How High-Power Microwave Irradiation Can (Economically) Assist Mechanical Drilling

Microwave irradiation of hard rocks conducted prior to conventional mechanical drilling could make mining and excavation processes more energy efficient, according to researchers at Institute of Mechanics of the Montanuniversität Leoben (Leoben University of Mining Sciences).

Scientists estimate that mechanical fragmentation or comminution of solid rock is just 1 to 2 percent efficient, with the majority of the spent energy dissipating as heat and noise. In order to improve the process, the technique of microwave irradiation prior to mechanical treatment has been explored for decades. Until lately, though, test results on the effect of propagated microwaves on rock samples were inconclusive.

Researchers at Leoben said in an earlier study that "in order to understand the full process from microwave propagation in the rock, to absorption, heating, thermal expansion and finally stress, the material has to be characterized in much greater detail."

So, they investigated the mineralogical composition and thermo-physical properties of basalt, granite and sandstone, and how the properties of these rock types absorb microwaves. High-power microwave irradiation tests of granite, in particular, demonstrate that with sufficient heating, a crack network can be established around the irradiation spot.

The Leoben researchers then conducted a comprehensive 3D numerical study to validate their earlier findings on granite, a typical heterogeneous hard rock found in excavations.

In the numerical simulation, a Voronoi tessellation algorithm was used to produce a realistic 3D microstructure that showed microwave-induced stresses in the granite samples. An FDTD (finite difference time domain) algorithm computed the electromagnetic field, and a finite element (FE) analysis calculated the thermal, as well as stress field. Other input, such as temperature-dependent thermo-mechanical and physical material properties, were added to complete the simulation.

The researchers ran simulations of short, intense micropulses lasting only a tenth of a second, and longer pulses of lower intensity that lasted 100 seconds.

“In the simulations, the short pulses showed a little more effect with the same amount of energy,” said Thomas Antretter, who is the principal investigator of the project funded by the Austrian Science Fund FWF.

A separate team corroborated the simulation's results through experiments using a microwave unit with an output of 25 kW, which is about 25 times the energy produced by a microwave oven.

“They actually irradiated rock samples under different conditions and for different lengths of time. It turned out that you can create crack patterns and that they will correlate well with the results of our simulation,” said Antretter.

If technical hurdles, such as fire safety issues, are overcome, microwaving hard rocks prior to mechanical breakage could significantly make mining and excavation more economical.

“We do not want to replace mechanical excavation completely, that would be impossible. But we can make it easier,” said Antretter.

A team at MIT also is exploring how powerful millimeter RF waves can penetrate hard rock by melting or vaporizing it, which could lower the cost of current mechanical drilling systems by ten times.