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4.2 Refiner Processes

Fig. 4.34 Selected examples of refiner plate designs.

Refiner plates are hard-wearing tools with a working lifetime of between 300

and 1000 h. Selected examples of mechanical pulp lines using the refiner process

are shown in Figs. 4.35–4.37.

Fig. 4.35 Single-stage TMP line with Metso-Double disc

refiners RGP 68DD (Union Bruk Norway, 1999).

1109

4 Mechanical Pulping Processes

Fig. 4.36 Two-stage TMP line with two-stage reject refining

and fractionation for LWC paper (Metso Paper).

Fig. 4.37 Modern CTMP refining in one or two stages (Metso Paper).

To date, the production of pulp for fluting has been the most common use of

semi-chemical hardwood pulps. The dominant process for this product is the neutral

sulfite semi-chemical process (NSSC), with sodium or ammonium sulfite as

cooking chemicals. The pulp yield is in the range of 70–80% depending on the

wood species. Birch, beech, maple, oak and eucalyptus are the most frequently

used hardwoods for fluting production, often as the sole component in furnish.

Primarily, the demand on a fluting pulp is high stiffness and good crush resistance.

A combination of short, stiff fibers and a high proportion of hemicellulose

In hardwoods makes them more favorable than softwoods for this purpose. A

semi-mechanical pulping process is illustrated schematically in Fig. 4.38.

1110

4.2 Refiner Processes

Fig. 4.38 Semi-mechanical pulping process.

1111

1113

5

Processing of Mechanical Pulp and Reject Handling:

Screening and Cleaning

Jurgen Blechschmidt and Sabine Heinemann

5.1

Basic Principles and Parameters

The aim of mechanical defibration of wood is the deliberation of fibers from the

wood, without their destruction. This happens only ideally, and in reality a mixture

of different fiber components and debris is created immediately after defibration,

and is characterized as follows:

_ Shives (unsuitable particles that must be separated from the

pulp).

_ Fibers, divided into:

Long fibers; length 800–4500 lm; width 25–80 lm

Short fibers; length 200–800 lm; width 2.5–25 lm

_ Fines, divided into:

Fibrillar fines (slime stuff); length up to 200 lm, width about

1 lm

Flake-like fines (flour stuff); length 20–30 lm; width 1–30 lm

Additionally, wood pieces of different dimensions and shapes, sand and other

nonwood particles are found (Fig. 5.1).

Separating by suitable screening brings the mechanical pulp to the following

composition, depending on its process parameters during defibration:

_ Shives content: 2–6% (extreme values up to 20%)

_ Fiber content: 50–80%

Long fiber content: 25–55%

Short fiber content:25–40%

_ Fines content: 20–50%

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Isbn: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co .

Handbook of Pulp

Edited by Herbert Sixta

Fig. 5.1 Ground wood components. (a) Long fibers;

(b) short fibers; (c) flake-like fines; (d) fibrillar fines.

There are two basic process principles for screening in mechanical pulping:

_ Pulp classification –that is, separation of the pulp into fractions

of different particles size (applied for shives removal fromthe pulp).

_ Separation according to particle density –that is, separation of

pulp and minerals such as sand and other heavy material (applied

for pulp cleaning).

The separation process can be characterized by a simple scheme, as shown in

Fig. 5.2.

Feed Accept

Reject

Fig. 5.2 Parameters of the screening principle.

5.2

Machines and Aggregates for Screening and Cleaning

No screening process can result in a complete or exact separation of accepts and

rejects. There are always particles retained in the reject that are smaller than the

separation size (slot width or mesh size), and there are also larger but slender and

flexible particles in the accept.