Behaviour of Certain Components in the Vacuum Drum Washer After the Do Stage in ECF Bleaching: Mill Tests, 2000 Pulping / Process & Product Quality Conference Proceedings
M. Sillanpaa, K. Ala-Kaila, M. Viirimaa, O. Dahl--Environmental concerns and tighter public regulations have created new pressures for reducing effluent loading from kraft pulp bleaching during the past decades. These requirements have forced both pulp mills and the related engineering industries to develop and optimize their pulping process equipment to meet these demands. This ongoing trend has already placed increasing requirements on pulp washing, particularly in bleaching.
The behaviour of chlorine compounds, sodium, TOC and COD in the vacuum drum washer after the D0 stage in ECF bleaching sequence is examined here. Three tests were performed at a pulp mill in northern Finland, two under normal bleaching conditions and the third test bypassing the oxygen
delignification stage before bleaching,
thus leading to a much higher chlorine
dioxide dose in the D0 stage.
The mill trials illustrated that inorganic chlorine, mainly chloride, constituted over half of the chlorine load in the liquor continuing with the ashed pulp from the D0 stage to the next bleaching stage. The amount of chlorine depended on the quality of the wash water, the chlorine dioxide dose in the preceding bleaching stage and the outlet consistency of the washed pulp. When the incoming kappa number in bleaching and the dose of chlorine dioxide in the D0 stage were higher, the amount of chlorine compounds in the effluent discharge was also higher. A certain amount of chlorine was attached to the fibre in a non-washable manner and was not removed during washing. The proportion of organic chlorine increased during washing and that of inorganic chlorine decreased, which suggests that inorganic chlorine is easier to remove than organic. The removal of sodium, COD and TOC was not efficient, because the second wash water contained large amounts of these compounds.