Selective Separation

As in screening, the selective separation of different types of solids contained in

the feed stream is of major importance for all contaminant removal and fractionation

applications. As described earlier, separation in a hydrocyclone depends

mainly on differences in the particles’ densities and specific surfaces. Hence, the

selectivity of separation in a contaminant removal application improves with the

density difference between debris and pulp.

Likewise, the density difference between individual pulp fibers determines how

selectively they can be separated in a cyclone. The apparent density of a pulp fiber

results from its diameter and wall thickness. Thick-walled, smaller-diameter fibers

(as found in softwood latewood) have a higher apparent density than thin-walled,

larger-diameter fibers (as found in softwood earlywood). When pulp is subjected

to fractionation in a cyclone, the thick-walled fibers will proceed preferably to the

underflow, forming the coarse fraction. Despite their lower apparent density, thinwalled

fibers are still heavier than the liquor. They report not only to the overflow (fine

fraction) but also to the underflow. It is therefore easier to obtain a relatively pure fine

fraction in the overflow than to obtain a pure coarse fraction in the underflow [23].

0 2 4 6 8 10

Proportion in each class, %

Fiber wall thickness, μm

Fine fraction

Feed

Coarse fraction

Fig. 6.22 Example of fiber wall thickness distributions of feed and fractions

after several stages of centrifugal cleaning;bleached softwood kraft pulp [23].

Separation in a hydrocyclone is influenced by a variety of factors such as the

complex fiber morphology, particle–particle interactions and short-circuit flows

within the cyclone. In practice, separation in a single cyclone is far from ideal,

and several stages of cleaning are needed to obtain fractions of significantly different

character, such as those illustrated in Fig. 6.22.

6.5