Tarmac Increase Production with New DUO Installation


DUO has recently installed a new 250tph aggregate processing plant for Tarmac Ltd at their Blashford site near Ringwood in Hampshire. The plant incorporates washing, screening and crushing solutions, and is supported by water treatment technologies to deliver Tarmac’s desired production levels and product quality.

Tarmac, the UK’s largest quarrying company and supplier of building materials to the construction industry, are a market leader in aggregate supply in the UK. The Blashford site is one of over 100 UK quarries currently operated by Tarmac and is currently producing 290,000 tonnes/yr of BSI approved quality sand & gravel.

Tarmac have been operating the site for 30 years and with planning permission for 6 million tonnes granted in 2008 it became obvious that a replant was necessary as the existing plant would not cope with the required production.

A Successful Tender
With several companies involved in the tender process, DUO and Tarmac worked together over a two year period to evaluate the various options. With over 30 years of experience in designing and installing plants of this type, the DUO solution offered the best value in terms of high quality equipment and installation. Also a critical factor in the successful tender was the ability of DUO to achieve both the health & safety and environmental standards set by Tarmac.

With the Tarmac goal of attaining their Zero Harm target, and where possible, exceeding legal health and safety requirements DUO worked throughout the tender, design and installation stages with Tarmac to minimize the risks to those on-site. The plant design also took into account the requirements of ISO 14001 in minimising any processes that would negatively affect the environment.

Richard Bedford – Unit Manager, commented, “the new plant gives us much more control and flexibility over our production, it enables us to produce more of the products we are selling; rather than just what is naturally found in the deposit."

The Installation
The plant is of modular design and has the facility to crush gravel products as required, to maximize the sand and 20mm products. Designed to process 320,000t/annum of sand and gravel, the new plant produces the required products of grit sand, 10mm, 20mm, 40mm and +40mm/-75mm.

Material is sourced from a dry extraction area 2.5 kilometres away, and is delivered by an existing ground conveyor to the new DUO wash plant where a fully galvanised radial conveyor stockpiles the material over a concrete recovery tunnel which provides a controlled feed onto the main conveyor utilising three heavy duty belt feeders.

The material from the recovery tunnel enters the first processing stage as it is delivered onto a Powerscreen 20 x 6 double-deck rinsing screen. Designed for maximum production efficiency, the rising screen splits the feed into three sizes: +75mm which is sent to a rejects bay and a -75mm/+5mm: and a sand.

“The Powerscreen Rising Screens are tried and trusted across multiple applications, particularly sand and gravel” explained John Bracken, the DUO Sales Manager who managed the tender process. He added “This is just one of several Powerscreen components utilized in the project; incorporating Powerscreen products ensures a high level of manufacturing quality and provides Tarmac with a brand they can trust”.

The sand from the rising screen is directed to a Finesmaster 120 compact sand plant. This versatile machine utilises a cyclone, instead of a bucket wheel, which removes clays, silts and slime from the sand and produces a dry sand. Depending on the feed material, sand production levels of up to 90 tph are achievable with the product, meeting the high standards that Tarmac and its customers’ demand.

The -75mm/+5mm material from the rising screen is fed into a Powerscrub 160 logwasher which creates a high speed scrubbing action through the rotation of two axles within a water-filled trough. Any unwanted materials, such as organics, that are mixed in with the feed material are floated off to a trash screen at the rear of the machine. Following the vigorous cleaning action the clean washed material is delivered to a dewatering screen before a being fed to another screen via a conveyor.

A second Powerscreen 20 x 6 Rinsing Screen, this time a triple deck, receives the material and splits it into four fractions: 10mm, 20mm, 40mm and + 40mm/-75mm. The material, which is again rinsed to remove any remaining contaminates, is delivered to stockpiling conveyors via a splitter box. The exception to this is the -75mm/+40mm material which is redirected to a surge bin.

This 20 ton surge bin, which is controllable by the plant operator, offers three options. The -75mm/+40mm can be fed into the VSI crusher. This machine handles approximately 20% of the daily feed, depositing crushed material onto the inclined re-feed conveyor for recirculation back into the system.

Alternatively, the material can be dropped into a rejects bay to be dealt with at the plant operator’s discretion. The third option is to store the material in the surge bin should the plant operator wish to temporarily delay the crushing operation.

Water Treatment
The washing system is supported by a DUO water treatment solution which provides a significant reduction in on-site water requirements, providing a substantial cost reduction over time. The water treatment solution includes a thickener tank, a clarified water tank, a flocculent dosing system and supporting equipment.

The first phase of the water treatment plant involves the pumping of the used water from the wash plant to the thickener tank. As the used water enters the thickener tank it is mixed with automatically prepared and automatically dosed flocculent. This flocculent operation is controlled on-site through the PLC and is housed in a flocculent cabin which was installed complete with internal heating and lighting.

With the ability to house a deep cone thickener tank, the design of the system negates the need for an internal rake system (still the system of choice for many less innovative companies). The tank offers lower maintenance costs as no moving parts are required; consequently minimising the risk of downtime.

With the thickened sludge pumped to a lagoon over 1 km away the clarified water weirs over the top of the thickener and is collected in an adjacent water storage tank, ready for re-use in the wash plant. This steel tank has been installed with ‘high/low’ sensors to provide the plant operators’ with maximum control over the plant’s operation.

The plant in its entirety is an impressive sight with all of the components benefiting from fully galvanised support structures. In addition to the galvanised support structures, bespoke handrails designed specifically to meet Tarmac’s requirements surround the walkways as well as the top deck of both screens and the entire circumference of the civils.

All of the supporting structures, walkways and handrails at the Blashford site were designed and manufactured by the DUO Manufacturing division based in Frome, Somerset.