GB2124512A - Improvements in or relating to granulators - Google Patents

Abstract

A granulator by which relatively large pieces of waste plastics material are reduced to granules by the cutting action of blades mounted on a horizontal rotor, against fixed blades 34, 35 (not shown) mounted on the surrounding housing 20. The working space of the granulator is in two horizontally-separated parts: raw material enters the first part vertically and is there subjected to a relatively coarse cutting action, after which it passes into the second part in which it is subjected to a relatively fine cutting action against fixed blades 39. Finished granules then leave the second part through an area of the surrounding housing wall which is formed as a screen 41. Part of the housing wall surrounding the first part of the space may also be formed as a screen to act as an additional outlet for finished granules, and the rotor blades may be arranged in a screw pattern to hel convey the material from the first to the second part of the space as well as to cut it. Fig. 3 shows the granulator with the rotor removed. <IMAGE>

Description

SPECIFICATION Improvements in or relating to granulators This invention relates to granulators, by which I mean machines used to break up a feed supply of relatively large and often irregularly shaped pieces of suitably rigid but low strength material into granules of relatively small and even size.

Granulators are particularly well known in the plastics industry where they are used to break pieces of waste plastics such as polythene and polystyrene into granules, either as a step in reprocessing the waste or - in the case of expanded polystyrene in particular -- in order to produce granules themselves useful as thermal insulating material.

Many of the granulators now in use in the plastics industry are of a general type that has been known for many years and which is illustrated diagrammatically in Figures 1 and 2 of the accompanying drawings: Figure 1 is a vertical section through the machine and Figure 2 is a section on the line Il-Il in Figure 1. A cylindrical drum 1 contains a rotor 2 rotatable about axis 3 by a motor (not shown). Raw material is fed to a hopper 4, from which it enters drum 1 in a generally tangential direction through a hole 5 in the wall 6 of the drum which coincides with the hopper outlet. Once within the drum, the pieces of raw material tend to travel clockwise within the annular space 7 which separates wall 6 from rotor 2.As they pass a first fixed or "dead" knife blade 8 mounted in wall 6, the pieces are repeatedly fractured and cut by the guillotining action against that blade of two moving blades 9 and 10 mounted on the surface of rotor 2. The resulting fragments of raw material tend to fall towards the lowest point 11 of annular space 7 and, under the influence of centrifugal force as well as gravity, to move towards and into contact with the screen 13 which forms the lowest arc of wall 6. Those that are small enough to do so then pass through the screen into a discharge outlet 12 which leads to a vessel (not shown) to receive the granules that are the product of the process.Pieces of raw material that are still too large to pass through the mesh of screen 13 when they first meet it are carried upward again within space 7 to be fractured for a second time by the guillotining action of moving blades 9, 10 against a second dead blade 14, the set of which is often arranged so that the action of the moving blades against it is finer than their action against dead blade 8. Granules that pass blade 14 then continue to move around space 7, passing first the hole 5 and then blade 8, and on meeting screen 1 3 once again a higher proportion than before will be small enough in size to pass through into outlet 12.

It should be noted that although Figures 1 and 2 show two moving blades 9, 10 and two dead blades 8 and 14, in practice there are usually more than two dead blades and sometimes many more than two moving blades. Also it is known in practice for the hopper 4 and screen 13 to be located differently relative to axis 3: for instance the screen sometimes extends from say 5 to 11 o'clock or from 6 to 12 o'clock instead of from about 4 to 8 o'clock as shown, and the hopper may be located nearer to 1 o'clock than where it is shown in Figure 1.

Although granulators of the kind just described are now long established in use and have proved reliable, it has become increasingly apparent that they have many inherent disadvantages. One such disadvantage is the short length of arc of screen 13: it cannot extend around the full circumference of the drum, because the inlet hole 5 occupies part of that circumference. Not only is it obviously true, at first sight, that a greater arc of screen would lead to greater throughput.It is well known in the art that throughput is related to the density of loading of raw material within the granulator drum: simply stated, the ideal state is for granules of finished or near-finished size to be packed densely in the part of the drum adjacent the screen, but for the part adjacent the inlet hole 5 to be relatively clear so as to offer minimum opposition to the entry of raw material from the hopper 4. With known granulators as shown in Figure 1 , this ideal tends not to be achieved.

Because throughput is restricted by the short screen length, the part of space 7 adjacent the screen 1 3 becomes so very full of granules that they cannot pass through the screen fast enough.

They are therefore frequently carried round and back into the other part of space 7, so blocking the entry of fresh raw material from the hopper 4.

A related disadvantage is that the flow of gas accompanying the particles as they are thus carried back into the upper part of space 7 tends to continue up the hopper 4 and to behave there as it would within a trumpet, so creating noise.

Yet a further disadvantage of the known granulator of Figure 1 is that the geometry allows no scope for successive cutting actions of increasing fineness: the moving blades 9 and 10 are similar to each other, as are the fixed blades 8 and 14, and the cutting action of either moving blade against either fixed blade is always the same.

The granulator of the present invention offers the prospect of obtaining a density distribution of material within the drum that is closer to the ideal mentioned above, and may also be less subject to the other disadvantages just mentioned. A granulator according to the invention comprises a cutting rotor rotatable about a horizontal axis within a generally cylindrical housing so as to define between rotor and housing a generally cylindrical working space in which raw material is broken up into granules by the interaction of fixed blades, mounted on the inner wall of the housing, against blades facing radially outwards and mounted on the rotor. Raw material enters the housing vertically through an inlet communicating with a first part of the working space.The wall of a second part of that space, axially removed from the first part, is formed as a screen to allow finished granules to leave the housing.

The screen may extend around substantially the whole of the circumference of the second part of the working space, and if desired part of the wall of the first part of the working space may also be formed as a screen, to act as an additional outlet for finished granules.

The blades on the rotor and the cooperating fixed blades mounted on the housing may be of different character in the first and second parts of the working space, for instance to give a coarse cutting action in the former and a finer action in the latter. The rotor blades may be arranged in screw or other pattern so that their action serves not only to cut the raw material but also to help propel the cut material axially through the housing from the first part of the working space towards the second.

The inlet may enter the housing in a direction having radial and/or tangential components.

The invention is further defined by the claims and will now be described, by way of example, with reference to the following further figures of drawings in which: Figure 3 is a vertical section through a granulator, with the rotor removed; Figure 4 is an elevation of the rotor; Figure 5 is a section on the line V-V in Figure 3, with the rotor shown in outline only; and Figure 6 is a section on the line VI--VI in Figure 3.

The granulator comprises a hollow cylindrical housing 20 with blind end walls 21 and 22.

Through these walls passes the shaft 23 of a rotor 24 rotatable about an axis 25 and driven by a motor 26, and supported in bearings 27. For clarity, almost all of the body of rotor 24 is omitted from Figure 1. Raw material enters housing 20 from a hopper 28 through a hole 29 formed in the cylindrical wall of the housing, the centre-line 30 of the hopper being vertical and lying along a radius relative to axis 25, although a more tangential placing of the hopper relative to the housing would be possible. Raw material falling under gravity from hopper 28 into housing 20 enters a generally cylindrical working space 31 between the rotor and the housing wall, in which it is broken down by the cutting action of a blade 32, carried by the hub 33 of rotor 24 against fixed blades 34, 35 mounted on the housing.On first entering space 31 from hopper 28, raw material enters what will be referred to as the "first part" 36 of that space lying directly beneath inlet 29 and to the right (as seen in Figure 3) of a radial plane indicated by broken line 37. While within first part 36 the raw material is subjected to the relatively coarse cutting action of fixed blades 34, 35, against moving blade 32, which is of spiral form and therefore serves also to help propel the coarse-cut material into a "second part" 38 of working space 31, which lies on the left-hand side of plane 37 and is thus axially separated from first part 36.In second part 38 the coarse-cut material is subjected to the finer cutting action of fixed blades 29 against moving blades 40, and the entire peripheral wall of housing 20 which bounds part 38 is formed as a screen 41 through which the finished product, that is to say raw material reduced to granules just small enough in size to pass the mesh of the screen, may leave the housing and pass by way of ducting (shown in outline only at 42, in Figures 3 and 6) to a collection point (not shown). As Figure 6 shows best, the fixed blades 29 project inwardly through apertures in screen 41 and are mounted on fixed supports 43 that are external to the housing 20.

The axial separation of the first and second parts 36 and 38 of working space 31 makes it possible for part 38 to be relatively densely packed with granules at or near finishing size, thus promoting high flow of those granules through screen 41, while part 36 is relatively less full of raw material because the material spends only a short time in that part before being propelled into second part 38 by reason not only of the screwfeed action of blade 32 but also of the predominant air flow from hopper 28, axially through housing 1 and out through screen 42, this air flow being indicated by arrows 44. The relative emptiness of first part 36 promotes free and unobstructed entry of raw material into that part from hopper 28. The axial component in predominant air flow 44 also minimises the chance of any part of the air flow generated by the spinning of the rotor being directed back up hopper 28, and thus generating noise.

If desired, part of the cylindrical housing wall that bounds first part 36 of the working space for instance the iower arc indicated by reference 45 in Figure 5 - could also be formed as a screen, similar to screen 41, n order to promote still greater throughput.

Claims (8)

1. A granulator comprising a cutting rotor rotatable about a horizontal axis within a generally cylindrical housing so as to define between rotor and housing a working space in which raw material is broken up into granules by the interaction of fixed blades mounted on the inner wall of the housing against blades facing radially outwards and mounted on the rotor, in which the working space is divided into first and second horizontally-separated parts, in which a vertical inlet for raw material is provided in the wall of the housing surrounding the first part of the working space, and in which at least part of the housing wall surrounding the second part of the working space is formed as a screen to allow finished granules to leave the granulator through it.

2. A granulator according to Claim 1 in which the screen extends around substantially the whole circumference of the housing surrounding the second part of the working space.

3. A granulator according to Claim 1 in which part of the housing surrounding the first part of the working space is also formed as a screen, to act as an additional outlet for finished granules.

4. A granulator according to Claim 1 in which the interactions between the fixed and rotating blades in the first and second parts of the working space are of different character, for instance to give a coarse cutting action in the former and a finer action in the latter.

5. A granulator according to Claim 1 in which the blades mounted on the rotor are arranged in screw or like pattern so that their action serves not only to cut the raw material but also to help propel the cut material axially through the housing from the first part of the working space towards the second.

6. A granulator according to Claim 1 in which the inlet is arranged radially relative to the rotor.

7. A granulator according to Claim 1 in which the inlet has a tangential component relative to the rotor.

8. A granulator according to Claim 1, substantially as described with reference to the accompanying drawings.