Study On The Effect Of Pulp Concentration On The Extraction Rate In Black Liquor Extraction

Paper industry is related to the basic raw material industry of national economy and its development is restricted by environmental pollution.In this paper,the influence of pulp concentration on the extraction rate of black liquor was studied by using the countercurrent washing process with red pulp and the black pulp as the raw material,and the lignin source pollution transfer in fiber slurry process flow was studied.The results have important scientific significance and universal realistic value for re-understanding and correctly locating the generation and control of pollution in biomass pulping process.Pollution feature and characteristic pollutants in black liquor of red pine chemical pulping were researched.It was found that black liquor has a complex organic composition with dark colour,and the BOD5 / CODCr was 0.25 which indicted that there were a lot of organics that could not be directly decomposed and utilized by microorganisms in black liquor.And phenolic compouds are main organic pollutants with up to 84.34% relative concentration which is the main source of pollution load.The influence of the concentration of the slurry on the washing process and the extraction rate of the black liquor were studied by three-stage countercurrent washing process.The results showed that the total solid content,the total organic matter content and the total inorganic matter content and the black liquor extraction rate was higer with the increase of slurry concentration,black liquor extraction rate of up to 98% with 35% slurry concentration,and high slurry concentration can have low water consumption and high alkali recovery under the same washing degree.With the increase of the slurry concentration,the organic pollutants left in the slurry was less,the kappa number of pulp was lower after three-stage washing,and the lignin content in bleaching process was less,the reduction of pollutants from the source into the follow-up bleaching and papermaking process,to reduce the bleaching effluent AOX,BOD5,CODCr load and the toxicity of waste water has a positive effect.A UV-visible spectrometry method to determine lignin content in black liquor was established to explore the contribution of lignin to the environmental pollution load.It was found that the lignin concentration in the black liquor was 67.5 g · L-1,the total amount of lignin accounted for 52.91% of the total organic matter with the contribution to CODCr content was 55.59% of the total CODCr content,and 1 kg of lignin produced about 1.43 Kg of CODCr,the results showed that lignin is the main source of pollution load for black liquor in traditional chemical pulping industry.Then the structural characteristics and molecular weight of lignin was studied.The results showed that the lignin only contained guaiacyl,no lilac and p-hydroxyphenyl,with typical character of coniferous wood.During the process of cooking,the molecular weight of the lignin was low as the broken of chemical connection between the lignin molecules,which indicated that the chlorine-substituted lignin was more active and the reaction activity was very strong and these can provide basic data and theoretical support for the behavior research of liquid phase lignin in the bleaching process and its contribution to the toxicity of waste water.