Separators and Classifiers… What do they do?

Pop quiz time… What do a merry-go-round, a slide, and air hockey have in common? Answer: They’re all things that can help explain what goes on in a separator. In the case of the cement plant, the separator separates fine particles from coarse particles. The fine particles are collected as product while the coarse particles are sent back for further grinding. The trick is to make sure that the stream of coarse particles doesn’t contain any fine particles and, likewise, to make sure that the stream of fine particles doesn’t contain any coarse particles. An efficient separator prevents over-grinding and the waste of energy that accompanies it. It also keeps product within specifications by making sure that the correct particle size is achieved. We’ll consider a conventional separator for the purpose of this discussion.

All separators use the same basic principles of physics that you probably studied in high school. The first principle is that the faster you spin something around in a circle, the more centrifugal force it has. Think back to your days at the play ground. If you sat on the outside of the merry-go-round, the faster that someone spun the merry-go-round, the more force that pushed you to the outside. Separators use the same principle to force particles to the outside of the device. The greater the centrifugal force; the more particles that are forced to the outside.

The second principle is illustrated by the slide. What goes up must come down. After the particles are flung out by centrifugal force, they impact the side of the separator and then fall down the side to be collected. They fall down because like anything else that has mass, they’re affected by gravity. You’ll notice that many separators have a cone-shaped lower portion. Those sloping sides, like the slides at a playground, help collect the coarse particles into the apex of the cone where it can then be conveyed back for further grinding.

If you’ve ever played air hockey you’ve probably noticed two important facts. Firstly, it’s possible to have a small hockey puck “float” on a stream of air. Secondly, something supported on a stream of air can be moved very smoothly and very quickly. Fine particles in a separator are suspended in a stream of air that ultimately conveys them to be collected as product. The greater the velocity of the air, the more particles that get entrained. Alternatively, the lower the air velocity, the less particles that get entrained. (Think about hooking up a leaf blower to that air hockey table and consider the size of the puck that you could use now!)

On conventional separators adjustments that impact either the centrifugal force or the drag force (which is acting in opposition to the centrifugal force) ultimately determine how coarse or how fine the separated material should be. The terms separators and classifiers are used interchangeably because they perform identical functions. The term classifier though typically refers to the later generation “high efficiency” separators. High efficiency classifiers use the same physical principles as the conventional separators although their internal configuration is quite a bit different. So just how are those adjustments made?

This information was extracted from presentations given at our Mill Grinding program. For more information on this program, click here.

Note: Although the terms centrifugal and centripetal are used interchangeably, please note that the correct term is centripetal force. (Isaac Newton was the first person to use that term.)