A blinding success
Hanson’s Forestwood Quarry overcomes problem of excessive blinding during wet weather thanks to Metso’s Trellex TFX rubber screen media
Wales is renowned for many things – but dry weather is not one of them. A combination of being close to the Atlantic and having lots of mountains means that it rains. A lot. This is good for growing grass to feed the country’s 10 million sheep but bad news when fine screening limestone using conventional steel wire mesh, which tends to clog and blind over in damp weather.
This was the problem facing Hanson’s Forestwood Quarry in Pontyclun, Mid Glamorgan. The blinding led to potential size and cleanliness quality problems with finished products, with the 10mm and smaller fractions in danger of being out of specification. It also meant that two or three times a day in bad weather the screen would have to be shut down and site engineers asked to climb into the screen and clear the blinding using rubber mallets. Taking up to an hour each time, this unpleasant job in a confined space also resulted in a significant loss of productivity, estimated to be between 20,000 and 30,000 tonnes a year.
Howell Bryant, quarry foreman, who has been at Forestwood since the early 1970s, commented: ‘You’d be in there on your hands and knees, and it was a dirty stinking job. It wasn’t a hard job, but you’d rather not do it three times a day. But since we fitted the new system we’ve only been in twice this year – they are a million times better’.
The ‘new’ system is Trellex TFX rubber mats (previously branded as Trellex Superflex) from Metso. Specified and supplied by Metso’s area manager Gareth Brooks, Trellex TFX is said to excel in conditions of up to 7% moisture, thanks to its flexibility, which helps prevent the material from blinding over.
Forestwood Quarry was opened in the 1950s and acquired by Hanson in 2001. The quarry’s plant was installed in the late 1990s by Unifloc and consists of a BJD rotary impact crusher, a SBM rotary impactor and four Hewitt & Robins screening modules.
A team of 10 men extracts and processes a quarter of a million tonnes of limestone (psv = 55) a year. One of eight Hanson quarries in the area, products include: 40mm, 28mm, 20mm, 14mm, 10mm, 6mm and dust. They are used for base course material, drainage material, capping and aggregate for a premix concrete plant that shares the site with the quarry. Interestingly, over 400 tonnes a month of 6mm stone is supplied for use as cat litter!
Richard Grey, production supervisor, was the driving force for the quarry’s switch from wire media to Trellex TFX rubber mats.
‘We were having a terrible problem with the Module 4 fine screen that produces 10mm, 6mm and dust,’ he said. ‘It was constantly blinding up in wet weather. The Trellex TFX system was more expensive but as it needs almost no unclogging – and therefore no production stoppages – the payback in terms of increased efficiency was very rapid. The lifespan of the rubber is also considerably more than the metal wire – by up to five times. Other than checking the tension and pinching up the bolts once a month, they will require almost no maintenance during their expected lifespan. This frees up our engineers for other duties, as they are not constantly being asked to clean the screens anymore.’
Flexible solution
The Trellex TFX system is available in several thicknesses with molded and punched apertures. Installed on screens with longitudinal supports, they can be fitted on cross and longitudinally tensioned screening machines. Their effectiveness lies in the extreme flexibility of the rubber screen ‘cloth’, where constant movement inhibits the material from blinding or pegging. The mats are reinforced with integral cords that maintain tension and provide accurate separations. No deck modifications are needed to the screen and panels are fitted with wear strips to protect the panel from wear from the camber bars. While particularly effective on limestone, Trellex TFX works well on a range of material types where blinding and pegging is a problem, including clay, coal, gravel and sand.
Metso supplied two engineers to fit the new deck and ensure it was installed properly. They also showed the quarry fitters how to install and maintain them correctly. Since then the Trellex TFX mats have been largely trouble free. No screen can cope in extremely wet conditions, and even the Trellex TFX mats can blind over, but only twice in eight months – rather than a twice daily occurrence. This guarantees that the aggregates are clean and of the right size whatever the weather, and as they are so durable they will not need to be replaced for some time. Richard Grey is so pleased with them that he is considering using Trellex TFX for the top deck of the screen as well.
