WO2012028291A1

WO2012028291A1 - Apparatus and method for mixing a powder with a liquid

Info

Publication number: WO2012028291A1
Application number: PCT/EP2011/004342
Authority: WO
Inventors: Michael Stratemann
Original assignee: Cargill, Incorporated
Priority date: 2010-09-01
Filing date: 2011-08-29
Publication date: 2012-03-08

Abstract

The present invention relates to an apparatus for mixing at least one powder with at least one liquid, comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with: - one or more liquid inlets, - one or more powder inlets, - mixing means, and - means for forming a liquid curtain at the interface between Zones A and B; wherein the powder inlet is positioned such that, in use, powder is fed into Zone B without passing through Zone A.

Description

APPARATUS AND METHOD FOR MIXING A POWDER WITH A LIQUID

Field of the Invention The present invention relates to an apparatus and to a method for mixing a powder with a liquid.

Background of the Invention The preparation of solutions, dispersions, slurries, suspensions and so on requires, at some stage, the mixing of a powder and a liquid. On an industrial scale, this usually occurs in a mixing tank. The liquid and powder are supplied separately to the tank and then combined, usually by means of some form of mixing elements. The mixing elements are typically positioned on a rotating shaft which passes through an opening at the top of the tank and is driven by an external motor.

The supply of a dry powder into the tank inevitably leads to the formation of airborne powder particles or dust. This, in turn, can lead to a number of problems: if the dust escapes the tank (for example through the opening at the top of the tank used to accommodate the rotating shaft), it can become a health hazard to those working in proximity. It can also create a very real explosion risk. Serious (and expensive) safety measures must therefore be taken. Furthermore, if the dust is allowed to accumulate (either within or outside of the tank), it could result in undesired bacterial growth and thus requires costly cleaning operations. One possible solution to these problems is to reduce the amount of dust that is able to escape from the tank by sealing the hole through which the rotating shaft passes, for example with a rubber seal. Unfortunately, these seals wear quickly and have to be replaced or repaired on a regular basis. They also do not solve the problem of dust accumulation within the tank.

There is therefore still a need in the art for an improved method of mixing a powder and a liquid, and an improved mixing apparatus therefore, which eliminates the problems associated with dusting. The present invention addresses these needs. Statements of the Invention

According to one aspect of the present invention, there is provided an apparatus for mixing at least one powder with at least one liquid, comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- one or more liquid inlets,

- one or more powder inlets,

- mixing means, and

- means for forming a liquid curtain at the interface between Zones A and B;

wherein the powder inlet is positioned such that, in use, powder is fed into Zone B without passing through Zone A.

According to another aspect of the present invention, there is provided a method for mixing at least one powder with at least one liquid in a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that it comprises the steps of:

- feeding at least a part of the liquid into the tank, at the interface between Zones A and B, in the form of a liquid curtain, and

- feeding the powder into the tank such that it reaches Zone B without passing through Zone A.

Figures

Figures 1 to 3 are simplified representations of possible embodiments of the present invention, in cross section.

Detailed Description

The present invention provides an apparatus for mixing at least one powder with at least one liquid, comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- one or more liquid inlets,

- one or more powder inlets,

- mixing means, and

- means for forming a liquid curtain at the interface between Zones A and B;

wherein the powder inlet is positioned such that, in use, powder is fed into Zone B without passing through Zone A. The terms "tank", "liquid inlet", "powder inlet", and "mixing means" will take their normal meanings in the art. Thus, the term "tank" refers to a receptacle or container of any shape and size suitable for holding a liquid. For the purposes of the present invention, the tank will be defined as having an upper portion (Zone A) and a lower portion (Zone B). The upper portion may or may not be closed. It is indeed a particular feature of the present invention that an open tank can be used without risking large quantities of dust from escaping during use. Preferably however, the tank will be closed (either with an integral or removable upper surface or lid). If it is closed, it will preferably have an opening suitable to accommodate a rotating shaft (as described below). It will also preferably comprise a means for releasing excess amounts of air or gas from inside the tank. The gas release means (or "outlet") may comprise, by way of example, a release valve or a filter. It will advantageously be positioned above the liquid curtain in Zone A.

The term "inlet" will refer to any means suitable for introducing a powder or a liquid into the tank. The tank will be provided with at least one liquid inlet and at least one powder inlet but may contain more. As will become apparent, certain embodiments may indeed require the use of multiple liquid inlets. It may also be desirable to include more than one powder inlet, for instance if more than one type of powder is being mixed (the term "powder" being used to refer to any particulate dry or substantially dry substance). Other inlets (e.g. gas inlets) may of course also be provided as required.

The term "mixing means" will refer to any means suitable to effect mixing of the at least one powder and at least one liquid within the tank. Preferably it will refer to a substantially vertical rotating shaft upon which are arranged one or more mixing elements (such as blades or paddles). In use, the mixing elements will preferably be submerged within the liquid to effect stirring or agitation while the rotating shaft extends upwardly out of the tank where it is driven by a motor or other mechanical means.

The term "liquid curtain" as used herein refers to a liquid spray which demarcates two zones within the tank: an upper portion (Zone A) above the spray, and a lower portion (Zone B) below the spray. A third, intermediate zone (the interface between Zones A and B), can be identified as that encompassed by the spray itself. Thus, the powder inlet may be positioned such that, in use, powder is fed directly into Zone B (below the spray) or into the spray itself, (i.e. into the intermediate zone at the interface between Zones A and B). The key is that the powder, once in the tank, must not pass through the tank's upper portion (Zone A). Indeed, equipment in which the powder inlet is positioned such that powder enters the tank in Zone A and then falls onto a liquid spray - as described in JP2001087603 for instance - will suffer from the dust problems that the present invention addresses.

Advantageously, the liquid curtain of the present invention will be formed in such a way as to effectively seal off Zone A from Zone B, i.e. so as to prevent airborne powder particles being fed into Zone B from escaping back into Zone A. Thus, when seen from above, the liquid curtain will preferably form a substantially uninterrupted surface, extending across the tank's full diameter in all directions. The liquid curtain can be achieved in a number of ways as will be described in more detail below.

The means for forming the liquid curtain may include any means for forming a liquid curtain as described above. According to a preferred embodiment, it will comprise one or more suitably shaped diverting elements positioned, at the interface between zones A and B, upon a shaft extending substantially vertically into the tank such that, in use, liquid arriving at the one or more elements, in a direction substantially parallel to the shaft, is diverted outwardly away from the shaft, thereby forming a liquid curtain. Suitable diverting elements may include, by way of example only, disks, blades and/or propellers. Advantageously, the shaft will be a rotating shaft. Even more advantageously, the shaft upon which the one or more diverting elements are positioned will be the same shaft used to drive the mixing elements within the tank.

Thus, according to one particular embodiment of the present invention, there is provided a mixing apparatus comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- mixing means comprising a rotating shaft upon which is positioned, at the interface between Zone A and Zone B, one or more diverting elements capable of forming a liquid curtain;

- one or more liquid inlets wherein at least one of the one or more liquid inlets is positioned such that, in use, liquid is fed onto the one or more diverting elements thereby forming a liquid curtain; and

- one or more powder inlets positioned such that, in use, powder is fed into Zone B without passing through Zone A.

According to this embodiment, the rotation of the shaft will contribute to the formation of the liquid curtain. The amount of liquid used to form the liquid curtain will depend on several factors including, in particular, the required volume of processing liquid. It is indeed an advantage of the present invention that the liquid used to form the liquid curtain may also be used for mixing with the powder. Thus, the process is efficient (no waste liquid) and environmentally friendly.

In any event, sufficient water should be used to reduce and, preferably, prevent powder being fed into Zone B from escaping back into Zone A. As will be apparent to a person skilled in the art, to achieve this, certain parameters may need to be taken into account and/or may need to be adjusted. These may include, for instance, the position and type (size of opening, etc.) of the one or more liquid inlets used to feed liquid onto the diverting element(s), the diameter of the tank, the rotational speed of the shaft, the size and configuration of the one or more diverting elements, the pressure of the liquid being fed onto them and the viscosity of that liquid.

According to another possible embodiment, the means for forming the liquid curtain will comprise one or more spray means (such as one or more nozzles) attached to or integrally formed with at least one liquid inlet positioned at the interface between Zone A and Zone B.

Thus, according to this embodiment of the present invention, there is provided a mixing apparatus which comprises a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- mixing means,

- one or more liquid inlets wherein at least one of the one or more liquid inlets is positioned at the interface between Zones A and B and is equipped with spray means capable of forming a liquid curtain, and

- at least one powder inlet positioned such that, in use, powder is fed into Zone B without passing through Zone A.

The at least one liquid inlet may be positioned, for example, at or close to the inner periphery of the tank (i.e. on or close to the substantially vertical inner walls of the tank) such that liquid is sprayed inwardly (e.g. towards the centre of the tank). Preferably, in this case, more than one liquid inlet equipped with spray means will be provided. When more than one liquid inlet with spray means is used, they will ideally be equally spaced around the tank's periphery to ensure maximum separation between Zone A and Zone B.

Alternatively (or in addition), the one or more liquid inlets with spray means could be positioned on or close to the rotating shaft such that liquid is sprayed outwardly towards the walls of the tank. Preferably, the spray means will be positioned on the shaft such that the liquid curtain is formed by rotation of the shaft. Here again, more than one spray means can be used to ensure proper formation of the liquid curtain.

As described above, the exact configuration of the one or more liquid inlets with spray means will be determined by a person skilled in the art based on a number of parameters including amount of process liquid needed, size of the tank, speed of rotation of the shaft, etc.

According to a further aspect of the present invention, there is provided a method for mixing at least one powder with at least one liquid in a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that it comprises the steps of:

- feeding the powder into the tank such that it reaches Zone B without passing through Zone A, i.e. either directly into the liquid curtain at the interface between Zones A and B or directly into Zone B.

This method is based on the insight, as described above, that a liquid which is to be mixed with a powder may also be used to form a liquid curtain, the falling droplets of which will contact any (rising or airborn) powder particles and then drag them downward into the accumulated liquid at the bottom of the tank. According to a preferred embodiment, the liquid curtain will be in the form of a rotating liquid curtain. Such a rotating liquid curtain may be provided by using a rotating disc and/or by one or more rotating nozzles.

Powders that may be mixed with a liquid using the apparatus and/or method of the present invention may be any particulate materials or mixtures thereof such as, but not limited to, starch, cereal flower, milk powder, cocoa powder, sugar, pigment, paint, pharmaceutically active compounds or excipients, saw dust, polymer particles, metal dust and so on. The particle size of the powder is not limited but will generally be within the range of about 0.01- 1000 Mm, such as 0.1-500pm, and in particular 1-100μητι. The liquid to be mixed with the powder may comprise, for instance, water, organic solvents, oils, dispersions, suspensions, solutions and mixtures thereof. It is noted that one or more powders and one or more liquids may be used in the mixing operation according to the present invention. When using more than one liquid it is optional that one of the two liquids is not to be mixed with powder and may be individually used and recovered.

According to one possible embodiment, the apparatus and/or method of the present invention will be used to mix starch and water to form a slurry. In that case, the starch will generally have an average particle size in the range of about 1 to 500 pm and more preferably an average particle size in the range of 2 to 100 pm. The starch will preferably be fed into the tank at a mass flow rate of about 10 to 5.000 kg/hour. According to a specific embodiment, the water curtain will be formed on a rotating disk, rotating at a rotational speed of about 375 to 3600 revolutions/minute. The amount of water applied to this rotational disk will preferably be about 10 to 75,000 litre/hour. The tank in which the starch slurry is formed may have a volume of about 10 to 5,000 litres. Advantageously, substantially no starch particles will either escape from or accumulate in the top of the mixing tank. Particular embodiments of the present invention will now be further described by reference to the annexed drawings (Figures 1 to 3) which are not to be considered as limiting the invention in any way.

Figure 1 shows an apparatus 1 according to the invention. The apparatus 1 comprises a tank 2 provided with an inlet 3 for liquid 4, a powder inlet 5 for powder 6, and mixing means 7. The mixing means comprise a rotational shaft 8 provided with mixing elements 9. The rotational shaft 8 is driven by a motor 10. The rotational shaft 8 passes through an opening 11 in the top wall 12 of the tank 2. The rotational shaft 8 is further provided with a rotational disc 13 onto which the liquid inlet 3 feeds the liquid 14 (which is of the same composition as liquid 4 supplied to inlet 3). Due to the rotational motion of the disc 13, the liquid 14 is brought into rotation and forms a rotational liquid curtain 15 in the tank 2 above a liquid 16. The powder inlet 5 feeds the powder 17 directly into the liquid curtain 15 and also beneath the liquid curtain 15 into a tank space 18 confined by the liquid curtain 15 and the liquid level 16 in the tank 2. Accordingly, powder particles 17 arriving in the tank 2 will be contacted with droplets of the liquid curtain 15 and dragged into the liquid level 16 where the powder particles are intimately mixed and thereby form a solution or slurry dependent on the solubility of the powder and the liquid used.

The apparatus 1 is provided with filter means 26 comprising a filter 27 connected via a filter inlet 28 with the interior of the tank 2 and via a filter outlet 29 with the surroundings. Accordingly, during initial filing the tank 2 with liquid 14 and powder 17, excess air present in the tank 2 can be released without liquid 14 and/or powder 17 being released to the outside. Figure 2 shows another apparatus 19. In this figure 2 structural elements similar or identical to those discussed in relation to figure 1 are identified with the same reference number.

In the apparatus 19, liquid 4 is fed into an inlet 20 having the form of a hollow shaft 20 feeding the liquid into a series of nozzles 21 spraying the liquid 14 in a rotational manner thereby forming a rotating liquid curtain 15. The powder 6 is fed via the powder inlet 5 beneath the rotational liquid curtain 15. In a manner similar to that discussed in relation to figure 1 , the powder 17 fed into the tank 2 is substantially prevented from escaping the tank 2 via the opening 11 in between the tank top wall 12 and the rotational shaft 20.

Figure 3 shows another apparatus 22 according to the present invention. In this case the inlet 23 for liquid 24 has the form of a nozzle 25 forming a liquid spray 15 in the tank 2. The inlet 5 feeds the powder 17 directly into the liquid curtain 15 and thereby again avoids the escape of liquid powder via the opening 1 1 between the tank top wall 12 and the rotational shaft 8.

Claims

1. An apparatus for mixing at least one powder with at least one liquid, comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- one or more liquid inlets,

- one or more powder inlets,

- mixing means, and

- means for forming a liquid curtain at the interface between Zones A and B;

2. An apparatus according to claim 1 , wherein the mixing means comprise one or more mixing elements positioned upon a rotating shaft.

3. An apparatus according to claim 1 or claim 2, wherein the one or more powder inlets are positioned such that, in use, powder is fed directly into Zone B and/or directly into the liquid curtain (at the interface between Zones A and B).

4. An apparatus according to any of the preceding claims, wherein the liquid curtain effectively seals off Zone A from Zone B.

5. An apparatus according to any one of the preceding claims, wherein the means for forming a liquid curtain comprises one or more diverting elements positioned, at the interface between Zones A and B, upon a shaft extending substantially vertically into the tank and wherein at least one of the one or more liquid inlets is positioned such that, in use, liquid is fed onto the one or more diverting elements to form a liquid curtain.

6. An apparatus according to claim 5, wherein the one or more diverting means are positioned upon a rotating shaft, preferably the rotating shaft of the mixing means.

7. An apparatus according to any one of claims 1 to 4, wherein the means for forming the liquid curtain comprises one or more spray means attached to or integrally formed with at least one of the one or more liquid intlets.

8. An apparatus according to claim 7, wherein the spray means comprises at least one nozzle.

9. An apparatus according to claim 7 or claim 8, wherein the one or more spray means are positioned:

- on or close to the inner periphery of the tank such that liquid is sprayed inwardly; and/or - on or close to the rotating shaft of the mixing means such that liquid is sprayed outwardly.

10. An apparatus according to any one of claims 7 to 9, wherein the one or more spray means are positioned on the rotating shaft of the mixing means.

11. An apparatus comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

12. An apparatus for mixing at least one powder with at least one liquid, comprising a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that the tank is provided with:

- mixing means,

13. A method for mixing at least one powder with at least one liquid in a tank having an upper portion (Zone A) and a lower portion (Zone B), characterised in that it comprises the steps of:

14. A method as claimed in claim 13, wherein the liquid curtain is a rotating liquid curtain.

15. An apparatus according to any one of claims 1 to 12 or a method according to claim 13 or claim 14, wherein the at least one powder has an average particle size in the range of 0.01 to 1000 Mm.