Thermal stresses and temperature distribution of granite under microwave treatment

Abstract

Efficient breakage of high strength rocks is a challenging task where conventional drilling and blasting methods are not preferred or allowed, for example, rock excavation in long tunnels at a great depth and urban environment sensitive to noise and vibration. Microwave treatment is an efficient and environmentally friendly method, which is considered to be one of the most promising ways to increase the performance of mechanical excavators, such as roadheaders and impact hammers. However, the mechanisms of microwave fracturing of granitic rocks have not been well investigated. In this study, Chinese granite was used as the research material. Its mineral composition and grain size were first characterized using petrographic thin section observation. Cubic granite samples were then heated in a single-mode industrial microwave. The temperature was captured using a high resolution infrared thermal camera. Then a realistic 3-D rock model with the same mineralogical properties was established using Voronoi tessellation. The multi-physics numerical model was calibrated using data from microwave heating tests. The effect of microwave power intensity, heating time, and mineral grain size on the temporal and spatial temperature distribution, and thermal stresses in the granite matrix was investigated in a comprehensive manner. The temperature gradients and thermal stresses were calculated. The maximum temperature gradient and the maximum tensile stress appeared at the interface between quartz and biotite. As either power level or heating time increased, the thermal gradient and the maximum tensile stress increased, and with same microwave energy output, greater temperature gradient and thermal stress could be generated during microwave heating at higher power levels for shorter durations. Moreover, the greater the grain size, the bigger the maximum tensile stress. The finding of the paper contributes the understanding of the mechanisms of microwave fracturing of rocks.