EP0155632B1 - Method and apparatus for separating material mixtures - Google Patents

Abstract

Process and apparatus for the separation of mixtures of substances by means of centrifugal forces, particularly for the filtration and/or sedimentation of solids from suspensions, wherein the material mixture is rotated in a cake-forming chamber until a cake of solid particles is formed. The resulting cake may thereafter be washed with a wash liquid, centrifuged and discharged from the cake-forming chamber. Discharge from the cake-forming chamber results in rearrangement of the solid particles of the cake. The cake of the rearranged particles is transferred to at least one further processing chamber and dried by centrifugation before being discharged from the apparatus.

Description

The invention relates to a method according to the preamble of claim 1 and to an apparatus for performing the method according to claim 5.

The use of centrifuges in the separation of substance mixtures is known in particular in chemistry, pharmacy and processing technology. Here, e.g. Thrust and peeler centrifuges used.

According to DE-OS 1 911 147, a centrifuge with a perforated centrifuge basket and a filter bag which can be inserted therein is known. It is a kind of peeling centrifuge in which the peeling process is replaced by an inverting process. While the liquid is being spun off, the centrifuge settles as a filter cake on the inner wall of the filter bag. When the centrifuge is emptied, the filter bag turns inside out more and more, starting from its upper end, continuously releasing part of the filter cake after the other. This is thrown into a space in the centrifuge housing and discharged from there. After completion of the emptying process, the entire width of the filter bag is turned inside out with its inner working surface.

However, the centrifuge must be emptied or the filter bag turned inside out at a greatly reduced speed in order to avoid damage to the filter bag during the turning up due to the high centrifugal forces caused by the centrifugal speed.

The reduced speed when emptying such a centrifuge results in a corresponding reduction in the material throughput.

Since the centrifugal basket for emptying is arranged so as to be axially displaceable by turning the filter bag inside out and the distance between the basket bottom and the upper end of the filter bag doubles, the axial space requirement of such a centrifuge is correspondingly large.

With the known pusher centrifuges, high throughputs or a high separation performance can be achieved, however, with a pusher centrifuge, the high product purities, the low product moisture levels, the good filtrate clarities and the high purity of the mother and washing liquid, e.g. can be achieved with a peeler centrifuge.

In the peeler centrifuge, a certain amount of mixture is poured into a filter drum, which is then rotated until the solid and liquid constituents are completely separated. The solid cake remaining in the drum is peeled off after the dry spin. With peeler centrifuges, the throughput, i.e. the separation performance, is usually relatively low. Since the individual batches are processed one after the other when the solid cake is peeled out, the speed of the peeling centrifuge must be changed frequently within the batch duration because of the different requirements of the individual steps, which can be very energy-intensive. Because of the high peeling forces, the design effort for such centrifuges may be very high. There is also the risk of unforeseeable chatter vibrations. Finally, grain protection in peeling centrifuges usually leaves much to be desired, since the peeling knives treat the particles of the solid cake extremely roughly. The throughput is generally adversely affected in peeling centrifuges by the remaining layer. This residual layer can become clogged with fine particles or lubricated by the peeling knife. The presence of a residual layer is necessary for peeler centrifuges due to the type of solids transport. In the peeling centrifuge, the solid particles form a rigid cake frame during the washing process and are not circulated.

Since pusher centrifuges work with a filtration chamber that is not closed off in the axial direction, it is not always possible to process mixtures of substances with low solids concentrations because of the risk of layer flooding. Shear centrifuges also do not produce filtrates that are largely freed of solids, since filter media with fine mesh sizes are difficult to use and a rigid bed of solids, like a peeling centrifuge, cannot form in the feed zone of the shear centrifuge due to the constant movement of the solid particles against each other and on the filter medium , which would largely prevent the loss of solids in the filtrate. The constantly moving gap between the moving floor and the filter medium is also unfavorable for achieving the clearest possible filtrates. Furthermore, a good separation of mother and washing liquid is difficult with push centrifuges. The grains or solid particles in a pusher centrifuge are often excessively stressed by the frequent pushing movements. In batch centrifuges, it is often not possible to add enough washing liquid to the cake, because otherwise the washing liquid would flood it and the washing liquid would then flow away unused.

In the cake formation zone the level of the liquid to be separated cannot be raised above the level of the solid cake. This also applies, for example, to a pusher centrifuge known from FR 1.226.010, in which the mixture of substances is centrifuged in a cake-forming room and the cake is successively transferred from the cake-forming room to other treatment rooms and spun there for dehumidification. According to this, the solid cake of the innermost or outermost stage can only be washed with a spray wash, but not with a flooding wash in which a closed liquid level forms over the solid cake. The festival Cloth cake can therefore not be flooded, neither by mother liquor nor by washing liquid. If the solid cake were flooded, the liquid would flow into the non-filtering solid discharge chamber of the innermost centrifuge stage and collect there unfiltered. This would wash out the cake of the innermost stage and wash it away, so that the suspension to be separated would pour through the trenches thus formed into the solid discharge area. A centrifuge operated in this way could therefore not fulfill its function.

In the case of a centrifuge according to FR 1.226.010, the solid cake can only be formed on a relatively small area of the cake formation area, as in conventional push centrifuges, namely in the gap between the already built-up and pushed forward solid dam and the retracting push floor.

The invention has for its object to propose a method for separating solid-liquid substance mixtures, including those with low solid concentrations, and to provide an apparatus for carrying out such a method, by means of which both high throughputs and a high separation performance as in a push centrifuge as well as high product purities, low product moisture levels, good filtrate clarity and high purity of the mother liquor and washing liquid as with a peeling centrifuge, while avoiding the problems described above with both centrifuge types and also achieving a grain protection superior to both centrifuge types.

This object is achieved by the characterizing features of claims 1 and 5.

Advantageous refinements are specified in the corresponding subclaims.

In the first step of the process, a solid cake is formed on the entire pre-dewatering area within the otherwise tightly closed space, which prevents the loss of fine particles in the filtrate. The filtration rate per unit area and thus the throughput is greater than with conventional push centrifuges, since the initial filling level of the suspension is higher. This advantage is due to the fact that the amount of liquid filtered per unit of time is approximately proportional to the liquid height above the filter medium, i.e. the higher the liquid level in the cake formation room, the higher the throughput of the centrifuge.

By forming the solid cake by flooding the entire pre-dewatering or filtration area, the effective area always remains the same, in contrast to conventional pusher centrifuges, in which the maximum possible throughput depends on the filterability of the cake in the application zone at the moment of the greatest compression and the smallest available filtration area. This means that suspensions with a much lower concentration can be processed than with known batch centrifuges. The filtration speed according to the method according to the invention is also considerably higher than in conventional push centrifuges because the solid cake is not axially compressed by the push floor during the filtration and thus the filtration is hindered.

When the solid cake is pre-dewatered, i.e. if there is no longer any liquid above the solid cake formed, in the second process step it is transferred in one go to one or more further treatment rooms. Here the cake is spun dry and discharged by mainly axial displacement, while another cake can already be formed in the cake forming room.

A high level of grain protection is achieved. With the transfer of the solid cake, it is rearranged, thereby loosening and reorienting the structure of the cake.

The proposed measure of transferring or rearranging the solid particles is based on the knowledge that microstructures, i.e. Form so-called "gussets" in which the capillary and adhesive forces outweigh the centrifugal forces so that the latter are unable to drive the liquid out of the solid cake. The rearrangement makes the gusset more accessible for washing liquid. So-called barrier layers, which also hinder the separation of the liquid, also form in the solid cake concentrically to the axis of rotation. As a result of the rearrangement, that is to say the loosening and reorientation of the structure of the solid cake, both the disadvantageous barrier layers and the aforementioned microstructures are dissolved, so that the material separation is significantly improved.

Before the transfer, the solid cake can be washed overflowing with any amount of washing liquid and spun between, which leads to a higher washing effectiveness than with the known push centrifuges. The mother liquor and washing liquid can be separated one after the other.

The method also has the advantage that the working hours of the two rooms can be adjusted under different depending on the mixture of substances to be separated so that the desired quality of the product can be achieved without a "jam" in the centrifuge. If it turns out, for example, that the spin drying takes an excessive amount of time in relation to the processes in the cake-forming room, more intensive pre-dehumidification in the cake-forming room can be achieved by an extended rotation period, so that the dry spin-drying takes less time. Overall, the operating parameters, in particular set the rotation times and batches in the individual rooms so that the throughput of the material through the centrifuge can be optimized without forming a so-called "bottleneck".

In a further advantageous embodiment it is provided that the cake formation space and the treatment space in the area between the drum rim and the sliding floor are approximately cylindrical and the treatment space is conical at least over a further area of its axial extent. In this arrangement, the cake formation space is thus arranged concentrically within the treatment space, so that the solid cake formed in the cake formation space falls radially outward into the treatment space with rearrangement. As a result of the conical design of the treatment room, the mass entering the conical section can be easily moved during the dry spin in the direction of the end section with the larger diameter of the treatment room.

The tangential solids discharge housing ensures gentle discharge of the solids. Advantageously, e.g. disc-shaped support fabric arranged with a sieve. This facilitates the outflow, in particular of the liquid that protrudes in the cake formation chamber.

A siphon system arranged on the drum rim with a siphon cup and a siphon peeling tube enables filtrate to drain through holes in the drum rim if there are no holes in the jacket of the inner drum and no side filtration.

• Fig. 1 einen Schnitt durch die schematisch dargestellte Vorrichtung und
• Fig. 2 einen Schnitt durch eine Ausgestaltung des in Fig. 1 umrandeten Bereiches A.

The method and the device are explained in detail below with reference to the drawing. It shows:

• Fig. 1 shows a section through the device shown schematically and
• 2 shows a section through an embodiment of the area A bordered in FIG. 1.

1, the device is designed as a horizontal centrifuge. A rotating, axially immovable outer drum 2 is fastened to the hollow shaft 1 and surrounds a treatment room 3. A filter is arranged on the inside of the outer drum 2. It has a cylindrical region 70a and an adjoining conical region 70b. At the end with the larger diameter of the conical region 70b, a tangential solid discharge housing 13 with a discharge nozzle 14 is arranged.

A filtrate housing 15 is used to hold the separated liquid.

An inner drum 5 with a thrust washer 34 is arranged concentrically with the outer drum 2 on an axially movable push rod 4 which is mounted in the hollow shaft 1. Both drums rotate about a common axis of rotation 30.

In the illustrated embodiment, the push rod 4 rotates at the same speed as the hollow shaft 1, so that the two drums 2 and 5 have the same angular velocity. The push rod 4 is axially movable in the direction of the double arrow 40, so that the inner drum 5 attached to it can also be moved axially.

A moving floor 8 with a sealing ring 9 is fixed in place on the outer drum 2 by means of a bolt ring 7. The push floor 8 tightly closes a cake formation space 32 within the inner drum 5 on an axial end face. The opposite end face of the cake formation space 32 is closed off by a drum rim 10 which is at a constant distance from the moving floor 8.

The mixture of substances to be separated is fed into the cake formation chamber 32 via an inlet pipe 12. The push rod 4 is set so that the cake formation space 32 is sealed off from the drum rim 10 on the one hand and from the push floor 8 on the other hand. A filter 6 connected to the inner drum 5 allows liquid to escape from the cake formation space 32, which is collected in the filtrate housing 15. A solid cake 50a forms in the interior 32 above the filter 6 and is flooded during the cake formation phase and the washing phase.

After a batch has been pre-separated in the cake formation space 32 after it has been filled in with cake formation by filtration / sedimentation, the cake can be pre-dehumidified by intermediate spinning. The solid cake 50a can then be washed submersed - if necessary several times - by means of a washing liquid.

Thereafter, the push rod 4 is moved to the right according to the arrow 40 in FIG. 1 together with the inner drum 5 and the filter 6, the drum rim 10 and the push floor 8 remaining stationary, so that the solid cake 50 a from the cake formation room 32 into the treatment room 3 falls. When the solid cake falls out of the cake formation space 32, the cake structures formed there, in particular poorly filtering barrier layers or areas with particularly high impurities, are dissolved and the solid particles rearrange. The push rod 4 moves to the right as shown in Fig. 1 to such an extent that the entire batch of solids from the cake formation space 32 falls on the cylindrical region 70a of the filter. The push rod 4 then moves to the left, the thrust washer 34 pushing the solids collected in the cylindrical region 70a of the filter to the left onto the conical region 70b of the filter. There, the solid cake 50b is dehumidified, the filter allowing the liquid to pass into the filtrate housing 15.

The inclination of the conical area 70b of the filter is selected so that the solid cake 50b can be moved with little thrust in the direction of the tangential solid discharge housing 13, from where the solids are discharged from the device via the discharge nozzle 14.

FIG. 2 shows the area A bordered in FIG. 1 in detail. In addition to the known filtration through bores 16 on the circumference of the inner drum 5, a so-called "side filtration" can be used: a supporting fabric 17 attached to the drum rim 10 with a sieve 18 facilitates the outflow, in particular of the liquid 60 that protrudes in the cake formation chamber (32) through bores 19.

A siphon system 36 fastened to the drum rim 10 with a siphon cup 20 and a siphon peeling tube 21 enables filtrate drainage through bores 25 in the absence of bores 16, 19 and / or in the absence of side filtration .

Claims (10)

1. Method for separating solid/liquid mixtures by means of a centrifuge, in particular for the filtering and/or settling out of solids from suspensions, in which from the mixture of substances in the delivery zone of the centrifuge, on a pre-drainage surface, a solids cake (50a) is formed, washed and then transferred in the wet state to another location within the centrifuge, characterised by the following stages of the method:

a) Formation of the solids cake (50a) by overflowing on the entire pre-drainage surface within a chamber (32), which is otherwise sealed hermetically;

b) Transfer of the entire pre-drained solids cake (50a) at centrifugal speed at one go or in one step into at least one further treatment chamber (3) for dry centrifuging and discharge.

2. Method according to Claim 1, characterised in that before the transfer, the solids cake (50a) is washed in an overflowing manner within the closed chamber and subjected to intermediate centrifuging.

3. Method according to Claim 2, characterised in that before washing, moisture is removed from the solids cake (50a) by intermediate centrifuging.

4. Method according to Claim 1, characterised in that the solids cake (50a) is discharged by centrifugal force into one or more treatment chambers and with further drainage on a suitable device for the extraction of moisture is moved towards the outlet.

5. Apparatus for carrying out the method according to Claims 1 to 4, consisting of a centrifuge with a filter drum and a cake-formation chamber (32), which serves for the formation of the solids cake (50a) and simultaneously for its pre-drainage, as well as of a treatment chamber (3) for the transfer and further treatment of the solids cake (50a), characterised in that the cake-formation chamber (32) is formed axially by a sliding base (8) and a drum rim (10) at a constant distance apart and radially by an axially displaceable filter (6) and the treatment chamber (3) is formed by an outer drum (2) attached to a hollow shaft (1) and that the filter (6) is attached within an inner drum (5), which in turn surrounds the cake-formation chamber (32) and which is attached to a push-rod (4) arranged concentrically in the hollow shaft (1) and mounted to slide axially.

6. Apparatus according to Claim 5, characterised in that in the region (70a) between the drum rim (10) and sliding base (8), the cake-formation chamber (32) and the treatment chamber (3) are approximately cylindrical and the treatment chamber (3) is constructed at least over a further region (70b) of its axial extent.

7. Apparatus according to Claim 5, characterised in that the internal drum (5) is provided with a sliding disc (34), which runs through the cylindrical region (70a) of the treatment chamber (3).

8. Apparatus according to one of Claims 5 to 7, characterised in that located at the open end section of the region (70b) of the treatment chamber (3) is a tangential solids discharge housing (13).

9. Apparatus according to one of Claims 5 to 8, characterised in that located on the drum rim (10) is a for example disc-shaped support cloth (17) with a screen (18).

10. Apparatus according to Claims 5 to 9, characterised in that located on the drum rim (10) is a syphon system (36) with a syphon cup (20) and a syphon peeling tube (21).

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