MechChem Africa talks to Kenny Mayhew-Ridgers, chief operating officer (COO) of Kwatani, about the company’s design philosophy for large scalping screens, which has resulted in a more than threefold extension of screen panel life, from eight weeks to more than six months.
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“Scalping screens typically sit between the primary and secondary crushers of iron-ore, manganese or platinum mines,” begins Mayhew-Ridgers. “Run-of-mine (ROM) ore delivered from a pit to a primary crusher has a size mix from massive boulders down to very small particles. Typically, using a gyratory crusher, the primary crusher breaks up the larger pieces of ore to a thickness of around 200 mm, ready for further processing in the secondary crusher,” he explains.
Before being passed on to the secondary crusher, however, the mixed size feed is instead diverted to a scalping screen. “Scalping screens relieve the secondary crusher by removing (scalping) the particles that do not need secondary crushing. Only the ‘overs’ that are still sized above the secondary’s size setting, typically 50 to 100 mm, are crushed a second time, while undersized material is removed by scalping screen and diverted further down the line,” Mayhew-Ridgers tells MechChem Africa.
“This immediately increases the capacity of the secondary crusher circuit because less material is being passed through. Also, fines tend to fill voids between course particles, reducing crushing pressures on larger particles. Fines also increase wear rates, reducing crusher life,” he adds. While mines used to take a modular approach using two or three smaller scalper units running in parallel, Mayhew-Ridgers says that today’s trend is to adopt a single line stream using one large screen to do all of the work. “This means that a large storage bin to store several hours of production is needed to cover for any downtime on the scalper. It might take eight hours to replace an exciter and a few worn screen panels, for example, so if the mine has a weekly halt to production, then we have regular opportunities for maintenance – but many mines don’t like halting production at all,” he continues.
“South African operators typically keep at least one spare unit that can be lifted in and out of the line with cranes, but the longer a scalper can continue to operate before it needs to be shut down and/or removed for maintenance, the better,” he tells MechChem Africa
Better designs for a longer life
According to Mayhew-Ridgers, scalping screens are exciter gearbox driven and they operate on a decline that can vary from traditionally quite steep to the gentler declines of modern units. “The steeper the decline, the more momentum the rocks have as they move across the deck and, while this improves throughput, from a screening perspective faster tends to be harder to control, so the stratification efficiency drops and the risk of blinding increases when the stroke of the screen eventually has to be set down in order to control the material flow,” he explains.
Broadly speaking, there are two schools of thought, he continues. Smaller screens use wire mesh screening media, which require steeper angles of decline. This presents the largest open screening area, which makes the scalper size efficient, that is, a smaller unit can theoretically deliver the same throughput.
The second option is to use rubber or polyurethane screen panels, which present less open area, but deliver a range of other advantages. “We at Kwatani have spent a lot of time and energy looking at integrated designs that include the screen panel, because panel design has to be closely matched to the design philosophy of the scalping screen itself in order to find the best balance between screening area, aperture layout and screen panel life,” he explains, adding that an understanding of the screen dynamics comes first. “Only then can one design a screen panel to best suits these dynamics,” he advises.
If using wire mesh, an elliptical motion on the screen surface is created using an unbalanced ‘swinging’ load on the exciter. This suits wire mesh because of its rigid aperture size, preventing most particle from blinding. The elliptical motion tends to throw any stuck particle back onto the deck, deblinding the screen, but a steeper angle of decline is required to make sure that the bottom edges of the mesh are lower than the top edges so particles can tumble over the apertures instead of becoming wedged.
“With larger scalpers such as single-line 7 500 t/h units, circular drives are not able to provide enough elliptical movement on the deck to create this effect, so linear motion exciters have to be used. The benefit here is the direction of vibration can be precisely chosen simply by adjusting the mounting angle of the exciter gearbox. But linear exciters cannot produce the elliptical tumbling motion needed to minimise blinding on wire mesh screens,” explains Mayhew-Ridgers.
“Abrasive materials also cause rapid wear of the wire mesh, while protecting the deck beams and runners underneath the mesh becomes complicated,” he adds.
“So for bigger screens, especially those being used with abrasive materials, we prefer to use bolt in rubber or polyurethane panels. Although these offer less open area, the panels put more material in direct contact with the ore, which can significantly slow the wear rate,” he says.
“A scalping screen with 7 500 t/h of ore being ‘bounced’ over its screens is being subjected to massive loads: over 40 t of material can be on the screen deck at any time. So, a very strong supporting structure is needed, not only to support the weight of 40 t of material, but also to accelerate it to between 4 and 5 Gs so enough energy is transferred to give stratification. And without stratification, the fines will not be removed and the machine will not function as a scalper, especially if the material is being moved through too fast.
“With rubber or polyurethane screens, a decline is still needed, but this must be gentler to prevent the material simply flowing over the screen. Very important, though, is the stiffness of the screen bed.”
Explaining, Mayhew-Ridgers says the linear exciters used for dry applications operate at lower speed with a bigger stroke, about
750 rpm with a 15 mm stroke. “A 15 mm stroke is small compared to a particle size of 150 mm. If the panel bed is too flexible, and it could sag up to 10 mm when accelerated, then a stuck particle would only experience 5.0 mm of the applied stroke, preventing material on the deck from being stratified.
“So, the stiffness of the supporting structure and the design of screen panels have to go hand in hand to get the results we need,” he informs MechChem Africa.
While polyurethane designed panels are strong and lightweight, he says the screening apertures in the panel tend to be too stiff for the heavy-duty scalping applications, so blinding becomes a bigger problem. “Rubber overcomes this problem, while also delivering improved wear life. Over the past eight years, we have come from a scalping design that used to have its screen panels replaced every six to eight weeks, to a custom rubber panel design that is lasting up to six months.
“A meticulously designed steel support structure aligns with our rubber screen panels to maximise the stiffness and the support area, while maximising the open area on our large scalping screens. Careful positioning and spacing of the deck beams also prevents under structure wear from the fines passing through,” he says, adding that since 2012, throughput of Kwatani’s scalper designs has been increased up to 1 500 t/h for the same size screens.
“We have also simplified panel replacement by incorporating a fastening mechanism on the underframe that enables the screen panels to be bolted in and removed from the top, without having to secure nuts underneath. This drastically improves safety and reduces panel replacement times – from two days to half a day for large screens – and it enables individual panels to be replaced quickly and in-situ when the need arises – within 10 minutes per panel, in some instances, taking into account that these panels weigh around 100 kg each and are bolted down with eight bolts to ensure the panel becomes part of the structural deck frame.”
As with all screen designs, it is important to ensure that scalping solutions are suited to the ore coming out of the mine. “Each ore has a different density, stickiness, wear, abrasion properties and much more, all of which need to be taken into account.
“We adopt a triangle approach that strives to bring together the ore properties, the screening media and the screen design parameters. It is by accommodating the parameters in all three of these areas that we are able to deliver optimised screening solutions that perform as efficiently as expected while delivering substantially improved wear life and lower operating costs,” concludes Mayhew-Ridgers.
