The influence of microwave irradiation on thermal properties and fracturing mechanism of basalt in rock excavation

Abstract

Microwave fracturing is a promising technique for facilitating the efficient exploitation of deep earth resources while reducing energy consumption and cutter wear during mechanical excavation. In this study, the thermal properties of basalt under six power levels are investigated and the mechanism of microwave fracturing is elucidated through real-time monitoring and microstructural analysis. The results show that the failure modes of basalt can be categorized into high-temperature melting failure (>300 ℃) and low-temperature burst failure (<200 ℃). High-power microwave irradiation not only altered the failure mode but also modified the relationship between temperature rise and time. The temperature distribution exhibits a wave pattern, making it more prone to inducing transverse tensile cracks. Dehydration of basalt is triggered when the temperature exceeds 200 ℃, which subsequently promotes the initiation of macroscopic cracks. Microscopically, microwave fracturing is mainly driven by thermal stresses, while steam pressure, especially under high-power conditions, plays a dominant role in the fracturing process. These results are anticipated to provide necessary theoretical and technical support for the efficient exploitation of deep earth resources.

1. Introduction

Global energy shortage is a huge challenge to the sustainable development of human beings, developing deep earth resources is an important link for current energy exploitation [1−3]. However, with the increase of the resource occurrence depth, the nonlinear growth of rock mass strength significantly leads to deep hard rock difficult breakage and high energy consumption of mechanical excavation, which seriously restrict the efficient exploration and mining of deep resources [4−6]. As an efficient and no secondary pollution new assisted rock excavation means [7−9], microwave fracturing technology has attracted a wide spread attention from scholars.

Microwave heating effect is a key factor causing rock damage. ZHENG et al [10], LU et al [11] and KAHRAMAN et al [12] systematically explored the influence of microwave power and time on the thermal characteristics of hard rock and found that temperature and heating rate were positively correlated with power and time: the higher the power, the lower the failure temperature. YAO et al [13] and HASSANI et al [14] reported that the heating depth index decreased under microwave surface irradiation, and they found that high power levels, water and small size samples significantly increased the heating rate. Furthermore, mineral composition and content are the most crucial parameters affecting the degree of microwave heating [15]. LU et al [16] and ZHAO et al [17] classified the microwave sensitivity of minerals according to the heating rate. WEI et al [18] observed that an increase in the content of microwave-sensitive minerals led to higher temperature and more severe internal damage. Therefore, it is evident that the degree of microwave heating is governed by a combination of microwave parameters and intrinsic rock properties, which greatly influences the extent of microwave-induced damage within the rock.

In the process of microwave fracturing, rocks exhibit an uneven temperature field [19, 20], with induced cracks typically forming at the boundaries between high and low temperature regions [21]. AHMADIHOSSEINI et al [22] discovered that the real and imaginary parts of the dielectric constant of the rock have a significant impact on the temperature distribution and energy absorption intensity. ZHAO et al [23] revealed that a maximum stress ring formed around the high-temperature zone, and when this stress exceeded the tensile strength of the rock, it led to the generation of radial cracks. Experimental results from SUN et al [2] ...