| A large number of pre-hydrolysate and black liquor are produced in the production process of pre-hydrolysis kraft-based dissolving pulp,which contain a large number of hemicellulose and lignin which can be used to produce a variety of high value-added products.However,the existence of lignin will limit the further utilization of hemicellulose,which needs to be separated and purified.Therefore,it is of great significance to explore the efficient,simple and low-cost separation and purification technology for the conversion of traditional pulp and paper mill to biomass refining platform.In this paper,Eucalyptus wood was used as raw material.Firstly,PAC(polyaluminium chloride)flocculation,activated carbon adsorption and ion-exchange were used to study the removal of lignin from the pre-hydrolysate.The changes of hemicellulose and lignin content with P factor(pre-hydrolysis strength),their changes before and after each treatment step were also discussed.The molecular weight and structure of lignin were characterized.Secondly,the extraction of hemicellulose from pre-hydrolysis liquor by ethanol precipitation method was studied.The effects of P factor,volume ratio of pre-hydrolysis liquor to ethanol,concentration coefficient,pH value and PEO concentration on the precipitation and purity of hemicellulose were investigated.On the basis of the research on the pre-hydrolysate of the research group,the joint treatment of the pre-hydrolysate and the black liquor was studied in this paper.By mixing the black liquor with the pre-hydrolysate,and after acidification,PAC flocculation and activated carbon adsorption,the preliminary separation of hemicellulose,acid soluble lignin and acid insoluble lignin was realized.The changes of acid-soluble lignin and acid-insoluble lignin in the process of treatment and the effect of lignin removal on the adsorption of activated carbon were discussed.The molecular weight and structure of all levels of lignin in the mixed liquor were characterized.The surface observation and specific surface area measurement of activated carbon were carried out.These experimental results provide a reference for the industrial application of pulp and paper mills.The results showed that the content of hemicellulose and lignin in the prehydrolysate increased first and then decreased with the increase of P factor.The content of total glycan reached the highest 16.8 g/L when p factor was 500,and the content of total sugar reached the highest 22.1 g/L when P factor was 700.The content of lignin first increased gradually and reached the highest value at P factor 900 and then decreased slightly.For the P-factor-500 pre hydrolysate,the content of lignin decreased from 6.45 to 5.68 g/L after flocculation treatment with 30 mg/L PAC,to 2.2 g/L after adsorption treatment with 10 g/L activated carbon,and to 0.3 g/L after adsorption treatment with ion exchange resin.PAC flocculation had no significant effect on the hemicellulose content.The hemicellulose content before and after activated carbon adsorption reduced from 22.10 to 18.95g/L,loss rate was 14 %.The content of hemicellulose before and after the ion exchange resin adsorption treatment reduced from 18.95 to15.60 g/L,loss rate was 15.1 %.Gel permeation chromatography showed that there was no significant difference in the molecular weight of lignin in the pre hydrolysate(the lignin of natrual precipitates,PAC removed,activated carbon adsorbed and adsorbed by ion exchange resin).The results of FTIR show that the main structure units of lignin in the pre-hydrolysis liquor are syringyl and guaiacyl,without phydroxyphenyl.The lignin obtained by natural precipitation and PAC flocculation does not contain alcohol hydroxyl,phenol hydroxyl and alkyl-alkyl ether bond.The structure units of lignin adsorbed by ion exchange resin are guaiacyl and have no alkyl-alkyl ether bond connection.The results show that when the pre-hydrolysis strength is P factor 200,the prehydrolysis liquor is the most advantageous to extract hemicellulose with ethanol precipitation.Increasing the volume ratio and concentration coefficient can significantly increase the precipitation of hemicellulose,and the optimal volume ratio and concentration coefficient are 1:10 and 90%,respectively.Acidification and PEO flocculation can significantly improve the hemicellulose products.The optimal pH value and PEO concentration are pH 2.0 and 400 mg / L,respectively.At this time,the hemicellulose purity can reach 85.5% and the precipitation amount is 5.0g/L.The pre hydrolysate is acidic,the mixed treatment of the pre-hydrolysate and the black liquor(volume ratio of 1:1)can play an acid-base neutralization role and reduce the acid consumption when acidifying the mixed liquor.The experimental results show that 95.4% of acid insoluble lignin(from 23.8 to 1.1 g / L)and 7% of acid soluble lignin can be removed by acidifying the mixed liquor to pH 6.0 and adding PAC for flocculation(the concentration of PAC in the mixed liquor is 300 mg/L).Compared with the method of acidification to reduce the pH to 3.0,the method of acidification combined with PAC flocculation reduced the dosage of 50% sulfuric acid from 45.8ml/l to 15.3 ml/l,saved 67% sulfuric acid.Furthermore,70.2% of acid-soluble lignin can be removed by the adsorption of activated carbon,but more crude hemicellulose was lost.It was found that the presence of acid-insoluble lignin will seriously weaken the adsorption capacity of activated carbon,especially when the content of acid insoluble lignin in the mixed liquor is higher than 1.5 g/L.The results of environmental scanning electron microscopy(SEM)and specific surface area measurement showed that when the content of acid-insoluble lignin in the mixed liquor was high,there was serious adhesion on the surface of activated carbon and the specific surface area decreased significantly.The experimental results provide an important reference for the study of acid-soluble lignin removal by activated carbon adsorption. |
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