An Introduction to Magnetic Properties of Ceramic Minerals: Part Two

In our previous blog, we provided an introduction to magnetic minerals used in ceramics, and discussed the problems these minerals can cause in ceramics applications without the use of magnetic separation. In this blog, we will further explore the different magnetic properties that minerals can possess.

Source of Magnetic Properties

The electrons of the atoms or ions within a mineral are what determine if the mineral will have magnetic properties. Following the principles of wave mechanics, the electron is considered as a current behaving like a wave when it is moving in a closed path around the nucleus. Here, this moving current generates a magnetic field. When a crystal is placed in an external non-uniform magnetic field, there is a force at work seeking to align the magnetic fields of the atoms and produce a magnetic moment for the whole crystal. The magnetic susceptibility χ, is the ratio of the resulting magnetic moment, M, to the strength of the external field, H.

χ = M/H

Diamagnetism and Paramagnetism

A diamagnetic mineral has a small negative value of χ and will be slightly repelled by a given magnetic field. Paramagnetic minerals, in contrast, have a small positive value of χ and as a result will be weakly attracted to a given magnetic field.

Diamagnetism is related to the way electrons are distributed in a given space, whereas paramagnetism is associated with the spins of the electrons. All atoms possess the property of diamagnetism. However, if an atom has incomplete electron shells (in the case of the transition elements), or contains an odd number of electrons, this will cause an imbalance of electron spins. As a result, the paramagnetic effect will overshadow the diamagnetic part of a mineral’s total magnetic susceptibility. Paramagnetism, as a note, is also found in metals where there is a cloud of free conduction electrons.

Typically, this type of behavior is only applied generally to crystals. This is because the internal crystal field as a whole modifies the magnetic effects. The electronic energy levels in a crystal are described as being split, and the total magnetic susceptibility relies on how the electrons are distributed in the different levels. Because of this, in complex compounds it is not possible to predict magnetic properties.

Iron-bearing structures in minerals are considered to be paramagnetic. However, there are some paramagnetic minerals that do not contain iron. Here, there are sufficient enough differences in magnetic susceptibility that separation is enabled using high-intensity magnetic separators.

The only one example of a diamagnetic mineral is bismuth. Meanwhile, paramagnetic minerals such as hematite and biotite mica are more widespread in ceramic feedstocks.

Ferromagnetism

Even in the absence of an applied magnetic field, ferromagnetic minerals will possess a magnetic moment. These minerals remain permanently magnetized and will be strongly attracted by even a weak magnetic field. Ferromagnetic substances also exist in unmagnetized conditions when, at room temperature, the interaction between neighboring atoms causes the electronic magnetic moments to remain permanently in alignment.

Iron, cobalt, nickel, and pyrrhotite are all typical examples of ferromagnetic minerals.

Antiferromagnetism and Ferrimagnetism

The way in which electrons align in certain crystals produces either an antiferromagnetic or a ferrimagnetic effect.

Antiferromagnetism

Antiferromagnetism occurs when adjacent atoms interact in a manner that aligns their spins in parallel, yet opposed directions called antiparallel spins. There is no permanent magnetic moment due to the fact that the two sets of moments cancel one another out. Examples of antiferromagnetism can be found in oxides such as nickel oxide and metals such as chromium.

Ferrimagnetism

Ferrimagnetism is a permanent moment caused by antiparallel alignment, in which the components in opposite directions are not equal. An example of a ferrimagnetic mineral is magnetite.

Magnetic Particles in Ceramics

Magnetic particles can cause severe consequences for ceramics producers if allowed to remain in ceramic materials. Fortunately, magnetic separation is an effective way of removing contaminant magnetic particles and protecting the integrity of ceramic goods. In our next blog, we will discuss the consequences of magnetic metal contamination for ceramics processors and explain how magnetic separation can be used to combat this problem.

For more information on different types of magnetic properties and how magnetic separation can be used to effectively remove contaminant particles in ceramics applications and many other industries, contact Bunting today.