QM June 2004 www.qmj.co.uk 13

More than 200,000 tonnes of materialhave passed through the CDEwashing plant at Irish Asphalt’sDuleek Quarry, in Co. Meath, since

its installation in the summer of 2003. In July2004 the plant’s ownership will transfer toIrish Asphalt, part of the Lagan HoldingsGroup, having been operated on a price-per-tonne basis by CDE since its installation.TheEuro 1.5 million contract represented a majorinvestment for Northern Ireland companyCDE, and while their processing systems havepreviously been chosen for applicationsranging from specialist sand classification toquarry-dust washing, the raw material atDuleek provided them with a considerablechallenge.

The Lagan Group are one of Ireland’s largestquarry operators with sites both north and

south of the border and a combinedproduction capacity of over 2 million tonnes.They are also a major supplier of asphalticmaterial, and their bitumen division is at theleading edge of binder and emulsion

By-product toCommercial OutputLagan Group increase production with CDE washing plant

Part of the new CDE washing and screening plant at the Lagan Group’s Duleek Quarry, Co. Meath, Ireland

The contrast between theclay-bound raw material (left)and the washed stone (right)

14 www.qmj.co.uk QM June 2004

technology. Lough Neagh in Northern Ireland,the largest freshwater lake in the British Isles,is the base for the Group's sand division, whileLagan’s concrete division operates one of themost modern concrete block and brick-makingplants in Europe.

The Duleek site is situated on what isessentially a Carboniferous limestone depositbut bands of heavy and extremely plastic clayare prevalent. Large stockpiles of overburdenand clay-bound material had been building upas waste by-products for some time and wereseriously compromising the efficiency of theoperation. Identifying and responding to this,Lagan’s regional quarry manager, Paul Tuite,initiated discussions with CDE in early 2003.Mr Tuite commented: ‘We demand the highest-

quality aggregates for both in-house use andexternal sales, and it was clear that ourexisting process could not achieve this. Ourpartnership with CDE has been rewarded withconsistent quality aggregates and productioncapacity has increased by more than 30% oninitial figures.’

Other prominent CDE washing plants andcontracts have been established throughoutIreland, the UK and Sweden, and theexperience gained from these, together withCDE’s growing reputation for innovation, ledto the Lagan Group contract.

The plant born of these negotiations hasalready attracted numerous visitors from theUK and further afield.What draws them is theconsistently high quality of the sand andaggregates produced by the plant, in spite ofthe poor-quality raw material. Up to 25% ofthe –90mm raw feed, and often up to 50% ofthe –5mm fraction, can be less than 75µm.Since final commissioning the plant hasconsistently supplied clean and dry outputscomprising 20mm, 14mm, 10mm, 6mm andconcrete sand, allowing Lagan to offercustomers commercial Type 1 and concreteconstituents, all from a material previouslyregarded as waste.

Processing plantA shovel-loaded 50-tonne capacity feedhopper and belt-feeder arrangementdischarges the raw feed material on to a 20mlong inclined conveyor.This feeds a 1.5m x4.0m CDE 2SG60 grizzly screen that removesany +90mm oversize, which falls into a walledbay. Previously tested grizzly bars had atendency to block owing to the highly plasticnature of the material, especially in wetweather.This led to a new design of high-

High-tensile offset finger barsare standard specification onCDE grizzly screens

The primary stage at Duleekshowing the grizzly screen,CDE Rotomax 1828 log-washer and primary rinsingscreen, with the Tesab mobileimpactor in the foreground

QM June 2004 www.qmj.co.uk 15

tensile offset finger-bars, which are now astandard CDE specification.These offermaximum flexibility and ensure that capacity isnot compromised, even with variations in theraw material.The –90mm material is conveyedby a second 20m conveyor to a CDE Rotomax1828 twin-shaft log-washer.This 8m longmachine is believed to be one of the largestlog-washers currently operating in Europe,with each shaft measuring 1.8m in diameterfrom tip to tip.The unit can accept up to 250tonnes/h of –90mm material, and retentiontime can be easily adjusted by altering theinclination and running speed, aided by thesingle motor and gearbox design. CDE say thatunlike many log-washers, the special Rotomaxdesign allows feed containing up to 30% –5mmmaterial to be efficiently handled without theneed for preliminary fines screening. Each shaftis fitted with heavy-duty chrome-molybdenumtips for maximum wear resilience.

Scrubbed material is fed directly to anR2M108 twin-deck primary rinsing screen.This1.8m x 6.0m unit was one of the first of CDE’slatest-generation rinsing screens, incorporatingrubber and polyurethane decks.The top deckis fully enclosed by a splash-proof cover,ensuring maximum water retention and aclean operative environment. Five spray-barsper deck (with individual controls) and flexiblerubber screen seals for each bar are included.The top deck separates +24mm material,which is conveyed to a mobile impact crusherfor reduction to commercial Clause 804specification.The underflow from the lowerdeck, comprising a –4mm water, sand and clayslurry, is flumed to the sump of a CDE C-62compact sand plant.

Another 20m conveyor carries the rinsing

screen’s –24mm +4mm intermediate productto a second log-washer, a 7m long Rotomax827 that removes any residual clay particlesfrom the aggregate; the success of this isevident from the clean nature of the finishedproducts.As with the primary log-washer, thespeed and inclination of this unit are easilyadjusted to accommodate variable feedmaterials and tonnages. Material from the log-washer is delivered directly to a dewateringscreen mounted over the final dry-classification screen.The Dw43 dewateringscreen is equipped with spray-bars in the first

General view of the entireCDE plant

Close-up of the second stageshowing (from top to bottom)the Rotomax 827 log-washer,Dw43 dewatering screen andD2M60 dry-classificationscreen

16 www.qmj.co.uk QM June 2004

screen section to give the material a final lightrinse.This unit includes modular polyurethanepanels and side protection as standard.Filtrates from both the dewatering screen andthe log-washer overflow to a ground sump forrecovery.The recovered water/fines slurry isreturned, by means of a Warman QV65SPsubmersible pump, to the primary rinsingscreen.This offers increased fluidization of thefeed material, retains water within the systemand is a valuable aid to site cleanliness andplant wash-down.Aggregates are classified intofour final products by a D2M60 1.5m x 4.0mtwin-deck dry screen with a split bottom

deck.The efficiency of the CDE dewateringscreen is highlighted by the unusually dryaggregates produced; moisture content isconsistently less than 5%.

Sand productionSand slurry leaving the primary rinsing screenis piped to the C-62 compact sand plant,which is rated at 120 tonnes/h.The compactsand plant is CDE’s best-known single productand has no trouble handling the high clayvolumes in the feed (the –4mm can contain upto 50% clay).The plant consists of ahydrocyclone mounted over a dewateringscreen and sump. Slurry is fed directly into thesump and is then sent to the custom-sizedhydrocyclone by a Warman 10/8E-M pump.Thesilt cut-point can be set to suit customers’specific product requirements, but here thesand consistently contains less than 3% –75µmmaterial, with typically 50% passing 600µm.Sand and residual water exit from the base ofthe hydrocyclone and pass to a dewateringscreen mounted underneath before the sand isdischarged, via a rubber-lined chute, to thesand conveyor at a typical 12% moisturecontent.The waste water containing clayspasses directly to an 8m diameter CDE T-08high-rate thickener, complete with a fullyautomatic polyelectrolyte dosing station, forfurther treatment.

Waste-water treatment is perhaps one ofthe most crucial elements of the entire plant.Given the dirty nature of the feed, manytraditional treatment systems would demandlarge volumes of water, which is not alwayspractical in quarry applications. Here, thecomplete plant operates at up to 220tonnes/h, which equates to 44 tonnes/h of

The C-62 compact sand plant

The compact sand plantalongside the 8m diameterT-08 high-rate thickener

QM June 2004 www.qmj.co.uk 17

waste solids. Polyelectrolyte dosing is carefullymonitored to ensure correct sludgesettlement/compaction without any risk offlocculant carry-over in the recycled water.Afreshwater storage tank is located adjacent tothe thickener and the primary rinsing screen,so only a relatively low-power recycled-waterpump is required.The sludge is sent to aremote lagoon for further settlement, thisbeing approximately 200m away with a 35mhead.The material is sent to the lagoon by aWarman 4/3C-AH pump. Final solids contentin the sludge is typically around 40–50%. CDEsay an interesting point to note is therelatively compact size of the T-08 thickenerunit.Alternative treatment systems would, theysay, generally require a much larger unit thanthe 8m diameter model specified here, but thisparticular unit has proven to be well able tocope with the 450m3/h of water within thecycle, and overall only 50m3/h of top-up wateris required.

CDE and the Lagan Group are currently indiscussions aimed at moving the plant to amore permanent on-site location.This move isexpected to see the addition of the latest-model CDE plate-and-frame filter press.Thistechnology will achieve maximum waterrecycling with top-up requirements reduced toa mere 20m3/h.The waste material will beoffered as easily handled dry cakes, completelyeliminating the need for settlement lagoons.Current investigations are exploring thepossibility of using this dried clay in Lagan’snewly commissioned cement plant.

Anticipating the plant’s handover from CDEin July 2004,Terry Lagan, director of IrishAsphalt, commented: ‘This CDE installationrepresents a new generation of washing plants

for problem materials…we are now able tofully exploit this quarry, handling materials thatwere previously stockpiled as waste — but,crucially, making commercial-grade productsfrom them.’

AcknowledgementThe editor would like to thank both the LaganGroup and Iain Walker of CDE for their co-operation in producing this article. �

The fully automatedpolyelectrolyte dosing station

One of the final stockpiles ofclean, dry aggregate