Deresination efficiency and loss of solids in mechanical pulp washing

In this paper, an approach for describing the behaviour of lipophilics and solid material in the washing stage of mechanical pulp is presented. This information is needed for the optimisation of washing processes in order to achieve a better balance between deresination efficiency and the loss of solid fibre material in the washing. Two independent and measurable elements were introduced which describe how much the deresination efficiency in the washing differs from the theoretical value, i.e., if all lipophilics would be in the dissolved form. These elements are the proportion of dissolved and colloidal lipophilics in the pulp and the retention of dissolved and colloidal lipophilics in the dewatering. The use of this classification system was justified by experimental results. The effect of different factors on the deresination efficiency was evaluated mainly through these basic elements. The factors studied were dewatering technique, mixing time and intensity, pH, degree of refining and the amount of wash water used. Also, the effect of filtrate circulation in the process and the freeness value of the pulp on the loss of solids in the washing was considered.