Oxalic Acid Formation in Oxygen Chemical Bleaching, 1999 Pulping Conference Proceedings

Juha Fiskari--One of the most important objectives of TCF bleaching (also known as Oxygen Chemical Bleaching, OCB) is to close the bleach plant and recycle the filtrates. This investigation studied the consequences of a partial closure of the liquid circulations in the bleaching sequence including the ozone and hydrogen peroxide stages, the focus being on the formation of oxalic acid. A series of experiments was performed with an oxygen delignified hardwood (birch, Betula) pulp sample from a Finnish pulp mill. The pulp was bleached with ozone and hydrogen peroxide. The filtrates were recirculated in order to simulate the liquid circulations of a partially closed bleach plant. The conditions (chemical charges, retention times, temperatures) were similar to those of an actual mill.

According to the results, the TOC levels of the filtrates increased significantly when they were recycled. Also the concentrations of oxalic acid increased. In the case of both of these properties, after the third recycling loop there was a limit above which the concentrations did not increase. Apparently, excessive oxalic acid formed in the oxidative bleaching precipitates as calcium oxalate. There were, however, major differences in the amounts formed in the different bleaching stages. The formation of oxalic acid in the PO stage was significantly higher than in the Z/Q stage.