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New Benninghoven Plant For Elstow Asphalt

Lafarge/Tarmac joint venture chooses TBA240 coating plant

With an investment of over £2million at an existing asphalt facility, a Lafarge/Tarmac joint venture has created Elstow Asphalt Ltd, a new coating plant operation in Bedford. Both companies looked at the Elstow site and agreed to augment the existing 100 tonnes/h Via Nova asphalt plant with a completely new plant.

The site itself is strategically linked to the M1 and M6 motorway network and also benefits from a rail connection, allowing a supply of crushed aggregates direct from Lafarge’s flagship Mountsorrel Quarry in Leicestershire. These were major factors that influenced the decision by Lafarge and Tarmac to jointly invest in a new state-of-the-art asphalt plant at Elstow.

To achieve the necessary mix specifications, production targets and environmental criteria, a Benninghoven model TBA240U asphalt plant equipped with a 4,000kg mixer was selected. Capable of delivering a wide range of standard and specialized mixes, the plant has a production output of well over 200 tonnes/h of dense basecourse at 5% initial moisture, and 170 tonnes/h of hot-rolled asphalt at 6% initial moisture.

The aggregate feed system consists of 12 cold-feed hoppers, each of 14m3 capacity, arranged as two six-compartment units, each fitted with a frequency-controlled belt feeder for accurate blending of the aggregates. Two of the hoppers in each section are for fines; these are equipped with hopper-wall vibrators to ease material flow. All the hoppers are sheeted on their sides, ends and roofs.

A 29m long collecting conveyor delivers aggregate materials on to a 13m long x 650mm wide inclined dryer feed conveyor, which discharges directly into the dryer drum.

The 2.5m diameter x 9m long dryer drum is fully lagged with 70mm thick Rockwool insulation and aluminium clad for heat retention and noise reduction. The drum is friction driven by four support rollers through individual 18.5kW geared motor units.

The burner is a Rax Jet 4 type, a well-proven Benninghoven unit that burns gas oil. This is equipped with a full range of operating and safety devices to deliver high-efficiency fuel usage. An air-inlet silencer is connected to the burner to suppress noise levels. The burner can also operate using waste oil as a fuel.

The plant’s dust-collection system, sized for the dryer unit and mixing section scavenging, is rated at 64,000Nm3/h and achieves emission levels of less than 25mg/m3. A primary skimmer unit mounted alongside the dryer separates any coarse dust from the airstream, which passes to the hot-stone elevator feed boot via a gravity flap valve and chute. A secondary system collects any remaining dust and transfers it through a bag-filter, screw conveyor and pneumatic conveyor to the plant’s reclaimed dust silo. Dust is also collected from nuisance points at the screen, weigh hoppers and elevator. As a result, the plant is equipped to meet the most stringent dust-control regulations.

Any excess dust collected can be discharged from the reclaimed dust silo through a rotary valve into a twin-shaft paddle conditioner where water is introduced to allow handling without creating fugitive dust.

Hot aggregates are transported from the dryer up to the screen by a vertical bucket-type elevator. This is totally enclosed and features twin-strand chains and wear-resistant steel buckets with replaceable liner plates at impact points. The elevator, which features externally located bearings and a safety backstop facility, is driven by a 37kW geared motor and toothed chain wheels at the elevator head.

Material discharged from the elevator passes on to a six-deck screen which features twin-shaft drive via two 19kW, ‘bolt-on’, low-maintenance motors with vee belts. The drive units are mounted outside the screen housing to allow high-temperature screening. Wide access doors, together with a rollaway chute, provide access to all six screen decks.

The hot-storage section comprises six bins with a total capacity of 80 tonnes. All the bins are insulated and clad and each one is fitted with continuous level indicators and an overflow chute. Discharge is by a pneumatically operated radial door, which accurately regulates material flow to the aggregate weigh hopper, from where the aggregates are discharged directly into the mixer.

Accurate weighing of ingredients is achieved by separate load-cell-mounted weigh hoppers; these comprise a 4,000kg capacity aggregate hopper, a 600kg capacity filler hopper and a 400-litre capacity weigh vessel for bitumen. Mixing takes place in an insulated 4,000kg capacity twin-shaft paddle mixer, with drive provided by two 55kW geared motor units through synchronized gears. A rapid and thorough mixing cycle is achieved by the mixer’s paddle arrangement, and a radiation pyrometer is situated at the mixer discharge to indicate mix temperature.

Mixed material discharged from the mixer falls into a 4,000kg capacity skip unit, which runs on horizontal tracks and distributes the material into six 50-tonne insulated compartments, all of which are fitted with high level warning indicators and electric trace heating around the cone and discharge doors. All the bins are mounted on load-cells to ensure accurate material loading, and a direct load-out hopper is positioned beneath the mixer.

Bitumen is stored adjacent to the plant in four vertical 80m3 capacity tanks. These are Benninghoven’s latest generation of high-thermal-efficiency units, which offer low running costs due to their stepped heating system design. An in-built loading system with high level probes offers fail-safe tank filling with no possibility of spillage.

Filler is stored in an external two-compartment vertical silo holding approximately 85m3 of reclaimed filler and 55m3 of imported filler. Both compartments are equipped with level indicators and shut-off valves.

A gravimetric additive system has been incorporated in the plant to feed granulated material from a 30m3 capacity silo. Each batch of granulate is accurately weighed in a load-cell-mounted weigh hopper before being discharged through a butterfly valve to the mixer.

A RAP system has also been incorporated in the facility to deliver precise percentages of recycled asphalt directly into the plant mixer. The system comprises a steep-sided, 10m3 capacity feed hopper fitted with an oversize grid and vibrators. A 4kW belt feeder mounted beneath the hopper delivers the RAP on to a 21m long transfer conveyor for delivery to a 20m high inclined elevator, which transports the RAP up to the mixer level where it is discharged on to a 3.5m long conveyor fitted with a belt-weighing system to provide accurate batch proportioning. An automatic vent duct at the mixer allows rapid extraction of steam during recycling operations.

The plant is controlled by a Benninghoven Online Batcher 3000 computer system, which displays all plant functions via colour graphics. The system, which is housed in an air-conditioned and heated control cabin overlooking the plant, is modem linked to Benninghoven and provides storage for up to 500 mix recipes. Administrative functions are carried out from offices located adjacent to the site’s new weighbridge.

Acknowledgement

Thanks are due to Nigel Moreton of Benninghoven UK Ltd and Greg Price, commercial and performance director aggregates and asphalt with Lafarge Aggregates.

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