Simply Red

Tarmac Northern’s Harden Quarry gears up for growth

Situated on a localized outcrop of ancient igneous rock on the edge of the Cheviot Hills, in the heart of the Northumberland National Park, Tarmac Northern’s Harden Quarry produces a modest output of around 150,000 tonnes a year. Yet the quarry’s size belies its standing in the marketplace, as Harden stone is used extensively throughout the UK and Europe and has even been exported to the Far East.

The key to the success of Harden stone is its highly distinctive red colouring, which makes it much sought after for high-value demarcation and decorative applications. Known as Harden Red, the stone - a highly fractured, 390 million-year-old mica-porphyrite with a PSV of 56, an ACV of 14, an AIV of 9, an AAV of 1.4 and a 10% fines value at 300kN – is unique within the Tarmac Group and one of only three similar sources of naturally occurring red material in the UK.

Harden Quarry first opened in the 1930s and was operated thereafter by a number of different owners before being acquired by Tilcon in the mid-1960s. Since the acquisition of Tarmac two years ago by Tilcon’s parent company, Anglo American, the 14.3ha site has operated under the banner of Tarmac Northern. With over 2.2 million tonnes of consented reserves and a further 1.4 million tonnes of identified reserves, in 1998 the quarry was granted a 25-year extension, subject to strict environmental conditions including the construction of a new access road and a number of tree-planting schemes.

Today the site is set for a new lease of life following a recent £1.25 million investment in a new, state-of-the-art processing plant capable of producing the full range of UK and European sizes. Additionally, to maximize product quality and realize full market potential, all aggregates are now washed during the crushing and screening process. Having made this investment, Tarmac are now looking to increase production at Harden by around 10,000 tonnes a year over the next two to three years.

New processing operations

Rather than replacing the quarry’s existing static primary crusher installation, which is soon to be dismantled along with the rest of the former plant, Harden now shares a fully mobile Lokotrak 105 tracked primary crusher with a number of other Tarmac sites. This machine is brought in four or five times a year and works on a pre-prepared blast pile for five or six weeks at a time to produce a stockpile of up to 30,000 tonnes of –125mm primary feed for the main processing plant.

Built by West Midlands-based TDA Design & Engineering Ltd and commissioned in October 2001, the new PLC-controlled crushing and screening plant is designed to process up to 100 tonnes/h. The primary crushed material is fed to the plant by wheel loader via a feed hopper and variable-speed belt feeder. This arrangement delivers the material on to a 750mm wide conveyor which in turn discharges on to a Finedoor 12ft x 5ft double-deck washing screen. A relieving deck removes –5mm material, which is delivered to stockpile via an Eagle Iron Works (EIW) 30 x 25 screw classifier/dewaterer, supplied by Syscom bv of the Netherlands, while all +5mm stone proceeds by conveyor to the secondary crusher –– a choke-fed Metso 800 cone with a 150mm opening.

The –40mm product from the secondary crusher is conveyed to a second Finedoor double-deck washing screen, this time a 16ft x 6ft version equipped with Tema Isenmann modular polyurethane panels. This unit also takes out –5mm material,  but this time 0–2mm and –5mm +2mm fractions are separated and fed to discrete product stockpiles via a pair of EIW 30 x 25 screw classifiers/dewaterers and two 600mm wide stockpiling conveyors.

The +5mm material from the second screen is conveyed to the final screening section, which comprises two Finedoor horizontal screens arranged in line –– a 16ft x 5ft double-deck unit and a 20ft x 5ft single-deck unit. These are mounted over four 4m wide toastrack storage bays which receive –8mm +5mm, –11mm +8mm, –16mm +11mm and –22mm +16mm final products.

The +22mm oversize from the final screen is either discharged to the ground or, alternatively, fed into a re-feed hopper for recirculation. In the latter case, the material is conveyed to a second Metso 800 cone crusher, this time with an opening of 50mm, before being conveyed back to the second washing screen in a closed-circuit arrangement.

Effluent treatment

Effluent from the EIW screw classifiers together with the super-fine material (nominally –30 microns) contained in the overflow from the site’s Svedala classifying cyclones is collected in a concrete-lined ground sump and then pumped to a modular, 6m diameter Parnaby deep-cone thickener. Here the solids settle to the bottom of the thickener with the aid of flocculent. The resulting clarified water overflowing the thickener is collected in a clarified-water tank and pumped back to the main plant for reuse. In total, the plant requires a constant water supply of 800 gal/min. With no mains water supply at Harden, any losses through the product are made up from a natural spring and the quarry’s own borehole.

The deep-cone thickener is fitted with a slowly rotating rake mechanism which draws the thickening sludge to the centre of the cone. From a central extraction well this sludge is drawn off and pumped to a 1.2m wide Parnaby Eco multi-roll filter press. The press has currently completed three months of a six-month extended trial, during which time it has performed well and coped with all the feed presented to it. The press is of a small, space-saving design, allowing it to be mounted inside a 20ft long secure container and making it easy to move should this be necessary.

An adjacent 20ft container houses the flocculent mixing and dosing system. This comprises a Ciba Speciality Chemicals low-cost powder unit with integral dosing pump and control panel. A second, smaller system is used to add cationic flocculent. This container also houses the electrical control panel. From here power is distributed to all the associated drives for the filter press, including the spray water pump, air compressor, filter cake conveyor and return water pump.

According to the manufacturers, Derek Parnaby Cyclones International Ltd, this configuration is capable of treating up to 10 tonnes/h of fine silts/clays contained in up to 250m3 of suspension, and with its containerized arrangement the plant can be quickly installed or moved. Typically, the filter press requires two days to off-load, install and commission, while the modular deep-cone thickener takes one week.

Markets and uses

Traditionally about 40% of Harden’s sales have been shipped overseas to destinations as far afield as Europe and the Far East. A consignment of 400 tonnes was recently dispatched to Japan –– Harden’s most distant customer. The biggest export market of all, however, is Holland, where the Dutch use the product to demarcate cycle lanes, bus lanes and footpaths. Other significant European customers are based in Belgium, Denmark, France and Germany.

Much of the stone is exported via Tarmac’s dedicated shiploading facility on the east coast at nearby Berwick-upon-Tweed. As well as exports, the stone is also delivered by sea to wharves on the river Thames to serve markets in the South.

Harden Red has a diverse range of uses but is particularly distinctive in asphalt mixes, such as Tarmac’s resin-bound Mastertint product. It has been used in many of the bus lanes, car parks and pedestrian zones in London and also features extensively on more prestigious projects such as The Mall and the courtyards of Buckingham Palace.

With its high-visibility characteristics, it is also well suited to use on central reservations and hard shoulders. In Scotland, the material was recently used in the central reservation on the M8 between Edinburgh and Glasgow.

In its unbound state, Harden Red is particularly popular as a decorative stone for landscaping purposes, as shown in a recent episode of the television garden makeover programme Ground Force.

Although the recent replant at Harden will result in increased production and additional sales, the importance of protecting the national park and its environment has not been forgotten. All the new equipment is hidden away in the base of the quarry, out of sight of the general public, and the more modern production techniques now being used are much quieter than the previous plant and generate considerably less dust.     

Acknowledgement

The editor wishes to thank Tarmac Northern Ltd for permission to visit the site and, in particular, Richard Maughan, special products manager, John Hall, regional engineering manager, and Adrian Parnaby of Derek Parnaby Cyclones International Ltd for their help in preparing this report.