EP0354846B1 - Screen for the purification and classification of a pulp suspension - Google Patents

Description

In the pulp industry and more particularly in the pulp industry from waste paper, a large number of sieves are used, either to separate the fibers forming the pulp from various impurities (called "contaminants") found in waste paper, in devices called "scrubbers"; or to sort the fibers according to their length in devices called "classifiers".

It is known to produce such sieves by providing them with holes, or slots, and it is also known by numerous patents such as FR 1,539,846; US 3,617 0̸0̸8; SE 72/11271; FR 840̸0̸658; FR-A-238488 and FR-A-2635125 to have upstream of the slits or holes made through the wall of the screen obstacles followed by grooves which, in cooperation with a hydrodynamic blade cause pulsations which improve the performance of the screen and prevent that it clogs.

However, these sieves, whether with holes or slits and whether or not they are fitted with obstacles, have so far been produced by machining solid plates with great difficulty.

Indeed, the slots and the holes must, for reasons of pressure drop and fouling be very short, that is to say of the order of 0 de, 5 to 1mm; on the other hand the sheets used in the current techniques are much thicker, of the order of 8 to 10̸ mm, for reasons of resistance and as the performances required by the users are increasing, the thicknesses of the sheets increase. It follows that it is necessary to machine a clearance in the thickness of the sheet and then clean and polish this clearance, these two operations representing the most important work that is carried out on the sheets.

The object of the present invention is to make it possible to produce very efficient sieves with relatively thin stainless steel sheets, of the order of 2 millimeters.

The technique, object of the present invention, which makes it possible to use such thin sheets while obtaining the necessary resistance, makes it possible to eliminate the essential clearances on thick sheets and greatly reduces the work of machining, cleaning and polishing . In addition, the consumption of raw material is lower.

This technique is based on the use of profiled elements whose cross-section is U-shaped. It is known from US Pat. No. 2,015,139 to make sieves by means of U-shaped profiles. This patent describes a flat plate consisting of 'U-shaped elements each having a bottom and two side walls. The elements are juxtaposed and held by welding two adjacent walls. The plate is machined in order to produce a succession of slots. But this machining also cuts the side walls and it is necessary to have stiffening bars to keep the flatness of the plate.

The present invention relates to a sieve for purification or hydrodynamic classification of paper pulp in a purifier or a classifier with a blade characterized by the juxtaposition of U-section elements comprising a flat bottom provided with perforations and two side walls, the elements U-shaped section being arranged such that they form a wall of revolution, in particular cylindrical, provided with slots or holes associated or not with grooves and obstacles.

The elements are arranged either parallel or perpendicular to the generatrices of the cylinder, or by forming with the direction of the generator of the cylinder an angle between 0 de and 90̸ °.

When the elements are parallel to the generatrices they are rectilinear and placed side by side; when they are perpendicular to said generatrices they are bent so as to be circular; when they are inclined relative to the generators they are wound in a spiral.

In the latter variant, the screen comprises at least one element with a U-shaped cross section. When the angle formed between the plane perpendicular to the longitudinal axis of the screen and the longitudinal axis of the element is close to 90 °, the screen comprises a plurality of inclined elements arranged in a spiral.

When the inclination α decreases, the screen may be formed with only one element wound in a spiral, the turns being contiguous.

The elements with a U-shaped cross section can be produced by pleating the sheet or else can be formed by U-shaped profiles which are placed side by side and held by any means.

In addition, in order to increase the rigidity of the cylinder, the screen comprises between two adjacent walls of two successive elements a flat element.

On the other hand, according to an alternative embodiment of the invention, the sieve can be produced by using U-shaped profiles whose wings are of unequal lengths but whose ends of said wings are in the same plane which is perpendicular to them so that the bottom of the U where the holes or slots are drilled is inclined; so that the perforated surface is inclined relative to the cylindrical surface of the screen.

If these asymmetrical section U are arranged along the generatrices of the cylindrical screen, obstacles are obtained which, depending on the direction of movement of the liquid, either slow it down or else cause the known effect of pulsation and / or swirling described in the previously cited patents.

If the asymmetrical U's are wound around a helix, we obtain a helical groove guiding the rejects (materials stopped by the sieve) towards their evacuation zone.

The asymmetrical U can be obtained by pleating a sheet or juxtaposition of separate elements as for symmetrical U.

• Figure 1: une vue schématique en perspective illustrant une portion de tamis réalisée en disposant côte à côte des profilés en U;
• Figure 2: une vue schématique d'une variante de réalisation de la figure 1;
• Figure 3: une vue schématique en perspective illustrant une portion de tamis réalisée par pliage de la tôle;
• Figure 4: une vue schématique d'une variante de réalisation de la figure 3;
• Figure 5: une vue schématique illustrant un tamis cylindrique selon l'invention dans lequel les éléments en U sont rectilignes et parallèles aux génératrices du cylindre.
• Figure 6: une vue schématique illustrant un tamis cylindrique selon l'invention dans lequel les éléments en U sont circulaires et perpendiculaires aux génératrices du cylindre.
• Figures 7a et 7b : deux vues schématiques illustrant un tamis cylindrique selon l'invention dans lequel les éléments en U juxtaposés sont disposés en spirale.
• Figure 8: une vue de profil du tamis de la figure 7.
• Figure 9: une vue de détail à grande échelle et en perspective illustrant la disposition des rainures et des fentes.
• Figure 10̸: une vue en élévation latérale de la figure 9
• Figure 11: une vue schématique d'une fraise réalisant simultanément la rainure et la fente.
• Figures 12 et 13 : deux vues de détail concernant un tamis selon la figure 6.
• Figure 14: une vue de détail correspondant au tamis de la figure 5.
• Figures 15,16,17 : trois variantes de réalisation de la jonction des profils.
• Figure 18: une vue schématique d'une variante de réalisation du tamis.
• Figure 19: une vue schématique de détail illustrant la juxtaposition de profilés à section en U dissymétriques.
• Figure 20̸: une vue en perspective de la figure 19.
• Figure 21: une vue schématique illustrant un tamis cylindrique réalisé au moyen de profilés en U dissymétriques disposés parallèlement à la génératrice du cylindre.
• Figure 22: une vue schématique illustrant un tamis cylindrique réalisé au moyen d'un seul profilé en U dissymétrique enroulé en spirale.
• Figures 23 et 24 : deux vues illustrant le mouvement relatif du liquide par rapport au tamis de la figure 20̸.
• Figure 25 : une vue partielle en perspective représentant une portion de paroi de tamis obtenu par pliage en réalisant des U dissymétriques.
• Figure 26 : une vue en coupe d'une variante de la figure 25.

By way of nonlimiting examples, the accompanying drawings are shown.

• Figure 1: a schematic perspective view illustrating a portion of sieve produced by placing side by side U-shaped profiles;
• Figure 2: a schematic view of an alternative embodiment of Figure 1;
• Figure 3: a schematic perspective view illustrating a portion of sieve produced by folding the sheet;
• Figure 4: a schematic view of an alternative embodiment of Figure 3;
• Figure 5: a schematic view illustrating a cylindrical screen according to the invention in which the U-shaped elements are rectilinear and parallel to the generatrices of the cylinder.
• Figure 6: a schematic view illustrating a cylindrical screen according to the invention in which the elements in U are circular and perpendicular to the generatrices of the cylinder.
• Figures 7a and 7b: two schematic views illustrating a cylindrical screen according to the invention in which the juxtaposed U-shaped elements are arranged in a spiral.
• Figure 8: a side view of the sieve of Figure 7.
• Figure 9: a large-scale detail view in perspective illustrating the arrangement of the grooves and slots.
• Figure 10̸: a side elevation view of Figure 9
• Figure 11: a schematic view of a cutter simultaneously making the groove and the slot.
• Figures 12 and 13: two detailed views of a sieve according to Figure 6.
• Figure 14: a detail view corresponding to the sieve of Figure 5.
• Figures 15,16,17: three alternative embodiments of the junction of the profiles.
• Figure 18: a schematic view of an alternative embodiment of the screen.
• Figure 19: a schematic detail view illustrating the juxtaposition of asymmetrical U-section profiles.
• Figure 20̸: a perspective view of Figure 19.
• Figure 21: a schematic view illustrating a cylindrical screen made by means of asymmetrical U-shaped profiles arranged parallel to the generator of the cylinder.
• Figure 22: a schematic view illustrating a cylindrical sieve produced by means of a single asymmetrical U-shaped profile wound in a spiral.
• Figures 23 and 24: two views illustrating the relative movement of the liquid relative to the screen of Figure 20 figure.
• Figure 25: a partial perspective view showing a portion of the screen wall obtained by folding by making asymmetrical U's.
• Figure 26: a sectional view of a variant of Figure 25.

Referring to these figures, it can be seen that according to the invention, a screen is produced from a sheet of thin thickness, between 1.5 and 2.5 mm, by juxtaposing elements 1, the cross section of which affects the shape of a U.

In Figures 1 and 2 we see that the screen consists of sections 1 in U, which are placed side by side. Each element 1 has a bottom 2 and two side walls 3; the elements 1 are joined to each other by their side walls 3, the bottoms 2 forming the cylindrical surface of the screen in which the holes, slots and / or grooves will be formed.

In FIGS. 3 and 4 it can be seen that the elements are produced by providing folds on a sheet metal plate, so as to also obtain side walls 3 and a bottom 2.

We can then practice (Figures 9 and 10̸) in said U-shaped grooves 5 and slots 6 perpendicular to the longitudinal axis of said elements, again perpendicular to the side walls 3.

The grooves 5 are therefore made in the bottoms 2, from the outer side opposite the ends of the side walls, perpendicular to their longitudinal axis but to a depth less than the thickness of said bottom 2, then a slot 6 is made in the bottom of the groove 5 to a depth greater than that of the thickness of the bottom 2 so as to pass through it. Preferably, a cutter is used consisting of two contiguous discs (FIG. 11), one 5a to make the groove 5 and the other 6a, of larger diameter to make the slot 6. A slot is thus obtained in a single operation 6 that is very exactly positioned relative to the groove, which is very important.

As can be seen in FIGS. 1 to 4, the U-shaped elements are made in such a way that at the bottom of the junction of two vertical walls 3 there is always a space 8 such that each groove 5 and slot 6 open out freely by their two ends in two empty spaces 8.

In the case of FIGS. 1 and 3, this requires that the radius of curvature R (FIGS. 9 and 10̸) of the surface making the junction between the bottom 2 and the side walls 3 is greater than the height "h" of the notch made in the wall 3 to pierce the bottom 2 when the slot 6 is made.

The same result can be obtained by connecting the walls 2 and 3 by oblique walls 9 as shown in FIG. 2 or even by not closing the folds of the sheet as shown in FIG. 4.

The advantage of this arrangement is that the slots 6 do not have end walls and that in this way they do not become blocked, neither during machining, nor during the use of the sieve by a heap of fibers.

So we get not only a very easy and very precise machining but also a screen that does not clog and that with a thinner sheet therefore less expensive.

In a first embodiment, the U-shaped elements can be straight and parallel to the generatrices of the cylinder as shown in FIG. 5 or else circular and perpendicular to these generatrices as shown in FIG. 6.

In the case of FIG. 5, it turns out that it is necessary to avoid that the U-shaped elements have too great a length because they then tend to bend. Elements of short length are then used so as to make a succession of small cylinders which are assembled on each other by circular rings 10̸, as shown in FIG. 14.

In the case of FIG. 6, the sheet is folded flat, and once the ribs corresponding to the walls 3 are made, the sheet is bent. It is practically impossible to achieve regular bending of a stainless steel sheet provided with ribs. However, we realized that if we proceed, after the creation of the folds, and before bending, the machining of the holes, the slots and possibly the grooves intended to make obstacles combined with the perforations (holes or slots), said bending takes place very easily and very regularly, thanks to the presence of the slots 6 and of the grooves 5. Then there is available at the lower part as well as at the upper part of the cylinder a fixing ring 11 which is fixed either to a closed fold as shown in Figure 12 or an open fold as shown in Figure 13. In a case as in the other the presence of this fold gives great elasticity to the assembly.

In a second embodiment (Figures 7a, 7b and 8), a cylinder screen is produced by a spiral winding of one or more U-shaped elements previously machined, that is to say comprising the perforations 6 (slots or holes) and possibly the grooves 5 previously described.

The sieve can be produced with a single element 1 of great length, produced either in one piece, or by butt welding a plurality of identical elements (FIG. 7a).

Preferably, the winding is carried out by fixing one of the ends of the element on a mandrel whose rotation makes the bending and the spiral winding. In this case, the turns have an inclination α relative to the plane perpendicular to the axis of the cylinder, slight by a few degrees.

The embodiment illustrates a cylindrical screen, but the invention is not limited to this conformation and extends to all forms of revolution, conical, cylindroconical, etc.

When the spiral winding of the element 1 is completed, the turns 13, 14 are joined together, so that they are strictly contiguous, in order to make any dough flow between two turns impossible.

During the step of perforating the elements, the grooves and / or the slots can be made not perpendicular to the longitudinal axis of the element, or to the side walls, but in a direction inclined by an angle α by relative to the perpendicular to the axis, this angle α being equal to the inclination of the turns of the sieve with respect to the plane 22 perpendicular to the longitudinal axis 23 of the sieve (fig. 8). This prior inclination α of the slots makes it possible to obtain slots parallel to the axis of revolution of the screen.

According to another embodiment, it is possible to produce a sieve by juxtaposing U-shaped elements. These elements being inclined in coils in spirals as illustrated in FIG. 7b. In this case, the angle α of inclination of the elements with the plane perpendicular to the longitudinal axis of the cylinder is close to 90̸ °.

Finally, the production of the sieve ends with the positioning, at each end of an end ring 18 which engages on the last turns and which determines a surface perpendicular to the axis of revolution of the sieve, as shown in Figures 1 and 2. These rings are intended to allow mounting of the screen in the body of the scrubber or classifier.

The assembly of the juxtaposed elements can be carried out according to several variants independently of the embodiment of the screen (rectilinear elements, figure 5, annular figure 6, or spiral figure 7).

In a first variant (Figures 15,16), the assembly is done either by conventional welding of the ends of the two adjacent side walls 3, with the addition of metal 15, or by continuous electrical welding of the adjacent wings.

In a second variant (FIG. 17), the assembly is carried out by the installation of a profile 16, or jumper, also in the general transverse shape of an inverted U, which comes to cap and pinch two adjacent wings.

The rider is continuous and, according to the embodiment, it is either rectilinear and maintains two adjacent walls over the entire length of the cylinder or between two crowns 10̸, or annular in the case of circular elements (Figure 6), or wound in spiral between two contiguous turns 13,14, all along the spiral

A third alternative embodiment is shown in Figure 18. In this alternative, the profiles are made with a thin strip, of the order of 0̸, 5 to 1 millimeter, the transverse dimensions of the profile being of the order of centimeter, for example with wings of 10̸ millimeters and a base of 20̸ millimeters; however, a sieve made with such a strip has a fairly great fragility. To stiffen it, provision is made to insert between two adjacent wings a flat metal element 17. This flat element is bent on edge and held contiguously between the walls 3 by electrical welding, preferably by continuous welding. This flat element 17 is approximately the same thickness as the strip, and its width is at least equal to the height of the wings. Preferably the flat element exceeds the wings by two or three times their height.

This variant has an important advantage: with U-shaped elements of small dimensions and small thickness, it is possible to make very small perforations (slots or holes). Slits of width ranging from one millimeter to ten microns can be obtained.

With such orders of magnitude, the deformation of the material during the bending of the element, has a great importance on the final section of the perforations: the metal on the concave side is compressed and the perforation closes, while on the opposite convex side it is stretched and the perforation opens. We then obtain for sieves with slits of perforations whose transverse shape is V which cooperates in the operation of the sieve.

The screen according to the invention has great technical and economic advantages. Technically, its realization is simple and can be largely automated. The use of thin metal sheet allows both reduced machining, therefore reduced metal loss, but also to achieve very fine perforations with precision and using traditional tools. Economically, such sieves are less costly in material, but above all much faster to produce; their construction cost is therefore significantly reduced.

Figures 19 to 24 relate to another alternative embodiment according to which the U-section profiles are no longer symmetrical as was the case in the previous figures but asymmetrical.

By asymmetrical U-shaped profiles is meant that the bottom 2 of the U, on which the perforations are made (holes or slits) and possibly the grooves associated therewith to produce obstacles, is inclined obliquely to the wings (instead of 'be perpendicular thereto) and that said wings 3a and 3b have unequal lengths so that their ends are in the same plane which is perpendicular to them. It is thus obtained that the bottom surface 2 of said profiles in which the perforations are made (holes or slots) and possibly the grooves 5 are inclined relative to the cylindrical surface of the screen.

Such asymmetrical U-shaped profiles and placed side by side are shown in Figures 19 and 20̸. In Figure 19, the unshaded area corresponds to the area where the perforations are made (holes or slots) and possibly the grooves.

These asymmetrical U-shaped profiles are used in exactly the same way as the previously described symmetrical U-shaped profiles. We can therefore either juxtapose them parallel to the generatrices of the cylinder as shown in Figure 21, or wrap them in a spiral as shown in Figure 22.

It will be noted that in the case of FIG. 21, obstacles are obtained which, depending on the direction of movement of the liquid or either slow it down (FIG. 23) or else cause the well-known effect of pulsation and / or swirl (FIG. 24 ).

All that has been described previously in relation to symmetrical U-shaped elements is applicable to asymmetrical U-shaped elements. In particular, it is possible to produce a sieve by folding by making asymmetrical U's, as shown in FIG. 25. By comparing FIGS. 25 and 26, it can be seen that one can have a sheet fold at each dissymmetrical U element, as shown in Figure 25, or have a sheet metal fold between several asymmetrical U-shaped elements (Fig. 26).

Claims (20)

1. Screen for the hydrodynamic purification or classification of paper pulp in a purifier or classifier with blades, characterised by the juxtaposition of components having a U-shaped cross-section comprising a flat base (2) provided with perforations and two lateral walls (3), the components (1) having a U-shaped cross-section being disposed such that they form a wall of revolution, which is in particular cylindrical, provided with slots or holes which may optionally be associated with grooves and/or obstacles.
2. Screen according to Claim 1, wherein the cross-section of the components is in the shape of a symmetrical U, i.e. comprising two parallel wings (3) of the same length and a base (2) perpendicular to the said wings in which the perforations and where applicable the grooves are provided.
3. Screen according to Claim 1, wherein the cross-section of the components is in the shape of an asymmetrical U, i.e. comprising two wings (3a, 3b) of unequal length, the base (2) in which the perforations (6) and where applicable the grooves (5) are provided being disposed obliquely relative to the wings such that the ends of the wings are in the same plane which is perpendicular thereto, and such that the said base is inclined relative to the cylindrical surface of the screen.
4. Screen according to Claim 1, characterised in that the components (1) are disposed either in a parallel manner or in a perpendicular manner relative to the generating lines of the cylinder, or forming an angle of between 0 and 90° with the direction of the generating lines of the cylinder.
5. Screen according to Claim 1, characterised in that it comprises at least one component (1) having a U-shaped cross-section which is curved and wound into a spiral.
6. Screen according to Claim 5, characterised in that it comprises a single component (1) having a U-shaped cross-section, wound into a spiral, the turns (13, 14) being joined and held tightly close together by any suitable means (15, 16).
7. Screen according to any one of the preceding claims, wherein the components (1) with a U-shaped cross-section are formed by U-shaped profiled sections.
8. Screen according to any one of Claims 1 to 5, wherein the components (1) comprise a series of U-shapes produced by the folding of a metal sheet.
9. Screen according to Claim 1, wherein the walls (2) and (3) of the components (1) are connected by oblique planes (9).
10. Screen according to Claim 1, wherein the cylinder is formed by a folded metal sheet, the folds not being closed.
11. Screen according to Claim 4, wherein the cylinder is secured at its lower and upper ends by a collar (12) secured to one of the edges of a fold.
12. Screen according to any one of Claims 1 to 5, characterised in that the adjacent components (1) are held contiguous by means of a weld (15) extending along the entire length of the juxtaposed area.
13. Screen according to any one of Claims 1 to 7, characterised in that the adjacent components (1) are held contiguous by a U-shaped profiled section (16) which clamps the adjacent walls (3) of the components and extends along the entire juxtaposed area.
14. Screen according to Claim 1, characterised in that the components (1) have small dimensions in cross-section, of the order of 10 mm and are of the order of 1 mm thick, and comprise perforations (6) in the form of slots of which the width is between approximately 10 µm and 1 mm.
15. Screen according to Claim 1, characterised in that it comprises between two adjacent walls (3) of U-shaped components a flat component (17), curved on its edge, for ensuring the rigidity of the wall of the said screen.
16. Screen according to Claim 15, characterised in that the thickness of the flat component (17) is similar to the thickness of the wings (5, 6) of the U-shape and the width thereof is at least equal to the height of the wings.
17. Screen according to any one of the preceding claims, characterised in that it comprises grooves (5) and slots (6) provided in the base (2) of the U-shaped components, perpendicular to the side walls (2); the grooves (5) having a depth which is less than that of the thickness of the sheet metal, the slots (6) being deeper.
18. Screen according to any one of the preceding claims, characterised in that the U-shaped components (1) are produced such that, at the base of the join between two vertical walls (3), there is provided a space (8) into which the ends of the grooves (5) and of the slots (6) lead.
19. Screen according to Claim 18, in which the radius of curvature (R) of the surface joining the base (2) to the side walls (3) is larger than the depth h of the flat indent produced in the base (2) and the said walls (3) in order to produce the slot (6).
20. Screen according to Claim 3, characterised in that the components comprise perforations in the form of slots (6) provided in the base of the U-shape, the axis (20) of the slots forming with the plane transverse to the component an angle a having a value substantially equal to the angle of inclination of the turns relative to the plane (22) perpendicular to the longitudinal axis of the screen.

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