Mechanism Of Alumina Hydroxide Crystallization During Ammonia Neutralization Of Ammonium Aluminum Sulfate Solution

Alumina is an important industrial material in China.Due to the lack of bauxite resources in China,about 50%of bauxite needs to be imported.China is a large coal consuming country.The fly ash produced by coal-fired power plants in the northwest of Shanxi province and the midwest of Inner Mongolia contains a large amount of aluminum,which is a high-value secondary resource.At present,there are many studies on the process of extracting alumina from high alumina fly ash.The ammonium hydrogen sulfate solution leaching method has the most industrialized prospect.The key step of this method is to neutralize the fly ash leaching solution with ammonia and precipitate aluminum from ammonium aluminum sulfate solution.However,due to the lack of theoretical guidance,the particle size of alumina prepared by this technology does not currently meet the requirements of electrolytic aluminum for alumina.In this paper,the thermodynamics of neutralization and aluminum precipitation process of ammonium aluminum sulfate solution in ammonia was calculated,the EH-p H diagram of Al-H₂O system was constructed,and the composition diagram of（NH₄）₂SO₄-Al₂（SO₄）₃-H₂O ternary system was constructed by isothermal dissolution equilibrium method.The crystallization kinetics of aluminum hydroxide produced by neutralization of aluminum ammonium sulfate solution with ammonia during aluminum precipitation was studied by batch dynamic method.Based on the particle number equilibrium equation,the data obtained from the experiment were fitted with 1st Opt software,and the crystallization kinetic model of aluminum hydroxide was established.The crystallization mechanism of aluminum hydroxide was analyzed based on the change law of particle size and morphology of crystallization products.The main conclusions were as follows:（1）The EH-p H diagram of Al-H₂O system in the temperature range of 25℃-45℃was determined.Aluminum mainly existed in the form of boehmite in the range of p H 4-11.（2）The composition diagrams of（NH₄）₂SO₄-Al₂（SO₄）₃-H₂O ternary system at 35℃and45℃were constructed.The composition diagram showed that the crystallization region of NH₄Al（SO₄）₂·12H₂O was much larger than that of Al₂（SO₄）₃·16H₂O and（NH₄）₂SO₄,and the crystallization region of NH₄Al（SO₄）₂·12H₂O increased with the decrease of temperature.（3）The crystallization kinetics model of aluminum hydroxide was established.Kinetic studies showed that the nucleation of aluminum hydroxide was mainly caused by the collision between crystals,and the nucleation and crystal growth of aluminum hydroxide were mainly controlled by diffusion.The decrease of reaction temperature,solution supersaturation and suspension density could inhibit crystal nucleation.The increase of reaction temperature and solution supersaturation could promote crystal nucleus growth.（4）Two parallel octahedral layers connected by hydroxyl groups were stacked orderly along the（020）crystal plane through interlayer hydrogen bond,and excess interlayer water made the layered structure form pores in the process of accumulation.The newly formed nuclei were adsorbed on the crystal plane and bonded with the hydroxyl on the surface layer of boehmite,which changed the preferred growth direction and eventually formed a porous spherical structure.