Sieves & Screens
What’s the attraction?
Paul Fears of Eriez Magnetics Europe, looks at overcoming the difficulties
of dry magnetic separation
Processing in a wet slurry commonly enables higher intensity
magnetic fields to be applied with better separation results, as achieved using High Intensity Magnetic Filters and Superconducting Powerfluxs. Nevertheless, many non-metallic mineral processors want to separate magnetic minerals and free iron when their product is in a dry state. This could be for a number of reasons including: The local availability of water; the cost of drying; the demands of the end customer and ever changing environmental issues
For dry processing, there are essentially four designs of magnetic separator proposed by Eriez which are a Rare Earth (RE) Roll Separator, a Magnetic Drum Separator, a Dynamic Drum Separator (DDS) or a Dry Vibrating Magnetic Filter (DVMF). Each has specific advantages and disadvantages and the most suitable solution is commonly determined following testing at an Eriez laboratory.
The design of magnetic separator very much depends upon the separation objective and the material characteristics. As shown on the chart, a material that is generally plus 80 micron and where paramagnetic and weakly magnetic minerals need to be separated will probably be well suited for processing over a Rare Earth Roll Separator. A much finer material, above 20 microns would be processed through a Dry Vibrating Magnetic Filter. This chart is only a guide and testing is always recommended to confirm what level of separation is possible
normally. This self-cleaning system is a simple and effective method of magnetic separation. Magnetically, the aim with a Rare Earth Roll Separator is to produce high peaks of concentrated field to which magnetically susceptible particles are drawn. A typical Rare Earth Roll is made up of magnetic rings or arcs interspaced with steel pole pieces. The magnetic flux is concentrated into the pole piece by pushing magnets with different polarities together. This produces peak fields of up to 21,000 gauss.
The belt is approx 0.15mm thick and this reduces the field to around 12,500 gauss. Therefore, it is essential to present a thin layer of material. It is important to know that if lower grade, cheaper Rare Earth magnets are used, the magnetic field will deteriorate with time. For this reason, Eriez only use the highest grades.
Magnetic Drums
The Magnetic Drum works on a very simple principle compared to the Rare Earth Roll Separator, using high intensity magnetic fields to defect magnetically susceptible particles and enable a separation. The main difference is that the Magnetic Drum does not have a belt and so requires less maintenance. In fact, once installed, the Magnetic Drum requires very little attention at all.
A Magnetic Drum has a magnetic arc fixed inside a rotating
Rare Earth Roll Separator
One of the most common high intensity magnetic separators used to process dry materials is the Rare Earth Roll Separator. This consists of a head roll constructed with strong Rare Earth permanent magnets and a belt to convey the material. This then operates as a simple conveyor.
Material is fed from a feed conveyor belt onto a Vibratory Feeder before being cascaded down a chute and into the magnetic field. Magnetically susceptible particles are attracted to the field, resulting in a change in their trajectory. This enables a separation. Non magnetic particles are unaffected and cascade
28 Solids & Bulk Handling • May 2010
shell. Material is fed, usually via a Vibratory Feeder, onto the drum shell. Magnetically susceptible particles are attracted and separated.
There are several models of Magnetic Drum to suit many different applications. Each model has a specific magnetic circuit design. For mineral processing, the best performing Magnetic Drums have been found to be the RR, RAS and RRS. The focus here is on the RRS as this generates the highest magnetic field and, in certain circumstances, gives results comparable to the Rare Earth Roll Separator. The RRS has a unique design of drum shell that enhances the magnetic field on the surface. The peak surface field is around 11,500 gauss,
www.solidsandbulk.co.uk
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