Advantage of a 300 mm diameter magnetic roll separator over 100 mm rolls?

The common opinion is, that using a 300 mm diameter magnetic roll separator you will get 4 times the capacity compared to a 100 mm roll diameter separator. That is a massive improvement – f.ex. 2 tons per hour per meter process width for silica sand on a 100 mm diameter roll you will get 8 tons per hour capacity for a 300 mm roll.

Quite an argument!

But is that really true? And are you facing other shortcomings, when using a 300 mm diameter roll?

The starting point is the roll itself. While 100 – 150 mm diameter rolls will be manufactured by using magnet discs and mild steel discs, the 300 mm roll is using segments of magnets – no discs. This is no shortcoming as long as you will compare the same magnetic roll configuration, which means thickness of magnets and thickness of mild steel discs.

In order to compare the efficiency of these different rolls, one also needs to be sure, that the height of the mineral bed fed onto the belt is comparable for both cases – in ideal case a monoparticular coverage of the belt for optimum separation.

One of the important parameter in magnetic separation efficiency is the roll speed – or, in other words – the centrifugal force. The weaker magnetic the material to be separated the lower the centrifugal force should be. Typically with 100 mm rolls one is separating at approx. 100 rpm weakly magnetic material.

100 rpm do mean, that the transport belt is traveling with 0,52 m/sec using a 100 mm diameter roll. At same rpm the belt is traveling with 1,57 m/sec for the 300 mm roll.

The corresponding centrifugal acceleration is for the 100 mm diameter roll 5,48 m/sec2 and 16,43 m/sec2 for the 300 mm roll – 3 times higher!

What does that mean? In order to separate weakly magnetic material one has to reduce the centrifugal force of the 300 mm diameter magnetic roll separator to 59 rpm in order to achieve approx. same centrifugal speed compared to the 100 mm diameter roll.

59 rpm do mean, that the belt is traveling on the 300 mm roll separator with 0,93 m/sec compared to 0,52 m/sec with the 100 mm roll separator with same centrifugal speed.

From that we can conclude: assuming the same height of mineral bed on the magnetic roll separator a 300 mm roll separator can only have maximum double the feed capacity compared to a 100 mm diameter roll asuming the same magnet roll configuration!

So: have a look to the magnet configuration – for typical silica sand and high purity quartz we are using 3 mm magnet discs and 1 mm mild steel discs in order to efficiently separate weakly magnetic material.

Magnet roll configuration will be the topic of our next summary coming soon.

Dr. Udo Jakobs

Dr. Jakobs GmbH