Experimental Study on Microwave Assisted Hard Rock Cutting of Granite

Hard rock cutting and mechanical excavation are common and widespread technologies that show a number of advantages over drilling and blasting technologies. Major amongst them is the continuous excavation instead of the blasting cycle. Extraction and loading takes place simultaneously, haulage can also be conducted while excavating. Furthermore, the surrounding rock mass is affected less, increasing safety as well as making scaling redundant. No blasting gases need to be vented and blasting noise does not apply to adjacent communities. The material excavated appears in a relatively distinct particle size distribution. Possible economic advantages in this regard are at hand – neither is the material pre-sized, nor do boulders affect the haulage chain’s effectivity. The major disadvantage, however, is the limitation of mechanical excavation machines to geotechnical conditions exceeding a certain abrasivity and rock strength. The aim of numerous research and innovation activities therefore is to overcome these barriers and to enable the technology to work efficiently also in tough conditions.

Firstly, this can be achieved by improving the quality of the cutting tools for example by introducing silicon carbide and diamond inserts. Secondly, completely new cutting concepts and principles are under consideration and development. These new concepts range from changing the cutting geometries to activation of the tool with artificial vibrations, undercutting and others. One third promising approach is to pre-weaken the rock mass in order to reduce the cutting resistance by introducing artificial cracks e.g. with the help of selective microwave irradiation.

Microwave treatment of rocks is driven by the absorption of microwaves by the rock, leading to a conversion of the electromagnetic energy into heat. Unlike classical convective heating, the heat flux is directly induced inside the material. Furthermore, different minerals show varying microwave absorbing abilities. These effects introduce temperature gradients into the rock that generate stress, which can exceed the strength of the rock, leading to the occurrence and propagation of cracks. The ability to transfer and absorb microwaves in a dielectric material is described by the complex dielectric constant \(\mathrm{\varepsilon }\) (permittivity):

where \(\mathrm{\varepsilon }_{0}\) defines the permittivity of vacuum, \(\mathrm{\kappa }_{\mathrm{r}}\) the real part of the relative permittivity and \(\mathrm{\kappa }_{\mathrm{i}}\) the imaginary part. The absorption of microwave energy within the material is mainly governed by \(\mathrm{\kappa }_{\mathrm{i}}\). According to Santamarina [1] typical values for hard rocks range from 10^-3 – 50 for \(\mathrm{\kappa }_{\mathrm{i}}\) and 2 – 10 for \(\mathrm{\kappa }_{\mathrm{r}}\) depending on various parameters (rock type, mineral distribution, microwave frequency, temperature, water content, …). Since rocks contain several minerals arranged in various distributions, different \(\mathrm{\kappa }_{\mathrm{i}}\) values appear in the material on the microstructure level. Typically well microwave absorbing minerals are plagioclase (\(\mathrm{\kappa }_{\mathrm{i}}\) = 0.004 - 0.32), pyroxene (\(\mathrm{\kappa }_{\mathrm{i}}\) = 1.62) and ilmenite (\(\mathrm{\kappa }_{\mathrm{i}}\) = 32.58) whereas quartz (\(\mathrm{\kappa }_{\mathrm{i}}\) = 0.0006 – 0.0033), orthoclase (\(\mathrm{\kappa }_{\mathrm{i}}\) = 0.00019) and muscovite (\(\mathrm{\kappa }_{\mathrm{i}}\) = 0.0006 – 0.0034) are poorly absorbing. Consequently, an inhomogeneous thermal field on the grain level is expected. Recently, several numerical studies of rocks with heterogeneous microstructures showed that the resulting stresses around the phase boundaries of strong-absorbing particles are high enough to initiate cracks which can propagate further into the material [2‐8]. It was already demonstrated that these thermally induced cracks may lead to a significant reduction in resistance during size reduction in comminution processes [9].

Despite the advances in microwave supported mineral processing no information is yet available on the possibilities of the technology in mechanical excavation. This paper will therefore introduce a combination of microwave pre-weakening and mechanical excavation with a conical pick and provide insights into the associated damage mechanisms.

Microwave irradiation leads to heating of the irradiated spots to surface temperatures of approximately 150 °C. It is assumed that the temperature beneath the surface is significantly higher (compare also [10]). The resulting crack pattern can be seen in Figure 1. A number of cracks are emerging from every single irradiation spot leading to an extensive cracks pattern all over the sample.

Linear cutting tests performed on an irradiated rock sample reveal a reduction of cutting forces associated with the microwave irradiation at a cutting depth of 4 mm with a spacing of 8 mm. Figure 3 shows the cutting forces acting on the pick when stripping one layer of granite. The figure shows a false coloured surface displaying the cutting forces as a function of the X‑Y position of the pick on the samples surface. To enhance visibility, the forces are smoothed using a moving average interpolation with a search radius of 50 mm. Black dots are highlighting the irradiated spots. The differences between the irradiated and the untreated parts of the block are clearly visible from these pictures. One can see that the entire area influenced by microwave irradiation shows reduced cutting forces when compared to the untreated area.

The cutting forces therefore range between 5.1 kN in the weakest to 7.3 kN in the toughest parts. As seen in Figure 4, the average cutting forces are 7.06 and 6.39 kN in untreated and irradiated part, respectively. This is equivalent to a reduction of 10 % caused by microwave irradiation.

It was already demonstrated by Restner and Gehring [11] that a change in rock mass properties, especially the Rock Mass Rating (RMR), will have a significant influence on the net cutting rate of and the specific pick consumption when cutting hard rocks. Microwave irradiation of a rock mass produced a number of artificial cracks, both on a macroscopic and a microscopic scale. Cracks can be caused by differential heating at grain boundaries, especially in the vicinity of the microwave beam. This was demonstrated for multiple rock types and ores in literature where extensive cracking on a grain-scale is reported for magnesite, andesite, dolomite and siderite [12] and a range of copper and lead-zinc ores (e.g. [13, 14]). Particularly the positive effect for copper processing was reported by various authors, e.g. [15‐17]. The presented results show that a range of larger cracks can also be introduced in some distance to the irradiation spot. Presumably this is due to stresses originating in the centre which are not just associated to different microwave absorption properties but also to physical phase changes like elongation of minerals under temperature load [6, 18].

The sum of these cracks represents a considerable reduction of RMR in the presented case, leading to a reduction of required cutting forces of approximately 10 % in the irradiated parts of a rock sample compared to the untreated parts of the very same sample.

In this paper we demonstrate the influence of microwave irradiation of granite, representing a typical hard rock, on the forces acting on a conical pick when cutting this rock in a linear cutting test rig. Thirty Seconds of microwave irradiation with 24 kW power in a chessboard-like pattern leads to a reduction of cutting forces by approximately 10 %. Furthermore, the technology shows numerous potential optimization points. Radiation times, spacing of radiation spots, adjustment of frequency can greatly increase the efficiency of a microwave treatment preceding mechanical excavation.