Hanson Complete Major Quarry Upgrade
New Benninghoven asphalt plant for Brayford Quarry
Brayford Quarry lies in a picturesque part of North Devon, some six miles north of South Molton on the A399. The site is owned and operated by Hanson UK, part of the Heidelberg Cement Group, and supplies both crushed stone and asphalt to established local markets.
Recent major investment in a new state-of-the-art asphalt plant from Benninghoven was part of a £6.4m upgrade of the site’s existing asphalt production facility to increase operating efficiency and realign output with changing market requirements.
Designed to provide significant operational benefits and cost savings, the new Benninghoven model TBA2000E plant, which replaces an ageing Parker unit, is able to produce a wide range of high-specification coated materials, including modern thin surfacings, stone-mastic asphalts and other specialized mixes, and will allow Hanson to provide a more comprehensive and efficient service to their main markets. The plant has a rated capacity of 120 tonnes/h at 6% initial moisture (DBM) and 80 tonnes/h capacity at 6% initial moisture (SMA).
The aggregate feed consists of two separate systems, one from the crushing plant and one from the plant’s own cold-feed hopper system, dependent on the PSV asphalt mixes required. Crushed and graded gritstone quarried at Brayford is fed to the plant via a conveyor which runs directly alongside the aggregate processing plant’s storage bins. The fully covered conveyor is mounted on trestles with safety pull wires along its walkways. This feeds on to an 18m long x 800mm wide transfer conveyor, which in turn delivers material on to the cold-feed conveyor and then on to a short dryer slinger conveyor.
Aggregates stored under cover are the site’s own together with 68 PSV gritstone from Hanson’s Craig Yr Hesg Quarry, in South Wales. These are fed into the six-hopper cold-feed storage systems where selected aggregate materials are discharged on to a 14m long x 800mm wide transfer conveyor which, in turn, delivers material on to the dryer slinger conveyor.
The aggregate dryer drum, which is 2.2m in diameter x 8m in length, is aluminium clad and fully lagged with 70mm thick insulation for heat retention and noise reduction. The drum is friction-driven by four support rollers powered by individual 11kW geared motor units.
The burner is a Benninghoven model RJ04 type, designed to burn both gas oil and Eco oil. This is equipped with a full range of operating and safety devices to deliver high-efficiency fuel usage, while an air inlet silencer connected to the burner helps to suppress noise levels. The burner and silencer are mounted on a track to allow the unit to slide back from the dryer for routine servicing.
A special Benninghoven-designed (PFO) processed fuel oil line heater unit complete with pumps, heater valves etc, all mounted on a skid base, is positioned alongside the duel-fuel storage tanks. The unit, which is ATEX certificated, boosts fuel to an atomizing temperature.
A surplus existing fabric filter, rated at 68,000Nm3/h, was utilized in the new plant’s dust-collection system. Sized for dryer unit and mixing section scavenging, the unit achieves emission levels of less than 20mg/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. The secondary fabric filter system collects any remaining dust and transfers it, via two screw conveyors, to the filler elevator. A dust monitor mounted on the exhaust stack provides a remote readout in the operator’s cabin. As a result, the plant meets 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, thereby providing an environmentally friendly method of purging this reclaimed dust. Excess filler can also be loaded directly into road tankers.
Hot aggregates are transported from the dryer up to the screen by a 32m vertical bucket-type elevator. This is totally enclosed and features a heavy-duty double-strand chain and wear-resistant steel buckets with replaceable liner plates at impact points. Elevator drive is by a 30kW geared motor and toothed chain wheels at the elevator head. The elevator features externally located bearings and a safety backstop facility, while a wide platform provides access for maintenance.
Material discharged from the elevator passes into a six-deck screen that features twin-shaft drive via two 13kW ‘bolt-on’ low-maintenance vibrating motors. These drive units are mounted outside the screen housing to permit the high-temperature screening of materials. Wide access doors, together with a roll-away chute, provide access to all six screen decks. An extraction fan ensures negative pressure is maintained in the screen housing for optimum dust control.
The hot-storage section has a total capacity of 150 tonnes and comprises six bins. The bins are all insulated and clad and each 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. A divert section with a horizontal rotary door allows graded sizes to be collected from either side of the hot-bin compartments.
Accurate weighing-out of ingredients is achieved by separate load-cell-mounted weigh hoppers, comprising a 2,000kg capacity aggregate hopper, a 300kg capacity filler weigh hopper, and a 200-litre capacity weigh vessel for bitumen. A 900 litres/min injection pump forces the bitumen into the insulated, 2,000kg capacity, twin-shaft paddle mixer, which is driven by two 22kW geared motor units through synchronized gears. A radiation pyrometer is situated at the mixer discharge to indicate mix temperature.
Mixed material discharged from the mixer falls into a 2,000kg skip unit that runs on horizontal tracks and delivers the material to a 150-tonne capacity storage section, arranged in three 50-tonne compartments. All three bins are equipped with high-level warning indicators and are insulated and fitted with electric trace heating around the discharge doors. A 6-tonne direct load-out hopper is positioned beneath the mixer. All the bins are mounted on load-cells to ensure accurate material loading.
Bitumen is stored in four tanks sited adjacent to the plant. These comprise two existing tanks of 60m3 capacity and two vertical 100m3 capacity tanks, which are Benninghoven’s latest electric high thermal efficiency units. These provide low running costs due to their stepped heating system design. An in-built loading system with high-level probes offers fail-safe filling, with no possibility of spillage, and complies with the latest RBA standards.
Filler is stored in an external, two-compartment vertical silo holding approximately 85m3 of reclaimed dust and 55m3 of imported filler. Both silos are equipped with level indicators and shut-off valves.
A gravimetric additive system is also incorporated in the plant to feed granulated material from a 30m3 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 similar-specification silo for pigment is sited alongside.
In addition, a RAP system has been incorporated in the facility to deliver precise percentages 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 24m long covered conveyor equipped with a belt weigher, to give accurate batches proportional to the weight of aggregates, which can be 20% of the mix at 3% moisture. The RAP is transported up to the mixer level via a vertical elevator. An automatic vent duct at the mixer allows rapid extraction of steam during recycling operations.
The plant is controlled by a Benninghoven on-line Batcher 3000 computer system that displays all plant functions via colour graphics. The system, which is housed in an existing control cabin adjacent to the plant, is modem linked and provides storage for up to 500 asphalt mix recipes.
All the asphalt plant and equipment at Brayford was supplied and installed in accordance with current CDM directives.
Acknowledgement
Thanks are due to Neil Pick, national engineering manager – asphalt, Hanson, and Nigel Moreton of Benninghoven UK Ltd, for their help in preparing this report.
