The aim of pulp washing in the bleach plant is to remove dissolved organic and inorganic materials, that would interfere with subsequent bleaching stages and raise the consumption of bleaching chemicals. Pulp washing in the bleach plant is mostly evaluated in the same manner as brownstock washing: i.e., using soda loss or COD as an indicator. Improving the efficiency of the bleach plant washers becomes even more critical in a closed bleach plant, where the consumption of bleaching chemicals is expected to rise. Process modeling using different indicators, obtained from laboratory tests, can be used to predict the effect of closure in the bleach plant and the recovery cycle.
In a 1995 survey of bleach plant washing practices in Canadian mills [1], the chemical demand in the following stage was the most frequently monitored variable, followed by mat consistency. The survey showed that, generally, chloride is an effective variable for monitoring the first C/D washing stage whereas, in extraction stages, sodium can be used to determine washer performance. Filtrate recycle and split showers can, however, complicate the determination of washer efficiency. Bleach plant effluent in Canadian mills surveyed ranged from 12 to 70 m3/adt, with a median of 28.4 m3/adt. Since bleaching effluents are sent to the biological treatment system, which can handle large variations in volumes and dissolved solids, bleach plant washer control has not been a major issue in open mills. As the trend towards reduction of effluents in the bleach plant continues, however, better control strategies will be required. Feedback control strategies used in brownstock washing can be used in bleach plant washers, if the required sensors exist. The carryover of a specific contaminant from the washing stage can be monitored by using the appropriate sensor, and shower flows can be manipulated to maintain pulp cleanliness [2].
The use of a displacement press, rather than a vacuum filter washer, in the bleach plant gives the opportunity for a reduction in the consumption of bleaching chemicals, water and steam. With higher discharge consistencies, it is easier to control pH, temperature and metal ions in the process. The consumption of alkali in the extraction stage is also reduced, because the volume of residual alkali-containing dilution liquor added to a high consistency pulp (~30%) after washing is much higher than the amount added to a pulp with a consistency of ~12%. The effluent volume of an open bleach plant can be reduced by more than 50% when filters are replaced by wash presses.
1. Towers, M. and Turner, P.A., Survey of bleach plant washing practices in Canadian mills, Pulp Pap. Can., 99(7), pp. 44-49, (1998).
2. Dence, C.W. and Reeve, D.W., Eds, Pulp Bleaching, Principles and Practice, Tappi Press, Atlanta,
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