Modern Quarrying Q2 2019

stroke ensures the material is lifted just enough to stratify (separate into layers of different sized material). Since the opening (and open area) of the screen is so small, a high frequency offers substantially more opportunities for the particles to find an open hole. Bed depth When it comes to bed depth – the thickness of the layer of material on a screen deck – Marais says that ideally a material bed depth at the discharge end of a screen should be a maximum of four times the aperture size of the screening media. “Too much material (high bed-depth) on a screen deck minimises the chances of under-size material finding an opening to fall through. Often relieving decks (a deck that’s used to limit the amount of material on the sizing deck) is used to lower the bed- depth on screens,” says Abelho. “As an illustration, a client screening out two products, a -40mm+6mm and a -6 mm product, may have to utilise a three-deck screen with a 20 mm relieving deck to ensure the bed depth on the 6 mm mesh is not too high. As a rule of thumb, the aperture above a deck shouldn’t be higher than a ratio of 3:1 or 5:1 (at a push), i.e. 6 mm square mesh should have a maximum 30 mm mesh. This not only ensures a good bed depth, but also minimises wear on the 6 mm mesh,” adds Abelho. He adds that too little material could cause the material to bounce too high and often minimising the chances of it finding an open aperture, especially on fine screening. Screening media When it comes to screening media, Abelho says knowing the type of media to be used is important as different types offer differ- ent open area percentages. Without a good knowledge of the open area, you can easily spec a screen that’s too small (or large). “The operating conditions, product shape, abrasiveness and final specifications are all significant when selecting the type of media to use. For example, the most popular media, plain square wire mesh, is good in dry conditions, but is easily blinded (when particles stick to the mesh effectively blocking your open area) by moisture when used with smaller apertures. More special- ised self-cleaning meshes are expensive but can be used in the rainy season, for exam- ple, when moisture is a problem.” Where the final product shape is not critical, a slotted mesh can be used. Slotted mesh has an opening with a width the same as your product size, but a length between

The smaller the aperture on your screen, the smaller the open area.

that efficiently screens out a 100 tph of -40 mm product may be too small to screen out 50 tph of a -6mm prod- uct. This is as a result of the 40 mm aperture mesh which has roughly 70% open area versus 40% on a typical 6 mm wire mesh,” explains Abelho. Speaking about Stroke (also referred to as throw) vs. frequency of a screen, Abelho says most screens operate at a maximum G-Force of up to about 5 g’s, with 4,6 g’s being a good industry average of the maxi- mum G-Force allowed on a screen to avoid it self-destructing. The amount of G-Force is a combination of the stroke and frequency, so typically a high frequency screen will have a short stroke, and vice-versa. “For larger aperture screening a longer stroke and lower frequency screen is more suitable. A longer stroke ensures there is less chance of pegging (material that gets stuck in the open aperture), while the lower frequency ensures a longer bearing life and gives the larger material a better opportunity to pass through the open aperture,” adds Abelho. For smaller aperture screening, Abelho says a short stroke and high frequency is advisable. A short

• Moisture – high moisture content hinders stratification; and • Bed Depth – the depth of the material being screened must not exceed 3-4 times the opening size of the screen aperture. According to Abelho, when it comes to screening efficiency, there are often factors that are crucial to a specific site but not necessarily a factor on another site, so site infor- mation is critical to make a proper assessment. “Calculating screening efficiency ‘by hand’ involve pages of formu- las (and different methods), it also requires detailed information of the PSD (Product Size Distribution) of the feed material. Modern programs such as Metso’s Bruno greatly simplify and automate these calcu- lations to a great extent. That said, we can scratch the surface of screen selection with some of the follow- ing basic factors that apply across the board and should be carefully considered,” says Abelho. Screen deck and stroke The smaller the aperture on your screening deck, the smaller the open area (the amount of open space in the screening media that allows your product to fall through). “A screen

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QUARTER 2 - 2019 MODERN QUARRYING