The screening, cleaning, flotation and washing processes are treated in this second part “Particle Separation Processes” of the review paper about “Stock Preparation”, which is focused on the process engineering aspects of the unit operations used in the production of virgin and recycled pulps. Chemical and physical- chemical aspects are out of the scope of this paper as well as the pulp dilution, transport and storage unit operations.
Particle separation processes refer to the unit operations aiming at separating different pulp components in order to remove or concentrate some of them in different fractions. The particle separation processes are essential in recycling to remove various contraries in a very large particle size range. Separation is mainly based on particle dimensions in screening and washing, on particle density in centrifugal cleaning and on particle surface properties in flotation. Fractionation is normally performed as an intermediate process between screening and washing, though the centrifugal process can also be used. Particle size and shape and hydrodynamic phenomena are decisive in the separation processes.
Fundamentals of centrifugal separation as well as flotation hydrodynamics have been extensively studied in the field of mineral processing, while fundamentals of pressure screening have not yet been investigated as much, since the technique is more recent and specific to the pulp and paper industry. The particle separation mechanisms are reviewed in this paper, with special emphasis placed on the description and analysis of physical mechanisms and on the theory developed about two main aspects:
– the large scale fibre suspension flows, which define particle transport and mixing;
– the particle separation micro-processes observed at the scale of the particles.
Finally, the effects of machine, operating and material param- eters are briefly analysed with respect to the theoretical background.