Adding Value at Torr Works

DUO Equipment install largest washing and water-management plant in Europe

Torr Works limestone quarry in Somerset, located approximately mid-way between Frome and Shepton Mallet, covers an area of some 200ha and includes 60ha that have been landscaped to blend in 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 annual output of Carboniferous limestone is despatched by rail, with up to 70 trains a week leaving the site each carrying, on average, 1,500 tonnes; local markets are also served within a 60–70km radius.

Production at Torr Works revolves around the use of a 1,200-tonne ‘walking’ primary crusher – the only one of its type in use in the UK today. Commissioned in 1986 and standing as tall as a seven-storey building, it comprises a 54in gyratory crusher mounted in a ‘walking’ frame manufactured by O&K. Capable of walking at 32m per hour, it typically moves a few metres each day following the working of the quarry face.

The walking crusher is linked to a flexible conveyor system that transports the crushed rock across the quarry floor to an 85,000-tonne capacity primary stockpile, which is around mile away from the furthest face.

Following a recent decision to wash and process the quarry’s large and growing stockpiles of primary scalpings to provide additional ‘added-value’ products, Aggregate Industries commissioned DUO (Europe) to provide a complete washing and water-management plant. Operating under CDM regulations as principal contractor on the turnkey project, DUO were responsible for all structural calculations, bespoke structural steelwork and the provision of conveyors, which were supplied by DUO Manufacturing.

Believed to be the largest facility of its kind in Europe, the new plant currently handles 600 tonnes/h of waste scalpings and produces one grade of aggregate and two grades of sand, although it has been designed with the capability to produce several sizes of aggregate with a just few minor modifications to the discharge chutes and the addition of further stockpile conveyors.

Material is drawn from the scalpings stockpiles and fed into a 50-tonne capacity feed hopper, which in turn feeds on to a 1,200mm wide x 27m long inclined belt conveyor. This feeds the material through a two-way discharge chute on to two in-line Cedarapids 6.0m x 1.8m three-deck primary rinsing screens. Supplied through the DUO dealership and well suited to large-scale production, these screens are the first of their type to be installed in the UK.

The inclusion of Cedarapids flat screens within the plant is said to provide significantly greater throughput compared with similar-sized inclined screens. Additionally, the use of flat screens provides easier access for maintenance and keeps the overall height of the plant to a minimum.

The design of the screen vibrator mechanism allows the angle of throw and amplitude of stroke to be adjusted, while adjustable pitched motor pulleys provide for easy speed adjustments to accommodate changes from coarse to fine material screening operations.

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

Any mid-size material from the rinsing screens is discharged into two Greystone log-washers, which mix the 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.

Featuring a spiral paddle along the log shaft, the Greystone log-washers provide continuous agitation of the material, delivering maximum scrubbing action, reducing energy requirements and eliminating intermittent shock loading on the shaft and gearbox. Heavy-duty flex couplings prevent any misalignment and provide maximum production and long wear life, as well as easy removal of the log shaft for maintenance or repair.

The special paddle base design features three bolt holes per paddle, while the Ni-Hard wear-resistant paddles themselves deliver extra strength to protect against breakage and maximize wear life. Shoe paddles mounted to the bases with Grade 8 bolts are installed at the proper angle to the extra-heavy ASTM seamless steel log shaft for maximum washing action and service life with minimum maintenance requirements.

The heat-treated shaft is fitted into a hard, stainless steel spacing sleeve and equipped 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 potential bearing failure due to seal leakage.

The log-washer shaft is driven by twin helical synchronized precision gears, which are totally enclosed within an oil bath gearbox. The unit is designed with externally mounted bearings for easy pressure lubrication, service and maintenance. Standard electric motors allow for a change of washing action with a simple change of the drive pulley on the motor, while shaft-mounted speed reducers with durable 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% efficiency rating per gear set. Dual-output shafts with timed gears allow for the interlocking of the paddles. In order to keep dirt out and oil in, the reducers are sealed with metallic double-lip seals.

Mid-size material is fed from the log-washers on to a final pair of in-line 6.0m x 1.8m Cedarapids part-rinser screens, which provide a final rinse. The resulting clean 5–20mm product is delivered to ground stockpiles by two 800mm wide x 24m long radial stockpile conveyors fitted with walkways along their full length and around the head drums.

Meanwhile, –5mm sand and water from the primary rinsing screens, log-washers and final part-rinsers is collected in two tanks located beneath the plant and pumped 170m across the site and up into a boiling box. From here it is fed under gravity into two Powerscreen Finesmaster 200 machines which classify and dewater the material to produce two grades of sand.

Water management – an essential part of the process – is dealt with by five Tecnoidea multi-plate filter presses, which provide maximum flexibility and throughput with rapid opening and closing times. Situated on a galvanized structure mounted 5.0m above the ground, these five presses provide an automatic silt-management system specifically designed and sized to minimize operator input and maintain low running costs.

As well as the five presses, the system includes three 7.0m diameter deep-cone thickeners to clarify the water, a 7.0m diameter deep-cone clarified water storage tank, an automatic flocculant mixing plant and an automatic flocculant dosing system, which regularly adjusts the volume of flocculant sent to the thickeners depending on the quantity of silt. Five in-line 80m3 capacity sludge buffer tanks with internal rakes complete the line-up.

Within each thickener tank the silt settles to the bottom of the cone and is automatically discharged into a silt buffer tank once it has reached the required density for pumping to the plate presses. In order to maintain throughput through the five presses, lime can be added if a large quantity of clay is present within the silt.

Lime addition is automatically controlled by means of a rotary valve. This feeds the lime on to a rotary screw, which in turn feeds it into a mixing tank; at this point water is added to create a lime solution. This is automatically distributed into the silt buffer tanks, as required, where it reacts with any clay material in the silt to facilitate water drainage in the press.

Once each press has been filled and the water drained, opening of the press allows the resulting silt cake to drop into a concrete bay below. The press then goes through a self-cleaning routine ready for another cycle. As in all such applications, DUO carefully selected the size of presses for Torr Works taking into account factors such as the silt content, clay content and settlement rate of the raw material.

In designing the whole plant, DUO also studied in detail the requirements for plant maintenance. In particular, maintenance of the five presses can be achieved with ease owing to the installation of fixed overhead lifting beams complete with electrically operated hoists, while access to the plate presses is via two staircases situated at each side of the structure with further access via wide, fully galvanized walkways.