| Thermal analysis is a group of analytical techniques in which physical properties of a substance are measured as a function of temperature.The traditional thermal analysis techniques include differential thermal analysis(DTA),differential scanning calorimetry(DSC)and thermogravimetric analysis(TG),etc.With the development of thermal analysis and computer technology,compared with the traditional thermal analysis technology,the current DSC/DTA-TG simultaneous thermal analysis technique can provide new technical means and evaluation criteria such as DTG,DDTA,and DDSC signals.In addition,DSC/DTA-TG-IR/MS can provide more useful information such as qualitative and a more complete quantitative data to explain the complex reaction mechanism of dehydration,dehydroxylation and recrystallization during clay minerals heating.Date to,the thermal analysis kinetics(TAK)based on the isothermal and non-isothermal data has become an important tool in the research field of heterogeneous systems.TAK has been widely used in the study of heat transfer reaction,structural evolution and high temperature pyrolysis of solid materials.Compared with the research in the fields of chemistry and materials science,the application of TAK in the field of mineral science is rare.Layered structure silicate minerals such as kaolinite,serpentite,talc,pyrophyllite,muscovite and montmorillonite is not only an important clay mineral,but also an important industrial mineral,which is widely used in many industrial fields.The physical and chemical properties of these minerals are closely related to the kinetic processes such as interlayer stability,dehydration,dehydroxylation and phase transition during heating.So far,the thermal effect characteristics and reaction mechanism of these minerals has been investigated extensively by many authors using conventional thermal analysis(TA),and a lot of research results have been obtained.However,by the restriction of the research methods and techniques,there are few reports,or the lack of in-depth and systematic research on the complex controlling mechanism such as multi-step dehydration reaction,overlying dehydration reaction,overlying reaction of dehydration and associated minerals decomposition,and multi-step phase transition during these minerals heating.These need further study.The multivariate nonlinear fitting technique in the thermal analysis kinetics(TAK)is the only one that can describe the multi-step reaction and determine the kinetic models and relative kinetic parameters in the whole reaction process.Therefore,the paper intended to choose kaolinite,antigorite,talc,pyrophyllite,muscovite and montmorillonite as the object,to study thermal decomposition mechanism of 1:1 and 2:1 type layered silicate minerals based on the non-isothermal thermal analysis data and multivariate nonlinear fitting technique combined with DSCTG-MS/FTIR and other technology.In addition,based on the results of previous studies,and combined with XRD,IR,SEM-EDS and DSC-TG-IR analysis,the most probable reaction mechanism and the relative kinetic parameters were analyzed and compared.This subject has important theory significance in the study of the complex mechanism such as multi-step dehydration reaction,overlying dehydration reaction,overlying reaction of dehydration and associated minerals decomposition,and multi-step phase transition during these minerals heating.Research methods and achievements provides new data for the further understanding the geological and mineralogical problems such as genesis and mineralization of clay minerals,and a scientific basis for the development and utilization of clay mineral resources.While it also accumulates useful experience for the application of TAK in mineral science,and explores new ways and methods for the research of mineral thermal transformation.The specific research work and conclusions are as follows:1.The effect of heating rate on the characteristics of dehydration,dehydroxylation and phase transformation of the studied layered silicate minerals was studied.(1)The results show that the heating rate has effect on thermal effect characteristic such as TG,DTG,DTA and DSC during dehydration,dehydroxylation or recrystallization of the studied layered silicate minerals,and the above reactions are kinetic processes.(2)There is only one valley in the DTG curve in the dehydroxylation process of kaolinite and montmorillonite,whereas there are two valleys in the DTG curve of antigorite,pyrophyllite,talc and Muscovite.Kaolinite has the lowest DTG peak temperature in the dehydroxylation process,and talc has the highest DTG peak temperature.There are similar temperature range in the dehydroxylation process of antigorite and montmorillonite,pyrophyllite and Muscovite,respectively.However,the temperature range of the former is narrower and the latter is wider.2.The activation energy,pre-exponential factor and reaction complexity during thermal transformation of the studied layered silicate minerals were investigated.(1)For the dehydration or dehydroxylation reactions of the studied layered silicate minerals,the value and change range of reaction activation energy of the dioctahedral minerals is obviously higher than of the trioctahedral minerals when the layer type(1:1 or 2:1)is the same;the reaction activation energy of 2:1 type minerals is slightly higher than of 1:1 type minerals when the octahedron ions characteristic(dioctahedron or trioctahedron)are the same;when the layer type and octahedron ions characteristic are the same,the value and change range of reaction activation energy of the cation containing minerals is higher,and the activation energy of the minerals with interlayer water is lowest.For the studied layered silicate minerals,the activation energy of dehydroxylation reaction is increased from montmorillonite,kaolinite,pyrophyllite,antigorite,muscovite to talc.(2)For the studied 1:1 and 2:1 type layered silicate minerals,the dehydroxylation process is a complex kinetic process.3.The most probable reaction mechanism model and relevant reaction parameters during dehydration or dehydroxylation process of the studied typical layered silicate minerals were determined.The results show that the dehydroxylation process of 1:1 type layered silicate minerals such as kaolinite and antigorite is controlled by a multi-step continuous mechanism,whereas 2:1 type layered silicate minerals,such as pyrophyllite,talc,muscovite,and montmorillonite dehydroxylation process is controlled by a multi-step competitive mechanism.1:1 typeAs 1:1 layered silicate mineral,the dehydroxylation process of kaolinite(dioctahedron)and antigorite(trioctahedron)are all three consecutive mechanisms,but kaolinite is controlled by the D3-Fn-Fn mechanisms,and antigorite is controlled by the Fn-D3-An mechanism.2:1 type(1)Pyrophyllite(dioctahedron)and talc(trioctahedron)There are competitive path in the dehydroxylation process of pyrophyllite and talc,and both of which are heterogeneous reactions.However,the different points are that the dehydroxylation process of pyrophyllite is three-step((?),Fn-Fn-Fn),and during the competition,the same reaction mechanism(Fn)are observed,and the activation energy of the reaction is close to 229.70-247.46 KJ/mol;whereas for talc,the dehydroxylation process is five-step((?),Fn-An-Fn-FnAn),and during the competition,the controlling mechanism is different,and the activation energy is higher than of pyrophyllite(402.24?338.86?563.31 KJ/mol).(2)Pyrophyllite(dioctahedron)and muscovite(dioctahedron,containing anions)There are two-step competitive path in the dehydroxylation process of pyrophyllite and muscovite,and both of which are heterogeneous reactions.And during the competition,the same reaction mechanism are observed.The dehydroxylation reaction of pyrophyllite and.The different points are that the dehydroxylation process of pyrophyllite is three-step((?),Fn-Fn-Fn),whereas muscovite is fourstep(Fn-Fn-An-An,(?)),and during the competition,pyrophyllite is controlled by Fn model and muscovite is controlled by An model,and the activation energy of pyrophyllite dehydroxylation(289.99?525.26 KJ/mol)is significantly lower than that of muscovite.(3)Pyrophyllite(dioctahedron)and montmorillonite(dioctahedron,containing anions and interlayer water)During the dehydroxylation of pyrophyllite and Ca-montmorillonite,there are the existence of two-step competitive path,and controlled by Fn model,and the reaction activation energy range is similar,for pyrophyllite,229.70?247.46 KJ/mol,and for montmorillonite,260?210 KJ/mol.4.The dehydroxylation process of pyrophyllite from Ermei,Fuzhou province under CO2 atmosphere is controlled by the four-step competition model,q:c,f,c;((?),Fn-Fn-FnFn)model.During competition,the dehydroxylation reaction in CO2 is one step more than in argon,air,and vacuum.The results confirmed that CO2 incorporation into pyrophyllite is controlled by Fn model,rather than a simple physical adsorption process.5.Non-isothermal crystallization kinetic analysis of during antigorite recrystallization based on JMA,SB(M,N)model,isoconversional and model-fitting methods was investigated.The results show that the isoconversional combined with multiple nonlinear regression methods is most suitable for describing the multiple crystallization behavior of layered silicate minerals.The most probable reaction mechanism was determined by the multiple nonlinear regression analysis during antigorite recrystallization,and followed two consecutive d:f((?)An-An)model,... |
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