| Biomass is fully utilized can reduce the dependence on non-renewable resources in daily life and industrial production.As a biomass resource with high yield in agricultural production,rice husk is rich with carbon and silicon.The effective separation of carbon resources and silicon resources is an effective way to enhance the added value of rice hull based products.In this paper,amorphous hydrated silica?SiO2·nH2O?was dissolved by the reaction of sodium hydroxide?NaOH?with it.The citric acid pretreatment and carbonization conditions of rice husk in the tube furnace were successively explored.This were to reduce the influence of metal inorganic salts on the thermal pyrolysis of organic compounds in rice husk and increase the dissolution rate of SiO2·nH2O.The influence of reaction factors on the dissolution of SiO2·nH2O and the control mechanism of its digestion process were researched.The rice husk is pretreated with a citric acid solution to dissolve the metal elements.The effects of citric acid solution concentration,temperature and time on the residual content of metal elements were studied by using single factor experiment.The optimal conditions of the pretreatment were concentration of 5%,temperature of 100°C,and time of 300min.The total content of metal elements decreased from 7.446%to 0.768%in rice husk,where the K content decreased from1.423%to 0.033%,Ca lowered from 2.616%to 0.050%,Fe reduced from 2.066%to 0.658%,the other metal elements from 1.341%decreased to 0.027%,but Si increased to 99.232%.Thermogravimetric analysis?TGA?showed that the thermal stability of rice husks pretreated with citric acid was increased,and the maximum pyrolysis rate of organic matter was improved.The effect of carbonization temperature,carbonization time and water vapor dosage on the dissolution of SiO2·nH2O during the carbonization process was investigated.The carbonization conditions were determined by response surface analysis based on the single factor experiments as carbonization temperature 650°C,carbonization time 50 min,and water vapor dosage 67g,the leaching rate about 97.98%.The influence of carbonization factors on the dissolution was carbonization temperature>carbonization temperature>water vapor dosage.And the carbonization time and water vapor dosage have remarkable interaction with the carbonization temperature on the leaching rate of SiO2·nH2O.Scanning electron microscopy?SEM?,infrared spectroscopy?FT-IR?and X-ray diffraction?XRD?Scanning electron microscopy?SEM?,infrared spectroscopy?FT-IR?and X-ray diffraction?XRD?showed that the loss of Si-OH in rice husk ash led to the agglomeration of SiO2·nH2O,but it was still amorphous state.The factors affecting the dissolution of SiO2·nH2O in the reaction of rice husk ash and NaOH solution were investigated.The optimal reaction conditions were determined by single factor experiment:NaOH solution concentration of 3%,reaction time of 180min,reaction temperature of 100°C,liquid-solid ratio of 20mL/g.The SiO2·nH2O dissolution rate in optimal condition was determined as 97.28%.The result by orthogonal test showed that the factors on the dissolution of SiO2·nH2O was liquid-solid ratio>reaction temperature>NaOH solution concentration>reaction time.The dissolution control mechanism of SiO2·nH2O was preliminarily analyze by the shrinkage core model of the liquid-solid phase reaction.In the rapid dissolution phase from 0 to 80 min,it was mainly controlled by the solid film diffusion.But in the slow dissolution stage of 80140min,the chemical reaction control effect is greater than the liquid film and solid film diffusion control. |
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