AU2011220386A1 - Detonator initiator - Google Patents

Abstract

A detonator which includes an explosive and an initiator for igniting the explosive, wherein the initiator includes a thermite composition and a tinder mixture which includes silicon and dibismuth trioxide.

Description

WO 2011/106803 PCT/ZA2011/000008 DETONATOR INITIATOR BACKGROUND OF THE INVENTION [0001] This invention relates to a detonator and to an initiating system for a detonator. 5 [0002] Conventional blasting caps or detonators contain lead-based primary explosives. These lead-based primary explosives are problematic to manufacture and due to enhanced sensitivity of the explosives to friction, heat and impact, many accidents may have been caused over the years. Furthermore, the use of lead based explosives is becoming environmentally unacceptable. A need exists for a less 10 sensitive initiating system in which the use of lead is, at least, reduced. [0003] It has been reported that detonators can use thermite-based compositions as initiators that will preclude the use of primary explosives. A thermal shock generated by a suitable thermite charge can under certain circumstances (e.g. narrow particle size distribution, confinement and specific tamped pressures) initiate an explosive 15 such as pentaerythritol tetranitrate (PETN) from deflagration to detonation in a very short period of time. A difficulty with a thermite-based initiator is that ignition thereof requires a high temperature, for example about 940 0 C for an Al-CuO thermite comprising coarse particles. [0004] Known techniques to ignite thermite compositions include the use of a 20 propane torch, magnesium metal strips, and a reaction between KMnO 4 and glycerine. These approaches are however unreliable and are not suited for use in a commercial detonator. Ignitors such as exploding bridge wires and laser techniques are effective but are expensive to implement.

WO 2011/106803 PCT/ZA2011/000008 2 [0005] It has been proposed to add a tinder mixture to the thermite to address the problems of the high temperature, and of the relatively long delay to ignition time periods. Barium nitrate and sulphur have been used as tinder for pyrotechnic compositions because of their excellent sensitivity to heat. The addition of barium 5 nitrate to a thermite composition increases the thermal effect, creates flame in burning and reduces the ignition temperature. A drawback, however, is that a barium compound also has a high toxicity that is exacerbated when inhalation occurs. [0006] Sulphur also acts as a tinder to facilitate ignition of a pyrotechnic but the presence of the low-melting volatile sulphur fuel tends to retard the burning rate 10 because the melting and vaporization steps of the endotherms absorb heat that would otherwise be available to raise the temperature of the unreacted mixture. An object of the present invention is to provide a detonator initiator in which the ignition temperature is significantly reduced. SUMMARY OF INVENTION 15 [0007] The invention provides a detonator which includes an explosive and an initiator for igniting the explosive, wherein the initiator includes a thermite composition and a tinder mixture which includes silicon and dibismuth trioxide. [0008] The thermite composition is preferably Al-CuO, in a stoichiometric ratio. [0009] Preferably the thermite is formed primarily from micron-sized particles. For 20 example the aluminium may be provided in powder form with the particles being smaller than 25 pm (800 mesh). [0010] The CuO is preferably micron-sized particles, typically less than 25 pm.

WO 2011/106803 PCT/ZA2011/000008 3 [0011] In the tinder the Bi 2

O

3 and the Si may also be micron-sized particles at less than 34 pm and 64 pm, respectively. [0012] In the initiator the thermite composition may be present in a weight percentage range of from 70% to 90% and the tinder may be present in a weight 5 percentage range of from 10% to 30%. [0013] Preferably the thermite composition constitutes approximately 80 wt % of the initiator and the tinder approximately 20 wt %. BRIEF DESCRIPTION OF THE DRAWINGS [0014] The invention is further described by way of examples with reference to the 10 accompanying drawings in which: Figures 1 and 2 are DTA responsive curves as a function of temperature produced upon ignition of respective thermite compositions; and Figures 3 and 4 illustrate respectively from one side and in cross-section detonators according to different forms of the invention. 15 DESCRIPTION OF PREFERRED EMBODIMENT [0015] The invention is concerned with a detonator which includes an initiator which is readily initiated by the application of heat. The applicant has selected the pyrotechnic Si-Bi 2 0 3 system as tinder for the thermite AI-CuO system because of the excellent sensitivity of the tinder to fire. The Si-Bi 2 0 3 is sensitive to fire and can also 20 be used as a short period delay with a burning speed of 150 mm per second in a composition comprising 80 wt % Bi20 3 and 20 wt % Si.

WO 2011/106803 PCT/ZA20111/000008 4 [0016] The temperature of the tinder reaction is sufficient to ignite a thermite reaction. Experimental work was conducted to determine an optimal composition of an AI-CuO thermite system mixed with Si and Bi20 3 (the tinder). [0017] Table 1 reflects ignition temperature data which were obtained for specific 5 mixtures of thermite (80 wt %) and tinder (20 wt %) formed from particles of different sizes. In Table 1 "coarse" denotes micrometer-sized (pm) particles for all types. In the case of aluminium (present in the form of flakes) "nano" applies only to the thickness dimension of the flakes. This type of aluminium material is referred to in Table 1 as fine. The nano silicon has a surface weighted mean particle size of 910 10 nm and a BET surface area of 10,1 m 2 /g. The sizing boundaries of the nano CuO particles were established as being less than 10 nm. Mixing of the thermite with the tinder in each case was accomplished using standard techniques. After adequate processing the initiator compositions were ignited. Appropriate instruments were used to perform a differential thermal analysis (DTA). 15 Table I Thermite Sensitizer ("tinder") Ignition 65.3% 14.7% 16% 4% temperature, Nature CuO Al Bi 2 0 3 Si 0 C coarse coarse coarse coarse 689 E nano fine nano nano 613 & 610 E nano fine coarse coarse 737 & 733 E coarse fine nano nano 714 0 nano coarse nano nano 912 0 * E = explosion; 0 = oxidation [0018] Figure 1 is a DTA response curve as a function of temperature for a stoichiometric AI-CuO mixture comprising coarse aluminium and nano-sized CuO 20 particles. A first endotherm has an onset temperature of 647 0 C - this corresponds to the melting of the aluminium metal (literature Al mp. 660.45 0 C). A second endotherm WO 2011/106803 PCT/ZA2011/000008 5 which occurred with an onset terrIperature of about 880*C is presumed to correspond to the melting and dissolution of the CuO in the aluminium melt (literature CuO mp. 13260C). This is followed by a fast exotherm with an onset temperature of about 9390C which is associated with the ignition temperature of the composition leading to 5 the aluminium oxidation reaction. [0019] Figure 2 is a curve of a DTA response of an initiator mixture, according to the invention which, as shown in Table 1, contains 80 wt % thermite (Al-CuO) and 20 wt % of the tinder mixture (Si-Bi 2

O

3 ). A strong exotherm occurs with an onset temperature of 6890C. This exotherm shows an extreme temperature rise which far 10 exceeds that observed for the Al-CuO thermite without the tinder. This is categorized as an explosion. [0020] The results of the investigations of particle size on the ignition temperature are summarized in Table 1. Satisfactory performance was obtained with the nano sized composition. 15 [0021] Subsequent investigations focussed on the effect of varying the stoichiometry of the thermite. The results are shown in Table 2. The constituents are given as weight %. The third entry in Table 2 corresponds to the thermite composition used for the tests summarized in Table 1. The lowest ignition temperature (6130C) is attained for the stoichiometric composition. 20 Table 2 Thermite Sensitizer ("tinder") Ignition temperature, Nature CuO Al Bi 2 0 3 Si 0C 20 60 16 4 741 0 16 64 16 4 622 E 14.7 65.3 16 4 613 E 12 68 16 4 621 E * E = explosion; 0 = oxidation WO 2011/106803 PCT/ZA2011/000008 6 [0022] Figure 3 illustrates a detonator 20 according to a first form of the invention which is based on the inventive principles described herein. The detonator includes a metallic tube 22, e.g. of aluminium, with an inner holder 24, also of aluminium. A 5 charge 26 of PETN is located at a blind end of the tube and tamped in position at 130kg. The holder 24 includes an outlet 30 and an inlet 32. A charge 34 of PETN, pressed at 70kg, is adjacent the outlet. A charge 36, made up of the thermite/tinder composition referred to and pressed at 90kg, is adjacent the charge 34. Optionally a time delay element 40, of any suitable composition, is loaded into the holder. A 10 starter 42, typically a mixture of red lead and silicon, is adjacent the inlet 32 and pressed at 130kg. [0023] The detonator 20 is used substantially in a conventional manner in that the starter 42 is fired by means of the energy output from a shock tube, not shown. If the delay element 40 is used then, after a predetermined delay period, the 15 thermite/tinder composition 36 is ignited. The firing impulse from the thermite composition leads to deflagration to detonation of the smaller PETN charge 34 which causes initiation of the larger PETN charge 26. [0024] Figure 4 shows a detonator 50 which does not include a delay element. The detonator has an aluminium tubular shell 52 with an open upper end and a closed 20 lower end 54. Contained in the shell is base charge 56 made up of 400mg of PETN pressed at 130kg (56A) and 400mg of loose PETN (56B). An initiating element, in the form of a tubular steel sleeve 60, is positioned inside the aluminium shell. The sleeve has an open first end which opposes the open upper end of the shell 52, and an opposed open second end. The sleeve contains a starter composition 62 which is WO 2011/106803 PCT/ZA2011/000008 7 similar to the starter 42 shown inFigure 3, a mixture 64 of tinder, thermite and PETN and a transition portion 66 of PETN. [0025] Highly satisfactory operation was achieved with the aluminium in powder form with the particles being smaller than 25 pm and with the CuO comprising 5 micron-sized particles less than 25 pm. The Bi 2

O

3 and Si respectively comprised micron-sized particles at less than 34 pm and 64 pm. [0026] The mixture 64 consists of 20% Al-CuO thermite/tinder and 80% PETN (75 to 180 micron particle size). The constituents are dry mixed and pressed at 90kg. The transition portion 66 comprises 140mg of PETN (75 to 180 micron particle size) 10 pressed at 70kg. [0027] As noted there is no delay composition. [0028] The detonator 50 illustrates similar desirable characteristics as the detonator 20. The ignition temperature of the detonator is reduced, for the reasons which have been given and, although the starter in each case includes red lead, overall the lead 15 requirement is also reduced.

Claims (9)

1. A detonator which includes an explosive and an initiator for igniting the explosive, wherein the initiator includes a thermite composition and a tinder mixture which includes silicon and dibismuth trioxide. 5

2. A detonator according to claim 1 wherein the thermite composition is Al-CuO, in a stoichiometric ratio.

3. A detonator according to claim 2 wherein the aluminium is in powder form with the particles being smaller than 25 pm (800 mesh).

4. A detonator according to claim 2 or 3 wherein the CuO comprises micron 10 sized particles less than 25 pm.

5. A detonator according to any one of claims 1 to 4 wherein the Bi 2 O 3 comprises micron-sized particles less than 34 pm.

6. A detonator according to any one of claims 1 to 5 wherein the Si comprises micron-sized particles less than 64 pm. 15

7. A detonator according to claim 1 wherein the thermite composition is present in a weight percentage range of from 70% to 90% and the tinder is present in a weight percentage range of from 10% to 30%.

8. A detonator according to claim I which includes a tubular shell, with an open end and a closed end, and starter inside the shell adjacent the open end, and 20 wherein the explosive is inside the shell adjacent the closed end and the thermite composition and the tinder mixture are between the starter and the explosive. WO 2011/106803 PCT/ZA2011/000008 9

9. A detonator according to claim 1 which includes a tubular shell with an open end and a closed end, a sleeve which is inside the shell and which has an open first end which opposes the open end of the shell and an opposed open second end, the explosive being inside the shell, between the closed end of 5 the shell and the open second end of the sleeve, a starter inside the sleeve adjacent the first end, and a transition portion of PETN inside the sleeve adjacent the open second end, and wherein the thermite composition and the tinder mixture are inside the sleeve between the starter and the transition portion. 10