WO1989002951A1

WO1989002951A1 - Method of impregnating lignocellulose material

Info

Publication number: WO1989002951A1
Application number: PCT/SE1988/000458
Authority: WO
Inventors: H. S. Ingemar Bystedt; Jan-Olof G. S. Sehlin
Original assignee: Sunds Defibrator Aktiebolag
Priority date: 1987-09-29
Filing date: 1988-09-08
Publication date: 1989-04-06
Also published as: SE8703744D0; AU2525088A; SE461796B; SE8703744L

Abstract

Lignocellulose material in the form of pieces with a length in the fibre direction of at least 100 mm is compressed perpendicularly to the fibre direction of the material to such a degree, that its pores substantially are compressed. The material then is allowed to expand in impregnation liquid. Thereafter the material is disintegrated to fibre bundles and/or individual fibres.

Description

Method of impregnating lignocellulose material

This invention relates to a method of impregnating lignocellulose material in piece form with a length in fibre direction of at least 100 mm. The lignocellulose material which, for example, can be different kinds of wood or bamboo, is intended for the manufacture of pulp to be used in different paper and cardboard qualities etc.

At all pulp production comprising treatment of the starting material with a liquid, which possibly in¬ cludes chemicals, it is of greatest importance, that the liquid gets into contact with the entire material- amount as intimately and uniformly as possible. This is normally achieved by chipping the material to chips, which then are soaked with the liquid. It is tried by different methods, such as vacuum evacuation, pre- -εteaming, hydrostatic pressure etc., to facilitate the penetration of the liquid into the chip pieces. The resistance of the fibre material to liquid pene¬ tration, the irregular form of the chip pieces etc., however, render it difficult to achieve rapid and uniform impregnation of the chips.

The complex of impregnation problems is intimately associated with the structure of the fibre material. In the following the structure of wood is dealt with, but the reasoning can be applied to other lignocell¬ ulose materials.

The solid wood substance has for all kinds of wood a density of about 1,50. The wood further includes a varying amount of pores in the form of lumens, i.e. the central cavity of the fibres and other wood cells, and other cavities, which are filled with air or liquid, so that the density of the dry wood varies between about 0,2 for balsa and 1,2 for guajacholz. The wood is hygroscopic and takes up water by chemical sorption, adsorption in the cell walls and capillary condensation in the submicroscopic structure of the wood substance. This water take-up, which takes place causing swelling, continues up to the so-called fibre saturation point, at which the submicroscopic capill¬ aries are filled with water. The moisture quotient of the wood, i.e. the moisture content based on bone dry wood, then is in an average 28$ and varies between 22$ for coniferous kind of wood, which is heavily resinous, and 3 -35% for certain deciduous kind of wood, a.o. beech, according to Kollmann, Technologie des Holzes und der Holzwerkstoffe, second edition, first volume, Springer Verlag 1951, page 39 . Above the fibre satur¬ ation point, the wood then can take up additional water by filling lumens and other cavities in the wood. This take-up takes place without increase in volume and depends on the porosity of the wood and can amout to 200-300% or more for highly porous kinds of wood. According to Kollmann, page 333, the following applies to wood, which has taken up water to the fibre saturat¬ ion point or above said point

^Rmax ⁼l÷θ'28?i,$0 = ^ °^ ^^/ά^ where Rmax is the maximum dry density of the wood free of pores at swelling maximum.

At impregnation it is desired to fill the pores of the wood with impregnation liquid as completely as possible.

Then the delignification at the reaction of the wood with the chemicals is rapid and uniform, so that the reaction time can be kept short and the pulp produced has a high and uniform quality.

The impregnation can be carried out in different ways, for example by atmospheric suction or by pressure impregnation, at which the liquid is pressed into the wood by means of hydrostatic pressure. These methods and similar ones have the disadvantage, that air re¬ maining in the pores is locked up and prevents the liquid to enter. It is, therefore, tried in different ways to drive the air out, by steaming, vacuum suction etc.

It is usual to impregnate wood in chip form by compress¬ ing it and cause it to expand in the liquid. In most cases this is carried out by compressing the chips in a compressing screw conveyor, for example according to SE-PS 160636. The application of the pressure, how¬ ever, which is required for effectively compressing the pores of the wood results in so strong friction forces and so great wear of the apparatus equipment, that it is practically unfeasible. The compression in a screw feeder for chips, therefore, in practice consists to the greatest extent of compacting the chips, so that air and possibly water between the chip pieces are driven out.

In order to achieve higher compression, it was also tried to press chips between rolls, for example accord¬ ing to CA-PS 677 4l8. Due to small particles of non- -unifor size and shape constituting the chips, however, this method is difficult and therefore not applied in practice.

Mechanical pulps are manufactured of wood in two ways, which are different in principle, viz. by grinding blocks or refining chips. At grinding, the block, i .^' e . the cross-cut logs, is pressed against a rotating grindstone while water is being added. At refining, the chips are disintegrated to pulp between two refin¬ ing discs rotating relative to each other. The method can be completed by heating the chips with steam at overpressure prior to and during the refining, so-called thermomechanical process, and/or impregnating and poss¬ ibly digesting the chips with a chemical solution prior to the refining, so-called chemi-mechanical process. By these methods, especially the chemi-mechanical one, long-fibred pulps are obtained, the strength properties of which are better than those obtained at grinding, but the optical properties are somewhat worse. The great disadvantage, however, is the very high energy consumption, which is substantially higher than at grind¬ ing. This is thought to be due to the fact, that the chip pieces whirl about between the refining discs without definite orientation of the fibres, while at grinding the block is held steady and is pressed against the grindstone with the fibres oriented in one and the same direction, i.e. in the plane of the grinding surface perpendicularly to its direction of movement. Only the outermost thin wood layer closest to the grinding surface is defibered, while the remain¬ ing part of the wood is unaffected and does not consume any unnecessary energy.

In order to make the groundwood pulp more long-fibred and stronger, the block was impregnated and digested, for example according to US-PS 2 713 5^0. This method, however, is very troublesome and results in an unimpregn- ated core, which assumes dark colour at digestion. Impregnated chips also have been ground, for example according to Tappi Journal, May 1987, vol. 70, No. 5, page 119-123. As the chip pieces are oriented random in relation to the grinding surface and its direction of movement, the resulting pulp is non-uniform and has high shives content.

It was found practically impossible to orient the chips at grinding, due to the shape of the chip pieces and their short extension in the fibre direction. In the patent application SE 860^769-3 a method has been proposed to manufacture mechanical pulps by pressing impregnated long wood against a defibering disc, for example a grinding disc. The wood had been chopped before the impregnation in order to facilitate the same.

The present invention has the object at this method and at other methods of manufacturing papermaking pulp from lignocellulose material in the form of pieces with a length in the fibre direction of at least 100 mm to achieve an impregnation which is better than at methods used heretofore. This and other objects are achieved by the method according to the invention as it is defined in the attached claims.

The invention is described in greater detail in the following, with reference to the accompanying drawings, in which Fig. 1 shows the moisture quotient of beech wood after compression with different pressures, Fig. 2 shows the density of beech wood after compression with different pressures and after expansion, Figs. 3 and shows an embodiment of a device for carrying out the method according to the invention, Fig. 3 being a vert¬ ical section and Fig. 4 a perspective sketch. The most effective and rapid way of impregnating the wood was found to compress the wood so that the pores are compressed,and then cause the wood to expand below the liquid level. When the pores then are restored, the liquid is sucked into them.

A number of experiments have been carried out to com¬ press beech wood. Blocks of the size 100x100x50 mm were heated to 100°C and thereafter compressed in a laboratory press. The following results were obtained, , based on 100 g bone-dry wood.

The moisture quotient and density after compression are also shown graphically in the diagrams in Figs. 1 and 2. They seem to asymptotically approach values, which well agree with those stated by Kollmann, see above. This implies, that at sufficiently high compress¬ ion lumen and other cavities are completely compressed, while the submicroscopic pores, which still contain the capillary bound water, remain.

The diagram in Fig. 2 also shows, that the curve for density is very flat at the beginning, proceeds steep in the range of about 5-15 MPa and then deflects to the asymptote.

The density after expansion to zero pressure also has been included in the diagram. This shows, that the greater part of the compression springs back, but for the higher pressure there remains a not insign¬ ificant deformation.

The conclusion from these experiments is, that pressures above about 5 MPa must be used for obtaining an eff¬ ective compression of the wood. A pressure of above 10, preferably 12,5-15 MPa or still higher, 15-30 MPa is desirable. Extrapolation of the curves in Figs. 1 and^"2, however, shows that pressures above about 25 MPa scarcely are motivated.

It also is worth to note, that so much liquid has been pressed out, that a dry solids content for the wood of more than 70% was achieved.

According to the embodiment shown in Figs. 3 and 4, the material pieces or staves .have a length in the fibre direction of at least 100 mm, suitably at least 200 mm and preferably at least 500 mm and a smallest measure across the fibre direction of at maximum 50 mm, preferably at maximum 25 mm. All staves 1, however, should be of equal length. They have first been soft¬ ened, for example by steaming.

The staves 1 are directed via a feed shaft 2 into the nip between two counter-rotating rolls 3. The fibre direction of the material is in parallel with the roll axles, and the rolls are slightly longer than the staves. The rolls are mounted in a trough 5 and rotated each by a motor (not shown). The trough is filled with impregnation liquid up to the nip. The nip is adjusted by taking up one roll against the other by means of hydraulic cylinders 4. The rolls can be fluted in order to facilitate the staves to be taken along.

Upon their arrival in the nip the staves are compressed so strongly, that the pores are compressed substant¬ ially entirely. Air, water and extractive matter are pressed out thereby. The staves at the same time are rolled out perpendicularly to the fibre direction and thereby assume a greater width, become thinner and get a larger surface. After the nip the material ex¬ pands again, whereby the pores are restored. The im¬ pregnation liquid then is sucked into the pores. The staves then are transported upward out of the bath by a conveyor 6 to be further processed, for example by digestion in steam phase. The staves thereby ace maintained, their fibres oriented in one and the same direction. Thereafter the material is disintegrated to fibre bundl¬ es and/or individual fibres. This is achieved prefer¬ ably by disintegration in a mechanical way to fibres and fibre fragments, for example by pressing the material with liquid added thereto against a rotating defibering member, such as a grindstone, with the fibre direction substantially in the same plane as the defibering surf¬ ace, against which the material is pressed, and perp¬ endicularly to the direction of movement of said surface.

The digestion or chemical delignification of the im¬ pregnated material possibly can be carried out to such an extent, that only insignificant or no mechanic¬ al aftertreatment at all is required for exposing the fibres.

For the impregnation different liquids can be used, for example water. Normally an aqueous solution is used, for example of a base, such as caustic soda, a per compound, such as hydroperoxide, a salt of sul¬ phurous acid, such as sodium sulphite or sodium hydrogen sulphite, or a mixture of some of these chemicals. As an additional effect of the high-pressure pressing the material is deformed, whereby the fibre bond is broken up. This renders it easier for the liquid to penetrate into the substance. Owing to the combined chemical and mechanical action, the fibres partially are set free, so that the continued fibre exposure is facilitated, which spares the fibres and reduces the energy consumption.

A number of experiments have been carried out, partly with aspen wood and partly with fir wood. Experiments with aspen wood

The aspen wood was dried to a dry solids content of 83%, which corresponds to a moisture quotient of 20$. The wood was sawn^'to^'pieces about 100 mm long in the fibre direction and 40x10 mm across the fibre direct¬ ion. The wood pieces were ground to pulp in a labor¬ atory grinder, partly untreated, partly after impregn¬ ation with water, partly after impregnation with a caustic soda solution. The following data applied to the impregnation: Softening in boiling water, min Pressure at wood compression,MPa Liquor concentration, NaOH g/1 Liquid temperature C Reaction time min

NaOH amount taken up in bone-dry wood % 0 2,3 4,7

Acidity of impregnated wood after neutralization with sodium bisulphite pH - 5,9 6,0

The ground wood was formed to sheets and tested. For comparison, also data for a typical refined chemi- mechanical pulp (CTMP) have been included. The following results tv&re obtained

The pulp from the untreated dry wood is very weak, the water-impregnated one is comparable with normal aspen groundwood pulp, while at impregnation with 4,7% NaOH a pulp is obtained, which is comparable with CTMP at refining of chips. The shiver content is very low. The most remarkable feature is that the energy consumption according to the invention was only about 0% of the energy consumption for the CTMP pulp.

Experiments with fir wood

A similar series of experiments was carried out with dried fir -wood, which also had a dry solids content of 83%. No softening in boiling water was carried out. The wood was impregnated with a solution of sodium sulphite and then digested in steam phase. The follow¬ ing data applied.

Pressure at wood compression MPa

Concentration Na₂S0, g/1

Impregnation temperature oC

Impregnation time min Amount of Na-SO,. taken up in bone-dry wood % Digestion temperature °C Digestion time min Acidity after digestion pH

The following test results were obtained for the ground pulps

Tensile index KNm/kg

2 Tear index Nm /kg

Diffuse blue reflect¬ ance factor % g₂ 67 66 6l Diffusion coefficient m /kg ₇₁ -₅₀ 40

Energy consumption kWh/t bone-dry 984 1290 977 24θ( As in the case of aspen wood, the pulp of the dry wood was very weak, so that it could not be formed to sheets at all. From the water-impregnated wood a normal groundwood pulp was obtained, while the pulp from the wood, which had been impregnated and digested, is comparable with CTMP. It is, however, slightly cut down, as the fibre fractioning shows. This deteriorates the strength, especially the tearing resistance. The conditions have to be optimized, especially for the grinding.

The energy consumption is only 40% of the energy consumption for CTMP, as in the case of aspen wood. Other embodiments are possible within the scope of the invention idea. The steaming prior to the compress¬ ion can be left out or replaced by some other softening, The compression can be carried out in a way other than between rolls, for example between two or more press plates. The impregnation liquid can be added in different ways, for example by spraying on the com¬ pressed fibre material.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the invention idea.

Claims

1. A method of impregnating lignocellulose material in the form of pieces with a length in the fibre dir¬ ection of at least 100 mm, c h a r a c t e r i z e d i n that the material first is compressed perpend¬ icularly to the fibre direction to such a degree, that its pores are substantially compressed, that the mat¬ erial then is allowed to expand in the impregnation liquid, and that the material thereafter is disinte¬ grated to fibre bundles and/or individual fibres.

2. A method as defined in claim 1, c h a r a c t ¬ e r i z e d i n that the pressure at the compress¬ ion of the material is at least 5 MPa, suitably at least 10 MPa, preferably 15-30 MPa.

3. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the thickness of the material pieces prior to the im¬ pregnation is reduced by cleaving along the fibre direction of the material to a smallest cross-section of at maximum 50 mm, preferably at maximum mm.

4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the material pieces are softened prior to the compression.

5. A method as defined in claim 4, c h a r a c t ¬ e r i z e d i n that the material pieces are steam¬ ed immediately prior to the compression.

6. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the compression takes place in the nip between two counter-rotating rolls.

7. A method as defined in claim 6, c h a r a c t ¬ e r i z e d i n that the fibre direction of the material is in parallel with the roll axles.

8. A method as defined in any one of the claims 1-5, c h a r a c t e r i z e d i n that the compression takes place between two or more press plates.

9. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the impregnated material is disintegrated in a mechanical way to individual fibres and fibre fragments.

10. A method as defined in claim 9, c h a r a c t - e r i z e d i n that the impregnated material is disintegrated by pressing it while adding liquid thereto against a rotating defibering member, such as a grindstone, with the fibre direction substant¬ ially in the same plane as the defibering surface, against which ifceis pressed, and perpendicularly to the direction of movement thereof.