Behavior of certain components in the vacuum drum washer after the first D stage in ECF bleaching.

Application: The correct choice of wash water can reduce the carry-over of chlorine compounds.

In tests performed at a pulp mill in northern Finland, trials were performed to determine the nature and amount of chlorine in washing. The researchers took samples of Scandinavian softwood kraft pulp from different points in the mill and measured the concentrations of total chlorine, chlorine dioxide, and other chlorine species, along with Na, TOC, and COD. They constructed mass balances for the different compounds in the surroundings of the vacuum drum washer after the first D stage. These mass balances showed how efficiently the compounds studied were removed in mill-scale washing.

The amount of chlorine was found to depend on the quality of the wash water, the chlorine dioxide dosage in the preceding bleaching stage, and the outlet consistency of the washed pulp. The proportion of organic chlorine increased during washing, but the proportion of inorganic chlorine decreased. The removal of sodium, COD, and TOC was not efficient because the secondary wash water to the first D stage contained large amounts of these compounds. S!

Sillanpaa, Sankari, and Dahl are researchers in the Dept. of Process Engineering, P.O. Box 90014, University of Oulu, Finland. Ala-Kaila is the manager of technology development at Metsa-Botnia, P.O. Box 165, 26101 Rauma, Finland. Email Sillanpaa at [e-mail].