Elemental Aluminum Hydrolysis Preparation Of High Purity Alumina

The high-purity alumina (≥ 99.99%) has been used in many fields. Currently the preparation methods of high-purity alumina are a great many, mainly inluding the sol-gel method, hot decompounding method, Bayer method, etc. However these methods more or less exist those problems, such as complicated process, high cost and pollution. The author researched into another path to prepare high-purity alumina--the high-purity aluminum hydrolyzes directly to produce high-purity alumina. Aluminum is used to prepare active aluminum powder, which hydrolyzes to become Al(0H)3.In order to getting the alumina with necessary purity and particle size distributing, Al(0H)3 must be calcined and milled. Through checking lately, this method is initated in the worldwide , which has simple process, low cost and free from pollution.This method has been applied in the Institution of Precious Metal. However, researching on the mechanism of this method is limited, and the attempt to optimize this technique is confined. Under the pecuniary aid of the nation and the province, the author researched on the mechanism thoroughly and writed this paper.The paper introduced the atomization equipment firstly, then the factors that effect on performance of the activated powder were discussed through experiments. The pivotal factors, pressure of atomization gas, rotation speed of turnplate, flux and degree of superheat of the melted aluminium, are analysed in detail. The experiment has detected that pressure and particle size had a reverse parabola relationship. Existing a critical pressure, being atomized pressure to near to this value, the particle size distributing is finest. The more high the rotation speed of the turnplate and the degree of superheat are, the more fine the particle size is;the more much the flux is, the more coarse the particle size is. After calculated the cooling speed, the paper switched to analyze the mechanizm of this method.Firstly, the thermodynamics behavior of the following reaction was calculated:2 Al+6H₂O→2Al(OH)₃+3H₂↑Aluminium has high activity itself, so this reaction can arises in normal temperature and pressure. But usually this reaction can't continue because of the effect of the close-grained oxide film in the surface.To promote the reaction, the film must be broken. XRD, SEM, Raman-spectral analysis found the hydrolyzation reaction is a crystallized process substantially. The equations are as follows:Al->Al3++3e A13++4H2O->A1(OH)4⁺4H+2H++2e-+H2TA1(OH)4⁺ H+<-? A1(OH)3+ H20The size of the aluminium powder is about 5um, so it has high surface energy;at the same time, the high cooling speed resultes in high-density crystal lattice disfigurement. These two factors cause the chemical bond of aluminium atom to be destroyed and the reactions would become easy.Hydrolyzation of aluminium is differ from the insute-oxidation in the surface of aluminium.Because the hydrolyzate is porous, the reaction couldn't be stopped. Moreover, M3+,Vt,M(OH)4,H2O are reactants and also resultants, so these reactions cited above areautocatalysis-reaction.On the basis of datum, confirm the first production(hexagnal bayerite)decompound to form supersaturation aqua, then the crystallized process is excitated.During Crystallized process the variation of the powder size has several stages, which is conduced by the crystal growth, agglomeration, the second nucleus and milling. Along with reaction time goes, the crystals become stable and the crystalline raises consumedly.The hydrolyzate is aluminum hydroxide, but a great deal of demanding currently on the market is a - AI2O3.Therefore, the hydrolyzate must be calcined. The process of phase transform is as follows: Al( OH)3 -A100H- n.-A12O3— Y -A12O3— 5 - A12O3^ 9 - A12O3^ a - A12O3The time of heat preservation had a notable influence on the phase transform progress. For example, during 400-800°C, if the heat preservation time was long, the process would be very complicated and would produce many transition phase, such as the r\ -, y -, 5 -, k - and x - phase;during 900-1000 ° C , if heat preservation time was long, it would produce a great deal of 9 -AI2O3. 0 - AI2O3 is stable relatively, and it's difficult to transform toa - AI2O3.According to such conclusions, the author thinks the particle size and the particle size distributing of the powder can be controled by a proper heat treatment process and the experiment also prove it's feasibility of this idea. But this process is so complex that many factors can't be controlled, which may cause performance of product undulate greatly. So this research inclines toward adopting compulsory classification process to control the particle size.To turbine-classification equipment, the classification particle size can calculate from the(18//F,following formula d = I——— . The classification accuracy and the pressures relate to closely.\ co rpExisted a pressure value, when the pressure is close to it, the classification efficiency is highest.Therefore, the calculation formula can be predigested to ? , among them, the k is anconstant relating to Vf, ^i, r, p.The formula elucidates an anti-function relation between rating granularity and rotate speed (co) this time. Therefore, in consideration of the operation should be as simple as possibly in the actual production, it inclines toward the pressure condition of fixing the rating equipments generally. So, the Vf within formula can also be looked as an constant. In the production, technicians carry out product of different classification particle size through accommodating the rotate speed.This research elaborated the new method and explained the theory fully the first time, which have graveness meaning to extend its applicable fields.