Arrangement

As mentioned above, the contaminant level in chemical pulp is far below the

mass reject ratio of industrial separation equipment. Consequently, a large portion

of acceptable fibers can be found in the reject of a single separator, together

with the contaminants. Economical constraints of pulping, however, require that

undesirable contaminants taken from the screening system carry along as few

good fibers as possible.

Hence, it is common to use a combination of separators, where, for instance, a

second screen is used to reduce the amount of good fibers in the reject of the first

screen, and a third screen to remove good fibers in the reject of the second one.

Such a simple cascade arrangement is shown in Fig. 6.26.

A

A

R

R

R

A

F

Fig. 6.26 Three-stage screening in cascade feedback arrangement.

F = Feed;A = Accept;R = Reject.

In a cascade system, the reject from one screen passes on to the feed of the

screen in the next stage. In a cascade feedback arrangement, only the accept of the

first stage proceeds to the downstream step in the pulp production process, while

the accepts of the other stages are in each case brought back to the feed of the

preceding stage (Fig. 6.26).

It should be noted that, in a cascade feedback screening system, sand accumulation

can lead to substantial wear and to the need for frequent exchange of screen

baskets. As screen slots become narrower, an increasing portion of the sand com-

596 6 Pulp Screening, Cleaning, and Fractionation

ing with the feed to the first screening stage is rejected. If the following stages

have screens of similar aperture size, the repeated rejection of sand effects a relative

increase in the sand concentration in the reject of each stage. If one of the

following stages has a screen of larger aperture size, sand may be accepted by this

screen and flow back to the preceding stage, where it is rejected again. Both of

these phenomena are inherent to screening systems operating with narrow slots.

Depending on the sand contamination of the pulp furnish, the installation of special

sand cleaners in between stages may be required to reduce the accumulation

of sand in the system.

Similar to pressure screening systems, hydrocyclones are normally arranged in

feedback cascades (Fig. 6.27). At four to five stages, cleaning systems often have

more stages than screening systems with two to four. This is stimulated by a lower

quantity of contaminants in the feed of cleaning systems and more difficult separation

tasks.

J

R

D

D

D

A

F

Fig. 6.27 Four-stage cleaning in cascade feedback arrangement

preceded by protecting pressure screen. F = System feed;J = Junk;

A = System accept;R = System reject;D = Dilution.

In a cascade feed-forward scenario, accepts from other stages are mixed with the

primary accept. Figure 6.28 illustrates a simple two-stage feed-forward system,

which is common for the barrier screening application of knot removal. The secondary

screen of the knot removal system is usually a piece of equipment which

combines several unit operations, including screening, washing and dewatering.

6.8 Systems for Contaminant Removal and Fractionation 597

A

A

R

R

F

Fig. 6.28 Two-stage screening in cascade feed forward

arrangement. F = Feed;A = Accept;R = Reject.

In a cascade feed-forward system for shive removal, the reject from the secondary

screen could be treated in a refiner, after which the accepts of the tertiary

screen could be combined with the accepts of the primary screen, while the rejects

of the tertiary screen go back to the refiner. However, the quality requirements of

chemical pulps do not, in most cases, allow feed-forward operation of shive

screening and, in some cases, not even reject refining.

6.8.2.2 Fiber Loss versus Efficiency

For an exemplary shive screening application where an incoming pulp contains

1% of shives, Fig. 6.29 shows the mass balance for pulp over a single screen,

assuming a 20% mass reject rate and a 90% shive removal efficiency. The reject

stream contains a huge amount of good fibers (in fact 95% of the rejected pulp)

and almost one-fifth of the good fibers from the feed pulp are lost to the reject.

Total pulp

Shives

500 t/d

5.0 t/d

Total pulp

Shives

400 t/d

0.5 t/d

Total pulp

Shives

100 t/d

4.5 t/d

Feed Accept

Reject

Fig. 6.29 Mass balance for single screen;20% mass reject

rate, 90% shive removal efficiency.

Keeping the same assumptions (i.e., 20% mass reject rate and 90% shive

removal efficiency in the primary screen), we can consider a three-stage screening

system operated in cascade feedback mode (Fig. 6.30). Due to the repeated screening

action, the amount of good fibers in the system reject is reduced to 1% of the

feed pulp. In general, the good fiber loss can be reduced by adding another

screening stage or by decreasing the reject ratio. However, the flow regime in the

pressure screen places a physical limit on both the reject ratio and the number of

stages in a multi-stage screening system. That is why there is a minimum loss of

598 6 Pulp Screening, Cleaning, and Fractionation

good fibers with the system reject from the last stage of a pressure screening cascade.

When the economic feasibility of equipment and operating costs versus the

loss of good fibers is taken into consideration, the number of stages in a screening

system for shive removal is typically three or four. As an indication, the related loss of

good fibers in everyday operation seldomfalls below the amount of rejected shives.

Total pulp

Shives

500 t/d

5.0 t/d

System feed

Total pulp

Shives

491 t/d

0.55 t/d

Primary accept

Total pulp

Shives

613 t/d

5.55 t/d

Primary feed

Total pulp

Shives

151 t/d

5.49 t/d

Secondary feed

Total pulp

Shives

123 t/d

5.0 t/d

Primary reject

Total pulp

Shives

28 t/d

0.49 t/d

Tertiary accept

Total pulp

Shives

38 t/d

4.94 t/d

Secondary reject

Total pulp

Shives

113 t/d

0.55 t/d

Secondary accept

Total pulp

Shives

9.4 t/d

4.45 t/d

Tertiary reject

PRIMARY SCREEN

SECONDARY

SCREEN

TERTIARY

SCREEN

Fig. 6.30 Mass balance for three-stage feedback cascade;20% primary

mass reject rate, 25% secondary and tertiary mass reject rates, 90% shive

removal efficiency in each screen.

When comparing the single-stage and three-stage screening balances depicted

in Figs. 6.29 and 6.30, another observation relates to the screening efficiency. Due

to the internal circulation within the three-stage system, the accepted pulp contains

10% more shives than the accept from the single-screen case. It should be

noted that multi-stage screening helps to minimize the loss of good fibers but at

the same time reduces the screening efficiency.

6.8.3

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