Study On Pyrolysis Characteristics Of Typical Hazardous Waste And Solid Waste From Aluminum Industry

With the rapid development of aluminum industry,the yield of spent cathode carbon block（SCCB）,a typical hazardous waste,and red mud（RM）,a general solid waste,has increased sharply.Because of the toxicity of fluorides in SCCB and the strong alkalinity of RM,the unreasonable disposal will cause serious pollution to ecological environment and do severe harm to organisms.Among the disposal techniques of solid waste,heat treatment such as pyrolysis and combustion has become a main treatment choice due to the advantages of simple process,quantity reduction,harmless disposal and resource recovery.Therefore,for the purpose of providing a theoretical reference to the research of waste control by waste and the harmless disposal in the field of heat treatment of SCCB and RM,the individual pyrolysis and co-pyrolysis characteristics,as well as the interaction effect of SCCB and RM were explored in this study.In order to understand the pyrolysis characteristics and kinetics of SCCB,RM and its mixture in detail,the theories and common methods of pyrolysis kinetics were employed on the basis of thermogravimetric experiment（TG）and thermogravimetric-Fourier transform infrared spectroscopy experiment（FTIR）.The main work and conclusions of this thesis are as follows:（1）Study on pyrolysis characteristics and kinetics of SCCB.Based on thermogravimetry and FTIR data,the pyrolysis behaviors,kinetics parameters and reaction model of SCCB were analyzed by multiple model free methods and SCE optimization algorithms under three heating rates.The results show that the pyrolysis of SCCB in nitrogen is concentrated in 1100-1500 K due to the emission of fluoride,and the combustion in air includes two stages:the oxidation of carbon and the escape of fluoride.FTIR analysis indicates that the main gas products of pure pyrolysis are CO,CO₂,HF and AlF₆^3-,while the main gases in air are CO₂ and HF.Activation energies calculated by multiple model free methods are 236.84-264.66 k J/mol in nitrogen,while 82.23-180.08k J/mol of first stage and 155.88-206.88 k J/mol of second stage in air.In addition,the multi-component reaction model was optimized by Shuffled Complex Evolution（SCE）algorithm based on the kinetic parameters obtained by FWO method.The optimized thermogravimetric and differential thermogravimetric（TG/DTG）curves in different atmospheres provide the consistent result with the experimental data under various heating rates.（2）Study on pyrolysis characteristics and kinetics of RM.The pyrolysis behaviors and kinetics of RM were studied based on thermogravimetric data at four heating rates and FTIR data at 20 K/min.Thermogravimetric analysis shows that the pyrolysis process of RM in nitrogen and air atmosphere is similar,including three stages,but a decomposition shoulder peak around 580-620 K appeared in air,which is formed by the dehydroxylation of goethite（FeOOH）.FTIR analysis shows that the main gas production of pyrolysis are H₂O and CO₂,and the gas emission peak corresponds to the pyrolysis peak of DTG curve.By employing FWO method coupled with CR method,the average activation energies of the three pyrolysis stages in nitrogen and air are calculated as 167.7,307.9,242.3 k J/mol and 162.2,265.8,214.1 k J/mol,the corresponding optimal reaction mechanism functions are determined as g（α）=[（1-α）^-1/3-1]²,g（α）=[-ln（1-α）]³,g（α）=α+（1-α）ln（1-α）and g（α）=[-ln（1-α）]³,g（α）=1-（1-α）^1/4,g（α）=[ln（1-α）]^1/2.The difference of results may be due to the influence of air atmosphere.（3）Study on co-pyrolysis characteristics,kinetics and interaction of SCCB and RM.The pyrolysis behavior and kinetic parameters of SCCB/RM mixture were studied based on thermogravimetric data at three heating rates.The interaction between SCCB and RM was analyzed by FTIR analysis,comparison of theoretical and experimental DTG curves,and comparison of individual pyrolysis and co-pyrolysis DTG curves.The results indicate that the co-pyrolysis process of SCCB and RM in air can be divided into four stages.Based on the deconvolution results of the overlapping pyrolysis peaks,the average activation energies E_a calculated by FWO method are 113.3,126.2,147.9 and 280.9 k J/mol,and the optimal reaction mechanism functions obtained by CR method are g（α）=[-ln（1-α）]²,g（α）=（1-α）ln（1-α）+α,g（α）=（1-α）^-1/2-1 and g（α）=（1-α）^-1/2-1,respectively.The positive value ofΔH andΔG of all the four stages indicate the oxidative pyrolysis process of mixture is non-spontaneous and require additional energy by thermodynamic analysis.The results of interaction analysis show that there is an inhibition effect before 800 K,a synergistic effect in the range of 800-1100 K and also an inhibition effect above 1100 K.