Adding Value To Scalpings

Hanson Aggregates invest £1 million in new washing plant at Batts Combe

Perched above the Somerset village of Cheddar and just a stone’s throw from the famous limestone gorge, Batts Combe Quarry has been producing quicklime and limestone products since the early 20th Century. Currently owned and operated by Hanson Aggregates, the site produces around 1 million tonnes of limestone a year, one third of which is destined for the quarry’s lime kiln with the remainder being processed for sale as coated stone or aggregate.

Like an increasing number of quarries, Batts Combe has recently undertaken steps to address the problem of what to do with its growing stockpiles of scalpings. Faced with a diminishing market for such material, due in part to the effects of the aggregates tax, Hanson have recently invested £1 million in the installation of a new washing and water treatment plant at the site. Although driven largely by commercial considerations, the decision to install the new plant has also benefited health and safety at Batts Combe by reducing the number of vehicle movements around the site.

Commissioned in January 2005 and designed to process up to 500 tonnes a day of –20mm primary crushed scalpings, the new washing plant is producing a clean –20mm +5mm product suitable for direct sale or further processing, together with a clean –5mm concrete sand. At the same time, the residual –75? fraction is being treated to produce a dry, easy-to-handle sludge cake.

New wash plant

The –20mm scalpings are drawn from their stockpile by wheel loader and fed into an adjacent 22-tonne capacity belt-feed hopper equipped with a 4m wide self-tipping grid. Supplied by LJH Group Ltd, the hopper is equipped with a 750mm wide variable-speed belt conveyor capable of discharging the scalpings at up to 80 tonnes/h on to an inclined wash plant feed conveyor.

Powered by a 7.5kW, shaft-mounted Flender geared motor, the 65m long by 600mm wide feed conveyor has an initial incline of 11° to provide sufficient clearance height for safe vehicular movements beneath. On arrival at the washing facility, the –20mm feed is discharged directly into the feed bin of a Haver & Boecker Hydro-Clean high-pressure washing system, supplied and installed by Mansfield Plant Maintenance Services Ltd. The feed bin is equipped with a level sensor that monitors the material level and controls the feed rate from the main belt feed hopper.

Designed to wash dirty and sticky raw material mixtures, the Hydro-Clean unit, which utilizes high-pressure water jets to ensure the complete dissolution of lumps of clay, produces clean aggregate materials within a highly compact footprint. The system has additional advantages over alternative washing systems including reduced energy and water consumption as well as a low-wear design.

The Hydro-Clean discharges on to a single-deck Haver Niagara linear-motion vibrating dewatering screen, also supplied by Mansfield Plant. Fitted with 5mm aperture polyurethane screen panels and equipped with an initial rinsing section, this unit discharges the –20mm +5mm material, via a flap gate, on to a 55m long x 600mm wide feed conveyor. Powered by a 5.5kW Flender shaft-mounted geared motor, this belt delivers the washed material to the quarry’s existing secondary crusher surge bin where it mixed with clean feed stocks for further processing. Alternatively, when this surge bin is full, the washed –20mm +5mm can be diverted to ground storage via a 15m long by 600mm wide stockpile conveyor.

All three conveyors, together with the wash plant’s steelwork support structure, stairs and walkways, were designed, supplied and installed by LJH. Each conveyor is equipped with a Beckford series 1001 single-idler belt weigher and all shafts are fitted with Cooper split bearings.

The –5mm underflow from the Niagara washing screen is flumed into a 2.1m diameter Linatex feed-regulating sump, whereupon the silt-laden overflow is piped to an in-ground effluent sump, while the coarser sand slurry is pumped from the base of the unit to a Linatex HK100 sand separator equipped with an extended feed box. The separator captures and discharges the useful sand component while removing the majority of unwanted –75? silt and clay slurry via the overflow, which is discharged to the in-ground effluent sump via the feed regulating sump.

The coarse underflow from the separator is delivered on to a vibrating Linatex VD6 compact dewatering screen fitted with a single, 5mm aperture polyurethane deck. This removes any remaining free water and discharges clean sharp sand (with a moisture content of around 12%) on to a cantilevered inclined stockpile conveyor. Underflow from the screen is directed back to the feed-regulating sump to prevent the loss of useful material.

Silt treatment

On command from level controls, the silt slurry collected in the in-ground effluent sump is pumped directly to a 5m diameter upward-flow thickener supplied and installed by Haith Industrial Ltd. This forms the first stage in a closed-loop system designed to handle 60m3/h of effluent (with a solids content of around 7.5% by weight) as well as surface water emanating from the wash plant and washed stockpiles, in order to provide clean supernatant water for reuse and a sludge cake for disposal.

Interlocked with the effluent delivery pump is a flocculent dosing pump connected to a fully automatic Ecomac powder-grade flocculent preparation system. The make-up system, which is topped up as required (currently one 25kg bag of polymer per month), prepares and ages the flocculent prior to dosing to the thickener. The addition of flocculent helps achieve solids-liquid separation within the thickener, giving rise to a thickened slurry underflow (42.5% by weight), which is discharged at the base of the cone-shaped vessel while the clarified supernatant water is discharged over weir plates at the top of the thickener. An internal rake system runs continually to maximize compaction and to ensure that the solids are directed down towards the cone outlet and underflow suction pump.

The clarified supernatant water flows by gravity into a 60m3 capacity clean water storage tank equipped with two pumps, one of which provides treated water for the automatic cloth-washing system on the filter press while the other returns treated water back to the scalpings washing plant for reuse.

The thickened underflow is extracted from the thickener/clarifier via a centrifugal pump and delivered into a 20m3 capacity slurry buffer tank with internal agitation paddles. Equipped with level probes to prevent overfilling and to provide run-dry protection for the filter press feed pump, this tank eliminates any surges and/or fluctuations in the thickened underflow and provides a homogeneous feed to the filter press, with sufficient sludge storage for approximately two-and-a-half hours or four press cycles.

From the buffer tank a fixed-speed feed pump, operating on command from the filter press PLC, delivers slurry to the feed ports of the Micronics Filtration Type 1-EBR 38-plate automatic sidebar press. This model is fitted with an electrically driven hydraulic system, supplied by Axis Hydraulics of Stoke-on-Trent, which provides the closing force of 345 bar required to seal the filter plates against the incoming filtration pressure of 7 bar delivered by the sludge feed pump.

The cycle time for the press is around 20–30min depending on sludge consistency, after which a vibrating plate-shifter mechanism discharges the 32mm thick cakes in around 6 min. Each press cycle produces some 2.5–3 tonnes of sludge cake, which drops into a three-sided concrete bund located below the portal-framed penthouse-type enclosure that houses the press. After discharge, the press plates close automatically ready for the next batch to be processed.

Periodic wash cycles are carried out (usually once a week) using an integrated filter-cloth washer. Utilizing potable water at 15 bar to wash the entire plate pack in sequence, this system ensures optimum performance from the filter cloths. All dirty water generated by the automatic cloth washing system is pumped back to the thickener feed launder, while the clean filtrate from the press itself is discharged by gravity into the treated water tank.

Control of the filter press is via an Allen-Bradley PLC connected to an Allen-Bradley PanelView 300 HMI, with control philosophy and electrics supplied by Sandbach-based process control specialists Procon. Siemens light guards fitted to both sides of the press provide automatic shutdown of the press should the operator intrude into the safety critical area during operation. In addition, mesh guards are fitted a both ends of the press.

Conclusion

Installation of the new wash plant at Batts Combe is providing Hanson with a number of benefits: it is helping to resolve the tip space issue; it has improved health and safety on site by reducing vehicle movements; and it is adding significant value to what had become little more than a waste product. With its lack of flaky material, the washed –20mm +5mm is particularly suitable as a concrete aggregate, and a proportion of the plant’s output is supplied to Hanson’s own ready-mixed concrete plants for this purpose, with the remainder being returned to the quarry’s main processing stream. A ready market has also been found for the –5mm concrete sand produced by the plant and Batts Combe is currently investigating the sand’s potential for use as a compost drainage medium.

Acknowledgement

The editor wishes to thank Hanson Aggregates for permission to visit the site and, in particular, Keith Savory, quarry manager, Dale Binning, plant fitter, and Paul Hawkins, wash plant operator, for their help in preparing this report.