Study On The Preparation Of Silicon From Coal Gangue Activated By Supercritical (Subcritical) Hydrothermal And Its Surface Modification

Coal gangue（CG）is a solid waste generated by coal mining and coal separation.The discharge of coal gangue generally accounts for 10%to 15%of the coal production in that year.China’s coal gangue reserves are huge,not only occupying a large amount of cultivated land,but also polluting the surrounding environment,so the comprehensive utilization of coal gangue is urgently needed.The preparation of silicon-based chemical products from coal gangue is one of the high value-added means of resource utilization.The composition of coal gangue is complicated,and it is necessary to activate it in order to effectively utilize the silicon element.Currently,the main activation methods include:mechanical activation,thermal activation,chemical activation,and complex activation.Complex activation can greatly improve the activation efficiency of coal gangue and promote the efficient utilization of mineral resources in coal gangue.In this thesis,coal gangue was used as raw material,and precipitated white carbon black were prepared by supercritical（subcritical）hydrothermal activation technology.Under the condition of high temperature hydrothermal,the combination of thermal activation and chemical activation was achieved by adding activator,and the hydrothermal environment was favorable for the dissolution of silicon,thus shortening the activation time.The effects of hydrothermal activation conditions（alkaline activator,activation temperature,particle size of coal gangue）and acid leaching conditions（concentration of hydrochloric acid,temperature,time and liquid-solid ratio）on the yield of silica were investigated.Secondly,it was found that the precipitated silica has high hydroxyl content on the surface which was easy to agglomerate and difficult to disperse in the organic phase,so the application was limited.In this paper,methyltrichlorosilane（MTS）was used as a modifier to improve its dispersion and hydrophobicity.The effects of temperature,time and concentration of modifier on the physical and chemical structure and properties of precipitated silica were investigated.The products were characterized by XRD,IR,SEM,N₂ adsorption and other characterization methods.Based on a large number of experiments,the following conclusions were obtained:（1）Three kinds of activators NaOH,Na₂CO₃ and KOH were used as activator during the process of supercritical hydrothermal activation of the raw coal gangue.The main crystalline phases of activated gangue were cancrinite,hydroxyl-cancrinite and kalsilite.The yield of silica was 39.22%,44.92%and 43.94%,respectively.Based on cost considerations,it is more suitable to use NaOH as an activator.（2）Activation temperature has a great influence on the main crystalline phases in activated coal gangue.A series of changes of kaolinite crystalline phases in raw coal gangue occurred with the increase of temperature under hydrothermal conditions（150³75°C）.The main crystalline phases were hydroxyl-carbonite after 300°C.The yield of product silica increased sharply with the increase of activation temperature,and the increase of yield was small when the activation temperature was above 300°C.（3）Using NaOH as activator during the process of hydrothermal activation of the raw coal gangue,the effect of particle size of coal gangue（0.75-0.20 mm）on the yield of silica was investigated.With the increase of particle size,the yield of silica increased first and then decreased,indicating that there was an optimum particle size of coal gangue under experimental conditions.（4）With the increase of modifier concentration（0.08-0.24 mol/L）,reaction time（10-120 min）and reaction temperature（25-85°C）,the activation degree and lipophilicity of modified silica increased gradually,but the growth trend was slower in the later stage.Therefore,the optimum modification conditions were as follows:the concentration of MTS was 0.20 mol/L,the reaction time was 90 min and the reaction temperature was 70°C.（5）The modified silica was still amorphous.Compared with that before modification,the specific surface area,pore volume and average pore size of the modified silica decreased.The hydroxyl groups on the surface of modified silica can be reduced by 88.3%⁹7.2%,and its hydrophobicity was enhanced.