Engineering Chemistry S Datta

344 ENGINEERING CHEMISTRY
(iii) RDX: Bechman process: (80% yield)
NO 2
CH —N—CH 2
2
N N O
Hexamine + HNO 3
(98%) + NH 4
NO 3
+ (CH 3
CO) 2
O ⎯⎯→ O N—N 2
— 2
CH 2 —N—CH 2
Fuses
Fuses are of two types:
(a) Safety fuse: Safety fuse is employed where electrical firing is not used. It consists of
a small-diameter core of black powder enclosed in a wrapper of water-proof fabric. It has an
approximate burning speed of 30 or 40 sec per ft. So when a fuse is used for blasting, a sufficient
length is used so that the shot-firers get time to reach to a point of safety.
(b) Detonating fuse. Detonating fuse has a velocity of detonation over 5000 metres per
sec. It consists of a charge of high velocity explosive like TNT in a small diameter lead tube.
The line of the fuse (cordean) is in contact with the charge throughout its length and hence
causes instantaneous detonation of the whole charge. A recent type of detonating fuse contains
pentaerythritol tetranitrate in a non-metallic wrapping and the velocity is as high as 7000
metres per sec. These fuses are used for exploding charges of explosive in deep holes.
Squibs
When deflagrating explosives like black powder are used, application of flame is
insufficient for firing. Squibs are used for this purpose. Squibs may be in metal or cardboard
tubes and may be open or closed at the firing end. Preferably, electric squibs are used.
Blasting Caps
Blasting caps are small cylindrical shells of copper or aluminium containing a detonating
charge of explosives. Blasting caps are used for initiating the explosion of dynamites and are
inserted into the explosive at the time of use.
There are three types of blasting caps as shown in Fig. 16.1. In ordinary fuse cap, a
length of safety fuse is inserted into the open end and the metal wall is crimped about the fuse.
The end of the fuse which is away from the cap is lighted and the spit of flame from the fuse in
the cap fires the ignition charge leading to detonation.
In electric blasting cap, two leading wires are connected within the cap by a fine wire of
high electrical resistance. On passing electric current this bridge wire is heated and becomes
incandescent and fires the ignition charge.
In the delay electric blasting cap, there is a delay charge of a slow burning composition,
inserted in the cap to delay the detonation of the cap charge.
This time interval can be varied from 15 to 20 sec. by using these delays. The composition
of the delays has been improved so that no gas is given off on combustion.
NO 2