Pulper for circulating a fibrous stock suspension

A pulper for circulating a fibrous stock suspension. The pulper includes at least one screen, having a screen surface which is provided with screen openings, and over which a rotor passes. The rotor is formed by a rotor head, to which a plurality of rotor blades are attached on the outer circumference. The rotor circle of the screen, which is passed over by the rotor wings, includes a plurality of bar-shaped elevations. In the process, pulping is to be improved, while wear is to be minimized, in that the angle of intersection between the working edge of the rotor blades pointing in the direction of rotation and the shear edge of the elevations pointing counter to the direction of rotation does not decrease radially outwardly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a pulper for circulating a fibrous stock suspension which is present in a vat.

2. Description of the Related Art

Pulpers are used to treat raw fiber materials, which are to be used in the form of a fibrous stock suspension in a machine for producing a fibrous web, and in particular a paper web. For example, fresh fibers or recovered paper fibers can be used as raw fiber materials in the process. For this purpose, the pulper and downstream devices are intended to comminute the raw fiber materials to yield individual fibers.

Pulpers generally include a screen plate including a rotor, which is to comminute the raw fiber material and suspend it in water. The accepted materials pass the screen openings, while the undesirable substances, such as foreign substances, are held back by the screen and removed.

The bars attached to such screens are usually intended to protect the screen element against wear. For this reason, these are also often made of particularly wear-resistant material, such as hard metal. In addition to the wear-reducing effect, however, they may also improve the function of the sorting device. Since the rotor blades are moved relatively closely past the screen element, the relative movement may create additional vortices, which favor the sorting opening being kept free. In the case of flake-containing fibrous stock, the cooperation between the rotor blade and the bar can achieve an extensive desirable pulping of the flakes.

Due to the wear, however, the distance between the rotor blades and the bars as well as the screen surface increases, which adversely affects pulping and efforts to keep the screen surface free, and thus also the specific energy requirement of the entire recovered paper treatment system. In addition, the wear increases with increasing contamination of the raw materials, so that the elements have to be replaced frequently.

What is needed in the art is a pulper which improves pulping, while at the same time minimizing wear and the energy requirement.

SUMMARY OF THE INVENTION

The invention provides a pulper that includes at least one screen, having a screen surface which is provided with screen openings and over which a rotor passes, wherein the rotor is formed by a rotor head, to which multiple rotor blades are attached on the outer circumference, and the rotor disk of the screen passed over by the rotor blades comprises multiple bar-shaped elevations.

According to the invention, the angle of intersection between the working edge of the rotor blades pointing in the direction of rotation and the shear edge of the elevations pointing counter to the direction of rotation remains constant radially outwardly, at least in sections, and/or increases, at least in sections.

In this way, it is achieved that the dirt along the shear edge of the elevation is able to migrate radially outwardly, and is not pinched between the rotor blades and the elevation. This promotes efforts to keep the screen free, and reduces wear. In addition, this counteracts further comminution of the impurities, in particular of plastic parts, which can thus be removed more easily. The reduced comminution also has process- and energy-related advantages.

The size, speed, and trajectory of the soil particles are of significance. If the soil particle is smaller than the distance between the rotor and the elevation, it is, in general, also smaller than the perforation in the screen, thus not resulting in any major problems.

If, in contrast, the soil particle is larger, it is to be removed to the outside, and not be pinched between the rotor and the elevation or the screen.

The contaminants are thus brought to a certain speed both by tractive forces in the suspension, and by contact with the rotor.

A distinction must be made between two cases. In the first case, the contaminant is not located in the immediate vicinity of the rotor and is entrained by the surrounding suspension. The forces acting on the contaminant, as well as the force acting by the contaminant on the screen or on the elevations, are then limited to the tractive forces in the suspension. The resulting wear on the elevation or the screen is low. Comminution of the contaminant itself will thus, in general, not take place. In the second case, direct contact occurs between the rotor, the contaminant, and the elevation or screen, wherein the introduced forces can increase considerably. As a result, the formation of the elevations according to the invention is significant.

Frequently, it already suffices when the angle of intersection between the working edge and the shear edge remains constant radially outwardly. So as to achieve the desired effect, however, it is advantageous in the process when the angle of intersection between the working edge and the shear edge remains constant radially outwardly across at least ⅔ of the length of the working edge, or even across the entire length of the working edge.

The least wear of the elevations arises when the constant angle of intersection between the working edge of the rotor and the shear edge of the elevation is 90°, since the fibers and impurities are then transported radially outwardly parallel to the respective elevation.

The transport of the impurities radially outwardly, however, can be intensified by radially outwardly increasing the angle of intersection between the working edge and the shear edge. The impurities are forced out of the wedge region.

It is advantageous in the process when the angle of intersection between the working edge and the shear edge increases radially outwardly across at least ⅔ of the length of the working edge, or even across the entire length of the working edge.

For pulping and wear, it has proven to be optimal when the angle of intersection between the working edge and the shear edge is between 80 and 120°, and preferably between 80 and 110°.

Depending on the fibrous stock and the process stage, the geometry of the elevations can be adapted in such a way that a greater deflaking or greater cleaning effect is achieved.

Depending on the type of the raw fiber material, or the shape and size of the vat, the screen surface or the rotor, it may be advantageous when the working edge of the rotor blades in the region of the passed-over screen surface and/or the shear edge of the elevations extends at least predominantly in a straight manner, or is at least predominantly formed by straight sections, or extends at least predominantly in a curved manner.

With respect to the load and the wear, but also for simplifying the design, it is advantageous when the screen has a rigid design.

Depending on the size of the vat, or the type and location of the rotor, it may be advantageous when the screen surface is curved or planar.

In the interest of a simple design, it often already suffices when the width of the elevations remains constant radially outwardly in the direction of rotation.

However, in particular in sections experiencing greater wear of the elevations, for example in the case of radially outwardly increasing angle of intersections between the working edge and the shear edge, it may be advantageous when also the width of the elevations increases radially outwardly, at least in sections, in the direction of rotation.

For efficient pulping, at least some, and preferably all, elevations should extend radially outwardly across the entire length of the working edges.

However, it may be advantageous, in particular with uneven wear of the elevations, when at least some elevations only extend radially outwardly across a section of the working edge.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly toFIG.1, there is shown a pulper. The pulper is formed by a cylindrical vat1that is open at the top, to which the raw fiber material and water are added. A drivable rotor5which includes multiple rotor blades7may ensure that the raw fiber material is mixed, comminuted, and suspended in water. The density of the stock in the pulper is between 2 and 7%.

The vat1is divided into two chambers by the screen3. The rejects at the screen3are discharged through the discharge connector13.

The accepts drawn off through the screen openings4via the accepts connector14in the form of the fibrous stock suspension2are further treated in downstream devices and processed into paper webs in a paper making machine.

The rotor5, or the rotor blades7thereof, during the rotation, pass over a surface of a screen3that has multiple screen openings4therein, thereby preventing clogging thereof.

The screen3, including the upstream rotor5, is usually situated at the bottom of the vat1, as is apparent inFIG.1.

However, it may also be advantageous or necessary to arrange this on a vat wall. Such horizontal pulpers are used in particular beneath the paper making machine during the treatment of scrap.

Due to the load experienced by the screen3, and to simplify production, it has a planar and rigid design. The screen openings4are circular, wherein, however, also oval or bean-shaped forms are possible, for example.

The rotor5, on the one hand, ensures that the raw fiber material is pulped and deflaked and, on the other hand, that potential impurities or foreign substances are cleared from the screen surface. In the process, the axis of rotation of the rotor5is situated perpendicularly on the screen surface.

The rotor5comprises a relatively large rotor head6, having by way of example six rotor blades7attached at the outer circumference thereof. By way of the large rotor head6, the formation of pockets of non-pulped, entrained raw material (plates or bales) can be effectively combatted.

On the side facing the rotor5, the screen3according toFIGS.1and2is provided with bar-shaped elevations8, which intensifies the pulping and cleaning action. If the raw materials have few impurities, the elevations8, as is apparent inFIG.8, can also include profiled, here saw tooth-shaped, shear edges12, which enhances deflaking.

These bar-shaped elevations8have a height of at least 2 mm above the screen surface and can be formed in one piece with the screen3, or as a separate element, or these can be formed in each case by a welding bead.

So as to simplify the use of as many rotor designs as possible, it may be advantageous to assemble the elevations8according toFIG.9from multiple straight sections, even if, as a result, the angle of intersection9only remains constant in sections and/or increases in sections.

The working edges11of the rotor blades7pointing in the direction of rotation10of the rotor5are, as is apparent inFIG.3, convexly curved in the direction of rotation10. If necessary, the working edges11, however, in accordance withFIG.5, can also, entirely or at least partially, comprise straight sections.

However, it is essential for the invention that the angle of intersection9between the working edge11of the rotor blades7and the shear edge12of the elevations8pointing counter to the direction of rotation10remains constant radially outwardly across the entire radial extension of the elevations8or, as is shown inFIG.3, increases radially outwardly and ranges between 90 and 110°.

FIG.4shows a variant in this regard, in which the angle of intersection9remains constant in the case of the elevation8illustrated with a continuous line, and increases in a radially outer section of the screen3in the case of the elevation8illustrated with a discontinuous line.

In particular in sections experiencing greater wear of the elevations8, for example in the case of radially outwardly increasing angles of intersection9between the working edge11and the shear edge12, it may be advantageous when the width of the elevations8increases radially outwardly in the direction of rotation, as is apparent inFIG.6.

In contrast thereto,FIG.7shows an approach in which the elevations8only extend across a radial section of the screen3. This allows optimizations, in particular in the case of irregular wear of the elevations8.

Regardless of the specific design of the working edge11of the rotor5and of the shear edge12of the elevation8, it is generally ensured that the impurities are pushed radially outwardly by the working edges11of the rotor blades7, along the shear edges12of the elevations8. Accordingly, the risk of dirt becoming pinched between the rotor blades7and the elevation8is decreased, which affects wear accordingly positively.

In addition, in this way also an optimization of the tractive and shear forces takes place, along with improved pulping and a reduced energy consumption.