The Formation of Oxalic Acid During Bleaching of Kraft Pulp

Abstract

Calcium oxalate scaling is a major practical problem in a paper mill, especially in the evaporators, pulp digesters, and the bleach room. The sources of oxalic acid were determined by analysis of mill samples. In the unbleached pulp mill, the source of oxalic acid is the wood. In addition, a major amount of oxalic acid was found to be formed in the bleach room. A laboratory bleaching study was undertaken to establish a quantitative relationship betweeen bleaching conditions and oxalic acid production. Hardwood kraft pulp was bleached using three sequences. The amount of oxalic acid produced was dependent upon the available lignin content of the pulp (Kappa number) and not the oxidant concentration or reaction temperature.