GB2235391A - Mixing apparatus and method - Google Patents

Abstract

A liquid ingredient is added (via 54) to a mixture of dry ingredients conveyed by a helical conveying means (12) at a point where there is a discontinuity (50 - 52) in the conveying means. Stirrer rods or wires (25a, 25b) are provided on the conveying means. <IMAGE>

Description

IMPROVED MIXING APPARATUS AND METHOD This invention relates to mixing apparatus of the kind described in International Application WO 87/05255 and has particular reference to improving the mixing performance of such apparatus especially, but not exclusively, in the face of a liquid addition to dry ingredients.

In the apparatus to which this invention relates, a conveying means in the form of an elongate member disposed helically about a rotating axis of the conveying means is located in a casing conforming to the envelope shape of the helically formed elongate member over a lower portion thereof, stirring members being provided on the conveying means which extend between at least some adjacent turns of the helically formed member.

The action of the conveying means produces a noncompacting environment, where the various dry ingredients are lightly tossed and effectively mixed as they are propelled along the casing.

We have found this apparatus to be highly effective at mixing several (e.g six or more) dry ingredients together so that the variations in proportions of the component ingredients in the mixture vary by less than +1% over a wide range of samples taken from the outlet of the mixing apparatus. We have found however, that the known apparatus is less effective at creating a uniform blend of ingredients when one (or more) of the ingredients is in liquid form.

When a liquid is added to mixed dry particles it should be uniformly distributed among the particles. With the mixing apparatus as described in WO 87/05255, when a liquid component was added in a similar manner to , but downstream of, the addition of the last dry particles in the mixture, at the point of entry of the liquid, there was a tendency for some particles to become saturated with liquid, whereas others remained dry. The immediate outcome of this was clumping, and the formation of some relatively large moist granules. The overall result was a less than desirable blending efficiency.

Surprisingly we have found that improved mixing results in apparatus of the kind described, if a liquid component of the mixture is applied (a) downstream of the point where at least a major part of the dry ingredients are brought into the conveying influence of the conveying means, (b) at a point where the said dry ingredients are less directly conveyed by the conveying means and (c) at a point upstream of a further period of conveying influence of the conveying means. In one simple arrangement, a significant discontinuity is provided in the conveying means part-way (e.g. half way) along its length, and the at least one liquid component is added to the pre-mixed dry ingredients as they move across the discontinuity. A discontinuity of (or of the order of) two complete turns of the helically disposed elongate member has been found to work satisfactorily.Some or all of the stirring members may be present in the discontinuity.

With a discontinuity or interruption in the helical elongate member, the dry ingredients and in-coming liquid component (preferably in the form of droplets) initially come into contact with each other at the bottom of the mixing chamber formed in the casing, rather than by random collision whilst failing. This produces more even wetting" of the dry ingredients, often without clumps being formed beyond the downstream end of the discontinuity.

Subsequently the helical rotation of the conveying means allows for continuous propulsion and further mixing of this uniformly moistened product.

By providing a discontinuity as described, when the conveyed tumbled particles reach the discontinuity, they will tend to fall to the bottom of the casing before travelling on, giving rise to a higher density of particles per unit length of the rotating axis than arises upstream of the discontinuity. Thus, a small and transient "sink" of the partially mixed materials is created, in which "sink" the liquid addition can be made and from whence further aerated propulsion and blending of the ingredients can begin.

In many instances, particularly where viscous liquids are added, it is important to continue at least some of the stirring members at least partially across the discontinuity so that the stirring members can continue the blending of dry ingredients and assist in transporting the ingredients across the discontinuity as well as acting to "fold in" the liquid ingredient to the well-mixed dry components. Providing stirring members (e.g. in the form of rods or wires) between turns of the helically formed member both inside the turns and outside the turns (i.e.

between the turns and the casing) on both upstream and downstream sides of the discontinuity and continuing said stirring members across the discontinuity is particularly advantageous. Each stirring member can be parallel to the rotating axis of the conveying means on each side of the discontinuity, and can extend in such parallel disposition across the discontinuity or can be slightly skew to the axis across the discontinuity.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of a known mixing apparatus, Figure 2 is a section on the line II-II of Figure 1, and Figure 3 is a schematic indication of the development represented by this invention.

The mixing apparatus shown in the drawing basically comprises an inefficient helical conveying means 12 moving particulate materials (with a great deal of "fall back" and "tumbling" in a blending section 13) from an input hopper 14 (via an input section 15) to a discharge chute 16 (via an output section 17). The section 13 of the conveying means 12 should be several times as long as the section 15 (typically between five and ten times as long).

The conveying means 12 comprises an elongate member 18 of circular cross-section wrapped helically at constant pitch about a central drive shaft 19. At intervals along its length, the member 18 is braced to the shaft 19 by radial arms 20. A drive wheel 21 is mounted on one end of the shaft 19 so that, in use of the blending device, the shaft 19 rotates (typically at a speed in the range 100 to 150 rpm) in the direction shown by the arrow 22.

The conveying means 12 is embraced throughout its length by a casing 23 which matches the cross-section of the lower half of the conveying means. 12 and supports the particulate materials being blended in a region where they are continually disturbed by the rotating turns of the member 18. In practice the depth of the layer of particulate material over the blending section 13 will be between one and two times the diameter of the cross-section of the member 18 (i.e. the lower part only of each turn of the helix is covered by the particulate material being blended).

To increase the agitation and tumbling of the particulate material in the casing 23 as it is conveyed in the direction of the arrow 24, stirring members are arranged between adjacent turns of the helix so that these are periodically moved through the shallow bed of material being conveyed towards the chute 16. As shown in Figure 1 the stirring means can be rods (or wires) 25a, 25b extending parallel to the axis of the shaft 19 at least over the blending section 13 and conveniently over all three sections 13, 15 and 17. One rod 25a can be located between turns inside each turn and the next rod 25b can be located between turns outside each turn. The rods 25a, 25b can be attached (e.g. welded) to each turn or to just some of the turns as may be required to give a stable structure.

Figure 2 shows a section on the line II-II of Figure 1 and indicates the open-topped nature of the casing 23.

Figure 1 shows three metering means 26a, 26b and 26c for feeding three different components of a mixture which is to be blended in the illustrated device 10. Any suitable means for supplying an accurately controlled rate of flow of material can be used but it is preferable to use in each metering means a screw conveyor (e.g. similar to the conveying means shown at 12 in Figure 1).

What has been described is taken from WO 87/05255 and works well with dry ingredients added via 26a, 26b and 26c. When, however, a liquid ingredient has to be added to mixed dry ingredients, problems can arise due to "clumping or other manifestations of non-uniform distribution of the liquid among the dry ingredients. These problems appear to persist irrespective of at which point along the blending section 13, the liquid addition is made.

However, by making the modification shown in Figure 3, and removing two complete turns of the member 18 at approximately the mid-point of the blending section 13, a significant discontinuity of the conveying of the dry materials arises when the dry ingredients reach the downstream end 50 of an initial section of the conveying means 12. The rods (or wires) 25a, 25b continue across the discontinuity so that the deeper bed of ingredients that forms when the influence of the turns of the member 18 is removed, is intensively blended in its passage across the discontinuity by the rods 25a, 25b. The upstream end 52 of the final section of the conveying means 12 marks the end of the discontinuity and the tumbling action of the rods (or wires) 25a, 25b is again augmented by the transporting action of the turns of the member 18.A dispenser for at least one liquid ingredient is provided at 54 so that the liquid is fed into a strongly blended bed of dry ingredients where the liquid can be well "folded" into the dry mixture. More than one dispenser 54 can be provided in the gap between the ends 50, 52 or a further discontinuity could be provided in the section 13 if a measure of mixing with a first liquid addition is required before a second liquid addition is made.

The invention will be further illustrated by the following Examples.

Example I Using equipment broadly as shown in Figures 1 and 2 three dry ingredients were added at 26a, 26b and 26c. The addition at 26a was 73% by weight of the total dry mix of powdered CaSO4 (ingredient A), the addition at 26b was 22% by weight of the total dry mix of powdered maize starch (ingredient B) and the addition at 26c was 5% by weight of the total dry mix of powdered degelatinised starch (ingredient C). The mixing apparatus was run for a period to allow steady conditions to prevail prior to the addition, via a peristaltic pump, of 20% by weight of the dry mixture of a coloured liquid into the mixture of dry ingredients.

The liquid was added at a point approximately half-way along the blending section 13. The output from the chute 16 in successive 30 second periods was collected in separate containers. Sample collection was only carried out for 2 minutes (four 30-second samples were taken) owing to the obvious non-uniformity of the colour of the moistened mixture. On opening the casing 23, it was clear that "caked" mixture was adhering to the conveying means 12 downstream of the point of addition of the liquid and a substantial number of lumps of moistened material were partially blocking the chute 16.

Example II After clearing out the caked material from the apparatus 10, the procedure of Example I was repeated, but with the point of ingress of the liquid moved further towards the chute 16 and the CaSO4 hopper positioned further from the liquid ingress point. This arrangement allowed better pre-mixing of the three dry ingredients but the caking and blocking problems experienced with Example I were still in evidence, although to a lesser extent, the accumulation of caked material on the outer surfaces of the downstream turns of the conveying means 12 allowing unmoistened dry material to flow through the centre of the conveying means without significant further blending beyond the point of ingress of the liquid ingredient.

Sample collection at 30 second intervals was undertaken as in Example I and the content of each container was analysed to determine the proportions of ingredients A, B and C in each collected sample of the blended mixture.

Adding all the samples together gave proportions with +1% of the intended proportions and the proportions in each individual sample were within a few % of the intended proportions.

Example III The apparatus shown in Figure 1 was modified by removing two complete turns of the elongate member 18 from the mid-point of the blending section 13 and arranging the output from the peristaltic pump to enter the casing 23 (via the dispenser 54) at a location just downstream of the end 50 of the upstream section of the conveying means 12.

Ingredients A and B were as in Examples I and II but ingredient C was dry vitamin B12 (a coloured powder) added, in an amount of 1% by weight of the total dry mix. The liquid addition was pure water added at a flow rate of 1 ml/second giving a liquid content in the resultant mixture of some 30% by weight.

As previously, samples from the chute 16 were separately collected but on this occasion, although samples were still taken at 30 second intervals, each sample was now the output of just 5 seconds duration. Six samples in all were taken (over a 3 minute period) and the pink colouration from the vitamin B12 appeared to be uniformly distributed throughout the last three collected samples.

There was no evidence of caking or clumping in any of the collected samples which appeared to be uniformly moist.

Examination of the interior of the casing 23 and of the conveying means 12 showed some signs of clumping near the point of entry of water but no external caking of the elongate member 18 could be observed downstream of the end 52 and there was no accumulation of lumps at the chute 16.

ExamDle IV The mixing apparatus used for Example III was cleared out and a mixture of three dry ingredients was again used but without the addition of any liquid in the discontinuity between the ends 50 and 52. Analysis of samples collected at the chute 16 showed uniformity of mixing which was at least as good as that obtained in experiments undertaken prior to removal of the two turns of the elongate member 18.

Claims (10)

1. Mixing apparatus comprising dry ingredient supply means, a mixture outlet, conveying means interposed between said supply means and said outlet, said conveying means being in the form of an elongate member disposed helically about a rotating axis of the conveying means and located in a casing conforming to the envelope shape of the helically formed elongate member over a lower portion thereof, characterised in that a discontinuity is provided in said helically formed elongate member and in that supply means for a liquid ingredient is located to add a liquid ingredient into the casing in the vicinity of the discontinuity.

2. Mixing apparatus as claimed in claim 1, in which stirring members are provided on the elongate member which extend between adjacent turns thereof, said stirring members extending at least partially across the discontinuity.

3. Mixing apparatus as claimed in claim 2, in which the stirring members extend fully across the discontinuity.

4. Mixing apparatus as claimed in any preceding claim, in which the discontinuity is provided substantially mid-way along the conveying means.

5. Mixing apparatus substantially as hereinbefore described with reference to Figure 3 of the accompanying drawing.

6. A method of applying at least one liquid component of a mixture to dry ingredients thereof which comprises applying the at least one liquid component (a) downstream of a point where at least a major part of the dry ingredients are brought into the conveying influence of a helical conveying means, (b) at a point where the said dry ingredients are less directly conveyed by the conveying means and (c) at a point upstream of a further period of conveyed ing influence of the helical conveying means.

7. A method as claimed in claim 6, in which a significant discontinuity is provided in the conveying means part-way along its length, and the at least one liquid component is added to the pre-mixed dry ingredients as they move across the discontinuity.

8. A method as claimed in claim 7, in which the conveying means comprises an elongate member disposed helically about its rotating axis with at least one stirring member extending between turns of the elongate member and the discontinuity represents a break in the helical turns of the elongate member but not in the at least one stirring member.

9. A method of producing a blend of dry ingredients and a liquid ingredient substantially as herein described in Example III.

10. A method of applying at least one liquid component of a mixture to dry ingredients thereof substantially as herein described with reference to Figure 3 of the accompanying drawing.