Lead azide explosive reaction

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

The increase in volume as gaseous products are formed in a chemical reaction is even larger if several gas molecules are produced from each reactant molecule, such as the formation of CO and CO, from a solid fuel (Fig. 4.17). Lead azide, Pb(N3)2, which is used as a detonator for explosives, suddenly releases a large volume of nitrogen gas when it is struck ... 

Commercially, lead azide is usually manufactured by precipitation in the presence of dextrine, which considerably modifies the crystalline nature of the product. The procedure adopted is to add a solution of dextrine to the reaction vessel, often with a proportion of the lead nitrate or lead acetate required in the reaction. The bulk solutions of lead nitrate and of sodium azide are, for safety reasons, usually in vessels on the opposite sides of a blast barrier. They are run into the reaction vessel at a controlled rate, the whole process being conducted remotely under conditions of safety for the operator. When precipitation is complete, the stirring is stopped and the precipitate allowed to settle the mother liquor is then decanted. The precipitate is washed several times with water until pure. The product contains about 95% lead azide and consists of rounded granules composed of small lead azide crystals it is as safe as most initiating explosives and can readily be handled with due care. 

Reaction with sodium azide even at —78°C leads to explosions attributed to fluorine azide formation. 

Lead azide explodes on heating at 350°C or on percussion. Its detonation velocity is 5.1 km/sec (Meyer, E. 1989. Chemistry of Hazardous Materials, 2nreaction with carbon disulfide and forms shock-sensitive copper and zinc azides when mixed with the solutions of copper and zinc salts (Patnaik, P. 1999. A Comprehensive Guide to the Hazardous Properties of Chemical Substances, ed. New York John Wdey). 

Primary explosives (also known as primary high explosives) differ from secondary explosives in that they undergo a very rapid transition from burning to detonation and have the ability to transmit the detonation to less sensitive explosives. Primary explosives will detonate when they are subjected to heat or shock. On detonation the molecules in the explosive dissociate and produce a tremendous amount of heat and/or shock. This will in turn initiate a second, more stable explosive. For these reasons, they are used in initiating devices. The reaction scheme for the decomposition of the primary explosive lead azide is given in Reaction 2.2. 

If small amounts of copper and hydrochloric acid are added to the reaction mixture, a white product is obtained. Mercury fulminate is stored under water. It is dried at 40 °C (104 °F) shortly before use. Owing to its excellent priming power, its high brisance, and to the fact that it can easily be detonated, mercury fulminate was the initial explosive most frequently used prior to the appearance of lead azide. It is used in compressed form in the manufacture of blasting caps and percussion caps. The material, the shells, and the caps are made of copper. 

While the environmental impact of cadmium azide in deep oil deposits is relatively low, the long-term use of Pb(N3)2 and lead styphnate in military training grounds has resulted in considerable lead contamination (see Ch. 1.2.3, see Fig. 1.17). On demand lead azide (ODLA) is available from the reaction of lead acetate and sodium azide. The recently introduced iron and copper complexes of the type [Cat]2 [Mn(NT)4(H20)2] ([Cat]+ = NH4, Na+ M = Fe, Cu NT = 5-nitrotetra-zolate) as green primary explosives [3] are relatively easily obtained and show similar initiator properties as those of lead azide (Tab. 2.2). 

The solutions are transferred to the reaction kettle through metered tubes, and the precipitated lead azide is washed and either filtered and dried or packed wet for shipment. In some countries the practice is to introduce the azide into detonators or explosive trains only at the place of manufacture in other countries, notably the U.S.A., transportation is permitted, and the packed azide may be stored and used in detonators or fuze trains at distant locations. However, while such peripheral practices and associated equipment are important, it is the control exercised during the metathesis in the kettle that affects the product and, more importantly, introduces subtle differences that make one form of lead azide different from another in performance characteristics, if not in gross physical appearance. Therefore, the emphasis of the present discussion will be on the reaction kettles and their associated equipment, and the following section will emphasize the control on the metathesis. 

Figure 1. Bench-scale apparatus of 600-ml maximum capacity for the preparation of lead azide and other sensitive explosives. The apparatus on the right is enclosed with a barricade during use and remotely operated through the control box shown in the left foreground. The turntable in the right foreground, when properly positioned beside the reaction apparatus at the rear, permits the selection of such operations as decant, wash, and filter. The stainless-steel reaction vessel is pivoted to pour into stations on the turntable. (Photo courtesy of ERDE, Waltham Abbey, England.)...

If one simply mixes a solution of lead nitrate (or acetate) with a solution of sodium azide, a precipitate of nearly pure lead azide forms which can be washed, filtered and dried, and utilized in a detonator or other explosive device. However, this lead azide is at best a flulTy fine powder (not unlike that produced by the reaction of hydrazoic acid with lead nitrite (Figure 9), very sensitive to electrostatic discharge, difficult to pour into small detonator cups, and difficult to compact by pressing. It is, however, a stable and powerful initiator. 

In the case of lead azide, Andreev [42] and Bowden and Yoffe [43] suggest that lead azide detonates immediately after being ignited and that a burning regime is absent. The theory of fracture that was subsequently developed to explain the initiation of fast reaction [44,45], and the previous observations lead to the conclusion that the shock initiation mechanism of this primary explosive is not likely to exhibit the same characteristics as those exhibited by the secondary explosives. However, examination of the shock sensitivity of dextrinated and polyvinyl lead azide to pulse durations vaiying from 0.1 to 4.0 psec shows that the initiation characteristics are indeed similar to those observed for heterogeneous explosives. 

It appears that insofar as growth to stable detonation is concerned lead azide displays characteristics similar to those of heterogeneous secondary explosives. The delayed stress excursions evident in measured stress profiles were interpreted as reactions behind the shock front. Reactions produce pressure waves which travel through the explosive at a velocity at least equal to its velocity of sound and interact with the undecomposed explosive ahead of the reaction front, causing a nonuniform rate of growth. 

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