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Duo Equipment installs the largest Washing and Water Management Plant in Europe

22nd January 2007

Torr Works Quarry is a carboniferous limestone quarry, adjacent to the A361 in Somerset, approximately mid-way between Frome and Shepton Mallet.
Covering an area of some 200 hectares, it includes 60 hectares which have been landscaped to blend with the surrounding countryside. Previously owned by Foster Yeoman the quarry is now operated by Bardon Aggregates, as part of the Aggregate Industries UK business.

Much of the quarry's current annual output is sent by rail - a great environmental benefit. Up to 70 trains per week leave Torr Works carrying on average 1,500 tonnes; local markets are also served within a radius of 60-70 kms.

Production at Torr Works revolves around the use of a huge primary crusher. Commissioned in 1986, it stands as tall as a 7 storey building and weighs 1200 tonnes, and is the only one of its type in use in the UK today. It consists of a 54" gyratory crusher which is mounted in a walking frame manufactured by O&K. Typically it will walk a few metres each day at 32 metres per hour to follow the working of the quarry face.

The use of this crusher is more efficient and environmentally friendly than the conventional haul truck operation as it is linked to a flexible conveyor system which transports the crushed rock across the quarry floor to the primary stockpile, up to a mile away, which is over 1.5 kms away from the furthest rock face.

The primary stock pile can hold 85,000 tonnes of aggregate, which varies in size from 225mm to 28mm, and has had the waste 'scalpings' screened.

These waste ‘scalpings' which can be used as fill material constantly add to the ever increasing stockpiles.

After a decision to further process and wash this mountain of ‘waste material' to provide additional ‘added-value' products Aggregate Industries approached DUO (Europe) to provide a complete washing and water management plant, who were successful in their bid to provide what is probably the largest plant of its kind in Europe.

DUO were the principal contractor of the turnkey project and operated under CDM regulations and were responsible for all of the structural calculations, bespoke steelwork and supply of all conveyors which were provided by DUO Manufacturing.

The plant currently handles 600 tonnes/hr of waste scalpings and produces one grade of aggregate and two sands; however it has been designed with the capability to produce several sizes of aggregate with some minor modifications to the discharge chutes and the addition of stocking conveyors.
Material is transferred from the scalpings stockpile and fed into a 50 tonne capacity feed hopper which feeds a 1200mm wide x 27m long inclined belt conveyor. It passes through a two-way discharge chute allowing material to drop onto either of the two in-line Cedarapid 6m x 1.8m three deck primary rinsing screens. These screens are the first to be installed in the UK and are supplied through the DUO dealership.

Cedarapids Screens.

The inclusion of Cedarapid flat screens within the plant has provided significantly greater throughput compared to the use of a similar size inclined screen.

Additionally, the overall height of the plant has been kept to a minimum and the utilisation of flat screens has provided easier access for maintenance.

The angle of throw and amplitude of stroke can also be adjusted due to the design of the vibrator mechanism and adjustable pitched motor pulley's provide for easy speed adjustment to accommodate changes from coarse to fine material screening operations.

Deck frames are huck-bolted to side sheets for greater durability under aggressive vibration; and high-strength alloy steel side sheets with spherical clamp bolt washers eliminate weld stress. The Cedarapids flat deck screens were also specially designed to accommodate modular polyurethane decks which are easily interchangeable.

Any midsize material passes from these rinsers is discharged into two Greystone Logwashers, these units mix incoming feed while scrubbing and washing the material at the same time. Any lumps of earth or clay are broken down during this process and carried away in the wash water. These units are ideally suited to large scale production and are the first to be installed in the UK.

Greystone Logwasher.

GreyStone Logwashers offer a spiral paddle positioning along the log shaft, providing continuous motion in the material for superior performance. The continuous agitation of the material results in maximum scrubbing action reduces energy requirements and eliminates intermittent shock loading on the shaft and gearbox. Heavy-duty flex couplings prevent misalignment and provide maximum production and long wear life. They also allow easy removal of the log shaft for maintenance or repair.

The unique paddle base design provides three bolt holes per paddle. The Ni-Hard wear-resistant paddles provide extra strength to eliminate paddle breakage and provide maximum wear life. Shoe paddles are mounted to the bases with Grade 8 bolts and installed at the proper angle to the extra-heavy ASTM seamless steel log shaft for maximum washing action and provide maximum service life with minimum maintenance requirements

The heat-treated shaft is fitted into a hard, stainless steel spacing sleeve and fitted with a heavy-duty rubber gasket that fits snugly between the seal retainer and tub. A specially designed slinger fits perpendicular to the shaft between the roller bearing and sleeve and turns with the shaft, adding protection to the outboard bearing and preventing bearing failure that may occur from seal leakage. The heavy, seamless steel log shafts have flanges welded to both ends that are then precision-ground to provide maximum service.

The Logwasher shaft is driven by twin-helical, synchronized precision gears and is totally enclosed within an oil-bath gearbox. The unit is designed with externally mounted bearings for easy pressure lubrication, service and maintenance. All standard electric motors enable a change of the washing action of the material with a simple change of the drive pulley on the motor.

Shaft-mounted speed reducers with rugged, cast-iron, corrosion-resistant housings provide positive gear alignment. The helical design of the gear teeth has a soft core that resists shock loads and provides a 98.5 percent efficiency rating per gear set. Dual out-put shafts with timed gears allow for inter-locking of paddles. In order to keep dirt and grime out and oil in, the reducers are sealed with metallic double-lip seals ensuring maximum life and trouble-free service.

Midsize material is then fed from each logwasher onto to the final twin in-line 6m x 1.8m Cedarapid part rinser screens to provide a final rinse. These screens split the material into an aggregate fraction, which is then delivered to their respective ground stockpiles by two radial 800mm wide x 24 metres long stockpile conveyors including walkway full length and around the head drum.

Sand Classification Plant.

All -5mm materials from the primary rinsing screens, logwashers and part rinsing screens is collected in two tanks under the plant and pumped 170 metres across the site up into a boiling box. From here it gravity feeds into two Powerscreen Fines Master 200 Compact Sand Plants which classify and dewater to produce two grades of sand.

Water management being an essential part of the project is dealt with by five Tecnoidea multi-plate filter presses which give maximum flexibility and throughput with rapid opening and closing times. Situated on a galvanised structure 5m from the ground these five presses provide an automatic silt management system that is specifically designed and sized to minimise operator input and maintain low running costs.

As well as the five presses, the system includes three 7m diameter deep cone thickeners to clarify the water, a 7m diameter deep cone clarified water storage tank, an automatic flocculent mixing plant and an automatic flocculent dosing system which regularly adjusts the volume of flocculent going to the thickeners depending on the quantity of silt. Five in-line 80m3 sludge buffer tanks with internal rakes complete the line-up.

Once in the thickener the silt settles to the bottom of the cone and is automatically discharged into a silt buffer tank when it has reached the desired density for pumping to the plate presses. Lime is then added if there is a large quantity of clay present within the silt.

The quantity of lime is automatically controlled using a rotary valve which then feeds the lime onto a rotary screw which in turn feeds the lime into a mixing tank. At this point water is added to the lime to create a solution which is automatically distributed into the silt buffer tank as required. The lime solution reacts with any clay material in the silt to facilitate the water drainage in the press.

The quantity of lime can also be adjusted depending on the quantity of clay in the silt to maintain throughput through the five presses.

When each press has filled and drained the water, it opens and the silt cake drops down into a concrete bay below; the press then self-cleans itself ready for another cycle. As in any application, when DUO selected the size of presses they took into account such factors as the silt content, clay content and settlement rate of the raw material. The flocculent mixing and dosing system maintains a continuous supply of clean water and automatically mixes a batch of flocculent solution when the level drops. An operator only has to add the powder to a storage bin.

In designing the whole plant DUO took great care and studied in detail the requirements for plant maintenance. In particular maintenance for the five presses can be achieved with ease through the installation of fixed overhead lifting beams complete with electrically operated hoists. Access to the plate presses is by two staircases situated at each side of the structure and provides further access onto walkways. All structural steelwork and walkways are fully galvanised.