MechChem Africa March-April 2021

⎪ Minerals processing and materials handling ⎪

itself in order to find the best balance be - tween 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 aperturesize, preventingmostparticle from blinding. The elliptical motion tends to throwany stuckparticlebackonto thedeck, deblinding the screen, but a steeper angle of decline is required tomake 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.

A Kwatani scalper screen for the iron ore sector on the test bench undergoing vigorous testing.

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 prob- lem. “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 customrubber panel design that is lasting up to sixmonths. “A meticulously designed steel support structure aligns with our rubber screen panels to maximise the stiffness and the support area, whilemaximising the open area on our large scalping screens. Careful positioning and spacing of the deck beams also prevents under struc- ture 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 fas - tening 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 – fromtwodays tohalf 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, tak- ing 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, abrasionproperties andmuchmore, all of which need to be taken into account. “We adopt a triangle approach that strives tobring together theore 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. q

“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 tominimise blinding onwiremesh screens,” explainsMayhew-Ridgers. “Abrasive materials also cause rapid wear of the wire mesh, while protecting thedeckbeams and runners underneath themeshbecomes 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 tomassive loads: over 40 t ofmaterial canbe 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 5Gs 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,

A Kwatani engineer overseeing the final testing of the scalper screen.

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