EP0801989B1 - Flotation method - Google Patents

Description

The invention relates to a flotation process for separating solids from a pulp-containing suspension according to the preamble of claim 1.

Methods of this type are e.g. known from DE-OS 34 01 161. Your application lies in the paper industry for the removal of printing inks, adhesives or other disturbing impurities, which mainly come from the waste paper. Doing so due to its hydrophobicity, the fiber is removed as a good substance in the suspension, while the contaminating solids are discarded with the foam. Because of this division of the solid streams into fibers and impurities is called selective flotation. There are other uses of such methods known in which as much of the solids as possible removed by flotation e.g. in the treatment of waste water generated by pressing in the Paper industry. In such cases one speaks of clarification flotation or because of Fumigation mechanism of relaxation flotation.

From US 4,490,259 a flotation apparatus is known in which a liquid first mixed with the required flotation chemicals and then in the actual flotation vessel is entered. The ventilation takes place only in the Flotation vessel, with the help of an electrolysis. In one within the Flotation vessel arranged mixing element, which has several diffuser-like steps then the gas bubbles are mixed with the suspension.

The flotation container according to EP 0 243 690 A, in which the Aeration of the suspension already inside the feed pipe into the flotation vessel he follows. The feed pipe is thus designed as a mixing pipe, whereby proposed is to distribute the air in the flowing suspension with the aid of orifices so that a kind of diffuser effect can be achieved again.

As already mentioned, such methods have been in use for a long time and have one achieved a relatively high standard of effectiveness. Nevertheless, the requirement arises further improvement of the effect.

The invention is therefore based on the object of a flotation process with a better separation effect and / or a lower specific energy requirement create. Better separation means that the amount of unwanted removed Components of the paper pulp suspension is further increased, that is, a better one Cleanliness of the paper pulp is achieved or less fiber loss or both.

This object is completely achieved by the features in claim 1.

The main effects of the method according to the invention can be described as follows:
The momentum exchange in the area of the mixing element is such that the specifically necessary energy dissipation takes place in order to promote the attachment of hydrophobic solid particles to the bubbles. Neither the air bubbles are adversely changed, nor are particles already attached to air bubbles torn off again. Rather, if desired, the bubble size can be better influenced by the method. It has been shown that a bubble size spectrum in accordance with the requirements, in particular in fiber suspensions, is difficult to produce or unstable by conventional injectors alone. In the new process, the entire energy conversion takes place under particularly favorable conditions, in particular in comparison to the known injectors, since the speeds can be lower because of the larger volumes involved. This is achieved by setting the flow velocity in the mixing element to a maximum of 2 m / sec, in special cases a maximum of 1 m / sec. It is also favorable to restrict the volume of the gas bubbles in the mixing element intended for flotation to at most three times the suspension volume.

The suspension located in the flotation container receives a rotational movement, since a suction which is effective within the flotation container is generated with the aid of the mixing element. The mixing element can have a relatively small mixing volume of at most 5%, based on the suspension volume in the flotation container. It can have a flow channel with a constant flow cross-section that is between 0.001 to 0.05 m ^{2 in} size. The inflow cross section into the mixing element is advantageously 1.5 to 5 times as large as the area of the inlet opening for the suspension in the flotation container.

Due to the suction mentioned, certain portions of the flotation tank can located suspension several times with the freshly poured in and fumigated Suspension stream are mixed. This recirculation movement therefore increases the Probability that solid particles to be separated with the entered Air bubbles can come into contact. This also separates the Flotation process improved. The suction effect described is also suitable for Deduct vertebrae, which in the prior art directly at the mouth of the Inlet pipe into the flotation tank can arise. These vortices disturb them Flotation and unnecessarily cost energy. If they are suctioned off according to the invention, their Energy can be used for fumigation and mixing.

On the other hand, the rotational movement is controlled in such a way that it does not cause flotation can harm. That means: The movement of the gas bubbles to the surface still takes place in the necessary manner instead, since the circulating flow is local to a small part the suspension in the flotation tank is limited. Preferably leaves the suspension the mixing element in approximately horizontal direction, which is also the case with vertical arranged inlet pipe applies.

Fig. 1

die Durchführung des Verfahrens anhand eines schematisch dargestellten Flotationsapparates;

Fig. 2

eine erfindungsgemäß verwendete Mischvorrichtung, schematisch;

Fig. 3

eine Variante mit geänderter Strömungsführung;

Fig. 4

eine weitere erfindungsgemäß verwendete Mischvorrichtung, schematisch;

Fig. 5, 6 und 7

schematisch: Weitere Ausführungsformen der erfindungsgemäßen Mischvorrichtung.

The invention and its advantages are explained with reference to drawings. Show:

Fig. 1

the implementation of the method using a schematically illustrated flotation apparatus;

Fig. 2

a mixing device used according to the invention, schematically;

Fig. 3

a variant with changed flow guidance;

Fig. 4

another mixing device used according to the invention, schematically;

5, 6 and 7

schematic: further embodiments of the mixing device according to the invention.

1 shows a schematically illustrated flotation apparatus, by means of which the method according to the invention can be explained. The flotation apparatus contains a flotation container 1 which is not completely drawn. When the method is carried out, this is largely filled with suspension, on the surface of which, as is known per se, a foam 8 is formed which contains as large a part as possible of the components to be flotated out. It can flow off via a foam weir as reject R. The suspension S 1 containing paper pulp enters the flotation container 1 through an inlet element 6 having an inlet opening 2. As shown here, it is mixed with gas G, for example air, before it enters the flotation container. According to the invention, the inflowing suspension is passed through a mixing element 3, the inflow opening 4 of which is located at a distance a from the inlet opening 2. This distance a is advantageously at most 1 m. As indicated by arrows, part of the suspension S 2 located in the flotation container 1 is sucked into the space between the inlet opening 2 and the inflow opening 4. This space has the
Function of a collecting room 7 (Fig. 2). The suspension cleaned by flotation exits the flotation container 1 as accept substance A through the outlet opening 5. In many cases this is a fiber suspension freed from foreign substances or in special cases clarified water, in which the largest possible proportion of all the solids contained has been removed by the flotation.

2, mixing element 3 and the associated inlet element 6 are somewhat more detailed, but still drawn schematically. The collecting space 7 is indicated by dashed lines between the inlet opening 2 and the inflow opening 4. In the direction of flow seen, the mixing element 3 has the extension c. A special feature is the offset b between the center lines of both openings. This offset can be used to regulate the Mixing effect can be changed, especially if the Changing the distance a is difficult. That would be the case with radial flow guidance Case, such as in Fig. 6 and 7. The offset b affects the recirculation of the Suspension within the flotation tank. The arrangement shown in Fig. 2 favors the suction of the suspension above the mixing element due to the offset Third

In the embodiment shown in Fig. 1, the inlet opening 2 is not flush with the wall of the flotation container 1, which can have advantages in flotation. In a modification of this, the inlet opening 2 can also be in the container wall lie.

Among other things, the size of the distance a determines the mixing and thus also the Flotation. Therefore, changing them can definitely be used as a control option of the flotation process can be used. 3 is a simple example of this shown, in which the mixing element 3 slidably connected to the inlet element 6 is. This shift can also be done with a motor and the motor as an actuator function of a control loop. The realization of the adjustability is different Wise possible and familiar to the expert.

Fig. 4 shows a very special device to use the method with particular simple means to perform. Here, namely, the mixing element 3 forms with the Inlet element 6 one unit. The suction of the already in the flotation tank located suspension takes place through elongated holes upstream from Mixing element 3 are attached. In this way, this is also simple Device a distance a between the inlet opening 2 and the inflow opening 4, these openings are indicated here as ovals.

In Fig. 5 the section through a flotation tank 1 'is shown, which is different 1 of which has an oval cross section. In addition, the suspension S 1 in this container is supplied and aerated by an inlet element 6 ', which is a has cylindrical inlet opening. This causes the suspension to float to flow radially from the inside out into the flotation container 1 '. This known per se Flow control has considerable advantages in flotation. The inlet element 6 ' can advantageously, as shown here, eccentrically attached in the flotation tank 1 ' his. The method according to the invention is also in such a flotation device applicable, the mixing element 3 'due to the radial inflow of the suspension is positioned essentially annularly around the inlet opening 2 '.

The conditions in the described area of the inlet element 6 'are somewhat shown in more detail in FIG. The mixing element has relative to the inlet opening 2 'a height offset. This is not necessary, but can be used to regulate the Mixing in the flotation tank, as already mentioned, can be used. The Mechanical implementation of this adjustment option is not shown here, it is familiar to the expert.

In cases where the suspension from the mixing element is in a radially outward direction directional flow emerges, the mixing element 3 "can also be of the type it is shown in Fig. 7. Here the suspension S 1 is through the inlet element 6 ' entered vertically into the flotation tank 1, with both the inlet opening 2 " as well as the inflow opening 4 "are horizontal Essentially vertical flow, in which a mixture of the already in the Flotation container suspension S 2 with the newly introduced suspension S 1 takes place. The redirection from the vertical to the horizontal takes place within of the mixing element 3 ″, which has a correspondingly designed channel. Here too can again, if desired, the mixing element 3 "relative to the inlet part 6 ' be slidably arranged to thereby control the flotation process.

Claims (13)

1. A flotation process for separating off solids from a suspension containing paper stock, in which first of all the gas bubbles intended for flotation are admixed to the suspension (S1) and these are then introduced through an inlet opening (2, 2', 2") into a flotation vessel (1, 1') and cleaned accepted stock (A) is discharged through an outlet opening (5), at least part of the solids present in the suspension (S1) being concentrated by flotation in a froth collecting on the surface and then being removed from the flotation vessel (1, 1'),
   characterised in that
   at least the predominant part of the suspension which has flowed into the flotation vessel (1, 1') is passed through a mixing element (3, 3', 3") such that before entering the mixing element (3, 3', 3") a portion of the suspension (S2) already present in the flotation vessel (1, 1') is drawn in and together therewith flows through the mixing element (3, 3', 3") at a rate of at most 2 m/sec, with mixing of these flows taking place.
2. A flotation process according to Claim 1, characterised in that the flow through the mixing element (3, 3', 3") takes place at a rate of at most 1 m/sec.
3. A flotation process according to Claim 1 or 2, characterised in that the mixing element (3, 3', 3") is located within the flotation vessel (1, 1'), its inflow opening (4, 4', 4") being at a distance (a) of at most 1 m from the inlet opening (2, 2', 2").
4. A flotation process according to Claim 1, 2 or 3, characterised in that the mixing volume within the mixing element (3, 3', 3") is at most 5% of the suspension volume in the flotation vessel (1, 1').
5. A flotation process according to one of the preceding claims, characterised in that the distance (a) between the inlet opening (2, 2', 2") and the inflow opening (4, 4', 4") of the mixing element (3, 3', 3") is adjustable.
6. A flotation process according to one of the preceding claims, characterised in that the position of the mixing element (3, 3', 3") relative to the inlet opening (2, 2', 2") is adjustable transversely to the direction of incoming flow, so that an offset (b) can be produced as desired.
7. A flotation process according to Claim 5 or 6, characterised in that the adjustment is altered during operation as a function of measured values which characterise the flotation effect.
8. A flotation process according to one of the preceding claims, characterised in that the suspension (S1) enters the flotation vessel (1, 1') through the inlet opening (2, 2', 2") in a horizontal flow.
9. A flotation process according to one of Claims 1 to 7, characterised in that the suspension (S1) enters the flotation vessel (1, 1') through the inlet opening (2, 2', 2") in a vertical flow.
10. A flotation process according to Claim 9, characterised in that the flow of suspension in the mixing element (3") is deflected such that it emerges therefrom radially outwards.
11. A flotation process according to one of Claims 1 to 7, characterised in that the suspension (S1) enters the flotation vessel (1') through the inlet opening (2') in a horizontal radial flow and that the flow through the mixing element (3') is radially from the inside to the outside.
12. A flotation process according to one of the preceding claims, characterised in that the volume of the gas bubbles intended for flotation in the mixing element (3, 3', 3") is at most three times the suspension volume.
13. A flotation process according to one of the preceding claims, characterised in that the suspension (S1) before introduction into the flotation vessel (1, 1') together with gas dissolving therein is placed under excess pressure and that the gas bubbles intended for flotation are produced by subsequently relieving the pressure on the suspension.

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