Case Study : Thompson's of Prudhoe
In common with many waste recycling companies, Thompson's of Prudhoe have become increasingly concerned that the high cost of dumping large tonnages of taxable waste in landfill sites was having a detrimental effect on their profitability. With this in mind, Thompson's Operation Manager, Aubrey Telfer and Craig Bateson, Site Manager of the company's Springwell Quarry, embarked upon a project which would eventually result in a reduction of between eighty and ninety percent, by weight, in the amount of material going to landfill.
The recycling plant at Springwell Quarry handles skip and C & D waste. For a number of years the incoming material had been initially split over a 40mm Trommel screen, with the 40mm down product being acceptable for use as cover material on both Thompson's own and other landfill sites in the area. However, with the tightening of legislation such material could no longer be used for this purpose since it contained paper, plastic, cardboard and polystyrene and it therefore became subject to Landfill Tax.
Having established that there was a throughput of some 25 tonnes an hour of this type of material it was considered viable to re-screen the 'fines'. A split which took out material at around 12 to 15mm would be ideal, since this soil-like material, with acceptably low levels of contaminants, could be used for cover material with no tax penalty.
Having decided upon this course of action, the choice of screening method became critical, not only as far as accuracy was concerned, but also the screening module would be required to operate with constantly changing materials and widely variable moisture conditions. A vibrating screen or another Trommel were both considered unsuitable, since when screening at such a small aperture the mesh would blind over when the material became damp. A tension deck was also considered, but the fact that it would involve physically changing the screen should it be necessary to increase or decrease the sample size meant that there would be a considerable amount of downtime.
In order to avoid all these problems the obvious choice was a star screen and after considering a number of different makes of star screen an excellent demonstration from Pearson Separation Machinery guaranteed the inclusion of a Pearson Star Screen in the re-designed plant.
back to top
(cont'd)
The Pearson Screen module consists of 18 rows of rotating stars within a 5m long chassis. Each shaft is 1560mm wide and is driven by independent electric motor, the speed of which can be controlled by an inverter, if required. The shafts run in sealed bearings, so little maintenance is required, other than a periodical cleaning of the star shafts. The stars are moulded from robust polyurethane which gives long service, whilst retaining the flexibility which helps to break down wet and matted materials, to allow more 'fines' to be screened.
The Pearson unit also features a unique adjustment mechanism which allows the operator to set the 'fines' to be removed between 10mm and 50mm. This is achieved by a simple mechanically linked system and adjustments can be made with the machine still in operation.
These features give the operator total control over 'fines' extraction since he can decide if the material extracted is too large or too small, and he can make the change instantly with no down time.
The combination of variable apertures and variable shaft speeds on the star screen meant that the plant would be capable of infinite adjustment. This is a huge consideration when handling this type of waste as no two loads are exactly the same.
With the 15mm down material removed, a 40 >15mm sample of material containing bricks, concrete, stones, wood, paper, plastics, cardboard, and polystyrene remains. However, the removal of this fines material had already reduced the weight of the material going to landfill by 40/45 % but there was still a considerable amount of valuable product left.
The next stage was to separate the light material from the heavy material to leave a useful inert fill material and a density separator was chosen to achieve this.
Separation by density is a simple process whereby the remaining material that comes over the star-screen is fed onto a vibrating feed table and over a defusing plate. High velocity air is blown up through the waste stream which has the effect of raising the light material to the top of the stream. The material is then fed over an air-knife which blows the light material out, leaving the heavy fraction to drop down onto a stockpiling conveyor.
(cont'd)
With the airflow to both the defuser plate and the air-knife being infinitely variable it is possible to determine the exact quality of the product extracted. For a very clean, heavy fraction it is possible to turn the air up to blow out all the sticks and small pieces of wood. Small stones will also be lost to the lights pile but a screening efficiency of 95% + can be achieved. Alternatively, a little less air will put the small stones and some of the wood into the heavies pile. If this is acceptable for use as a rough hardcore/fill material, then there is even less weight going to landfill. It is also possible to safely change these settings while the machine is in operation.
Having decided on the exact layout of the new waste handling line it was necessary to combine these components in a robust structure and it was with the help of Master Magnets/I.R.S. that a framework was designed and built to link the whole system together in a structure that would protect the machinery and ensure that the material was transferred from one section of the operation to another with the minimum of spillage. In addition all the relevant areas would need to be easily and safely accessible for maintenance, cleaning and adjustment.
Master Magnets are best known as one of the leading suppliers of magnetic separators and metal detectors in the UK, exporting their products all parts of the world. With acquisition of Integrated Recycling Systems Ltd., Master Magnets has the expertise to design and manufacture complete plants for recycling and other metal separation processes.
The Master's were therefore contacted to design and manufacture the support structure, including access stairways, walkways and safety guarding. In addition two belt conveyors were designed to fit within the operating area to efficiently transport the material.
Shortly after the commissioning of the system Thompsons were able to realise in full the anticipated benefits it would bring. When compared to the old method of working there has been a reduction by some 80 - 90 %, by weight, in the amount of material going to landfill. With the landfill tax in excess of £20 per ton it is estimated that the plant will recoup the capital expenditure in less than 6 months.
Thompson's new plant at Springwell Quarry