Pulp washing - Determining local properties using a new measurement technique
There are many incentives for making the pulp washing process as efficient as possible: an efficient separation of the pulp from the black liquor will save a substantial amount of energy, reduce costs by reducing the need for make-up chemicals and improve the bleaching process. There are many discrepancies in the literature, however, as to how different parameters influence washing efficiency. It was thought that a new measurement technique which enables the concentration of different species to be measured inside the pulp bed during the washing process, would provide new data that could shed some light over the area of pulp washing. It was shown that the measurement technique worked well for the materials tested: sulphite, sulphate and mechanical pulp and calcium silicate, respectively. The equipment functions better for materials with a long washing time, (i.e. calcium silicate and mechanical pulp) since the measurement error is dependent on the measurement time. Local dispersion coefficients could be calculated for mechanical pulp; it was shown that the local dispersion coefficient at the top of the bed was influenced by inlet mixing. The filtrate measurements showed that the washing of mechanical pulp beds differs substantially from that of sulphite and sulphate pulp beds. The dispersion coefficient for mechanical pulp was, on average, a factor of 10 smaller than the dispersion coefficients for the chemical pulp. The explanation for this can be found in the difference in bed structure due to differences in fibre properties.