Microwave heating is the process by which an electromagnetic operation in a certain range heats the dielectric material.

Initially it was mainly used in the food industry to cook or heat food materials. It became clear that there is a great potential for other applications in subsequent processes.For this reason, early examples of microwave techniques have begun to be seen in geological areas such as mineral processing,oil shale extraction, soil remediation, and even more in recycling processes such as waste treatment. Especially, the emolliating and fracture of rocks is one of the important examples of using microwave in the field of geology. The creation of effective equipment for emolliating and fracture of rocks is complex, but completely realized the problem.

microwave assisted disc cutter

Figure 1. Microwave-assisted disc cutter concept of a TBM.

Microwave processing technologies of various materials have already been worked on. In fact, a lot of research has been done on the use of microwave energy to destroy and emolliating rocks with the USSR in the ’80s. These researches have been applied in particular to provide thermomechanical destruction of frozen soils.[1] In the 1960s, the concept of using microwave energy to generate heat for rock drilling was introduced, but due to the technical issues at the time, it was not economically accepted and the research stopped. Microwave applications have primarily been investigated for the mineral processing industry to reduce energy requirements during comminution of ores and increase the liberation of valuable mineral particles for enhanced separation.[2] In the past it has been proved that mechanical cracking is the most economical method and therefore it is the most used method among many rock breaking methods. However, as existing technologies progressed and raw materials become less scarce and valuable, new methods of rock breaking have developed. The amount of heat produced by a microwave in a particular rock depends on the microwave power level and exposure time, and on the mineral and chemical composition of the rock. Some minerals, such as pyrite, absorb the microwave, and some, like Calcite, are microwave-resistant. Differential volumetric expansion of the mineral components of the rocks during heating creates stress along the grain boundaries and causes internal cracks which may weaken the rock. For example, in a gigantic Norwegian ilmenite ore after microwave treatment, the bond work index has fallen to 90%.[2] Microwave radiation has also been proven to reduce uniaxial pressure stress (UCS) and tensile strength, as seen in many microwave sensitive materials with many experiments.

microwave system

Figure 2. View of variable 3 kW power microwave system

Tunneling machine (TBM) is a machine used to excavate tunnels with a circular cross section along various soil and rock layers. Moreover, they can be used for micro tunneling. Hard rock breakage in mining and civilian applications, the performance of mechanical equipment such as tunnel drilling machines (TBM’s) has a high degree of bit wear and maintenance. Pre-conditioning to weaken natural rock before mechanical breakage is an advantageous method for reducing bit wear, maintenance requirements and costs. Flame torch-assisted TBMs are considered to be economical in terms of equipment, it can not be regarded as the best method for today’s technology due to their high fuel consumption and smoke emissions. Preconditioning with microwave energy offers several advantages over the flame torch method.First of all,it does not increase fuel consumption or produce emissions because electricity source is available on the machine.

It is also easy to control because of the quick power on and off switch.

norite cylinder

Figure 3. Norite cylinder cracked after (a) 120 s after microwave treatment at 5 kW power.

The properties of a specific microwave system depend on the intended application. The MET Advanced Technologies Company can develop the desired microwave system in the direction of the customer’s request. Commercial microwave ovens use up to 3 kW of low power, and consist of only one magnetron, a short waveguide and a closed metallic cavity. Industrial microwave systems use higher power levels (3-200 kW) and consist of magnetron, isolator, power meter waveguides, tuners, cavities and generators. The magnetron produces microwave energy, which is then transmitted to the cavity by waveguides, which are long hollow rods made of a dielectric material to reconstruct all microwaves. The physical dimensions of the magnetron, isolator, waveguide and radio receiver are directly related to the microwave frequency. For example, a microwave with a frequency of 2.45 GHz has a wavelength of 12.2 cm.[2]

norite cylinder

Figure 4. Norite cylinder cracked (b) 65 s after microwave treatment at 5 kW power.

The heat generated by the microwave is mainly dependent on the frequency and intensity of the electric field. [3] Represent to field conditions and to give an example, a custom-made microwave system was designed with variable power up to 3 kW at a constant frequency of 2.45 GHz.

The norite samples irradiated by the microwave are shown in Figure 3 and Figure 4. Microwave irradiation causes a number of cracks in the surface of norite blocks. A main crack is horizontally distributed and many secondary cracks are vertically distributed. Later that,discs of norites cracked in the cross section of the radius after treatment at 5 kW power for 120s and 65s under different microwave irradiation time.


[1] Microwave Heating for Emolliating and Fracture of Rocks, Aleksander Prokopenko, May 17th 2010, DOI: 10.5772/14067, https://www.intechopen.com/books/advances-in-induction-and-microwave-heating-of-mineral-and-organic-materials/microwave-heating-for-emolliating-and-fracture-of-rocks

[2] The influence of microwave irradiation on rocks for microwave-assisted underground excavation, Ferri Hassani,Pejman M.Nekoovaght,NimaGharib, February 2016, Pages 1-15, https://www.sciencedirect.com/science/article/pii/S1674775515001341?via%3Dihub

[3] Study on the Effect of Microwave Irradiation on Rock Strength, Qin Like , Dai Jun and Teng Pengfei,June 13th 2015, http://www.jestr.org/downloads/Volume8Issue4/fulltext84142015.pdf