Choosing the right screens for your operation

Selection of screen type

Selecting a screen is not something that happens in isolation. To be sure of making the right choice, you need to make a comprehensive analysis, one that examines your screen or screens within the context of your entire operation. Only when you analyze screening alongside your other machinery and in terms of the overall aims of your operation will you be able to optimize effectiveness.

In addition, with the huge amount of variables involved, it’s unlikely that the ideal solution exists. It’s inevitable that compromises will have to made, and that you will need to balance factors like set-up costs, running costs, efficiency, and accuracy.

That said, it is possible to set out some general principles for selecting a screen type. Three factors that will always need to be considered are the maximum feed size, the general application, and the desired separation. Or, to put it in the simplest terms, what you start with, what you’re planning to do to it, and what you hope to have at the end!

This table brings together these three factors and shows which combinations are suitable for Sandvik’s range of stationary screens (more stars indicates greater suitability).

Beyond this, you’ll need to consider issues specific to your own operation. For example, do you have limited space in which to install the screen? Do you have any preferences regarding screening media? Are you intending to wash the feed material during screening? These are just some of the factors that will determine which type of screen is most appropriate.

The decision-making process can be complex and challenging, which is one the reasons why Sandvik have introduced Peak Screening. It is a program that – in addition to supplying equipment, parts and servicing – offers expert consultation to optimize screening productivity. You can also apply for a free, no-obligation performance review by contacting your local Sandvik representative.

Screen area and width

As well as identifying the type of screen most suitable for your operation, you will have to make a decision about its size. There are a lot of considerations to take into account, so we have dedicated an entire chapter to finding the right screen area, here

Nevertheless, it’s worth giving some consideration to screen width at this stage because it is such an important factor in effective screening. The screen width is one of four factors that determines the bed depth (that is, the depth of material) over the whole screen. The other factors are the feed rate, the bulk density, and the speed that the material travels (this, in turn, is affected by factors including the angle of the deck, the vibration mechanism, and the screening media). The screen width, then, should be predicated on a calculation involving these factors.

The screen width, feed rate, material speed, and bulk density determine bed thickness.

If the bed depth is too thin or too thick, screening cannot be done effectively. This is true at all points on the screen.

A thick bed means that material will move more slowly along the deck and bounce less, allowing for better selection. However, if the bed is too thick then stratification will not happen properly and the material won’t have enough opportunities to fall through the apertures. At the other end of the scale, a thin bed will stratify more completely and peg less, but if the deck is too thin then the material will bounce along the deck, leading to poor screening accuracy.

Therefore, screening experts normally think about bed depth in terms of limits, at both the feed and discharge ends of the screen. As a rule of thumb, the upper limit for bed depth is typically 10 times the desired separation at the feed end. At the discharge end, the bed depth often is between 0.75 and 4 times the desired separation. More accurate limits depends on the separation size.

Finally, it’s important not to fall into the trap of choosing a wider screen than necessary. It can be tempting to think that a larger screen is a safer choice but, for the reasons outlined above, effectiveness and productivity can be compromised by an excessively wide screen.