IES84543Y1 - Improvements in and relating to waste handling - Google Patents

Description

lmgrovements in and relating to waste handling Introduction The present invention relates to a method of treating waste material including garbage and plastic bags and in particular to a rotary screen for use in such a method.

Waste recycling and in particular the recycling of domestic waste, including the waste from service industries such as hotels and restaurants, is becoming a difficult problem. The problem is caused by the fact that waste is generally not homogenous and indeed a lot of the waste is delivered for recycling in plastics bags. Such waste material also includes industrial waste which is not produced by the industrial process per se, but is incidental thereto, such waste comprising packaging materials, namely wood, plastics, paper, cardboard and indeed food as a waste product of food producers and of the canteens, etc. of such operations. Almost all of these materials are biodegradable except plastics. Unfortunately, some of the waste materials take some considerable time to degrade, unless they have been prior treated.

It is known to use a rotary screen, often called a trommel screen, to separate such garbage into course and fines material. Often the garbage has been pre-shredded, ground or otherwise treated. Such screens are, for example, disclosed in US Patent Specification No. 4,533,054 (Sommer et al). The screen includes knives or blades which project from the interior of the screen cloth to cut or abrade the material.

These blades are usually relatively sharp. In some cases, for example, they may have a serrated edge or saw tooth edge such as disclosed in US Patent Specification No. 4,509,693 (Riemann et al). Another typical construction is shown in US Patent Specification No. 5,638,959 (Sommer et al). The idea is that the knife equipped trommel screen rips open plastics bags and thus allows the material to fall out of the bags to be separated within the screen. This works relatively well as far as the cutting of the plastics bags is concerned. However, there are problems in that because the knives are relatively sharp, they tend to enter into pieces of wood and other more rigid material and then to carry that material around the screen so that after a while, the screen gets totally clogged and is relatively inefficient in use. It works very well when the waste material is relatively homogenous, such as wood trimmings.

In many operations, when the waste material is subjected to grinding, abrasion or shredding, such as delivering the garbage between a pair of counter-rotating spaced- apart shafts, each carrying a plurality of radially and circumferentially arranged hammer-like projecting flails. A typical example of such a shredding machine is described in our co-pending British Patent Application No. 0219813.3 These work very well for disintegrating hard materials, however, unfortunately plastics bags and the like tend to be squashed in these machines and are often not opened. Thus it is necessary for the rotary screen to have some way of opening the bags.

As stated above, it has been found that the rotary screens of the construction, such as described in the prior art, do not work very well in that because the blades are relatively sharp, they tend to stick into harder materials which are not dislodged from the blades.

The present invention is directed towards overcoming this problem.

Statements of Invention According to the invention, there is provided a rotary screen for waste material including garbage in plastics bags comprising a substantially cylindrical drum mounting a perforated screen cloth; and a plurality of inwardly directed elongate teeth projecting beyond the screen cloth mounted spaced-apart circumferentially and axially within the screen, wherein each tooth comprises a substantially triangular flat spear head-like blade having a relatively thick inner portion of at least 5mm converging into a blunt leading edge, the converging sides of the blade subtending an angle greater than 60° and terminating in a rounded free distal end.

The advantage of the present invention is that because the blades are relatively blunt, they will not carry around with them harder materials such as pieces of wood and the like while, at the same time, they are sufficiently sharp to abrade and cut the bags.

However, even more importantly, as the bags are carried around the screen, they tend to fall off any respective blade penetrating them onto another blade below where again they are again penetrated, while harder material will not be carried around by the blades. Indeed, when it falls off one blade, when it is in contact with a blade, it will fall onto a blade below but will not impinge thereon. Thus, the harder material tends to be abraded somewhat by the blades while being carried through the screen.

In one embodiment of the invention, the leading edge of the tooth is rounded.

In a still further embodiment, the leading edge of the tooth subtends an angle greater than 90° and indeed may subtend an angle of the order of 120°.

In another embodiment of the invention, the teeth extend from adjacent a proximal entrance end of the screen for of the order of two thirds of the length of the screen and in another embodiment of the order of one quarter of the length of the screen.

In another embodiment of the invention, the teeth project at an angle inclined to the radius of the screen.

In the rotary screen according to the present invention, a cover plate is mounted above the screen and spaced-apart a relatively small distance therefrom to prevent delivery of material out of the screen.

In a still further embodiment of the invention, brushes are mounted on the exterior of the screen in contact with the screen cloth to clean the screen.

In a still further embodiment of the invention, the screen is inclined downwardly from entrance to exit and ideally between 2° and 5° to the horizontal.

Detailed Descrigtion of the Invention The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: F ig. 1 is a plan layout of a waste recycling plant according to the invention, Fig. 2 is a side view of portion of the waste recycling plant, Fig. 3 is a side view in the direction of the arrow lll-lll of Fig. 2, Fig. 4 is a side view in the direction of the arrow IV-IV of Fig. 2, Fig. 5 is a view of part of a screen with portion removed showing the interior screen cloth in accordance with the invention, Fig. 6 is a side view of a tooth used in the screen, Fig. 7 is a sectional view along the line Vll-VII of Fig. 6, and Fig. 8 is a sectional view along the line Vlll-VIII of Fig. 6.

Referring to the drawings and initially to Figs. 1 to 4 thereof, there is illustrated a waste material processing plant, indicated generally by the reference numeral 1. The waste material processing plant 1 comprises various stages, all indicated generally by reference numerals, namely, a waste storage station 2, a shredding station 3, a screening intake station 4, a screen station 5 and a further processing station 6.

Most of the equipment is generally conventional, except for the screening station 5.

Various features of the mechanical loading equipment such as shovels, grabs, and so on, as well as trucks and vehicles, are illustrated but may not necessarily be identified as there is no need to do so. Refuse is not identified by any reference numeral and indeed is not shown, except as a stockpile of waste.

The waste storage station 2 is a standard stockpile of waste material and garbage such as described already. The shredding station 3 comprises a shredder 10 such as, for example, a shredder described in our co-pending Patent Application No. 02198133 comprising a plurality of hammer-like projecting flails which beat and abrade the material. There is also included a conveyor 11 for feeding the screening intake station 4.

The screening intake station 4 also includes an intake hopper 12 feeding a conveyor 13 which in turn feeds the screening station 5 which will be described in more detail later. The screening station 5 in turn feeds, through a conveyor 14, the further processing station 6 which includes a conveyor 15 feeding either baler 16 or further processing equipment 17.

The screening station 5 comprises a rotary trommel screen, indicated generally by the reference numeral 20, fed by an intake hopper 21 and mounted on a framework 22 below which there is shown a trailer. It will be noted that the screen 20 is inclined at an angle of approximately between 2 and 5° to the horizontal to allow the delivery of the material out the screen. Most of the features of the screen 20 are not described as they are relatively standard. The screen 20 comprises various screen cloths 30 formed from perforated metal plates, usually with square holes of the order of 40mm mesh. The screen cloth 30 therefore comprises a plurality of openings 31 and has mounted thereon a number of inwardly directed teeth 35 (Figs. 5 to 8 inclusive). Each tooth 35 comprises a base plate 36 mounted on the screen cloth 30 by bolts 37. triangular flat spear head-like blade 38 having a relatively blunt leading edge 39 and a Each tooth 35 in effect comprises a triangular plate 38 having a rounded free distal point or end 40. Generally speaking, the trailing edge 41 of the blade would also be constructed in the same way as the leading edge so that the blade can be easily reversed as it wears.

It will be appreciated that the free end of the triangular flat blade 38 may be flat rather than rounded. it will be noted that the leading edge 39 is rounded and that the leading edge subtends an angle in the embodiment shown of the order of 90°. The leading edge would generally subtend an angle greater than 60° and may be as blunt as to subtend an angle of 120°. It will also be noted that the point 40 is rounded and thus will not penetrate easily into hard material but obviously will penetrate easily into plastics bags.

Generally, the teeth 35, many of which are not illustrated, extend from adjacent a proximal entrance end of the screen 20 for of the order of two thirds of the length of the screen. They may extend only one quarter of the length of the screen. Ideally, the teeth are angled to the screen cloth to further assist in cutting the materials.

The screen 20 further includes a removable top cover 50 and associated cleaning brushes 51. In operation, material is delivered from the waste storage station 2, either directly into the hopper 12 or from the shredding station 3 via the conveyor 11.

The waste material is delivered into the screen 20, which screen 20 will agitate the waste by means of the teeth 35, which teeth 35 will easily puncture plastic bags and the like and indeed will assist in the puncturing because they will tend not to carry the plastics bags around the screen but will allow the plastic bags to drop off them and to impinge onto further teeth below. This ensures the bags are tossed and mixed as they travel through the screen. This separates the contents of the bags and other waste into fine and oversize material. At the same time, the teeth being blunt will not hold harder material such as wood and the like. Thus, the screen can be operated to abrade considerably the waste material and garbage. The fines will be delivered our the screen cloth 30 onto the trailer and the remainder of the oversize material will be delivered out the conveyor 14 for further processing in the further processing plant 17 or for baling for fuel or other disposal. The material suitable for compost, namely, the material delivered into the trailer, can be removed for processing as compost.

When compost is being made, it will be appreciated that after the compost has been made, it can be further ground, screened and shredded as necessary. Indeed, it can be screened to remove any materials that have not be composted. It will also be appreciated that the compost material may be ground and shredded further prior to composting.

The term “relatively blunt” is used in this specification in the sense that the blade widens out away from the edge subtending an angle greater than 60° which itself is sharp, it is the point or distal end which is rounded. Thus, the blade acts to cut plastics bags and abrade and impact on the contents of the bags and loose garbage.

This will cause the waste material to be carried around the screen and be separated by the blades. This is entirely different to the manner in which existing screens are constructed for this task.

If you have a sharp blade that subtends say angle of 10° or so, it then forms a narrow leading edge which, even when worn is still relatively sharp, in the sense that it still offers a thin blade to the material. Further, it will still provide a relatively sharp thin piercing device.

What must be appreciated is that the construction of teeth of the present invention is based on teaching differing from conventional wisdom. It is based on the realisation that the plastics bags and their contents do mt need to be cut up in the most efficient way but that what is required is to release the contents of the plastics bags, separate the contents and other waste in the screen, while not clogging up the screen.

The conventional teaching is that the more you cut up the waste and pulverise it, the better. Unfortunately, the more you try to cut up the waste, the sharper and narrower are the teeth required. These in turn penetrate some of the waste and then retain it on them as they rotate, effectively nullifying the blade in the sense it cannot cut into other waste and additionally blocking parts of the screen cloth. Even when such teeth or blades, in use, become relatively blunt at the edge, they are still relatively thin and will penetrate and retain waste on them.

There is a considerable difference in action of a thin previously sharp blade or tooth and one which may be sharp at its outer edge, in the sense of two flat surfaces meeting along a line if the flat surfaces subtend a large angle of the order of 60° or even 120°.

Some of the blades of the prior art, when blunt, could be said to resemble the teeth of the present invention. However, even when the knives of US Patent Specification No. 5,638,959 (Sommer et al) wears and the lip of the knife becomes blunt, the knife is still narrow and thus if it cuts into articles admittedly with more difficulty than when sharp, it is still likely to carry the waste around the drum.

The present invention does not simply provide blunter teeth in the sense of worn teeth of the prior art but it provides wider teeth. Indeed, it is not that the present invention provides a blunter tooth in the sense that it terminates in a rounded edge, but that it is less penetrating in its action.

Further, the provision of the rounded distal end is all important. Bags falling on the distal end will be easily pierced, while harder objects will tend to bounce off the teeth.

Again, because of their general shape, the sharp blades of the prior art, even when worn, will penetrate and hold these harder pieces of waste.

It is also important to appreciate that the present construction of screen is provided not to pulverise or pulp waste material, but simply to separate it. The plastics bags should be opened and their contents separated, while at the same time delivering the bags and the harder oversize waste material out of the screen as quickly as possible to prevent clogging of the screen.

In the specification the terms “comprise, comprises, comprised and comprising" or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail within the scope of the claims.

Claims (1)

1. CLAIMS A rotary screen for waste material including garbage in plastics bags comprising a substantially cylindrical drum mounting a perforated screen cloth; and a plurality of inwardly directed elongate teeth projecting beyond the screen cloth mounted spaced-apart circumferentially and axially within the screen, wherein each tooth comprises a substantially triangular flat spear head-like blade having a relatively thick inner portion of at least 5mm converging into a blunt leading edge, the converging sides of the blade subtending an angle greater than 60° and terminating in a rounded free distal end and wherein cleaning brushes are mounted on the exterior of the screen cloth in contact with the screen cloth. A screen as claimed in any preceding claim, in which the leading edge of the tooth subtends an angle of the order of 120°. A rotary screen as claimed in any preceding claim, in which the teeth extend from adjacent a proximal entrance end of the screen for of the order of two thirds of the length of the screen. A rotary screen as claimed in any preceding claim, in which the teeth project at an angle inclined to the radius of the screen. A rotary screen substantially as described herein with reference to and as illustrated in the accompanying drawings.