| Coal gangue is a kind of solid waste discharged through coal mining and coal washing process,which accounts for a large proportion in the whole solid wastes mass.It has become one of the largest industrial wastes in China and brought a series of serious social,environmental and economic problems.Besides,coal gangue contains abundant aluminum resources and a large number of available mineral components such as kaolin,which is a kind of solid waste resources with high value of utilization.However,the utilization efficiency of coal gangue is not high due to its low activity.Calcination is an important activation technology of coal gangue,and fluidized suspension calcination can provide a new way for calcination activation of coal gangue and preparation of calcined kaolin to realize comprehensive utilization.At present,most of the existent studies only examined the optimization of calcination process conditions and cold simulation,and few researches focused on the key scientific issues in the fluidized suspension calcination,such as thermodynamics and kinetics mechanisms,phase and microstructure evolution,crystal structure change,pore structure characteristics transfer,and so on.Consequently,in order to solve the above problems,the suspension calcination of coal gangue collected from Shuozhou,Shanxi Province,China were investigated using X-ray diffraction,infrared spectrum analysis,comprehensive thermal analysis,specific surface area and pore structure analysis,X-ray photoelectron spectroscopy,nuclear magnetic resonance and scanning electron microscopy etc.A systematical study was carried out by theoretical analysis,experimental researches and computing simulation,and some achievements,having obviously scientific and technological significance,were obtained.The mineralogical properties of coal gangue were analyzed by chemical analysis,XRD,FTIR,TG-DSC and SEM.The results demonstrated that the main components are Al2O3 and SiO2,the mass fractions are 37.85%and 47.38%respectively,and the C content is 1.68%,the loss on ignition is 14.78%.The crystalline minerals in the ore are mainly kaolinite and quartz.The coal gangue belongs to the typical kaolin-coal gangue.The thermodynamics of coal gangue calcination process was calculated and analyzed systematically by using HSC Chemistry 6.0 thermodynamic software and DSC differential thermal test.The results show that thermal decomposition of kaolinite can proceed spontaneously when temperature is above 947 K.The calculation results of reaction enthalpy show that the thermal decomposition reaction of kaolinite is endothermic reaction,while the combustion reaction of carbon,the oxidation reaction of pyrite and the recrystallization reaction of metakaolinite are exothermic reaction.The influence mechanism of calcination factors and impurities on the physicochemical properties of calcined products was studied by suspension calciner based on thermodynamic analysis and simulation calculation.The chemical oxygen demand(COD value)of products can be used to characterize the carbon residue of calcined products.The leaching rate of aluminium can be used to characterize the chemical reaction activity.The whiteness value is an important application index of calcined kaolin.The results show that suspension calcination temperature and time are positively correlated with whiteness value and negatively correlated with COD value of products.Increasing calcination temperature and prolonging calcination time are beneficial to increase of whiteness and decrease of COD value.The leaching rate of active aluminium of calcined product is higher under the calcination temperature with range of 600?900? and the calcined product is inactivated after 1000?.The increase of oxidizing atmosphere and air flow rate is conducive to the increase of whiteness value,decrease of COD value and increase of reaction activity.Under the same calcination temperature and time,the properties of suspension calcination products are uniform,whiteness and COD values are lower and activity is higher than that of static calcination products.The whiteness value of calcination products decreases with the increase of carbonaceous,hematite and pyrite.The granularity of calcined material decreases,which is conducive to the ncrease of whiteness value,decrease of COD value and increase of reaction activity.The experiments show that the calcined kaolin products with whiteness value of 90.3,COD value of 280.83 ?g·g-1 and aluminum leaching rate of 85.36%can be obtained,when the raw ore with an average particle size of 18.69 ?m calcined at suspension calcination temperature of 900?,calcination time of 70 min and the air gas flow rate of 200 mL/min.It is determined by non-isothermal kinetics and isothermal kinetics studies that the optimal reaction kinetics function of coal gangue calcination process is F3 model,which is controlled by chemical reaction order,the integral form is g(?)=(1-?)-2-1,and the differential form is f(?)=1/2(1-?)3.The activation energy Ea obtained by non-isothermal kinetics is 184.2 kJ/mol,and the pre-exponential factor lnA is 19.87 s-1.The activation energy Ea obtained by isothermal kinetics is 170.68 kJ/mol,and the pre-exponential factor lnA is 20.57 s-1.The increase of carbon content in coal gangue can not change the reaction mechanism function during calcination,and the mechanism function is still F3 chemical reaction model,but the activation energy and pre-exponential factor are increased.XRD,FTIR,PSD,BET and SEM were used to characterize the transformation of mineral phase,crystal structure,particle size,pore structure and micro-morphology in different stages of calcination process under different conditions.The results show that the relative thermal decomposition rate of kaolinite is above 90%and the product has high activity when the coal gangue calcined by suspension calcination at 700?900?,and the stable mullite phase will be formed after 1000?.Increasing calcination temperature(?900?),prolonging calcination time and increasing carbon content contribute to dehydroxylation of kaolinite,increase in average particle size of the particles,destruction of pore structure,decrease of specific surface area and total pore volume,and destruction of layered ordered structure on product surface.Besides,the mechanism of thermal activation was studied by means of XPS and NMR analysis.The results show that the tetrahedral coordinates of silicon has no change during suspension calcination,and the layered structure of silicon-oxygen tetrahedron is maintained,but the degree of order decreases.Polyhedra structure of aluminoxy-polyhedral transforms from hexacoordinate to tetracoordinate and pentacoordinate,and the degree of order becomes worse.Meanwhile,the binding energy of aluminum and silicon on the surface of suspension calcination products decreases,and the Gibbs free energy of the system decreases.The thermal activation of coal gangue is attributed to the formation of thermodynamically unstable system of suspension calcination products.The research results in this paper provide a new technical approach for the thermal activation of coal gangue and the preparation of calcined kaolin,and provide theoretical support for the further development and engineering application of suspension calcination technology of coal gangue. |
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