One of Britain’s newest asphalt coating plants is winning favour with customers keen on receiving efficient service delivery and custom-designed material, reports Mike Walter
Contractors from northern England in need of quality asphalt at short notice are among the beneficiaries of a coating plant established in Lancashire by a dynamic new company. Baxter Asphalt opened their facility at Poulton-le-Flyde, near Blackpool, in July 2001 – thought to be the first asphalt plant to be installed in Britain for near three years.
The new facility, a 60 tonnes/h Parker Blackmix plant, was set up to provide asphalt for parent company Baxter Construction and to supply materials to a wide range of external clients including housebuilders, road contractors and local authorities.
A large proportion of Baxter Asphalt’s clients are local road-surfacing contractors, to whom material is delivered within a 30km radius. In addition, minor-works contractors can turn up and drive away with small quantities of asphalt.
Operations manager Peter Metcalfe explains that the company is geared towards providing clients with material that can be collected on demand. ‘The asphalt industry is renowned for keeping customers who come to collect material waiting,’ he says. ‘We find that a lot of our customers are fed up with going to conventional asphalt plants and waiting far too long for three tonnes of material. We, on the other hand, can produce and hand over that amount in around 5min.’
One of the reasons why Baxter Asphalt can assure their customers of a swift service is the close working relationship they have established with bitumen specialists Nynas. Two grades of binder (40/60 and 250/330) are delivered by Nynas on a frequent basis, using tankers from their own fleet based at their refinery in Cheshire. The two companies also work to a well honed just-in-time schedule.
Customers themselves are encouraged to play their part in ensuring prompt service delivery. Most minor-works contractors who collect material for use on driveway repair or road reconstruction, for instance, ring ahead to book their order, which can be as little as half a tonne. According to Baxter Asphalt’s commercial manager, David Heaton, maintaining good levels of communication with both customers and suppliers helps the company to meet its client’s needs.
Baxter Asphalt manufacture their own proprietary range of stone-mastic asphalt products, including blends particularly suited to industrial and road use, and produce conventional macadams and standard wearing course mixes.
They also provide a bespoke service to clients who want an asphalt blend to satisfy specific criteria. Peter Metcalfe explains: ‘More and more, customers want their asphalt to meet an end-product specification.
‘For example, a contractor laying asphalt for use by the industrial sector may want a material with heavy deformation resistance, but may not require the material to demonstrate such a high level of skid-resistance, while other contractors may require asphalt with improved skid-resistance and added texture depth.
‘We develop products to serve a purpose and provide solutions to our client’s problems.’
Virgin aggregates brought to the coating facility are kept in covered storage to keep dust levels down and to ensure the quality of the material remains consistently high. A total of 14 different sizes and types of aggregate, from 6mm single size up to 28mm, are stored at the plant and fed into six cold-feed bins. A sophisticated computer control system monitors the level of material in each bin and, when a particular blend of asphalt needs to be mixed, discharges the required amount of aggregate.
A variable-speed conveyor feeds the material into a rotary dryer where the aggregate is heated to the required mix temperature. The material is then screened and weighed before entering the mixer. Bitumen is added before the hot material is carried up to one of two discharge hoppers ready for loading into waiting vehicles The plant also has the ability to add fibres into the asphalt mix, and coloured pellets can be introduced when using a clear binder or conventional bituminous material.
Demand for material produced at the plant during its first few months of service has been ‘fast and furious’, says Mr Metcalfe. As a result Baxter Asphalt have increased their first annual production target from 25,000-30,000 tonnes to around 40,000-50,000 tonnes.
‘The new facility has the capability to produce up to 700 tonnes a day and last autumn was very busy. We are now looking forward to what we hope will be a further increase in demand as we approach the spring.’
A valued client of Baxter Asphalt is Blackpool Borough Council, which, says David Heaton, is one of the most forward-thinking authorities in Britain with respect to specifying the latest asphalt technology.
‘The council’s use of innovative products, and the fact that the area is likely to undergo great change in the next few years, bodes well for Baxter Asphalt in potentially increasing its supply of material to Blackpool.
‘There are plans to turn the town’s seafront into a Las Vegas-style resort and, if that were to go ahead, it could mean a great opportunity for suppliers to provide a range of coloured asphalts for pedestrian walkways and the promenade.’
The new coating plant
Baxter Asphalt’s new 60 tonnes/h Blackmix plant, supplied by Parker Plant Ltd of Leicester, is able to produce a wide range of conventional coated macadams to BS4987 and hot-rolled asphalts to BS594, as well as more specialized surfacing products such as stone-mastic asphalts, thin surfacings and pigmented and fluxed materials, in both large and small quantities, to cater for the ‘grocery’ business. This flexibility of mixes was a high priority during plant selection.
Design parameters for the new plant also called for an installation that would provide exceptional flexibility of mix specifications to cover the increase in technically advanced specialized coloured asphalt coatings in red, green and blue, as well as being able to meet the most stringent of environmental controls. Due to the plant’s location in an urban area, particular emphasis was placed on the reduction of noise and dust emissions from the site, thus ensuring a high degree of environmental control. As a result, the mixing tower is fully enclosed in a sheeted factory-style building that is of stack-up design. The tower comprises individual pre-assembled modules that house all the main units of the plant, each module sitting on the section beneath and bolted together.
Sized aggregates (limestone, gritstone etc) are stored in bays that are fully covered to keep out the weather. This ensures that plant feed materials have a low moisture content which improves product workability and quality within the processing cycle.
The plant is fed from an in-line hopper unit incorporating six 500mm wide variable-speed belt feeders. All the feeders have side-walled belts to eliminate spillage and all operate independently to allow for product blending. Drive is via a 2.2kW electric motor and reduction gear unit. Electric vibrator motors are fitted to the sand hoppers to assist material flow. The entire feed system is housed in a covered structure with both side and roof sheeting to help keep the aggregates dry prior to heating.
An inclined ‘S’-bend conveyor, fitted with a full-length roof and side sheeting, transfers material directly into the feed ring of the rotary dryer. The aggregates are dried and heated in this 1.5m diameter x 6.5m long rotary drum, which is fully insulated and clad to provide effective heat retention. The cylinder rotates on two machined-steel roller paths mounted around the outside of the cylinder using expansion fixings. Drive is provided by an 18.5kW motor through a reduction gear unit and final chain drive. The dryer is equipped with specially designed lifting flights to provide an even curtain of material across the whole drum diameter. A radiation pyrometer and thermocouple mounted in the discharge chute constantly monitors aggregate temperatures leaving the drum.
A highly efficient Parker Turbo-jet proportioning oil burner is fitted at the discharge end of the dryer. This incorporates flame-failure detection, automatic and manual flame control, and has a 10:1 turndown ratio. The burner is track mounted, allowing it to be withdrawn from the dryer cylinder for routine nozzle inspection and maintenance. An attenuator is fitted at the burner end to reduce noise.
A totally enclosed hot-stone elevator equipped with continuously-linked 300mm wide steel buckets lifts the hot material up to the screen level in the tower. The dust-tight elevator casing includes feed and discharge chutes fitted with replaceable steel liner plates at the main impact points. Drive is from the headshaft via an electrical motor and reduction gear unit (fitted with a backstop device) to traction wheels and a chain.
The screen is a 1.2m x 3.0m Parker Rapide type equipped with three-and-a-half decks, giving six sizes of aggregates, plus oversize. This inclined screen is independently mounted on coil spring units and incorporates quick-release clamps for ease of mesh changing. Drive is from an 11kW motor, through multiple vee belts, to an eccentric shaft assembly. Screen amplitude can be adjusted by the addition of balance weights.
In the tower, six hot-storage bins with a total capacity of 17 tonnes hold the aggregates prior to weighing. The storage bins are insulated and clad for effective heat retention. Pneumatically operated radial doors regulate the flow of the various sizes of aggregates to the weigh hopper. Each compartment is fitted with a continuous material-level sensor and an overflow chute.
Separate weigh hoppers, all mounted on load-cells, are provided for aggregates, filler and bitumen, with remote weight indications shown on digital displays in the control cabin. The aggregate weigh hopper is capable of receiving a full batch from any one bin. Bitumen is stored in two 30,000-litre capacity electrically heated tanks sited adjacent to the plant. When required, bitumen is weighed out in a heated and insulated weigh vessel and discharged via a delivery pipe to the mixer.
Filler is fed from a vertical imported-filler silo via a screw conveyor to the weigh hopper or as reclaimed dust from the bottom hopper of the bag filter via a flap door and screw conveyor.
The 1,000kg capacity paddle mixer is of the twin-shaft type, giving a fast, thorough mixing and coating of the material. Direct drive is provided by heavy-duty 30kW motor/gear units located at one end of the paddle shafts, and by synchronized gear wheels at the other.
A 1,000kg wire-rope-driven skip, running on an inclined track, distributes the mixed materials into a two-compartment, 50-tonne capacity (split 20 tonnes and 30 tonnes) storage facility. Each hopper is insulated and equipped with electric trace heating to prevent stored materials from hardening. A 2,000kg capacity spoiled-batch hopper is incorporated alongside the main storage hoppers. Each silo is fitted with a load-cell weight sensor to give accurate digital readings to the plant operator.
Primary dust collection is achieved by means of a structure-mounted skimmer that takes out coarse airborne particles, transferring them, via a flap vale, to the hot-material elevator feed chute.
A secondary system, collects any remaining dust and transfers it, via bag filters, to the filler weigh hopper. Any excess dust is conveyed to a twin-shaft conditioner where water is added by spray jets and the resulting mixture discharged to a skip.
The design of the filter allows dust-laden air that enters the cabinet to be collected on special high-temperature cloth filter bags which are systematically ‘cleaned’ down, the dust falling into a bottom trough hopper. Temperature probes fitted to the ducting prior to the filter are linked to controllers which protect the filter bags from high gas temperatures. As a result, emission levels from the plant meet the most stringent of European dust-control regulations.
Emissions testing recently carried out by local authority Wyre Borough Council revealed dust emissions to be 1.5mg/m3, well within the limit of 100mg/m3.
A Flomac pelleted additive system for special mixes consists of a silo loaded by 500kg bags. The pellets are blown by pneumatic conveyor into a batch weigh hopper and fed directly into the plant paddle mixer.
The plant is controlled by an advanced PC-based computer control system that offers the latest hardware technology within a user-friendly package. The system provides high-resolution colour graphic displays of all plant functions. The computer is linked to an industrial PLC that interfaces with the plant to control all weighing and mixing operations. Up to 1,000 standard mix recipes are stored in the system and data on aggregate, filler and bitumen usage are logged to allow operational analysis at a later date.
The plant is equipped with a recycled asphalt (RAP) feed system comprising a 9.5m3 capacity feed hopper fitted with a 250mm bar grid, belt feeder, and transfer conveyor to a RAP elevator. The 400mm wide bucket elevator carries material up to the plant’s mixer level and discharges on to a 3.5m long horizontal weigh conveyor. Material is delivered directly into the paddle mixer through a steel chute. The system can accommodate a maximum of 15-20% of suitable granulated reclaimed asphalt. When such material is added, a steam-evacuation system, comprising a remotely operated shut-off door, removes steam from the mixer and transfers it into the ducting prior to the bag filter.
Lorries are required to follow a one-way system through the site and are weighed in and out on a 50-tonne weighbridge. Administrative and commercial functions are carried out from Portakabin offices sited next to the weighbridge.
Thanks are due to Nigel Moreton, Parker Plant Ltd and Baxter Asphalt for their help in preparing this plant report.