Washing Plant boosts production at Tarmac Quarry

First published in the September 2015 issue of Quarry Management as 'Clean Sweep at Broom Quarry'

Installation of the world’s first TWS AggreSand 206 modular washing plant boosts production at Tarmac’s Broom South site

Tarmac’s Broom South sand and gravel site, in Bedfordshire, boasts the world’s first AggreSand 206 modular washing plant from Terex Washing Systems (TWS). Seamlessly combined with an AggreScrub 150, it is processing in excess of 350 tonnes/h to produce three grades of aggregate and two sands.

Tarmac regional operations director Andy Bate commented: ‘This plant has the capability to process double the capacity of the previous fixed plant. It features a very innovative and compact design and provides us with the ultimate in flexibility, especially on sand production, so we can make two sands at varying cuts. We currently produce a 0–4mm concrete sand and a 0–2mm fine sand.’

High capacity, high efficiency and the ability to operate within planning constraints were among Tarmac’s key criteria for the processing plant at Broom, which meant it had to be relatively low level with a low ground pressure footprint. ‘What we got with TWS and Duo plc was a highly efficient plant offering ultimate flexibility and the ability to produce the variations and high specifications we needed,’ said Mr Bate. ‘Working in conjunction with them allowed us to deliver a high-capacity, cost-effective solution to meet varying market needs. We are seeing an increasing market demand in this particular area, but when the need arises we can take this model and transfer it anywhere within our industrial footprint across Tarmac.’

Alex Moss, managing director of Duo plc, commented: ‘This plant has allowed us to work as one in developing a solution with the client. Critical to this process was understanding what Tarmac’s specific demands were so we could bring to the table a solution to meet their needs. Acquiring information on their feed material, end-users and market demands was fundamental in developing a solution specifically for them.’

The AggreSand concept of a modular, pre-wired and pre-plumbed installation offering quick set-up times proved particularly intriguing to Tarmac as a solution to their specific needs, a solution far removed from other wash plant installations they experienced in the past.

The material processed at Broom South requires scrubbing in order to produce a clean aggregate suitable for drainage, decorative stone and concrete production, and an AggreSand 165 was originally selected and installed in June 2014 to meet this need. Although this had been successfully producing thousands of tonnes of high-quality clean aggregate and two in-spec mortar and concrete sands, continued strengthening of the UK aggregates market prompted Tarmac to approach Duo plc once again to investigate how production could be further increased.

At around this time TWS had just announced details of their larger-capacity AggreSand 206, as well as the AggreScrub 150, a paring that was set to become the ideal combination to meet Tarmac’s specific requirements. TWS application and design engineers, together with Duo plc, visited the Broom site to identify and suggest a plant that would address Tarmac’s specific needs. The seamless combination of the new AggreSand 206 and AggreScrub 150 proved particularly appealing to Tarmac in terms of access, serviceability and modularity. Based on the recognized efficiencies and productivity of the previously installed AggreSand 165, combined with the additional capabilities of the larger-capacity AggreSand 206, Tarmac opted for this combination to provide the extra output required. 

AggreSand 206

The new plant consists of a bespoke feed system which can be fed, via a 12ft vibrating grid, by either wheel loader or Tarmac’s existing land conveyor and radial feed conveyor. The main feed conveyor, which is 23m long x 1,050mm wide and inclined at 18°, is driven by two 11kW electric motors and Bonfiglioli gearboxes, mounted on either side of the drive drum, and features Cooper split bearings. The conveyor has an integrated wash box with spray bar to pre-soak the feed material before it reaches the rinsing screen.

The AggreSand 206 at Broom is a three-deck version that utilizes 10 individually controlled spray bars on each deck. The two-bearing screen is fitted with polyurethane modular media on all three decks. The top deck acts as a protection deck, removing any +80mm material. The middle deck is a 40mm passing deck while the bottom deck is split (2mm and 5mm) to produce the two required sand fractions. The unit is fitted with two short conveyors which deliver the oversize and mid-size materials to two 65ft stockpiling conveyors. Chutework on the AggreSand 206 employs the well tested ‘rock-on-rock’ dead-box system, which results in excellent wear properties. 

As the feed material at Broom has a high sand to stone ratio, resulting in a sand throughput approaching 350 tonnes/h, TWS provided an innovative solution whereby the AggreSand 206 single-grade 200 tonnes/h sand plant produces the clean concrete sand, while a TWS FM120C unit is utilized alongside the AggreSand unit to produce the masonry sand. The AggreSand 206 utilizes one 45kW 250/200 pump and two G4-660mm cyclones to produce the clean, in-spec concrete sand, while the FM120C utilizes a 30kW 200/150 pump and one G4-660mm cyclone to produce the in-spec mortar sand. These two sands are  produced on the split bottom deck of the AggreSand aggregate screen. 

A specially designed rubber-lined catchbox allows blending to take place so that an acceptable coarse and fine sand ratio can be achieved. The fine material is captured in one section of the two-grade catchbox and piped to the FM120C units’s sump tank. Material between 40mm and 80mm in size is washed, stockpiled and crushed on a periodic basis, while the material between 5mm and 40mm is fed, via a specially extended (16m) inclined conveyor (17°), to the AggreScrub 150 for scrubbing and the removal of sticky clay agglomerates. 

AggreScrub 150

The AggreScrub 150 consists of an 8m hull, equipped with two rotating shafts fitted with 30mm abrasion-resistant blades. The blades convey the material from the feed end of the scrubber to the aggregate rinsing/sizing screen. As the material travels through the hull the abrasive stone-on-stone action scrubs the aggregate clean and breaks up any clay, while any organics or plastics are floated off and dewatered on the AggreScrub’s 6ft x 2ft trash screen. Underflow from the trash screen is received in a catchbox beneath the screen and gravity fed to the sump tank under the hull.

Meanwhile, scrubbed aggregate exits the main discharge of the AggreScrub 150 on to a two-deck 12ft x 5ft part-rinsing screen fitted with polyurethane modular media to produce 5–10mm, 10–20mm and 20–40mm clean aggregates. The three grades are stockpiled via three TC6532 conveyors. As indicated, the initial section of this screen is fitted with spray bars to rinse off any sand/grit residues produced during scrubbing. This is captured in a partial catchbox under the screen and piped to the sump tank below the hull to join the trash screen underflow. This sump tank is fitted with a 100/100 centrifugal pump which sends the dirty water back to the AggreSand 206 sand plant to recover any sand and maximize the reuse of process water. 

The control system for this particular AggreSand wash plant is housed in a centralized cabin on site, which is one of the options offered by TWS on all AggreSand units; alternatively, the control panel can be mounted on the machine.

Miles Dobson, head of manufacturing at Tarmac, commented: ‘The new plant has given us a great deal of flexibility in the market. We started here with a smaller plant and then, after looking at the change in the market, opted for a larger plant that allows more flexibility to better serve the growing demand.

‘Moreover, we have built a strong relationship with Duo plc and TWS, and are excited about further installations under way at other Tarmac sites. The fantastic thing about the AggreSand plant is its modular design, which allows us to be fast and flexible in the market. It was very swift to install and is running efficiently and producing what we anticipated.’ 

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