Evaporation of Acidic Effluent from ECF Bleaching: Pilot Tests, 1998 Environmental Conference Proceedings

O. Dahl, J. Niinimäki H. Kuopanportti,

University of Oulu

The applicability of evaporation for the treatment of acidic effluent from ECF bleaching was examined in pilot tests using a concentrator with a rated capacity of about 120 litres of condensate per hour. The concentrator had originally been developed for concentrating black liquor concentration by up to 90%. The effluent used in the experiments was derived from real Do (first chlorine dioxide stage) acidic stage of ECF bleaching. The results were evaluated by analysing the quality of the condensate and concentrate as a function of total concentrate solids from 0.3% to 5%. The results indicate that the quality of the condensate decreased as total solids in the oncentrate increased. The total carry-over of substances causing conductivity was about 30%, while the value for COD was 40% and that for TOC 34%.

About 60% of the COD can be attributed to methanol, formic acid and acetic acid carry-over during evaporation. The high carry-over of inorganic and organic material can be explained as exceeding considerably the operating point of the design capacity.

The main precipitate component of the concentrate was calcium oxalate hydrate, the amount of which was greatly dependent on total solids in the concentrate during evaporation. No clear scaling problems were observed in the evaporation equipment after the 30-hour pilot test, but there was some evidence of a scale-up of barium salts when the equipment was washed with alkalised de-ionised water.

Regardless of the relatively high amounts of inorganic and organic components in the condensate, our laboratory-scale re-use test confirmed that the quality of the condensate was good enough for re-use in the final chlorine dioxide stage of the ECF bleaching process concerning reagent consumption and main pulp properties.