Study On Growth Unit In Neutralization Crystallization Process Of Aluminum Hydroxide

Fly ash is the solid waste of thermal power plants.With the accelerating development of the power industry,the accumulation of fly ash is increasing year by year.The massive discharge of fly ash has problems such as land occupation,environmental pollution,and damage to human health.However,the abundant high-aluminum coal stored in the northwestern part of my country and the western part of Inner Mongolia.These high-aluminum coals are burned by thermal power plants to form fly ash with an alumina content of more than 40%,which was an important aluminum-containing resource.The technology of extracting alumina from ammonium bisulfate wet leaching fly ash has the characteristics of mild process conditions,recyclable materials,and low energy consumption,and has good application prospects.The extraction of alumina by ammonium bisulfate wet leaching of fly ash mainly includes two parts:fly ash leaching and leaching solution neutralization and crystallization.The main component of the leaching solution is ammonium aluminum sulfate solution.The ammonium aluminum sulfate solution is added to ammonia water to prepare coarse-particle aluminum hydroxide,which is the key to neutralization and crystallization.This dissertation mainly studied the neutralization process of its ammonium aluminum sulfate solution,and explored the process conditions for the preparation of alumina precursor pseudoboehmite by neutralization of ammonia water and ammonium aluminum sulfate solution.The stable form of the aluminum ion monomer was determined by combining the structure of the solution and the structure of the pseudoboehmite.Based on the first principles,the structural form of stable existence of growth units were calculated.The most favorable combination mode models were obtained through the bond orientation analysis.The growth unit was dehydrated and condensed into a crystal nucleus.And the properties of each crystal face of pseudoboehmite were explored,and the relationship between the appearance of different crystal faces and the crystal morphology were obtained.On the basis of the predicted equilibrium morphology and growth morphology of pseudoboehmite,considering the influence of solution temperature on the morphology,the growth mechanism of the pseudoboehmite morphology obtained from the experiment were discussed.The main results obtained are as follows:（1）It was determined that the crystalline product of the neutralization process of ammonium aluminum sulfate and ammonia solution was pseudoboehmite.The average particle size of pseudoboehmite is 26.1μm.The morphology was mainly lamellar superimposed flower-like morphology and spherical morphology.The aluminum in the crystalline product mainly exists in the bonding form of Al-O₆ and Al-OH.（2）The main bonding forms of aluminum in the solution with a neutralized crystal p H of7.0 are Al-O₆,Al-H₂O and Al-OH.The aluminum ion monomers with higher binding energy under different coordination numbers are Al（OH）₄^-and Al O₂^-.The six-coordinate aluminum monomer Al O（OH）（H₂O）₄ has the greatest thermodynamic stability.（3）Al₅O₅（OH）₅（OH₂）₉ is the growth unit that produce pseudoboehmite in ammonium aluminum sulfate and ammonia solution.The optimal molecular configuration of growth unit Al₅O₅（OH）₅（OH₂）₉ was determined.The nucleation was formed by the superposition of the growth units.（4）Through first-principles calculations,it was determined that the pseudoboehmite（131）surface is the most stable,with a surface energy value of 0.0474 J/m²,and the corresponding HRTEM only displays the（131）surface.The（200）plane has the largest absolute value of adsorption energy（0.2106 e V/unit）,and the diffraction peak intensity corresponding to the crystal plane is the highest.