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Paul Fears, managing director, Eriez Magnetics, explains why, in the ceramics industry, iron spots are a common problem.

Iron spots are caused by ferrous contaminants being present either in the body or glaze and can prove very costly for producers.

Contamination can occur either naturally from iron bearing minerals such as Magnetite and Hematite, or be introduced in the process through milling, screening or plant deterioration.

Eriez’s task is to remove that contamination irrespective of the source and this means magnetic separation at not just one but several locations within a plant.

The best way to address the problem of iron contamination is to remove it before it gets into the plant from the raw material infeed.

Suppliers could be instructed to improve the quality of their product but primary separation of tramp metal such as nuts, bolts, wire and smaller iron will still be needed.

Larger tramp can be separated using a suspended magnet while smaller iron can be separated using a Plate Magnet.

The type of suspended magnet that would need to be installed has a stronger field than standard suspended magnets.

Eriez makes two types of suspended magnet, with the CP Magnet being most common unit manufactured.

However, it is not suitable for this type of separation.

The Eriez OP Magnet is far stronger, giving better separation and protection of screens and ball mills.

This magnet would be installed over a conveyor belt containing the glaze or body to remove any large contaminants.

Once the material has been mixed, milled and screened it is ready to be passed through the strongest magnetic separator possible.

The Eriez High Intensity Filter, a high-gradient magnetic separator, is capable of capturing fine ferrous and paramagnetic particles in liquids and slurries.

It generates a background magnetic field of around 5,000 gauss.

When intensified in the matrix, this increases to 15-18,000 gauss, which is enough attraction to draw fine and weakly magnetic particles.

This has been proven to provide the best level of iron separation possible.

Not only is the separation performance exceptional but the unit is totally automatic.

Therefore, maximum magnetic separation can be guaranteed at all times.

On manual clean units such as tubes, users have to rely on the operators to clean them.

Calculations have shown that tubes need to be cleaned every 15 minutes to maintain separation performance.

This is physically impractical.

For the HI filter, maintenance is minimal – maybe once a month to check the matrix.

The next stage of the process is typically a spray dryer.

Spray Driers are notorious for introducing fine iron into the material, usually bonded to the spray-dried particle.

Therefore, before storage, the spray-dried ceramic should be passed over a high-intensity magnetic separator.

Eriez recommend the use of a rare earth drum magnet.

As material reaches the drum, the magnetic field attracts and holds ferrous particles to the drum shell.

As the drum revolves, it carries the material through the stationary magnetic field.

The non-magnetic material falls freely from the shell, while ferrous particles are held firmly until they are carried out of the magnetic field.

Many companies promote the use of rare earth rolls on the spray-dried material, which is a high-intensity, high-gradient, permanent magnetic separator for the optimum separation of paramagnetic particles from dry products.

Advantages of using magnetic drums are: throughput is higher on a drum than a roll; magnetic field is deeper on a drum than a roll; maintenance is significantly lower on a drum; RE rolls need regular belt replacements, whereas drums do not.

The aim is to separate free iron and iron attached to spray-dried particles.

For this you do not need the additional strength of the RE roll and, therefore, a magnetic drum is the best solution.

As a final check prior to the presses, Eriez recommends the installation of simple rare earth magnets.

Usually this is directly above the presses.

These simple systems will remove any iron from the final storage silos and transportation system, and ensure that the product is clean before pressing.

Examples of the magnetic systems to be placed are either an elbow magnet above the press, a cartridge magnet below the silo or a tube magnet.

The preparation and, therefore, magnetic treatment of glaze is very similar.

Glaze is regarded by many companies as being more important than body, as defects can be seen more readily.

Many companies install the HI filter on the glaze line first and see immediate results.

Very often the HI filter can be supplied on a moveable base to transport from one glaze tank to another.

Most HI filters being used for glaze production have at least a 5,000 gauss background field.

The model HI25-50 is the most common unit.

After the HI filter has removed all the fine difficult paramagnetic minerals and abraded stainless steel, transportation of the material usually results in putting back small amounts of free process iron.

As a final check, users can install a ceramic trap (or CMT) at the glaze station to remove free iron.

The ceramic magnetic trap was designed by Eriez to utilise every spare millimetre of magnetic strength while forcing the operators to clean the magnets.

Magnetic tubes are inserted into a slightly larger diameter tube and glaze passes between the surface of the magnetic tube and the inside of the outer tube.

Glaze can be either pumped from the bottom or fed from the top.

As the magnetics build up on the magnetic tube surface the gap narrows and when the magnetic tube is full of iron the gap closes, preventing the passing of material and forcing the operator to clean the magnets.

Separation far exceeds any small electrofilter such as the Ferro Filter.

One other option is the B Trap.

Although not giving the same level of separation performance as the ceramic trap, it is used extensively in the ceramics industry in pipelines.

Some companies have even installed these types of traps on their water pipes, as that is the source of high amounts of free iron.

In conclusion, magnetic separation is an important part of any ceramics industry process line.

Removing ferrous contamination at several locations in the plant ensures the end product is contamination free and of the highest quality, while also reducing costs for producers.

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