Study On Kraft Pulping And Lignin Extraction Of Eucalyptus

A novel two-stage kraft cooking process was developed which benefited lignin extraction from black liquor.In the first stage,acidic substance produced from wood chips during cooking made the pH of black liquor reduce that was benefical to the extraction of lignin.In the second stage,the cooking conditions were modified to ensure the pulp yield and kappa number.Extraction of lignin from black liquor was studied by focused-beam reflectance measurement(FBRM)and UV spectrophotometer.Eucalyptus grandis urophylla was used to optimize cooking parameters.The two-stage cooking process which benefited lignin extraction was obtained by optimizing the cooking conditions.The optimum cooking conditions were determined as follows:the first stage added 66%of black liquor(based on total liquor),the heating up time was 30 min,the time at maximum temperature was 30 min,the maximum temperature was 130?140?,liquor to wood ratio was 5:1;In the second stage,the alkali charge was 20%(as NaOH),sulfidity was 35%,and the heating up time was 90 min,the time at maximum temperature was 90 min.the maximum temperature was 160?,liquor to wood ratio was 5:1.The two-stage cooking process for eucalyptus grandis urophylla could be operated stably by optimizing the parameters under laboratory conditions.The result showed that the pH of black liquor from the first stage colud be reduced to about 10.5 which was in favor of lignin extraction by acid.Compared with traditional kraft cooking,the alkali charge of the two-stage process decreased by 2%(on wood)and the kappa number decreased by 1.4,while the pulp yield was similar.The optium pH of lignin extracted from the first black liquor was 9.0.The consumption of sulfuric acid is about 5.6kg/t.Compared with the black liquor from traditional kraft cooking,the sulfuric acid consumpyion decreased by 61%.UV absorbance decreased by about 25%after centrifugation,solids can be obtained by centrifugation about 30kg/t.UV absorbance decreased by about 27.6%after membrane separation.Experiment with eucalyptus globulus shows that the pH of black liquor from the first stage colud be reduced to about 10.0 by raising the first stage maximum temperature to 140?.Compared with traditional kraft cooking,the pulp yield(51.1%)was decreased by 0.7%,the kappa number(15.4)decreased by 2.1 and the alkali charge decreased by 4%(on wood).It takes sulfuric acid about 3.8kg/t to adjust pH to 9.0.Compared with the black liquor from traditional kraft cooking,the sulfuric acid consumpition decreased by 74%,UV absorbance decreased by about 23%after centrifugation,solids can be obtained by centrifugation about 30kg/t.UV absorbance decreased by about 27%after membrane separation.The pH of black liquor from the first stage colud be reduced to about 9.0 by reducing the amount of black liquor and extending the holding time.Compared with traditional kraft cooking,the kappa number increased by 1.1 and the time at maximum temperature in second stage decreased by 10 min while the pulp yield was similar.Lignin can be separated directly by centrifugation.UV absorbance decreased by about 12%after centrifugation,solids can be obtained by centrifugation about 7kg/t.UV absorbance decreased by about 15%after membrane separation.The residual alkali of black liquor from the first stage colud be reduced to about 0.25-0.30g/L by reducing the alkali charge in the second stage.Compared with traditional kraft cooking,the kappa number increased by 0.9,the time at maximum temperature increased by 30 min and the alkali charge decreased by 6.5%(on wood).Solids can be obtained by centrifugation about 18.5kg/t.It takes sulfuric acid about 2.5 kg/t to adjust pH to 9.0.The infrared spectra and thermogravimetric properties of lignin extracted from the first stage black liquor under alkaline conditions are similar to those of lignin extracted from traditional black liquor by traditional acidification.