Elements Dissolution Of Coal Fly Ash Activated By The Roasting With Sodium And The Further Preparation Of Aerogel

Coal fly ash,as the ash of coal-burning plant,is a kind of industrial solid waste produced in large quantities in China.The discharge and storage of a large amount of coal fly ash not only encroaches on land and wastes resources,but also cause serious environmental pollution and ecological damage.Coal fly ash contains a large amount of aluminum and silicon resources,which can be used as raw materials for production of aerogels.The preparation of aerogel using coal fly ash can save the production cost of aerogel,help to further expand the new way of comprehensive utilization of coal fly ash and has practical significance for the development of the utilization of coal fly ash resources.In this paper,coal fly ash based aerogels with excellent properties were successfully prepared by using coal fly ash activated by the roasting with sodium as raw materials through acid leaching,impurity removal,sol-gel,surface modification and atmospheric pressure drying.During the acid leaching process,the dissolution of aluminium,silicon and sodium under different acid leaching conditions was investigated,and the selective removal of sodium chloride was achieved by adding ethanol to the acid leaching process,based on which a new method for the preparation of aerogel from coal fly ash was developed.During the sol-gel process,the effect of additives and temperature on the gelation time of aluminium and silanol solutions and the formation of their spatial network structure were studied,and the pore structure development pattern of aluminium and silanol solutions was analysed.In the surface modification process,the influence of different surface modifiers and their environmental factors on the quality of coal fly ash-based aerogel products was studied,and the changes in pore structure and capillary pressure during the surface modification of wet gels and atmospheric pressure drying were analysed.The main research work and conclusions are as follows:（1）The effects of different hydrochloric acid concentrations,temperatures and solid liquid ratio on the dissolution of aluminium,silicon and sodium from coal fly ash activated by the roasting with sodium were investigated,meanwhile,based on the low solubility of sodium chloride in ethanol,the effects of ethanol addition on the dissolution of aluminium,silicon and sodium from coal fly ash activated by the roasting with sodium were investigated,and a method for the simultaneous removal of sodium from coal fly ash activated by the roasting with sodium by hydrochloric acid-ethanol acid leaching was proposed.The results showed that at a hydrochloric acid concentration of 4 mol/L,a temperature of 20℃ and a solid-liquid ratio of 1:7,the reaction was basically complete,and the dissolution rates of aluminium,silicon and sodium reached the highest,which were 88%,84%and 97%respectively.Meanwhile,the addition of ethanol could remove the sodium chloride impurities simultaneously during the acid leaching process,but the excessive addition would also affect the aluminium and silicon dissolution,and when the alcohol/water volume ratio was 1.40,the aluminium and silicon dissolution rates remained at 76%and 84%,while the sodium dissolution rate was below 10%,and it could be used as a precursor solution for the subsequent preparation of aerogel.（2）The effects of different ammonia and ethylene oxide additions and different temperatures on the gelation time of aluminosilicate alcohol solution and their formation of spatial network structures were investigated using aluminosilicate alcohol solution obtained by acid leaching of coal fly ash activated by the roasting with sodium as raw materials.The results showed that,in the sol-gel process,with the addition of ammonia and propylene oxide or the increase of temperature,the mutual binding rate of sol particles increased significantly and the gelation time all showed a decreasing trend,which could reach 7 min,10 min and 18 h respectively in the experimental range.The density of the aerogel tends to decrease and then increase with the addition of ammonia and ethylene oxide and the temperature.The reason for this may be that the rapid rate of binding leads to agglomeration of the gel nuclei and the uneven structure of the gel network generates internal stresses,which in turn makes it more likely that the holes will collapse when drying.（3）The aerogels were prepared from wet gels made from aluminosilicate alcohol solution.The quality of coal fly ash-based aerogel products made with different modifiers（TMCS,HMDSO,DMDES）in the surface modification process were investigated under different conditions of temperature,modifier dosage and modification time,respectively.The results show that all three modifiers can replace Si-OH on the surface of the wet gel with Si-CH₃to avoid excessive shrinkage of capillary pores during atmospheric pressure drying of the wet gel,thus improving the performance of coal fly ash-based aerogel products.The density of the TMCS and HMDSO modified aerogel samples did not vary significantly with temperature and decreased with time and dosage,while the density of the DMDES modified aerogel samples increased with temperature and decreased equally with time and dosage.Comparing the three modifiers,the properties of HMDSO modified aerogel samples were better.The density,the specific surface area,the pore size and the pore volume of the resulting fly ash-based aerogel were 164 mg/cm³,817.95 m²/g,9.17nm and 1.71 cm³/g respectively when the modifier to wet gel volume ratio was 0.6:1 and the gel was modified at 35℃ for 30 h.