Study On The Structural Changes Of Lignin In The Process Of Component Separation And The Impact On Pollution Load

As an important part of the national economy,the paper industry has created many environmental problems while making a great contribution to the national GDP.Therefore,it is of great significance to study the pollution mechanism during pulping delignification.Lignin is the substance that needs to be separated from cellulose during the component separation process,and it is often dissolved in waste liquid and is one of the main sources of pollution.In this study,lignin,an important component of lignocellulosic materials,was used as the research object,and eucalyptus was used as the raw material to separate and remove lignin by the traditional soda method and the 1,4 butanediol organic solvent method.By setting different delignification durations to stage the delignification process,the structural changes of lignin in waste liquor under different delignification levels were studied,and the influence of the lignin structure changes on the waste liquor pollution was explored.The research results provide basic data for the upgrading of cleaner production technologies in the pulp and paper industry.Firstly,the properties of pulp and black liquor obtained at different delignification levels in the traditional soda delignification process were studied.The results show that the Kappa number of soda pulp decreased from 27.93 to 9.33 with the prolongation of delignification time?1h,2h,3h,4h and 5h?,and the concentrations of lignin in black liquor were 23 g/L,24 g/L,41g/L,54 g/L and 51 g/L respectively,the lignin removal rate of the raw material increased significantly while the pulp yield decreased.Secondly,the chemical structure of the 5 alkali lignin was studied by element analysis,thermogravimetry?TG?,gel chromatography?GPC?,nuclear magnetic resonance?NMR?and other instruments.The results show that the structure of alkali lignin obtained with different degrees of delignification has changed a lot.Elemental analysis results show that as the degree of delignification deepens,the C element content of the purified lignin samples in the black liquor continues to increase while the O element content shows a downward trend,which indicates that a large number of oxygen-containing groups have been removed,while chemical bonds such as ether bonds are broken.The results of GPC,TG and 2D NMR spectroscopy showed that the lignin removed from the raw materials had both degradation and condensation reactions in the waste liquid.In the early stage of delignification,the degraded fragments undergo a condensation after the?-O-4',?-O-4',?-?'and?-5'bonds of lignin are broken,and several typical condensation structure,such as biphenyl sub-structure,diaryl ether,vinyl aryl ether,and diaryl methylene structure were formed,so the molecular weight distribution of alkali lignin is relatively wide,ranging from 0 Da to 30,000 Da.The alkali lignin in the waste liquid will be further degraded into small molecular substances under the strong alkali effect and high temperature conditions.The molecular weight of lignin is mainly in the range of 0 Da to 15000Da.At this stage,the degradation of lignin is the main reaction.At the same time,the result of the pollution load shows that the COD_cr?mg/g?produced by unit mass of alkali lignin are1354.95 mg/g,1360.73 mg/g,1423.56 mg/g,1157.90 mg/g and 1495.88 mg/g,respectively,it presents a trend of earlier increase and later decrease,which is same as the trend of the contribution rate of alkali lignin components to the total COD_cr of waste liquid.Data analysis results show that the Mw,O element content,?-O-4'bond content and lignin S/G of alkali lignin have a greater influence on the COD_cr produced by unit mass lignin,and the relationship is:COD_cr produced by unit mass of alkali lignin=0.05 Mw?Da?-45.92 O?%?-10.98?-O-4'?%?+1500.88 S/G+448.88.The error between the calculated value and the measured value of the formula is less than 0.7%,which provides theoretical guidance for pollution control.The lignin in the raw materials is removed by the organic solvent pulping method,which is the typical environmentally friendly delignification technology.In the experiment,1,4butanediol was used as a solvent to study the changes of lignin structure and the environmental pollution load of the delignification process,also it was compared with the soda method.It was found that compared with the traditional soda pulp,the solvent pulp has a higher Kappa number,and the degree of delignification is much lower than that of the soda method.This is due to the strong alkali effect during the delignification process of the alkaline method,the lignin?-O-4',?-O-4',?-?'and?-5'bonds can be broken The element is degraded into small molecules and easily dissolved.However,in the process of solvent delignification,the?-O-4'bond is easier to break,but the lignin structure connected by?-?'and?-5'is relatively stable,and it is not easy to be 1,4 butanediol solvent damage.In addition,with the extension of the holding time,the pulp yield is higher than that of the alkali method,indicating that compared with the alkali method,on the basis of ensuring a relatively high pulp yield,the cooking time can be appropriately shortened and energy consumption can be reduced.The structure change of lignin in waste liquid is relatively small,and the thermal performance of solvent lignin is obviously better than that of alkali lignin,and its molecular weight distribution is more uniform.The structure change of lignin in waste liquid is relatively small,and the thermal performance of solvent method lignin is obviously better than that of alkali lignin,and its molecular weight distribution is more uniform.³¹P-NMR results show that there is no obvious condensation reaction of lignin in the waste liquid of 1,4 butanediol solvent method.It provides a theoretical basis for the subsequent separation and high-value utilization of lignin in waste liquid.The pollution load results show that the contribution of 1,4 butanediol lignin to the total COD_cr of waste liquid is small,and the pollution value generated by 1,4butanediol lignin is much smaller than that of alkali lignin.The CODcr?mg/g?produced by unit mass of 1,4 butanediol lignin is:1602.27 mg/g,1739.82 mg/g,1740.41 mg/g,1757.39 mg/g and 2187.74 mg/g,which is 1.1 to 1.5 times to alkali lignin.The CODcr produced by unit mass of 1,4 butanediol lignin=-1.36 Mw?Da?-901.48 O?%?-2703.78 S/G+2294.11 aliphatic OH?%?+41353,the error of this formula is less than 0.3%,it can be used for theoretical guidance in environmental governance.