Studies On Magnesium-based Hydrogen Peroxide Bleaching And Mechanisms Of Deinked Pulp

Virgin fiber shortage in China’s paper industry is obvious, however, the waste paper pulpconsumption proportion remains at64%or even higher, so the waste paper pulp should befully used. At present, conventional NaOH-based hydrogen peroxide bleaching of deinkedpulp has such problems as higher pollution, invalid decomposition of hydrogen peroxide,yellowing reversion, and scaling. Hence, Mg(OH)2-based hydrogen peroxide bleaching andmechanisms of deinked pulp were researched in this paper. The process conditions ofMg(OH)2-based hydrogen peroxide bleaching of deinked pulp were optimized. The effects ofsulfuric acid and EDTA chelating treatments on Mg(OH)2-based hydrogen peroxide bleachingwere discussed. More specifically, the surface properties of Mg(OH)2-based hydrogenperoxide bleached pulp and its yellowing reversion mechanisms as well as the formation ofoxalate during the peroxide bleaching process were investigated.First, gradual replacement of sodium hydroxide with magnesium hydroxide as the alkalisource during the hydrogen peroxide bleaching process of deinked pulp was studied. Underthe conditions of10%consistency and3%H2O2charge, when50%NaOH was replaced withhigh-purity Mg(OH)2in peroxide bleaching, the bleached deinked pulp was produced with thehighest brightness gain (13%ISO) and tensile index increase (5N·m/g). When Mg(OH)2replacement ratio increased from50%to100%, peroxide consumption decreased12%-58%.When Mg(OH)2replacement ratio was50%-100%, the COD load of bleaching filtratesdecreased24%-39%.Second, the effects of sulfuric acid treatment (A) and EDTA chelating treatment (Q) onMg(OH)2-based peroxide bleaching of deinked pulp were compared. The metal contents ofthe unbleached pulp were removed after acid-washing or chelating. The transition metal ions(such as Mn, Fe, and Cu) removal from unbleached pulp was grater in a chelating stage thanin an acid-washing stage, while Mg removal was greater in an acid-washing stage. At thesame Mg(OH)2replacement ratio, more residual peroxide and lower COD were in QP filtratesthan those in AP filtrates. QP pulp had higher strength properties than AP pulp, while AP pulphad higher brightness and bulk. ATR-FTIR analysis showed that the carbonyl groups in ligninwere removed during A or Q treatment and peroxide bleaching. And carbohydrate was alsodegraded to a certain degree.Third, acetone extraction, enzyme (laccase/mediator or lipase) treatment and XPS,SEM-EDS surface analysis techniques were combined to explore the surface properties ofMg(OH)2-based peroxide bleaching of deinked pulp. The low-resolution of XPS and SEM-EDS showed that some trace elements such as calcium, silicon, aluminum were presenton the sample surface, in addition to the main elements of carbon and oxygen. PMg(OH)2bleached pulp had less surface lignin and extractives than PNaOHbleached pulp. The extractionimproved pulp brightness and tensile index, but had little effect on residual ink removal.Enzyme-treated pulp surface was covered by the redeposited lignin or extractives. However,when the enzyme-treated pulps were bleached, the redeposited lignin or extractives weresignificantly removed. Lipase-treated bleached pulp had better properties and lower ERICvalues than laccase/mediator-treated bleached pulp, further indicating that lipase can not onlyremove surface lignin and extractives, but also more residual ink contaminants.Then, UV light-induced yellowing and ATR-FTIR were combinedly used to acceleratethe yellowing of the paper, evaluate its yellowing properties and reveal its yellowing reversionmechanism. The pulp brightness loss, P.C. number and chromatic aberration had a linearrelationship with b*. During the irradiation of UV light, the pulp brightness loss, P.C. numberand chromatic aberration increased quickly, and then slowed down. The photoyellowingmathematic model of deinked pulp was founded. After UV irradiation for360min, the bandintensity at1729cm-1increased distinctively and a new band at1674cm-1appeared. Thisindicated that p-quinone groups were produced during the irradiation process, thus resultingin paper yellowing. The band intensity at1674cm-1of PMg(OH)2bleached pulp was lower thanthat of PNaOHbleached pulp, which indicated that the brightness stability of PMg(OH)2pulp wasbetter than PNaOHpulp.Finally, the formation rules and influencing factors of the fouling factor-oxalate-produced during peroxide bleaching of deinked pulp were explored, and the oxalate formationmultiple regression equation was founded. At the same peroxide dosage, the oxalate formationincreased linearly with alkli charge increasing, PNaOHbleaching produced more oxalate thanPMg(OH)2bleaching, and the formation rate of the former was much faster. As to the samebrightness target, PNaOHbleaching produced more oxalate than PMg(OH)2bleaching. The oxalateformation multiple regression equation was founded: Y=-47.772+10.174X1+4.674X2+0.820X3+13.664X4. Mg(OH)2dosage (X1), peroxide dosage (X2), temperature (X3) and lnt (X4)had positive correlation with oxalate formation (Y).