An investigation of factors affecting pulp washing efficiency

Washing is an important operation in post-pulping and inter-stage bleaching operations to remove and recover chemical-rich liquors. There are two basic mechanisms of washing, namely, compression and displacement. The transport of liquor or removal of unwanted liquors from the formed fibre pad is complicated by several factors that have been isolated and investigated. Experimental data are presented together with critical analysis of the key factors affecting displacement pulp washing efficiency. The experimental results are grouped under process options, liquor property options, and novel experiments to elucidate other possible mechanisms that could affect washing. These include the effect of vibration, fibre packing, fibre orientation, fibre type, and model fibre experiments. The studies form the basis for a new pulp washing process termed vapour-phase displacement washing. Some preliminary data are given to show the potential of this new method.