2.1. General

The objective in fractionation is to classify the pulp selectively according to certain fibre properties and to separate the flows appropriately. The separated fractions can then be used in tailor-made paper products, either directly or after further processing. Although the goal of fractionation is primarily good selectivity, the yield of the fractions must also be reasonable, otherwise the complexity of the system and the investment and production costs may become too high. Yield and selectivity generally follow opposite trends, which leads to a compromise between the two in practical situations.

Pulp fractionation differs from cleaning, i.e. removal of impurities from the pulp, although the difference may not always be clear. The pulp may also be fractionated in cleaning, but this is not necessarily the desired effect (Forbes 1987). Cleaning and fractionation may or may not be interconnected in the same process stage. The aim of cleaning is to remove material which cannot be used at all or only after extensive processing, while causing as low a fibre loss as possible, as determined by the mass reject rate (Karnis 1997). That of fractionation, on the other hand, is to produce fractions having different fibre properties, which can be utilised separately in different products or enhanced by proper processing and used separately or recombined in certain forms.

There are only a limited number of practical fractionation processes, so that fractionators can be divided into two categories according to their operating principle (Karnis 1997): devices in which separation is based on mechanical barriers, and devices in which it is based on the hydrodynamics of the suspension in the fractionator. Different constructions have been developed in both categories, but the main methods even today are still pressure screening in the former category and centrifugal cleaning in the latter. Other types of fractionation process have also been tested, including flotation (Eckert et al. 1997), a wheel plate atomiser (Moller et al. 1980), liquid column flow (Olgård 1970), and a rotating cone (Rewatkar & Masliyah 1999), but they are not in general industrial use at present.

This work focuses on pressure screening, and the following sections in this chapter will give a general insight into the process.