Pulp displacement washing - An investigation of the influence of fibre and bed properties using a novel measurment technique

Doctoral thesis, 2008

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 increase energy efficiency, reduce costs by minimizing the need for make-up chemicals and improve the bleaching process. Research in this area has, however, been more or less neglected and in the existing literature there are many discrepancies as to how different parameters influence the washing process. A more systematic way of studying pulp washing was therefore required. A new measurement technique which makes it possible to measure concentrations and porosity within the pulp bed during the washing process was therefore developed. The measurement technique has been used for kraft, sulphite and mechanical pulps and calcium silicate (validation experiment). The technique worked well for all of the materials, but especially so for materials where the washing process took a long time, since the measurement error is dependent on the measuring time. The new measurement technique was used to obtain local displacement curves. The displacement curves were used to calculate local dispersion coefficients which were compared with the dispersion coefficients obtained from measurements on the resulting filtrate. The local dispersion coefficients in general followed the same trends as the filtrate dispersion coefficients. Using the new measurement technique the influence of washing efficiency on pulp type, flocculation and the presence of a porosity gradient was examined. It was thereafter investigated how compressing kraft pulp to different dry contents with variations in pH and temperature influences its strength properties. The finding was that if the alkali content and the temperature were kept low, compaction did not cause any permanent damage even though the compression pressure was high. A simulation study was performed in order to investigate how compressing the pulp before washing influenced the wash process and it was shown that the dilution factor could be reduced substantially whilst the cleanliness of the exiting pulp suspension remained the same leading to a substantial cost reduction.