NON-PRIMARY EXPLOSIVES DETONATORS (NPED)
AN ECO-FRIENDLY INITIATING SYSTEM FOR COMMERCIAL BLASTING IS THE NEED-OF-THE-HOUR.
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By: Partha Das Sharma (E.mail: sharmapd1@gmail.com)
Introduction: Detonators are used for initiating high explosives during any of the blasting operation carried out for excavation work. Conventional detonators have base charge of PETN (Penta Erythritol Tetra Nitrate) with a priming charge of a heat-sensitive, detonable, primary explosive (please refer Note below given on primary explosives), such as ASA (Lead Azide, Lead Styphnate and Aluminum powder) which is placed above and adjacent to the base charge. In a delay detonator, adjacent the primary explosive is an amount of a deflagrating or burning composition of sufficient quantity to provide a desired delay time.
Strength depends upon amount of base charge. Aluminum or Copper shells of about 6 to 10 mm dia., are used. It is detonated either by safety fuse in case of plain detonators, by electric current in case of electric detonators, by signal tube in case of NONEL and by electronic circuit in case of Electronic detonators. Length of the shell is kept to accommodate fuse head, neoprene plug (for electric instantaneous detonators) and length of delay element used as per the delay timing (incase of delay detonators). Above the delay composition (if present) is an electric match (an electric fuse head of Nickel-Chromium-Iron alloy bridge wire soldered to the ends of two brass foils separated by an insulating piece of plastic board in case of electric detonators), a low energy detonating cord or shock wave conductor (such as shock tube), or the like, retained in the open end of the metallic shell.
Detonators, including electronic, electric and non-electric types, are widely used in mining, quarrying and other blasting operations. In-hole detonators are generally used to initiate an explosive charge which has been placed in a borehole, while surface detonators are generally used outside of the borehole to initiate one or more explosive initiating signal means such as shock tube or detonating cord.
Surface detonators are generally identical to in-hole detonators with the exception that the base charge of high explosive is preferably reduced or omitted to give lower output. The output is preferably reduced to a level sufficient to initiate adjacent shock tube, detonating cord and without, for example, throwing excessive amounts of shrapnel which can damage nearby lengths of shock tube or cord. This feature of output control is a desirable practice in the design of detonators in order to control the energy output of in-hole and surface detonators.
Thus, for the purposes of this specification, a primary explosive (as indicated in the box) is defined as an explosive substance which readily develops complete detonation from
Primary Explosive, a sensitive explosive which almost always detonates by single ignition such as spark, flame, impact and other primary heat sources of appropriate magnitude.
Primary explosives can detonate by the action of a relatively weak mechanical shock or by a spark. If used in the form of blasting caps (detonator), they initiate the main explosive. An initiating explosive must be highly brisant and must have a high triggering velocity. The most important primary explosives are Mercury Fulminate, Lead Azide, Lead Trinitroresorcinate, Silver Azide, Diazodinitrophenol, and Tetrazene, which is used as an additive in primers.
Initiating charges must be transported only if they are already pressed into capsules. The latter are usually made of aluminum, and sometimes of copper, white plastic capsules are used for special purposes.
Primary explosives, have a low deflagration to detonation transition, and go from burning to detonation very quickly. They differ as to sensitivity and the shock given off. Primary explosives are reactive to different materials, care should be taken when handling primary’s that are in the natural state.
a. Lead Azide
Uses – Manufacture of detonators. Properties:
v Excellent initiating agent for high explosives, more efficient than mercury fulminate. v Generally covered with Lead Styphnate for sensitivity.
v Used as intermediate charge.
v Good initiator for sensitive booster charges, RDX, PETN, Tetryl
v Reactive to copper in the presence of moisture. Formation of Copper Azide, which is extremely
sensitive.
v Color, white to buff with rounded aggregates.
VOD: 16,745 ft/sec. b. Lead Styphnate
Uses – Manufacture of detonators Properties:
v Lead Styphnate is a poor initiator, but it is easily ignited by fire or by a static charge. v Used as priming layer which causes Lead Azide to explode from a flash.
v Reddish – Brown crystals.
VOD: 17,000 ft/sec c. Mercury Fulminate
Mercury Fulminate appears to have been prepared for the first time by Hohann Kunckel von Lowenstern. (1630 - 1703). The preparation and properties of mercury fulminate were described in much detail by Edward Howard in 1800 in a paper presented to the Royal Society of London. Preparation of Mercury Fulminate is carried out by a process essentially the same as described by Howard.
Uses – Manufacture of detonators Properties:
v Used as a base charge in the past. v Not used extensively
v Color is White – Grey or Light Grey with a yellow tint.
VOD: 14,780 ft/sec
d. Other Primary Initiating
stimuli such as flame, conductive heating, impact, friction or static electrical discharge, even in the absence of any confinement. In contrast, a secondary explosive can generally
only be detonated if it is present in larger quantities or if contained within heavy
confinement such as a heavy walled metal container, or by being exposed to significant shock wave or mechanical impact. Some of the examples of secondary explosives are
PETN, Cyclo-Tri-Methylene-Trinitramine (RDX), Cyclo-Tetra-Methylene-Tetranitramine (HMX), Tri-Nitrophenyl-Methylnitramine (Tetryl) and Tri-Nitro-Toluene (TNT) or mixtures of two or more of these and/or other similar substances.
Shortcomings of use of Lead / Mercury based primary explosives: This use of Lead Azide as a heat-sensitive, primary explosive in the preparation of surface and in-hole detonators, and in particular, the use of Lead / Mercury-containing materials has several serious disadvantages. These include
A) Even the presence of a small charge of primary explosive makes a conventional detonator potentially hazardous to handle because of its sensitivity to mechanical deformation or impact;
B) The manufacture of the detonator requires the production and handling of significant quantities of sensitive materials which require costly handling procedures.
C) Detonator manufacturing plants must address the health risks of dealing with potentially toxic materials such as Lead, and address the proper disposal of these toxic materials.
D) The possibility of contaminating the Mines environment and atmosphere can not be ruled out because of presence of potentially toxic materials such as Lead, Mercury etc., in the detonators used for blast.
Accordingly, due to the desirability of minimizing or eliminating the use of primary explosives during the production and use of detonators, and with the potential adverse safety and/or toxicity reasons, it would be desirable to provide a detonator which was essentially free from primary explosives, and in particular, Lead Azide.
Development of Primary Explosive-free Detonators: One approach to the eliminating the primary explosives from detonators has been the development of Primary Explosive-free Detonators or Non-primary explosives detonators (NPED). This system relies on the establishment of conditions in the detonator which cause a Secondary Explosive to undergo a "Deflagration to Detonation Transition" (DDT) reaction.
In these DDT detonators, a deflagration reaction is initiated directly in a secondary explosive by a thermal reaction with an igniting device, such as the Flame-front from a shock tube, or directly from a heated bridge wire. By incorporating suitable confinement to the secondary explosive, and/or control of Particle size, morphology, density and formulation of the secondary explosive as well as careful selection of the initiation means and detonator design, this deflagration reaction is caused to transfer to a detonation reaction. The detonation provides sufficient force to initiate an adjacent base charge of
the detonator and in turn, the detonator directly initiates a shock tube or length of detonating cord attached to it.
In this context, references of few Patents would be quite interesting. These include - U. S. Patent No. 2,400,103 (Cobb); U. S. Patent No. 3,096,714 (Yuill), U. S. Patent No. 4,727,808 (WangetaL), U. S. Patent No. 4,316,412 (Dinegar and Kirkham), PCT Patent Publication No. W097/22571 (Dumenko) published June 26,1997, US Patent No. 5385098 (Lindquist et), and a related European application numbered as EP-A1-0365503 (Lindquist et gl.).
Improved DDT detonator still requires strong confinement, and in particular, back-confinement of the initiating element. The back back-confinement consists of a cup-shaped confinement shell (with an aperture), which surrounds the initiating element. Use of this cup-shaped confinement shell, together with the preferred use of back-pressing of the initiating element (as prescribed by some of the above researchers), adds to the manufacturing cost and complexity of the completed detonator.
Thus, while DDT detonators have shown promise for the replacement of standard primary explosive-containing detonators, their reliability and ease-of-manufacture have led to continued interest in developing additional types of primary explosive-free detonators. In order to overcome these difficulties, other researchers have proposed devices such as "flyer" plates or incorporation to use of lasers. However, these have met with little commercial success due to operational and manufacturing difficulties.
Therefore, it would be desirable to provide a composition for use in detonators which is preferably free from primary explosives and which can be operated without requiring non-standard levels of confinement. Several approaches have been made to modify the construction of detonators and following steps would be important to discuss in this respect:
i) Improvement in igniting device at the open end of the shell,
ii) Development in initiating element comprising an initiation portion and optionally a transition portion to obtain high-burn-rate with substantially high pressure to the tune of more than 5 cm/sec and more than 50 atmospheres respectively. This high-burn-rate pressuring initiator or more suitably provide a rapid increase in pressure so as to provide a shock wave suitable for initiating of the porous, powdered explosives.
Preferred materials suitable for use in the high-burn-rate pressurising initiator include materials such as Potassium Picrate, Potassium Styphnate, Lead Styphnate, Potassium Trinitrobenzoate, or alkali or alcaline earth metal salts of Nitro-Aromatic compounds and in particular Nitrophenols or Nitrobenzoates or mixtures thereof.
With this preferred mixture high burn rates are possible while also providing good levels of pressurization. This combination provides an improved ability to effect the transition from deflagration to detonation when combined with the PETN. This improved ability can be achieved with decreased levels of confinement and without the need for back-pressing. Thus, this preferred mixture provides clear advantages over the DDT formulations for development of NPED.
iii) Development of suitable base charge characterized in that said initiation portion, which should at least partially contained within a confinement sleeve and comprises an intimate mixture of a relatively large particle size, porous, powdered explosive having interstitial spaces, and a relatively small particle size, high-burn-rate, pressurizing initiator located within said interstitial spaces. (The term "Porous Powdered Explosive" refers to an explosive material, which when loosely poured into a container, allows air to pass through the material without a substantial amount of air resistance. Preferred materials useful as porous powdered explosives include materials such as PETN, RDX, HMX, Tetryl, TNT, or combinations thereof. Most preferably, however, the porous powdered explosive is PETN.
Discussion on Delays system required: Further, the detonators may be electronic, electric or non-electric. The detonator may also be a Delay Detonator. Detonators may comprise pyrotechnic delay element / a series of delay elements, an electronic timing circuit, or some other device, to cause a time delay between initiation of the igniting device and the subsequent initiation of the initiation portion and/or base charge. However, the detonator may also be an instantaneous, non-delay type.
It should be noted that the materials used in the production of the delay element / a series of delay elements are typically not gas-generating or produce very little gas during combustion. This is in distinct contrast to the high-burn-rate pressurising initiator used in the intimate mixture of the present invention.
In fact, in electronic system of delay detonator, due to absence of pyrotechnic delay element and as the initiation of the igniting device is located after the electronic timing; the high-burn-rate pressuring initiator can effectively be directed to the porous powdered base charge directly. Therefore, adopting this state of the art system with electronic delay detonator is much easier and affordable. This makes an electronic detonator more eco friendly, apart from having its inherent advantages such as precise and accurate delay timing.
Conclusion: In order to develop eco-friendly non-primary explosives detonators a high level of research is required. In developed countries like USA, Canada, Australia etc., the use of conventional detonators are being restricted due to the various disadvantages associated with it. In India also we should encourage develop and use of NON-PRIMARY EXPLOSIVES DETONATORS.
Author’s Bio-data: Author is, at present, in charge of Marketing & Technical Services in SOLAR GROUP OF EXPLOSIVES INDUSTRIES, NAGPUR, engaged in manufacturing and supply of almost all range of Cartridge Explosives, Bulk explosives & detonators. This company within a very short span of time, it has emerged as one of the leading manufacturer, supplier and exporter of Explosives & Detonators in India.
Author is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization a couple years back. Author can be contacted at E-mail: sharmapd1@rediffmail.com and sharmapd1@gmail.com
Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company.
