Geoscientist’s Toolkit: Frantz Magnetic Separator

Frantz magnetic separator.  Image credit: Bill Mitchell (CC-BY).
Frantz magnetic separator. Image credit: Bill Mitchell (CC-BY).

When a sample for geochemical analysis gets in to the lab, often one of the first priorities is to separate the mineral(s) of interest in the rock (e.g. zircon, potassium feldspar, or quartz) from the other minerals. Once a rock has been crushed and milled to single-grain size, the sample is ready for separation.

One of the first methods employed is magnetic separation, which will separate the more magnetic (paramagnetic) minerals from the less magnetic (diamagnetic).

For magnetic separation, a Frantz magnetic separator is used (see figures). It has a chute which is tilted both down its long axis (right to left in the picture) and its short axis (far to near in the picture). With the chute alone, the samples would end up in the non-magnetic (near, pink) bucket from gravity. However, a strong electromagnet is used (big black things above and below chute) which holds the paramagnetic materials up against the force of gravity, directing them into the far chute.

Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon).  Image credit: Bill Mitchell (CC-BY).
Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon). Image credit: Bill Mitchell (CC-BY).

Both the feed chute and the main chute have vibrating motors attached, so that the grains get slowly bounced around and move gradually down. The electromagnet provides enough force to keep the paramagnetic minerals in the upper (far) part of the main chute. By adjusting the current running through the electromagnet, the threshold for magnetic/non-magnetic can be controlled.

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