Enzymatic Treatment Of DCS In Bleached Deinked Pulp Of Old Newsprint

During the production of newsprint using old newprint as the raw material, the close of white recycling system results in a buildup of dissolved and colloidal substances (DCS) in the whitewater, which influences machine runnability and paper quality. The objective of this study was to investigate the efficiency of enzymatic treatment to remove DCS of deinked pulp (DIP). The characteristics and chemical components of the DCS in the white water prepared from bleached de-inked pulp (BDIP) were determined and three types of enzymes (pectinase, lipase and laccase) were used respectively or matched for DCS treatment to obtain the optimum treatment conditions. In addition, the composition change of DCS after enzymatic treating was identified by GC-MS to study the mechanism of DCS degradation and the effect of enzymatic treatment of DCS on pulp dewatering, sizing degree and strength and brightness of the paper were studied.DCS water prepared from BDIP was characterized with several sorts of chemical substance termed high dissolved substance (DS) , content of which were as high as 74% in total and they served as the main effects in increasing cationic demand (CD) of white water. Various extractives were detected by GC-MS (more than forty types) and most of these were fatty acid and resins. As the consequent of white water recycling, inorganic electrolytes like monnovalent (Na⁺, K⁺) were accumulated and resulted in high concentration, while bi- and trivalent ions (Ca²⁺ , Fe³⁺, Al³⁺) were kept lower, occurring with a decrease appeared in DCS average diameter size and CS content.Pectinase KJ-544010 from Novozymes performed better in DCS treatment than other three pectinases and the optimum pectinase treatment conditions were as follows: 60℃, pH 7.5, pectinase dosage 100 U·L^-1, reaction time 2 h. Treatment of DCS with pectinase could obviously reduce the CD about 50%, and caused a decrease of average DCS particle size and an increase in electrical conductivity at same time. In addition, GC-MS analysis of the extractives in the solution phase after pectinase treatmennt showed more galacturonic acid was released but the concentration of other substances changed little, which indicated that pectinase hydrolyzed pectic substance specifically and resulted in a reduction of CD value and an improvement of retention aid efficiency.Lipase could induce an obvious decrease of DCS turbidity and an increase in CD, and the optimum treatment conditions were pH 7.5, 50℃, lipase dosage 50 U·L^-1 and reaction time 2 h. Degradation of esters was proved by GC-MS analysis of the treated DCS water and the content of glycerol, fatty acid and resin acid were detected in a considerable increase. The degradation of esters in DCS by lipase treatment was useful to the pitch control.Laccase and mediators treatment of DCS led to the significant reduction of lignin substances in DCS water obtained from BDIP , and the VIO (Violuric acid) was the most effective mediator. The optimum laccase treatment conditions were as follows: the laccase dosoge 50 U·L^-1, pH 7.5, temperature 50℃, reaction time 2 h and air pumping. Lignin concentration in DCS decreased 26.6% and CD value changed slightly with laccase/VIO treatment. GC-MS analysis results further proved that not only lignin but also esters, some fatty acids as well as resin acids in DCS could be degradated by laccase, which showed that laccase/mediator system could be used for DCS control. laccase/mediator could be used to remove the lignin and improve the paper strength.Matched use of pectinase, laccase and lipase in the DCS treatment was optimized by orthogonal experiment of the processing conditions and an expected efficiency were obtained at a low enzyme dosage as well as short reaction time. The CD value, lignin concentration, tubidity and particle size of DCS decreased after treatment of DCS with matched enzymes treatment, but this improvement was less than the sum efficiency of each enzyme under their optimum conditions. Enzymatic treatment of DCS in this way could effectively improve retention aids efficiency, dewatering of pulp, fines retention and paper properities like porosity, tear strength, bursting strength as well as breaking length. However, the treatment could result in a slightly decrease of paper whiteness and internal sizing efficiency. It was shown that the substances produced by lipase degradation were detrimental to the bond of AKD and fibers, which led to the reduction of paper sizing degree.