Improving The Peroxide Bleaching Efficiency Of Aspen CTMP By Using Chitosan And Its Mechanism

Aspen bleached chemithero-mechanical pulp （BCTMP） are widely produced andused in the pulp and paper mills in north of China. During the production of BCTMP,peroxide is widely used in the pulp bleaching. The hydrogen peroxide decompositioncatalyzed by the transition metal ions such as Fe²⁺/Fe³⁺, Cu²⁺and Mn²⁺negatively affectthe bleaching efficiency. The chelating agents such as EDTA and DTPA are used asstabilizers in peroxide bleaching for the improvement of peroxide bleaching efficiency,but the low biodegradability and low chelating efficiency at alkaline condition limitstheir application in alkaline peroxide bleaching.Chitosan （CS） and chitosan oligosaccharide （COS） was used in the alkalineperoxide bleaching of aspen CTMP, and the effects of CS and COS on the peroxidebleaching efficiency was studied. The free radicals formed in peroxide bleaching weredetected by electron paramagnanetic resonance （EPR）, and the adsorption and chelationof different transition metal ions by CS and COS was discussed.The mechanism forimprovement of peroxide bleaching efficiency of aspen CTMP by using CS and COSwas primarily investigated.The results showed that, compared to the control without CS or COS addition, theperoxide bleaching efficiency of aspen CTMP was improved with low level addition ofCS or COS, and both the brightness of bleached pulp and residual peroxide increased.The effects of CS and COS on bleaching improvement depended on the degree ofdeacetylation of CS and the molecular weight of COS, which was improved with theincrease of the degree of deacetylation of CS and the molecular weight of COS. Thehydroxyl radical concentration in peroxide bleaching system decreased with theaddition of CS, and it was slightly affected by COS addition.The effects of CS and COS on the peroxide bleaching efficiency of aspen CTMP inpresence of different transition metal ions such as Fe²⁺/Fe³⁺, Cu²⁺and Mn²⁺werediscussed and compared. The results showed that, the greatest effects of CS on theperoxide bleaching efficiency were obtained in presence of Fe²⁺/Fe³⁺, and least effectsof CS on the bleaching efficiency were obtained in presence of Mn²⁺. The effects ofCOS on the bleaching efficiency varied slightly in presence of different transition metalions. The results of adsorption of different transition metal ions on the CS showedthat, Fe, Cu and Mn ions could be adsorbed by CS in the conditions of alkalineperoxide bleaching, and the adsorption transition metal hydroxide colloid on the CSsurface was also found by AFM. The stability of hydrogen peroxide was evaluatedin presence of the transition metal ions at alkaline conditions, and the resultsshowed that, the decomposition of peroxide caused by Cu, Fe or Mn ions was allinhibited by both CS and COS.