A thermal method to fracture rocks

Int. J. Rock Mech. Min. Sci. & Geomech. Abstr.

Printedin GreatBritain.All rightsreserved

Vol.28, No. 6, pp. 509-514,1991

0148-9062/91 $3.00+0.00 Copyright© 1991PergamonPresspie

Technical Note A Thermal Method to Fracture Rocks A. I. OLIVA~f J. H. BALANC,~,N'{" M. E. NAVARROt

v. SOSAt

L. A. MALDONADOt P. BARTOLOt R. E. CASTRO# J. A. AZAMAR-BARRIOS#

INTRODUCTION

former, the detonation produces a high-pressure wave propagating along the whole charge, and in the latter Yucaffm (M6xico) State's surface is predominantly this wave is attenuated. karstic [1], i.e. it is a cacareous-type and has many We can mention several facts on which the effectivecracks. The covering soil layer is thin and comes from ness of the explosion depends: confinement, density, the limestone that, when dissolved in water, produces a diameter of perforation, mass of explosive. The sensisubstance called "Terra Rosa", which is typical in rainy tivity of the explosive is very important for the user's tropical regions. security. An explosion must be conducted easily under The peninsula's soil is highly permeable: this causes specific conditions, but difficult or impossible to occur one phenomenon seen only in this part of Mexico: the during the careful fabrication, shipment, storage and vertical erosion (formation of fissures and cavities) preparation of charges. Explosive power is classified in causes the limed material to be dragged by pluvial water, terms of percentage of nitroglycerine or equivalent making the formation of vegetation very difficult. This present in the explosive powder. phenomenon is seen in a large portion of the state. Since the rock existing in Yucathn's state is considered During the rainy seasons (especially during storms), of high enough quality to fabricate materials for conwater filtering drags the soil; this leaves the rocks struction, it is very important to perform tests to detercontinuously uncovered. minate its resistance under forces of compression of Given all the kinds of Yucatecan soils, it is possible to stress [3], as well as its absorption percentage, dry classify them according to their thickness: the "Tzck'El" volumetric weight and relative density. These three latter or thin, localized in central and northern regions of the tests are also known as the Index Tests of the rock. state; the "K'ankab-Tzck-El", medium-thick and localDetermination of the rock's water absorption is also ized all along the peninsula; and the "K'ankab" or deep, important to know in relation to the quality of the localized mainly in the southern part of the state and material; generally speaking, low water absorptions some areas of Panab~t and Tizimin villages. correspond to compact and resistible materials, unless The companies aiming to fabricate materials for they present planes of weakness. masonry and building construction in Yucaffm's peninOtherwise, there exist porous materials with low densula have employed for years rocks as a raw material or sities and high absorptions that are quiet deformable; for preparing concrete components. To extract these they are easily distinguishable because of the presence of rocks from quarries, dynamite is used. Explosives [2] are pores and visible cavities. If the dry volumetric weight [4] employed to break and reduce the rocks to sizes that is less than 1 g/cm~, materials are porous and have low allow the fragments to be handled easily. Charges of densities; for values exceeding this figure, materials have explosives are put along perforations in the rocks, and higher densities. detonated manually or by using an electric detonator. If The so-called "quicklime" is fabricated by burning the the reaction is complete, i.e. if it occurs instantaneously stones directly with fire [5]. As a result of this process, all along the body of the charge, the explosion will be an a whitish talcum-like residue is formed. One variety of effective one. However, a deflagration occurs when this, "extinguished lime", is one of the most common reacting particles are separated from non-reacting parmaterials used in the construction industry. During the ticles or simply when the exposive material burns. The fabrication of the lime, the stone exposed to fire first difference between these two reactions is that in the loses quarry water at temperatures between 120 and 150°C. At approx. 900°C, dissociation of CaO and CO2 tCentro de Investigaci6n y de Estudios Avanzados del Instituto Politrcnico National, Unidad Mrrida. A.P. 73, Cordemex,97310 takes place; the latter is transferred and wasted in the atmosphere in the form of gas, and the former kept as Mrrida, Yucatan, Mrxico. 509

510

OLIVA et al.:

TECHNICAL NOTE

ThermocoupLe

Ceromic cover :

0

,i ¸

'.g

(

'

'?

Rock

C

C .,.

/

Oo

~Ceramic rod

C (

- Resistive element

(

o.

'

Fig. I. Scheme of a rock and the thermal cartridge inside a perforation, with terminals connected to the power supply. The position of the thermocouple is also shown•

a profitable product. The CaO so obtained is known as quicklime. Knowing properties of the different types of stones, we can optimize the method to attack and destroy them. DESCRIPTION OF THE THERMAL METHOD

The traditional method for extracting rocks, as already mentioned, uses explosive material put into perforations spread along the whole surface of the rocks. The use of these explosives is highly restricted by Mexican law because of the danger existing when the explosions occur near urban zones, roads, etc. In the thermal method, every perforation in the rock is employed to receive an electrical resistance instead of filling them in with explosives. The terminals of the resistance are connected to an electrical power supply to produce heating by Joule effect. In this way, it is possible to reach a local temperature of 900°C in a period of 30 min. Under the appropriate conditions of time and power, the rock becomes fractured. Hence, we call this resistive element the "thermal cartridge". In Fig. 1 is shown a schematic diagram of the application of this method, indicating its components. The thermal cartridge is built from a resistive element, made of a high-melting point material (nichromel, in this case) wound on a ceramic axis to obtain rigidity. This resistive element is protected with a ceramic cover in order to

avoid oxidation from direct contact with the environment; this results in increasing its lifetime. The amount of times one thermal cartridge can be used will depend on the care taken in its use. A thermal cartridge is introduced in each perforation in such a way that it fills in the whole perforation, without leaving any free space to the walls. This method can be employed in a multiple and simultaneous way when extraction of large rocks from a quarry is desired. For this purpose it is enough to make a series of perforations distributed over the whole rock, and once the respective thermal cartridges are heated and the rock is fractured, the resulting fissures are joined; a lever makes the extraction very easy. Table 1 Stone Volume of stone (m3) Diameter of hole (cm) Depth of hole (cm) Supplied power (W) Rate of supplying power Time for appearance of first fracture (min) Temperature T (K) when first fracture appeared Total heating time (min) Final temperature (K)

Soft 0.018 0.95 20 540 full

Hard 0.07 2.54 25 1000 600 W/hr

55

35

1126 90 1197

1027 102 1513

Buried (3 holes) 1.5 2.54 55, 60, 63 1500 full, 380 W/hr, full 30, 40, 60 1082, 1063, 1148 150 1231, 1253, 1258

OLIVA et al.: TECHNICAL NOTE

511

Fig. 2. (a) Soft stone already broken after sudden heating, The sample was fragmented along the bigger fracture. Note the burned zone in the perforation. (b) Hard stone fragmented. Radial fractures from the perforation can be appreciated.

MEASUREMENTS AND DISCUSSION F r a c t u r e results

Fractures were obtained for several stones with slightly different densities (2. I-2.7 g/cm 9). We report the results obtained for three samples that will be referenced as "soft", " h a r d " and "buried". Three perforations were

made in the buried stone. These three holes were aligned and separated by a distance of 60 cm, approximately. They had different depths, the purpose being to observe how deep a fracture could be induced in this rock. Different thermal treatments were given to these stones. The parameters of the treatments were: rate for supplying power (gradual, or full from the beginning),

512

{)llVA c; ai.: TECHNICAL NOTE

supplied power and total time of heating. The temperature inside the perforation (T) was measured every 5 rain using a K-type thermocouple and a T E G A M 871 thermometer. An HP-6456B power supply was employed. The stones manifested the first fracture after a certain time, which is also reported in Table 1. We find that fractures are always obtained, regardless of the conditions of heating. The best results (least time for fracture) were obtained with 1000-1500 W, full, supplied from the beginning. In each case, fractures were radial. Both in the soft and hard stones, it was possible to break them in a few pieces with a hammer after heating. In Fig. 2a is shown the soft stone once broken. It is important to note how thin the burned (dark) zone is. This leads to the important result that the treatment did not modify the mechanical characteristics of the sample beyond this small zone. The same for the hard stone is observed in Fig. 2b. In the buried rock, it was observed that the three fractures extended along the whole depth of each hole. Also, the fractures joined each other; this makes it easy to separate and extract smaller

rocks from the ground. This can be appreciated in l=ig. 3 for the 55 cm hole, after removal of one of the smaller parts of the rock.

Chemical analysis By means of X-ray photoelectron spectroscopy (XPSI, we can find the chemical composition of the sample after the heating treatment. In Fig. 4, an XPS spectrum is presented. The expected Ca and O concentration in this region is confirmed, and we also found carbon residuals from unreacted CaCO3.

Thermal characteristics Since there was no information concerning the thermal characteristics of rocks in Yucatfin (as far as we know), it was necessary to measure these properties, and then look for the application of this method to break other materials with similar characteristics. To do this, the experimental procedure was the following: one side of the stone was heated, and the temperatures of both sides were measured; a 100 W light source was used as

Fig. 3. Detail of one perforation in the buried stone. Several fractures along the whole perforation (55 cm long) can be seen.

OLIVA et al.:

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Fig. 4. XPS spectrum of the hard rock (burned region). Only Ca and O are present, just as expected. Traces of C are observed. The unmarked peaks correspond to Ca and O, also.

a heater. As the stone was heated, an increasing difference of temperatures on both sides of the stone was established. In Fig. 5 a typical heating curve of these stones is shown. Once the steady state was reached, the thermal conductivity coefficient K was determined from Fourier's law:

K=

QX

a ( r 2 - 7",)'

where Q = heat supplied to the stone, X and A are the width and the transversal area of the stone, and T~, T] are the final temperatures on both sides of the stone.

25

20

15

I

0

10

2!0

= 30 Heating

time

........

4 d' 0

5 !0

60

(rain)

Fig. 5. Typical heating curve of a hard stone for measurement of thermal characteristics. Power supplied: 30 W; width: 4 crn; transversal area: 78 crn ~.

514

OLIVA et al.: TECHNICAL NOTE

Hard stone Soft stone Granite (6) Marble (6)

Table 2 cp(cal/g/K) 0.34 0.4 0.2 0.21

K(W/m/K)

7 6 2.9 2.5

An estimation of the specific heat cp can be obtained using the relation: W c° = m ( d T / d t ) o "

Here, m = mass of the stone, W = power supplied to the stone and ( d T / d t ) o is the initial slope of the heating curve. Results for different stones studied in this work are presented in Table 2, as well as values for other materials. We think that it was possible to fracture the stones by the thermal method because of their very high values of specific heat and thermal conductivity. In fact, the high gradient of temperature generated along the rock during the heating causes severe internal strains in the rocks, and this causes radial fractures. It would be interesting to know how marble and granite respond to this treatment; we have just begun to conduct some experiments in this direction. CONCLUSIONS AND PERSPECTIVES It was possible to fracture stones without explosives by using a "thermal cartridge". Fractures for both small

and buried rocks were obtained; the latter sample is quite similar to those found in quarries. Temperatures as high as 900°C are necessary to fracture the stones. The heating can be fast, and the calcination of the stone occurs only near perforation. We think that this happens because of the thermal characteristics of the stones, since a great thermal gradient is introduced into the samples and the resulting strains are high enough to break the material. With this method, risks are drastically lowered and losses of material are diminished too. Extrapolation to industrial scales and other materials is very promising. However, it will be necessary to conduct a more detailed study to optimize the method.

Accepted for publication 4 May 1991.

REFERENCES

1. Government of State of Yucatfin (Secretaryof Plan). Monography (1983). 2. Merrit F. S. Civil Engineer's Manual, Vol. 2. McGraw-Hill, New York (1984). 3. Soils T. J. A. Indice de correlaci6n entre el mrdulo de elastieidad de las rocas a compresi6n y a tensirn. Monography, Engineering Faculty, University of Yucatfin (1985). 4. Dzul M. F. Anfilisis de resultados en pruebas de roca caliza. Monography, Engineering Faculty, University of Yueatfi.n (1988). 5. Uc and V/tzquezA. A. Estudio de la cal en Yucat/m. Monography, Engineering Faculty, University of Yucat/m (1985). 6. Perry C. Chemical Engineer's Manual (Sth Edn), pp. 3-279.