Basic System Design Principles

We have seen above that there are various purposes for operating a screening or

cleaning system. While fractionation is of increasing interest, most applications

594 6 Pulp Screening, Cleaning, and Fractionation

still target the removal of large, heavy-weight, or light-weight contaminants. There

is a fundamental difference between contaminant removal and fractionation with

respect to the amount of material to be separated. After fractionation, the smaller

pulp fraction is seldom less than 20% of the pulp in the feed stream. In contrast,

the contaminants to be removed during screening or cleaning are typically no

more than 3% of the feed stream pulp.

Both contaminant removal and fractionation are subject to the condition that

the rejected portion contains only a minimum of the acceptable portion. Modern

screening and cleaning equipment removes unwanted matter quite efficiently

from the feed stream and produces an accept stream of high purity. In order to

achieve this, the reject stream must contain a relatively large amount of acceptable

material in addition to the matter to be rejected.

In a contaminant-removal system, economic reasons call for the minimization

of good fibers lost with the removed contaminants. Such systems usually consist

of a number of separators which can be operated in different arrangements. On

the one hand, contaminant removal is usually most efficient in a cascade feedback

arrangement. On the other hand, generally accepted rules for designing fractionation

systems are yet to be developed. In fact, it is uncertain if such rules will ever

exist, as fractionation tasks are custom-designed for a particular application.

In many cases, the design of separation systems is based on experience and

rules of thumb, because the interrelation of equipment, operating and pulp furnish

parameters is not yet fully understood. The resulting systems are often safe

to operate, but do not necessarily represent the best process solution and economy.

Screening and cleaning systems tend generally to be complex because of the

large number of design and operating parameters. Their function is challenged by

the circumstance that the optimum performance of the system is typically

achieved with equipment working near its point of failure (i.e., plugging). As

mechanistic models are further developed, the basic understanding of effects on

screen capacity, reject thickening and screening efficiency will improve. Computer

simulation provides valuable support in this respect [19,29].

In the following sections we will examine some common systems for contaminant

removal, as well as a few potential fractionation systems. However, before

doing this it may be appropriate to highlight some general aspects regarding the

design of separation systems.

Slotted screen baskets are quite susceptible to damage by junk material such as

metal bolts or rocks. A damaged screen basket leads to inferior screening efficiency

and requires costly replacement. Therefore, it has proven advantageous to

protect slotted screens from junk by the installation of an upstream perforated

screen. A protective screen is also highly recommended for cleaning systems to

avoid damage or blocking of hydrocyclone cones. When the amount of junk material

is low, protective screens can be operated with intermittent reject discharge.

As a result of reject thickening, industrial separation techniques involve dilution

at various points, both in the form of internal dilution to the equipment and

in the form of dilution between stages. The objective of dilution is first, to avoid

6.8 Systems for Contaminant Removal and Fractionation 595

plugging at the reject outlet and second, to adjust the feed consistency between

stages. It should be noted that most of the illustrations in this chapter lack such

dilution streams in order to avoid unnecessary complexity.

6.8.2

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