There’s no such thing as one-size-fits-all when it comes to screening media. With so many factors that can impact on their effectiveness, you need to give careful consideration to your entire operation if you want to make the best choice. In this article, we’ve outlined some of the key variables that should influence your decision-making.
To begin the selection process, you should first consider the general type of screening that’s being done. This will make it easier to narrow down your options. We’ve separated screening into four broad categories:
Coarse screening in the plant’s primary stage. Also known as scalping, the aim here is to remove natural fines from the feed before crushing. Because it deals with heavy, coarse material, the screen and the screening media need to be extremely robust. Self-supporting screening media, made from steel-reinforced rubber, may be well-suited to the task. A grizzly screen may also be an option here.
A screen, or screens, with multiple decks to direct material to multiple crushers. A free-fall screen might be in operation here, which would offer a range of different media options.
Screening in a closed circuit with a crusher. During this stage, oversize material from the screen is fed into a crusher then conveyed back into the screen. Managing this circulating load is a crucial aspect of this stage, and for maximum efficiency you need to find a screening solution that removes undersize particles at the same rate as they are created by the crusher.
Separation into final product fractions. There is an emphasis on product quality at this stage so its important to have a screen and screening material that provide accuracy. It’s also useful to have screen material with good anti-blinding properties here, especially if conditions are difficult.
The next decision to make involves the type of material used in the screen media. Most modern screen media is made from rubber, polyurethane, or steel – or some kind of combination of these elements. They all deliver unique qualities:
Rubber is an especially useful material for screens that need to absorb impact, for example, in coarse screening.
There is a wide range of synthetic and natural rubbers available, all with unique characteristics. The hardness of the rubber (measured in shore A) is one of the key factors that determine its applicability. Harder rubbers (around 60 shore) absorb impact and have a long wear life, while softer rubbers (around 40 shore) are more flexible, more resistant to abrasion, and less prone to blinding.
Polyurethane also has a range of qualities. Thermoset polyurethane (typically open cast) is generally more durable and resistant to abrasion, whereas thermoplastic polyurethane (typically injection molded) is often used in washing screens or wet screens.
Steel is still commonly used in screening media, for example, in the woven wire mesh in tensioned screens. One advantage of this setup is that the steel wires are thin, which means that there are a lot of holes for undersize material to pass through. This media is also normally cheaper than rubber or polyurethane alternatives. On the negative side, it has a short wear life so is likely to be replaced every one to two weeks.
An often overlooked drawback of steel screening media is noise. Screens that use steel are considerably louder than rubber or polyurethane alternatives, so demand additional consideration of employee safety.
Steel is also used for the bars in grizzly screens, for punch plate screen panels, and to reinforce panels with rubber or polyurethane exteriors.
Thinner panels can handle a higher capacity, are more accurate, and are less prone to blinding and pegging. That might suggest that thinner panels are always preferable, but the factor that weighs in favor of thicker panels is wear life. Thicker panels last longer, so you have to make a trade-off between the costs of replacing panels (including both the cost of the panels and the impact of shutting down operations to replace them) against the benefits of more efficient screening.
You also need to consider the relationship between panel thickness, deck inclination, and separation. The illustration below shows two deck setups for separating fractions smaller than 10mm, one with a thin wire mesh and one with a thicker rubber or polyurethane panel.
You can see from this example that when there is an incline, thicker panels need larger apertures to achieve the same separation. If you were to increase the incline further, the necessary hole size would increase even more. Similarly, if you increase the panel thickness the necessary hole size increases. It’s a reminder, if one was needed, that hole size is dependent on a lot more than just the desired separation.
The shape and pattern of the apertures on a screen panel can be significant. Here’s a summary of the characteristics of the most common layouts.
1. Square holes, in line. Used under normal conditions.
2. Square holes, staggered. This helps prevent fines tracking with high fines content or short screens. It has a slightly reduced open area compared to square holes in line.
3, 4. Slotted along the flow. These deliver higher capacity but lower accuracy (oversize control). They can help reduce pegging, when the apertures are small.
5, 6. Slotted across the flow. For use in wet screening and dewatering
7, 8, 9. Round holes. For use with coarse crushed material. They offer longer wear life, but have a higher risk of pegging.
As we mentioned when we discussed panel thickness, above, finding the right aperture size is more complex that just matching it to the separation size. To calculate the correct hole size, you need to take into account the following factors:
Clearly there are a lot of things to consider when selecting screening media, so Sandvik have put together this handy look-up guide to make it a little easier to find the best screening media for your needs.
However, to make sure that you’re maximizing the productivity of your screening operation it’s best to take some expert advice. Sandvik’s Peak Screening service is a complete package that combines high-performing technology, performance guarantees and ongoing access to advice.
You can get started with a free performance review: click here for more details.