Many mills suffer from insufficient brown stock washing, most often noted as high carry-over. But pulp washing is not only about getting a lower carry-over into the bleach plant, or in case of an integrated unbleached mill, to the paper machine. Both oxygen delignification and the operation of pre-oxygen washers are affected by poor washing and a high dry solids content in the liquors circulating between the stages. To compensate for insufficient washing some mills run with a higher than normal dilution factor, but then must accept an increased flow to the evaporators with a lower than necessary dry solids. Brown stock washing can be improved by adding one or two new wash stages. This washing improvement usually opens up for a reduced overall dilution factor, a reduced flow (and higher solids) to the evaporators, and a better or equal wash result. However, before additional washing is added, a thorough modeling of the whole wash line must be made to ensure that the new washer(s) is added in an optimal position. An incorrectly placed washing device can increase the carry over into the oxygen delignification or increase the dry solids in areas where this is not wanted even if the overall carryover is reduced. This paper describes the impact on the fiberline that various solutions (such as adding a wash press, or a pressure diffuser) can have on the overall wash efficiency, looking specifically at the carry over between wash stages and the carry over into the oxygen delignification.
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