Paul Fears, Managing Director, Eriez Magnetics Europe Ltd

Abstract
The magnetic treatment of industrial minerals is conducted in both wet and dry processes.  The decision to process wet or dry is determined by one of two factors; how the end customer wishes to receive the material or the process route needed to achieve the end customer specification.  In many cases, non metallic mineral processors prefer to process in a dry state, but the present technology only enables purification when as a slurry.  This especially applies when dealing with material with finer particle sizes such as kaolin or calcium carbonate.  This paper discusses three magnetic separation techniques presently used to treat dry non-metallic minerals, specifically looking at fine particle processing.

The Rare Earth Roll Separator is an industry standard for high intensity dry magnetic separation especially when processing feldspar and silica sand.  There have been many developments in magnetic material and design that enable higher capacities and improved levels of separation.

A Rare Earth Magnetic Drum, model RRS, produces high surface fields similar to those of the Rare Earth Roll Separators and has many additional benefits including installation, maintenance and spares.  Setting key operating parameters enables the treatment of fine materials with a limited loss of product to the magnetic fraction.  The RRS has been used to process a wide range of industrial minerals

For specific fine powder applications, especially with high value materials, the Dry Vibratory Magnetic Filter (DVMF) is the only option.  Based on the High Intensity Magnetic Filter, which is used extensively in the ceramic and mineral industries to process slurries, the DVMF is designed to capture magnetic particles on a batch process basis.  Successful applications include talc and silica flour purification.

Introduction
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
-      Ever changing environmental issues

For dry processing, there are essentially 4 designs of magnetic separator, 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.

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.

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 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, for example only use the highest grades.
Rare Earth Rolls can be built with magnetic widths of up to 1.5m.

In Europe, there are three different designs of Rare Earth Roll Separator.  The difference is simply the diameter of the head magnetic roll.  The 3 models are the:

-     RE300 (300mm dia)
-     RE100 (100mm dia)
-     RE75 (76mm dia)

As shown with these results on a silica sand, the RE300 Rare Earth Roll enables over 40% extra capacity per metre than the 75mm diameter model.  For a mineral processor, this means less units are required for a plant, resulting in less maintenance and less spares. 

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 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, which is very similar to that of a Rare Earth Roll Separator.

Looking at a case history where the objective was achieving maximum Fe2O3 removal at 330 kg/hr with a maximum product loss of 1%, milled zirconium powder processed on the Rare Earth Roll Separator resulted in a high product loss, mainly due to electrostatic attraction on the belt.  Eriez laboratory technicians then ran the material over the RRS Drum.  After extensive test work, two key issues arose:
1.      The electromagnetic Vibratory Feeder, critical to the separation, was having difficulty feeding an even layer of the fine material onto the drum surface.
2.      There was still too much product loss with the normal rotation speed of the Drum.
The initial results, with the standard Vibratory Feeder and Drum speed, showed a 47% reduction in magnetics.  However, once the Vibratory Feeder was redesigned with a ‘Airflow’ tray and the RRS Drum was rotated at around 100m/sec, the reduction in magnetics increased to 75% with only 0.6% of the feed ending up in the separated fraction.  The ‘Airflow’ tray aerates the material, improving the flow characteristics and ensuring an even feed of material onto the drum.

One test result that proved surprising was on a silica sand from North Africa.
Silica Sand would typically be purified using a multi-stage Rare Earth Roll Separator and the 84ppm feed had been reduced to 54ppm with the 300mm diameter model.
However, tests on a 3-pass RRS Drum system reduced the iron content to 44ppm.
The triple-pass RRS Drum system is operating well at site.

Dynamic Drum Separator (DDS)
An intriguing development for finer powder processing came in the form of the Dynamic Drum Separator or DDS.  Even though only suitable for the removal of free iron, the DDS has proven very successful.  The most common applications have come from hard mineral processing, such as refractory minerals including alumina, corundum and silicon carbide.

With a DDS, there is a conveyor with a hollow head pulley.  Inside the head pulley is a strong magnetic rotor that rotates at high speeds.  Material is conveyed into the magnetic field with free iron attracted to the rotating magnetic field and deposited away from the clean product under the belt.  The magic of the separation occurs immediately when the material enters the zone.  Examining a close-up of the separation zone illustrates why the DDS being so well suited to fine particle separation.  As the magnetic particles enter the rotating field, the free iron particle spins, colliding with neighbouring particles.  This collision helps liberates them from the non-magnetic powder, enabling a clean separation of iron.

The following case history looks at a silicon carbide sample with a free iron contents of 2.7% where the objective was to reduce to <0.1% free iron at 500kg/hr with a minimum product loss.  Previous tests conducted with Rare Earth Roll Separators and Magnetic Drums had separated the magnetics, but with an unacceptable high product loss.  Minimising this loss was crucial to the economics of the project.  Tests in the laboratory and, ultimately, production results at site showed that by passing a material with a feed of 20000ppm magnetics over the DDS it was possible to reduce the magnetics content to below 190ppm.  This equates to a 99% magnetics removal.  Additionally, the amount of feed discharged into the magnetics was only 5%, constituting an acceptable loss of good product.  This was achieved with a feedrate of just under 3 tonnes per hour per metre.

Dry Vibrating Magnetic Filter (DVMF)
For the finest powder processing of industrial minerals such as talc, the Dry Vibrating Magnetic Filter is employed.  This is an electromagnet with a background field of up to 5000 gauss. 

The electromagnetic coil of the DVMF generates a magnetic field, focused in on its centre.  A magnetic matrix is positioned in this central core.  The DVMF operates on a batch processing basis. Product is fed in from the top and magnetics captured on the matrix.  The magnet is then turned off and the matrix vibrated, releasing the captured magnetics.

One successful installation is processing talc in South America.  The separation objectives were similar to most other projects, requiring maximum iron removal with a minimum loss of product.  Handling talc on traditional magnetic separators such as the RE Roll or Magnetic Drum has previously proven difficult.  The results exactly met the customer’s objectives.  The Fe2O3 content of a product up to 3.5% iron could be reduced to less than 0.5%.  Also, and equally important, the talc recovery was between 96% and 98%. 

On a project in the UK, the objective was to reduce the iron content of a magnesia alumina spinel.  Once again, the material was very fine and unsuitable for other types of magnetic separator.  Tests had shown that the product loss and separation performance were simply not good enough.  In this particular test, a 50kg feed with 14ppm magnetics was processed, removing over 100 g of magnetics with some entrapped product.  However, when fired, only 2 iron spots were found in the end product.  This met all the separation objectives and the customer ordered a production unit.

In conclusion, selecting the best magnetic separator for specific separation objectives is best achieved by conducting controlled tests.  Advice can then be given on what process would enable the best separation and, ultimately, give process guarantees.

Eriez Magnetics Europe Limited
Bedwas
Caerphilly

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