Study On Silica Removal Mechanism Of Low-temperature Pre-causticizing In Green Liquor From Straw Pulping

Owing to the shortage of wood resources,non-wood materials such as straw is widely used for pulping and papermaking.However,non-wood fiber bring a series of problems in the process of pulping and papermaking because of its high silica content,which was known as “silica influence”.Mainly as follows: The viscosity of Na₂SiO₃ increases during the evaporation process of black liquor,which makes evaporation of black liquor difficult and increases energy consumption.Na₂SiO₃ in green liquor reacts with Ca（OH）2 to form CaSiO₃ during causticizing section,leading to the lime mud difficult to wash and containing high alkali.CaSiO₃ needs high temperature to decompose and transform a glass-like compound to enfold calcium carbonate when the lime mud is calcined,resulting in insufficient calcination of lime mud and making quicklime cannot be recycled for causticizing process.In summary,there is still no effective approach to solve the problem caused by “silica influence”.The kinetics of causticizing reaction in green liquor from straw pulping was studied.Low-temperature pre-causticizing was proposed to remove silica from green liquor,so as to provide theoretical guidance for the reuse of lime mud.For the first above purpose,the effect of causticizing temperature on the concentration of CO₃^2-and SiO₃^2-（as SiO2）in green liquor was studied.It was found that causticizing temperature had no significant effect on the reaction rate between Na₂SiO₃ and Ca（OH）2,but it affected the reaction rate between Na₂CO₃ and Ca（OH）2 obviously.And the reaction kinetics of Ca（OH）2 with Na₂CO₃,Ca（OH）2 with Na₂SiO₃ were studied.The results showed that the reaction between Ca（OH）2 with Na₂CO₃,Ca（OH）2 with Na₂SiO₃ were both second order reaction,and the activation energy of the reaction between Na₂SiO₃ and Ca（OH）2（2.30 KJ/mol）was smaller than that between Na₂CO₃ and Ca（OH）2（4.10 KJ/mol）.Afterwards,removing silica from green liquor by low-temperature（20℃）pre-causticizing and compared with other temperatures were studied.It was indicated that pre-causticizing at high-temperature had negative effect on silica removal and increased the loss of Na₂CO₃ in green liquor.Low-temperature was beneficial for silica removal and retaining of CO₃^2-.89.27% of desilication rate and 10.26% of causticizing efficiency were achieved when the silica removal process were under the following conditions: the quicklime added amount of 20%,temperature of 20℃,stirring rate of 400 rpm,reaction time of 60 min.Subsequently,after removing silica from green liquor with pre-causticizing,Ca（OH）2 reacted with Na₂CO₃ from green liquor to form causticizing calcium carbonate（CCC）.And the particle size distribution,specific surface area,crystal morphology,infrared spectrum and morphology of CCC were analyzed.The results showed that with increasing of pre-causticizing quicklime,the PSD curve of CCC became smooth and the CCC particles were uniform.When the pre-causticizing quicklime increased from 0% to 30%,the specific surface area of CCC decreased from 9.98 m²/g to 2.41 m²/g,decreased nearly by 76%.According to XRD results,it can be seen that no matter how much the pre-causticizing quicklime added,the crystals of CCC were both calcite and aragonite.CaSiO₃ formed as amorphous in CCC.CCC formed from pre-causticizing at 20℃ had lower silica content than that at 90℃ according to FTIR and SEM results.The morphology of CCC changed gradually from amorphous to pillar-like particles with increasing of pre-causticizing quicklime.Eventually,we reported the properties of CCC with different pre-causticizing temperature and added amount of pre-causticizing,as well as its impact on paper quality when used as filler.The results showed that the increase of pre-causticizing quicklime addition and the decrease of pre-causticizing temperature were beneficial to increase the whiteness of CCC and decrease the sedimentation volume and oil absorption of CCC.When CCC used as filler,addition of pre-causticizing quicklime and pre-causticizing temperature had no significant effect on optical properties and tensile strength of paper,while affected sizing effect obviously.When the pre-causticizing quicklime increased from 0% to 20%（pre-causticizing temperature of 20℃）,the Cobb value of paper decreased from 152 g/m² to 30 g/m².