Characteristics Of Silica Producing And High-temperature Drying Of Pretreated Rice Husk

Rice husk contains abundant silica（SiO₂）.Pretreatment of rice husk by soaking in water or acid solution before combustion can obtain rice husk ash with high SiO₂ purity and high specific surface area.After pretreatment,rice husk has a high moisture content（>60%）,which is usually reduced to less than 13%by hot air drying with high energy consumption before burning.The direct smoldering of pretreated wet rice husk can not only save energy consumption in the drying process,but also produce SiO₂with good quality,which has a good application prospect.However,there are few relevant researches at present.Centrifugal dewatering is an efficient and economical dewatering technology.Centrifugal dewatering of pretreated rice husk before smoldering is benefitcial to recover the heat generated from the smoldering of rice husk.The drying process is very important in smoldering,and the drying process is the key to achieving self-sustaining smoldering.However,there is a lack of separate studies on the drying process in smoldering.In response to the above problems,the following aspects of work have been carried out in this paper.（1）Accumulation smoldering experiments were carried out for untreated,water-washed and acid-washed rice husks in a muffle furnace at different pile thicknesses and temperatures,and the properties of the prepared rice husk ash were tested.The results showed that pretreatment promoted the accumulation smoldering of rice husk,reduced the residual char content in the ash and increased the SiO₂ purity and specific surface area of the ash.Under the conditions of 550°C and 8 cm pile thickness,the residual char content of untreated rice husk accumulation smoldering ash was 8.3%,the SiO₂ purity was 89.05%and the specific surface area was 99.45 m²/g.The residual carbon content in the ash after accumulation smoldering of water-washed and acid-washed rice husk was reduced to 0.5%and 0.6%,respectively,while the SiO₂ purity was increased to 94.77%and 98.10%,respectively,and the specific surface area was increased to 165.00 m²/g and 278.78 m²/g,respectively.（2）Centrifugal dewatering experiments were carried out in a centrifuge with washed rice husks.The effects of centrifuge speed and dewatering time on the effect of centrifugal dewatering of rice husks were compared,and the energy consumption of the centrifugal dewatering process was calculated.The results showed that:when the speed is higher than400 r/min,the wet basis moisture content of rice husk can be reduced from 60%to about 40%by centrifugal dewatering for 30 min;When the speed is higher than 1600 r/min,increasing the speed enhances the dewatering effect less;In order to ensure the economy of centrifugal dewatering,the dewatering time should not exceed 5 min.It is economical to use centrifugal dewatering to reduce the moisture content of the rice husk before smoldering.（3）Thin layer high-temperature drying experiments were carried out on rice husk to compare the effects of moisture content and temperature during the thin layer high-temperature drying process of rice husk.By fitting the experimental results to common thin layer drying models,the optimized thin layer high-temperature drying model was determined,and the drying parameters and activation energy under different conditions were calculated.The results show that Logarithmic model is the optimal model for the thin layer high-temperature drying process of rice husk;under the experimental conditions,the moisture diffusion coefficient of rice husk ranges from 4.34×10^-9 to 1.14×10^-8 m²/min,which increases with the increase of temperature;the activation energy of drying ranges from 15.54 to 19.84 k J/mol,which decreases with the increase of moisture content.（4）A deep-bed high-temperature drying experiment was conducted on rice husk to compare the effect of temperature during the drying process.At the same time,the quality changes and temperature changes during the experiment were investigated.It was found that the maximum mass loss rate appeared earlier during the drying process as the drying temperature increased;when the drying temperature was higher than 125℃,along with the drying process,the temperature in the deep-bed near the external heat source increased rapidly,decreased slowly,decreased rapidly,and increased again;when the drying temperature was higher than 175℃,some rice husks occurred pyrolysis during the drying experiment.