Study On Dissolved And Colloidal Substances In Newsprint Mill Process Water-Characterization, Stability And Enzymatic Control

During the production of chemi-mechanical pulp and deinked pulp,dissolved and colloidal substances(DCS) are released from the fibers into the process waters or pulp-water systems.The white water systems are increasingly closed,which results in the accumulation of DCS in the water systems.The increased amounts of DCS will affect the pulp and papermaking processes and the paper quality.Thus,the composition,release mechanisms and stability of DCS from Masson pine CTMP(chemi-thermo-mechanical pulp) and ONP (old newsprint) deinked pulps were studied in detail.Additionally,The efficiency and mechanisms of enzymes to treat DCS were investigated.Firstly,the components of DCS from Masson pine CTMP and ONP deinked pulps were analyzed qualitatively and quantitatively.Results showed that the RFAs(resin and fatty acids) in DCS of Masson pine CTMP were mainly composed of oleic acid,linoleic acid,DHA (dehydroabietic acid),pimaric acid,abietic acid and oxidized products of DHA.The ratio of fatty acids to resin acids in FR-DCS(DCS of the first refiner pulps),CTMP-DCS(DCS of unbleached CTMP),BCTMP-DCS(DCS of peroxide bleached pulps) were 1:2.8,1:2.1 and 1:3.8,respectively.About 90%of RFAs in DCS of deinked pulps were resin acids,among which DHA and abietic acid were the predominant ones.Based on the proportion of sugar units determined,the carbohydrates in DCS of Masson pine CTMP contained galactoglucomannans and considerable amounts of arabinolglucoxylans and arabinogalactans.More pectic substances were released during the peroxide bleaching.The main sugar units in DCS of deinked pulps were 4-O-methyl-glucuronic acid and xylose.Secondly,the release mechanisms of DCS from Masson pine pulps and deinked pulps were investigated.It was found that screw press could extrude the wood resins from chips non-selectively.RFAs in the first refiner pulps were not dispersed into the water phase proportionally,and much smaller amount of abietic acid was released.The amount of fatty acids in CTMP-DCS was increased up to 50%of total RFAs.This indicated that fatty acids were more easily released from fibers than resin acids in that condition.The release of DCS from unbleached CTMP fibers was strongly affected by the initial pH value of the suspension. As the pH value was above 8.0,much more wood resins and pectic substance were released, leading to the increase of turbidity and CD value of DCS.The ratio of fatty acids to resin acids in DCS of deinked pulps almost remained the same at 1:9.However,the proportion of resin acids components in No.4 sample(DCS of peroxide bleached pulp) was different from those of another three samples(DCS of unbleached pulps, 1~#-3~#),with DHA being a relative minor component,due to the large decrease of DHA concentration.Then representative models of wood resins,i.e.triglyceride(TG),fatty acid(O) and resin acids(R) and synthetic polymer(PA) were selected to study the stability of resins and synthetic polymer in DCS in the presence of simple electrolytes,cationic polymer,or fillers. Furthermore,effects of polysaccharides on the stability of model emulsions under the conditions mentioned right above were investigated in detail.Results showed that metal ions (sodium ions and calcium ions) could lead to the aggregation of model emulsions stabilized by electrostatic mechanism.The residual turbidity of TG emulsion decreased rapidly upon addition of simple electrolytes,which indicated that TG was much more easily destabilized than R and O emulsions.The residual turbidity of three model emulsions(R,O,TG) decreased with the increasing dosage of CPAM(cationic polyacrylamide).At a certain dosage of CAPM,the model emulsions would be completely aggregated.If more CPAM was added,the emulsion would re-stabilize.Fillers(PCC and talc) could adsorb the colloids in the model emulsions,resulting in the aggregation/flocculation of colloids.At pH 7.0,emulsion O had the best stability against PCC-induced aggregation,with emulsion R being the worst;as for talc,emulsion O were the best with TG being the worst.Neutral polysaccharide GM had a positive effect on the stability of three model emulsions in the presence of simple electrolytes,cationic polymer,or fillers.The stability of emulsion R and O could even be enhanced to a level that metal ions could not induce the aggregation of colloids.GM could slow down/baffle the direct contact of CPAM with colloids to a certain extent,but could not keep the colloids from being attacked by CPAM. There could be an affinity between GM and the added talc particles,which affected the absorption of colloids onto the surface of talc particles.This affinity was favorable for the stability of emulsions.Acidic polysaccharide PGA had positive effect on the stability of three model emulsions (R,O,TG) against sodium-induced aggregation by increasing the surface charge density of colloids.The efficiency of stabilizing emulsion R was the highest,with TG being the lowest. Interaction between calcium ions and PGA could barely be explained by a charge neutralization mechanism.Insoluble aggregates were formed between calcium ions and the carboxyl groups of PGA,leading to a co-aggregation of colloids and the aggregates.However, once the added GM was enough to wrap the colloids,the adverse effect of PGA would be limited within a certain range.Interactions between colloids of three model emulsions and their effects on the mixture stability were investigated.The interactions between colloids depended on the preparation procedure of mixture.There were no observable interactions between colloids at pH 5.0,if pre-emulsified models were mixed together(procedure 1) without addition of calcium ions. However,some interactions might exist when the colloids were de-stabilized upon addition of calcium ions into the mixture.The mixture(procedure 2) made of pre-mixed ethanol solution of model resins and distilled water was different from that of pre-emulsified models.At both pH 5.0 and 7.0,there existed considerable interactions between components(model R,O,TG) without addition of calcium ions.Two-component interactions(R-TG,O-TG,R-O) were relatively strong as the proportion of the third component(O,R,TG) was low.R-TG and O-TG interaction contributions to the total turbidity of mixture were positive,while that of R-O interaction was negative.The absolute contribution of R-TG interaction was the largest,with that of R-O being the smallest.The interactions between components resulted in the formation of two/three-component colloids,but not a single-component ones.At pH 5.0,the residual turbidity of mixtures of procedure 2 was almost linearly correlated to the proportion of three models upon addition of calcium ions,with the interaction contributions to the turbidity being negligible.This further confirmed the structure model of two/three-component colloids.At pH 7.0,two-component interactions had adverse effect on the mixture stability against calcium-induced aggregation,especially as the proportion of TG was high.Finally,the efficiency and mechanism of two lipases and two pectinases as well as to degrade or hydrolyze model substances and DCS from Masson pine BCTMP and deinked pulps were evaluated.The optimal conditions for pectate lyase(PGL) treatment were as follows:pH 9.0,60℃,120min.PGL could effectively degrade the pectic acid in the DCS of Masson pine BCTMP and bleached DIP,resulting in a decrease of about 40 percent in CD value.A minimum average DP(degree of polymerization) of 6 was required for pectic acid to interact strongly with cationic polymers and reduce the efficiency of the latter.Alkaline pectin lyase(PL) could efficiently degrade the pectin model.A decrease of 80 percent in CD value could be achieved at a dosage of 200 U(pectin model 5 mg).The optimal conditions for PL treatment were pH 9.0,60℃,120min.PL could effectively reduce the CD value of DCS from Masson pine BCTMP.Treatment of DCS from Masson pine BCTMP with PL could not prevent DCS from complete aggregation by calcium ions,although the rate of aggregation was lower.Pectic substances in DCS of BCTMP were mainly composed of unmethylated pectic acids.Two lipases(LP1 and LP2) could effectively hydrolyze triglyceride.The optimal conditions for LP1 and LP2 treatment were pH 7.0,70℃and pH 6.0,60℃,respectively.All of three ester bonds in triglyceride could be hydrolyzed by these two lipase,with the final product of glycerol and free fatty acid.The ability of lipases to hydrolyze triglyceride was strongly affected by fatty acids added.Laccase could degrade the conjugated resin acids and unsaturated fatty acids in model emulsions(R,O,TG) to some extend in the presence of ABTS(2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate)) or VIO(violuric acid).Palustric acid and abietic acid in DCS of Masson pine CTMP could be partially removed by lacaase treatment without addition of external mediator.