Study On The Preparation And Formation Mechanism Of Highly Active Silic

Due to its excellent physical and chemical properties,silicon and its compounds have important applications in the fields of metallurgy,chemical industry,light industry,semiconductor,communication,new energy and so on.The abundant silica in the earth’s crust is the main source of silicon.Silica,which is abundant in the earth’s crust,is the main source of silicon.However,various silicon compounds are difficult to synthesize by direct depolymerization of silica because of the chemical stability of silica.Most silicon compounds and products must be produced from metal silicon（also known as industrial silicon）.Silicon metallurgy is a high energy consumption process,which needs to convert silicon dioxide into metal silicon through carbothermal reduction reaction at 1900℃.Using active silica as raw material to directly and cheaply synthesize various silicon compounds and products to avoid high temperature carbothermic reduction process is the forefront of research in related fields.The research and development of active silica is conducive to improving the energy-intensive status of the silicon industry from the source.Kaolinite is abundant in the earth’s crust and widely distributed.With the development of coal mines,metals and other minerals,kaolinite based solid wastes with kaolinite as the main mineral component are often produced.The efficient resource utilization of these solid wastes can completely eliminate the harm to the environment and produce good environmental,economic and social benefits.Kaolinite contains a large amount of alumina resources,and has been studied as the most important potential substitute resource for bauxite to extract alumina from it,but the large amount of silica resources contained in it has not been effectively utilized.This paper takes kaolinite as the research object,and carries out the preparation technology and theoretical research of high-activity silica,which has important theoretical significance and practical value.In this study,silica with high chemical reactivity was prepared by using kaolinite as raw material.The super high chemical reactivity of active silica was characterized by studying the characteristics of its rapid reaction with dilute strong alkali solution at room temperature.The effects of conditions and parameters in the process of direct acid leaching of kaolinite,thermal activation acid leaching,mechanical activation acid leaching and calcination of active silica on the chemical reaction activity,crystal structure,morphology,particle size,specific surface area,pore structure and other physicochemical structures of silica were designed and studied by X-ray diffraction（XRD）,infrared spectroscopy（FT-IR）²⁹Si high-resolution magic angle rotating solid-state nuclear magnetic resonance analysis(²⁹Si MAS NMR),synchronous thermal analysis（TG-DSC）,SEM,TEM,bet adsorption analysis,quantum chemical calculation and other means to study in detail the production path of high active silica,reveal its formation mechanism and activation mechanism,and analyze the essential reason for its extraordinary high chemical reaction activity.The main research contents and results of this study are as follows:（1）Research and develop a new idea to prepare high-activity silica from ore raw materials,and prepare high-activity silica by directly leaching kaolinite with sulfuric acid.The prepared high-activity silica can react completely with dilute Na OH solution within 2 h at room temperature of 25°C,and has high chemical reactivity.（2）Kaolinite is formed by closely stacking different unit layers along the c-axis,which seriously hinders the contact between solvent and ore.The pre-intercalation of sulfuric acid between the kaolinite layers enables the simultaneous reaction of alumina and acid at the edges and interiors of the crystals to facilitate the leaching of aluminum.The concentration of sulfuric acid solution significantly affects the intercalation behavior,and concentrated sulfuric acid（98%）has the highest intercalation efficiency.The first-principles simulation results show that the intercalation of sulfuric acid molecules between kaolinite layers can proceed spontaneously from the perspective of energy,the intercalation of sulfuric acid between layers of the kaolinite structure is driven by both hydrogen bonding and electrostatic adsorption.（3）The effects of sodium hydroxide solution concentration,reaction temperature,liquid-solid ratio and reaction time on the reaction rate of silica and sodium hydroxide were studied.Through alkali dissolution experiment,the chemical reaction activities of silica prepared by different acid extraction processes of kaolinite and silica from different sources and crystal forms were compared,and the formation pathway of high active silica and its extraordinary high chemical reaction activity were studied and determined.By studying the kinetic mechanism of the reaction between active silica and sodium hydroxide solution,the essence affecting the reaction rate of high active silica and sodium hydroxide solution is revealed.（4）The study confirmed the formation mechanism of active silica.The ²⁹Si MAS NMR chemical shift of kaolinite is-91.36 ppm,and the silicon oxide in it belongs to the chemical environment of the Q³ structure（（SiO）₃SiOAl）.After the kaolinite is directly acid leached to extract the aluminum oxide layer,the layered structure of the silicon oxide layer is maintained.First,a Q³ structure silanol（（SiO）₃SiOH）with one hydroxyl group is formed,and then the structure can occur adjacent to the same layer.The polycondensation between silicon-oxygen tetrahedra or the polycondensation between silicon-oxygen tetrahedra between different layers forms two kinds of ²⁹Si MAS NMR chemical shifts of-115.87 ppm and-106.96 ppm Q⁴structure（（SiO）₃SiOSi）dioxide Silicon,there is also a very small amount of silanols that have not undergone polycondensation.²⁹Si MAS NMR chemical shift is-95.63 ppm,which are components of high-activity silica.The breaking of the Si—O—Al chemical bond connecting the silicon oxide layer and the aluminum oxide layer in kaolinite is one of the important reasons for the improvement of the chemical reactivity of silicon oxide.In the process of spontaneous polycondensation of Q³-structured silica with broken Si—O—Al bonds to Q⁴-structured silica,activation occurs again.According to the different polycondensation methods,there is a chemical reactivity between the final active silicas.Difference,the chemical reactivity of active silica with ²⁹Si MAS NMR chemical shift of-115.87 ppm is higher than that of-106.96 ppm.（5）The formation and activation mechanism of highly active silica was verified by uantum chemical calculation studies.The breaking of the Si—O—Al bond in kaolinite results in a significant decrease in the strength of the silicon-oxygen bond in the silicon oxide.After breaking the Si—O—Al chemical bond,the corresponding Q³-structured silicon-oxygen layer subsequently undergoes polycondensation of adjacent silicon-oxygen tetrahedra in the same layer and polycondensation between silicon-oxygen tetrahedra between different layers,resulting in the formation of active silica with two different chemical environments.Both forms of polycondensation lead to a further decrease in the strength of the Si—O chemical bond.The above-mentioned dual activation leads to a significant reduction in the strength of the Si—O chemical bond in kaolinite,and ultimately leads to the formation of highly reactive silica.