Auto-causticizing Alkali Recovery For Non-wood Black Liquor And The Mechanism Of Silica Removal Via Combustion

Silica problem is one bottleneck in non-wood pulp and papermaking mill.As a result,it is difficult for the comprehensive utilization and recycle for the the white mud drived from the allali recovery.In this study,a facile approach for the silica removal by the addition of MgO and Mg SO4 in black liquor via the combustion process was demonstrated.Meanwhile,sodium tetraborate was added into the black liquor for the auto-causticizing.According to this approach,it can be completed for the auto-causticing reactions and the silica can be mostly removed simultaneously.(1)The VIE value ananlysisVolumetric Isothermal Expansivity(VIE)is one of the important parameters for the combustion of the black liquor.In the study,we focued on the analysis of the VIE by response surface methodology(RSM).The results showed that four factors including combustion temperature,combustion time,the concentration of of black liquor,and the weight of black liquor(presented by the significace)were closely related to VIE value.The optimized parameters are as follows: combustion temperature and time are 360? and 40 min seperately,the concentration of the blacak liquor is 55 wt.%,and the weight of black liquor is 5g.(2)Study on the dynamic viscoelasticity of the black liquorThe dynamic viscoelasticity of bamboo kraft pulping black liquor under a high temperatures of 98 °C and solid concentrations(60 to 80 wt.%)was studied.Rotational rheometer analysis data indicated that the viscoelastic motion law with a high solid concentration in black liquor was in accordance with the Kelvin model,and black liquor with a medium concentration conformed to the Maxwell model.As a result,the temperature and solid concentration greatly affected the dynamic viscoelasticity of bamboo kraft pulping black liquor.For instance,the modulus increased as the solids concentration increased,indicating that the viscous component of black liquor was prominent.For this reason,it was easier to soften the bamboo black liquor,providing a favorable condition for the pure viscosity of black liquor.Steady shear thinning was apparent during the frequency shear process.The dynamic viscosity,storage viscosity,and complex viscosity of the black liquor decreased as the shear frequency increased.The quantitative relationship between dynamic viscosity and angular frequency can be described using the Power Law model.(3)ICP-AES method for the determination of inorganic elements inblack liquorA facile and reliable approach for the determination of major inorganic elements including Si,Al,Ca,Mg and B in bamboo kraft black liquor by using inductively coupled plasma atomic emission spectrometry(ICP-AES)with microwave digestion was demonstrated.The results showed that black liquor samples pretreated under the digestion system with the digestion solution HNO3 and H2O2(volume ratio 10:4,HF was added when determing silica)resulted in the conversion of silicides into reactive silica.The resulant samples can be suitable for the silica analysis by ICP-AES.The ICP-AES results showed that the silica element can be readily determined with a high accuracy(recoveries was as as high as 98% to 103%).This approach would enable a rapid analysis of the total silica content in non-wood lignocellulosic waste liquid,thus paving the method for a comprehensive utilization of the spent liquid resource in a more efficient way.(4)Silica removal by MgO and MgSO4MgO and MgSO4 were added into the black liquor in order to remove silica during the combustion process.The results showed that,MgO and MgSO4 can both reduce the silica content,but a better result can be obtained when using MgSO4.The silica removal rate can be 33% when MgO was added,and the rate can be 82% when the dosage of MgSO4 is 2.0%.The product derived form the combustion process using Mg O as the silica removal agent is the substance like Mg2 Si,while the product is probably Mg2Al4Si5O18 using MgSO4 as the silica removal agent.The size of this precipitate is about 21.20?m,and the surface area is 47.30 m2/g.The TG-FT-IR results showed that,maximum weight loss can be obtained in the temperature of 210? and 840?.The products derived from this process are CO2?CH4?H2O?aldehyde ketone compounds,alcohols,and phenol compounds.(5)Auto-causticizing and silica removal simultaneously.The auto-causticizing effects of the sodium tetraborate and the synergistic effect of MgSO4 and sodium tetraborate were studied.It was found that,the auto-causticizing rate can be 65% with a 15% dosage of sodium tetraborate.In this condition,the ratio of B/Na is 0.2.Moreover,the synergistic effect between sodium tetraborate and MgSO4 can be observed during the combustiong process.The product derived from the process contains O?Mg?Al?Si?B?Ca,which possible structural formula are Ca(Si2O5),CaB2(SiO4),and Ca2B2O5 etc.