Study On Treatment And Utilization Of Flax Alkaline Sulfite Spent Liquor

Alkaline sulfite pulping process has some characteristics including good pulp brightness,good bleachability,high yield,high pulp strength,and wide applicability for many wood species.However,high effluent load of spent liquor produced in the pulping process leads to the difficulty of discharge,which greatly limits the production of pulp.Therefore,in this study,flax alkaline sulfite spent liquor was used as raw material,the combined processes of flocculation,biological and Fenton treatment was used to reduce the effluent load of spent liquor,and the lignin in spent liquor was isolated and further processed into active carbon for high value utilization.First,the effects of different coagulants on the properties of spent liquor were investigated.Through optimization,it was found that#3 PAC performed well on the removal of chemical oxygen demand(COD),color,and lignin from spent liquor.When the dosage of#3 PAC was 7.0 g/L and the pH of spent liquor was 7.0,the COD,color,and lignin removal were 49.7%,95.9%,and 59.4%,respectively.Secondly,the optimal coagulant(#3 PAC)and coagulant aid(PAM)were used together to treat the spent liquor.The effects of conditions of coagulation-flocculation process on the removal of COD,color,and lignin from the spent liquor was investigated.The results showed that,under the optimal conditions(#3 PAC dosage of 5.0 g/L,CPAM dosage of 2.0 mg/L,pH 7.0)of coagulation-flocculation treatment,the highest removal for COD,color,and lignin were achieved,i.e.,49.5%,95.4%,and 54.1%,respectively.Then,the treated spent liquor after coagulation-flocculation process was subjected to subsequential activated sludge process and Fenton treatment.After activated sludge process,the removal of COD,lignin,and UV254 were 50.9%,24.3%,and 16.4%,respectively.Then,after Fenton treatment,the removal of COD,color,lignin,and UV254 of the spent liquor were 54.7%,86.6%,67.2%,and 79.8%were achieved,respectively.Finally,the separation,characterization,and application of lignin were carried out:(1)the lignin was isolated and characterized from flax alkaline sulfite spent liquor by two methods including acidification and ultrafiltration.The results showed that the typical aromatic ring structure and functional groups of lignin were found in both lignin samples recovered by acid precipitation and ultrafiltration;lignin separated by acid precipitation had better thermal stability;(2)lignin-based activated carbon was prepared by potassium hydroxide(KOH)activation from lignin separated by acid precipitation,and the properties of lignin-based activated carbon were characterized.The results showed that lignin-based activated carbon was highly disordered,had hierarchical pore structures with the coexisting of micropores and mesopores,and had a large number of microporous structures.The BET surface area,total pore volume,and micropore volume of lignin-based activated carbon by KOH activation were 1881.12 m2/g,0.806 cm3/g,and 0.692 cm3/g,respectively.It is expected that lignin-based activated carbon material would have potential application in adsorption of heavy metal ions from wastewater.