Technical Field

This invention is concerned with a device for blend¬ ing a mixture of particulate materials.

Many industries have a requirement for a blended mixture of particulate materials containing accurate proportions of the materials. The food, chemical, pharma¬ ceutical, agricultural and horticultural industries at least have such requirements.

Summary of the Invention

According to the present invention a device for continuously blending particulate materials includes a plurality of metering means, each to meter a particulate material, and blending means to receive and blend the metered quantities of particulate material delivered from the respective metering means, said blending means comprising a conveying means in the form of an elongate member disposed helically about a rotating axis of the conveying means, the cross-section of the elongate member being of arcuate form at least on the side thereof facing the conveying direction of the conveying means, the con¬ veying means being located in a casing conforming to the envelope shape of the helically formed elongate member over a lower portion thereof.

Conveniently the pitch of the helically formed number is constant throughout its length and the elongate member is desirably of circular cross-section. Preferably stirr¬ ing members are provided on the conveying means which extend between at least some adjacent turns of the helic¬ ally formed member. Conveniently some of the stirring members are located on the outside of the respective helical turns (i.e. between the conveying means and its casing) and some on the inside of the respective

helical turns (i.e. between those turns and the axis of rotation of the conveying means). The stirring members can, with advantage, be disposed parallel to the axis of the conveying means and an aligned series of stirring

5 members extending from end to end of the conveying means can be provided by a single bar, rod or wire extending from the first turn to the last turn of the helically formed member. Conveniently stirring members extending the full length of the helically formed member alternate

I^Q in being secured inside and outside the helical turns.

The metering means can conveniently each also com¬ prise a respective conveying means formed from an elongate member of at least part-arcuate cross-section helically disposed and mounted for rotation about its helical axis.

15 Whereas the helically formed member of the conveying means of the blending device is designed to run at high speed in a casing containing particulate material to a depth of not much more than that required to cover the lower part of each turn of the elongate member, the

20 conveying means in each metering conveying means is de¬ signed to run at slow speed (e.g. one tenth of the speed of the blending conveying means) in a casing containing . particulate material to a depth completely covering each turn of the elongate member at least at the inlet end

25 of the metering conveying means. The amount of material dispensed from each metering conveying means into the blending means can be adjusted by varying the speed of rotation and/or the pitch of the elongate member forming the conveying means. 0 A blending device according to the invention may be used not only to blend the particulate materials, but also to blend a dosed minor proportion of liquid inert to the other components of the blend. By 'inert' is meant both chemically and physically inert including 35 at least to the extent that it is not a solvent for any solid component of the blend. It will be appreciated

that the blending conveying means not only blends the components fed to it at its upstream end but conveys the mixture of particulate materials to the downstream end of the conveying means from whence it is discharged as a well blended mixture.

The provision of a cross-section of at least part arcuate form for the elongate member of the conveying means is important to the blending process. When a flat surface of the elongate member is presented in the conveying direction (e.g. a member of rectangular cross- section is used), it has been found that the material is liable to pack between the helical turns and the rela¬ tive movement of the particles during their conveyance along the helical member, necessary to making a thorough blend, is inhibited.

Where each of the plurality of metering means also comprises a conveying helix in a casing, the elongate members forming these helices may be of any convenient cross-sectional shape (e.g. of flat, rectangular or round section). The helical conveying means may be driven at correlated speeds to deliver appropriate proportions of particulate materials, or the pitches of the helices may be varied and all conveying means driven at similar speeds of rotation to achieve the same purpose.

Where a component is required to form only a small proportion of a final blend, it can be desirable to meter such a component using a smaller diameter helix for the conveying means (and a smaller diameter casing) compared with that used to deliver a or the major component(s) of the blend, rather than rely solely on employing differ¬ ent speeds of rotation or different pitches of similarly sized helices.

Two streams of blended particulate materials may

be obtained from a blending device in which the helix of the conveying means changes hand mid-way along its length. Such a helical conveying means having only one drive but receiving metered materials onto each different handed length adjacent to the centre of the length delivers two blends, one from each end of the helical conveying means.

Brief Description of the Drawings

The invention will be further described by way of example, with reference to the accompanying drawings, in which

Figure 1 is a sectional side elevation of a first embodiment of blending device made in accordance with the invention;

Figure 2 is a transverse sectional view of the blend- ing device of Figure 1 taken on the line II-II of Figure

1,

Figure 3 is a schematic side elevation of an arrange¬ ment embodying six metering devices and the blending device of Figure 1,

Figure 4 is an end view of the arrangement shown in Figure 3.

Figure 5 is a scrap view showing how an adjustable pitch of a conveying helix can be arranged, and

Figure 6 shows some suitable cross-sections for the elongate member forming a conveying means of a blend- ing device.

Description of Preferred Embodiments

The blending device 10 shown in Figure 1

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 part¬ iculate 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 conven¬ iently 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 (eg 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 indicate 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 similar conveying means to that shown at 12 in Figure 1 .

Figures 3 and 4 show such an arrangement in which two main metering conveyors 26A, 26B are combined with four secondary metering conveyors 26C - 26F. Hoppers 27A to 27F are provided above each conveyor and contain enough material to cover the turns of the respective conveying means at all times during use of the arrange¬ ment. Each conveyor 26A to 26F discharges its output into the hopper 14 of the blending device 10, the con¬ veyors 26C - 26F discharging into a downwardly converg¬ ing duct 28 which opens into the hopper 14. Since the secondary metering conveyors 26C - 26F are intended to feed minor proportions of the ingredients- of the mixture blended in the device 10 _/ they can be of shorter length and smaller diameter than the conveyors 26A and 26B in¬ tended to meter major components of the blend.

Each metering conveyor 26A - 26F and the blending conveying means can be driven from a single prime mover

(e.g. an electric motor). To provide different rotational speeds for the different conveyors, gearboxes of variable ratio can be used as can chain and sprocket drives or toothed wheel and toothed belt drives. However, in pract- ice it has been found that varying the pitch of the helical turns of any given metering conveyor is a very useful way of fine tuning the metering rate at which it delivers its component to the blending device and accordingly each metering conveyor can be constructed as shown in the scrap section of Figure 5. With this arrangement one end of the helically disposed rod (18) can be moved axially with respect to the shaft (19) by an end plate 30 and a nut 31 screw-threaded onto a tapped end 32 of the shaft. If the arrangement shown in Figure 5 is used, any radial supports (e.g. the arms 20) of the helically disposed member and any stirring members (25a, 25b) provided must be arranged to accommodate the slight axial movement made by each turn of the helix when the plate 30 is axially adjusted. Thus a rod forming a stirring member could be fixedly secured at one end to a turn of the helix and slidably mounted at all other turns. The blending conveying means could include a construction such as shown in Figure 5 but it is not necessary to provide such a pitch adjustment facility in the blending device. A sliding device can be used in place of the threaded arrangement 30-32 to provide for helix pitch adjustment.

The arrangement shown in Figures 3 and 4 can also be used to add small volumes of liquid (e.g. a liquid surfactant) to a mixture of ingredients to be blended and an inlet region for such a liquid is indicated at 33 in Figure 3. A 1% v/v addition of liquid surfactant has been found to provide "particle coating" of the blended mixture.

The blending device 10 may be specifically arranged

- 8 - to ensure back mixing of the particulate materials e.g. it may be inclined upwardly towards the discharge end thereof and/or fitted with contra-wound helix sections.

A mixture of xanthan gum powder and sodium carboxy

5 methyl cellulose particles fed to the hopper 14 of the device 10 in the proportion of 10:1 was found to be blended by the device to an accuracy of + 1% over six random samples of the blend.

In a typical example of an apparatus according to 10 Figures 3 and 4 - the metering conveyors 26C - 26F were each variable pitch helices of 51 mm (2 inches) diameter of 3.2 mm (% inch) diameter round section rod of 9.5 mm (% inch) pitch and the conveyors 26A, 26B were of variable pitch helices of 200 mm (8 inches) diameter of 9.5 mm 15 (I inch) diameter round section rod at 63.5 mm (2^ inch) pitch. The elongate member 18 in the blending device 10 was of the same size, pitch and rod diameter as the conveyors 26A, 26B but was of fixed pitch. The overall length of the blending device was some 2000 mm.

20 Figure 6 shows some suitable cross-sectional shapes for the member 18 of the blending device. The section

'40 is circular, the sections 41 and 42 elliptical, and the sections 43 to 45 part arcuate with a flat section facing away from the conveying direction 24.