*2.2. Thermal Pretreatment via Electromagnetic Waves (Dielectric Heating)*

The transfer of energy without the need of a specific material medium of propagation is termed electromagnetic (EM) waves. EM waves are categorized based on wavelength (λ) and frequency (*f*) (Figure 5), since all EM waves travel at the same speed (*c* = *f*λ).

Among the EM spectrum, radio waves and microwaves (MW) are being used for heating applications in some industries, such as food, geological exploration, pharmaceutical, plastic, construction, medical therapy, and mining. Radio wave heating is usually referred to as radiofrequency (RF) heating [49]. Both RF and MW techniques are referred to as dielectric heating because their heating mechanism relies on the dielectric properties of the material to be heated. Materials with significant dielectric properties respond to dielectric heating. In mining, dielectric heating has gained the attention of researchers because of its peculiar advantages over conventional heating, such as the production of clean energy, fast processing, heating uniformity, and better heat penetration to the ore matrix.


**Figure 5.** Electromagnetic spectrum.

In contrast to heating through furnaces, which employ heat transferred through convection, conduction, and radiation from the material's surface, dielectric heating involves energy conversion from electromagnetic into heat energy. When material is subjected to an electromagnetic field, electric and magnetic polarization may arise within the frequency of the electromagnetic device used in the process, which may lead to heating. This heating occurs in different mechanisms: dipolar polarization, conduction, and interfacial polarization. In dipolar polarization, the atomic dipole reorientation occurs, but there exists a phase difference between the dipoles and the electric field orientation, which leads to the heating of the material. When a material consisting of a significant electrical conductor is subjected to an electromagnetic field, ions or electrons move which leads to electric polarization. This causes the heating of the material due to its electrical resistance. In such cases, the heating mechanism is through conduction (like that of conventional heating), and a high microwave power and long residence time will be required to make a significant effect on the rock's strength reduction. In rocks with heating behavior such as Goethite (hydrated iron ore), the iron mineral heats, but the bulk ore must be heated to a point at which the hydroxy ion of the water molecules is released. For interfacial polarization, a sample consisting of conducting and non-conducting material with a significant dielectric property is subjected to an electromagnetic field such that both dipolar polarization and the conduction heating mechanism occur. Therefore, when ore is subjected to an electromagnetic filed, both the atomic dipole and electric polarization may occur, leading to microcracks. The extent of the microcracks depend on whether there is an existing fracture and the arrangement of crystals and even the crystal shape, which may help to increase the intergranular cracking.

The main difference between the microwave and radiofrequency means of dielectric heating is their frequency range, which determines the device's configurations. Microwave radiation has a higher frequency compared to the radiofrequency (Figure 5). However, within the frequency range of radio waves, 10–100 MHz is mostly employed for a heating purpose [49]. The typical device set up for the two approaches is as presented in Figure 6. Both methods have been explored for the dielectric heating of ore with the sole aim to reduce its strength and consequently, the comminution energy can be minimized.

**Figure 6.** (**a**) Ore pretreatment in a microwave oven (modified after [50]). (**b**) Ore pretreatment in a radiofrequency device [51].
