EP1050342B1 - Method for removing impurities from an aqueous paper fiber suspension - Google Patents

Abstract

Selective flotation stages (1,2) are used to separate foreign matter from a fiber suspension (S). The downstream flotation s (2) uses the residual air contained in the cleaned suspension flow (A1) from the first stage (1), without additional air to give bubbles for flotation. The second and downstream stage (2) acts as a flotation cyclone, with a centrifugal acceleration of the hydrocyclone of a v which is 5-200 times the rate of the spin of the earth, or 10-100 times. At the second and downstream flotation stage (2), the suspension from the first stage (1) is fed at a tangent into a cylinder. The flotation foam (R2) formed at the second stage (2) the cleaned suspension (A2) move out of the stage (2) at the axially opposite end to the inlet. The flotation foam (R2) is taken axially from the side of the second flotation stage (2) which also has the inflow, and the cleaned suspension (A2) is taken axia from the opposite side. The weaker force field used is the gravity of the earth. The cleaned suspension (A1) from the first flotation stage (1) is passed directly to the downstream flotation stage (2), without dwelling in a vat.

Description

The invention relates to a method for removing contaminants from an aqueous paper fiber suspension according to the preamble of claim 1.

Processes of the type mentioned are used to separate out of a paper fiber suspension at least part of the unwanted solid particles suspended therein, so-called impurities. As is known, in a flotation, a foam or scum containing the substances to be excreted is formed. A typical application of such a method is the treatment of an aqueous fiber suspension obtained from printed waste paper in which the ink particles are already detached from fibers, so that they can be floated. The flotation process described here exploits the differences between papermaking pulp and unwanted particulate matter such that the pulp remains in the fiber suspension due to its rather hydrophilic nature, while the particulate matter in question is hydrophobic and therefore enters the foam together with the air bubbles. In addition to the ink particles there are also a variety of other substances that are hydrophobic and therefore can be separated by flotation of the pulp. Such substances are in particular adhesives, fine plastic particles and possibly also resins. Since the flotation process mentioned here separates fibers from impurities, this is called selective flotation. The term "flotation deinking", which is also used, is generally used not only for the removal of ink particles (ink), but also more generally for the selective flotation of impurities from pulp suspensions.

JP 60134090 A shows a combination of a flotation cyclone with a subsequently connected separating vessel (secondary process vessel). The flotation foam of the flotation cyclone is passed to the second container for aftertreatment.

Particularly good effect have processes with two successive Flotationsschritten whose action are each differently adapted to certain types of contaminants. Thus, it is known from EP 0 888 819 A1 (DE 197 28 393) to carry out first a flotation in the normal earth gravity field and then another with the aid of a flotation cyclone. This combination allows the removal of impurities whose particle size is present in a broad spectrum. In the second sub-step, in particular, the smaller particles present in the pulp suspension are floated by an optimally tuned Starkfeld flotation device whose force field is stronger than earth gravity. It can then be e.g. also detect such ink particles that were bonded to the fibers by oil-based binders.

DE 44 26 159 A1 describes a process in which the pulp suspension to be purified first passes into a flotation cyclone operating with centrifugal forces and then into flotation cells of conventional design which operate essentially solely on the basis of gravity. The flotation foam of the latter flotation cells - possibly also of the flotation cyclone - is purified in another flotation cyclone which forms the secondary stage.

Although these two methods work advantageously in combination with different flotation steps, they are relatively expensive since each substep contains a complete flotation with renewed aeration.

The invention is therefore based on the object to provide a flotation, with which the spectrum of floated impurity particles is large, without too high equipment and operational effort is needed. In special versions should also be done a venting of the suspension.

This object is achieved by the characterizing features of claim 1 in its entirety.

The inventive method has the advantage that a wide range of sizes in the flotation can be detected by - as known per se - at least two optimized to a respective specific size range sub-steps are connected in series. In this case, the second part of the step without further ventilation of the suspension is carried out, which significantly reduces the cost of equipment, energy and process control. The air content in the accept of the upstream flotation step is utilized for further flotation. It is of particular advantage that the downstream flotation step is carried out in a stronger force field, e.g. that of a flotation cyclone. Such apparatus can be particularly effective with the help of small bubbles relatively fine impurities precipitate.

Another advantage of the method is that only the residual air that is still present and often undesirable in the accept of the upstream flotation step must be removed by the downstream flotation step so that a low air content suspension is formed. This affects e.g. during operation of subsequent disc filter for thickening of the suspension from low.

In the illustrations of some publications, the aeration is not plotted at the individual flotation steps, see e.g. in figures of DE 197 28 393 A1. The person skilled in the art is, however, undoubtedly certain that such simple details have been omitted here merely for reasons of simpler representation. The same applies, e.g. for pumps, control equipment and fasteners.

Fig. 1

ein Schema des erfindungsgemäßen Verfahrens;

Fig. 2

eine Variante des Verfahrens;

Fig. 3

schematisch: Eine erfindungsgemäße Anlage;

Fig. 4

schematisch: Flotationszylinder der Fig. 3 in Ansicht von oben.

The invention will be explained with reference to drawings. Showing:

Fig. 1

a scheme of the method according to the invention;

Fig. 2

a variant of the method;

Fig. 3

schematically: A plant according to the invention;

Fig. 4

schematically: flotation cylinder of Fig. 3 in top view.

Fig. 1 shows a very simple scheme with an upstream flotation step 1 and a downstream flotation step 2. The pulp suspension S, before being fed to the upstream flotation step 1, is mixed with air L in a mixer 3 so that air bubbles of the desired size and number arise. The processes of the flotation step 1 are known; a flotation foam R1 is formed, which contains a large part of the impurities, and an accept A1, which is formed by the purified pulp suspension. This is fed without further ventilation via the pump 5 to the downstream flotation step 2. While in the example shown here the upstream flotation step 1 is operated solely in the earth's gravity field, a centrifugal field is generated in the downstream flotation step 2 by rotation of the suspension. As a result, as already described, finer impurities can be separated by means of smaller air bubbles, so that they pass into the flotation foam R2. This centrifugal field is advantageously not so strong that there is a risk of separating air bubbles and contaminant particles. Favorable values are between 10 and 100 times the acceleration of gravity. The pulp suspension which has been purified again in this way is fed to accepts A2 for further use. The two flotation foams R1 and R2 are removed together here as reject R and discarded. As is known, however, such rejects can also be treated separately or jointly in one or more secondary stages in order to avoid fiber losses.

The method shown schematically in Fig. 2 differs in some ways from that in Fig. 1 and shows another way to realize the invention. In this case, the upstream flotation step 1 consists of a plurality of flotation devices connected in series. Such a circuit is common in many cases and includes, for example, five to eight flotation processes connected in series. In this case, the accept, before it comes from the previous in the next flotation, promoted by a pump and mixed again with air. The accept A1 originating from this upstream flotation step 1 passes into a flotation cyclone which performs the downstream flotation step 2. Unlike in the example according to FIG. 1, here the flotation foam R2 is taken from the same side on the axial side, on which the accept A1 is also introduced into the flotation cyclone. According to the invention, there is no re-ventilation. The accept A2 originating from this flotation step is drawn off at the axially opposite end, either tangentially or axially (dashed arrow).

FIG. 3 shows the flotation devices which can be used for the flotation steps in somewhat more detail. The gravitational effect flotation cell (flotation step 1) has an oval shape and forms a substantially closed housing. As a result, the air L escaping the flotation foam R1 can be sucked in again inside the flotation vessel. The mixer 3 'for air L and pulp suspension S is part of a mixing tube, which dips into the suspension. Of course, another ventilation may be provided. The downstream flotation step 2 is carried out in a cylinder 6 in which the inflowing suspension, that is to say the accept A1, of the first flotation step is pumped in without further aeration and is pumped e.g. is set in rotation by a tangential inlet. Both the flotation foam R2 and the accept A2 are here withdrawn at the opposite end of the cylinder 6 in the axial direction of the inlet. Another solution in this regard is shown in FIG. 2. To control the amount of foam, as shown here, a throttling of the accepts A2 by means of an adjustable throttle valve 7 can be made. Again, both flotation foams R1 and R2 are brought together again, which is already advantageous because then also the air exiting from the flotation foam R2 can get into the mixer 3 '. Instead of air L, a special gas can also be used.

FIG. 4 shows in plan view that the apparatus used for the downstream flotation step 2 is a cylinder 6. One recognizes the tangential inlet for the accept A1 of the upstream flotation step (1) and the tangential exit for the accept A2.

Claims (9)

1. A method for removing anionic trash from an aqueous pulp suspension (S) using a plurality of selective flotation steps (1, 2), of which an upstream flotation step (1) is operated with a weaker field of force than the flotation step (2) immediately downstream, and
   in which the accepted stock (A1) of the upstream flotation step (1) is guided into the downstream flotation step (2),
   characterised in that
   the downstream flotation step (2) is operated using the residual air contained in the accepted stock (A1) of the upstream flotation step (1) and without further addition of air.
2. A method according to Claim 1,
   characterised in that
   the downstream flotation step (2) is carried out in a flotation cyclone.
3. A method according to Claim 2,
   characterised in that
   the centrifugal acceleration in the hydrocyclone is set to values of between five and 200 times the acceleration due to gravity.
4. A method according to Claim 3,
   characterised in that
   the centrifugal acceleration in the hydrocyclone is set to values of between ten and 100 times the acceleration due to gravity.
5. A method according to Claim 1,
   characterised in that
   the accepted stock (A1) for the downstream flotation step (2) is introduced tangentially into a cylinder (6).
6. A method according to Claim 2, 3, 4 or 5,
   characterised in that
   both the flotation froth (R2) formed in the downstream flotation step (2) and the accepted stock (A2) on the side axially opposite the inlet are removed from the flotation device used in the downstream flotation step (2).
7. A method according to Claim 2, 3, 4, 5 or 6,
   characterised in that
   the flotation froth (R2) formed in the downstream flotation step (2) is removed from the flotation device which is used axially on the side on which the inlet is also located, and the accepted stock (A2) on the axially opposite side.
8. A method according to one of the preceding claims,
   characterised in that
   the weaker field of force is the Earth's field of gravity.
9. A method according to one of the preceding claims,
   characterised in that
   the accepted stock (A1) from the upstream flotation step (1) is guided directly and without a vat to the downstream flotation step (2).

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