EP0327469B1 - Process for defibrillating lignocellulosic material - Google Patents

Abstract

Process for mechanical fibre removal from a lignocellulose material obtained by a preliminary impregnation treatment of chippings by a reagent which promotes softening of the lignin. Said process is characterized in that it consists in passing the material through an apparatus (20) of the crusher type with 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.

Description

The invention relates to a process for defibrating a cellulosic material which has undergone an impregnation pretreatment aimed at softening the lignin, with a view to obtaining a paper pulp.

The various methods of manufacturing paper pulp, in particular from wood chips or woodchips, consist first of all in reducing the latter to the state of fibers.

To obtain this separation of the fibers, the wood can be disaggregated purely mechanically by means of grinders. This produces high-performance mechanical pulps because most of the constituents of wood are found.

This separation can also be obtained by elimination of lignin, which ensures the adhesion of the fibers to each other, by means of chemical reagents.

By subjecting the shavings to cooking for a certain period of time in apparatuses called digesters - with a continuous or discontinuous principle - in the presence of an appropriate reagent, the lignin and other soluble constituents are completely dissolved so as to retain only the cellulose whose fibers are perfectly insulated, the disintegration of chemical pulp is obtained by simple blowing.

Between these two extreme processes, processes are known which involve both mechanical work and chemical work to varying degrees in combination, if necessary, with the action of heat which aims to soften the binding lignin. .

Thus, before the mechanical defibration treatment, the platelets are softened by the action of a chemical reagent or water vapor at high temperature, and so-called mechanical chemical, thermo-mechanical or chemico-thermo-mechanical pastes are obtained. .

Mechanical processing is generally carried out by disc refiners or the like which are used to individualize the fibers. Depending on the operating conditions, the disc refiners also allow the release of fibrils which will improve the mechanical resistance of the paper obtained.

The invention relates to a method for mechanically treating fibrous materials resulting from an impregnation pretreatment, in particular with a chemical agent, promoting the softening of lignin.

In particular, it relates to a process for preparing a paper pulp from wood chips consisting in pretreating said wood chips in a machine of the twin-screw type where they are subjected to a succession of compressions / releases with injection of one or more solutions of at least one chemical agent and then to defibrate the material from the machine by mechanical means, characterized in that after defibering the material is subjected to a classification operation, and in that the classification rejections are subjected to defibration in an apparatus of the hammer mill type.

Hammer mills are devices already known per se. Thus patent GB 752,534 describes such a grinder capable of being used for defibering a coarse paste previously impregnated.

The crusher advantageously replaces the disc refiners, for example, commonly used in this case.

• Par son principe de fonctionnement, les fibres soumises au traitement mécanique sont uniquement celles qui forment des agglomérats qu'il s'agit d'individualiser, alors que les fibres déjà défaites passent rapidement au travers de la grille sans avoir aucun traitement mécanique. Ce phénomène est de ce fait favorable à la conservation d'un maximum de fibres longues. En outre, le taux de bûchettes de la pâte acceptée par le classement est réduit à une faible valeur.
• La deuxième raison est liée à une consommation d'énergie plus faible que dans les raffineurs à disques classiques, toutes conditions étant égales par ailleurs. Cela peut s'expliquer éventuellement par le fait que pour le traitement d'une même quantité initiale de matière, les fibres individualisées dès le début de l'opération ne sont pas soumises à l'action répétée des marteaux car elles sont rapidement évacuées. Il s'ensuit que l'énergie mécanique n'est effectivement dépensée que sur une quantité plus faible de matière. En outre, on travaille la pâte à basse concentration, il n'est pas nécessaire de concentrer les rejets de classement.

This for two important reasons:

• By its operating principle, the fibers subjected to the mechanical treatment are only those which form agglomerates which it is a question of individualizing, whereas the fibers already defeated pass quickly through the grid without having any mechanical treatment. This phenomenon is therefore favorable for the conservation of a maximum of long fibers. In addition, the rate of dough logs accepted by the classification is reduced to a low value.
• The second reason is linked to lower energy consumption than in conventional disc refiners, all conditions being equal. This can possibly be explained by the fact that for the treatment of the same initial quantity of material, the fibers individualized from the start of the operation are not subjected to the repeated action of the hammers because they are quickly evacuated. It follows that mechanical energy is effectively expended only on a smaller quantity of matter. In addition, the paste is worked at low concentration, it is not necessary to concentrate the classification rejections.

The pre-treatment to which the wafers are subjected before defibering them in the hammer mill is thus carried out at the interior of a twin-screw machine according to the teaching of patent FR 2,418,295. It appears, in fact, that by the short-term impregnation which occurs inside the machine, leading to a softening of the fiber connections, the optimal conditions are met for a good operation of a hammer apparatus which has tendency to open the fibers very quickly and fairly easily.

• la figure 1 est une représentation schématique partielle d'une installation de fabrication de pâte à papier à partir de plaquettes de bois :
• la figure 2 est une vue de face, en coupe transversale, d'un broyeur à marteaux susceptible d'être employé dans le cadre de l'invention.

Other details and advantages of the method will appear on reading the following description of an embodiment of the invention given by way of example and illustrated by the appended drawings where:

• FIG. 1 is a partial schematic representation of an installation for manufacturing paper pulp from wood chips:
• Figure 2 is a front view, in cross section, of a hammer mill capable of being used in the context of the invention.

FIG. 1 schematically represents part of an installation for producing high-yield cardboard pulp. The arrows in bold line indicate the material flows.

It includes a machine of the twin-screw type (1) supplied with wooden chips by a hopper (3) whose capacity is determined to guarantee a satisfactory supply, even in the event of a short-term interruption of the chain for handling the machine.

The recovery of the plates under the hopper is carried out by a twin-screw extractor (5) whose speed of rotation is controlled by that of the twin-screw machine to provide it with a supply suitable for all operating modes. The machine rotors are themselves driven by a motor (7).

Holes (9) are provided along the envelope of the twin-screw machine to allow the injection of a chemical reagent - which in this case is a solution of neutral sulfite of sodium hydroxide - inside the different treatment pregnant. Likewise, orifices (11) are provided for injecting the quantity of water necessary for controlling the dryness of the material being treated.
Downstream, the machine opens into an outlet chute (13) whose shape allows the distribution of the material flow towards the inlet of two hammer mills (20). A detailed description of these devices is given below. The installation example illustrated by the diagram refers to two grinders in parallel, but this number is not limiting. A single shredder may suffice if necessary.

The defibration operation in hammer mills requires a supply of water at (22) to reduce the dryness of the material. The raw pulp from the crushers falls into a vat (30) where it is stirred and taken up by a pump (32) to be directed to a battery of binders, two in this case (40,42), arranged in series.

The classification rejects are subjected to grinding in a third hammer mill (50) of the same type as the previous ones, also supplied with make-up water in (52) for the control of the dryness of the material which after grinding is recycled to the raw dough tank (30) through the conduit (56).

The accepted dough leaves the classification in (44) towards the suction of dough pumps to undergo various subsequent treatments before being accepted in the stock of pulp of the cardboard machine. It is thus, in a manner known per se, purified, for example by passage through cyclone depurator groups arranged in cascade, then directed onto thickeners making it possible to adjust its dryness to the value required for its passage through the cardboard machine.

FIG. 2 illustrates a model of hammer mills capable of being used in the process according to the invention.

It comprises two rotors (200) mounted on two horizontal shafts (202) inside a frame (204) open both in its upper part at (206) for the admission of the pulp to be defibrated and in its lower part (208) for the evacuation of the latter after treatment. Each rotor consists of parallel discs (210) separated by spacers (only one disc per rotor is visible in the figure). They are connected at their periphery by pins (214) parallel to the shaft. One spindle in two carries a row of parallelepiped hammers (216) oriented radially along their major axis (only one per row is visible in the figure). The hammers have a certain degree of freedom of rotation around their support axis.

In the example shown, each rotor is thus provided with four rows of hammers.

On either side of the vertical plane passing through the shaft of each rotor are disposed a central harrow (220) and a lateral harrow (222) whose teeth are interposed between the planes of rotation of the hammers, and extend substantially to the periphery of the discs.

A grid (230) in the form of a half cylinder longitudinally covers part of the lower half of each rotor (200) at a short distance from the surface swept by the free ends of the hammers. In this way the hammers cooperate with the peripheral grid to break and defibrate the material remaining in the device. Each grid is pierced with calibrated orifices (holes or slits) so as to allow the passage of individualized fibers while retaining the maximum of logs. The fibers having passed through the grid are entrained towards the outlet opening (208).

The calibration of these orifices (holes or slits) is decisive for obtaining the expected result. It is desirable that the diameter of the holes be between 1 and 5 mm, or when it is a slot that their width is between 0.4 and 2 mm. When the shape of the orifices is not cylindrical, these values apply to their smallest transverse dimension.

Nozzles (240) for injecting water are disposed across the intake opening (206).

Sheets (250) extend the cylindrical grids (230) to confine the enclosure swept by the rotor and limit projections. They are articulated around a longitudinal axis to give access to the rotor when maintenance is required.

The installation described above makes it possible in particular to obtain high-yield cardboard pulps, of the order of 92%, for example from fresh spruce.

The operation is as follows:

The plates are prepared before their admission into the hopper (3) they are sieved so that only those which have the appropriate dimensions are accepted, washed and free of all foreign matter.

They pass from the hopper to the entry of the twin-screw machine by the extractors.

This machine consists, in accordance with the teaching of patent FR 2,418,295, of two parallel screws whose threads are identical and which are driven in the same direction of rotation inside a sheath.

The threads consist of successive zones with different pitches. In a first section, the threading is first of all direct pitch for driving the material downstream, then the pitch is reversed to brake the material until it forms a plug.

The material passes in a second section to the other through calibrated windows, formed in the thread immediately downstream of the plug area, undergoing relaxation. The size of the windows is chosen so as not to reduce the length of the fibers but to simply split the plates.

In the second section the thread is again direct pitch, then reversed to form a second plug. The machine thus comprises a certain number of sections which follow one another from upstream to downstream.

Inside each section, the material goes through a series of alternating phases of pressure build-ups followed by pressure drops which allow the material to be impregnated with the desired reagent.

Indeed, because of the rotation of the screws in the same direction, the progression of the material from upstream to downstream, is accompanied by a alternation of compressions / loosening when it passes from one screw to another.

During each compression phase - which occurs inside the threads of a screw preceding the portion where they mesh with the threads of the other screw - the material tends to expel the liquid trapped between the fibers. During the following expansion - when it has passed between the threads of the other screw - the liquid again fills the interfiber spaces.

The excess liquid released during the compression phases can possibly be eliminated by filters placed in suitable places.

This crushing of the material inside each section, but also from one section to another, is thus favorable for its impregnation to the core by the reactive liquid.

By an appropriate choice of the characteristics of the machine and its operation: threads, speed of rotation of the screws, length of the braking zone, gauge of the windows, etc ..., one can raise the pressure inside machine up to high values, of the order of 100 bars, and obtain an appropriate temperature of the material of between 60 and 100 °. In this way, the platelets are impregnated at the heart without damaging the fibers.

Controlled quantities of water are introduced inside the different sections so that the advancement of the material occurs under optimal conditions.

Thus, while the wafers, at their introduction, have a humidity of 35 to 50%, the product produced by the machine comes out with a dryness of 25 to 30%. After such treatment, the material is only partially defibrated. Under the operating conditions of the exemplary embodiment presently illustrated, the rate of sticks is 55%. In order to perfect the individualization of the fibers while preserving their quality of length as much as possible, they are treated mechanically by means of the invention. The material is thus conveyed to the mills, where the pretreated fiber bundles are subjected, in accordance with the invention, to a mechanical separation treatment in the mill by the action of hammers cooperating with the harrows and the cylindrical grid.

Upon entering the mill, the material is sprayed with water by means of the nozzles so that at the outlet its dryness is reduced to a value between 2 and 20%.

The adjustment of the dryness allows an adjustment of the treatment in relation to the rate of sticks and to the classification BAUER MAC NETT.

By the action of the hammers sweeping the grid, the fibers which are not yet open are easily undone while the fibers which have already been individualized pass quickly through the calibrated orifices without having had the time to undergo mechanical stresses capable of cutting them. The production of a pulp with preservation of the length of the fibers is one of the important advantages of the process since only the heaps of fibers to be treated are subjected to the action of the mill. This is of course a statistical estimate. But the results show that, overall, the proportion of long fibers resulting from the treatment can in the present case be as high as 54%.

By long fibers is meant the proportion of fibers which are collected through a 28 MESH sieve. This result should be compared to that of a mechanical grinding wheel pulp for which the proportion of fibers collected through a 28 MESH sieve is 22%.

Another advantage of using a hammer mill under these conditions is the significant gain in energy achieved compared to defibering carried out in conventional refiners which, moreover, at an equivalent degree of refining, do not allow the conservation of such a high proportion of long fibers.

To illustrate the advantage of using a hammer mill, the same amount of wood was subjected to four different treatments, the first treatment A corresponding to that of the invention where the wafers were impregnated with neutral sulfite of sodium in a twin-screw machine then defibrated in a grinder of the type described above, the grid of which is pierced with holes of 2.5 mm in diameter.

In a second treatment B, the same raw paste leaving a twin-screw pretreatment is passed through two refiners in series of the DEFIBRATOR and BAUER 411 type to obtain a paste having the same degree of drainage.

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

Finally in a fourth treatment D, a wheel paste was produced.

The characteristics of BAUER MAC NETT are mentioned below for each pasta.

It can be seen that the treatment recommended by the invention produces a pulp whose proportion of long fibers collected in the T28 sieve is 54.4%, that is to say more than 10% greater than that obtained in conventional refiners with a proportion of fines less than 20%, this for equivalent degrees of drainage.

If we return to the description of the treatment of the material in the installation illustrated in Figure 1, the raw dough from the crusher whose dryness fell for example to 2%, only has a rate of sticks of the around 15%. The dough is collected in a vat room with classification rejects where it undergoes continuous brewing. At the exit of the vat, the dough is pumped towards the battery of binders which will eliminate the sticks. Thus the dough accepted by the classification contains only about 0.6% of sticks and can be subjected to conventional operations of purification and thickening before delivery to the stock machine.

The classification rejects are subjected to grinding in a hammer mill (50) similar to the first two (20), an addition of water (32) reduces the dryness of the dough which is returned to the vat (30).

The process which has just been described has the advantage of producing a high-yield pulp whose fiber length is kept to the maximum with a reduced energy expenditure. It is obvious that the hammer mills can be fed with other raw doughs than those resulting from a pretreatment with a twin-screw machine as long as this dough has sufficient impregnation to the core for optimal use in the shredders.

Finally, the installation may include a combination of conventional refiners and hammer mills if necessary.

Claims (6)

1. A method for the preparation of a paper pulp from wood chips consisting in pretreating these wood chips in a machine of the dual screw type (1) in which they are subjected to a sequence of compressions and pressure reductions with the injection of one or a plurality of solutions of at least one chemical agent, then in defibrating the material from the machine using a mechanical means, characterized in that after defibration the material is subject to a grading operation and in that the grading rejects are subject to defibration in an apparatus (50) of the hammer mill type.

2. A method as claimed in claim 1, characterized in that the moisture content of the grading rejects subject to defibration is controlled by the injection of water into the hammer mill such that on discharge from the mill the dry content of the machine is between 2 and 20%.

3. A method as claimed in claim 2, characterized in that after passing through the hammer mill the grading rejects are recycled upstream of the grading apparatus (40, 42).

4. A method as claimed in claim 1, characterized in that the defibration of the material from the machine of the dual screw type (1) is carried out in a moist atmosphere in an apparatus (20) of the hammer mill type, the dry content of the material on output from the apparatus being between 2 and 20%.

5. A method as claimed in one of the preceding claims, characterized in that the orifices of the grid of the hammer mill are holes with a diameter of between 1 and 5 mm.

6. A method as claimed in one of claims 1 to 3, characterized in that the orifices of the grid of the hammer mill are slots with a width of between 0.4 and 2 mm.

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