Process And Mechanism Of Alumina Extraction From High-alumina Fly Ash By Ammonia Method

Currently,the fly ash in china is mainly used for construction,building materials and agriculture and so on,which belongs to a low added value industry.Especially,it is a big waste for high-alumina fly ash in which alumina content is greater than 30%,So the extract of alumina from high-alumina fly ash,not only enhances the value of fly ash and bring huge economic benefits,but also solves the problem of shortage of bauxite resources.It has great practical significance and long-term strategic significance.At present,the alumina extracting technology from fly ash can be divided into three categories:alkaline method,acid method and ammonia method.Alkaline method mainly includes limestone sintering and pre-deification soda lime sintering;acid method includes sulfuric acid method and hydrochloric acid method;ammonia method is ammonium sulfate sintering method.Compared with alkaline method and acid method,ammonia method has some advantages such as low energy consumption,weak-acid reaction system,the reducing extraction process,suitable for handling all types of high-alumina fly ash,and comprehensive utilization of slag,etc.In this paper,aiming to alumina production technology based on aluminum sulfate phase,a detailed study on ammonia-method sintering and leaching,iron removing of solution,growth process of aluminum hydroxide crystal was carried out and the kinetics of alumina production process were analyzed in detail.For armmonia method sintering and leaching process,the appropriate mechanical activation conditions were determined by the analysis of the properties of sintered products:the main factor to affect the extraction rate of Al2O3 was sintering temperature,and the second factor was heating time and holding time.The alumina extraction rate of fly ash was 86.02%at the optimal conditions,namely sintering temperature of 580?,heating time of 50 min,holding time of 120 min.For iron removal process of fly ash by magnetic separation method and leaching liquid,the relevant results showed that only magnetic iron could not meet the requirements of alumina production.Ammonium alum method and goethite method was necessary for in-depth iron removal of fly ash,the suitable process conditions of ammonium alum method are the pH value of 1.5,the reaction time of 6 h,and ammoniojarosite seeds of 80 g/L.The optimum conditions for goethite method iron removal are the reaction temperature of 90?,the aluminum ion concentration of 40 g/L,pH value of 2.5,the reaction time of 8 h,3%hydrogen peroxide used as the oxidant,dosing interval of 30 min,and seed dosage 8 g/L(leaching solution).Under these conditions,the iron ion concentration in the leaching liquid reduced to 14.4 mg/L,which meet the requirements of the alumina production.The aluminum ion is almost no loss in the whole process of removal iron ion.For investigating the growth mechanism of aluminum hydroxide crystal by the reaction of ammonia and aluminum sulfate,some parameters such as aluminum ion concentration,reaction temperature,the added way of ammonia and dispersant;coefficient of aluminum hydroxide crystal seed,pH value of the solution,and reaction time during the process conditions were considered.The main factors of affecting product(aluminum hydroxide)d50 and affecting the type dimension D.were studied by RSM(Response Surface Method).The results show that the main factor of affecting product(aluminum hydroxide)d50 is the reaction temperature,followed by reaction time,dispersant amount,and pH value of solution.The main factors that affecting the type dimension D is the dispersant amount,followed by pH value of solution,reaction temperature,and reaction time.The kinetic results of ammonia sintering shows that mixed sintering process of ammonium sulfate with fly ash can be divided into two stages.At the initial stage,sintering process is controlled by the chemical reaction;at the later stage,sintersing process is controlled by the interfacial diffusion.Kinetics of iron removing process of the solution shows that the ammonium jarosite method iron removal process and goethite method iron removal process can be divided into two stages,the different stages should be in line with the second-degree reacting rate equation,The study on the kinetics of aluminum hydroxide crystal growth shows that the entire precipitation process can be divided into two parts,the crystal nucleus growth and grain growth.Diffusion-controlled mechanism is the dominant for aluminum hydroxide nucleation growth and grain growth.The corresponding results provide some new ways for the process and mechanism of alumina extraction from high-alumina fly ash.