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, GA, pp. 569-596 and pp. 649-673, (1996). 

Brownstock washing is a key unit operation, affecting the efficiencies of both bleaching and the recovery cycle. Washing becomes even more critical with system closure, where alkaline/acid bleaching filtrates are recycled to the brown fibre line. The operation of brownstock washing has to be optimized prior to closure, to prevent excessive carryover and significant increase in the consumption of bleaching chemicals. Several approaches can be used to improve brownstock washing and reduce the carryover of black liquor to the bleach plant. These include: increased shower flow, additional shower bars, closed screen rooms and addition of defoamer and washing aid chemicals. Reconfiguring existing stages to employ filtrate segregation may also provide incremental improvement in washing efficiency. Process modeling can be used to predict the results of washing optimization and various system closure scenarios, ahead of mill trials. Process control is another cost-effective approach for improving washer efficiency. Sufficient filtrate surge tank capacity is recommended to balance accumulations in pulp storage tanks, especially during production rate changes. Sensors are now available to measure the entrained air in pulp suspensions and, therefore, provide better control in washing operations*. Many older mills are using original washers, which tend to be overloaded. However, modifications made to original washing equipment can allow mills to run at increased production levels. The new generation of washing equipment provides improved efficiency, reduced operating and capital costs and minimal environmental impact. Some of the new alternatives to the rotary vacuum drum washer include the Drum Displacer^TM, the Pressure Diffuser, the Chemi-Washer, the Compaction Baffle Filter and the Displacement Press.  
*R. Thompson and T. Mahmood, Recent developments in mill implementations of water reduction strategies, in “Water Use Reduction in the Pulp and Paper Industry”, 2^nd edition, PAPRICAN, December 2001.