Study On Alkali Pre-extraction Of Bagasse Hemicellulose And Its Effect On Subsequent Pulping Performance

The papermaking material bagasse contains about 30% hemicellulose,nearly 70%hemicellulose is dissolved in the black liquor after cooking in the alkaline pulping process.This part of hemicellulose will increase the viscosity of black liquor and raise the energy consumption and water consumption of black liquor extraction.It is relatively easy for the black liquor to scale the evaporator in the evaporation stage.When the black liquor is concentrated,its solid content is not high,and it contains a lot of hemicellulose,and the combustion heat value of the hemicellulose is only half of that of the lignin.This process reduces the thermal efficiency of the alkali recovery furnace;in addition,the dissolved hemicellulose degrades into isosaccharide acid during high temperature cooking and increases the consumption of alkali for cooking.The process of extraction and purification of hemicellulose before pulping was investigated in this study.Before alkaline pulping,extraction of hemicellulose from alkali solution at room temperature.When the alkali concentration was 40 ~ 200g/L,the liquor to bagasse ratio was8:1,and the extraction time was 1 h,it was found that the extraction rate of hemicellulose increased as alkali concentration elevated.When the alkali concentration was in 100g/L,the extraction rate of hemicellulose reached the maximum and then started to slow down.During extraction,the consumption of alkali increased with hemicellulose extraction.Under the condition that the total alkali(preextraction+alkali amount for cooking)charge remained constant,the cooking yield decreased slightly and the hemicellulose content in the pulp decreased a little compared with the traditional cooking method(one-time).After alkali concentration of 80-100g/L extraction,the cooking yield decreased obviously,and the content of hemicellulose in the pulp was very low.The delignification efficiency of cooking was improved after alkali concentration of 60g/L extraction.When alkali concentration of80-100g/L was used,the efficiency of cooking and delignification decreased because of insufficient alkali for cooking.Therefore,in the stage of pre-extraction,it is not recommended to select a high concentration(more than 60g/L)of alkali.Hemicellulose pre-extraction consumed a portion of the alkali.The extracted bagasse for subsequent cooking can reduce the alkali charge.Compared with traditional black liquor,the viscosity of bagasse black liquor extracted by 60g/L decreased from 16.91 cP to 13.56 cP,the solid content also decreased from 118.89g/L to 79.05g/L,and the calorific value increased from 12.91MJ/Kg to 13.62 MJ/Kg.Although the initial freeness of bagasse pulp extracted by60g/L alkali was higher than that of traditional pulp,and the last freeness of pre-extractedpulp was lower than that of traditional pulps at the same PFI revolutions.This finding indicated that it was easy to beat the pulp by alkali extraction.The tensile index and burst index of traditional bagasse pulp were higher than that of alkali extraction bagasse pulp,while the tear index was slightly higher than traditional bagasse pulp.Hemicellulose was separated and characterized by ultrafiltration and ethanol precipitation of extraction solution extracted from bagasse with concentrated 60g/L base at the room temperature.At the room temperature,two kinds of polysulfone membranes with relative molecular weight of 3000 Da and 1000 Da were used for ultrafiltration.The experimental results show that the ultrafiltration velocity increases with flow rate and pressure;however the effect of pressure is greater.After ultrafiltration,the organic matter content in the retention solution was about 3.5 times higher than that in the original solution,and the recovery rate of hemicellulose was about 64%.The samples of hemicellulose were tested by ion chromatography,UV spectrum and gel chromatography.The results showed that the hemicellulose extracted by normal temperature alkaline solution was polymerized.The weight average molecular weight of the sample is 45300.The sample contains about 3.7% lignin.The main component is xylan,reaching 91.39%.The recovery rate of ethanol is 74.2% and 87.6%respectively in vacuum distillation and atmospheric distillation.The former is about 7 times faster than the latter.The feasibility of alkali pre-extraction of hemicellulose before pulping was explored.The utilization efficiency of plant fiber resources in China was studied by combining pulping and papermaking with biomass refining.It is of practical of long-term significance to take environment production and economic benefits into consideration for both the paper industry and forest industry.