WO1990000213A1

WO1990000213A1 - Lignocellulosic material and a method for its manufacture

Info

Publication number: WO1990000213A1
Application number: PCT/SE1989/000357
Authority: WO
Inventors: Jonas Arne Ingvar Lindahl
Original assignee: Mo Och Domsjö Aktiebolag
Priority date: 1988-06-29
Filing date: 1989-06-21
Publication date: 1990-01-11
Also published as: SE8802450L; SE8802450D0; SE461472B

Abstract

Lignocellulosic material of sheet structure and a dry solids content > 80 %, preferably > 85 %. The material is characterized by a density which lies within the range of 400-575 kg/m3 measured in accordance with the standard method SCAN-P7:75 and a disintegration ability calculated as the amount of electrical energy consumed during disintegration of the material in a hammer mill of between 30 and 60 kWh/ton pulp, preferably 35-50 kWh/ton pulp, and in that subsequent to being disintegrated the material has a bulk density of 15-23 cm3/g measured in accordance with the standard method SCAN-C33:80 and a network strength of 2-4.5 N. The invention also relates to a method for producing a continuously wet-formed web of lignocellulosic material, comprising the steps of dewatering and pressing the web to a dry solids content of about 30-60 %, followed by drying the web. The method is characterized by compressing the wet formed web having a dry solids content of at least 50 %, preferably at least 80 % with a linear pressure resulting in a density increase of more than 10 %.

Description

Lignocellulosic material and a method for its manufacture

Technical Field

This invention pertains to lignocellulosic material hav¬ ing a sheet structure which renders the material primarily suited for dry disintegration (i.e. dry shredding) in the manufacture of absorbent materials (for instance fluff). The material can be used also in the manufacture of certain types of board and paper. One particular embodiment of the invention relates to a method which can be applied when thickening and wet forming pulp suspensions on an endless wire followed by drying.

By sheet structure is meant that the lignocellulosic material is in the form of a coherent pulp web which has been rolled up (reel pulp) or divided into sheet form (sheet pulp) . Although the web or sheet thickness may vary, the thickness will lie within the range of 0.1-8 mm.

All percentage values given for concentrations and dry solids content are in percent by weight (w/w) .

Drawings figure 1 is a schematic block view which illustrates a known process for the manufacturing of sheet structures from pulp suspensions. Reference to this known process is made below, under the heading "Standpoint of Techniques".

Figures 2 and 3 illustrate two embodiments of the inven¬ tion. These Figures are referred to in the following de- scription of the inventive embodiments, under the heading "The Invention" . Standpoint of Techniques

When producing pulp of the kinds with which this inven¬ tion is concerned, the pulp is obtained in the form of an aqueous suspension at a pulp consistency (pc) of 0.5-5%, and in which the pulp suspension is delivered to a wet forming and drying machine, via a buffer store (buffer tower) on which a pulp web is wet formed. The known technique by means of which a pulp web is wet formed is illustrated in Figure 1. The pulp suspension is led through a conduit 2, from the buffer tower 1 to a headbox 3 installed in front of the wet forming machine. The suspension exiting from the headbox is distributed across a wire 4 (wire section) provided on the wet forming machine, said wire being moved at a constant speed, e.g. a speed which ranges from 30 to 200 m/min, de¬ pending on the desired thickness of the pulp web. The wire functions as a filter and is effective in dewatering the pulp web in said wire section to a dry solids content of 20-35%. The pulp web is then passed into a press section 5-7, where the pulp web is pressed and dewatered mechanic- ally between the rolls of one or more roll pairs 5-7. When leaving the press section, the pulp web will normally have a dry solids content of 40-50%. According to this known tech¬ nique, the pulp web is then passed into a drying section 8, where further water is removed, such as to bring the pulp web to a dry solids content of 85-95%. The pulp web is dried in the drying section 8, by passing the pulp web between, or over and/or under a plurality of heated cylinders, or is dried by being carried on hot air. The resultant thickness of the pulp web άs achieved in the drying section. Certain types of pulp web can have a tendency of returning to a more loosely knitted or more bulky structure (less compact), the density of the pulp web being lowered to a greater extent than would otherwise have been the case if the thickness of the pulp web had not been influenced in this stage of the process. When applying known techniques, densities in the range

3 of from 300 to 380 kg/m have been achieved in the case of high yield pulps, such as chemithermomechanical pulps

(CTMP). Corresponding density values for chemical pulps are

₃ normally from 500 to 750 kg/m .

After the drying section 8, the pulp web can be divided into sheets, which are stacked in bales (sheet pulp, bale pulp) or the pulp web may alternatively be rolled up onto rolls of relatively large diameter (so-called reeling drums). The resultant so-called jumbo reel is then divided up and coiled into reels of smaller diameter.

The bales and also the reels are then packaged in thick paper or plastic, prior to being transported to a converter.

A third alternative is to disintegrate the pulp web into flakes subsequent to the pressing stage, i.e. immediately after leaving the press section 5-7. The flakes are blown to a drier and then pressed into plates, which normally have a thickness within the interval 10-20 cm. The density of flake

3 dried pulp is normally from 600 to 800 kg/m .

Examples of how pulp can be dewatered, dried and de¬ livered to the consumer are described and illustrated in US-A- ,112,587 (sequence: wet forming with subsequent de- watering between press rolls and drying by passing and pressing the web around drying cylinders, dry disintegra- tion, dry forming, compression, sheeting and baling), US-A-3,047,453 (pelletising) and JS-A-3,627,630 (sequence: dewatering, fluffing and drying with warm air) .

The pulp can be further processed to fluffed form for liquid absorption purposes by a converter. The pulp is thereby disintegrated (defibrated, dry shredded) in, for instance, a disc refiner or a hammer mill, whereafter the disintegrated pulp is blown to a dry forming station, in which the desired end product is formed, this end product being, for instance, soft crepe paper, absorption pads for use in disposable diapers, pant protectors, etc. Disintegra¬ tion ability, measured as the electrical energy consumption ιo of a hammer mill in the manufacture of known types of fluff pulp is normally from 45 to 70 kWh per ton of pulp having a dry solids content in excess of 85%. In order to eliminate the risk of obtaining a poor fluff quality, the disintegra¬ ting of the pulp should not reduce the weight percentage of

" ^~ long fibres by more than 30%. By long fibres is meant here and in the following fibres which are retained on a net structure having a mesh of 30, in accordance with Bauer-McNett.

Technical Problems

20 When manufacturing pulp for making fluff, it is essen¬ tial that the dry pulp can be readily disintegrated (readily dry shredded) . When the amount of energy required herefor is excessive, there is an increased risk of the fibres shorten¬ ing, with subsequent impairment of fluff quality and an in-

25 crease in dust formation (impaired working environment), while increasing disintegration cost at the same time.

It is known that the pressing of a wet formed pulp web to dry solids contents of within the range of 20-50% will greatly increase the density of the pulp web and also in 30 the formation of hydrogen bonds between the fibres. At the same time, the web is imparted a hardness i.a. from the hydrogen bonds formed, which renders the material less suited for liquid absorbing purposes. In an effort to over¬ come this drawback, the pulp suspension has been admixed with chemicals (softeners) which render it difficult for hydrogen bonds to form during the pressing, drying, and storage of the pulp. These softeners, however, have in¬ creased the overall production costs.

The Invention

The invention provides a cellulosic material whose pro¬ perties enable the aforesaid problems to be alleviated with- out the use of auxiliary chemicals herefor. The material is defined as a lignocellulosic material having a sheet struc¬ ture and a dry solids content above 80%, preferably above

85%. The material is characterized by a density which lies

3 within the range of 400-575 kg/m measured in accordance with the standard method SCAN-P7:75, and a disintegrating ability of 30-60 kWh/ton pulp, preferably 35-50 kWh/ton pulp, calculated as the amount of electrical energy consumed when disintegrating the material in a hammer mill. The dis¬ integrated lignocellulosic material shall have a bulk den-

3 sity of 15-23 cm /g, measured according to the standard method SCAN-C33:80 and a network strength measured in accordance with the standard method SCAN-C33:80, which is 2-4.5 N, such as 2.8-4.5 N. The word disintegration as used above corresponds to dry shredding the lignocellulosic mate- rial of the invention to fluffed form, which in turn means that the weight proportion of long fibres shall preferably remain unchanged, but it is acceptable as long as it does not decrease by more than 30% during the disintegration.

The lignocellulosic material is primarily a high yield pulp, such as chemimechanical pulp (CMP), chemithermo- mechanical pulp (CTMP), groundwood pulp (SGW) and refiner pulp (RMP) . The pulp will normally contain a high proportion of long fibres, i.e. >40% (w/w) is retained in accordance with Bauer-McNett on a net structure having a mesh of 30. The pulps concerned here are normally derived from softwood (preferably spruce and pine) and in certain cases from soft¬ wood as aspen.

The invention also relates to a method for producing the aforedescribed lignocellulosic material. The method can also be applied to other types of pulp for instance unbleached and bleached chemical pulps made according to the sulphite or sulphate methods. The pulps concerned may also be long- -fibre fractions of the aforesaid pulp types. Pulp mixtures of said pulps can also be used. Depending on the type of lignocellulosic material, it is possible to produce sheet

3 structures having densities in the range of 400-900 kg/m .

The method is illustrated in respect of two exemplifying embodiments thereof (Figures 2 and 3). The reference nume¬ rals recited below refer to these two Figures. This aspect of the invention can be defined according to known tech¬ niques as a method for manufacturing lignocellulosic pulp of sheet structure, in which a pulp suspension having a pulp consistency of 0.5-5% is formed into a pulp web, and com¬ prises the steps of (i) wet forming and dewatering the pulp suspension on a wire section 4, (ii) pressing the wet formed web to a dry solids content that may be 30-60%, normally about 40-50% in a press stage 5-7; (iii) drying the web in a drying section 8 to a dry solids content that may be above 80%, preferably 85-95%; and (iv) rolling the thus obtained web into reel form, or cutting the web into sheets that are baled. The method according to the invention is character¬ ized in that the pulp web, when said pulp web has a dry solids content in excess of 50%, like in excess of 80% and preferably within the range of 85-95% and primarily sub¬ sequent to the drying stage but prior to being cut, is compressed in a compression stage 9-10 at a linear pressure which will result in a density increase of 10% or more. Nor- mally this implies the application of a linear pressure greater than 30 kN/m measured across the width of the pulp web, and suitably beneath 300 kN/m. The preferred range is 50-200 kN/m. Pulp web compression can be effected in diffe¬ rent types of presses 9-10; for instance the pulp web may be passed and compressed between two surfaces. One way of per¬ forming this is to let the web pass through one or more roll stands whose rolls are made of a suitable hard material, such as steel. Among other presses which can be used in this connection can be mentioned belt presses equipped with flex- ible, converging double plates of the kind capable of being used in the manufacture of chipboard. When the dry solids content of the pulp is 50-95% at the commencement of the compression stage, compression can be effected .suitably while heating the web. When the dry solids content is 85% or higher, the increase in dry solids content resulting from the heating process should not, however, exceed 2%.

Figure 2 illustrates an embodiment in which compression is effected subsequent to drying. Figure 3 illustrates the same embodiment, but with the difference that heating of the pulp web is effected subsequent to compressing the pulp web.

At the time at which the convention period expired high yield pulps, preferably CTMP and SGW, were preferred as lig¬ nocellulosic material. At the same time the most preferred embodiments of the inventive method were to perform the com- pression in a press roll nip or in a belt press, in both cases with a linear pressure within the range of 50-200 kN/m.

Example 1

Reels of CTMP were produced in accordance with known techniques in a pulp mill, the CTMP being intended for the subsequent manufacture of disposable diapers. Peroxide- -bleached, water thinned CTMP having a freeness, according to the standard method SCAN-C21:65 of 650 ml CSF (Canadian Standard Freeness), was passed through a conduit 2, (see Figure 1) to a headbox 3, and delivered therefrom to a wire section 4. The pulp suspension entering the headbox had a pulp consistency (pc) of 1.2%, and the suspension was de- watered on the wire to a dry solids content of 26%. The pulp web was then pressed in three stages 5, 6 and 7. Upon com¬ pletion of the first press stage, the pulp web had a dry solids content of 34%, upon completion of the second press stage 40% and upon completion of the third press stage 44%. The pulp web was then dried to a dry solids content of 91% and rolled onto a reeling drum. The resultant jumbo-reel was divided into smaller and narrower reels. Samples were taken from the jumbo-reel, for the purpose of analyzing the pro¬ perties of the pulp web, the results of these analyses being set forth in Table 1 below.

When practising the present invention, the aforede- scribed procedure was followed up to and including the dry¬ ing stage, i.e. up to and including the drying section 8. The dry pulp web (see Figure 2) was then passed to a con¬ tinuously operating press section 9, 10. The press section comprised two roll pairs 9 and 10 respectively, the rolls being made of steel. The pressure exerted by respective roll pairs 9 and 10 on the pulp web was controlled hydraulically. During the test, a linear pressure of 80 kN/m was measured during passage of the pulp through both the first roll pair 9 and the second roll pair 10. The pulp had a dry solids content of 91.1% prior to passing through the roll pairs 9 and 10, and a dry solids content of 91.3% when leaving the roll pairs. The pulp web exiting from the press section 9-10 was rolled-up on a reeling drum. The resultant jumbo-reel was divided into narrower and smaller reels, for delivery to respective consumers, e.g. converters. Samples were taken from the jumbo-reel and analyzed, the results of these analyses being set forth in Table 1 below.

A comparison between the aforesaid reels showed that the reels produced in accordance with the invention had a markedly smaller diameter than the reels produced in accord¬ ance with the known technique. Measurements taken from seve- ral reels showed that the reels produced in accordance with the invention had a diameter which was, on average, roughly 25% smaller than the reels produced in accordance with the known technique.

Table 1

Pulp sheet properties Known technique According to the invention

2 Grammage, g/m 661 663

Thickness, mm 2.01 1.49

3 Density, kg/m 328 445

As will be seen from these results, the pulp sheet produced in accordance with the invention was substantially thinner than that of the sheet produced in accordance with the known technique. The method of the invention also had a pronounced effect on pulp density and the increase, expressed in rela¬ tive percent, is about 35%. Considering that the pulp web was compressed in a dry state and at a speed of 58 m/minute, the result is surprisingly good.

Rolls produced in the aforedescribed manner and having a width of 280 mm were transported to a diaper mill,, where the sheets were disintegrated into fluff in a hammer mill. It was found that the electrical energy consumed when disinteg¬ rating the inventive reel pulp was less than that required to disintegrate the reel pulp produced in accordance with the known technique. Thus, the specific electrical energy consumption of the hammer mill reached 43 kWh/ton when dis¬ integrating t^ne inventive reel pulp, whereas an electrical energy input of 58 kWh/ton was required to disintegrate the reel pulp manufactured in accordance with the known tech- nique. The result is very surprising in view of the fact that the fluff pulp, formed to a sheet structure in accord¬ ance with the invention, had a substantially higher density than the sheet structure formed according to the known tech¬ nique.

Samples were taken from both of the aforesaid pulps and analyzed in accordance with the standard method SCAN-C33:80, for the purposes of determining the pulp properties if im¬ portance for liquid absorption purposes. The results ob¬ tained are set forth in Table 2 below.

Table 2

Fluffed Pulp Properties Known technique Accordin to the invention

it will be seen from Table 2 that the fluff obtained from the two pulps had substantially identical properties, de¬ spite the fact that the pulp produced in accordance with the invention had a much higher density than the other pulp and consumed far less electrical energy during the disintegra- tion process. Example 2

In a further test carried out in accordance with the invention, the possibility was explored of pressing a wet pulp web to a lower dry solids content, relative to Example 1, subsequent to the pulp web leaving the third press stage, i.e. lower than 44%, and therewith obtain a lower pulp den¬ sity, relative to Example 1, prior to the drying stage. The pulp web was pressed in the first stage to a dry solids con¬ tent of 34%, in the second press stage to a dry solids con- tent of 38%, and in the third- press stage to a dry solids content of 41%. The pulp web was then dried to a dry solids content of 91% and then compressed to a higher density, in accordance with the invention. This slightly modified method was found to give the same results as the earlier test carried out in accordance with Example 1. One exception, however, resided in the amount of electrical energy consumed when converting the pulp web to fluff. Thus, the specific electrical energy consumption was only 39 kWh/ton, i.e. 19 kWh/ton lower than the electrical energy required to dis- integrate conventionally manufactured reel pulp.

Example 3

Long-fibre spruce pulp obtained by fractionation in accordance with US-A-4,562,969 was produced in a groundwood pulp mill. The fractionated pulp was placed in a container and transported to the CTMP-mill according to Example 1. The pulp was slushed in a pulper (not shown in the Figure) in a conventional manner and then diluted with water to a pulp consistency of 1.2%. The pulp suspension was passed through a conduit 2 (see Figure 2) to a headbox 3 and deposited on a wire . The pulp suspension was dewatered on the wire to a pulp concentration (pc) of 26%. The pulp web was then pressed in three stages. Upon completion of the first press stage, the pulp web had a dry solids content of 33%, upon completion of the second press stage a dry solids content of 40%, and upon completion of the third press stage a dry solids content of 43%. The pulp web was then dried to a dry solids content of 91% and rolled onto a reeling drum. Samples were taken for analyzing the properties of the pulp. The results of these analyses are set forth in Table 3 below.

In accordance with the invention, a further test was carried out in which the pulp was pressed in three stages to a dry solids content of 44%. Subsequent to being partially dried in a drying section 8 to a dry solids content of 63%, the pulp web (see Figure 3) was passed to a continually ope¬ rating press section 9, 10, in which the pulp web was com¬ pressed. The press section compr-irsed double roll pairs made of steel. The pressure exerted by respective roll pairs 9 and 10 on the pulp web was controlled hydraulically. During the test, a linear pressure of 80 kN/m was measured during passage of the pulp web through the first roll pair 9, and a linear pressure of 120 kN/m during passage of the pulp web through the other roll pair 10. No change in the dry solids content of the pulp web was observed. The pulp web com¬ pressed in accordance with the invention was then further dried in a second drying section 11, to a dry solids content of 91.3%. The pulp web was then reeled onto a reeling drum. Smaller and narrower rolls were produced from the jumbo-reel and delivered to consumers, e.g. converters.

Analysis samples were taken from the jumbo-reel, the results of which analyses are set forth in Table 3. A com¬ parison made between the reels showed that the diameter of the reels produced in accordance with the invention were, on average, about 30% smaller than the diameters of the reels produced in accordance with the known technique. Table 3

Pulp sheet properties Known Technique According to the Invention

2

Graimiage, g/m 650 649

Thickness, πrn 1.80 1.24

3

Density, kg/m 361 523

It will be seen from these results that the thickness of the sheet structure was substantially reduced when practising the inventive method. The effect of the method on pulp den¬ sity is surprisingly high and the resultant density in- crease, expressed in relative percent, is about - 5-%rτ ^~Tn view of the fact that the pulp web was compressed in a dry state and at a speed of 58 m/minute, the result is surprisingly good.

The pulp reels produced in accordance with the afore- going had a width of 280 mm and were transported to a diaper manufacturing mill, where the sheet pulp was disintegrated to fluff form in a hammer mill. It was found that the elect¬ rical energy consumed in the disintegration process was lower in the case of the inventive reel pulp than in the case of the pulp produced in accordance with the known tech¬ nique. Thus, the specific electrical energy consumption was calculated to be 41 kWh/ton when disintegrating the inven¬ tive reel pulp, whereas the reel pulp manufactured in accordance with the known technique consumed 56 kWh/ton energy. The result is surprising when considering that the reel pulp produced in accordance with the invention had a much higher density than the reel pulp produced in accord¬ ance with the known technique. Samples were taken from the fluffed pulps and analyzed in accordance with the standard method SCAN-C33:80 in order to investigate properties significant for absorption purposes. The results obtained are set forth in Table 4 below.

Table 4

Fluffed Pulp Properties Known Techni ue Accordin to the invention

It will be seen from Table 4 that there was no difference between the different reel pulps with regard to the proper¬ ties investigated. This indicates that the lignocellulosic material of the invention can replace known types of fluff pulp without giving measurable drawbacks of the end product.

Claims

1. A lignocellulosic material of sheet structure and a dry solids content >80%, preferably >85%, c h a r a c t e r -

3 i z e d by a density within the range of 400-575 kg/m measured in accordance with the standard method SCAN-P7:75 and a disintegration ability calculated as the electrical energy consumption when disintegrating the material in a hammer mill of between 30 and 60 kWh/ton pulp, preferably

35-50 kWh/ton pulp; and by a bulk density subsequent to

3 disintegration of 15-23 cm /g, measured in accordance with the standard method SCAN-C33:80, and a network strength of

2-4.5 N.

2. A lignocellulosic material according to Claim 1, c h a r a c t e r i z e d in that the cellulose material is a high yield pulp, such as a chemimechanical pulp, a chemithermomechanical pulp, a groundwood pulp or a refiner pulp.

3. A method for manufacturing a continuous wet formed web of lignocellulosic material, comprising the steps of dewatering and pressing the web to a dry solids content of about 30-60% and subsequently drying the web to a dry solids content >80%, c h a r a c t e r i z e d by compressing the wet formed web between two surfaces at a dry solids content of at least 50%, preferably >80%, with a linear pressure effective in increasing the density by more than 10%.

4. A method according to Claim 3, c h a r a c t e r i z e d in that the linear pressure is 30-300, preferably 50-200 kN/m.

5. A method according to any one of Claims 3-4, c h a r a c t e r i z e d by effecting said compression between one or more roll pairs.

6. A method according to any one of Claims 3-4, c h a r a c t e r i z e d by effecting said compression in a belt press.

7. A method according to any one of Claims 3-6, c h a r a c t e r i z e d by heating the pulp during or subsequent to said compression stage.