Study On The Preparation Of Metallurgical Grade Alumina By The Thermal Decomposition Of Aluminum Chloride Hexahydrate

China is one of the main producer and exporter of primary aluminium.The output of smelting grade alumina in China has risen up to 61 million metric tons by the end of 2016.With the increase of alumina production capacity,it is necessary to seek for other possible methods for smelting grade alumina manufacture since China is lack of high quality bauxite.In the recent years,the acid method for the extraction of alumina from aluminum-rich coal fly ash has drawn more and more attention.Based on the acid process,a series of study on the preparation of smelting grade alumina with the thermal decomposition of aluminum chloride hexahydrate(AlCl3·6H2O)was carried out as introduced in this thesis.In this work,thermodynamic analysis of AlCl3·6H2O was firstly studied with the FactSage software.Then the non-isothermal and isothermal kinetics for the thermal decomposition process of AlCl3·6H2O was investigated with differential scanning calorimetry-thermogravimetric analysis(DSC-TGA)method.And the morphology and compositional evolution of the intermediate products were investigated through X-ray diffraction,scanning electron microscope and chemical analysis technologies.The thermal decomposition process of AlCl3·6H2O is estimated to be a one-step endothermic reaction.The onset temperature is approximately 363K(90℃),and the main decomposition temperature ranges from 363K(90℃)to 623K(350℃),within which interval the weight loss rate is about 70%.Non-isothermal kinetic analysis showed that the decomposition mechanism of AlCl3·6H2O tends to follow the modified Avrami-Erofeev function,and the simulated conversion function can be expressed as G(α)=[-1n(1-α)]1/m.The average activation energy ’E’ is derived as 59.7 kJ/mol,and the kinetic exponent ’1/m’and pre-exponential factor ’A’ are 0,72 and 6.67×105 min-1,respectively.Contrast to the experimental data,the kinetic model as well as the calculated prameters were proved to be credible.However,The result of isothermal kinetic analysis showed that the average activation energy‘E’ is 75.8 kJ/mol,whichi is a bit higher than that calculated from the Kissinger/FWO equations in the the non-isothermal analysis.This is probably caused by the temperature increase during the non-isothermal experimental process,which means the reaction activation energy is substiantially affected by the temperature.The water vapor starts to release at the very early stage of the decomposition process,and the hydrogen chloride emission increases fast when the heating temperature rises above 373K(100℃).The conversion rate of AlCl3·6H2O to alumina reaches approximately 96%when the temperature goes up to 623K(350℃),however,the product maintains as amorphous state until γ/θ-Al2O3 crystals are formed at around 1123K(850℃).The pyrolysis product obtained at 1273K(1000℃)for 60min behaves a chlorine content of 0.067%,and the loss on ignition of 3.13%.Studies on the effects of gas partial pressure on the decomposition rate of AlCl3·6H2O showed that the water vapor atmosphere could significantly accelerate the decomposition rate,and the aluminium hydroxide phase was observed with XRD method under both 423K(150℃),60min and 433K(160℃),60min conditions.