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(11) | EP 0 070 932 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Initiatory explosive for detonators and method of preparing the same |
| (57) An initiatory explosive suitable for use in a detonator in place of the known ASA
mixture is composed of isogonous mixed crystals of lead azide and lead styphnate which
can be produced by introducing simultaneously into a reactor vessel a solution containing
lead acetate and a solution containing a water-soluble styphnate and sodium azide
so as to cause them to react in the presence of a crystal habit modifier such as dextrin
or carboxymethyl cellulose which will cause the resulting precipitate to be a homogeneous
crystalline material composed of isogonous mixed crystals. |
[0001] This invention relates to detonators for explosives and is concerned with an initiatory explosive (also known as a primary explosive) suitable for use in such detonators and a method of preparing it.
[0002] Detonators for explosives normally consist of a metal tube closed at one end and containing a base charge of a highly brisant explosive, for example PETN (an abbreviation for pentaerythritol tetranitrate), Tetryl (2,4,6-trinitrophenyl methyl nitramine) or RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), on.which is disposed a smaller sensitive priming charge composed ' of an initiatory explosive which can be set off, for example, by a spark from an electrically ignited fusehead or by heat or flame from a safety fuse.
[0003] Presently commercially available detonators, particularly those which are set off with a safety fuse, most commonly have a priming charge composed of an initiatory explosive of ASA (an abbreviation for a mixture of lead azide (Pb(N3)2), lead styphnate (lead 2,4,6-trinitro-resorcinate) and aluminium). ASA normally contains about 70 parts by weight of the lead azide, about 30 parts by weight of the lead styphnate and about 2.5 percent by weight (of the ASA) of fine light aluminium powder.
[0004] Lead styphnate is mixed with lead azide to combine the desirable properties of flame sensitiveness of lead styphnate and the high disruptive power of lead azide. Lead azide by itself has just an acceptable level of flame sensitiveness if its purity is more than 98%. However, such a pure lead azide is relatively more sensitive to friction and impact and is more hazardous to manufacture and handle. Pure lead.azide is used in military explosives where the disruptive power of the material is required to be as high as possible. Commercial lead azide is manufactured using hydrophilic colloids such as dextrin, gelatin, or carboxymethyl cellulose as desensitisers and crystal habit modifiers. These reduce the purity of lead azide to about 95% but render it safer to handle. Such lead azide is however not adequately sensitive to ignition from a safety fuse and therefore cannot be used for ordinary non-electric detonators.
[0005] Lead styphnate is much more flame sensitive than lead azide. Its disruptive power is however lower. It suffers from another serious drawback in that it is not only highly prone to static electrification during its handling in the dry state, but it is also capable of being fired at very low levels (140 ergs) of static electrical energy. Handling of dry lead styphnate is therefore hazardous and accidents are quite frequent in filling operations. When lead styphnate is mixed with lead azide in the manufacture of ASA mixture, the resultant mixture also has similar hazards of static electrification and firing by low level static electrical energy.
[0006] There are other serious problems in making the ASA mixture. In practice lead azide and lead styphnate are mixed together using both dry and wet mixing methods. In the wet method, the mixture is likely to be inadequately homogeneous. In the dry method, the mixing operation carries a serious hazard. Even if mixing is carried out by remote control, there is no assurance of homogeneity of the mixture.
[0007] Attempts have been made by a few workers in this field to co-precipitate lead azide and lead styphnate together in order to avoid the mixing operation. These attempts have not however been successful. The conditions for precipitation of lead azide and lead styphnate are different. The former is precipitated in a neutral medium while the latter requires an acidic medium in the final stage of precipitation. Lead azide and lead styphnate crystals formed concurrently in a solution are prone to segregation during washing of the wet material and its drying and sieving. Under conditions suitable for the formation of lead azide, it is difficult to precipitate desired quantities of lead styphnate simultaneously.
[0008] It is an object of the present invention to provide a binary initiatory explosive in which the constituents have the same crystal habit and whose crystals can grow into each other forming what are known as isogonous, or mixed or layer crystals.
[0009] According to the invention, there is provided a method of preparing an initiatory explosive comprising lead azide and lead styphnate, which comprises bringing into contact an aqueous solution of lead acetate and an aqueous solution of a water-soluble styphnate and sodium azide in the presence of at least one crystal habit modifier so as to cause the solutions to react and thereby form a precipitate which is a homogeneous crystalline material composed of isogonous mixed crystals of lead azide and lead styphnate.
[0010] In the method of the present invention, the precipitation conditions are such as to produce both lead azide and lead styphnate in the same crystal habit capable of forming isogonous or mixed crystals. The special feature of this invention is the homogeneity of the intergrown crystals which have the appearance of a single species irrespective of the proportions of the constituent lead azide and lead styphnate present in the material.
[0011] In a preferred embodiment of the invention, a solution containing lead acetate and a solution containing a water-soluble styphnate and sodium azide are introduced simultaneously into a reactor vessel in carefully controlled concentrations. Available free alkali required during the reaction may be provided by adding an appropriate amount of basic lead acetate to the lead acetate solution. Alternatively, the alkali may be provided by addition to the azide/styphnate solution of potassium hydroxide. Control of available alkalinity is, however, better if it is in the form of basic lead acetate.
[0012] The water-soluble styphnate may be potassium styphnate, magnesium styphnate or ammonium styphnate and may be prepared in situ by reaction of styphnic acid with an appropriate base, e.g. potassium hydroxide or aqueous ammonia.
[0013] The crystal habit modifier may be incorporated in the lead acetate solution and/or in the azide/ styphnate solution, and/or may be present in the reactor vessel into which the two solutions are introduced. Thus two or more different modifiers can be employed if desired. Examples of suitable crystal habit modifiers for use in the present invention are dextrin and carboxymethyl cellulose.
[0014] The pH of the reacting solutions, (which will generally be approximately neutral, i.e. 6.5 to 7.5, when basic lead acetate is used to control the available free alkali) is controlled and crystal habit modifiers are added to ensure that isogonous mixed crystals of lead azide and lead styphnate are formed, resulting in a uniform free flowing product without much crystal debris or dust.
[0015] The amount of lead styphnate precipitated together with lead azide will depend on the free alkali available during the reaction and hence can be controlled by varying either the quantity of basic lead acetate added to the lead acetate solution or the quantity of potassium hydroxide added to the azide/ styphnate solution. A large variety of products useful as initiatory explosives is thus possible. However for practical application a styphnate content of 20% to 30% is generally aimed at in the final product, to ensure both adequate disruptive power and flame sensitiveness.
[0016] In the following paragraphs, the initiatory explosive prepared in accordance with the invention is designated as LSA.
[0017] LSA prepared in accordance with the invention has adequate flame sensitiveness for firing either by a safety fuse or by an electric fusehead. It does not develop significant or dangerous levels of static charge during handling and the level of energy in terms of a static charge required to fire the material is very much higher than that needed for ASA. LSA is also quite satisfactory as regards its sensitiveness to friction and mechanical impact as shown in the following table:-
[0018] LSA can be used by itself as a priming charge in detonators and there is no necessity to mix it with aluminium powder, or graphite or anti-static agents.
[0019] LSA made according to this invention is thus considerably superior to ASA in regard to ease of manufacture, safety in handling and homogeneity without in any way detracting from the performance characteristics desired for an initiatory explosive for electric and non-electric detonators.
Example 1
SOLUTION A
[0021] A solution of lead acetate (100 g/l) was prepared with distilled or deionised water. The pH of the solution was 5.5 to 5.9. To 10 litres of this solution, 500 ml. of a dextrin solution (15% W/V) in water were added.
SOLUTION B
[0022] A solution of sodium azide (25 g/l)was prepared in distilled or deionised water. To 10 litres of this solution, 75 g. of solid potassium hydroxide were added followed by 84 g. of dry styphnic acid. The resulting solution was stirred until all the styphnic acid had dissolved. The free alkali (calculated as potassium hydroxide) in the final solution was 2.5 - 2.8 g/l. 350 ml. of an aqueous solution (1.5% W/V) of carboxymethyl cellulose were then added.
PRECIPITATION
[0023] One litre of water was placed in a jacketted vessel provided with a stirrer and 400 ml. of an aqueous dextrin solution (15% W/V) were added. The stirrer was started and maintained at 60-70 RPM. Water at 40 + 20C was circulated in the jacket of the vessel. Solution A (8.5 litres) and solution B (8.0 litres) were then run into the vessel simultaneously, at a uniform rate, so that the total addition time was between 25-30 minutes. A dosing pump can be used in a conventional manner for this purpose. After the addition had been completed, the stirring was continued for a further 5 minutes. The contents of the precipitation vessel were then allowed to cool to ambient temperature and to settle. The resulting precipitated crystals were washed with water in the vessel three times with decantation in between washings. The resulting product was then sieved through a 52 mesh sieve, filtered and dried.
[0024] Yield: 450 - 500 g. Analysis - Styphnate (calculated as basic lead styphnate) 20-22% : Azide (calculated as PbN6) 68-70% ; Lead content 62-64%.
Example 2
SOLUTION A
[0025] A solution of lead acetate (250 g/1) was prepared in distilled or deionised water. The pH of the solution was between 5.5 and 5.9. To 5 litres of this solution were added 235 ml. of an aqueous dextrin solution (15% W/V) and 700 ml, of an aqueous basic lead acetate solution (20% W/V). The pH of the final solution was between 6.5 and 7.3.
SOLUTION B
[0026] A solution of sodium azide (86 g/1) was prepared in distilled or deionised water. To 5 litres of this solution were added 180 ml. of ammonia solution (25% W/V) followed by 150 g. of dry styphnic acid. The resulting solution was stirred until all the styphnic acid had dissolved. Ammonia if present in excess (as detected by smell) was removed by warming. The solution was then cooled and 600 ml. of an aqueous solution (3% W/V) of carboxymethyl cellulose were added. The pH of the final solution was adjusted to between 7 and 7.3 with ammonia or acetic acid.
PRECIPITATION
[0027] The precipitation was carried out as described in Example 1 using 300 ml. of an aqueous solution of dextrin, 3.4 litres of solution A and 3.2 litres of solution B. The contents of the precipitation vessel were then allowed to cool to ambient temperature and to settle. The resulting precipitated crystals were washed in the vessel with water three times, with decantation in between washings. The resulting product was then sieved through a 52 mesh sieve, filtered and dried.
1. A method 01 preparing an initiatory explosive comprising lead azide and lead styphnate, which
comprises bringing into contact an aqueous solution of lead acetate and an aqueous
solution of a water-soluble styphnate and sodium azide in the presence of at least
one crystal habit modifier so as to cause the solutions to react and thereby form
a precipitate which is a homogeneous crystalline material composed of isogonous mixed
crystals of lead azide and lead styphnate.
2. A method according to Claim 1, wherein the lead acetate solution also comprises
basic lead acetate to provide available free alkali during the reaction.
3. A method according to Claim 1, wherein the azide/styphnate solution also comprises
potassium hydroxide to provide available free alkali during the reaction.
4. A method according to Claim 1, 2 or 3, wherein the water-soluble styphnate is potassium
styphnate or ammonium styphnate.
5. A method according to Claim 1, 2, 3 or 4, wherein the crystal habit modifier is
dextrin or carboxymethyl cellulose, or a mixture thereof.
6. An initiatory explosive comprising isogonous mixed crystals of lead azide and lead
styphnate whenever prepared by the method claimed in any preceding claim.
7. A detonator containing as a priming charge an initiatory explosive as claimed in
claim 6.