Cámaras de Aire una real alternativa en Voladura de Rocas Eduardo Araya Chávez, Ing. Minas, International Technologies, Chile
RESUMEN El Principal objetivo de la Voladura de Roca es la fragmentación del macizo rocoso, en una adecuada distribución de tamaños, con un mínimo de daños a las paredes e infraestructuras y a un costo razonable. La Energía disponible es una columna explosiva que no siempre se distribuye en forma eficiente a lo largo de un pozo de voladura de roca. Las cámaras de aire en sus diferentes configuraciones ayudan a distribuir mejor la carga explosiva. International Technologies ha realizado un aporte a este fenómeno con la creación de cámaras inferiores. Con el uso de cámaras de Aire se puede conseguir los siguientes beneficios: •
Reducir la longitud del taco
•
Retener la energía en el pozo
•
Disminuir sub drill
•
Reducir consumo de explosivo
•
Distribución de fragmentación más uniforme
•
Reducir Fly rock y onda aérea
•
Disminuir daño a las paredes finales
•
Disminuir contaminación de emulsión por el material del taco
HISTORIA CAMARAS DE AIRE Desde el comienzo de la era industrial, en voladura de rocas también se busco optimizar el uso de la energía explosiva, es así que tenemos algunos de los principales avances: +100
Robert B. Hopler; Used underground
1940
Mel’Nikov & Marchenko; Surface Blasts
1961
Univ. of Missouri; Mid-column air decks
1980
Univ. of Maryland; Modelling South Africa; Pre splitting Atlas Powder Co;
Field tests
1987
Chiappetta & Mammele; Air Deck
1999
Intec; Test in Evergreen
2001
R. F. Chiappetta; Nueva Técnica Voladura
2006
W.L. Fourney;
Borehole Pressures in an Air Decked
Todos estos estudios y pruebas empíricas resultaron en el desarrollo de: Nuevas Técnicas de Voladuras Productos explosivos Accesorios especiales Algunos de ellos son: •
Cargas desacopladas
•
Pre Splitting
•
Voladura de Contorno en Minería Subterránea
•
Emulsiones de Pequeño diámetro para Precortes
•
Explosivos de baja densidad
•
Air decking o cámaras de aire
•
Elementos o accesorios plásticos, bolsas, retenedores de taco
CÁMARA DE AIRE INFERIOR Las cámaras de aire o air decking se implementaron en la minería de carbón en EEUU. Para conocer un poco de estas importantes operaciones, una bre ve reseña. El combustible para la generación termoeléctrica en EEUU es el carbón. Los tres principales estados productores de carbón en EEUU son: 1. West Virginia 2. Kentucky 3. Wyoming Los métodos de explotación en estas minas de carbón son: Open pit tradicional y el Casting, este último consiste en explotar la capa superior estéril del manto de interés, con perforación, voladura y desplazamiento con Draglines y Buldózer del material. La capa superior de estéril pude tener espesores de 10 a 30 m como el caso de la foto , Figura 1
Figura 1: Mina Carbón en Kentucky.
Durante la segunda mitad de la década 1990, el bajo precio del carbón y alto costo de explotación, comienza a inquietar a las compañías m ineras de carbón de EEUU. Uno de los problemas que complica la operación es la alta dilución generado por la detonación de los hoyos de voladura en el contacto estéril – carbón. Como se aprecia en Figura 2
Figura 2; Voladura Tradicional en Casting La faena minera Evergreen tenía un esquema de explotación con perforación en gran diámetro, hasta una profundidad de 2 a 3 m sobre el manto de carbón, la capa estéril que cubría el remanente de mineral era extraída con perforación y voladura en pequeño diámetro, faena lenta y de alto costo. Figura 3
Figura 3; Mina Evergreen en West Virginia En esta faena se realizan las primeras pruebas con cámara de aire inferior, la perforación primaria de producción comienza a disminuir la distancia al manto de carbón, hasta llegar
a eliminar dicha capa, con buenos resultados de piso y eliminando el daño al manto de carbón. Los resultados obtenidos se explicaban dado la alta estratificación del cuerpo mineralizado, entonces se decide probar el sistema en una faena sin estratificación, es decir, un cuerpo masivo o porfido cuprífero. Se busca además realizar un estudio m ás acabado para entender lo que estaba pasado al fondo del taladro con la cámara de aire . Este estudio se le solicita a la empresa BAI. COMO FUNCIONA LAS CAMARAS DE AIRE Para obtener una explicación técnica del proceso logrado empíricamente, se solicita a BAI (Blasting Analysis International) un estudio . El trabajo es realizado por R. F. Chiappetta, y posteriormente presentado y publicado por ISEE en New Orleans el 2004 . Una breve reseña de este estudio se entrega a continuación. El estudio se realiza en una faena minera del estado de Pennsylvania. Se efectúan varios test a los cuales se miden los mismos parámetros para poder evaluar y comparar. Para evaluar de usan varios instrumentos que se detallan a continuación: Figura 4 1. Borehole Inspection Camera 2. TDR Based VOD Recorder 3. High-Speed Video Camera 4. Digital Fragmentation Analysis 5. Conventional & Laser Surveying 6. Digital Seismographs
Figura 4: Instrumentos para evaluar voladuras
Los test realizados se muestran en la figura 5 siguiente
Figura 5: Esquema pruebas con cámaras de aire Para medir la velocidad de detonación V0D se perfora desde la cara libre del banco un pozo de 3” inclinado para interceptar al pozo vertical y extraer por ahí el cable coaxial, el esquema se muestra en Figura 6.
Figura 6: Esquema para medir V0d en cámara inferior
Todas las pruebas anteriores se realizan pozo a pozo, para evaluar: desplazamiento, sobre quiebre, fragmentación, vibración, se realiza el siguiente esquema Figura 7.
Figura 7: Pruebas voladura completa Estas pruebas se realizan con sistema de perforación y voladura tradicional, es decir con sub drill y sin cámara inferior y se compara con perforación sin sub drill y cámara inferior.
Figura 8: Esquema para medir vibraciones
RESULTADOS OBTENIDOS Con el estudio realizado por BAI se comprueba que las cámaras de aire inferior transforman la Voladura Tradicional compresiva en una nueva Técnica de Voladura Tensional. Los principales resultados son: 1. Aumento de la presión de barreno en la zona de la cámara. 2. Se transforma la energía potencial del aire en Energía cinética. 3. Se reduce el factor de carga 4. Se disminuye las vibraciones 5. Se reduce Fly Rock 6. Se disminuye la Onda aérea.
Figura 9: V0D en cámara de aire
La presión de barreno en la zona de la cámara se aumenta entre 2 a 7 veces Figura 10.
Figura 10: Aumento de Presión en cámara de aire La misma situación ocurría cuando se comenzaron a fabricar los primeros detonadores noneles a inicio de los 90 Figura 11.
Figura 11: Aumento de presión
La energía potencial del aire se transforma en energía cinética, esto se determina con el VOD medido en pozo inclinado de 3” Figura 12.
Figura 12: Transformación de Energia La vibración medida en voladuras con cámaras de aire resultaron 30% menor.
Figura 13: Vibración medida
La fragmentación resultante es más homogénea y de menor granulometría.
Figura 14: Resultados de Fragmentación
TRABAJOS PUBLICADOS Existen diversos trabajos técnicos y memorias de titulo que se han realizado aplicando la Técnicas de las Cámaras de Aire inferior. Al ingresar a la Web nos encontramos con los siguientes trabajos: 1. Use of Air-Decks to Reduce Sub drillings in Escondida Mine Author: Correa E. C. Source: Fragblast, Volume 7, Number 2, June 2003, pp. 79-86(8) Publisher: Taylor and Francis Ltd Abstract: This paper aims to show the results and comparison obtained from blasting using overdrilled blastholes and blasting using a bottom of the blasthole located air-deck. The effect of using a blasthole air-deck, on medium to low hardness rock found in the western area of the Escondida pit, helped maintain the required grade level even after loading with heavy equipment. Additional benefits include satisfactory fragmentation of the blasted material. 2. The use of air decks in production blasting in an open pit coal mine Authors: Jhanwar J.C.1 ; Jethwa J.L.2 Source: Geotechnical and Geological Engineering, Volume 18, Number 4, 2000 , pp. 269287(19) Publisher: Springer Abstract: The influence of air deck blasting on blast performance and blast econom ics and its feasibility has been studied in the production blasting of soft and medium strength sandstone overburden rocks in an open pit coal mine in India. The air deck blasting technique was very effective in soft and medium strength rocks. Its main effects resulted in reducing fines, in producing more uniform fragmentation and in improving blast economics. The fines were reduced by 60–70% in homogeneous sandstones. Oversize boulders were reduced by 80% and shovel loading efficiency was improved by 20 –40% in blocky sandstones. The explosive cost was reduced by 10–35% dependent on the type of rock mass. Throw, backbreak and ground vibration were reduced by 10–35%, 50–80% and 30–94% respectively. For a particular rock mass and blast design environment, air deck length (ADL) significantly influenced the fragmentation. ADL as represented by air deck factor (ADF) in the range of 0.10–0.35 times the original charge length (OCL) produced optimum results. ADF beyond 0.35 resulted in poor fragmentation and in inadequate burden movement. Language: English Document Type: Regular paper
Affiliations: 1: Central Mining Research Institute Regional Centre, 54-B, Shankar Nagar, Nagpur – 440 010, India. E-mail:
[email protected] 2: Central Mining Research Institute Regional Centre, 54-B, Shankar Nagar, Nagpur – 440 010, India 3. Borehole pressures in an air decked situation Authors: Fourney, W.1 ; Bihr, S.1 ; Leiste, U.1 Source: Fragblast, Volume 10, Numbers 1-2, -2/March-June 2006 , pp. 47-60(14) Publisher: Taylor and Francis Ltd Abstract: In the late 1970s and early 1980s in conjunction with other oil and gas well stimulation studies, personnel from the Dynamic Effects Laboratory performed model testing to demonstrate the effectiveness of utilizing an open section of borehole just before a plug. We called the process stem induced fracturing. The open section beneath the stem was used to increase the pressure magnitude and spread out the duration of the pressure pulse. This technique was later utilized by Frank Chiapetta [Chiappetta, R.F. and Mammele, M.E., 1987, Analytical high-speed photography to evaluate air decks, stemming retention and gas confinement in pre-splitting, reclamation and gross motion applications. Proceedings of 2nd International Symposium on Rock Fragmentation by Blasting , Keystone, Colorado, USA, 257 - 309] in the fracture and fragmentation of rock in quarry blasting situations. He called his technique air deck blasting. In fact, Frank found that the Russians had previously discovered the same technique. There is currently interest in utilizing the same technique with an open hole beneath the explosive at the bottom of the bore hole to better remove the toe in a fragmentation shot. This paper reviews the development of stem induced fracturing and describes a series of model tests conducted to measure borehole pressure at points along a borehole when an explosive charge is detonated at the midpoint of the borehole. Tests were conducted in both stiff boreholes (aluminum) and less rigid boreholes (PMMA). Pressure time profiles were measured at the charge site, midway between the charge and the bottom of the hole, at the stemming at the top of the borehole, and at the bottom of the borehole. Crack initiation sites and crack propagation were also determined in the PMMA models. Some high speed pictures were taken of the event in the PMMA. Keywords: Borehole pressure; Air decks; Stem-induced fracturing; Toe blasting; Model testing Document Type: Research article DOI: 10.1080/13855140600858198 Affiliations: 1: Dynamic Effects Laboratory, University of Maryland, College Park, Maryland, 20740, USA
4. Influence of air-deck blasting on fragmentation in jointed rocks in an open-pit manganese mine J. C. Jhanwar , , J. L. Jethwa and A. H. Reddy a
Central Mining Research Institute Regional Centre, 54 -B, Shankar Nagar, Nagpur,
Maharastra 440 010, India
b
Department of Mining Engineering, Visvesvaraya Regional
College of Engineering, Nagpur 440 010, India Received 14 January 1999; accepted 13 September 1999. Available online 24 May 2000. Abstract The influence of air-deck blasting on fragmentation in jointed rock masses of a manganese open-pit mine has been investigated. It is revealed that air-deck blasting improves the degree of fragmentation and produces more uniform fragmentation compared with conventional blasting. Mean fragment size (MFS) reduces from a range of 0.36 –1.0 m to a range of 0.27–0.51 m. Fragmentation index (FI) and blast-induced fragmentation increase from 1.38 to 4.34 and from 37–65% to 66–100% respectively. Empirical correlations have been developed to estimate MFS and FI from conventional and air-deck blasting on the basis of Bieniawski's rock mass rating, powder factor and ratio of spacing to burden. Results indicate that air-deck blasting could be more effective in very low to low strength moderately jointed rock masses than in medium strength highly jointed rock masses. 5. Application of Air Decks in Production Blasting to Improve Fragmentation and Economics of an Open Pit Mine Publicación Geotechnical and Geological Engineering Editor Springer Netherlands ISSN 0960-3182 (Print) 1573-1529 (Online) Fascículo/ejemplar/número Volume 17, Number 1 / marzo de 1999 DOI 10.1023/A:1008899928839 Páginas 37-57
6. Application of Air Decks in Production Blasting to Improve Fragmentation and Economics of an Open Pit Mine J.C. Jhanwar1, A.K. Cakraborty1, H.N. Anireddy2 and J.L. Jethwa2 (1) Central Mining Research Institute Regional Centre, 54 -B, Shankar Nagar, Nagpur-, 440 010, India (2) Department of Mining Engineering, VRCE, Nagpur, India Abstract In blasting with air decks, repeated oscillation of shock waves within the air gap increases the time over which it acts on the surrounding rock mass by a factor at between 2 and 5. The ultimate effect lies in increasing the crack network in the surrounding rock and reducing the burden movement. Trials of air deck blasting in the structurally unfavourable footwall side of an open pit manganese mine has resulted in substantial improvements in fragmentation and blast economics. Better fragmentation resulted in improved shovel loading efficiency by 50–60%. Secondary blasting was almost eliminated. Use of ANFO explosive with this technique reduced explosive cost by 31.6%. Other benefits included reductions in overbreak, throw and ground vibration of the order of 60–70, 65 –85 and 44% respectively. This paper reviews the theory of air deck blasting and describes in detail the air deck blast trials conducted in a manganese open pit mine in India. The blast performance data have been analysed to evaluate the benefits of air decking over conventional blasting. air deck - fragmentation - blasting - open pit mine
A Further Study on the Mechanism of Air decking Authors: Lu W.; Hustrulid W. Source: Fragblast, Volume 7, Number 4, December 01, 2003 , pp. 231 -255(25) Publisher: Taylor and Francis Ltd Abstract: Airdecking is used in mining for two quite different applications. One is to enhance the fragmentation by amplifying the induced fracturing and the second is for pre-split blasting in which the borehole fracturing is reduced. This paper deals with the first of these effects. A forth coming paper will describe pre-splitting by airdecking. The use of air decks to enhance rock fragmentation and so to reduce explosive costs has been the practice for quite long time. Although a number of studies has been conducted to verify the advantages of blasting with air decks and to investigate the mechanisms involved, the proposed mechanisms still cannot explain clearly the phenomena observed in practice and the design approach adopted for this kind of blasting is still primary based on rules-ofthumb. In this paper, the theory of shock tubes is adopted to (a) investigate the processes of the expanding detonation products, (b) study the interactions between the explosion products and the stemming or bottom of blasthole, and (c) to decide the distribution of the changing pressure of explosion products along blasthole. Numerical simulation and theoretical analyses are then performed to study the physical process of blasting with air decks. Finally, a reasonable value for the airdecking ratio is decided theoretically. It is shown that the pressure -unloading process caused by the propagation of the rarefaction wave and the reflected rarefaction waves in the detonation products plays an important role in the enhanced fragmentation of rock when blasting with air decks. The unloading process can induce tensile stresses of rather high magnitude in the rock mass surrounding blasthole. This favors fracturing of the rock. The reflected shock wave with a magnitude of gas pressure higher than that of the average detonation pressure in a fully charged blasthole acts as the main energy source to break the rock in the air deck and stemming portions. The second and succeeding strain waves induced by the unloading or reloading of the pressure within the blasthole also contribute to form the initial fracture network in the rock around the blasthole. It is also revealed that there exists a reasonable range of values for the airdecking ratio. For ANFO, this value varies from 0.13-0.40.
