Microwave-induced modifications of properties of rocks and fragmentation

Rock fragmentation is a process which is applied in big volumes for tunnelling, extraction of valuable ores, and processing of minerals. An efficient rock fragmentation process is characterized by a high performance rate and by low consumption of tools and breakage energy. The very attractive issue with using microwaves in this respect is the fact, that microwaves in rock have a certain penetration depth, introducing artificial cracks not only close to the surface but to a certain depth which is of the same order as mechanical treatment. Preparatory work has shown that modifications range from the generation of large cracks to spallation and even to total breakage of samples depending on the type and origin of the rocks. This work also has shown the necessity for an improved understanding of the basics of the processes and for optimizing the conditions for applications. The final goal of the present proposal is aimed at conclusions about future applications for rock fragmentation by using cw high-power microwaves. For approaching this final goal, individual goals are: (a) Establishing a data base of the relevant rock properties - dielectric permittivity, thermal diffusivity, thermal expansion and specific heat capacity. (b) A simulation package which consistently includes microwave absorption, microwave heating, distributions of temperature and stresses. (c) Conditions for selective heating and strong local stresses with pulsed irradiation. (d) Conditions for selective straining as a novel aspect taking into account the a-ß phase transformation of quartz at 573C which is accompanied by a volume change. (e) Conclusions for optimizing the microwave irradiation conditions for the damage of rocks which enables more efficient and cost effective fragmentation. The interdisciplinary collaboration of physicists, mechanical engineers, mining engineers and geologists and the different types of simulations with input from experiments will provide a profound basis for the development of a consistent simulation package and are a novel integral approach to the problem of microwave rock interaction, the resulting rock heating and the damage. The work proposed is considered as a potentially successful path towards the main focus "Production Technologies" of the TRP with special emphasis on new products and production tools, e.g., microwave-assisted mechanical cutting or even microwave-induced fragmentation only, flexibility on the basis of adjustable irradiation conditions, establishment of competence for more efficient materials production. An industrial partner for future activities with the goal of developing hybrid excavation and comminution machines, using a combination of mechanical breakage with microwaves will be provided with profound knowledge for applications.

Excavation processes in mining and tunneling are very costly, especially if the rock material is very hard. This is primarily due to the fact that only 1% of the invested energy is converted into the creation of new surfaces, i.e., the fragmentation of the rock. Almost the entire rest is lost as frictional heat generated by the tool scraping over the rock. In an effort to increase excavation efficiency engineers have long been considering more economical alternatives. One idea is to heat up the rock by means of microwave irradiation thereby creating thermal stresses high enough to break or at least generate cracks in the rock so that subsequent mechanical processes already cut into a partly pre-damaged and hence softer material. While the principle has been around for some decades, its potential merit has never fully been quantified despite various research activities on that issue. The investigations performed in the framework of the FWF project TRP 284-N30 now managed to provide that previously missing quantitative information by virtue of a thorough experimental as well as theoretical analysis of the physical phenomena coming into play. This included a) the propagation of the electro-magnetic waves emitted by a high-power microwave source into the rock-material, b) locally varying heating due to absorption of the microwaves, c) redistribution and evolution of the temperature field due to heat conduction, d) thermal expansion of the differently heated areas followed by mechanical strains and stresses inside the rock that locally exceed the rock?s strength thus leading to micro-cracks that eventually coalesce and form a dominant crack. The project also covered the much more complex case, when the heterogeneous nature of natural rocks is taken into account. Then the individual constituents of a rock exhibit different material properties significantly amplifying the temperature gradients and stresses hence accelerating the formation of cracks. The theoretical investigations were carried out by numerically solving the equations governing the phenomena enumerated above. This required massive computational resources, provided by the high-performance cluster available at Montanuniversität Leoben. In order to ensure the predictive quality of the numerical results accurate material data were determined in sophisticated experiments conducted in collaboration with other research institutions (Österreichisches Gießerei Insitut, Seibersdorf Labor GmbH) or using equipment designed and manufactured at in-house facilities. The results of this truly interdisciplinary work, with contributions from the fields of continuum-mechanics, microwave physics and mining engineering, allow for the first time reliable predictions as documented by an excellent agreement with crack patterns found in full-scale microwave irradiation experiments. The methodology developed in the project thus lays the theoretical foundation for the potential use of microwave-assisted rock fragmentation technologies on an industrial scale.