Magnetic Properties of Ceramic Minerals – Bunting Magnetics Europe Ltd


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Magnetic minerals and elegant iron luxuriate in plagued ceramic producers for the explanation that first glazed pot used to be produced many thousands of years in the past. Early ceramicists adopted give an explanation for and brilliant patterns to disguise any imperfections precipitated by such contamination. Over time, the build apart a query to for white or single coloured ceramic merchandise elevated, making disguising any imperfection an increasing number of tough.

Figure 1 – Iron speck contamination in fired ceramic tableware. Photo © Paul Fears Images

Ceramic producers became to technology, with magnetic separators and screens, to title after which purchase the problematic minerals and elegant iron. Many ceramic uncooked enviornment topic deposits, equivalent to feldspar, silica sand, ball clay and kaolin, luxuriate in problematic magnetic minerals incorporated micas (muscovite and biotite), hematite, chromite, and iron-stained quartz.  Additionally, free iron is at all times launched into the scheme from the crash or failure of job plant. Figuring out the minerals’ magnetic properties and their reaction internal a magnetic enviornment outlined the magnetic separator create job. 

The behaviour minerals demonstrate in a magnetic enviornment is classed under one of 5 headings:

  • Diamagnetic;
  • Paramagnetic;
  • Ferromagnetic;
  • Anti-ferromagnetic;
  • Ferrimagnetic;

These classifications give an explanation for whether or no longer separation is in all probability with a magnetic separator.

Figure 2 – 3 Fractions after magnetic separation: non-magnetics, middlings and magnetics.  Photo © Paul Fears Images

Source of Magnetic Properties

The magnetic properties of any mineral reside in the electrons of the atoms or ions. In step with the ideas of wave-mechanics, the electron, transferring in a closed direction about the nucleus, is assumed to be as a most contemporary behaving love a wave, and this transferring most contemporary generates a magnetic enviornment. When a crystal is placed in an exterior non-uniform magnetic enviornment, there might per chance be a pressure working to align the magnetic fields of the atoms to have faith a magnetic moment for the entire crystal. The magnetic susceptibility χ, is the ratio of the resulting magnetic moment, M, to the energy of the exterior enviornment, H. χ = M/H

Diamagnetism and Paramagnetism

Diamagnetic minerals luxuriate in a cramped detrimental trace of χ and are reasonably repelled by the magnetic enviornment. In incompatibility, paramagnetic minerals luxuriate in a cramped sure trace of χ and are weakly attracted by the enviornment.

Paramagnetism is linked to the spins of the electrons, whereas diamagnetism is linked to their distribution in say. Diamagnetism is a property possessed by all atoms. On the opposite hand, when the atom contains an irregular alternative of electrons, or has incomplete electron shells (as in the transition substances) imbalance of the electron spins causes the paramagnetic enact to overshadow the diamagnetic piece of the total magnetic susceptibility. Paramagnetism is furthermore discovered in metals the save there might per chance be a cloud of free conduction electrons.

This behaviour is easiest applied in most cases to crystals on myth of the internal crystal enviornment, as a total modifies, the magnetic outcomes. The electronic energy levels in a crystal are described as being crash up and the total magnetic susceptibility is dependent on the distribution of the electrons in the assorted levels. Therefore, in advanced compounds, it is miles never in all probability to predict the magnetic properties.

In minerals, iron-bearing structures are paramagnetic. On the opposite hand, there are paramagnetic minerals with out iron. The diversities in magnetic susceptibility are sufficient to enable separation the utilize of high-intensity magnetic separators.

Bismuth is the most tremendous one instance of a diamagnetic mineral. Paramagnetic minerals are more customary in ceramic feedstocks and embody Hematite and Biotite Mica.

Figure 3 – Paramagnetic minerals separated the utilize of a Electromagnetic Filter. Photo © Paul Fears Images


Ferromagnetic minerals possess a magnetic moment even in the absence of an applied magnetic enviornment. They’re strongly attracted by even a previous school magnetic enviornment and reside permanently magnetised. On the opposite hand, ferromagnetic substances furthermore exist in unmagnetised circumstances when, at room temperature, the electronic magnetic moments are permanently in alignment as a outcomes of interaction between neighbouring atoms.

Iron, Cobalt, Nickel and Pyrrhotite are conventional examples of the ferromagnetic species.

Antiferromagnetism and Ferrimagnetism

The style whereby electrons align in sure crystals produces both an antiferromagnetic or ferrimagnetic enact.


This occurs when adjoining atoms have interaction in a style to align the spins in parallel nevertheless adversarial directions called antiparallel spins.  The 2 sets of moments abolish every other and there might per chance be no longer any longer any eternal magnetic moment. Conventional examples embody metals equivalent to Chromium and oxides equivalent to Nickel Oxide (NiO).


Antiparallel alignment, whereby the substances in opposite directions are no longer equal, ends in a eternal moment which is ferrimagnetism. Magnetite is a genuinely valuable instance of a ferrimagnetic mineral.

Desk 1 – Magnetic properties of ceramic linked minerals

Final consequence of Magnetic Particle Contamination

The presence of magnetic particles in ceramic physique or glaze, both in mineral build or as free-iron, causes a big vary of points during the firing processing. In the past, such flaws were covered by ornate and brilliant glazes and decoration. On the opposite hand, contemporary British ceramics producers stop no longer luxuriate in the beautiful of enforcing such systems.

Magnetic particles trigger both structural and floor defects, as well to coloration and brightness points. More cost-effective trace, industrially produced British ceramics on the entire luxuriate in very viewed shaded spots and associated glaze deformation. Much less viewed shaded or brown particle specking is furthermore customary. Such defects happen when magnetic minerals or free-iron is most contemporary in the glaze during the firing job. The heat is absorbed and retained by the particle, inflicting a darkening in coloration and expansion. The particle might per chance well per chance also furthermore ‘pop’, inflicting defects in the surrounding glaze.

Yet another enact is the changing of the last fired glaze coloration and darkening of the brightness, especially when white or off-white, due to the presence of elegant magnetic particles.

Defects on ceramic tiles, tableware and sanitaryware, which depend on ideal look, consequence in both partial develop into or total rejection and recycling.  In crops with inadequate systems to purchase the magnetic particles, such charges might per chance well per chance also furthermore be above 10%. Here’s dear in both time and offers.

Non-viewed magnetic contamination stays problematic. Magnetic particles remaining in the physique sooner than firing abilities the similar reaction to heat as these in the glaze. This weakens the surrounding ceramic development, main to elegant cracks that diagram over time. Failure of a ceramic product due to fracturing is usually traced help to loads of magnetic particles, which shall be then viewed alongside the rupture line. A single magnetic particle is sufficient to trigger such catastrophic failure.

This poses a distress for loads of high-performance British ceramics, especially as the contamination is just not any longer viewed from the outset. Failure easiest occurs over time.

Magnetic Separation of Minerals

Figuring out the magnetic properties of the minerals in a particular ceramic uncooked enviornment topic is extremely valuable when optimising magnetic separation effectivity. The wide vary of magnetic separator designs utilize diverse strengths and forms of magnetic enviornment to have faith a separation. Several designs of magnetic separator might per chance well per chance also feature in a single mineral processing operation.

Desk 2 – High intensity magnetic separators regularly out of date to job ceramic uncooked offers, physique and glazes

The vary of high-intensity magnetic separators is divided into dry and wet processing. Many ceramic uncooked offers, equivalent to feldspar, zircon and silica sand are handled and processed in a dry say. Most ceramic mineral processors magnetically handle their personal reserves as elevated specification offers generate elevated income. High-intensity magnetic separators purchase magnetic minerals and free iron. Designs embody the Uncommon Earth Roll Magnetic Separator and Uncommon Earth Drum Magnet, which both utilise eternal magnets, or the electromagnetic Precipitated Roll Magnetic Separator, which is more fit correct for facing high-temperature offers (e.g. straight after drying).

Throughout the manufacturing plant, the spray dry ceramic job regularly introduces free elegant iron into the scheme. Both the Uncommon Earth Roll Magnetic Separators and Drum Magnets are employed to purchase such problematic contamination.

As soon as the ceramic uncooked offers had been milled and mixed into slips, physique and glazes, additional magnetic extraction takes space. The milling job on the entire liberates entrapped mineral iron, whereas free iron is reintroduced from the scheme (e.g. in the course of screening, pumps, and so forth).

Electromagnetic Filters, producing high-magnetic fields, cleanse both the physique and the glaze. Bigger-sized objects, for high capacities, present the last separation stage for getting rid of very elegant and weakly magnetic minerals from physique. Miniature-sized Electromagnetic Filters, on the entire on mounted on moveable frames, separate elegant iron and previous school magnetics from the glaze.  On the opposite hand, care is well-known, as some glaze colouring is magnetic.

As a remaining take a look at and to forestall any viewed contamination on the glaze, neodymium uncommon earth Tube Magnets or Magnetic Liquid Traps with neodymium magnets are positioned as shut to or internal the glazing location. This captures any magnetic particles launched to the glaze in the course of transportation to the location.

Figure 4 – Bunting Electromagnetic Filter in a British ceramics plant. Photo © Bunting

Future Mineral Reserves

As previous school ceramic mineral reserves are depleted and more advanced deposits are mined, idea the mineralogy through magnetic susceptibility is vitally valuable. Magnetic separator suppliers work intently with mineral processors and plant designers, on the entire on the venture feasibility stage, offering advice on the optimum create of magnetic separator and stage internal the scheme. This on the entire involves intensive take a look at work in magnetic separation laboratories and at space.

The ongoing build apart a query to for white and pattern-free British ceramics intention that there might per chance be no longer any leisure for ceramic producers and uncooked enviornment topic suppliers in the fight to eradicate magnetic minerals and free iron.

Extra records on high intensity magnetic separators for the British ceramics and mineral processing sectors is on hand on the Bunting online web page

Bunting-Redditch online web page –

Visit Bunting Magnetics Europe Ltd on stand 17-327.

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