Halkyn Quarry Replant Completed

Major redevelopment scheme safeguards limestone production at North Wales site for the next 30 years

CEMEX have recently completed the largest single investment project in their UK aggregates business with the replacement of the original processing plant and quarry infrastructure at Pant-y-Pwll Dwr Quarry (aka Halkyn Quarry), installed in the 1950s, with a new state-of-the-art processing facility. As well as delivering modern standards of production efficiency and environmental control, the scheme has also successfully achieved its overriding aim of releasing 32 million tonnes of reserves that had previously been made inaccessible by the location of the former processing plant, thereby safeguarding production continuity at the site for the next 30 years.

The quarry itself is situated on the northern flank of Halkyn mountain in Flintshire, North Wales, a wild grazing landscape steeped in industrial history that lies between the Clwydian mountain range and the Dee estuary, close to the village of Pentre Halkyn and some 15 miles west-north-west of Chester. The site covers a total area of 51ha, with some 32ha currently active.

Halkyn mountain and its natural resources have been exploited by man for many centuries. Over the years this corner of North Wales has given up vast quantities of lead, chert and limestone. Evidence of lead mining dates as far back as Roman times, but it was not until the early 1600s that mining in this area really began to flourish. As the mines grew in both depth and extent, flooding of the tunnels and shafts was a constant danger to the miners, a problem that was partly solved by the advent of steam-powered Cornish pumping engines at the turn of the 18th century.

The ultimate solution to this problem, however, was the construction of the 10-mile long Milwr Tunnel from sea level at Bagillt in the north to Loggerheads in the south. Started in 1897 and completed in the mid-1900s, this tunnel effectively drained 60 miles of mine passages, some up to 800ft below ground level, and even today is believed to influence the level of the water table around Halkyn mountain. During their heyday between 1845 and 1938, the Halkyn mines produced nearly 500,000 tons of lead, representing nearly 10% of the UK’s total production at that time.

Early quarrying in the area was carried out on a small scale by locals digging limestone for buildings and walls, or for grinding into a powder to use as fertilizer. Gradually, as demand for building stone and agricultural lime increased throughout the 1900s, larger quarries developed and eventually supplanted the once booming mining operations, which finally came to an end in 1987. Today there are three active commercial quarrying operations on Halkyn mountain, supplying materials to construction sites across north-west England and North Wales.

Halkyn replant

Initial planning for the replant at Halkyn Quarry began in the mid-1980s when permission for an extension to the site was granted, although it was some 10 years later before the project finally began in earnest. Between 1996 and 2001 the design of the new plant was re-engineered several times, with projected costs ranging from £10 million to £20 million. The scheme eventually selected represented a total investment of £13.5 million.

Approval to start the replant was granted in 2002 although this was subsequently deferred for a year. Following extensive best-value design exercises, the main contracts were awarded to a restricted number of suppliers during March and April 2004, with Whitwick Engineering (Coalville) Ltd appointed as main contractors for the mechanical engineering and materials handling aspects, and RMB Contractors Ltd and Glenace Electrical Ltd looking after the civil works and electrics respectively. A project-management team led by CEMEX Central Engineering Services’ head of engineering, Andrew Boam, was established to ensure tight cost control and smooth delivery of the various phases of construction.

Work on site commenced in August 2004 with the project being conducted in accordance with current CDM and health and safety regulations. One of the first tasks was the creation of a new access road to increase site safety and efficiency and to allow work to commence on the civil engineering works for the new infrastructure. The physical and logistical demands of the redevelopment were considerable. The new primary crusher had to be situated on the floor of quarry void some 600m from the position of old primary plant, which itself was located at an elevation 100m higher on the quarry rim. The scheme also called for a new 1km long trestle-mounted conveyor incorporating a 43m long high-level gantry section to deliver material to the new secondary crusher at the top of the quarry. In total, almost 3,000 tonnes of steel were used in the new structures.

As a testament to the skill and professionalism of the teams working on site, no lost-time incidents were recorded throughout the 15-month construction period, and according to Andrew Boam, the various contractors’ approach to CDM and health and safety issues was ‘exemplary’. Formal commissioning of the new plant began in December 2005 and continued through to February 2006. To ensure production continuity, the old processing plant remained in operation until the new facility was fully commissioned, during which time employees underwent intensive training on the new plant’s computerized systems and processes. Unlike its predecessor, the replacement plant is fully automated for maximum production flexibility and features comprehensive diagnostic capabilities. Decommissioning and dismantling of the old plant commenced at the end of July this year.

Processing operations

The Carboniferous Limestone worked at Halkyn Quarry lies predominantly within the Loggerheads Limestone Formation and extends to a depth of at least 22m AOD, which is some 232m below the level of the main plant. The deposit itself comprises a very clean, largely clay-free material with a crushing value of 22, an LA abrasion test value of 21 and a particle flakiness index of 6. Despite the bulk of the reserves lying at considerable depth, the quarry suffers from very little groundwater infiltration, as the area is thought to be drained by the Milwr Tunnel, which is located 550m east of the quarry at a depth of 10m AOD. Nevertheless, some water-filled fissures or pockets are anticipated during development of the site, which may have some bearing on future extraction plans.

During each 10h loading shift, blasted limestone is loaded into three 40-tonne Cat 771D rigid dumptrucks by a Cat 988G wheel loader for the current 400m haul from the face to the new primary crusher installation, a complete load-and-haul cycle taking around 9min. At the primary station the material is dumped into a 150-tonne feed hopper equipped with a Kleemann+Reiner 1,250mm x 6,000mm reciprocating tray feeder, which in turn discharges on to a Kleemann+Reiner twin-deck grizzly screen with bars set at 150mm. All +150mm material is fed directly into a Kleemann+Reiner SHH15 primary impact crusher, which has a throughput capacity of 525 tonnes/h. Any oversize boulders are handled by a PRF Engineering rock breaker mounted adjacent to the crusher.

Scalpings rejoin the –150mm crusher product for delivery to a 900-tonne capacity surge bin located at an intermediate elevation within the quarry. Featuring an overflow stockpile conveyor and equipped with two Skako feeders at its base, this vessel on its own is designed to provide around 1.5h of production capacity in the event of a primary section breakdown or planned shutdown. In addition, a re-feed hopper situated adjacent to the surge bin allows the overflow stockpile material to be shovel-fed back on to the feed conveyor to the secondary plant selector screen when necessary.

On arrival at the top of the secondary plant, situated on the quarry rim, the primary crushed material is a discharged into a twin-deck Don Valley selector screen, which is used to split the material into +75mm, –75mm +40mm and –40mm fractions. The two coarser sizes are discharged into two 230-tonne capacity selector bins equipped with 450 tonnes/h Skako vibratory tray feeders, which in turn feed a 180-tonne surge bin mounted above the secondary crusher. The –40mm fraction is discharged into a third selector bin from where it can be fed directly on to the main screen house feed conveyor or, alternatively, to a lorry load-out conveyor rated at 850 tonnes/h or to a 2,000-tonne ground stockpile.

The 450 tonnes/h Kleemann+Reiner SNH20 secondary impactor discharges its –40mm product on to the main screen house conveyor. Final screening is carried out by three dust-encapsulated Don Valley screens — a 3,430mm x 7,000mm triple-deck unit, a 3,000mm x 7,000mm double-deck unit, and a 2,400mm x 5,000mm single-deck unit — all equipped with Tema Isenmann modular panels. A highly effective dust-extraction system, supplied by Enviroflo Engineering Ltd, ensures that the main screen house remains virtually dust-free at all times.

The three main screens deliver eight final product sizes: +40mm; –40mm +28mm; –28mm +20mm; –20mm +14mm; –14mm +10mm; –10mm +6mm; –6mm +4mm; and –4mm to dust. These are discharged into a series of eight storage bins with capacities ranging from 250 tonnes up to 700 tonnes. The method of bin discharge varies depending on the product characteristics, with the finest fraction being discharged by belt feeders and the coarsest fraction by vibro-feeders. The other six bins are discharged via highly accurate Rowecon sliding-gate feeders.

Any of the sizes from the storage bins can be sent to a Kleemann+Reiner SNH12 tertiary impactor for re-crushing in a closed-circuit arrangement, with the –20mm tertiary crushed product rejoining the secondary crushed material and –40mm fraction on the main screen house feed conveyor.

The new plant is equipped with two product blending/lorry loading conveyor lines, both rated at 1,000 tonnes/h. These are controlled automatically by a Rowecon software system, which utilizes smart cards to allow drivers to collect their loads direct from the load-out bays without having to leave their cabs. The accuracy of the bin discharge/load-out system is such that drivers rarely have to top-up or tip off material, making turnaround times much quicker. An additional stock-out conveyor line is available to feed three 3,500-tonne capacity partly covered storage bays with any product size or blend direct from the final storage bins at a rate of up to 500 tonnes/h. Alternatively, this line can be diverted to a 20,000-tonne ground stockpile.

Apart from the primary crusher, which has its own dedicated control room, all other plant operations and load-out functions are monitored and controlled from a second control room located adjacent to the lorry load-out bays. The new plant is fully automated with a Scada control system and incorporates monitoring devices to detect bin levels, conveyor loads, belt slippage, blocked chutes etc. All the control system architecture was developed in house by CEMEX, resulting in a substantial six-figure cost saving on this aspect of the project.

Redevelopment of the quarry infrastructure also included the construction of new administration facilities, complete with a laboratory and meeting room, and a new weighbridge office, as well the installation of a new 50-tonne capacity BridgeMont weighbridge from Avery Weigh-Tronix and Hippowash wheel-washing systems from CDM Steels. In addition, a new workshop and parts store together with canteen and mess facilities for the site’s 26 staff were included in the scheme.

Conclusion

Halkyn Quarry currently has planning consent to produce up to 850,000 tonnes of limestone per year, the bulk of which is used to meet ready-mixed concrete, coated material and general construction requirements in the markets of Deeside, Wirral, Liverpool, Warrington, north-east Wales and south Cheshire. However, the new plant has been designed and engineered with the potential capacity to produce up to 1.4 million tonnes per year to cater for possible future development and production needs.

By releasing previously sterilized reserves to help meet such needs, the replant has secured the future of the quarry for the next 30 years, while at the same time production and operational efficiency at the site has been optimized, particularly in key areas such as materials handling, operator access and egress, and environmental control.

Reduction in dust output, in particular, will be a major environmental and commercial benefit at Halkyn. In addition to the highly effective final screen house dust-extraction system, Enviroflo Engineering have supplied equally beneficial dry foam dust-suppression systems for all crusher feed and discharge points, as well as water spray dust-suppression for transfer points and discharge chutes on the lorry load-out and stocking conveyors, the combined effect of which has been a notable reduction in the site’s overall level of emissions.

Due to the nature and depth of the reserves at Halkyn, active restoration of the extraction area does not form part of the site’s current operations, however the quarry’s waste tips and other areas have been restored and returned to common grazing land, thereby maintaining the ‘commoners rights’ that have existed on Halkyn mountain since medieval times.

Acknowledgement

The editor wishes to thank CEMEX for permission to visit the site and, in particular, Andrew Boam, head of engineering, and Mick Ripley, quarry manager, for their help in preparing this article.