Oxalic acid formation during ECF and TCF bleaching of kraft pulp

Formation of oxalic acid during bleaching of softwood kraft pulp was studie d. The bleaching agents used were oxygen, hydrogen peroxide, chlorine dioxi de, peracetic acid, and ozone. Formation of oxalic acid increased linearly with the reduction in kappa number. However, bleaching with peracetic acid produced less oxalic acid than the other bleaching agents. In ozone bleachi ng, about one-third of the oxalic acid formed is adsorbed on the pulp and n ot released until pulp pH is increased during a later stage, where it could combine with calcium ions to produce a calcium oxalate scale.