Mechanical Technology February 2015

⎪ Materials handling and minerals processing ⎪

Local trommel screens succeed internationally

Multotec is a global leader in trommel screen design and manufacture and has been supplying the market for more than 20 years. Says screening product manager, Anthony Yell: “We deal with most of the major mill manufacturers globally. It is a highly specialised segment that was developed in South Africa and Multotec has built a reputation that now enables us to supply internationally.”

M ultotec’s Anthony Yell says the company has manu- facturing plants in South Africa and Chile and can also supply spare parts from Australia, Chile and the US, which reduces the supply chain dramatically and extends its geographic footprint. “We have sup- plied mill and scrubber trommel screens to the four corners of the earth, from South America to Papua New Guinea and from Siberia to Kazakhstan. There are not many places where they are not prevalent as the Multotec product has proved to be highly reliable.” Yell explains that mill and scrubber trommel screens fit onto the end of a grinding mill or a scrubber by means of a bolt on flange. Sizes range from 950 mm by 1 200 mm up to a massive 6.0 m by 6.0 m. Mill trommel screens are supplied typically into the platinum, gold and copper sectors while scrubber trommel screens are being supplied mainly into the diamond sector. Self-driven trommel screens have their own drive arrangement and are supplied mainly into the heavy mineral sands sector to remove organic material from beach sand, for example, as well as to water treatment and coal briquetting plants. Self-driven trommel screens are supported on both ends, sitting on a ma- chine tyre and with rollers at the bottom. They are driven through the discharge end by either electric or hydraulic drives. “The latter has proved to be very reliable and allows for the flexibility of fully vari- able speed,” Yell says. Trommel screens are designed using software such as finite element analysis (FEA) in order to calculate the stresses in the structure for a full rotation. These stresses must be under a maximum permissible level in order to optimise the fatigue life. Mill and scrubber trommel screens have a design life in excess of five years, while self-driven trommel screens

have a design life of ten years. Yell explains that BS 7608:1993 for Fatigue Design Assessment of Steel Structures is ad- hered to in the design pro- cess. Self-driven trommel screens are also designed

A SAG mill trommel frame with cast polyurethane screen panels and a pinless fastening system.

using FEA and can also be supplied with a feed and discharge arrangement in accordance with specific customer requirements,” Yell says. FEA is also used to calculate the requirement for special cross bracing to counter torsional twisting. “When the mill or scrubber starts up, the trommel wants to turn in the opposite direction for a split second. If the trommel is not braced sufficiently, torsional twisting will occur, which will weaken the trommel structure and ultimately lead to prema- ture failure. In addition, the load in a trommel is shifting continuously, placing ongoing stress on the trommel, which complicates the structure,” Yell says. It is such complexity that means that every single trommel screen is unique. “Multotec custom designs each prod- uct to suit the specific application. We size it; we check the load; we examine the material lump size. When a client requests a trommel screen, our first step is to determine the main parameters. There may be a lot of variation in the di- ameter of the bolt on flange diameter, for example, as it depends on the size of the mill. Thus there is not a standard range apart from the basic design parameters that we use.” Such is Multotec’s experience in this regard that it prides itself on its ability to turn customer quotes around in 24 hours. “We use sophisticated programmes to design the equipment needed, taking into account the basic FEA requirements at tender stage, whereupon an accurate drawing, price and quote can be gener- ated. The speed and efficiency with

which we can respond to a customer’s enquiry put us streets ahead of the rest of the market.” Yell says the key to the success of Multotec’s trommel screens lies in main- taining the integrity of the rubber lining through regular maintenance. “Trommel screens are generally rubber lined. It is critical to ensure the integrity of that rubber lining. If it wears out it needs to be repaired, as a trommel screen can fail prematurely if its structure is eroded.” The rubber lining protects the trommel screen from corrosion and wear and is fundamental to an extended trommel life. “If they are maintained properly, trom- mel screens will last for many years. We have had trommel screens running for over ten years that are still operational,” Yell says. Customers can return trommel screens in order for the rubber lining to be stripped off and the structure subjected to non-destructive testing to determine the integrity of the trommel structure. “If it is still within specification it can be shot blasted and relined, whereupon it will be good to go for many more years,” Yell explains. Multotec has the added advantage of being able to supply polyurethane panels for smaller feed sizes where the main wear factor is sliding abrasion and rubber panels where impact is the wear mechanism. “We design and manufac- ture both rubber and polyurethane panels in house,” Yell says. He adds that a recent trend in mineral processing has been that mills are tending to get big- ger and therefore trommel screens are becoming larger as well. “This can pose

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Mechanical Technology — February 2015