Mechanical Technology February 2015

⎪ Materials handling and minerals processing ⎪

Above: Anthony Yell, screening product manager, Multotec, with a SAG mill trommel with modular Injection moulded screen panels. Right: The 3A self-driven trommel installation at Grande Côte, Senegal.

is installed and replaced from inside the trommel, generally using polyurethane pins and sleeves. Our solutions mean that that the panels are easy to remove and replace safely. Another issue related to panel wear is ‘pebble porting’ in semi-autogenous (SAG) milling, which sometimes results in hard rounded pebbles that cannot be economically ground down any further being ejected from the mill and sent to secondary crushing. These rounded pebbles, with diameters from 50 mm to 80 mm, can account for up to 20 % of the trommel feed needed to be screened out. “They tend to be abrasive and cause excessive panel wear. Multotec has developed a range of panels that can handle these pebbles and still achieve acceptable life,” Yell says. “The most important factor for us is to understand the trommel duty and the load coming off the end, because we have to design the front-end of the trommel to be able to take this load.” Yell points out. “We have focused investment in developing designs for many years and our trommel screen designs have proven themselves to be exceptionally reliable,” Yell says. “As long as they are maintained properly in terms of the rubber lining and panel replacement then they will last a long time.” q

the high wear rates of the screen media due to the high peripheral speed of the trommel screen itself.” In addition, the trend towards larger mills means higher flow rates and velocities. “Double the velocity on a polyurethane screen equates to an increase in the wear rate by a factor of five,” Yell notes. “We have invested extensive research and development into producing panels that last longer, as with large copper pro- cessing plants where the large tonnages mean an average run time of 12 to 14 weeks without any stoppages for panel change outs. It has been our objective to develop panels that can last that length of time,” says Yell. This usually requires that trommel screen panels become bigger and heavier. “On the smaller trommel screens we use modular injection moulded panels of 305 mm by 305 mm up to around 2.5 m in diameter. For sizes above that it is more cost effective to manufacture larger panels in either rubber or polyurethane. The chal- lenge is manufacturing panels that are not too large or too heavy for two people to lift and install. Otherwise specialised lifting equipment is required which then complicates the re-lining procedure.” From a safety perspective, Yell notes that all the panels are fastened from inside the trommel screen. Everything

challenges in terms of acceptable life of screen media. However, we do have solutions for this,” Yell says. “Mill manufacturers often increase throughputs by accelerating the dis- charge rate out of mills by using so called pulp lifters, which function a bit like a pump impeller to propel the material out of the mill at a higher velocity. The trom- mel design implication is that we need to stop the feed landing half way down the trommel screen as this reduces trommel efficiency and causes high wear to the screen media. “In order to combat this we have designed what we call a baffle plate placed at the feed end of the trom- mel that forces the feed down onto the trommel surface to improve screening efficiency,” Yell continues. “If the trommel screen rotates in one direction only, we recommend installing an advancing spiral or scroll in order to be able to control the retention time of the material in the trommel. “The scroll pitch can be designed to increase or reduce the retention time accordingly. Furthermore, single or twin start scrolls can be used to further vary the retention time and im- prove screening efficiency when required. There is a great deal that can be done to optimise output,” Yell says. “One of the biggest problems we have today with the larger trommel screens is

Mechanical Technology — February 2015

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