PROPERTIES OF EXPLOSIVES

Explosives are used in underground mines and quarries to break coal and other rocks. An explosive is a solid or a liquid substance or a mixture of substances which change themselves instantaneously into a large volume of gases at high temperature and pressure when a flame, heat or sudden shock (detonation) is applied to it. The high pressure that is built up is capable of doing the work of breaking rock. Detonation is a process of giving sufficiently violent shock to the explosive to bring about an almost instantaneous rearrangement of atoms. 

The chemical energy in an explosive is released and converted suddenly into heat and mechanical energy when heat, flame or detonation is applied and this conversion is due to a chemical reaction which is essentially a process of oxidation. An explosive, by virtue of its constituents, contains enough oxygen necessary for complete oxidation. A commercial explosive contains, apart from the explosive substance or explosive mixture, the following materials:

1.   Combustible matter such, as wood meal, fibre, sulphur, charcoal, etc.

2.  Oxidising agents, such as sodium nitrate, ammonium nitrate, potassium nitrate, etc.

3.   Stabilizers such as magnesium and calcium carbonates.

4.  Anti-setting agents to prevent caking of salts.

5.  Sensitizers, like metallic powders.

The following properties of an explosive are of interest to its user:

1. Strength: This is a measure of the amount of energy released by an explosive during blasting and hence its ability to do useful work. The relative strength or power of an explosive is given by the term weight strength in the case of explosives manufactured by ICI India Limited and a few other explosive manufacturers. The" weight strength, in the case of 1CI explosives, indicates the strength of any weight of explosive compared with the same weight of Blasting Gelatine which is taken as standard because it is the most powerful commercial explosive manufactured by –ICI. The weight strength of Blasting Gelatine is 100. At present, Blasting Gelatine is not in the regular manufacturing range of 1CI India Limited.

The Ballistic mortar is calibrated initially with standard Blasting Gelatine and subsequently the weight strengths of other explosives are determined with respect to the above calibration.

Indian Oxygen Ltd. and Indian Detonators Ltd. do not use the term weight strength to indicate the relative strength of their explosives.

2.  Velocity of detonation: It is the rate at which the detonation wave   passes    through   a   column   of explosive   and   this   is   of considerable importance as the shock energy of detonation increases rapidly with this velocity. Most of the high explosives, permitted explosives and slurry explosives used in the mines have a velocity of detonation ranging between 2500 and 5000 metres per second. For high explosives which are used as boosters, the V.O.D. is high, e.g. O.C.G. - 6000 m/s; Primer - 7000 m/s.

It should be noted that the basic principle of detonation is-more intimate the contact between the oxidizer and fuel, the higher is the V.O.D.

3. Density: The density is important when selecting an explosive for a particular use. With a high density explosive the energy of the shot is concentrated — a desirable feature in tunnelling and mining; operations in hard ground. On the other hand when the output of lump coal from mine is important, it is advisable to use a low density explosive, which distributes the energy along the shothole.

4. Water resistance: Explosives differ widely in resistance to water and moisture penetration. Some explosives deteriorate rapidly under wet conditions, but others are designed to stand water long enough to enable the work to be done. When blasting is to be performed under wet conditions a gelatinous or slurry explosive should be used. The higher the nitro-glycerine content of an explosive, the better its water resistance properties

5.  Sensitivity: An explosive is required to be insensitive to normal handling, shock and friction, but must remain sufficiently sensitive to be satisfactorily detonated and capable of propagating satisfactorily, cartridge to cartridge and even over short gaps such as may occur in practice.

6.  Fume characteristics: Explosives which are to be used where ventilation is restricted must produce a minimum of harmful gases in the   products   of detonation.   Slurry   explosives   and   AN based explosive are preferable to the NG based ones.

7.  Legal permission: Only permitted explosives of proper type should be used in underground coal mines.