can enhance the conversion of Al(OH)₄^-→Al₂O(OH)₆^2-→Al(OH)₃ and improves the seeded agglomeration process of sodium aluminate liquors.