WO1990005208A1

WO1990005208A1 - Process for reducing the consumption of energy during the mechanical production of paper pulp

Info

Publication number: WO1990005208A1
Application number: PCT/SE1989/000636
Authority: WO
Inventors: Sven Ljungbo
Original assignee: Sven Ljungbo
Priority date: 1988-11-10
Filing date: 1989-11-07
Publication date: 1990-05-17
Also published as: AU4509789A

Abstract

The invention relates to a new method for producing paper pulp by mechanical means and refining the paper pulp. It consists in saturating channels - lumina, pores - in the raw fibrous material, which may be wood, straw, plant leaves or conventionally produced paper pulp with water and then freezing the water in the fibres to ice, whereupon the mechanical processing - defibration or refining - is carried out at this freezing temperature.

Description

Process for reducing the consumption of energy during the mechanical -production of paper pulp

.ϊixh present methods, mechanical production of paper pulp by, for instance, the thermomechanical (TMP) or chemothermomechanical or grinding methods requires a great deal of mechanical energy. Certainly it has proved possible to reduce, with the grinding methods, the consumption of mechanical energy by increasing, with the so-called pressure- rinding method, the process temperature to such a degree as to cause it to amount, in the same way as with the TMP method, to about 120°C. A further reduction in energy consumption has been achieved with the so-called super-pressure-grinding method by increasing the temperature to about - A ° C.

Nevertheless, the consumption of mechanical energy is, with these methods for producing mechanical paper pulp,undesirably high.

According to the present invention it is now possible to increase the saving cf mechanical energy by instead lowering the process temperature, with these and similar methods of producing paper pulp. Even a reduction to less than +5°C results in a 'certain saving. However, a considerable reduction in energy consumption is achieved if the temperature is lowered to 0°C or less. The energy saving is maximised if the cavities in the raw fibrous material - with wood mainly lumina, and with stalks or leaves^' the corresponding channels - are filled entirely or partly with water, prior to freezing to a temperature of less than 0°C.

The reason why the energy consumption drops at lower temperatures is not established, but the following connection seems likely:

When the temperature drops, the wood or straw becomes more rigid and approaches a brittle state. As a result defibration, largely a process of crushing, can be achieved more easily.

If the cavities in the fibres are filled with water and then frozen to ice, the ice expands as the temperature declines further, whereas the walls of the fibres shrink. This causes stresses in the walls of the fibres, as a result of which they split more easily during mechanical processing.

With defibration/crushing, the threads of ice within the fibres probably act in the manner of wedges.

The raw fibrous material can be filled with water in various ways.

It is for instance possible to evacuate the raw fibrous material under water until the air has been removed from the fibres and replaced by water.

If the raw fibrous material is fairly finely dispersed - wood chips, chopped stalks cr leaves - it is sufficient to let it lie in the water for some time. During this process one can accelerate the penetration of water by adding to the water 1 - 2 pro mille of some wetting agent and/or a similar amount of caustic soda solution or a different alkali. Also vibrating the ^' aqueous mixture will speed up penetration of the water.

The following examples illustrate some embodiments of the invention without limiting the latter.

Example 1

White-wood chips are introduced into a tank with water containing 0.1 sodium oleate and 0.1 9^ sodium^"hydroxide. A vibrator rod is lowered into the tank and vibrates its contents. Once most of the chips have sunk to the ground, the remaining chips are skimmed off, whereupon the water-filled chips from the bottom are taken up and centrifuged so as to remove the surface water. The chips are then frozen by blowing air at a temperature of -10°C through them.

The frozen chips are then defibrated within a defibrator cooled down to a temperature of -10°C, air at -10°C being likewise blown through said defibrator during the defibration process.

The fibrous mass obtained is again frozen to -10°C and then refined in a breaker roll refiner or turbo-refiner or perfecting engine or in another grinding apparatus, which is also cooled to -10°G.

Soap and caustic soda solution are washed away from the paper pulp produced. If necessary, the remaining soap residues are precipitated by means of aluminium sulphate or calcium chloride, before the pulp is used for the production of paper.

Example 2

Wooden blocks are saturated with water by lowering them into the water containing soap and soda solution as in example 1 and evacuating the air from the tank at the same time as its contents are vibrated.

The water-logged wooden blocks are frozen to a temperature of -10°C, whereupon they are ground in a grinder, which is also kept at a temperature of -10°C.

The paper pulp produced is refined as in example 1.

Example 3

Wooden chips are saturated with water as in example 1 and frozen* o a temperature of -30°C. Then they are lowered into methylchloride at the same temperature and subjected to powerful high-frequency vibrations or ultrasound.

As much of the methylchloride as possible is pressed out of the resultant paper pulp for further use, whereupon the pulp is stirred out in the water. The residual methylchloride decocted during this process is recovered, and the pulp is used, possibly after refining, for the production of paper.

**•

Example 4

Wooden chips are saturated with water by submersion in water and simultaneous vibration. Once most of the chips have sunk to the bottom, the remaining floating chips are skimmed off, whereupon the more or less water¬ logged chips which have sunk to the bottom are freed from surface water and frozen to a temperature between -5 and -30°C. However it is also possible, prior to the subsequent crushing process, to freeze the chips to lower temperatures, but this requires more energy without yielding any significant advantages.

The chips are frozen in known manner according to a process developed - by Infrasonik AB in Stockholm, by causing the chips to drop over the bunch of cooling tubes in a vertical shaft while at the same time generating infrasound in the, shaft.

Then the frozen chips are crushed in a roll-type crusher or roller press, with the rollers and other devices which come into contact with the chips cooled so as to keep them at a temperature between -5 and^' -30°C.

This cooling effect can be brought about by blowing through cold air or by cooling fluid from a refrigeration machine flowing through ducts within the crushing tool.

Only one passage through the roll-type crusher will not suffice to process the fibres adequately. That is why one passes the crushed fibrous mass several times through the mill or passes it through several roll-type crushers in sequence. In the latter case the gap between the rolls must be reduced from mill to mill, since the fibrous mass is discharged from every mill as a compact, dimensionally stable, frozen cake, the thickness of which equals the width of the gap. Alternatively it is also possible to tear the cake apart and to feed it as a granular mass into the next mill or to circulate it within the mill already passed.

It is not necessary to process the fibrous mass with roll-type crushers or roller presses in every stage. After the initial crushing operation processing, which corresponds to refining with the present TMP and grinding processes, can be effected with the aid of other crushing machines such as rodmills, vibrating rodmills, ballmills, vibrating crushers with flat or grooved jaws, mills with vibrating rolls and other machines for crushing or grinding the material to granulate or powder.

It is however essential for these machines too to be cooled so that the fibrous mass remains frozen throughout the entire process. It may be possible to waive this requirement in the final stages, of the process, if the degree of refining required is small.

Also in the first crushing stage of the process use can be made of some of these other crushing machines. Especially stamping crushers, vibrating crushers, vibrating rodmills, roller presses or roll-type crushers with vibrating rolls are suitable alternatives for conventional roll-type crushers, at the first stage of crushing. Also other machines can be used for breaking up the water-logged, frozen raw fibrous material such as cutters for milling wood to thin chips, which, if constantly frozen, can readily be refined to paper pulp, hammer mills and other machines for disintegrating materials.

Naturally the process according to the invention can be combined with a light chemical treatment of the fibrous mass so as to achieve correspondence wi h CTMP. This chemical treatment can then be applied either prior to freezing and defibration of the raw fibrous material or after the mechanical production of the paper pulp or so that. he chemical impregnation takes place prior to freezing - defibration, whereas heating with a view to carrying out the chemical process is effected after defibration or refining.

The process according to the invention of freezing the mass of water¬ logged fibres prior to mechanical processing of a fibrous raw material for paper and of processing at freezing temperatures can also be used for refining chemical paper pulp or of ground, TIv-P or CTMP pulp produced according to conventional methods.

It is known from US 2638415 and US 4 **12888 how to freeze moisture- saturated paper pulp and cause it to thaw after several hours in the frozen state. By this means one achieves certain improvements of the characteristics of the pulp. However, there is no grinding subsequent to the freezing process, and these patents do not therefore anticipate the present invention. πs 3771 729, US 3 774855 and US 4073443 describe machines used for pulverising materials which are difficult to crush such as elastomers, certain pigments or certain spices at very low temperatures, i.e. cryo- temperatures. They do not mention the possibility of defibrating water_¬ logged, frozen fibrous material so as to produce a mass the fibrous structure of which is retained and which can be used by way of paper pulp. They do not therefore anticipate the present invention.

Claims

1. Process for mechanically producing paper pulp from the fibrous raw materials wood, straw or plant leaves, characterised in that one first introduces water into the inner channels such as lumina and pores of the raw fibrous material, then cools the raw fibrous material in such a way that the water freezes to ice, and then, while maintaining the freezing temperature, freezes the raw fibrous material mechanically so that it is broken up into fibres and fibre fragments.

2. Process according to claim 1, characterised in that mechanical processing consists mainly in a combination of crushing and frictional processing.

3. Process according to claim 1, characterised in that the mechanical processing is largely of tearing type.

4. Process according to claim 1, characterised in that the mechanical processing is effected by means of ultrasound.

5. Process according to claim 1, characterised in that the mechanical processing is effected mainly by crushing. β. Process according to claim 1, characterised in that the process is used for refining conventionally produced paper pulp of the types sulphate pulp, sulphite pulp, semi-chemical pulp, bleached pulp, ground pulp, thermomechanical pulp and chemo-thermomechanical pulp.