ISSN 1062-7391, Journal of Mining Science, 2013, Vol. 49, No. 5, pp. 749–756. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.A. Leonenko, G.V. Sekisov, A.Yu. Cheban, S.A. Shemyakin, A.P. Kuz’menko, I.V. Silyutin, 2013, published in Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, 2013, No. 5, pp. 80–90.
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ROCK FAILURE
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Rock Failure under Laser Radiation N. A. Leonenkoa, G. V. Sekisova, A. Yu. Chebana, S. A. Shemyakinb, A. P. Kuz’menkoa, and I. V. Silyutinb a
Institute of Mining, Far East Branch, Russian Academy of Sciences, ul. Turgeneva 51, Khabarovsk, 680000 Russia e-mail:
[email protected] b Pacific National University, Shared Center for Laser and Optic Technology, ul. Tikhookeanskaya 136, Khabarovsk, 680035 Russia e-mail:
[email protected] Received May 13, 2013
Abstract—The review of the laser application practices in mining is followed by the discussion of experimental treatment of carbonate rocks by the continuous fiber-optic ytterbium laser radiation at output capability of 600 W. Local disintegration of carbonate rocks is estimated with a view to show the possibility and practicability of the laser-aided means and technologies in mineral mining and processing. Keywords: Laser radiation, fiber-optic laser, rocks, cutting energy efficiency, laser radiation rate.
INTRODUCTION
The present-day science and technology worldwide are stepping-up and enhancing studies into interactions of energy and matter with a view to create innovative technologies and design highperformance equipment. These intents are especially apparent in the area of quantum electronics and laser technology that are enjoying wide application in treatment of various solids. The laser technology market is thick, flexible and progressive [1]. Within half a century passed from the creation of laser and till its practical implementation, the key point was and remains the high-performance laser radiation interaction with solid, including rocks. The related research was initiated in the late 1960s in USA, USSR [2–5] and other countries, using laser with output power 0.5–1.0 kW. By the mid 1980s the interest in the problem weakened due to inapproachability of higher power lasers (over 10 kW), the absence of movable laser systems, low efficiency of the existing gas laser (in operation in the 1980s): their efficiency was 3–5% as against the modern fiber laser efficiency up to 30%. At the turn of the century, the International Forum on Advanced High-Power Laser and Applications AHPLA’99 in Osaka presented papers on application of high-power laser in mining and launched sessions on laser drilling and failure of rocks. Mainly, those were presentations by Japanese researchers from the Tokai University. The researchers that tested continuous СО2-laser 13 kW in power in tuff and granite drilling noted that laser drilling rate in granite was decreased from 1.8 to 0.7 mm/s when hole length grew from 3 to 12 cm [7–9]. The average rate of 8-m long drilling by СО2-laser 50 kW in output power was 2.76 mm/s in tuff and 1.23 mm/s in granite. The laser drilling rate in tuff is 2 times higher than machine holing. The tests on cutting sandstone and granite by СО2-laser with output power of 10 kW were performed at the following parameters: the cutting rate—3 m/min, the cutting thickness—100 mm (maximum cutting rate of 63 mm was reached at the cutting rate of 1.7 mm/s). Granite drilling was performed by laser beam radiation, which caused cracking and made the specimen brittle, and then mechanical drilling with drill diameter to 2 cm. The YAG double-laser with total output power of 6 kW was tested in situ. In 7 min laser radiation made a hole 2 cm in diameter in rock layer 20 cm thick at the drilling rate of 5 mm/s. 749
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In 1994 after USA adopted the program on application of the Cold War technologies to industry, American researchers started investigations of laser hole drilling in rocks, e.g., trial ground White Sand, studies of interaction of laser radiation with rocks, HF(DF)-laser MIRACL with output power of 0.5–1.2 MW. The tests showed increased rate of drilling with laser as compared with the conventional hole-making methods, which freshened the interest to the use of laser radiation in mining [10]. The Russia–America joint project in 1997–1999 to analyze interaction of laser radiation with rocks that typically enclose oil-bearing formations using infrared СО- and СО2-laser was aimed at designing of laser drilling rigs. The project was implemented by the scientists from Colorado School of Mines, Gas Research Institute (now Gas Technology Institute), Lebedev’s Physical Institute and Moscow State Mining University [12–14]. The typical oil-enclosing rocks were sandstone, limestone, shale and granite. The laser radiation energy density and strength were 1 kJ/cm2 and 107 W/cm2, respectively. The process of laser flare on the surface of specimens was analyzed using the high-rate photographic recording method, and the laser flare propagation velocity was measured. The differences in the processes that are induced in silicates by CO- and СО2-laser radiation were identified. The authors [15] analyzed reflection and absorption spectra of rocks before and after laser treatment. Until recently, material treatment mainly used high-power СО2-laser. The compiled set of industrial laser technologies based on СО2-lasers displayed high-quality cutting of wider range of materials. However, СО2-lasers were bulky, lacked fiber outlet, wanted high efficiency and quality of the beam and, thus, gave way to the fiber lasers that were the breakthrough in the laser physics [16]. Within a few years, the output power of the fiber lasers was enhanced from hundreds watts to several tens of kilowatts. The technological set of the fiber laser technologies is upon the anvil [17], and the industries that use laser treatment of material face the problem of choosing parameters of the fiber lasers to be the best suited with the assigned tasks. The fiber laser technology set is being compiled for different metals and polymeric materials. More than 30 years, the Russian scientists from the Lebedev’s Physical Institute, Prokhorov’s Institute of General Physics, Institute of Problems of Laser and Information Technologies and the Moscow State Mining University have been studying laser radiation interaction with rocks (mainly, silicate rocks treated by infrared CO- and CO2-lasers). The Institute of Mining of the Far East Branch RAS and the Pacific State University carry out the team research aimed at development of laser technologies in mining [18–25], using the ytterbium fiber laser of continuous radiation at maximum output power of 600 W (manufactured by IRE-Polus, affiliate of IPG Photonics Corporation). The equipment is to the courtesy of the Shared Center for Laser and Optical Technologies, Russian State Scientific Center for Robotics and Technical Cybernetics, Saint-Petersburg, whose mission is to handle challenges in the framework of top-priority trends in education, science, technology and technique in Russian Federation. The continuous fiber lasers feature high efficiency (to 30% if charged and to 70–80% after optical injection), small size, reliability and quality of beam radiation. The high surface-to-volume ratio of the fiber waveguide greatly simplifies the procedure of the laser cooling. The application horizons of the high-power fiber lasers are broad. These lasers are used to treat materials (cutting, welding, drilling, etc.) in automotive, space and other industries. American institute GTI [10] advanced an idea of rock cutting and destructing by lasers. The conception was backed up by the research accomplished in 2006 into tunneling in silica and carbonate rocks using fiber lasers as well as into low energy laser drilling in sandstone and limestone [10, 26, 27]. The GTI report on the research findings is available at the site of the US Department of Energy: www.osit.gov. JOURNAL OF MINING SCIENCE Vol. 49 No. 5 2013
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After the first success of the high-power laser trials [11, 14], GTI carried out tests with industrial laser equipment. It has been found out that the industrial ytterbium fiber laser can break rock at the energy level comparable with the machinery disintegration energy [27]. Fiber lasers have become the lead technical device used in mining, tunneling, cutting and drilling in rocks and concrete. Ytterbium fiber lasers transfer cutting energy for 5 to 200 m in rocks, at a certain degree of accuracy. The experiments with fiber lasers manufactured by IPG Photonics (www.ipgphotonics.com), with power of 3.0 and 5.4 kW showed that the fiber laser appeared more efficient than the other types of lasers in drilling in limestone and sandstone [27]. The experiments used limestone and sandstone blocks 50.8×12.7×12.7 cm. The 3.0 kW power fiber laser beam penetration in sandstone reached 7.6 cm, with the hole diameter of 2.54 cm after 62 s of the focused radiation. Currently, GTI emphasizes cutting down in costs of exploration and drilling (since 2011) owing to the use of the laser energy, with the power of fiber lasers reaching 50 kW. Besides, the ecological impact has been mitigated. Capacity of a laser is estimated by its cutting rate. This article estimates laser-aided disintegration of carbonate rocks with the aim at developing geotechnologies featuring energy preparedness, economical effect and ecological safety. Below the authors present the research performed at the Institute of Mining, Far East Branch of the Russian Academy of Sciences, and at the Pacific State University. The tests of laser-aided disintegration of rocks took specimens of limestone and pink dolomite from Londokovsky deposit, Sopka I-II, Jewish Autonomous Region. The limestone specimen had density 2.32 t/m3, freeze resistance FR 100, 200, and strength to 60 MPa; the dolomite specimens had density 2.77 t/m3, FR 100, 200, 300, and strength to 80 MPa. The brick-shaped specimens 40 mm wide and long were cut from the rock mass and fixed on a test table with the adjustable advance. The advance rate toward the laser beam radiation was 2.0, 1.0 and 0.5 mm/s. The test laser was ytterbium fiber laser LS-06, IRE-Polus, with output power of 600 W. The radiation was continuous, with wavelength of 1070 nm, using the fiber optic GBH-type connector 50 µm in diameter. The connector and cutting face spacing was 4 mm; the radiation power was varied within 29 and 95% of the laser rated power of 150, 300, 450 and 570 W; maximum and minimum laser power densities were 15·106 and 4·106 W/cm2, respectively. There were three experimental series per 12 cutting regimes (three advance rates and four radiation powers), which made up 36 experiments totally. After the cutting was completed, the cut depth was measured with the accuracy to 0.1 mm. For higher measurement accuracy, the cut depth (40 mm) was divided into segments 10 mm long, and the maximum and minimum penetration depths were estimated in each of the segments. In case of microflaws of inclusions, the radiation penetration depth deviated by 0.5–2 mm from the average value. By the results of 8 measurements (for each of the three specimens), the average depth of cut with the given cutting regime was identified with the accuracy to 0.01 mm. The experiments also specified main regimes and parameters of laser radiation, and sequence of its effect on the specimens. The depth of cut grew with increasing laser radiation power (see Fig. 1). For instance, in cutting the limestone specimen at the cutting rate of 0.5 mm/s, radiation power P of 150 W, the cut depth h was 6.90 mm (Fig. 1a); whereas at the double power (to P = 300 W), the cut depth increased to 13.27 mm, i.e., by 92%. At three times as much power (to P = 450 W), the cut depth was 19.44 mm, i.e. grew by 182%. At radiation power 570 W, the cut depth was 24.15 mm, i.e. 250%, which is probably due to extra energy required to heat the specimens, availability of heterogeneities, roughness of cutting face and focal distance bias. JOURNAL OF MINING SCIENCE Vol. 49 No. 5 2013
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Fig. 1. The curves of the laser radiation cut depth h and the laser radiation power P in (a) limestone and (b) dolomite at different cutting rates: 1—2.0 mm/s; 2—1.0 mm/s; 3—0.5 mm/s.
As compared with limestone, dolomite is 17% higher in density and 25–33% higher in strength. Other conditions equal, the dolomite specimen cut depth by laser is greatly smaller than that in the limestone specimen. For example, at cutting rate of 0.5 mm/s and laser radiation power of 570 W, the cut depth in limestone is 24.15 mm and the cut depth in dolomite is 20.05 mm, i.e. 17% smaller. After disintegration the cut surface area S was estimated. The energy consumption E was calculated as the product of P and the cutting time t. Depending on the cutting rate v, the cutting time was varied between 20 and 80 s. The efficiency of the laser disintegration of the rock specimens can be expressed in terms of the specific cutting energy expressed as a ratio of the cutting energy consumption to the cut surface area. The experimental data are compiled in the table. The specific energy of cutting depends both on the cutting rate and radiation power. As the cutting rate is increased, the specific energy of the process lowers (see Fig. 2): at the radiation power of 450 W in limestone, the specific cutting energy is 41.28 J/mm2 at the cutting rate of 2 mm/s and 46.29 J/mm2 at 0.5 mm/s, i.e. 12% higher. Parameters of limestone and dolomite cutting by laser radiation
150 300 450 570
40 40 40 40
3.62 7.02 10.37 12.98
150 300 450 570
80 80 80 80
6.90 13.27 19.44 24.15
Cutting rate 2 mm/s 76.4 65.2 148.0 126.0 218.0 185.6 272.4 232.0 Cutting rate 1 mm/s 3.09 144.8 123.6 5.98 280.8 239.2 8.79 414.8 351.6 10.96 519.2 438.4 Cutting rate 0.5 mm/s 5.79 276.0 231.6 11.12 530.8 444.8 16.24 777.6 649.6 20.05 966.0 802.0 1.63 3.15 4.64 5.80
dolomite
1.91 3.70 5.45 6.81
Cutting energy consumption, J
limestone
20 20 20 20
dolomite
150 300 450 570
Specific cutting energy, J/mm2
Cut surface area, mm2 limestone
Cutting time, s
limestone
Radiation power, W
dolomite
Cut depth, mm
3000 6000 9000 11400
39.26 40.54 41.28 41.85
46.01 47.62 48.49 49.14
6000 12000 18000 22800
41.44 42.74 43.39 43.91
48.54 50.16 51.19 52.01
12000 24000 36000 45600
43.48 45.21 46.29 47.20
51.81 53.96 55.42 56.86
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Fig. 2. The specific energy Esp of laser disintegration of (a) limestone and (b) dolomite versus the laser radiation power P at different cutting velocities: 1—2 mm/s; 2—1 mm/s; 3—0.5 mm/s.
Fig. 3. The dot element electron images of the original specimens: (a) limestone; (b) dolomite.
In the same cutting regimes, the specific cutting energy is higher in dolomite as against limestone: at the radiation power of 450 W and the cutting rate of 1 mm/s; the specific cutting energy is 51.19 and 43.39 J/mm2 in dolomite and limestone, i.e., the latter is 18% higher than the former. The surface of the test rock specimens before and after the laser-aided disintegration were examined using LEO EVO 1455VPSE microscope, Carl Zeiss, Germany, equipped with energydispersive INCA-ENERGY analyzer meant for qualitative and quantitative elemental analysis. The test-sensitivity is 0.1%; electron beam width is 20–30 nm; penetration depth is 1 µm. JOURNAL OF MINING SCIENCE Vol. 49 No. 5 2013
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Fig. 4. The dot element electron image of the laser-treated limestone specimen (radiation power 150 W) and the spectrograms of locally bounded objects.
The elemental composition was estimated within the locally bounded sites on the surfaces of the limestone and dolomite specimens, by the energy-dispersive spectral values. The electron microscopic images of the original specimens, with the hypothesized elemental composition of the local sites are shown in Fig. 3. In terms of stoichiometric oxygen, the original limestone specimens contained: 30–49% of calcium oxide; 58–70% of carbon dioxide, and under 0.3% of admixed manganese and magnesium; the original dolomite contained: 13% equally of calcium oxide and magnesium and 75% of carbon dioxide. Visualization of the microscopic observations in Fig. 4 displays appreciable difference in the surfaces untreated and treated by laser radiation (spectrum 1 and spectrum 2 in Fig. 4, respectively). The increase in the laser radiation strength from 4·106 to 8·106 W/cm2 does not entail much change in the spectra of calcium, carbon and oxygen (Fig. 4). The quantitative composition of oxides of alkaline-earth elements (Са, Mg) in the laser-treated surface layer grows only by 2–3%. The laser heating of the test rock specimen and then free cooling results in formation of surface fractal structures containing the original elements and the new ones, such as K, Al, Si, Zr (Fig. 5).
Fig. 5. The dot element electron image of fractal structures on the surface of the laser-treated rock specimen. JOURNAL OF MINING SCIENCE Vol. 49 No. 5 2013
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Fig. 6. The fractal structures in carbonate rocks with the completely outburnt carbon after the laser radiation with the strength of 80·106 W/cm2.
Under the laser radiation strength of 15·106 W/cm2, the subsurface layer carbon burns out completely (Fig. 6), which is explained by the ultimate thermal stress reached in the lattice in the laser-aided disintegration zone. So, the process of rock disintegration by laser radiation was carried out by the fiber laser with output power of 600 W; maximum radiation strength of 15·106 W/cm2, focused radiation diameter of 70 µm; the specific energy of laser cutting ranged between 40 and 60 J/mm2 in the cutting rate range 2–5 mm/s. CONCLUSIONS
Fiber lasers show wider horizons in material treatment and enjoy unlimited application in industry. The present-day series-produced fiber lasers have power from 100 W to 10 kW. From the world experience, the market segment of the fiber lasers 3 to 10 kW in power expands. These high-power lasers leave other laser types behind in all commercially significant characteristics, do not need equipment to be adjusted, require no highly qualified maintenance and are computer-connectable and, thus, admit programmed numerical control. The high-power lasers easily penetrate mining process flows owing to low energy consumption, simple operation and high endurance. The presented research findings allow a substantiated approach to selecting efficient regimes of rock disintegration by laser radiation, as well as the qualitative and quantitative estimate of chemical transformations under fast radiation-induced thermal processes. ACKNOWLEDGMENTS
This work was supported by the Russian Foundation for Basic Research, grant no. 13-05-00586, and by the Far East Branch of the Russian Academy of Sciences, project no. 12-III-A-08-179. REFERENCES 1. Panchenko, V.Ya., Lazernye tekhnologii obrabotki materialov: sovremennye problemy fundamental’nykh issledovanii i prikladnykh razrabotok (Laser Technologies of Material Treatment: Current Fundamental and Applied Problems). Moscow: Fizmatlit, 2009. 2. Mukhamedgalieva, A.F., Bondar’, A.M., Ziborova, T.A., Baranov, R.I., and Panin, M.I., Continuous CO2Laser Radiation Effect on Quartz Minerals and Quartz-Bearing Rocks, Kvant. Elektron., 1975, no. 1. 3. Mukhamedgalieva, A.F., Bondar’, A.M., and Ziborova, T.A., Potash Feldspar KA1Sí308 IR Adsorption Spectrum Deformation under СО2-Laser Radiation, Zh. Tekh. Fiz., 1976, vol. 2, no. 1 4. Mukhamedgalieva, A.F. and Bondar’, A.M., Laser-Simulated Reactions on the Surface of Quartz and Some Other Minerals, Fiz., Khim., Mekh., 1983, no. 5. JOURNAL OF MINING SCIENCE Vol. 49 No. 5 2013
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