The first two bleaching stages are the most important ones, because they remove most of the lignin and inorganic material from the pulp (Dence & Reeve 1996). Therefore, efficient washing in the first two washers provides good chemical and economically profitable conditions for bleaching. Inefficient washing increases the consumption of bleaching chemicals in the following stage and reduces pulp brightness.
Furthermore, the importance of washing depends on the bleaching chemical applied in the next bleaching stage. Pulp washing between the ozone and chlorine dioxide stages is not necessary and it is possible to reduce the need for chlorine dioxide in the AD stage by means of acid treatment without interstage washing (Henricson 1997, Chirat & Lachenal 1997, Fuhrmann et al 1998, Pikka & Vehmaa 2002). An ozone stage, on the other hand, can be quite sensitive to impurities as Vuorinen et al (1997) have proved. For example, filtrate from the A-stage is very detrimental in the subsequent ozone stage and washing is beneficial between the A and ozone stages. Since ozone reacts very sensitively with dissolved organic solid (Lindholm & Malkov 1997), only a low carry-over of these solids can be tolerated in bleaching sequences that employ this chemical. This leads to an increased demand for efficient washing before the ozone stage.
Beside transition metals, reduced sugars have also been observed to retard peroxide bleaching and accelerate the catalytic decomposition of hydrogen peroxide, so their removal requires good washing (Heikkila et al 2000). On mill scale, it has been observed that the bleaching efficiency of the peroxide stage is reduce due to reactions with circulated dissolved material (Fuhrmann et al 2000).
After the final D-stage, washing is essential to minimise the carry-over of corrosive material into the pulp dryer or paper machine and to minimise the brightness reversion of the pulp in storage towers.
(Source: Silanpaa, Mervi (2005). Studies on Washing in Kraft Pulp Bleaching. Finland: Oulu University Press)
