GB2164331A

GB2164331A - Waste disposal apparatus

Info

Publication number: GB2164331A
Application number: GB8419555A
Authority: GB
Inventors: Thomas Richard Wotton
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-01
Filing date: 1984-08-01
Publication date: 1986-03-19
Also published as: JPS6140888A; GB2164331B; GB8419555D0

Abstract

A waste disposal apparatus includes a shredder 4 to shred waste material to a required product size. The shredded material is passed through a magnetic separator 8 to a pneumatic separator 16 which separates the light fraction from the heavy fraction. An aerobic process is carried out on the heavy fraction in a digestor tower 18 to convert the heavy fraction into compost. The light fraction is fed to a pelletiser 36 which converts the light fraction into pellets of refuse derived fuel. <IMAGE>

Description

SPECIFICATION Waste disposal process Technical field The present invention relates to a waste disposable process, and is particularly concerned with but not restricted to a process for disposing of household and commercial waste.

Two conventional methods of disposing of household waste are: (a) By the tipping of raw rubbish, and (b) By incineration.

Tipping is the more commonly used method of disposal but this method has unattractive features particularly where it is not properly controlled. It can provide a source of water pollution, and it can become an attraction for disease carrying insects and vermin.

The incineration of waste is expensive.

It has also been proposed to provide a waste disposal process which can produce compost. With this previously proposed process a significant amount of the waste material has to be rejected as being of no further use.

Object of the invention It is the aim of the invention to provide a waste disposal process in which the aforementioned difficulties are alleviated.

Statements of invention The present invention relates to a waste disposal process comprising shredding the waste material to a required product size, separating the ferrous material from the waste material, separating the light fraction from the heavy fraction, feeding the heavy fraction through the top of a digestor tower, carrying out an aerobic process in the tower under controlled conditions to convert the heavy fraction into compost, and feeding the light fraction to a pelletizer to form pellets of refuse derived fuel.

The pelletiser may comprise a feed shredder and a pre-dryer to reduce the moisture content of the material.

The pellets may be cooled and screened, the pellets above a preselected size being bagged or stored in bulk.The pellets below a preselected size are recirculated into the pelletiser for pettelisation.

Figure in the drawing An embodiment of the invention will now be described by way of example with reference to the accompanying illustrative block schematic drawing of a composting plant.

Detailed description of the drawing Referring to the drawing, waste material to be processed is brought to a plant reception area (2) where it is weighed. Two weighbridges mounted on either side of a weighbridge office are located at this reception area (2), one weighbridge or Quin' traffic and the other weighbridge for QOut' traffic.

Each weighbridge is of the load cell type and is sunk into a shallow pit so that the weighbridge platform is flush with the ground level. Each weighbridge has anti-skid deck plates of self draining pattern which are mounted on a heavy duty steel superstructure of welded construction. This superstructure is mounted on high grade load cells and each weighbridge will include restrainers, kerbing and bearing plates. Each weighbridge is graduated up to 50,000 k.g. in 10 k.g. divisions on a digital weight indicator having numerals which are 30 mm high.

The weighbridges are arranged automatically to print out four copies of a suitable size ticket together with a tallyroll. Both the tickets and the tallyroll will record the required data, such as for example date and time, Quin' and QOut' weights, the material code and the automatic calculation of the nett weight.

The weighed material is tipped onto a concrete apron and passed onto two slow moving horizontal plate feeders which are each two meters wide and ten meters long. These plate feeders are driven by units which can provide variable speeds and torque control. These feeders are mounted in a shallow reception hopper so as to be approximately at ground level thereby enabling items such as oxygen bottles which will cause damage to the plant to be removed by hand.

These slow moving feeders onto two inclined plate feeders designed to operate at approximately three times that of the slower moving feeders.

These inclined plate feeders carry the waste material into the primary shredders (4), and they have continuous side skirt plates terminating at the shredder hood.

Each primary shredder (4) is a hammer mill having a throat size of 120 cm x 150 cm, and is driven at 850 rpm by a 350 HP motor capable of reducing approximately 55 tonnes of waste per hour to the required shredded size.

Each hammer mill possess a feed hood complete with anti flyback curtain, a vee belt drive complete with motor pulley, pulveriser pulley and vee belts, a 350 HP slipring motor complete with slide rails, suitable for a 50 Hz supply, a vee belt drive guard and a fabricated base frame.

Two belt conveyors (6) which are 1.20 m wide convey the shredded material from the shredders (4) to magnetic separators (8). There are two magnetic separators (8), one at the head of each belt conveyor (6). These magnetic separators (8) remove magnetizeable material from the shredded waste material. The remainder of this shredded waste material is then conveyed to two rotary trommel screens (10) which are approximately 2 m diameter by 7 m long and are driven by variable speed drive units incorporating torque control.Each trommel screen (10) is mounted in a 3 mm thick mild steel dust enclosure, and is located on a platform provided with a ground level walkway and all necessary chutes and supports.

The smaller sized material - Qthe fines' - is collected from the trommel screens (10) by a fines conveyor (12) and fed onto main feed conveyor (14).

The oversized material from the trommel screens (10) is passed to a vibrational pneumatic separator (16) which separates the light material, such as papers and plastics, from the heavier material. This heavier material is passed to the main feed conveyor (14) where it combines with the fines from the trommel screens (10). The main feed conveyor (14) comprises a single inclined belt 1.40 m wide which is completely enclosed where it runs outside a building. This conveyor (14) transports rejected fine material from the trommel screen (10) and heavier material from the separator (16) to digestor towers (18). A chute is located at the heading of the main feed conveyor (14) to feed into the number 1 or the number 2 digestor tower. The main feed conveyor (14) is supported on a series of trestles manufactured in braced frames.A trestle is located at the head of the conveyor (14) adjacent to the digestor towers (18) so as to take the main reaction from the head of the conveyor (14) in order to minimize the load on the towers (18). These frames also support a three drum type gravity take-up device.

The two glass coated, steel, open-topped towers (18) are provided with an internal diameter of 9 m and a height of 24 m. These towers (18) are made of glass which is fused to steel sheets which are bolted together with plastics-headed high tensile steel bolts, nuts and washers. The lap joints between the steel sheets are sealed with a specialised mastic sealant.

The tower structure includes a vertical ladder with five intermediate access platforms. A top cage located on the ladder and a walkway is provided to connect the main walkway with a horizontal catwalk. An opening 23 cm square is provided in each tower (18) with a cover plate and circular screw flanges. A door is mounted in the base sheet of each tower (18) to allow access under the steel floor.

A mild steel plate floor is located 980 mm above the base of each tower (18), and this floor is perforated to allow the passage of air therethrough.

Five thermocouples are provided at equally spaced vertical points along the height of each tower (18), and these thermocouples are connected to a six point recording thermometer in a control room. The six temperatures recorded are the ambient temperature, together with the internal temperature of the material in the tower at the five points. This information is critical to the maintainance of the maximum aerobic efficiency of the plant. The recordings enable operators to effect required temperatures for pathogen kill and sterilisation and these recordings identify any low temperature areas within the tower (18) which require additional treatment by the insertion of localised air pokers. In addition, gas analysis equipment is available to analyse samples of the gases emitted from the top of a tower (18).

Each tower (18) operates on a continuous basis with 100 tonnes of shredded material being fed into the top of the tower (18) daily from the conveyor (14), and the equivalent amount (approximately 60 tonnes) of compost being removed from the base of the tower (18). Air is provided by air compressors, in response to the aforementioned temperature readings, and passed into a plenum chamber in the base of a tower (18). This air then passes through the perforated floor of the tower (18) and is allowed to percolate through the material and along the full height of the tower (18) hereby effecting complete aerobic decomposition.

The material resides in a tower (18) for a period of 14 days, but primary decomposition takes place over a period of approximately 10 days. The last four days spent in the tower (18) complete the total degeneration of the material and replace the curing period required by other systems.

The composted material is removed from the base of a tower (18) by a rotary extractor having a capacity of 15-20 tonnes per hour, and this composted material is passed along a feed conveyor (20) to two product crushers (22). The feed conveyor (20) is 1 m wide and is provided with full length skirts. The product crushers (22) are designed to reduce the compost material to a size less than 15 mm, and these crushers are driven by 25 HP motors.

The material is then passed from the product crusher (22) to a vibrating compost screen (24) 1.8 m wide x 4.5 m long included in a 3 mm mild steel dust enclosure. This screen (24) is provided complete with a supporting structure, sheeted housing, access platforms and the necessary chutes. This compost screen (24) has a 15 mm mesh and the material passing though this screen is the final composted product which is passed along a fine compost conveyor (26) to bagging plant (28). This conveyor (26) is 1.2 m wide and the bagging plant (28) is complete will full length skirts.

Oversize material is returned from the screen (24) along a return conveyor (30) to the reception area (2) for recirculation and reprocessing. The conveyor (30) is 1 m and is provided with full length skirts and a walkway on one side.

The bagging plant (28) is capable of filling 3x50 k.g. bags per minute and this bagging plant (28) includes a belt feed automatic weigher with a discharge chute and a foot valve operated pneumatic bag holder. The plant (28) also includes a continuous band heat sealer with a sealing conveyor, and an inclined conveyor to elevate sealed bags to should height for easy handling. The light fraction of the material from the separator (16) is passed to twin pelletising plants Each pelletising plant comprises a feed shredder (32) and a pre-dryer (34) to reduce the moisture content of the material to approximately 15-. The material is then pelletised in a pelletiser (36) and cooled in an after-cooler (38).

The pellets are then screened in a screen (40), and the pellets above a preselected size are emitted from the screen (40) and either bagged or stored in bulk for sale. The fine material passing through the screen (40) is recirculated into the pelletiser (36) for re-pelletisation.

The pellets from the screen (40) which have been produced by the pelletiser plants constitute refuse derived fuel, and on a tonnage basis approximately 30- of the input tonnage to the plant is produced as compost, and 30- as refuse derived fuel. The re maining tonnage is lost in the process as water vapour, emitted gases (mainly Co2) and extracted ferrous metals.

Claims

1. A waste disposal process comprising shredding the waste material to a required product size, separating the ferrous material from the waste material, separating the light fraction from the heavy fraction, feeding the heavy fraction through the top of a digestor tower, carrying out an aerobic process in the tower under controlled conditions to convert the heavy fraction into compost, and feeding the light fraction to a pelletiser to form pellets of refuse derived fuel.

2. A process as claimed in Claim 1 including shredding the light fraction in the pelletiser.

3. A process as claimed in Claim 2 including pre-drying the shredded light fraction.

4. A process as claimed in any preceding Claim including cooling and screening the pellets.

5. A process as claimed in any preceding Claim including recirculating the pellets below a preselected size into the pelletiser for re-pelletisation.

6. A process as claimed in any preceding Claim in which the ferrous material is separated magnetically.

7. A process as claimed in any preceding Claim in which the compost is crushed and screened.

8. Apparatus for carrying out a waste disposal process including a shredder to shred the waste material to a required product size, a first separator to separate the ferrous material from the waste material, a second separator to separate the light fraction from the heavy fraction, a digestor tower in which an aerobic process can be carried out under controlled conditions to convert the heavy fraction into compost, and a pelletiser to form pellets of refuse derived fuel from the light fraction.

9. An apparatus as claimed in Claim 8 in which the pelletiser comprises a feed shredder.

10. An apparatus as claimed in Claim 9 in which the pelletiser comprises a pre-dryer.

11. An apparatus as claimed in any one of Claims 8 to 10 including an after-cooler and a screen to cool and screen the pellets.

12. An apparatus as claimed in any one of Claims 8 to 11 in which the first separator is a magnetic separator.

13. An apparatus as claimed in any one of Claims 8 to 12 including a crusher and a screen to crush and screen the compost from the digestor tower.

14. A waste disposal process substantially as herein described with reference to the accompanying drawing.

15. A waste disposal apparatus substantially as herein described and shown in the accompanying drawing.