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Originalarbeit
BHM (2017) Vol. 162 (2): 77–81
DOI 10.1007/s00501-016-0569-0
© The Author(s) 2017. This article is available
at SpringerLink with Open Access
Experimental Study on Microwave Assisted Hard Rock
Cutting of Granite
Philipp Hartlieb1 and Bruno Grafe2
1Chair of Mining Engineering and Mineral Economics, Montanuniversitaet Leoben, Leoben, Österreich
2Institute of Mining and Special Civil Engineering, Freiberg University of Mining and Technology, Freiberg,
Deutschland
Published online January 5, 2017
Abstract: This paper describes the influence of high-power
microwave irradiation on the forces needed to cut granite
with a conical pick. Granite samples have been irradiated
with 24 kW at 2450 MHz with an open ended waveguide.
The effect of microwave treatment was assessed by linear
cutting tests with a conical pick.
Microwave irradiation lead to an extensive network of
cracks. The linear cutting tests provide a comparison of
treated and non-treated parts and show that peak as well
as average forces needed for cutting this very hard and
abrasive material are reduced significantly. The average
cutting force is reduced from7.06 to 6.39 kN, equalling 10%,
from the untreated to the irradiated part, respectively.
Keywords: Microwave irradiation, Rock cutting,
Alternative rock breakage, Alternative excavation
Experimentelle Untersuchungen zum mikrowellenun-
terstützten Hartgesteinsschneiden am Beispiel von
Granit
Zusammenfassung: Diese Arbeit beschreibt den Einfluss
von Hochenergie-Mikrowellenbestrahlung auf die beim
Schneiden von Granit auftretenden Kräfte. Granitproben
wurdenmitMikrowellen bei 2450MHzmit 24 kWmit einem
offenen Hohlleiter bestrahlt. Der Effekt dieser Bestrahlung
wurde mit einem linearen Schneidprüfstand bestimmt.
Es zeigt sich, dass dieMikrowellenbestrahlung zu einem
ausgeprägten Rissnetzwerk im Gestein führt. Die durch-
geführten Schneidversuche an mit Mikrowellen behan-
delten und unbehandelten Bereichen zeigen, dass sowohl
die gemessenen Spitzen- als auch die Durchschnittskräf-
Dipl.-Ing. Dr. P. Hartlieb ()
Chair of Mining Engineering and Mineral Economics,
Montanuniversitaet Leoben,
Franz Josef Strasse 18,
8700 Leoben, Österreich
philipp.hartlieb@unileoben.ac.at
te durch die Bestrahlung deutlich reduziert werden. Die
durchschnittliche Schneidkraft verringert sich demnach von
7,06 im unbestrahlten auf 6,39 kN im bestrahlten Bereich.
Dies entspricht eine Reduktion der Kräfte von 10 %.
Schlüsselwörter: Mikrowellenbestrahlung,
Hartgesteinsgewinnung, Schneidkraft, Alternative
Gesteinszerkleinerung
1. Introduction
Hard rock cutting and mechanical excavation are common
and widespread technologies that show a number of ad-
vantages over drilling and blasting technologies. Major
amongst them is the continuous excavation instead of the
blasting cycle. Extraction and loading takes place simulta-
neously, 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. Thematerial 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, how-
ever, is the limitation of mechanical excavation machines
to geotechnical conditions exceeding a certain abrasivity
and rock strength. The aim of numerous research and in-
novation 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 de-
velopment. These new concepts range from changing the
cutting geometries to activation of the tool with artificial
BHM (2017), 162. Jg., Heft 2 © The Author(s) Hartlieb & Grafe 77
Originalarbeit
Figure 1: Radiationpattern (left); Crackdistribution (middle);Radiation chamberwithwaveguide (right)
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 absorp-
tion of microwaves by the rock, leading to a conversion of
the electromagnetic energy into heat. Unlike classical con-
vective heating, the heat flux is directly induced inside the
material. Furthermore, differentminerals show varyingmi-
crowave absorbing abilities. These effects introduce tem-
perature gradients into the rock that generate stress, which
can exceed the strength of the rock, leading to the occur-
rence and propagation of cracks. The ability to transfer and
absorb microwaves in a dielectric material is described by
the complex dielectric constant є (permittivity):
є = єr + iєi = є0 (κr + iκi) ,
where є0 defines the permittivity of vacuum, κr the real
part of the relative permittivity and κi the imaginary part.
The absorption of microwave energy within the material is
mainly governed by κi . According to Santamarina [1] typ-
ical values for hard rocks range from 10-3 – 50 for κi and
2 – 10 for κr depending on various parameters (rock type,
mineral distribution, microwave frequency, temperature,
water content, . . . ). Since rocks contain several minerals
arranged in various distributions, different κi values appear
in the material on the microstructure level. Typically well
microwave absorbing minerals are plagioclase (κi = 0.004 -
0.32), pyroxene (κi = 1.62) and ilmenite (κi = 32.58) whereas
quartz (κi = 0.0006 – 0.0033), orthoclase (κi = 0.00019) and
muscovite (κi = 0.0006 – 0.0034) are poorly absorbing. Con-
sequently, 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 thematerial [2–8]. It was already
demonstrated that these thermally inducedcracksmay 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 possibili-
ties of the technology inmechanical 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 mecha-
nisms.
2. Methods and Material
2.1 Rock Samples
Thedescribed testswereperformedonsocalled“Neuhauser
Granite”. This granite is composed of 27 % quartz, 53 %
feldspar and20%micas. Singleminerals can beup to4mm
in size. The texture is granular with some xenomorphic
crystals. The rock shows a uniaxial compressive strength
of 210 MPa and a Cerchar abrasivity index of 4.2. Com-
pletely intact (crack free) samples were cut to blocks of
50x50x30 cm for further investigations.
2.2 High-power Microwave Irradiation
The high-powermicrowave irradiationwas performedwith
a 24 kW microwave source operating at a frequency of
2450 MHz with an open ended waveguide directed at the
rock surface. The apparatus allows for treating blocks with
dimensions of 50x50x30 cm (30 cm in depth). The pre-
cisely moveable x-y-table enables irradiating specific spots
at the rock surface. Corresponding to the size of the wave-
guide, these spots have dimensions of approximately 5 cm
in diameter.
Irradiation tests were performed in a chessboard-like
pattern consisting of three lines. Two lines had a spacing of
4.5 cm towards each other, the third one 9 cm. Each of the
red spots in the figure represents an irradiation spot. The
spacing between the spots in one rowwas 7.5 cm. The lines
represent the cracks emerging from those irradiation spots
as can be seen in the photograph. The irradiation parame-
ters were chosen to be 24 kW with 2450 MHz for a time of
30 s at each spot (Figure 1).
78 Hartlieb & Grafe © The Author(s) BHM (2017), 162. Jg., Heft 2
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Figure 2: Linear rock cutting
standHSX1000-50ofTUBerg-
akademieFreiberg
Figure 3: Floatingmediancalculationofcutting forces forpartly radiatedgraniteblock (XandYaxis: surfaceof theblock,Z-axis:movingaveragecutting
force, search radius=50mm)
2.3 Linear Cutting Test Rig
The cutting test rig HSX-1000-50 was specifically pro-
duced and manufactured by the company ASW GmbH
for TU Bergakademie Freiberg (Figure 2). It is used for
carrying out comprehensive rock cutting tests aimed at the
optimization of the rock cutting process (decreasing spe-
cific energy of rock destruction, maximizing cutting output,
reducing wear and tear of cutting tools, reduction of dust
and noise emissions). Programmable movement of a table
with a sample in cutting direction, the X-axis, and of the
tool holder in the respective Y and Z-axes enables cutting
tests to be carried out in awide range of cutting parameters
and trajectories. The relatively big size of rock samples (of
up to 60x120x50 cm) provides high precision and reliability
of given results. The rig is equipped with a laser surface
scanner, which extends scientific issues to be solved. The
power of 60 kW, flexible operating modes and modular
design of HSX-1000-50 allows for different types of cutting
tools to be tested. In operation mode, cutting and pene-
tration forces of up to 50 kN can be measured with piezo
sensors at a maximum sampling rate of 10,000 kHz while
the maximum cutting speed lies at 1.7 m/s.
3. Results
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 sig-
nificantly higher (compare also [10]). The resulting crack
pattern can be seen in Figure 1. A number of cracks are
BHM (2017), 162. Jg., Heft 2 © The Author(s) Hartlieb & Grafe 79
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Figure 4: Meancutting forceand its reduction comparing radiatedand
untreatedpartof specimen
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 8mm. 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 untreat-
ed and irradiated part, respectively. This is equivalent to
a reduction of 10 % caused by microwave irradiation.
4. Discussion
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 macro-
scopic 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 exten-
sive cracking on a grain-scale is reported formagnesite, an-
desite, dolomite and siderite [12] and a range of copper and
lead-zinc ores (e.g. [13, 14]). Particularly the positive effect
for copper processingwas reported byvarious authors, e.g.
[15–17]. The presented results show that a range of larger
cracks can also be introduced in some distance to the irra-
diation spot. Presumably this is due to stresses originating
in the centre which are not just associated to different mi-
crowave absorption properties but also to physical phase
changes likeelongationofmineralsunder temperature load
[6, 18].
The sum of these cracks represents a considerable re-
duction of RMR in the presented case, leading to a reduc-
tion 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.
5. Conclusions
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 ofmicrowave irradi-
ation 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.
Open access funding provided by Montanuniversität
Leoben.
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