Rock fragmentation through microwave energy has received much more attention recently for potentially improving energy efficiency and delivering environmental benefits. A theoretical model containing pre-existing micro cracks is established to investigate the origin of damage and failure of hard rock irradiated by microwave. The radius of propagating cracks and damage of hard rock during heating process are quantified. It shows that the critical microwave energy required for fracture initiation is related to radius and density of pre-existing cracks. The propagation behaviour of micro cracks can be divided into two stage. In the first stage, with the extension of micro cracks, the required microwave energy is decreasing. The absorbed microwave energy is transformed into surface energy and kinetic energy of the propagating cracks. In the second stage, after meeting a critical value of the radius, the required microwave energy is increasing with the propagation of cracks. The absorbed microwave energy is only transformed into surface energy of the propagating cracks. The greater the density of micro cracks, the more microwave energy required for crack growth. The study of strength reduction for hard rock is predicted according to the crack propagation. The study makes contributions to industrial implementation of microwaves in rock breakage.
Mechanical rock comminution technology which is widely used in underground opening, tunneling and mining is an energy intensive process. The efficiency of the process is closely associated with rock mechanical properties, especially strength and abrasivity. Various means of artificially altering the rock properties have been investigated during the last decade to increase breakage machine's life (Hartlieb and Bock 2018; Mardoukhi et al. 2017). The technologies involve high-pressure water jets (Jeng et al. 2004; Ciccu and Grosso 2010), microwaves (Kingman and Rowson 1998) and high-power lasers (Buckstegge et al. 2016). Microwave heating technology offers following advantages over other technologies: instantaneous control; highly safety and automation; non-contact and rapid heating; selective and volumetric heating (Bermúdez et al. 2015). Breakage efficiency can be improved significantly after microwave irradiation.