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Replant at BlashfordMajor plant upgrade boosts production capacity for Hampshire sandand gravel quarry

The new DUO sand and gravel plant at Tarmac’s Blashford Quarry

Located some three miles north-west ofRingwood, in Hampshire, Blashford Quarrylies within the 7,000 acres of rolling farmland,

woodland and parkland that make up SomerleyEstate, seat of the earls of Normanton since 1825 andhome to the current custodian, Viscount Somerton,son of the sixth earl. The estate contains substantialreserves of sand and gravel and has provided avaluable source of building materials for the NewForest area and beyond since before World War II.Tarmac Ltd, the current leaseholders of Blashford

Quarry, have operated the site for around 30 years,producing products for the company’s on-sitebagging plant and for use in Tarmac’s ready-mixedconcrete plants across the region, as well as for saleto builders’ merchants and contractors in theRingwood–Bournemouth area and along the southcoast. At present, extraction operations at Blashfordare centred on Nea Farm, where a deep deposit ofhigh-quality, largely silt-free sand with a 10–20% gritcontent is being worked to an average depth of 5min dry conditions above the water table. An extensivefield conveyor system, which was the subject of amajor upgrade during 2006/7, transports the ‘as dug’material 2.5km from the extraction site to theprocessing plant situated adjacent to the A338Salisbury Road.With extraction of the Nea Farm deposit due to be

completed in around four years time, in 2008 Tarmacsuccessfully secured planning permission for theextraction of a further 6 million tonnes of reservesat Plumley Wood, ensuring the continuation ofoperations on the Somerley Estate for a period of upto 25 years. At their furthest extent in around 10 years’time, extraction operations at Plumley Wood willreach a point 2.5km beyond the current dig at NeaFarm and approximately 5km (or around one hour’sconveyor travel) from the processing plant. Furthermodifications and additions to the to the site’salready extensive field conveyor system are currentlybeing installed to accommodate the move to PlumleyWood.Moreover, with extraction in this new area

scheduled to commence in late summer 2012, fouryears ago Tarmac identified an opportunity to replaceBlashford’s former processing plant – comprising abarrel washer, sizing screens and open crushingcircuit capable of producing up to 290,000 tonnes perannum – with a more modern, higher-capacityfacility. Following a competitive tender process,DUO were selected to supply the new plant andsubsequently worked closely with Tarmac over a two-year period to evaluate various options capable ofmeeting the desired production levels and productquality.A key factor in DUO being awarded the tender ‰

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was their ability to achieve the required health, safetyand environmental standards set by Tarmac, whosegoal is to meet, and where possible exceed, legalhealth and safety requirements with the aim ofachieving their target of Zero Harm. Throughout thetender, design and installation stages, DUO workedclosely with Tarmac to minimize the risks to those onsite, and to reduce any environmental impacts.

Conveyor upgradeSix years ago, ahead of the more recent plantdevelopments, Tarmac upgraded Blashford’s existing2.5km field conveyor system to harmonize the driveson the 14 conveyors which make up the system andto increase the handling capacity from 150 to 350tonnes/h, while providing the additional benefits ofimproved reliability and reduced maintenance costs.The staged upgrade commenced in January 2006 andwas carried out by Worksop-based Canning Conveyor,who were responsible for all design, manufacturingand installation work. After extending the face section with a new 120m

long x 750mm wide field conveyor, Canning splitanother existing conveyor and interfaced a new 20mlong elevated lattice section with walkway andelectrically operated two-way chute. This new sectionis powered by a ceramic-lagged Canning SuperDrivemotorized drive drum, including a Rulmeca jibdischarge and high- and low-tension idler pulleyscomplete with internal sealed-for-life roller bearings,and features a polyurethane primary belt scraper anda tungsten blade secondary scraper.This upgrade was followed by the modification of a

horizontal conveyor, which entailed the incorporationof a new 22m long x 750mm wide elevated transferconveyor complete with a tail-end loading section.Driven by a 22kW Canning SuperDrive motorized drivedrum, this section was fitted with new primary andsecondary belt scrapers and an emergency-stoppull-wire system.Canning completed the upgrade work with a new

troughed-belt radial stockpile conveyor completewith walkway assembly and head platform aroundthree sides. Powered by a ceramic-lagged 22kWSuperDrive motorized drive drum, this inclinedlattice-frame conveyor is designed to handle up to 250tonnes/h from the field conveyor. In addition, Canning were also responsible for

retrofitting SuperDrive units along the entire lengthof the existing conveyor system. This included 10

Part of Blashford’s extensiveand recently upgraded fieldconveyor system

View showing the main surgepile, rinsing screen, log-washer and sand plant

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Special Report

ceramic-lagged SuperDrive motorized drive drumsalong with replacement jib discharge and high- andlow-tension bend drums. A total of 34 new non-drivedrums were also installed, along with high-densitypolymer suspended idler sets with polymer safetyhandles, new primary and secondary belt scrapers,as required, and new heavy-duty mesh guardingwhere appropriate. Lately, Canning Conveyor havebeen back on site to carry out the extension of theexisting system to Plumley Wood.

New sand and gravel plantWith groundworks for the new wash plant completedin August 2010, DUO commenced installation worklater that year and commissioned the new facility inJune 2011. The plant is designed to produce up to320,000 tonnes per annum of washed, high-qualitysand and gravel products comprising sharp sand and10mm, 20mm and 40mm single sizes, as well as–75mm +40mm crusher bypass. The 250 tonnes/hplant is of modular design and has the facility to crushthe gravel, as required, to maximize the production ofsand, 10mm and 20mm products in line with demand.The recently upgraded 2.5km field conveyor delivers

the sand and gravel to the processing plant area wherea fully galvanized 33m long x 900mm wide radialstockpile conveyor discharges the feed to the mainsurge pile. Three 7.5kW heavy-duty belt feedersmounted within a 46m long square-section concreterecovery tunnel beneath the surge pile provide acontrolled feed on to an 87m long x 900mm widereclaim/catenary conveyor (100m radius), which reaches a maximum inclination of 18° at its headend. The catenary conveyor discharges directly into a

Powerscreen 20ft x 6ft double-deck rinsing screenwhich washes and splits the feed into three sizes:+75mm oversize, which is discharged into a rejects bay;–75mm +5mm; and –5mm sand. The –5mm fractionis directed to an adjacent Finesmaster 120 compactsand plant, which utilizes a hydrocyclone and integraldewatering screen to remove any silts, clays and slime,while up to 90 tonnes/h of clean, dry sharp sand endproduct is fed to a 2,000-tonne stockpile by a 21m longx 800mm wide radial conveyor. Any excess moisturewithin the sand stockpile is drawn out by a triple take-off manifold moisture-reduction system suppliedand installed by Moisture Reduction Systems Ltd.Meanwhile, the –75mm +5mm material from the

rising screen is fed directly into a Powerscrub 120Rlog-washer for scrubbing. At this point, any unwantedmaterials, such as organics, that are mixed in with thefeed material are floated off via a 6ft x 2ft trash screenat the rear of the log-washer, while the clean –75mm+5mm material exits the front of the log-washer viaan 8ft x 4ft dewatering screen. A 33.5m long x 800mmwide inclined static conveyor delivers the material toa second Powerscreen 20ft x 6ft screen, this time adry triple-deck version, which splits the material intothree final single size fractions (10mm, 20mm, 40mm)and –75mm +40mm oversize.The 10mm and 20mm single sizes are delivered to

their respective 2,000-tonne stockpiles by 21m longx 800mm wide radial stockpile conveyors, while the40mm product is delivered to a 500-tonne stockpileby a similar sized static stockpile conveyor. Meanwhile,the oversize material is redirected, via a 35.5m longx 800m wide inclined conveyor, to a 20-tonne capacitycrusher surge bin.

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Crushing circuitThe crusher surge bin allows the –75mm +40mmmaterial to be choke fed, via a Rotex vibratory feeder,into a Sandvik vertical-shaft impactor (VSI) forcrushing before being recirculated back to theplant’s main feed (catenary) conveyor, via a 37.m longx 800mm wide inclined re-feed conveyor, in a closed-circuit arrangement. Typically, around 20%(approximately 300 tonnes) of total daily productionat Blashford passes through the crusher. If necessary,a crusher bypass chute can be opened to allow ‰

The Finesmaster 120 compactsand plant

View showing the dry screenand feed conveyor to thecrusher surge bin

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the –75mm +40mm material to drop from the surgebin into a concrete bay below. The CV216 VSI selected for Blashford is part of

Sandvik’s six-model CV200 series, designed toensure increased capacity and consistent productshape and size with reduced power consumption andmaintenance requirements. Significantly, thecrusher’s autogenous rock-on-rock crushingtechnique is said to offer several major advantagesin the production of ‘manufactured’ sand, including

a consistently cubical product shape.For Tarmac, a key benefit of the CV216 is its low

environmental impact thanks to Sandvik’s patentedBi-Flow system. Used in conjunction with high-performance rotors and wear parts, this systemoffers an improvement in throughput capacity but withreduced power consumption, thereby deliveringelectricity cost savings and lowering the site’s CO2emissions per tonne.The crusher re-feed conveyor, main conveyor, all

three single-size product stockpile conveyors and thesand stockpile conveyor are all equipped with PreciaMolen belt weighers for accurate productionmonitoring and control. The entire plant features fullygalvanized support structures, while bespokehandrails, designed specifically to meet Tarmac’srequirements, surround all walkways and theperimeter of the civils. All of these supportingstructures, walkways and handrails were designedand manufactured by DUO’s in-house manufacturingdivision, based in Frome, Somerset.

Water treatmentThe sand and gravel washing system at Blashford issupported by a DUO water-treatment systemcomprising a 7m diameter deep-cone thickener tank,3.5m diameter clarified water tank, flocculent dosingsystem and all the necessary ancillary equipment. Waste water from the rinsing screen, log-washer

and dewatering screen is pumped, via the sand plantand ground sump, to the deep-cone thickener tankwhere it is mixed with a flocculent solution. Thesystem’s automated flocculent preparation and dosingoperation is controlled by a PLC housed in theflocculent cabin, which was installed complete withinternal heating and lighting.The use of a ‘deep cone’ thickener tank design

eliminates the need for an internal rake system andany its associated drive mechanisms, therebyminimizing the risk of downtime and reducingmaintenance costs as no moving parts are involved.Thickened sludge is pumped to the site’s silt

The Sandvik CV216 vertical-shaft impactor

Crushed material isrecirculated in a closed-circuitarrangement

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Special Report

View showing the single-sizeproduct stockpilingarragement

lagoon while clarified water weirs over the top of thethickener tank and is collected in a 100m3 capacitycorrugated steel storage tank equipped with ‘high/low’sensors, ready to be reused in the washing plant.Overall, the new sand and gravel plant not only

provides a greater production capacity than itspredecessor, it also gives Tarmac much more controland flexibility over production, allowing the companyto produce more of the products that are in greatestdemand, rather than being limited to what naturallyoccurs in the deposit. In a recent further development at Blashford, a new

DUO washing plant is currently being commissionedat Burnt Hill, a former extraction site located betweenNea Farm and the new Plumley Wood extension. This

‘intermediate’ wash plant will remove the bulk of thesilts from the ‘as dug’ Plumley Wood deposit, therebyproviding a cleaner feed to the main processingplant, while the waste silts and fines themselves willbe used to raise and reinstate ground levels after sandextraction in the new extension, as part of theprogressive restoration of the site. Further detailsabout this latest development will be reported in afuture edition of QM.

AcknowledgementThe editor wishes to thank Tarmac Ltd for permissionto visit the site and, in particular, Richard Bedford, unitmanager, and Matt Walters of DUO (Europe) plc, fortheir help in preparing this report.

General view of the new plantshowing the deep-conethickener that forms part ofthe water-treatment system