TCF bleaching of softwood kraft pulp Part 5. Investigation of EDTA-metal complexes in chelation filtrates

EDTA-metal complexes in filtrates from chelating stages have been investiga ted by means of reversed-phase ion-pair liquid chromatography It was possib le to separate EDTA complexes of Fe3+, Mn2+ aid Cu2+. The effect of EDTA on the pretreatment of oxygen delignified softwood kraft pulps with low manga nese content was studied. The low manganese content was obtained by applyin g an loll exchange pretreatment [(Mg+Ca)] in which manganese ions are displ aced from the pulp by an excess of magnesium and calcium ions or by an acid pretreatment at pH 2 (A) in which the protonation of carboxylate groups in pulp leads to a removal of manganese ions. The pulps prepared, along with a water pretreated reference (W), were then subjected to an EDTA pretreatme nt prior to hydrogen peroxide bleaching. Analyses of the metal ion content in pulps and of EDTA-metal complexes in Q-stage filtrates in combination wi th results from peroxide bleachings suggest that the manganese present in p ulp decreases the brightness gain obtained in peroxide bleaching, but has a minor effect on the viscosity drop during bleaching. The results indicate that the main effect of EDTA, besides manganese removal, is that catalytica lly active iron and/or copper is removed from the pulp.