EP0070782B1 - Method of making a cellulosic fibre pulp for non paperlike applications - Google Patents

Abstract

1. Process for producing a pulp of cellulose fibres not to be used for paper-making, by passing wood chips into a pulping machine comprising at least two screws (1, 2) having identical threads meshing in one another and driven in rotation on the inside of a sleeve (3) provided with an upstream chip inflow orifice (31) and with a downstream pulped-material cutflow orifice (32), the threads (11, 21) of the screws (1, 2) having varied pitches determining a succession of processing zones (I, II etc.), each comprising a section (A) with a direct pitch for driving the material downstream and a section (B) with a reversed pitch for increasing pressure as a result of the braking of the material, the threads with reverse pitches being provided with apertures (12, 22) for the downstream passage of the material which has reached the desired degree of pulping, the sleeve (3) being provided with filtering parts (34) located upstream of the reverse-pitch braking sections (B), at least in the first two processing zones (I, II), in order to carry out drying of the material, characterised in that the material ist first brought to the desired degree of pulping by being passed through the first processing zones (I, II, III), which incorporate reverse-pitch sections (B) provided with apertures, the width of which decreases from one zone to the next to a width of the order of 4 to 5 mm, in that, at the same time, drying together with filtration is carried out to a degree of dryness of between 35% and 55%, and in that the shives and clusters of fibres are subsequently eliminated in the same machine, without reducing the length of the fibres and without carrying out any fibrillation, by passing the pulped and dried material through at least two additional processing zones (IV, V), which incorporate reverse-pitch braking sections provided with apertures having the same width of the order of 4 to 5 mm as in the last pulping zone (III), and a homegenous pulp consisting solely of isolated fibres which have not undergone any fibrillation is obtained at the outlet of the machine.

Description

The subject of the invention is a method and an installation for manufacturing a pulp of cellulosic fibers which can be used especially in non-paper fields without any treatment other than drying.

A number of processes have been known for a long time for manufacturing cellulose pulp, in particular from wood chips and which consist, by various means, of breaking up the chips in order to isolate the fibers. These processes have been specially developed for the manufacture of paper pulp.

To obtain the insulation of the fibers, the chips can be disaggregated purely mechanically by means of grinding wheels, for example. The so-called mechanical pastes are then obtained. However, it may be interesting to try to eliminate the lignin which ensures the adhesion of the fibers to each other by means of chemical reagents. In this case, the chips undergo cooking for a fairly long time in devices called digesters allowing the reagent to attack the ligin which dissolves in the liquid phase. If the chemical treatment is sufficiently advanced to obtain complete isolation of the fibers, the so-called chemical pulps are obtained which are easily disintegrated, for example by blowing with compressed air.

However, mechanical treatment and chemical treatment can also be combined. We then obtain semi-chemical pastes which use, in a first stage, a chemical reagent attacking lignin then, in a second stage, mechanical energy, generally in devices called disc shredders.

It is also possible to soften the chips before the mechanical treatment by the action of a chemical reagent or steam at high temperature and the so-called mechanical-chemical and thermo-mechanical pastes are obtained.

In all cases, the paste obtained at the end of the defibration treatment is in the form of a very dilute suspension which must be washed, bleached and then wrung out.

The Depositing Company has been developing for a few years a new process and a new machine for defibrating wood chips. This machine described, in particular in French patent no. 2319737, comprises at least two identical threaded screws meshing in one another and rotated inside a sheath provided with an upstream inlet orifice for the chips and a downstream outlet for the defibrated material; the threads of the screws have varied pitches which determine a succession of treatment zones each comprising a direct pitch section in which the material is entrained downstream and an inverted pitch section which slows down the advancement of the material and consequently determines an increase in pressure. In this section with inverted steps, the nets are however provided with windows allowing the passage downstream of a certain quantity of material. According to the preceding invention, the widths of the windows are adjusted so as to keep the material insufficiently upstream. the material having reached the desired degree of defibration, most of the defibration work being obtained by a combination of compression and shearing effects in the compressed areas, in particular due to the fact that, in these zones, the material is entrained around the threads passing from one screw to another. It is thus possible to progressively defiberate the chips by reducing the width of the windows from one braking zone to the next. Generally, correct defibration is obtained by passing through three or four sections with reverse pitch.

On the other hand, an important advantage lies in the fact that it is possible to carry out, inside the same machine, the bleaching of the dough during the defibration of the chips as well as the broad and a wringing to a degree at least 35% dryness. In document FR-A-2451 963, washing of the dough is carried out at the same time as defibration, in the successive zones of the machine. This therefore includes a number of successive defibration zones comprising a compression section and an inverted pitch section, and the sheath surrounding the screws is provided with filtering zones in the parts preceding the inverted pitch sections. It is in fact observed that the liquid phase which is expressed in the compression zones can go upstream in the less compressed zones, passing between the periphery of the threads and the internal wall of the sheath to the filtering zones where it is discharged.

The dough thus produced contains a certain number of impurities, in particular knots, heaps of fibers and small, insufficiently defibrated pieces called buchettes. This is why the dough also passes through a disintegrator which performs final defibration and then through various sorting devices.

In addition, the raw pulp is made up of fibers normally too coarse to be usable directly for the manufacture of paper and must therefore undergo a so-called refining treatment in a refining pile or a refiner. During this treatment, and in particular under the action of water, a so-called fibrillation effect is obtained which promotes the adhesion of the fibers to each other during the manufacture of paper.

All these operations which have just been described briefly have been specially developed for the manufacture of paper. However, cellulose fiber pulps can have other uses. In particular, in addition to the chemical uses of the cellulose thus produced, the pulps of fibers produced by the papermaking route serve as raw material for the manufacture of absorbent products of the cellulose wadding type or of molded products in which the fibers isolated by the defibration treatment are mixed with a thermosetting binder.

In these non-papermaking applications, it is also necessary to remove impurities, fiber cores and buchettes. On the other hand, it is generally also necessary to remove most of the resin, in particular in order to obtain absorbent products or to avoid clogging of the installations for the subsequent removal of water.

For this purpose, hitherto known methods have been used for the manufacture of paper pulp, although in these other applications, the pulp of fibers need not necessarily have the same properties. In particular, if, in order to obtain the complete defibration of the dough, it is passed through devices similar to conventional refiners, a fibrillation effect is obtained which is not necessary and can even be annoying because it reduces the rate of dehydration .

Furthermore, sorting and refining are carried out in very dilute suspension, which has no disadvantage in the manufacture of paper which in any case requires feeding the machines with a very dilute suspension in order to obtain a good fiber distribution. On the other hand, in non-paper applications, it is advantageous to reduce as much as possible the content of the pulp in liquid, the elimination of which requires a great expenditure of energy.

The subject of the invention is a new process allowing the final defibration of the pulp by means better suited to the production of fiber pulp for non-paper use and which, moreover, requires simpler equipment and much less bulky and a much lower energy expenditure.

Furthermore, while the pulp manufacturing facilities are only provided for very large productions, the new method according to the invention makes it possible to adapt much more flexibly to productions of medium importance.

In accordance with the invention, the defibration treatment being carried out in a screw machine of known type, after having obtained the desired degree of defibration by passage through the first treatment zones, the scabs and fiber bundles are completely eliminated by passage of the material in at least two additional treatment zones, until a homogeneous paste consisting only of isolated fibers which have not undergone fibrillation is obtained.

It has already been indicated above that, in a screw machine of the type described in French patent 2,319,737, the defibration treatment is carried out gradually by passing through several treatment zones each consisting of a direct-pitch drive section. followed by a braking section provided with inverted pitch threads in which windows are provided whose width makes it possible to control the passage downstream of the material having reached the desired degree of defibration. If you start with wood chips, you can reach the degree of defibration of a normal pulp by passing through three or four treatment zones. On the other hand, according to a very interesting characteristic of this machine, the wood chips can be fed to the upstream orifice of the machine with a very small amount of water and, in particular, when the chips are soaked with water before defibration, as is usually the case, the quantity of liquid phase which they contain is generally sufficient, the material being able to be entrained from one treatment zone to another and defibrated under good conditions up to a 55% dryness.

As in the other mechanical defibration processes, the dough thus produced contains a certain quantity of fiber bundles and insufficiently defibrated sticks, yet the defibration treatment cannot be continued by further reducing the width of the windows because, in some cases, the quality of the dough would be reduced by excessive shortening of the fibers. This is why, when the fiber pulp produced is intended for the manufacture of paper, it is subjected to a refining operation by conventional means.

However, it has been found that if the defibrated pulp is passed through a certain number of additional treatment zones without changing the width of the windows of the braking sections with reverse pitch, it is possible to completely eliminate the nozzles without excessively reducing the length of the fibers. This surprising effect can be explained by the fact that the length of the fibers depends essentially on the width of the windows and therefore is not much modified if this width is not reduced in the following areas, and that on the other hand , if the defibrated pulp is passed through at least two additional treatment zones, thanks to the relatively high degree of dryness, the sections with inverted steps continue to determine an increase in pressure of the material and consequently to subject the buchettes to a work of shearing and torsion under pressure which makes it possible to eliminate them completely and to obtain at the exit of the machine a homogeneous paste made up only of isolated fibers.

Admittedly, this passage into additional treatment zones does not replace refining because the fibrillation effect sought for paper pulps is not obtained, but, precisely, in non-paper applications, fibrillation is not necessary and can sometimes be harmful. On the other hand, it is particularly advantageous to obtain continuously a pulp of fibers devoid of scraps and relatively dry at the outlet of a single space-saving machine supplied with wood chips.

However, this type of machine also has the very advantageous advantage of also carrying out, if necessary, the elimination of most of the resin contained in the wood. This elimination of the resin is particularly useful for obtaining absorbent materials of the cellulose wadding type. Until pre sent, this type of product was made from a chemical paste for which the cooking before was carried out with reagents chosen so as to effect the elimination of the resin at the same time as that of the lignin. The chemical route also gave the advantage of providing a very fine paste devoid of buchettes. However, chemical pulp is expensive and attempts have also been made to make absorbent materials from thermo-mechanical or mechanical-chemical pulp. In this case, the resin must be removed by a reagent, generally before the defibration treatment. However, such pulps contain a certain amount of flossers which are not suitable for the usual uses of cellulose wadding and which, as said above, must be eliminated by means known in the paper industry.

However, it has been found that, when the defibration treatment is carried out in a machine with several screws, if resinous wood shavings are used as raw material whose age relative to the time of cutting is chosen so as the resin peels off from the channels which contain it, the removal of most of the resin can be carried out mechanically at the same time as defibration in the first two treatment zones.

In practice, wood chips that are less than three days old or about six months or more should be treated. In fact, in the first case, the resin has not yet become very viscous and can escape from the channels and, in the second case, the resin has, on the contrary, become hard and can come off from the wall of the channels.

. Thus, the passage in the compression zones, that is to say at the end of the drive sections and in the braking sections makes it possible to express the resin which dissolves in the liquid phase expressed at the same time and can be discharged with it through the filtration orifices provided upstream of the compressed zones.

While, normally, the expulsion of the resin by compression should require a pressure of the order of 500 bars, it is found that, in a machine with several screws, the same result is obtained with a pressure of only 150 to 200 bars . In fact, the mechanical defibration treatment to which the shavings are subjected in the compressed areas allows, by the combined shearing, compression and pressure effect that is obtained, to loosen the resin from the channels into small particles which mix in the liquid phase and evacuate with them.

This mechanical effect is amplified if a reagent promoting the detachment of the resin is introduced into the sheath.

However, it has been observed that the introduction of a bleaching reagent such as a mixture of hydrogen peroxide and sodium peroxide, without sequestering agent, activates the elimination of the resin. It will be noted that the bleaching reagent is, in this case, introduced from the first treatment zone at the start of defibering, which allows it to impregnate the chips in the first drive zone with the first pressure build-up. However, the bleaching reagent is also introduced into the second treatment zone where the elimination of the resin is continued and the pulp is at the same time bleached.

It is also advantageous to provide the first two treatment zones with filtration orifices. In this way, the pulp is concentrated in the first zone by eliminating part of the liquid phase with the resin and, in the second zone, a washing liquid can be injected which activates the elimination of impurities. However, a certain quantity of bleaching reagent is allowed to pass downstream, which makes it possible to complete it in the following treatment zones so as to obtain, at the outlet of the machine, a defibrated, refined and bleached pulp, which is particularly interesting when making cellulose wadding.

Other chemical reagents can also be used to promote the removal of the resin. Thus, for example, it was found that it was advantageous to subject the chips to a sulfonation pretreatment before the defibration treatment. One can for example carry out a short treatment with a mono-sulfite or bi-sulfite. The shavings having undergone this prior sulfonation are introduced into the first treatment zone of the machine where a washing liquid is also injected. It is thus possible to carry out a rinsing in the first training zone then, after the first defibering step carried out in the first braking section, a bleaching in the second treatment zone and possibly in the following zones. Before leaving the machine, a final rinse can be carried out in the last treatment area.

Each inverted step section, by the compression of the material it causes, determines the formation of a continuous plug, even gas tight. This is why it is also possible to use a volatile solvent to activate the elimination of the resin.

Obtaining a relatively dry product at the outlet of the machine is particularly advantageous in the case of non-paper applications for which drying on a filter cloth is not suitable.

Indeed, generally, it is preferred to use as raw material a pulp of fibers dried in filter presses and therefore being in the form of plates. The fibers are mechanically detached from these plates and form a wad which is then carded. The non-defibrated parts must be eliminated by a dry sorting, which decreases the overall yield. Thanks to the process according to the invention on the contrary, an already refined and already wrung dough is obtained which requires only additional drying, for example, by hot air to obtain a dry fluff for before being carded in a conventional manner but not containing non-defibrated parts.

In the case where the fiber pulp is mixed with a binder for the production of molded objects, it is possible to directly use the pulp leaving the machine with the degree of dryness of at least 35% and to carry out the drying. final by pressure in a mold with a filtering wall. By carrying out the refining without the fibrillation effect, the risk of clogging of the filtering walls is reduced and the speed of dehydration is increased. On the other hand, the elimination of the resin and of the water promotes the mixing of the fibers with the binder.

The figures show schematically and by way of example a machine for carrying out the process. FIG. 1 is a view in longitudinal section through the axis of a screw, FIG. 2 being a view in cross section along II, II of FIG. 1.

The machine comprises two screws 1 and 2 with parallel axes and provided with identical threads which penetrate one into the other. A sheath 3 which envelops the two screws is provided at one end with a supply orifice 31 and at the other end with a discharge orifice 32. The two screws are rotated in the direction allowing, depending on the not threads, to entrain the material introduced through the orifice 31 to the outlet orifice 32.

The threads of the screws have various pitches which determine successive treatment zones, respectively I, II, III ... etc. In the example shown, the machine comprises six treatment zones followed by a zone VII for discharging the treated material towards the outlet orifice 32.

Each treatment zone comprises a zone A whose pitch determines, as a function of the direction of rotation of the screw, the drive downstream of the material and a zone B whose pitch is reversed so as to determine a braking of the material and therefore compression at the end of zone A and in zone B.

As shown in FIG. 2, the threads 11 and 21 of the screws in zone B are provided with openings 12, 22 forming windows which extend from the shaft of the screw to the periphery of the thread. The width of the windows decreases when going downstream. Thus, for example, if one treats a paste having undergone a pretreatment of impregnation with a reagent, for the softening of the chips, the windows can have a width comprised between 15 and 20 mm in the first braking section and from 4 to 5 mm at the exit of the last defibration zone, i.e. zone III or IV. This width is kept in the braking sections of the following zones V and VI.

On the other hand, orifices 33, generally placed just in front of the braking sections B1, B2, etc., that is to say in the compressed areas, make it possible to inject various reactants under pressure and, in the first two treatment zones 1 and II, filter parts 34 are provided in the sheath upstream of the compressed zones.

Thus, it was possible to treat in such a machine coniferous wood chips of more than six months having undergone a pre-treatment of sulfonation before their introduction into the machine. By the orifice 33, a washing liquid was introduced so as to carry out a rinsing in the zone AI, the spent liquid being evacuated by the filtering part 34. By the orifice 35 in the zone II a reagent was introduced. bleaching, for example a mixture of hydrogen peroxide and sodium hydroxide, in a proportion of the order of 2% of the weight of dry matter. A second spin is carried out in a filtering part of the barrel, to a dryness level of 35%, which is then maintained until it leaves the machine.

At the exit from the second treatment zone, most of the resin had been removed by filtering parts since only 0.7% of the weight of dry matter remained. The pulp obtained from the machine was completely defibrated and contained approximately 35% of long fibers passing through the 18 MESH sieve and 35% of short fibers passing through the 48 MESH sieve. Thanks to the elimination of the resin, this paste was particularly suitable for the manufacture of an absorbent material since its absorption coefficient defined according to the standards in force was only 10.

Of course, the invention defined by the claims is not limited only to this embodiment described only by way of example, nor only to the applications which have been mentioned. This is how the fiber pulps thus produced could also serve as a raw material for nonwoven products.

Claims (11)

1. Process for producing a pulp of cellulose fibres not to be used for paper-making, by passing wood chips into a pulping machine comprising at least two screws (1, 2) having identical threads meshing in one another and driven in rotation on the inside of a sleeve (3) provided with an upstream chip inflow orifice (31) and with a downstream pulped-material outflow orifice (32), the threads (11, 21) of the screws (1, 2) having varied pitches determining a succession of processing zones (I, II etc.), each comprising a section (A) with a direct pitch for driving the material downstream and a section (B) with a reversed pitch for increasing pressure as a result of the braking of the material, the threads with reverse pitches being provided with apertures (12,22) for the downstream passage of the material which has reached the desired degree of pulping, the sleeve (3) being provided with filtering parts (34) located upstream of the reverse- pitch braking sections (B), at least in the first two processing zones (I, II), in order to carry out drying of the material, characterised in that the material ist first brought to the desired degree of pulping by being passed through the first processing zones (I, II, III), which incorporate reverse-pitch sections (B) provided with apertures, the width of which decreases from one zone to the next to a width of the order of 4 to 5 mm, in that, at the same time, drying together with filtration is carried out to a degree of dryness of between 35% and 55%, and in that the shives and clusters of fibres are subsequently eliminated in the same machine, without reducing the length of the fibres and without carrying out any fibrillation, by passing the pulped and dried material through at least two additional processing zones (IV, V), which incorporate reverse-pitch braking sections provided with apertures having the same width of the order of 4 to 5 mm as in the last pulping zone (III), and a homegenous pulp consisting solely of isolated fibres which have not undergone any fibrillation is obtained at the outlet of the machine.

2. Process for producing a pulp of fibres according to Claim 1, characterised in that the machine is supplied with resinous wood chips, of which the age in relation to the cutting period is selected so that the resin detaches from the channels containing it, and in that most of the resin is eliminated mechanically at the same time as pulping in the first two processing zones (I, 11).

3. Process for producing a pulp of fibres according to Claim 2, characterised in that a bleaching reagent is injected at least into the first drive section (A1), to assist the detachment of the resin and at the same time bleach the pulp during the pulping operation.

4. Process for producing a pulp of fibres according to one of Claims 2 and 3, characterised in that a washing liquid is injected into the sleeve (3) downstream of the first processing zone (I), to carry out washing at least in the second drive section (A2), the spent liquid being discharged via the filtration orifices (34) made in the sleeve (3).

5. Process for producing a pulp of fibres according to Claim 1, characterised in that a volatile solvent is injected into the sleeve (3) downstream of the first reverse-pitch braking section (B1

6. Production process according to Claim 1, characterised in that the material is passed through at least five processing zones.

7. Production process according to Claim 3, characterised in that the injected bleaching reagent is a mixture of oxygenated water and an alkaline product.

8. Production process according to Claim 1, characterised in that before being supplied to the pulping machine, the chips are subjected to a prior sulphonation treatment.

9. Production process according to Claim 1, characterised in that the age of the wood chips is less than three days after cutting, so that the resin contained has remained fluid.

10. Production process according to Claim 1, characterised in that the age of the wood chips is at least six months after cutting, so that the resin contained has hardened.

11. Machine for producing a pulp of cellulose fibres not to be used for paper-making, comprising at least two screws (1, 2) having identical threads meshing in one another and driven in rotation on the inside of a sleeve (3) provided with an upstream chip inflow orifice (31) and with a downstream pulped-material outflow orifice (32), the threads (11, 21) of the screws (1, 2) having varied pitches determining a succession of processing zones (1, II, etc.), each comprising a section (A) with a direct pitch for driving the material downstream and a section (B) with a reverse pitch for increasing pressure as a result of the braking of the material, the reverse-pitch threads being provided with apertures (12, 22) for the downstream passage of the material which has reached the desired degree of pulping, the sleeve (3) being provided with filtering parts (34) located upstream of the reversepitch braking sections (B), at least in the first two processing zones (I, II), to carry out drying of the material, characterised in that it incorporates, downstream of the first chip-pulping and drying part, a second part for eliminating the shives and clusters of fibres without any fibrillation effect, the first part comprising at least three pulping zones, in which the reverse-pitch sections (B1, B2, B3) are provided with apertures (12, 22), the width of which decreases from one section to the next to a width of the order of 4 to 5 mm in the last section (B3), and the second part comprising at least two additional processing zones (IV, V) incorporating the reverse-pitch sections (B4, B5) provided with apertures (12, 22) having the same width of the order of 4to 5 mm as in the last pulping zone (III).

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