Washing during kraft pulping can be divided in two separate areas each with this own distinct features, namely brownstock washing and washing in bleaching. The main purposes of brownstock washing are to remove dissolved organic and inorganic substances that would contaminate the pulp during subsequent steps in the process and to recover the maximum amount of spent chemicals with the minimum dilution. Efficient washing improves the recovery of spent chemical, reduces the consumption of reagents in the subsequent bleaching, and essentially limits effluent load from the plant.
The aim of washing in bleaching is to remove both dissolved organic and inorganic matter, which will disturb the subsequent bleaching stage and enhance the consumption of bleaching reagent. As the optimum chemical conditions in successive bleaching stages vary, one very important purpose of washing is to render the conditions more suitable for the next bleaching stage. This may include modifying the pH, the metal content of the pulp and the temperature. Thus, an additional aim of washing is also to serve as chemical conditioning and a heat exchanger during the purification procedure.
During the past decade, environmental concern have created new pressures to reduce effluent loading from kraft pulp bleaching. Pulp mills also have intrinsic needs to cut down their use of water and reduce effluent. Efficient and rational water use to provides improve heat balance and chemical recovery. Intrinsic and extrinsic demands have forced both pulp mills and the related engineering industries to develop and optimise pulping process equipment to meet these demands. Improve pulp washers and washing system connected with the introduction of oxygen-based reagents have in particular provided considerably improved tools for the reuse of process water and have thereby essentially reduced effluent discharge or even allowed the implementation of a closed water circulation system in kraft pulp bleaching.
This ongoing trend has placed increasing requirements on pulp washing particularly in the interstage washing in bleaching. Less fresh water is used for pulp washing, but at the same time the quality and cleanliness of the pulp should remain high. The more closed the water re-circulation system is, the dirtier the wash water will be during bleaching, probably resulting in an increased reagent consumption and poorer pulp quality. Since more expensive bleaching chemicals are use and the loss of bleaching chemicals is not acceptable, more specific washing is needed.
(Source: Silanpaa, Mervi (2005). Studies on Washing in Kraft Pulp Bleaching. Finland: Oulu University Press)
