Fibre consistency

The fibre consistency determines how many fibres will absorb the, in each impact, applied energy. Normally the fibres are refined at a consistency of 3.5 to 4.5% fibres in water.

Refining at a high fibre consistency means that there are many fibres in the refiner gap to absorb the applied energy.

If the consistency is decreased, the number of fibres being able to absorb the applied energy decreases. The stress on the individual fibres becomes greater. The risk for damaging the fibres increases.

When refining the fibre walls are treated. The primary wall, P, is more or less peeled off. The same happens to the outer secondary wall, S1. The secondary wall, S2, with its large number of OH-groups, is released. The outer part of the S2 layer starts to split up and the fibrils partially come loose. This is called outer fibrillation.

Even the inner part of the fibre wall, S2, is influenced by the refining. The fibre wall starts to delaminate; called inner fibrillation. When the outer fibre walls P and S1 are removed and the fibre wall S2 is delaminated, water can penetrate. The fibre wall begins to swell. The water absorption increases the fibre’s softness and pliancy.

Later, when the water between and inside the soft fibres evaporates in the drying section fibres will easily collapse. When the web is finally dried the fibre surfaces come in close contacts and bind strongly to each other.

Thus, the main reason to refine is to attain an outer and an inner fibrillation but some undesired things happen.

When the fibre wall is split up, parts of the wall are peeled off and a lot of fine material is created. The fibre is bent and wrinkled. The fibre may even be pressed together so hard that it will be cut off.

A too intensive refining may be detrimental, the average fibre length decreases, the fibre walls are weakened, the fine material content becomes too high.

The fibre changes may lead to, a stock difficult to dewater, a lower paper strength.

During the refining some substances are released from the fibres. Small amounts of lignin, hemicellulose and wood resin will dissolve or disperse in the water.

source: Lennart Stolpe

Refining

The main reason to refine is to increase the capability of the fibres to form hydrogen bonds. Unrefined fibres are stiff and the amount of OH groups on the fibre surface is low.

If paper is made from unrefined fibres the direct contact area between the fibres in a sheet will be small. There will be only a few hydrogen bonds created, the paper would be weak. During the refining the outer fibre walls are broken up to make it easier for the fibre to absorb water and to
swell. The outer surface area increases and the fibre becomes softer. The number of OH groups on the surface increases

Large areas of direct contact and many OH groups make it possible to form more hydrogen bonds. Consequently, a paper made from refined fibres is strong. Further in the process the fibre’s inner cavity, lumen, may fall in and the fibre will resemble a band. The fibre collapses.

Stock preparation

The part of the paper making process in which the pulp is treated mechanically and sometimes chemically is called the stock preparation department. To be able to produce a good and strong paper from chemical fibres, the fibres must be refined.

In the stock preparation department different types of pulps are mixed, for example long fibre
softwood with short fibre hardwood. To improve the paper properties or to make the paper production less expensive, different kinds of chemicals and fillers are added.

Before the pulp is pumped to the paper machine it must be diluted, cleaned, and maybe deaerated and screened a last time.

Paper machine

Paper Machine Overview

Forming zone (wire part)

Remark:

1. Headbox
2. Forming board
3. Foils
4. Wet suction boxes
5. Sym-Former
6. Dry suction boxes
7. Couch roll
8. Pick up roll
9. Wire driving
10. Auto guide roll
11. Wire roll
12. Breast roll
13. Wire (forming fabric)
14. Web

Press Zone

Remark

1. Couch roll
2. Felt
3. Press roll
4. Web (red line)
5. Cylinder Dryer

Drying Zone

Source: Metso

Papermaking

This page contains sites of learn about papermaking.

1. Papermaking Wet-End Chemistry is explained more specific at ncsu
2. Basic about papermaking
3. Paper physics information from Charles Green, with an emphasis on paper curl, dimensional stability, and shrinkage of xerographic papers.
4. Free Journal about papermaking

Dry suction boxes

The vacuum level in the dry suction boxes is higher than in the wet ones. In most cases the levels are between 10 and 30 kPa (1 −3 metres columns of water).

The water can be removed only at a limited rate. If the suction increases too much, the wire is pressed harder against the blades of the suction box, and the friction becomes greater. The increased friction causes the wire and the blades of the suction box to get unnecessarily worn and the force moving the wire strongly increases.

The blades of the suction boxes are quite plane. Now the fibres are totally fixed and it is not necessary to create turbulence.

Wet suction boxes

The more the fibre network grows, the more difficult it will be to drain the stock. A way to Overcome the increasing drainage resistance is to gather the foils in closed boxes and to let them work in a vacuum.

The underpressure between the foils makes the suction pulses longer and more water is sucked
through the fibre net.

The vacuum in the suction boxes is increased keeping step with the growth of the fibre network.
When the wire has passed the last suction box, the free stock is in most cases gone and the water mirror on the wire has disappeared. This point is called the wet line. All fibres are now fixed in the fibre network and the forming is in principle finished.