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THE ROOTS OF OXALIC ACID FORMATION


April 1, 2000
By Pulp & Paper Canada

KARLSTAD, Sweden — An article in TAPPI JOURNAL (February 2000) by Anna Alexander, Stora Enso Research, Monica Ek, and Goran Gellerstedt, Royal Institute of Technology, describes Oxalic acid formation…

KARLSTAD, Sweden — An article in TAPPI JOURNAL (February 2000) by Anna Alexander, Stora Enso Research, Monica Ek, and Goran Gellerstedt, Royal Institute of Technology, describes Oxalic acid formation during ECF and TCF bleaching of kraft pulp. The bleaching agents used were oxygen, hydrogen peroxide, chlorine dioxide, peracetic acid and ozone. The formation of oxalic acid in softwood kraft pulp bleaching increased linearly with the reduction of kappa number. However, bleaching with peracetic acid produced less oxalic acid than other bleaching agents. In ozone bleaching, about one third of the oxalic acid formed is adsorbed on the pulp and not released until pulp pH is increased during a later stage, where it could combine with calcium ions to produce a calcium oxalate scale. Mills that are increasing closure in the bleach plant can avoid scaling problems by tailoring their operations to minimize formation of oxalic acid.

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