Numerical Study on the Weakening Effects of Thermal Pretreatments Application on Granite

Rock breakage or fracturing is a key process in mining, tunnelling, mineral engineering, and in geothermal energy recovery. A common issue encountered during traditional, mechanical fracturing is tool wear, which lengthens the duration of the operations and raises the overall costs. Moreover, rock breakage, especially when dealing with hard rocks, is an energy-intensive and inefficient process. This is note- worthy for rock comminution (crushing and grinding) as well, since it accounts for a large part of the total energy consumption in mining processes.

Thus, several rock pretreatment methods, consisting of application of physical agents to rocks and ores in order to weaken them, have been developed in the last decades, for the scope of rock fracturing efficiency improvement and consequently for equipment preservation and energy saving. These pretreatments utilize physical agents such as high pressure water-jets, lasers, plasma, ultrasounds, etc. Other pre- treatments employ thermal agents, which apply heat energy through conduction, convection, or radiation to induce cracks which weaken the rock and facilitate rock breakage.

In this work, the weakening effect of thermal pretreatments on mechanical properties and strength of granite-like rocks was studied numerically. Two different types of thermal pretreatments were explored: one using "conventional" heating (conduction and convection) and the other using microwave irradiation. Conventional heating was realised through simulations of high-power heat flux applied for a very short time to the outer surface of cylindrical specimens. Microwave irradiation was obtained through modelling of a multi-mode microwave oven, in which numerical samples were inserted and heated for longer times.

In this thesis, two models were used to describe rock fracture: an embedded dis- continuity model and a damage-viscoplasticity model. The thermo-mechanical (conventional heating) or electromagnetic - thermo - mechanical problems (microwave heating) representing these real-life physical phenomena were split in two or three steps, respectively, and solved separately by adopting a staggered approach. Two solution methods of the global thermo-mechanical part of the problem were developed: an explicit-explicit dynamic scheme and an implicit-implicit quasi-static scheme. The behaviour of the models and the solution methods was tested in 2D and 3D numerical simulations of thermal pretreatments application on granite specimens.

The results showed that these models are able to capture the temperature field and the crack/damage distribution in the heated rock. Moreover, the stress-strain curves from the uniaxial tension and compression tests performed on pretreated specimens showed that thermal pretreatments can be a feasible way to weaken rock before subjecting them to comminution. This analysis provides a viable starting point for developing real-life cases simulations of thermal pretreatments application on rocks.