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Microwave fracturing and assisted mechanical breakage of rocks has been demonstrated to be efficient and cost-effective, holding great potential in rock excavation. However, the size effect on the fracturing process using open-ended microwave antennas has not been well studied or understood. To address this issue, a dielectric-loaded converging antenna is used to heat and fracture diorite specimens of different dimensions. The study investigates the effect of specimen plane size and thickness, as well as microwave energy, on the crack characteristics. The results show that as specimen plane size increases, fracturing becomes more localized with shorter and narrower cracks. Conversely, specimen thickness is negatively correlated with crack characteristics on the microwave-treated surface. The study also defines and examines the representative elementary volume (REV) of microwave fracturing using the antenna. Regression models are developed and validated to predict crack characteristics such as number, density, total length, and maximum width. The paper concludes by discussing the arrangement of microwave radiations on rock masses with different joint spacings using the antenna. Highlights: The effect of specimen plane size and thickness, as well as microwave energy, on the crack characteristics is comprehensively studied. The representative elementary volume (REV) of microwave fracturing using the DLCWA is defined and studied. Regression models are developed to predict crack characteristics such as number, density, total length, and maximum width. The arrangement of microwave radiation points of rock masses with different joint spacings using the DLCWA is proposed. [ABSTRACT FROM AUTHOR]

Microwave heating has been considered as one of the promising rock breaking technologies. However, the non-uniform temperature distribution of microwave treated rocks has not been quantitatively studied and the microwave rock fracturing mechanisms have not been systematically revealed. In this paper, microwave heating tests were implemented using a 6 kW industrial microwave system. The ratio of maximum temperature to average temperature ( R M / A ) was studied to characterize the non-uniformity of temperature. The temperature gradient and maximum temperature difference were calculated to quantitatively characterize the thermal stress. Experimental results indicate that the granite fracture temperature threshold is about 200 °C and cracking typically occurs at a minimum temperature gradient of 6 °C/mm. As either power level or irradiation time increases, the thermal gradient and temperature difference increase. For a fixed microwave energy, microwave heating at higher power levels for a shorter duration has a higher R M / A and a greater temperature difference and temperature gradient, which explains that the combination of high power and short heating time yields better rock weakening effects. [ABSTRACT FROM AUTHOR]

Microwave fracturing of hard rocks holds great promise in the civil, mining and tunnelling industries. The role of heating and cooling in the fracturing of rocks and when and where cracks initiate from and propagate to remain unclear and need to be addressed for future field applications of the technology. This study treated an alkali feldspar granite using a 6 kW industrial microwave source and a customised open-ended dielectric-loaded converging waveguide antenna. The real-time acoustic emission (AE) characteristics in the microwave heating and natural cooling phases were recorded and investigated. The surface temperature and P-wave velocity reduction of the specimens were also measured to quantify the thermal damage. The fracturing of granite is found to be tensile failure and is heating-dominated. Although a considerable amount of AE hits and events was detected in the cooling phase, they were of low energy, and therefore, the role of cooling in rock fracturing by open-ended microwave can be neglected. Fractures initiated from the exterior of the antenna and propagated towards the edges and the interior of the specimens. The cracks obtained from the AE localisation were in good agreement with those observed. With the increase of power level, the time for crack initiation was shortened, confirming the high-power effect. Highlights: Acoustic emission activities were measured in the microwave heating and cooling phases on granite blocks. Microwave fracturing of the granite is found to be heating-dominated. The microwave-induced rock fracturing is primarily tensile failure. The test results confirm the high-power effect in microwave fracturing of rocks. [ABSTRACT FROM AUTHOR]

Microwave fracturing has a great potential in assisting excavators in breaking hard and abrasive rocks. The key to utilizing this technology is to design an antenna that is capable of fracturing low loss rocks at relatively low power levels in a short duration. This article presents the design of a dielectric-loaded converging waveguide antenna (DLCWA) composed of a pyramid-shaped alumina infilling and a converging waveguide. The design of the DLCWA is optimized based on the maximum temperature, the high-temperature volume, and the effective working distance. To validate the design, a prototype antenna is manufactured and used for microwave heating and fracturing tests. The numerical simulations and experimental tests demonstrate that the designed DLCWA has unparalleled advantages at the radiation distance of 0–30 mm over the WR430 waveguide and a 30 mm unloaded converging waveguide antenna. With improved performance in power focusing and directivity, the proposed DLCWA is by far the most suitable antenna for microwave fracturing of low loss hard rocks. [ABSTRACT FROM AUTHOR]

Microwave fracturing of rocks prior to mechanical breakage has the potential to significantly enhance rock breakage efficiency while reducing the wear of cutters/bits of mechanical excavators. However, rocks in nature require a vastly different microwave power intensity to be fractured, and they tend to fail in varying patterns under different power intensities. At present, there is no established suggest method for selecting the most suitable microwave antenna to fracture a specific rock in the field. In this study, we compared the technical characteristics of the four open-ended waveguide-based antennas: namely, the horn antenna, the standard waveguide antenna, the converging waveguide antenna, and the dielectric-loaded converging waveguide antenna. Using a 6 kW microwave source and these antennas, we treated four types of hard rocks. We investigated the impact of antenna type on rock fracturing and found that depending on the rock type and antenna used, rocks were typically fractured or weakened in the pattern of cracking, spalling, melting, or a combination thereof. Based on our observations, we have, for the first time, proposed a method for selecting the optimal antenna to fracture rocks with varying microwave fracturability indexes. This paves the way for the commercialization of microwave-assisted mechanical rock breakage. • Compared the technical merits of four types of open-ended waveguide-based microwave antennas. • Evaluated the antenna performance by treating four types of rocks with different microwave fracturability indexes (MFIs). • Proposed a method for selecting the optimal antenna for different types of rocks based on their MFIs. [ABSTRACT FROM AUTHOR]

Microwave heating is a promising technique in assisting the breakage of hard and abrasive rocks. It features rapid heating and selective heating, enabling generation of large enough thermal stresses to induce damage in rocks. This paper investigated the effect of microwave treatment on the heating rate, spatial temperature distribution, ultrasonic wave velocity, elastic modulus and load bearing capacity of three igneous rocks (i.e., gabbro, monzonite and granite). The crack pattern and crack density were also studied using petrographic thin-section observations. Experimental results indicate that the single-mode microwave system can effectively and efficiently weaken the three rocks by generating cracks or melting/shattering the specimens. The heating rates and thermal gradients increase as either exposure time or power level increases. When heated at 2 kW for 120 s, the P-wave velocities of gabbro and monzonite can be reduced to 44% and 51%, respectively, S-wave velocities reduced to 34% and 62%, Young's modulus reduced to 13% and 27%, load bearing capacity reduced to 34% and 43%. In contrast, granite specimens were more prone to violent failures at high power levels. Petrographic observations imply that the dominant crack types in gabbro, monzonite and granite are intergranular & transgranular cracks, intergranular cracks, and transgranular cracks, respectively. [ABSTRACT FROM AUTHOR]

The thermal damage induced by microwave treatment in rocks has not been well explored, especially when using the dielectric-loaded converging waveguide antennas (DLCWAs). In this paper, the effects of microwave power level, heating time and working distance on the temperature increase, ultrasonic wave velocity reduction, and more importantly, the fracturing characteristics and the damage zone, are comprehensively investigated using a 6 kW microwave source and a customized DLCWA. A novel method is also proposed to adjust the P-wave velocity (V p) reduction across the thickness of the specimens and to determine the equivalent damage zone. The results show that the temperature and high temperature area, crack number, length and density, and the V p reduction rate as well as the damage zone all increase as either the power level or the heating time increases and decrease as the working distance increases. The damage zone can be divided into the crushed zone, fracture zone and discrete fragment zone based on the crack characteristics and the crack generation mechanisms. The method proposed is demonstrated to be reliable in predicting the damage zone induced by microwave heating. • Used a dielectric loaded converging waveguide antenna to fracture a biotite diorite. • Investigated the effects of microwave power level, heating time and working distance on thermal fracturing. • Proposed a novel method to estimate the P-wave velocity distribution across the total thickness. • Determined the equivalent thermal damage zone based on the spatial P-wave velocity reduction. [ABSTRACT FROM AUTHOR]