Tarmac, the UK?s largest quarrying company and supplier of building materials to the construction industry, has recently installed a 250 tonne per hour aggregate processing plant at its Blashford site in Hampshire, UK. The plant incorporates washing, screening and crushing solutions and is supported by water treatment technologies to deliver Tarmac?s desired production levels and the high quality product it requires.
Tarmac is a market leader in aggregate supply in the UK. The Blashford site is one of more than 100 UK quarries that Tarmac operates and the site is currently producing 290,000 tonnes per year of British Standards approved quality sand and gravel.
Tarmac has been operating the Blashford site for 30 years and with planning permission for six million tonnes granted in 2008, it became obvious that a replant was necessary because the existing plant would not cope with the required production.
SUCCESSFUL TENDER
With several companies involved in the tender process, DUO, a supplier of comprehensive package solutions for the quarrying, aggregate and recycling industries, and Tarmac evaluated the various options over a two year period. With more than 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. A critical factor in DUO?s successful tender was its ability to achieve Tarmac?s strict health and safety and environmental standards.
With the Tarmac goal of attaining a zero harm target and, where possible, exceeding legal health and safety requirements, DUO worked throughout the tender, design and installation stages with Tarmac to minimise 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.
?The new plant gives us much more control and flexibility over our production,? unit manager Richard Bedford explained. ?And 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 sand processing plant is of a modular design and has the facility to crush gravel products as required, to maximise the sand and 20mm products. Designed to process 320,000 tonnes per annum of sand and gravel, the new plant produces the required products of grit sand, 10mm, 20mm, 40mm and +40mm/-75mm.
{{image2-a:r-w:225}}Material is sourced from a dry extraction area 2.5km 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 rinsing screen splits the feed into three sizes: +75mm, which is sent to a rejects bay, a -75mm/+5mm, and sand.
?The Powerscreen rinsing screens are tried and trusted across multiple applications, particularly sand and gravel,? DUO sales manager John Bracken, who managed the tender process, explained. ?This is just one of several Powerscreen components utilised in the project. Incorporating Powerscreen products ensures a high level of manufacturing quality and provides Tarmac with a brand it can trust.?
The sand from the rinsing screen is directed to a Finesmaster 120 compact sand plant. This machine utilises a cyclone, instead of a bucket wheel, which removes clays, silts and slime from the sand and produces 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 rinsing 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 it is 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 -75mm/+40mm. 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 that is redirected to a surge bin.
This 20-tonne surge bin, which is controlled by the plant operator, offers three options. The -75mm/+40mm can be fed into the VSI crusher. This machine handles about 20 per cent 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 that 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 flocculant 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 flocculant. This flocculant operation is controlled on-site through the PLC and is housed in a flocculant cabin, which was installed complete with internal heating and lighting.
{{image3-a:r-w:225}}With the ability to house a deep cone thickener tank, the design of the system negates the need for an internal rake. The tank offers lower maintenance costs because no moving parts are required, consequently minimising the risk of downtime.
With the thickened sludge pumped to a lagoon more than a kilometre away, the clarified water weirs over the top of the thickener and is collected in an adjacent water storage tank, ready for reuse 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 impressive, with all the components benefitting from fully galvanised support structures. In addition to the galvanised support structures, bespoke handrails designed to meet Tarmac?s requirements surround the walkways as well as the top deck of both screens and the entire circumference of the civils.
The exclusive dealer for Terex Washing Systems in Queensland, New South Wales, Victoria, Tasmania and South Australia is Finlay Screening and Crushing Systems. The distributor for Powerscreen products in Australia is The Lincom Group.
Source: Finlay Crushing & Screening/Terex Washing Systems