The Basic Study On Preparation Of Hydrated Silica From Construction Waste As Silicon Source

With the rapid development of cities and the renewal of buildings,the emission of construction waste continues to grow.Huge amounts of construction waste not only consume land resources,but also cause environmental pollution.Recovery of silica from construction waste is of great importance for construction of an environment-friendly society as the construction wastes contain a lot of silica.This paper focused on the recycling of silicon in construction waste.A method for preparing hydrated silica was proposed.Silicon tetrafluoride was first produced by leaching silica from construction wastes with fluorosilicic acid and hydrochloric acid.Then it was hydrolyzed or neutralized.The reaction mechanism,thermodynamics,kinetics,and process conditions of the acid leaching process and the hydrolysis and neutralization process of silicon tetrafluoride were analyzed and explained.The hydrated silica products were characterized and analyzed.It provides theoretical and data references for the process of recycling hydrated silica from construction wastes.First,the process of leaching waste clay bricks with fluorosilicic acid was studied,and the effect of process conditions on the silica leaching rate was investigated.The results showed that the silica leaching rate could reach 74.1%in this case,which the concentration of fluorosilicic acid was 3.05 mol/L,the liquid-solid ratio was 9:1,the reaction temperature was 130?,and the reaction time was 2.5 h.The shrinking core model was used to analyze the kinetics of leaching of waste clay brick by fluorosilicic acid.The leaching process was controlled by chemical reaction with the apparent activation energy of 42.3 kJ/mol.The acid leaching residue mainly contained crystalline silica.Secondly,the process of waste concrete leaching with hydrochloric acid and fluosilicic acid was discussed.The decomposition rate of waste concrete could reach 28.04%when the concentration of hydrochloric acid was 2mol/L,the liquid-solid ratio was 4:1,the reaction temperature was 20?,and the reaction time was 10 minutes.The silica leaching rate could reach 79.16%when the concentration of fluorosilicic acid was 3.05 mol/L,the liquid-solid ratio was 9:1,the reaction temperature was 130?,and the reaction time was 2.5 h.The results of kinetic analysis showed that the process is mainly controlled by chemical reaction,and the apparent activation energy of the reaction is 28.4 kJ/mol.The main components of hydrochloric acid leaching residue were quartz,albite and glass,and the primary solid phase of fluorosilicic acid leaching residue was quartziferous silica.Subsequently,the preparation mechanism and silicic acid polymerization process of hydrated silica prepared by silicon tetrafluoride were studied.The effects of pH value,external fluoride on silica gel time and silicic acid polymerization rate were discussed,respectively.The effect of pH value on the silica gel time was most obvious in these factors.The silica gel time was prolonged with the increase of pH value,when the pH value was lower than 2.Then,the silica gel time decreased with pH value increased from 2 to 8.However,the gel time increased greatly with increasing pH value,when the pH value was higher than 8.The isoelectric point of the silicic acid colloidal particles was close to 2,and the maximum silica gel time appears at the isoelectric point.The addition of fluoride salt could greatly shorten the silica gel time and cause the isoelectric point move to a higher pH value.The effects of process conditions on the properties of hydrated silica prepared by hydrolysis method were investigated,and the appropriate reaction conditions were chosen.The hydrated silica products prepared without aging,replacing with absolute alcohol and drying at 300? for 2 h,its specific surface area could reach 348.73 m2/g,with the pore diameter of 16.79 nm,the DBP oil absorption value of 2.3 ml/g.The silica purity was 92.4%,which mainly composed of amorphous silica,and the particle size was 1.48?m.The product conformed to the Class A standard for precipitated hydrated silica products in the national standard.The influences of process conditions on the properties of hydrated silica prepared by neutralization method were discussed.The specific surface area of hydrated silica product prepared by the neutralization method could reach 269.77 m2/g,with the reaction ending pH value at 8,aging at 20? for 0.5 h,replacing with absolute alcohol for 2 h,drying at 350? for 2 h.The silica product was mainly composed of amorphous silica with purity of 93.1%,DBP oil absorption value of 2.21 ml/g,and particle size of 955 nm,which satisfied the Class A standard for precipitated hydrated silica products in the national standard.