Mechanical screening can never be 100% accurate, but even so, if you aren’t satisfied with your product there could be a straightforward explanation and a painless remedy. It is important to understand the root cause of any problems so the right solutions can be found. In the article below we’ve identified some of the common causes of inefficient screening, as well as outlining some remedial actions.
One of the most common issues for screen operators is that output doesn’t match their expectations. Regardless of whether there is too much undersize in the overflow, too much oversize in the underflow, or even a combination of both, it’s advisable to first check these three areas:
It’s important to keep a close eye on the feed material. If the material delivered from the crusher isn’t appropriate to the screen set-up, then unexpected results will arise. For example, perhaps the crusher is producing material that exceeds the maximum feed size or is delivering a load which is too large for the screen to process. When the product from a screen is unsatisfactory, it is easy to assume that the screen is not operating correctly, but often the source of the problem lies earlier in the process.
Depending on the type of screen that’s being used, there can be a number of adjustable settings available to you. If we take Sandvik’s SL horizontal screen series as an example, it’s possible to change the inclination angle as well as several attributes of the screen motion (the shape of the stroke, the stroke angle, the stroke length and the rotation speed). There may also be an issue related to the screening media. It may be worn, damaged, or blocked, but it might also be possible that the media is not compatible with screening that’s being attempting. For example, the material, aperture size, or aperture shape might not be appropriate.
A further complication is that these factors are interdependent. For example, if the inclination angle to the horizontal is increased, then the effective aperture will be reduced, so screening media with larger apertures may be required to achieve the same separation. With this in mind, it may be worth seeking expert advice when configuring or re-configuring screen settings.
As the angle of inclination increases, the space for particles to fall through effectively becomes smaller
A critical look at the arrangement of the feed material on the screen decks can often be revealing. If you notice that the material is not evenly distributed across the width of the screen, then screening performance is almost certainly sub-optimal. This can often happen if material is loaded directly onto the screen without a feedbox. Off-center loading can also occur when different feed materials from different chutes are loaded onto one screen. In both these scenarios, the likely result will be an increase in the carryover of undersize material.
Bed depth also greatly affects screening. You can find a more detailed discussion of bed depth in the The right screen for the job chapter of the Knowledge Hub, but to quickly summarize, the depth should be proportional to the desired separation. The depth decreases as the material travels along the screen deck, of course, so the optimal screen depth is different at the feed and discharge ends of the screen. At the feed end, the upper limit for bed depth is typically 10 times the desired separation, while at the discharge end the upper limit is typically 4 times the desired separation.
If the material bed is too deep, material will not have enough time to stratify properly and there will therefore be excessive carryover of undersize material. Overloading the screen in this way can also result in increased wear to the screening media and shorter life, and can even restrict the movement of the screen, and thus reduce its effectiveness. If the material bed is too shallow, then material will bounce along the screen deck, without sufficient opportunities to fall through an aperture. The end result – excessive undersize material in the overflow – is likely to be the same.
In this example, the feed is concentrated on the middle of the deck, and the material bed is too thin.
If the screening media gets partially blocked by feed material, results will be sub-optimal at best. There are two ways that this can happen: pegging and blinding.
Pegging is when particles get wedged in the holes on the screen media, inhibiting screening performance
It mostly happens with stones that are of a similar size to the apertures, and also with flaky or elongated material. Carrot-shaped particles (oblong stones) with one end wider than the aperture size, and one end thinner) can be especially troublesome.
Pegging can often be alleviated by adjusting the crusher and/or the screen settings. If there are a lot of near-size stones in the feed size distribution, it may be worth changing the crusher settings to avoid this. With the screen, increasing the stroke length, stroke angle, and/or the screen inclination can be effective.
Screening media can also be a factor here. If the screening media is excessively thick, that makes it easier for particles to become lodged. Aperture shape and pattern can also be significant – slotted holes in a staggered pattern are less likely to become pegged. The angle on the sides of molded apertures also reduces the pegging risk. As with blinding, more flexible screening media can provide additional movement to prevent particles from settling.
Blinding happens when small particles in the feed stick to each other and to the screening material through moisture or, sometimes, static electricity. Apertures can be blocked as a result.
Blinding tends to happen during difficult screening applications, for example when dealing with feed material that contains both moisture and a high content of fines. Material that contains clay, and combinations of dry crushed and wet natural materials are also prone to blinding.
Fortunately, there are a number of ways to address blinding. One method is to increase the movement of the surface of the deck, so that any build-up is continually dispersed. This can be done by increasing the rotation speed of the screen’s motion. Changing to more flexible screening media can have a similar effect – the additional vibration from flexible material can stop any moist material from settling. Sandvik’s WN4000 and WX7000 ranges both provide these anti-blinding qualities.
You could even look into the possibility of attaching a ball deck. Sitting below the screen deck, this is a cage containing rubber balls which bounce against the underside of the screening media, stopping particle build-up.
This chapter of the Knowledge Hub has hopefully reinforced that there are a lot of variables that can affect screening performance. To summarize, here’s a table that shows the significant factors that you should take into account when assessing the effectiveness of your screening operation.
