Tarmac invest in state-of-the-art processing technology at key East Anglian sand and gravel operation

Located on the outskirts of the Essex county town from which it takes its name, Colchester Quarry is Tarmac's largest and most important unit in the East Anglian region, with sand and gravel, ready-mixed concrete, asphalt, recycling, dry silo mortar (DSM) and landfill operations, together with a regional administration office, all occupying various parts of the site. The site has been in existence for around 35 years, although it originally comprised two discrete and separately owned quarries (Bellhouse and Stanway) located on either side of Warren Lane. Later, having both been acquired by Tarmac, the two sites were combined and their operations linked by field conveyor via a tunnel under the road.

Until recently extraction operations were centered on the former Stanway site, with material being conveyed to Bellhouse for processing and distribution. However, with diminishing useable material at Stanway and significant reserves still remaining beneath the 20-year-old Bellhouse processing plant, some two and a half years ago Tarmac took the decision to, in effect, reverse the operation.

In order to release the sterilized deposit at Bellhouse, the company opted to construct a brand new state-of-the-art sand and gravel plant on previously worked land at Stanway, shut down and dismantle the Bellhouse processing facility and then reverse the existing field conveyor to supply the new plant, thereby adding to (and eventually supplanting) the current supply of raw feed from the Stanway site.All of these changes were permissible under the site's existing planning consent.

Based on previous successful projects, one of the two main contracts was awarded to sand plant specialists Linatex. The brief from Tarmac was to deliver a plant capable of producing two sizes of gravel (20mm and 10mm) and two grades of sand to precise specifications, the latter comprising around 85% of the plant's total output.

In the early 1980s Linatex introduced their ‘sand on recipe' concept, whereby raw sand would be separated into distinct fractions and stored in silos. Subsequent mixing of these fractions in the correct proportions could then be carried out to produce any required particle size distribution in the final product. Since then further improvements in computer control and automation have led to the development of the Linatex dual in-line blending system (DIBS), which allows two or three separated fractions to be accurately re-blended by computer in one continuous process, with the computer storing the settings for any desired final product.

Designed, built, supplied, installed and commissioned by Linatex, the new computer-controlled sand-classification and dewatering system at Colchester Quarry is capable of handling up to 600,000 tonnes per annum (327 tonnes/h) of -5mm feed material to produce two main specification-controlled products: a close-specification concrete sand, designated ‘T5', for use in airport aprons and runways; and a dry silo mortar sand for use by the new on-site DSM plant. The sand plant is also capable of producing other quality-controlled BS sands for the concrete, mortar and asphalt markets.

The -5mm sand fraction from the washing plant (which can represent as much as 92% of the as-raised raw feed) is presented to the Linatex system, together with 454m³/h (2,000 gal/min) of process water, via a flume arrangement. The feed slurry is discharged directly into a 4.6m diameter feed-regulating sump, which ensures correct mixing and distribution of the feed solids within the process water stream and delivers a consistent feed to the downstream classification processes. A 250/200 Linapump centrifugal slurry pump draws the feed slurry from the sump and delivers it to a 760mm (1030) hydrocyclone, which acts as a pre-wash to remove the majority of unwanted -75µ silts and clays early in the process, thereby reducing downstream loading.

The underflow from the hydrocyclone is directed to a 3.0m x 7.0m HG30/70 single-deck sizing screen which, owing to the requirement for 100% passing 1mm for the DSM mortar sand product, performs a single cut at 1mm to produce a -5mm +1mm grit sand as the first separated fraction. This is collected by a 2.4m diameter sump and delivered by a 100/100 Linapump to a 610mm (HK125) separator for dewatering and presentation to the grit sand blending tank.

Meanwhile, the -1mm underflow from the sizing screen passes to a second 4.6m diameter feed-regulating sump and 250/200 pump for delivery, via two 760mm (830) hydrocyclones, to two 3.0m diameter T-type classifiers (hydrosizers) where the second classification is carried out. Utilizing the phenomenon of ‘hindered settling', the T-type classifiers are configured to perform a ‘cut' at 250µ.

The coarse fraction (-1mm +250µ) underflow is directed to a blending tank as the second discrete fraction for blending purposes, while the -250µ fine fraction overflow from the T-type classifier is fed, via a 4.5m diameter sump and 250/200 Linapump, to two 762mm (HK150) separators where it is dewatered and presented to two fine-fraction blending tanks.

As settling ponds are employed to handle the effluent discharge, the entire system has been configured to maximize process water recirculation. Effluent discharges from the processing equipment are directed to a collecting tank from which two 250/200 Linapump units, arranged in series and with a combined power rating of 235kW, draw and pump 800m³/h (3,500 gal/min) of process water over 1.2km through 327mm bore pipework to the site's settling lagoons. For level control purposes, the first pump in line is inverter controlled and linked to ultrasonic level probes in the collecting tank.

Dual in-line blending system (DIBS)

The grit, coarse sand and fine sand fractions held in the fluidized blending tanks provide buffer capacity and allow the sands to be mixed together in a continuous blending process. All the tanks are monitored using density pressure transmitters for fraction availability and are fitted with electropneumatic discharge valves connected to a state-of-the-art computer control system featuring a PC visualization package. This allows simultaneous production of the primary and secondary sand products to accurate specifications and ensures maximum utilization of the feed stock. Changing product specifications, once they have been set up in the computer, is a simple automated process.

The -5mm +1mm grit fraction primarily discharges to a VD18 dewatering screen for delivery, via a radial conveyor, to a 4,700-tonne capacity ‘T5' concrete sand stockpile.The coarse and fine fractions are blended in the correct proportions by PLC controlled valves, while any unused grit fraction is directed to a VD9 dewatering screen for discharge by static conveyor to a small residue grit sand stockpile. This material can be reintroduced into the feed when there is a deficiency of this fraction in the natural deposit.

The plant's secondary product, DSM sand, is blended according to required proportions and discharged to another 4,700-tonne stockpile via a VD15 dewatering screen and radial conveyor. Any unused fine or coarse fraction is discharged on to a VD12 dewatering screen for stockpiling by static conveyor as a residue.

Following minor post-installation adjustments by Linatex, the plant is now successfully producing the required specification products irrespective of the variable nature of the feed material. Nevertheless, Tarmac's carefully structured approach to the working of the deposit and the blending of the feed material within a target grading envelope is helping to maximize the quantities of the final products produced, while at the same time minimizing the rejection of material to the two residue stockpiles.

Linatex acknowledge QMJ magazine for the permission to reproduce this article in part form.

Further Information:

Linatex Ltd
Wilkinson House,
Galway Road
Blackbushe Business Park
Yateley
Camberley
Hants
GU46 6GE
UK

T: 01252 743000
F: 01252 743030

www.linatex.com