AU620378B2 - Process for removing the fibre from a lignocellulose material - Google Patents

Description

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OPI DATE 25/08/89 APPLN. ID 30'473 89 ORAAOJP DATE 28/09/89 PCT NUMBER PCT/FR89/00043 PCT ORG7

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DEMANDE INTERNATIONALE PUBLIE~iRTU 1AATEZ'E JdaRATION EN MATIERE DE BREVETS (PCT) (51) Classification internationale des brevets 4 (11) Nuiunro de publication iriternationale: WNO 89/ 07169 D21B 1114, 1/02, D21D 1/32 Al (43) Date de publication Internationale: 10 aofit 1989 (10.08.89) (21) Numnro de la demntinde internationale: PCT/FR89/00043 (81) Ftats d~sign~s: AU, BR, DK, Fl, JP, NO, US.

(22) Date de d.,p6t iternational: 3 f~vrier 1989 (03,02.89) Publi~e A vec rapport de recherche internationale.

(31) Numf~ro de la dernande prioritaire: 88/01333 Avant l'expiration du d~1ai pr~vu pour la modification des revendicatlons, sera repub~ie si de telles modifica- (32) Date de priorit6: 5 f~vrier 1988 (05.02,88) tions sont regues.

(33) ?9,ys de prioriti; FR (71) D~posant (,pour tous les Elats d~sign~s saiV UV5J. BE- GHIN-SAY [FR/FRI; F-59239 Thumeries (ER), (72) laventeur; et Inventeur/Diposant (US seiulement) :DELAROCHE, Jean, Paul [FR/FR]; 12, rue de Sally, F-62112 Corbehem (FR).

(74) Mandatalre: DAVID, Daniel; B3 ghin-Say, 54, avenue Hoche, F-75008 Paris (FR).

(54)Tltle.- PROCESS FOR REMOVING THE FIBRE FROM A LIGNOCELLULOSE MATERIAL (54)Titre: PROCEDE DE DEFIBRAGE D'tJNE MATIERE LIGNOCELLULOSIQUE 3 j I 7, 2 220 52 Sos (57) Abstract 3 Process for mechanical fibre removal from a lignocellulose materia' obtained by a preliminary impreantion treatment atf chippings by a reagent which promotvs softening of the lignin, S'.d process Is characterized in thsdt agc~nsists in passing the matderial through an apparatus (20) of the crusher type Ah hammers and a cylindrical grid. This process ensures that the pulp obtained has a maximum~ of long fibres, The process also offers a lower energy consumption than in conventional disk-type refiners.

(57) Abrkg6 P, Le proc~d6 de d~f Ibrage me'anique d'une mati~re Ilgnocellulosique obtenue par un pr~traltertt d'impr~gnation de paqUettes par un agent r~actif favorisant te ramollissement de Ia lignine est caract~rls6 en ce qu'il consiste A faire passer la mati~re A travers Un appareil du type broyeur A marteaux et A grille cylindrique, GrAce A ce proc~d6, on conserve Ia pAte obtenue un maximum de fibres longues. Le proc~d6 se traduit en outre par une consommation d'6nergle plus faible que dans les raffineurs disques classiques.

PROCESS FOR GRINDING A LIGNOCELLULOSIC MATERIAL The invention relates to a process for grinding a cellulosic material which has been subjected to an impregnation pretreatment aimed at softening the lignin with a view to obtaining a papermaking pulp.

The various processes for manufacturing papermaking pulp, especially from wood platelets or chips consist, initially, in reducing the latter into the state of fibres.

To obtain this fibre separation, the wood can be disintegrated by a purely mechanical route by means of stone grinders. Mechanical pulps are thus obtained in a high yield because most of the constituents of the wood are recovered.

This separation can also be obtained by removing the lignin which ensures that the fibres adhere together, by means of chemical reactants.

By submitting the chips to a cooking operation for a certain time in devices known as digesters on a 20 continuous or noncontinuous principle in the presence S* of an appropriate reactant, lignin and other soluble constituents are dissolved completely, leaving only cellulose, whose fibres are perfectly isolated, the disintegration of chemical pulps is obtained merely by 25 blowing, Between these two extreme processes, there are known processes involving both mechanical work and chemical work, combined to various extents, if appropriate, with the action of heat, the purpose of which is to soften the binding lignin.

Thus, before the mechanical grinding treatment, the platelets are softened by the action of a chemical reactant or of steam at an elevated temperature and socalled mechanochemical, thermomechanical o3 chemical thermomechanical pulps are obtained.

The mechanical treatment is generally ensured by disc or equivalent 7efiners, the purpose of which is to separate the individual fibres. Depending on the ope:;ating conditions, disc refiners also permit the release of xr ct fibrils, which will improve the mechanical strength of la the paper obtained.

According to the present invention there is provided a process for the preparation of a papermaking pulp from wood platelets, consisting in pretreating the said wood platelets in a twin-screw type machine, where they are subjected to a succession of compressions/releases with the injection of one or more solutions of at least one chemical agentA and then grinding the material produced in the machine by a mechanical means, wherein, after grinding, the material is subjected to a classification operation and the resulting classification rejects are subjected to a grinding operation in an apparatus of the hammer mill type.

Hammer mills are devices which are already known per se, but as far as the applicant company is aware, they have never been employed in the conditions which are recommended, that is to say for grinding a previously impregnated coarse pulp.

The grinder can advantageously replace the disc refiners, for example, which are usually employed in this case.

This is for two important reasons: S" because of its operating principle, the fibres subjected to the mechanical treatment are solely those which form agglomerates which are to be separated ;e individually, while thb fibres which are already disintegrated pass quickly through the grid without having undergone any mechanical treatment. As a result of this, this phenomenon promotes the preservation of a maximum of long fibres.

the second reason is linked with an energy "consumption which is smaller than in traditional disc refiners, all conditions being otherwise equal. This may be explainable by the fact that in the case of the treatment of an equal initial quantity of material, the fibres which are separated individually as soon as the operation begins are not subjected to the repeated action of the hammers because they are quickly removed. As a 4, result, the mechanical energy is actually expended only on -2- SWO 89/07 16 9 SPCT/FR89/00043 2/2 6 .~,~-iu~d~-*i;aur..~muKllo*nrP~ yi a smaller quantity of material.

According to the invention, the pretreatment to which the platelets are subjected before being ground in the hammer mill is performed inside a r t*iY 1 x

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-2a- 3 twin-screw machi.ne according to the teaching of patent FR-2,418,295. It appears in fact that, owing to the impregnation of a short duration, which takes place inside the machine, resulting in a softening of the interfibre bonds, optimum conditions are combined for a satisfactory use of an apparatus with hammers, which tends to open the fibres very quickly and quite easily.

Other details and advantages of the process will appear on reading, which follows, of the description of an embodiment of the invention, given by way of example and illustrated by the appended drawings, where: Figure 1 is a partial diagrammatic illustration of a plant for the marvnfacture of papermaking pulp from wood 15 platelets; Figure 2 is a front view, in cross-section, of a hammer mill capable of being employed within the scope of the invention.

Figure 1 shows diagrammatically a part of a plant for the production of board pulp in a high yield. The thick arrows show the flow of materials.

It comprises a machine of the twin-screw type (1) fed with wood platelets from a hopper whose capacity is determined to guarantee a satisfactory feed, even in the event of a brief line stoppage for machine maintenance.

The platelets are taken up under the hopper by a twin-screw extractor whose speed of rotation is servo-controlled by that of the twin-screw machine so as to provide it with a suitable feed under all operating conditions. The miachine rotors are themselves driven by a motor Orifices are arranged along the casing of the twin-screw machine to permit the injection of a chemical reactant which, in this case, is a neutral sodium sulphite solution into the various treatment enclosures. Similarly, orifices (11) are provided for injecting the cuantity of water needed to control the dry content of the material being processed. i 4 Downstream, the machine opens into an outlet spout (13) whose shape enables the flow of material to be distributed towards the entry of two hammer mills A detailed description of these devices is given later. The plant example illustrated by the diagram refers to two mills in parallel, but this number is not limiting. A single mill may suffice, where appropriate.

The grinding operation in the hammer mills requires an input of water at (22) to reduce the dry content of the material. The raw pulp produced by the mills falls into a chest (30) where it is stirred and taken up again by a pump (32) to be conveyed towards a battery of classifiers, two in the present case (40, 42), which are arranged in series.

The classification rejects are subjected to a *grinding operation in a third hammer mill (50) of the same type as those above, also fed with make-up water at (52) to control the dry content of the material which, after being ground, is recycled towards Lhe?. raw pulp 20 chest (30) via the conduit (56).

Pulp which is accepted leaves the classification at (44) towards the suction of the pulp pumps in order to S" undergo various subsequent treatments therein before entering the pulp stock of the board machine. It is thus 25 purified in a manner known per se, for example, by S'.passing through cyclone purifier units arranged in a cascade, and is then conveyed towards thickeners enabling its dry content to be adjusted to the value required for its passage in the board machine.

Figure 2 illustrates a model of hammer mills which can be employed in the process in accordance with the invention, It comprises two rotors (200) mounted on two horizontal shafts (202) inside a framework (204) which is open both in its upper part at (206) to enable the pulp to be ground to enter and i.i its lower part (208) to discharge the latter after treatment.

Each rotor consists of parallel discs (210) separated by spacers (only one disc per rotor can be seen "a

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*S in the figure). At their periphery they are connected by rods (214) which are parallel to the shaft. Alternate rods carry a row of parallelepipedal hammers (216) pointing radially along their long axis (only one per row can be seen in the figure). The hammers have a certain degree of freedom of rotation around their support shaft.

In the example illustrated, each rotor is thus equipped with four rows of hammers.

A central harrow (220) and a side harrow (222) are arranged on both sides of the vertical plane passing through the shaft of each rotor, their teeth being interposed between the planes of rotation of the hammers, and extending substantially as far as the periphery of the discs.

15 A grid (230) of semicylindrical shape overlaps lengthwise a part of the lower half of each rotor (200) at a small distance from the area swept by the free ends of the hammers. The hammers thus interact with the peripheral grid to break up and grind the material residing in the apparatus. Each grid is pierced with calibrated orifices (holes or slits) so as to permit the passage of the fibres which have been individually separated, while retaining as many shives as possible.

The fibres which have thus passed through the grid are entrained towards the outlet opening (208).

The calibration of these orifices (holes or slits) determines whether the expected result is obtained. It is desirable that the diameter of the holes be between 1 and 5 mm, or else, when slits are involved, that their width be between 0.4 and 2 mm. When the holes are not cylindrical in shape, these values apply to their shortest transverse dimension.

Nozzles (240) for injecting water are arranged across the entry opening (206).

Metal sheets (250) extend the cylindrical grids (230) to confine the enclosure swept by the rotor and to limit scattering. They are articulated around a lengthi4 wise shaft to permit access to the rotor when its ,J maintenance has to be carried out.

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The plant described above makes it possible, in particular, to obtain board pulps in a high yield, of the order of 92 from, for example, fresh spruce.

The operation is as follows: The platele:s are prepared before entering the hopper they are screened so that the only ones accepted are those which have appropriate dimensions and are washed and rid of any foreign matter.

They travel from the hopper ho the entry of the twin-screw machine via the extractors, This machine consists, in accordance with the teaching of patent FR-2,418,295, of two parallel screws whose flights are identical and which are driven in the same direction of rotation inside a barrel.

15 The flights consist of successive zones of different pitches. In a first section, the flight is firstly of direct pitch for driving the material in the downstream direction, and the pitch is then reversed to hold up the material until a plug is formed.

20 The material passes in a second section to the other (sic) through calibrated ports arranged In the flight immediately downstream of the plug zone, being subjected to a hold-up. The size of the ports is chosen so as not to result in a decrease in the length of the fibres but merely in fractionation of the platelets.

In the second section the flight is again of direct pitch, but reversed to form a second plug. The machine thus comprises a certain number of sections which succeed each other from upstream in the downstream direction.

Within each section, the material passes through a series of alternate stages of pressure rises followed by pressure drops, which enable the material to be impregnated throughout with the desired reactant.

In fact, because of the rotation of the screws in the same direction, the forward t'ravel of the material from upstream in the downstream direction is accompanied KA4. by an alternation of compressions/releases &t the time when the latter moves from one screw to the other.

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r 7 b During each compression stage which is produced within the flights of a screw preceding the portion where they mesh with the flights of the other screw the material tends to expel the liquid trapped between the fibres. During the expansion which follows when it moves between the flights of the other screw the liquid refills the spaces between tile fibres again.

The excess liquid released during the compression stages may be removed, if desired, through filters placed in appropriate places.

This trituration of the material within each section, but also from one section to another, thus promotes its thorough impregnation with the reactant liquid.

15 By an appropriate choice of the machine characteristics and of its operation: pitch of the flights, speed of rotation of the screws, length of the hold-up zone, calibre of the ports, and the like, the pressure inside the machine can be raised to high values, of the order of 100 bars, and an appropriate temperature of the C material, oi between 60 and 100°, can be obtained. A thoough impregnation of the platelets is produced in .this way, without damaging the fibres.

Controlled quantities of water are introduced 25 into the various sections so as to result in the material go. travelling forward in optimum conditions.

Thus, while the platelets have a moisture content of 35 to 50 when they enter, the product processed by the machine leaves it with a dry content of 25 to 30 After a treatment of this kind, the material is only partially ground. In the operating conditions of the exemplary embodiment illustrated at present, the shive content is 55 In order to complete the individual separation of the fibres while maintaining their length quality as much as possible, they are treated mechanically by the means of the invention, The material is thus conveyed towards the mills, where the bundles of pretreated fibres are subjected, in accordance with the invention, to a mechanical separation treatment in the 9 9 .9.

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-8 mill by the action of the hammers interacting with the harrows and the cylindrical grid.

on entering the mill, the material is sprayed with water by means of nozzles so that, when it leaves, its dry content is reduced to a value of between 2 and The adjustment of the dry content permits the treatment to be regulated in relation to the shive content and to the Bauer Mac Nett classification.

The f ibres which are not yet open are easily disintegrated by the action oi the hammers sweeping the grid, while the f ibres which are already individual ._y separated pass quickly through the calibrated orif~ices witbout having had the time to be subjected to mechanical 15 stresses capable of shearing them'. The production of a pulp with conservation of the fibre length is one of the great advantages of the process, because only the piles of fibres to be treated are subjected to the action, of the mill. This is, of course, a statistical estimate.

20 However, results show that, overall, the proportion of long fibres arising from the treatment can be as high as 54 in the prosent case.

Long fibres means the proportion of fibres which c6ze collected on the 28-mesh screen. This result should 25 be compared with that of a mechanical stone-ground pulp, in the case of which the proportion of fibres collected on a 28-mesh screen is 22 Another advantage of the use of a hammer mill in these conditions is the great ener'gy saving produced when compared with a grinding operation performed in conventional refiners which, furthermore, do not enable an equally high proportion of long fibres to be preserved, at an equivalent degree of refining.

To illustrate the advantage of the use of a hammer mill, an equal quantity of wood Was subjected to four different treatments, the first treatment A 4 corresponding to that of the invention, where the platelets were impregnated with neutral sodium sulphite in a twin-screw machine and were then ground in a mill of the h F T 9 type described above, whose grid is pierced by holes mm in diameter.

In a second treatment B, the same raw pulp leaving a twin-screw pretreatment is passed through two refiners in series, of the Defibrator and Bauer 411, type, to obtain a pulp having the same degree of draining off.

In a third treatment C, a chip pulp is passed through the same refiners.

Lastly, in a fourth treatment D, a stone-ground pulp was produced.

The Bauer Mac Nett characteristics for each of the pulps are cited below.

A B C D 14-m 31.6) 25.3) 9.8) 8.7) (54.4 (49.1 (35.3 (22.8 28-m 22.8) 23.8) 25.5) 14.1) 48-m 16.3 18.7 21.3 21.4 200-m 9.9 12.0 16.9 28.3 fines 16.9 20.1 26.5 27.5 shives 10.2 14.7 8.6 8.9

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It is found that the treatment recommended by the invention produces a pulp in which the proportion of long fibres collected on the 28-m screen is 54.4 that is more than 10 higher than that obtained in conventional refiners, with a proportion of fines of below 20 this being for equivalent degrees of draining off.

If we return to the description of the treatment of the material in the plant illustrated in Figure 1, the raw pulp produced in the grinder, whose dry content has fallen, for example, to 2 now has a shive content only of the order of 15 The pulp is collected in a chest with the classification rejects, where it is subjected to continuous stirring On leaving the chest, the pulp is Spumped towards the battery of classifiers which will t 10 remove the shives. Thus, the pulp accepted by the classification now contains only approximately 0.6 of shives and may be subjected to the conventional purification and thickening operations before being released to the machine stock.

The classification rejects are subjected to a grinding operation in a hammer mill (50) similar to the first two an addition of water (32) reduces the dry content of the pulp, which is returned to the chest The process just described has the advantage of producing a pulp in a high yield, in which the length of the fibres is preserved as much as possible with a reduced energy cost. It is obvious that the hammer mills can be fed with raw pulps other than those produced by a 15 twin-screw machine pretreatment, as long as this pulp exhibits a thorough impregnation which is sufficient for an optimum utilization in the mills.

Finally, the plant may comprise a combination of traditional refiners and of hammer mills if this is found to be necessary, e S 6

Claims (6)

1. A process for the preparation of a papermaking pulp from wood platelets, consisting in pretreating the said wood platelets in a twin-screw type machine, where they ar6 subjected to a succession of compressions/releases with the injection of one or more solutions of at least SU\Ca_\Q c.r £otc\>>3 tLM_ -o-c P\atbeSs i one chemical, agent. and then grinding the material produced in the machine by a mechanical means, wherein, after grinding, the materal is subjected to a classification operation\and the resulting classification rejects are subjected to a grinding operation in an apparatus of the hammer mill type.

2. A process according to claim 1, wherein the moisture content of the classification rejects subjected to the 15 grinding is controlled by injecting water into the hammer mill so that on leaving the said mill the dry content of the machine is between 2 and

3. A process according to claim 2, wherein after passing through the hammer mill, the classification S 20 rejects are recycled upstream of the classification davices.

4. A process according to any one of claims 1 to 3, A .W herein the grinding of the material from the twin-screw type machine is performed in a moist medium in an apparatus of the hammer mill and cylindrical grid type, the dry content of the material leaving the apparatus being between 2 and A process according to claim 4, wherein the orifices of .Le grid are holes with a diameter of between 1 and

6. A process according to claim 4, wherein the orifices I of the grid are slits with a width of between 0.4 and 2mm.

7. A process according to claim 1, substantially as herein described with reference to the accompanying drawings. DATED: !3 July, 1991 PHILLIPS ORMONDE FITZPATRICK Attorneys For: BEGHIN SAY S.A. RU w 1555Z c l -do-l -j ~ii i __i