GRA opt for new Trellex screen system
Following a careful period of testing and assessment, Valence-based Granulats Rhone Alpes (GRA), part of the Vicat Group, have become the first sand and gravel plant in France to switch to the new Trellex modular step screen system developed by Metso Minerals.
GRA wash and separate some 650,000 tonnes of material to produce eight grades of crushed limestone and a further three grades of silica rolling material at their site each year, to supply to Vicat Group’s concrete and cement operations. Crushed stone is screened into European grades of 0/2, 0/4, 4/6, 6/10, 10/14, 10/20, 0/20 and 0/31.5, and the natural rolling material to grades of 0/4, 5.6/11.2 and 10/20.
Following successful operation over the past 18 months on a 2m section of GRA’s Metso Dragon VP3 five-screen separator installation, the top deck has now been fitted entirely with Trellex 300 LS polyurethane modular step screen media.
‘Having experienced damage to the existing frame of our machine due to poor assembly of the previous screen system, we were very disappointed,’ said plant manager Patrick Sahy. ‘But Metso adapted special rails to accommodate their new media very quickly and easily.’
The Trellex ‘snap-on’ frame is based on steel profiles that are easy to install and require no maintenance. The screen frame is protected by the modules and both the profiles and the modules are lightweight and easy to handle, minimizing the risk of injury and accidents.
The new screen has a total length of 5m over the 1.5m working width of the deck, to which 300 tonnes of rolled, natural stone are fed every hour. In addition to being graded, the stone is also washed on the top deck, with water fed both from the feedbox and sprayed on to the deck at intervals.
‘Metso provided the new screen media with holes of 25mm to separate stones of 22.4mm and below. The calculation of the hole size was based on the strict specifications laid down by the concrete industry and had to take into account the thickness of the media itself,’ explained Metso Minerals’ regional sales manager, Alain Trouillet.
Polyurethane offers a number of benefits over other media, including its high resistance to sliding wear, especially in wet and fine applications, such as those being carried out by GRA. The low friction on the surface when water is employed further enhances the throughput, while the special properties of the precision-moulded apertures ensure consistent accurate screening.
Each step of the new Metso screen is completely flat, with no cambers or dips to hold back fines. Material is spread evenly over the entire surface to ensure that every square centimetre of screening area contributes to better performance and more accurate screening. The cascade generated by the step design also continuously breaks up the layer of feed material so that smaller particles are screened out faster. Stratification is maximized and fines are minimized from step to step, resulting in faster screening and more effective separation of critical sizes.
Another innovative feature of the modular system is the snap-in-place deflector, which is used for areas of the deck where no holes are present, thereby ensuring no suitably-sized stones escape calibration.
Already installed at major mineral-processing facilities across Europe, Metso Minerals say the new Trellex system has demonstrated its ability to deliver a powerful three-way pay-off for fine and intermediate materials, providing better screening accuracy, maximum service life and higher throughput.
Metso Minerals, Wear Protection & Conveying, PO Box 74, SE-231 21 Trelleborg, Sweden; tel: +46 410 525 73; fax: +46 705 582 573.