Study On The Preparation Of Bulk SiO₂ Aerogels From Coal Gangue And Their Properties

Coal gangue is a solid waste produced during coal mining.At present,China has accumulated about 4 billion tons of coal gangue.The massive accumulation of coal gangue has caused many environmental problems:the waste of land resources,the pollution of groundwater,the pollution of the environment,and the geological disasters caused by landslides and collapses of coal gangue bodies.Various problems have seriously damaged the living environment of human beings.Silica aerogel is a material with extremely high porosity(80%to 99%)and ultra-low density(0.003 to 0.5 g/cm3),and extremely low thermal conductivity(10 to 40 mW/(m·K)),The porous nano-network structure material with a large specific surface area(500 to 1000 m2/g)has good application prospects in aviation,construction,environmental protection and other fields.However,due to the high preparation cost and complicated process flow,the application range is limited,making aerogel not widely used.In this study,coal gangue from Pubai County,Weinan City,Shaanxi Province was used as raw material to prepare fine-alumina and alkali-soluble sodium silicate solution after high-temperature roasting and acid leaching to prepare sodium silicate solution.Atmospheric drying method and sol-gel method were used to prepare low density,high performance and large bulk SiO2 aerogels.X-ray diffraction,scanning electron microscopy and thermogravimetry-differential thermal analysis were used to study the influence of preparation factors on the activation of coal gangue,aluminum extraction and the performance of SiO2 aerogel.The formation mechanism of SiO2 aerogel was also studied.Research indicates:(1)The key factor in the activation of coal gangue is temperature,and the particle size and cooling method have little effect on the activation of coal gangue.The coal gangue has been successfully activated when the roasting temperature reaches 500℃;(2)Coal gangue calcining temperature 550℃,hydrochloric acid leaching time 180min,hydrochloric acid concentration 6 mol/L and acid-leaching liquid ratio of 1/6 when the gangue extraction rate of aluminum reaches the maximum;(3)The concentration of SiO2 was the highest when the concentration of sodium hydroxide was 8 mol/L,the best was the hydrochloric acid gel,and the gel density was the lowest when the concentration of the acid gel was 9.5 mol/L.The acetamide,ethylene glycol,and the hexadecane were used.The silica gel modified and vacuum dried with dimer silica,the density and morphology of SiO2 aerogel were the best.(4)The best preparation method of SiO2 aerogel:calcined gangue powder with particle size of 200 mesh was calcined at 550℃ for 2 h,2 g of which was acid leached with 6 mol/L hydrochloric acid and acid leaching time was 180 min.The acid leaching residue was dissolved with 6 mol/L sodium hydroxide,the alkali dissolution time was 4 h,2 ml was taken,and ethyl orthosilicate(0.5 mL)was added.Ethylene glycol(a drop of about 0.1 mL)was used.)and acetamide(amount 0.15 g),controlled pH 7～8,trimethyl chlorosilane and hexamethyldisiloxane(volume ratio 1:5)for substitution and modification,surface modification and solvent replacement The time was 24 h and 72 h,respectively.Vacuum drying(70～,24 h)resulted in a bulk SiO2 xerogel.(5)The bulk SiO2 aerogel was prepared under the optimal preparation conditions,with an average density of 0.09～0.24 g/cm3,a porosity of 90%～98%,a high hydrophobicity,a contact angle of 147.6°,and SiO2.The 5 h adsorption rate of aerogel to oil(eating oil or xylene)is about 500%;the optimal thermal conductivity is 0.02480163 W/m·K,which meets the requirements for super-insulating materials.