EP0719585B1 - Agitator mill with separator for retaining milling beads - Google Patents

Abstract

Stirred grinding mill with continuous flow between inlet(4) and outlet (21) includes a rotating shaft carrying agitators (3). The mill vessel (1) is charged with grinding beads, which are retained by a rotary separator located on the shaft, in advance of the outlet. The separator has radial openings which extend from an annular, internal region about the shaft, through to an outer, annular space forming part of the main grinding volume. In this novel design, the rotary (6) separator is designed to classify by centrifugal force. The radial openings are formed by gaps between adjacent rotor blades (10). The finest particles achieve the outlet by axial travel, but coarse material is flung outwards to return to the mill. Also claimed is a grinding circuit, in which the mill described, has a centrifuge (or similar) at the outlet, to return any oversize to the lower input of the mill, and deliver the fines as product.

Description

In the production of plastics, paints, toners, pharmaceuticals, food, etc., various substances have to be broken down into ever smaller particle finenesses and mixed with one another or dispersed in liquids.
In the manufacture of paper e.g. B. the paper web emerging from a paper machine is coated with a lime-water suspension which has a fineness of approximately 5 μm. In the production of this coating agent (hereinafter called slurry), broken limestone is minced in fine stages in opencast or civil engineering, down to the fineness mentioned above.

Agitator mills are used to manufacture such fine products, especially the slurry. According to the increasing delicacy ever smaller grinding pearls are used, whereby the Contact points between the too coarse particles and two each Grinding beads, and thus the grinding effect can be increased. Each smaller but the grinding pearls are, the more difficult it is hold back in the grinding container so that only the sufficient fine lime particles together with the suspension water in the Fine material or finished product, but not grinding pearls. This difficulty is compounded by the fact that the grinding beads become smaller and smaller in the course of operation due to wear.

In the course of the development of the present invention, it was recognized that the grinding effect is improved if the grinding beads do not have essentially the same size, but if grinding beads of different sizes work together in the mill. This increases the likelihood that the differently sized limestone particles will be ground between appropriately sized grinding beads, ie larger particles between larger grinding beads and smaller particles between smaller grinding beads. It is important here that with increasing fineness of the limestone particles to be comminuted, ever finer grinding pearls are also present.
It should be considered that the grinding beads become smaller and smaller in the grinding process due to abrasion. While the mill is in operation, it is most practical - together with the coarse dispersion to be divided - to add grinding beads with a suitable initial, larger diameter, which continuously become smaller within the mill over the course of their operating time, which results in a particle size distribution from larger to medium to small and small Grinding pearls in the mill results. Initially, however, a mixture of grinding beads of different sizes can be put into the mill.

In particular, it was recognized that the best grinding effect was achieved when the grinding beads are completely removed by the abrasion in the mill and even ground up, i.e. when the grinding pearls in for so long the grinder are held back until they reach the upper grain size of the desired fine suspension, i.e. the end product. With the previous mills, however, this is practically difficult to achieve.

Usually, to retain the grinding beads, sieves are arranged on the outlet side of the grinding container between the grinding chamber and the fine material outlet.
The sieve openings thus determine the size of the particles in the fine material: larger particles are retained by the sieve, finer particles pass through. The finest sieves that can be used in practice have sieve openings of approx. 100 µm. This means that the fineness of the lime particles as well as the continuous grinding beads is about 100 µm. This grain size is too large for many applications, a finer grain size of less than 40 µm is desired.

In the course of operation, however, the screen openings and soon the sieves clogged altogether. It then forms on the sieve a screen covering that acts as a filter. This means that in the course this process of clogging and building up the screen a pressure loss is built up. This increases the throughput, that is the amount of finished product achieved per unit of time, from. Therefore, the screens have to be backwashed very often, to remove the sieve or filter cake again, which leads to downtimes and thus loss of production.

Because of the difficulties involved, attempts were made decades ago to manage the retention of the grinding beads without sieves or filters:
From German publication 20 20 649 an agitator mill with a separator according to the generic term mentioned at the beginning has become known.
This separator is essentially formed by a cylindrical ring which is provided with a series of more or less radial bores distributed uniformly over the circumference. A hub is formed on the lower end of this radially pierced ring, with which the separator is fastened to the shaft. A seal is inserted between the top of the separator and the top cover of the grinder.

By means of such a separator, the grinding beads are to be retained in the grinding chamber solely on account of the centrifugal force, so that the sieves or filters with the difficulties mentioned at the outset could be avoided. However, this separator has not found its way into practice.
This is probably due to the fact that the radial passages - despite their large number - offer a cross section that is far too small for the fine suspension flowing through. The cross-section in each of the many, relatively narrow bores is correspondingly small, that is to say the flow velocity is correspondingly high, so that the centrifugal force cannot have sufficient effect.
It should be borne in mind here that the centrifugal force increases with the square of the radius, that is to say it is greatest radially on the outside and decreases radially inward in the square. Accordingly, the restraint can essentially only take place on the outer circumference, where the centrifugal force is correspondingly greatest: those particles that have been dragged into a radial bore by the flow are immediately subject to a centrifugal force that decreases in square, while the flow force corresponds to the narrow cross-section high is:
Particles caught in a radial hole no longer have a chance to be thrown outwards. This is all the more true if, according to the exemplary embodiments of this document, the separator body is surrounded by a sieve or filter, that is to say every radial bore is covered by a filter through which a particle - if it could get into the bore radially on the inside at all - returns again , would have to be thrown out through the sieve.

Other prior attempts to create pearl retention without sieves or filters are that between the grinding chamber and Outlet space provided several plates with a small distance from each other are, which each form a gap between each other.

The gap width is smaller than the pearls to be retained, Smaller grinding pearls can pass through together with the fine material drain the column. However, even when using one A large number of such plates or gaps in the flow cross section too small, so that there are difficulties in operation.

In contrast, the object of the invention is an agitator mill to create with a bead separator, the Separation limit corresponds to the upper fine grain size with which Sieves or filters for the retention of grinding beads are avoided, and which nevertheless ensures that no grinding pearls or Dispersant particles get into the finished product can.

According to the invention, this object is essentially achieved in that the separator is designed in the manner of a classifying rotor of a centrifugal classifier.
The bottom of the rotor is closed by a lower disc, which is firmly connected to the agitator shaft.
The coarse suspension fed into the mill at the bottom is ground to the desired fineness on the way up and flows into the upper annular space between the housing wall and the classifying rotor.

The regrind is brought together by the blades of the classifier rotor with the grinding beads contained in it in the upper annulus in Accelerated circumferential direction and thus to a larger one Brought peripheral speed. The grinding beads as well as the still too coarse particles of the regrind are due to their larger Mass by centrifugal force in the outer annulus, i.e. in the Grinding container retained. Each together with the Fine material flow coming close to the rotor circumference Bead is ground by the blades in the circumferential direction accelerated, d. H. the particle is gripped accordingly stronger centrifugal force acting radially outwards, thus the particle is thrown off tangentially, i.e. in the grinding container held back. The fines, however, come between two Blade of the classifier / separator rotor through in the radially inner outlet space and from there into the outlet line.

Using a centrifugal force separator, the coarser particles can be separated or retained at the required small separation limit of approx. 40 µm and below. As is known, the separation limit of a centrifugal force rotor becomes smaller the higher the speed and the higher the radius, since it is known that the centrifugal force increases with the speed and the square of the radius. Thus, with increasing speed and increasing radius, ever finer particles are retained by the centrifugal force against the flow generated by a vacuum or suction in the outlet line.
The separation or restraint takes place on the outer circumference of the classifying rotor. Uniform separation conditions are thus obtained on a relatively large separating surface corresponding to the radius and the axial height of the classifying rotor, so that it is all the less that large particles are entrained into the fine material under irregular flow or irregular centrifugal force. So there is no or only a little too coarse grain (grinding beads or lime particles) in the fine material.

A correspondingly dimensioned rotor can be advantageous directly be moored on the shaft of the agitator, so that no separate Drive for the classifier rotor is required. Here but with this design, the speed of the classifying rotor is equal to Speed of the grinder is obtained for the desired fine Separation limit of approx. 40 µm a larger radius of the rotor than that Radius of the grinder. The classifier rotor is therefore in one Diameter corresponding to the wider upper part of the housing or housed in a housing attachment (claim 2). It is it favorable for a steady flow and especially for a decrease in the particles rejected by the classifier rotor when the Annulus between classifier rotor and housing wall relatively large is.

However, several smaller classifier rotors can also be accommodated in a correspondingly large upper housing part, each of which can be driven separately at the desired speed, independently of the agitator. (Claim 3)
Or a classifier rotor is rotatably mounted on the agitator shaft and driven by the agitator shaft by a separate drive or via a countershaft. (Claim 4)
Practical testing has shown that there is surprisingly little grain that is too coarse, that is, faulty grain, in the fine material that has passed through the classifying rotor. This very important result can be attributed, among other things, to the following effect: The temperature in the mill rises to over 100 ° C due to the friction. So far, steam bubbles have formed in the mill. The steam bubbles disrupt the flow at the outlet of the mill, the flow does not remain uniform, the speed changes, so coarser particles are pressed or torn through the sieves or the partial sieve covering at various points.
In contrast, the classifying rotor used according to the invention builds up pressure due to the centrifugal force. During operation, this results in an overpressure of a few bar in the direction of flow outside the classifying rotor. At this overpressure, the boiling temperature rises accordingly. Thus, thanks to the classifying rotor as a retaining element, the formation of vapor bubbles and the transport of the faulty grain based on these into the inside of the classifying rotor are avoided.

Figur 1

zeigt einen axialen Schnitt durch eine Rührwerksmühle, die an ihrem Auslaßende mit einem Sichter-Separator gemäß der Erfindung versehen ist;

Figur 2

zeigt vergrößert, im axialen Schnitt den oberen Bereich der Mühle mit dem erfindungsgemäßen Sichter-Separator;

Figur 3

ist der radiale Schnitt nach Linie III-III in Fig. 2.

Figur 4

zeigt den Schnitt nach Linie IV-IV in Fig. 2, nämlich den Schnitt durch die Dichtung zwischen Sichterrotor und oberem Gehäusestirnring;

Figur 5

zeigt im Schnitt nach Linie V-V in Fig. 2 die Dichtung zwischen der Welle und dem oberen Gehäusedeckel.

Figur 6

zeigt eine weitere Ausgestaltung der Erfindung in einer Darstellung gemäß Fig. 2.

Figur 7

zeigt einen Mühlenkreislauf mit einer Mühle im wesentlichen nach Fig. 6.

In a further embodiment, a relaxation space provided with a steam outlet is provided on the outlet side of the classifying rotor for receiving and discharging the water vapor arising behind the rotor blades due to the pressure drop. (Claim 5)
As is well known, different quality requirements are placed on the end product or fines depending on the intended use. There are cases in which coarse-grain defective grain is particularly harmful.
In a further embodiment of the invention, a mill circuit is therefore proposed, in which the fine material drawn off by means of (at least) a classifying rotor on the outlet side of the grinding container is not provided directly as the end product, as was previously the case, but is fed onto a fine sieve or onto a centrifuge. The fine portion of this fine sieve or centrifuge now serves as the end product from which any coarse grain has been removed; the coarse grain from the centrifuge is fed back into the mill, advantageously together with the raw dispersion. (Claim 6)
The seal between the upper end ring of the classifier rotor and the upper container wall can be made in any way that is proven in the prior art, for. B. by labyrinth seals. A special seal is characterized in that a ring of radially extending rods is provided as a seal against an upper, annular wall of the grinding container on the upper end ring of the classifier rotor. (Claim 7)
In an analogous manner, the seal between the agitator shaft and the upper housing cover can also be made using such a set of radial centrifugal webs. (Claim 8)
Exemplary embodiments of the invention are described below with reference to the drawing.

Figure 1

shows an axial section through an agitator mill, which is provided at its outlet end with a classifier separator according to the invention;

Figure 2

shows enlarged, in axial section the upper region of the mill with the separator according to the invention;

Figure 3

is the radial section along line III-III in Fig. 2nd

Figure 4

shows the section along line IV-IV in Figure 2, namely the section through the seal between the classifier rotor and the upper housing end ring;

Figure 5

shows in section along line VV in Fig. 2, the seal between the shaft and the upper housing cover.

Figure 6

shows a further embodiment of the invention in a representation according to FIG. 2.

Figure 7

shows a mill circuit with a mill essentially according to FIG. 6.

The agitator bead mill consists of one in the example case vertical, cylindrical grinding container 1, in which an agitator is rotatable. The agitator consists of the agitator shaft 2, the with annular or radially extending stirring elements 3 is equipped. The raw suspension to be ground is fed through a Inlet port 4 fed to the lower end of the grinding container. In the grinding container is a filling of grinding beads 5, the in the course of operation to an ever finer particle size are worn out or rubbed off. For the grinding effect is the interaction of large and small grinding beads Cheap. If possible, the grinding beads should be in the Grinder will be held back until the smaller grinding beads from the larger ones have been reduced to the fineness of the fine material are. According to the invention there is a classifying rotor 6 on the Outlet side, in the example at the upper end of the grinding container, arranged.

1 and 2, the classifier rotor sits firmly on the agitator shaft, is thus driven by the agitator shaft 2, so that no separate drive is required. To at the given Speed a higher centrifugal force and thus finer separation limit reach, the diameter of the classifying rotor 6 is larger than that Diameter or the radial extent of the stirring elements 3. Apart one of which is a relatively wide one that surrounds the classifying rotor Annulus 7 favorable for the separation effect. The classifier rotor 6 is located therefore in an upper extension 8 of the housing; a conical transition part 9 leads from the normal outer diameter of the grinding container over to the larger outside diameter for grinding pearl and coarse grain retention.

The rotor blades 10 or webs of the classifying rotor 6 are between a lower end ring 11 and an upper end ring 12 captured. The lower end ring is on a support disk 13 attached, the radially inner edge of the agitator shaft 2 or is attached to a sleeve 2a fixed on this. The upper end ring 12 of the rotor is provided with radial arms 14 a ring 15 also attached to the sleeve 2a.

On the upper end ring 12 of the classifier rotor there is a ring of radial rods 16, with which the sifting rotor 6 above one Separating ring 17 is sealed, which is the upper end of the Grinding room 7 or the coarse material space separates from the special Fine goods collection space 18, which in turn is formed between this separating ring 17, a peripheral wall 19 and a flat conical upper housing wall 20. A opens into this peripheral wall Fine material line 21 out of the fine material space 18, through which the ground in the mill and by means of the classifier rotor 6 of the Fine slurry still separated into coarse material flows away. The agitator (and classifier) shaft 2 / 2a carries below the upper one End wall a ring 22; in the space between this Ring 22 and the upper end wall 20 is another Wreath of radial rods 23, whereby the fine material space 18 opposite the outside atmosphere is sealed.

In the embodiment according to FIG. 6, the classifying rotor 6 is by means of two bearings 24, 25 rotatable on the agitator shaft 2 stored. The lower bearing 25 is an axial bore 26th and a radial bore 26a a detergent, e.g. B. water or a dispersant required in the mill anyway.

The classifier rotor can be driven by the agitator shaft or be derived from the drive of the agitator shaft, e.g. B. means a gear, with which the speed of the agitator shaft translates the required higher speed of the classifier rotor becomes.

7 is above the outlet of the classifier rotor special relaxation space or steam collecting space 28 is provided, which is used in particular to absorb water vapor, due to the pressure drop behind the rotor blades 10 forms. Through an outlet 29 can be above the Fine water vapor escaping from the fine suspension mirror.

Are also random in that by means of the classifier rotor separated fine slurry product still a number of coarse-grained off-grain particles to be available. The amount of this missing grain depends from the operating conditions. In particular, you have to at higher Expect throughput with a higher incidence of defective grain. Such missing grain is particularly harmful with certain products.

In order to eliminate such missing grain - and thus the one by the Use of the classifier rotor as a pearl and coarse grain retention device high throughput possible even with difficult products or to achieve relationships, will be in a further embodiment the invention of the fine material outlet 21 of the agitator mill with a Centrifuge 30 connected, by means of which these missing particles be separated. The coarse material outlet 31 of the centrifuge is So connected to the inlet 4 of the agitator mill, during the Fines outlet 32 of the centrifuge the final fines end product delivers. (Fig. 7)

Reference list Agitator mill

1

Grinding bowl

2nd

Agitator shaft

2a

Sleeve / hollow shaft

3rd

Stirrer

4th

Inlet supports

5

Grinding beads

6

Classifier rotor

7

Annulus

8th

upper extension

9

conical transition part

10th

Rotor blade

11

lower brow ring

12th

upper brow ring

13

Support disc

14

radial arm

15

ring

16

radial rods

17th

Separating ring

18th

Fine collection room

19th

Peripheral wall

20th

Housing wall

21

Fine goods management

22

ring

23

radial rods

24th

Bearings from rotor on
Agitator shaft 2

25th

lower bearing

26.26a

Holes for
Detergent

27

- - - -

28

Steam collecting room

29

Steam outlet

30th

centrifuge

31

Coarse material outlet

32

Fines outlet of 30

Claims (8)

1. Agitator mill in whose grinding container (1) provided with an inlet (4) and an outlet (21) and charged with grinding beads (5) an agitator shaft (2) equipped with agitator members (3) is rotatable, on which shaft (2) there sits, upstream of the outlet, in order to hold back the grinding beads, a separator (6 to 14) provided with radial passages which extend between a radially outer annular chamber (7), belonging to the grinding chamber, and an inner outlet chamber (18) surrounding the agitator shaft in an annular manner, characterised in that the separator is constituted by a rotor in the form of a centrifugal force separator rotor (6), the passages being formed by the intermediate spaces or gaps between each two separator rotor blades (10).
2. Agitator mill according to claim l, characterised in that the separator rotor (6) is arranged in an upper portion (8) of the grinding container (1), which portion has a widened diameter, or in a widened housing extension.
3. Agitator mill in whose grinding container provided with an inlet and an outlet and charged with grinding beads an agitator shaft equipped with agitator members is rotatable, a device for holding back the grinding beads being provided on the outlet side of the grinding container, characterised in that, in an upper portion (8) of the grinding container (1), which portion has a widened diameter, or in a widened housing extension, at least one separately drivable separator rotor (6) is provided for holding back the grinding beads.
4. Agitator mill according to claim 3, characterised in that a separator rotor (6) is supported rotatably on the agitator shaft (2) and is driven by a separate drive or by the agitator shaft by means of a transmission gear or the like.
5. Agitator mill according to any one of claims 1 to 4, characterised in that a pressure-relief and vapour-collecting chamber (28) provided with a vapour outlet (29) is provided on the outlet side of the separator rotor in order to receive and remove the steam resulting from the drop in pressure behind the rotor blades (10).
6. Mill circuit having at least one agitator mill according to any one of claims 1 to 5, characterised in that the fine material outlet (21) of the agitator mill is connected to a centrifuge or like separating device (30), of which the coarse material outlet (31) is connected to the inlet (4) of the agitator mill, while its fine material outlet (32) supplies the fine material end product.
7. Agitator mill, preferably according to any one of claims 1 to 5, characterised in that a collar of radially extending rods (16) is provided on the upper end ring (12) of the separator rotor (6) as a seal against an upper, circular, wall (17) of the grinding container (1).
8. Agitator mill according to claim 7, characterised in that a collar of radial rods (23) which are arranged on a carrying ring (22) surrounding the shaft is provided as a seal between the agitator shaft (2) and the upper housing cover (20).

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