Rock breakage is a key process in mining, mineral engineering, and in geothermal energy recovery industries. New and more efficient drilling and comminution methods are needed to save energy and extend the working life of mechanical tools and equipment. Thermal pretreatments of rock, which consist in heating rock in furnaces or muffles (“conventional” heating), or in microwave ovens, can be a viable way to facilitate mechanical fracturing, since such non-mechanical loads weaken rock by causing cracks and damage.
Thermal pretreatments may be useful for example to exploit geothermal heat resources in Scandinavia OR Nordic countries, where the bedrock is made of hard granite. This geological setting is very challenging for conventional drilling techniques since the boreholes reach depths of up to 5 to 7 kilometers. Therefore, the basic costs of the operations become critical due to the wear of the drill bits and for the time which is needed to replace them.
Another example of potential application of thermal pretreatments is microwave preconditioning in mineral extraction and processing since ores often contain both minerals that do not react to microwave heating (transparent minerals) and microwave absorbing minerals. The thermal stress caused by microwave irradiation can facilitate the separation of precious metals and minerals from gangue material.
For her doctoral dissertation, Martina Pressacco used computational methods to model conventional heating and microwave irradiation on granite specimens. She started by modelling conventional heating of 2D sections of granite and by obtaining the crack, stress, and temperature distribution inside the specimens. Next, she modelled microwave irradiation on 3D granite samples and derived the temperature, stress, and damage distribution. Finally, she compared the different outcomes of conventional and microwave irradiation pretreatments on a granite specimen in terms of ratio between induced damage and spent energy.
“With the upcoming green transition, there is an increase in demand for raw and critical materials. There is a growing interest towards green and cleaner energy sources as well. Our study on thermal pretreatments of granite may help address these urgent issues. Despite being a theoretical feasibilty study, it can pave the way for potential more complex numerical analyses of real case studies. Numerical modeling can help to improve these methods by highlighting the mechanisms underlying rock fracturing due to thermal loads,” Pressacco says.
Public defence on Tuesday 28 November
The doctoral dissertation of MSc. Martina Pressacco in the field of Structural Mechanics titled “Numerical Study on the Weakening Effects of Thermal Pretreatments Application on Granite” will be publicly examined at the Faculty of Built Environment of Tampere University at 12 o’clock on Tuesday, 28 November 2023 at Hervanta campus, Tietotalo, Lecture Hall TB104 (Korkeakoulunkatu 1, Tampere). The Opponent will be Professor Heinz Konietzky from Freiberg University of Mining and Technology, Germany. The Custos will be Senior Research Fellow, Docent Timo Saksala from Tampere University, Finland.