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Screens & Feeders



Clean graded rock separated from fines.
Clean graded rock separated from fines.
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Heavy duty trommels leave no stone unturned

The resurgence of the trommel screen has coincided with an upsurge in interest from quarry operators about reworking their waste reserves.
Demand for good quality stone in the UK has been rising steadily and although in many areas the quarrying industry is in the middle of reduced outputs, operators are still looking at ways to maximise their potential reserves in order to make their operations both economic and profitable.

Restrictions on new quarry developments mean that keeping production up with demand has become a challenge. Companies are now looking at the vast hidden reserves of waste left behind as the quarrying of more accessible good rock became the prime objective. As the good rock reserves grow less abundant, new methods, both in cleaning and sizing, have had to be developed to capitalise on these material assets.

The development of the heavy duty trommel screen by UK supplier Stelex Construction has spearheaded this recent trend and has resulted in very high recovery rates of good, clean rock from overburden waste which, in turn, have led to an increase in company operating profits.

A SHORT HISTORY
Trommel screens have been used since mechanised quarrying began in the late 18th century. Back then, the screens consisted of circular steel meshes bolted together to form a single barrel that rotated around a long, central shaft set with a slight inclination. Crushed material travelling within the cylinder was graded out, according to the size of the apertures making up the barrel. These screens were used extensively until vibrating flat bed types came onto the market in the 1950s. 

The development of bigger, more robust, heavy duty, high capacity trommels in the 1980s for the production of rip rap and armour stone paved the way for trommels purpose-built to handle high tonnages of material and lump sizes of up to one metre. The rolling action of the barrel breaks up any unwanted clays to produce a clean product. The barrel is itself self-cleaning and the entire unit can operate without manual intervention.

In 2006, Stelex, with over 15 years of experience in the manufacture of this type of screen, decided to redesign and expand its Hercules product range to greatly improve the machine, both in built quality and operating performance. The resulting generation of trommels feature a “live drive barrel” that eliminates the need for gear wheels and chains and offers a smoother, more reliable and maintenance free drive to the barrel.

Other features include a hard steel feed hopper with high wear resistance internals, which can accommodate up to 160 tonnes, as well as a variable speed reciprocating feed table, which is supported on rollers and moves material into the barrel via hydraulic rams. The barrels themselves are manufactured from high grade steel bars welded to form segments that are then bolted together. Aperture sizes are made up to suit individual customers’ product requirements.

As well as cleaning dirty and contaminated feed materials, trommels have been used in both quarrying and mining for taking out fines prior to material being fed to a crushing station. They have also been used to produce high volumes of sea/river defence rock and sized material to make up Gabian baskets for road and rail cutting support.

INTERNATIONAL CASE STUDIES

Qatar
Here, the duty was to deliver 30 million tonnes of rock for the construction of a new breakwater to provide additional deep water facilities for large container vessels at the Ras Laffan port.

The machines supplied were two model HT232s, each with a barrel diameter of 2.3m and capable of handling throughputs of up to 1000 tonnes per hour of as-dug rock. Operating 24 hours per day, seven days per week, reliability and low maintenance was the key to this project. Both machines supplied clean sized rock to a fleet of transport vehicles for delivery to the construction yards. Drive for both the feeder and barrel was provided by a separate remotely stationed electric/hydraulic power pack. As part of the contract the machines were specified to work in temperatures of up to 50oC. These machines are the largest Hercules trommels operating outside the UK.

Southern USA
Here, the duty was to clean as-dug quarried materials to produce rip rap sizes of 304mm (12 inches), 457mm (18 inches) and oversize at a rate of 500 tonnes per hour. The machine, a model HT182M, is the latest generation of the fully mobile trommel, designed to be brought in, set up quickly and put to work, then towed to another site to repeat the operation. The clean graded rock was loaded from the quarry into large river barges which each carry 10,000 tonnes. The barges are linked together to transport the material down the Mississippi river to New Orleans to repair the levees damaged by Hurricane Katrina. The contract called for over two million tonnes of rip rap to be supplied.

Germany
Here, a quarry operator needed a machine to clean and size as-dug materials from waste in a large limestone quarry in preparation for cement manufacture.

A mobile HT182M with a 1.8m diameter barrel mounted on a special support frame  to segregate products was supplied for  this task.

France
Two customers with recycling plants were experiencing very high wear rates on their existing screens and were not able to  cope with heavy builders’ waste, including bricks and concrete, from industrial and domestic skips.

For primary sorting, Stelex installed heavy duty trommels fitted with heavy duty perforated plate segments, together with chutes and bin walls to eliminate contamination and enable screened materials to be conveyed away for secondary sorting.

In 2009, Stelex launched a new design of recycling barrel that features easily replaceable punch plate aperture panels. The panels slide easily into the barrel and are locked in place by heavy duty lifters. The benefits of such machines lie in the recycling market where operators need a compact, well engineered yet heavy duty machine with a ‘switch-on and forget’ reliability.

FUTURE PROSPECTS
The increasing requirement to reclaim and recycle a greater proportion of primary rock, and the need to clean materials containing heavy soils and clays will ensure that heavy duty trommels will continue to play a major role in a wide spectrum of applications within the extractive industries. The latest machines have proved to be extremely efficient in delivering clean rock products and will pay back the investment involved.

Nigel Moreton is the principal of NMC Media, which provides a full range of press and PR services for the UK quarrying, mining, construction and recycling industries.

This article is reprinted with the kind permission of Quarry Management (UK). It originally appeared in QM in June 2009.









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Tuesday, 15 October, 2019 2:42am
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