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Double barrelled aggregate processing
1st February 2008

Double barrelled aggregate processingWith new twin aggregate processing lines increasing throughput at its Northfleet Wharf site, CEMEX UK, the building materials provider, aims to meet the demands of its customers.

Northfleet Wharf is a marine aggregates facility situated on the south bank of the Thames at Northfleet in Kent. Owned and operated by CEMEX UK the site has recently been redeveloped as part of its current national investment programme of over £60 million this year.

The 35 years old washing plant being run on the site was increasingly demanding in terms of maintenance and efficiency. The decision was made to take the old plant down and install a temporary Powerscreen washing system.

During installation of the temporary plant the decision was taken to install a new, upgraded plant to meet the increased demand for marine dredged materials. The plant had to be able to process marine aggregates to a specific quality at an output of 400 tph. Having asked DUO Europe plc to install the temporary system CEMEX UK turned to the company to provide the new plant.

New site services were installed in late 2006 along with new concrete foundation pads allowing DUO to begin the plant build in January 2007. The temporary Powerscreen plant ran until the new plant commenced operations in August. The site has also added a silt plant to manage the waste.

The plant in action

Material arrives at Northfleet from the Thames estuary and the south coast. From the as-dredged material stockpile, process material is fed to the new 400 tph processing plant. Material is fed via loading shovel to a 50 t capacity feed hopper. This hopper is fitted with a 150 mm rejects grid and variable speed drag out belt feeding onto a 30 m long x 1000 mm wide inclined conveyor rated at 400 tph. From here the -150 mm material is fed into a two way bifurcated discharge chute with a shut off gate at each leg. The chute will allow a split of material at approximately 50/50 to be sent forward to the two double-deck 20 x 6 Cedarapids Horizontal rinsing screens, which are positioned side by side.

From the screens the oversize +20 mm material is scalped off and discharged onto an 11 m long x 800 mm wide reversible conveyor. Rated at 180 tph, this conveyor either stocks the oversize material or passes it forward for crushing via a 21 m long x 800 mm wide inclined conveyor. This conveyor, rated at 180 tph, feeds a 15 t crusher surge bin. The crusher, a Pegson 1000 Automax, is surge fed via a vibrating tray feeder. The crushed material is fed onto a 7.7 m long x 800 mm wide inclined conveyor. Rated at 180 tph this conveyor feeds onto the 30.9 m long x 1000 mm wide inclined conveyor where it is blended with the as-dredged feed material.

The -6 mm material is flumed to two Powerscreen Fines Master 200 sand plants. The waste silt is flumed to the water management plant. The dewatered sand is blended from both Fines Masters onto a 20 m long x 800 mm catenary conveyor. Rated at 280 tph this conveyor feeds onto a 24 m x 800 mm wide powered radial stockpile conveyor.

The -20 ­­mm +6 mm aggregate is sent forward from each Cedarapids Screen independently via a pair of 24 m long x 800 mm wide inclined conveyors.  Rated at 260 tph these conveyors together feed two 20 x 6 Cedarapids horizontal screens.The material from both top decks is fed onto a 30 m long x 800 mm wide stocking conveyor giving a 20 mm product. The material from the bottom deck is fed onto a 24 m long x 800 mm wide powered radial stockpile conveyor giving a 10 mm product.

Control and performance

Plant operation is via a touch-screen, with full start/stop, alarm, history recording and fault diagnostic capabilities. The plant design allows operation either in full twin-line specification or with one of the two processing lines shut down, allowing maintenance to be carried out on one section while the other half continues operating.

Staff keep the site remarkably clean and tidy even after the plant has running for several days; CEMEX UK worked hard with the commissioning team to ensure that there were no leaks in the plant from the outset.

The new plant allows the Northfleet site to now produce the same amount of material over a 10 hour period that it was producing over a 20 hour period with the old system. Furthermore, staff are finding the new plant to be much easier to maintain and more accessible. Built on concrete pads on what is essentially marsh ground the plant runs  smoothly, vibration is virtually non-existent and it is quiet enough to achieve 400 tph with no more noise than the site's old plant produced with only half of that output.

The effectiveness of the horizontal screens has impressed staff at Northfleet who at first wondered how they would handle the anticipated volume but have now, since witnessing their efficiency in operation, seen just how they do it. The screen set-up is standard but because sand is supplied to the Northfleet site's block works, which requires a slightly coarser material for block production, some modifications to the screen media were needed. Furthermore, being in a saltwater environment CEMEX UK insisted that the entire framework be galvanised. The 60% improvement in throughput compared to the old plant means the Northfleet site can meet market demand on a 10 hour day as opposed to a 20 hour day, which offers the potential to run at double shift as the market increases.

Recycled water and waste management

Some 80% of the water used in the aggregate processing plant is recycled. The water used for aggregate washing is part river / part mains water, depending on salt content, which is monitored on a daily basis. The water held in a tank is then pumped across to the grading screens and the primary screens.

As it is washed the waste water flows into a sump between the two Fines Masters, from where it is then pumped into the top of a deep cone thickener. Simultaneously the batching pumps inject the flocculent solution into the delivery pipe. As a result, inside the thickener tank the sludge will drop while the clean water weirs over the top of the thickener into a clarified water tank and is reused by the wash plant. The flocculent system is automatic therefore the operator only needs to keep the hopper full of powder. The underflow from the thickener is discharged into the sludge tank. The sludge is then pumped directly to the silt press.

The Tecnoidea 68 plate, fully automatic silt press - the largest available - was installed at the Northfleet Site in February 2007. As the sludge is pumped up the press is closed. Once the water has drained the press reaches full capacity, it automatically opens and drops the resulting silt cakes. It shakes itself clean, closes itself back and starts pressing again. The cycle takes approximately 20 minutes and each cycle contains approximately 6 tonnes of sludge. That works out at roughly 1% of the overall production. The dryness of the silt cakes can be altered depending on the level of dryness required for the pressed cake. The filtrate water from the press is returned to the clarified water tank.

Prior to the new silt plant all of the silt was deposited into silt lagoons that had to be dug out using an old dragline.  The silt press and the returns water system means that the dirty water used for washing the material is reused after being cleaned through the press.

Silt cake falling from the bottom of the press is stored at the site boundaries but will eventually go to landfill. In the future, the site is looking to reintroduce the silt back into the product or sell it off as another useful product, though there is someway to go with developing this.

The DUO Equipment division through their UK dealership supplied the Powerscreen Finesmaster sand plants, all Cedarapids screens, Pegson cone crusher, Technoidea Silt Press and Water Management System.Working alongside, the DUO Manufacturing division (LJH) designed and manufactured all the fixed conveyors including the installation of fully galvanized heavy-gauge steelwork with guarding throughout.

DUO - PSWS commissioned Screening Consultancy & Supplies (SCS) to supply and install SCS Polyurethane Modular systems to the 20 x 6 Double Deck Cedarapid Screens. SCS also installed rubber wear resistant linings to protect the Cedarapid screen frames, drive tubes, support chassis and feed and discharge trays. SCS has been contracted by CEMEX UK to supply and maintain the screening surfaces and rubber wear resistant screen and chute linings for a period of 2 years.