Method for producing a mat of cellulose fibres with a controlled level of mineral material for bitumen-impregnated roofing elements, and suitable device

Abstract

A method for continuously producing a mat of cellulose fibers intended for producing roofing elements made from bitumen-impregnated cellulose fibers, from a dilute slurry of cellulose fibers spread by a headbox on a dewatering fabric of a forming table having a predefined width, the dewatering fabric moving between an inlet of the forming table at the headbox side and an outlet of the forming table, moving the dilute slurry towards the outlet, gradually removing the liquids of the dilute through the dewatering fabric during the movement to obtain, at the outlet, the mat having a thickness of at least 2 mm, the dilute slurry having a certain level of pre-existing mineral materials. The method involves adding mineral material to the cellulose fibers by spraying a controlled quantity of aqueous solution or dispersion of mineral fillers over the width of the forming table onto the surface of the dilute slurry.

Claims

1. A method for the continuous manufacturing of a mat of cellulosic fibres from a dilute slurry of cellulosic fibres (3) spread by a headbox (6) on a dewatering fabric (8) of a forming table (1) having a determined width, the dewatering fabric moving between an inlet (6) of the forming table (1) on the headbox side and an outlet (9) of the forming table, carrying along the dilute slurry of cellulosic fibres (3) towards the outlet, the liquids of said dilute slurry of cellulosic fibres being gradually removed through the dewatering fabric during said movement in order to obtain at the outlet said mat, the dilute slurry (3) having a certain level of pre-existing mineral materials and mineral material is added to the cellulosic fibres by spraying with sprays of an aqueous solution or dispersion of mineral fillers, over the width of the forming table, onto the surface of the dilute slurry having already lost a part of its initial liquids, wherein the mat of cellulosic fibres is intended for the making of roofing elements made of cellulose fibres impregnated with bitumen and has a thickness of at least 2 mm at the outlet of the forming table (1), the line or zone of impact of the drops of the sprays of aqueous solution or dispersion of mineral fillers on the dilute slurry is placed at such a level, along the forming table, that the dilute slurry has lost at least 25% in weight of its initial liquids as they were present at the outlet of the headbox or has a mass concentration of solid per liter of dilute slurry higher than 33% with respect to the initial one, the quantity of added mineral fillers is controlled in order to obtain in the roofing element a determined level of total mineral materials resulting from the addition of the mineral fillers to the pre-existing mineral materials that it was possible to preserve in the mat, said control leading in producing a dry mat, with at least 99% of dryness, including between 15% and 25% in weight of total mineral materials, the sprays formed by the spraying nozzles are inclined with respect to the vertical and oriented in the direction of, or opposite to the direction of, displacement of said dilute slurry of cellulosic fibres, from the top to the bottom, and the kinetic energy of the drops of the sprayed aqueous solution or dispersion of mineral fillers at their arrival on the surface of the dilute slurry having already lost a part of its initial liquids is reduced by implementation of mirror nozzles (5).

2. The method according to claim 1, wherein the spraying of the aqueous solution or dispersion of mineral fillers is obtained by implementation of at least one linear spraying ramp (2) arranged over the width of the forming table (1), transversally to the feeding direction of the dilute slurry of cellulosic fibres, said at least one ramp being arranged at determined height(s) above the dilute slurry of cellulosic fibres (3) spread on the dewatering fabric, the aqueous solution or dispersion of mineral fillers being pressurized for being sprayed through spraying nozzles (5).

3. The method according to claim 1, wherein the spraying ramp (2) includes a plurality of spraying nozzles (5) transversally distributed to ensure an overlapping of the impacts of the drops coming from each pair of adjacent nozzles on the surface of the dilute slurry having already lost a part of its initial liquids.

4. The method according to claim 1, wherein the sprays formed by the spraying nozzles are inclined with respect to the vertical and oriented in the direction opposite to the direction of displacement of said dilute slurry of cellulosic fibres, from the top to the bottom.

5. The method according to claim 1, wherein the dilute slurry having already lost a part of its initial liquid, able to receive the spraying of aqueous solution or dispersion of mineral fillers, has a mean mass concentration of solid materials per liter of liquid comprised between 15 g/l and 30 g/l.

6. The method according to claim 1, wherein the line or zone of impact of the drops of the sprays of aqueous solution or dispersion of mineral fillers is placed at such a level, along the forming table, that the essential of the mineral fillers added remains towards the surface of the mat, the part of mineral fillers having been able to diffuse into the thickness of the mat not exceeding half the height of said mat.

7. The method according to claim 1, wherein it is added by spraying a quantity of mineral fillers such that the dry mat, with at least 99% of dryness, includes about 20% in weight of total mineral materials.

8. The method according to claim 1, wherein the mineral material of the mineral fillers is selected from the group consisting of kaolin, calcium carbonate, talcum, calcination residues, and mixtures thereof.

9. The method according to claim 1, wherein the sprays formed by the spraying nozzles are inclined with respect to the vertical and oriented in the direction of displacement of said dilute slurry of cellulosic fibres, from the top to the bottom.

Description

(1) The present invention will now be exemplified, without being limited thereby, by the following description of embodiments and implementations in relation with:

(2) FIG. 1, which shows a forming table on which advances a dilute slurry of cellulosic fibres that is currently losing its liquids and with one spraying ramp arranged above the forming table, a parallel graph showing the concentration in g/l and the dryness level in %, respectively, of the slurry along the table,

(3) FIG. 2, which shows a transverse view of the forming table with its spraying ramp and the circuit for feeding the spraying nozzles with the dilute solution of mineral fillers.

(4) In FIG. 1, the forming table 1 is viewed laterally with its spraying ramp 2 of mineral fillers. On this forming table circulates a dewatering fabric covered with a dilute slurry of cellulosic fibres 3 fed and spread more upstream on said dewatering fabric by a headbox 6. In FIG. 1, the dilute slurry of cellulosic fibres 3 that is carried along by the dewatering fabric circulates from the right to the left as indicated by the machine direction arrow. The dilute slurry of cellulosic fibres 3 hence includes more liquids on the right side than on the left side because these liquids leak by passing through dewatering fabric. That way, several zones can be distinguished along the forming table 1, between its inlet, on the upstream and headbox side, and its outlet, on the downstream side, firstly a first liquid zone then a transition zone, then a zone qualified as solid zone. In FIG. 1, the limits between these zones have been represented by precise lines but, in practice, it is understood that the passage between two zones is not abrupt. The passage between the transition zone and the solid zone is called water line and corresponds to the passage from a bright aspect to a matt aspect of the surface of the dilute slurry of cellulosic fibres 3.

(5) In this example, the spraying ramp 2 is placed at 2.6 meters from the inlet of the forming table, in the transition zone. It results therefrom that the zone of impact of the drops of the sprays of aqueous solution or dispersion of mineral fillers is located at such a level, along the forming table, that the dilute slurry has lost at least 25% in weight of its initial liquids, which corresponds to a mass concentration of solid per liter of dilute slurry higher than 33% with respect to the initial one, as shown in the following table (values before spraying for the measurements in the spraying zone):

(6) TABLE-US-00001 Component versus location Headbox Sraying Variation Water 985 g 740 g 25% Fibres + fillers 15 g 15 g Concentration 15 g/l 20 g/l +33%

(7) The spraying ramp 2 is a linear ramp that includes a set of aligned spraying nozzles 5 of the mirror type. The ramp 2 for the spraying of mineral fillers is installed on a gantry mounted as a bridge, over and crosswise, perpendicularly, with respect to the circulating dewatering fabric.

(8) By way of example of mirror nozzle, it can be mentioned the flat-spray nozzles of the series K (ref. KWG, KHW, KIW . . . ) of the PNR company, on its website www.pnr-nozzles.com. The spraying ramp 2 allows the spraying of an aqueous solution or dispersion of mineral fillers over the width of the forming table, onto the surface of the dilute slurry having already lost a part of its initial liquids. Each nozzle 5 can be made operative or not and its flow rate can be adjusted individually, thanks to a corresponding tap 4.

(9) As can be seen in FIG. 2, the aqueous solution or dispersion of mineral fillers 7 is pressurized to be sent in the spraying ramp. The aqueous solution or dispersion of mineral fillers 7 is stored, or even prepared, in a tank that includes mixing and stirring means so as to maintain a certain homogeneity in said solution or dispersion.

(10) Due to the inclined orientation of each nozzle 5 to produce a spray from the top and outlet of the table (downstream) to the bottom and inlet of table (on the headbox/upstream side), the fan spray of sprayed drops is hence oriented from the top to the bottom in the direction opposite to the direction of circulation of the dilute slurry of cellulosic fibres 3. A mirror nozzle produces a spray of drops in a direction that is more or less perpendicular to the main axis of the nozzle, according to the structure thereof. Hence, the drops fall on the surface of the dilute slurry of cellulosic fibres 3 more upstream/towards the inlet, on the headbox side, of the table with respect to the place where the spraying ramp 2 is located. Thanks to the use of mirror nozzles, the kinetic energy of the drops arriving onto the surface of the dilute slurry of cellulosic fibres 3 is reduced, which makes it possible to avoid the destructuration of the mat of cellulosic fibres in course of formation.

(11) Preferably, the fan sprays of each nozzle 5 overlap at least in part each other at the place where they arrive on the surface of the dilute slurry of cellulosic fibres 3. Still more preferentially, the overlapping or absence of overlappingthe sprays are only contiguousis adjusted so that the addition of mineral fillers is homogeneous over the width of the mat. It is understood that this adjustment may be manual or automated and may be obtained in various manner by varying the spraying pressure, the position in height or the orientation of the ramp . . . . In other variants, a tracer or a colorant, visible or not, but which can be detected, is added to the mineral fillers and it is controlled that it is uniformly distributed over the width of the mat. This tracer may be a substance useful for another function but that is detected for the purpose of controlling the spraying uniformity.

(12) The place along the forming table 1 where the drops arrive on the surface of the dilute slurry of cellulosic fibres 3 may be chosen according to various criteria. In all the cases, it is avoided to add mineral fillers too much towards the beginning of the forming table because the dilute slurry of cellulosic fibres 3 is still too wet/liquid. Likewise, it is avoided to add mineral fillers too much towards the end of the forming table because the dilute slurry of cellulosic fibres 3 has this time lost too much liquids and the mineral fillers won't be correctly absorbed by the mat, and moreover, the addition of new liquids, which are brought by the spraying, to the dilute slurry of cellulosic fibres 3 that has already lost the most part of its liquids would not be profitable. Hence, the addition of mineral fillers is made whereas the dilute slurry of cellulosic fibres 3 has a mean mass concentration of solid materials per liter of liquid comprised between 15 g/l and 30 g/l, and preferably of 20 g/l.

(13) Thanks to the invention, the total level of mineral materials in the finished product may be controlled and adjusted according to the needs. This control and adjustment may be made automatic or semi-automatic as a function of the availability of a sensor making it possible to measure in continuous or periodically the level of mineral materials in the mat or the finished product. It is understood that the more the measurement is performed upstream, close to the ramp, the more the level of mineral fillers in the production will be stable over time. However, this measurement must not be too close to the ramp in the case where a later loss of liquids would cause modifications of this level after the measurement and except absence of correction for this possible later loss.