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Eriez provides tests at its laboratory to determine the most suitable solution for dry magnetic treatment of industrial minerals.

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’s 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 applies when dealing with material with finer particle sizes, such as kaolin or calcium carbonate.

Three main magnetic-separation techniques are presently used to treat dry non-metallic minerals.

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 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 captures magnetic particles on a batch-process basis.

Successful applications include talc and silica-flour purification.

Processing in a wet slurry 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: local availability of water, cost of drying, demands of the end customer and changing environmental issues.

For dry processing, there are 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 depends upon the separation objective and the material characteristics.

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.

Testing is always recommended to confirm what level of separation is possible.

The rare earth roll separator is one of the most common high-intensity magnetic separators used to process dry materials.

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 together magnets with different polarities.

This produces peak fields of up to 21,000 gauss.

The belt is approximately 0.15mm thick, which reduces the field to around 12,500 gauss.

It is therefore essential to present a thin layer of material.

If lower grade, cheaper rare earth magnets are used, the magnetic field will deteriorate over time, which is why Eriez only uses the highest grades.

Rare earth rolls can be built with magnetic widths up to 1.5m.

In Europe, there are three different designs of rare earth roll separator: RE300 (300mm diameter) RE100 (100mm diameter) and RE75 (76mm diameter).

The RE300 rare earth roll enables more than 40 per cent extra capacity per metre than the 75mm-diameter model.

For a mineral processor, this means fewer units are required for a plant, resulting in less maintenance and fewer spares.

The magnetic drum works on a simple principle compared with the rare earth roll separator, using high-intensity magnetic fields to defect magnetically susceptible particles and enable a separation.

The magnetic drum does not have a belt, so requires less maintenance.

A magnetic drum has a magnetic arc fixed inside a rotating shell.

Material is fed, usually by 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 are 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’s drum shell enhances the magnetic field on the surface.

The peak surface field is around 11,500 gauss, which is similar to that of a rare earth roll separator.

Looking at a case history where the objective was achieving maximum Fe2O3 removal at 330kg/hr with a maximum product loss of one per cent, 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 issues arose: the electromagnetic vibratory feeder, critical to the separation, was having difficulty feeding an even layer of the fine material onto the drum surface; and 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 per cent reduction in magnetics.

However, once the vibratory feeder was redesigned with an ‘airflow’ tray and the RRS drum was rotated at around 100m/sec, the reduction in magnetics increased to 75 per cent with only 0.6 per cent 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 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 three-pass RRS drum system reduced the iron content to 44ppm.

The triple-pass RRS drum system is operating well at site.

An intriguing development for finer-powder processing came in the form of the dynamic drum separator or DDS.

The DDS is only suitable for the removal of free iron but 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.

As the magnetic particles enter the rotating field, the free iron particle spins, colliding with neighbouring particles.

This collision helps to liberate them from the non-magnetic powder, enabling a clean separation of iron.

The following case history looks at a silicon-carbide sample with free iron contents of 2.7 per cent, where the objective was to reduce to less than 0.1 per cent 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 a 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 per cent magnetics removal.

Additionally, the amount of feed discharged into the magnetics was only five per cent, constituting an acceptable loss of good product.

This was achieved with a feedrate of less than three tonnes per hour per metre.

The dry vibrating magnetic filter is suitable for the finest powder processing of industrial minerals such as talc.

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 are captured on the matrix.

The magnet is then turned off and the matrix vibrated, releasing the captured magnetics.

One successful installation processes talc in South America.

The separation objectives required maximum iron removal with a minimum loss of product.

The Fe2O3 content of a product up to 3.5 per cent iron could be reduced to less than 0.5 per cent.

Equally important, the talc recovery was between 96 and 98 per cent.

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 more than 100g of magnetics, with some entrapped product.

However, when fired, only two iron spots were found in the end product.

This met all the separation objectives and the customer ordered a production unit.

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