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  Explosives, Chemistry tutorial

Introduction:

The explosive is a chemical mixture or compound which whenever exploded by action of heat or impact provides large volume of gases in a very short time at high pressure and temperature. All the commercial explosives are widely categorized into two groups: Low explosives and high explosives.

Explosives:

Explosives are the materials which decompose quickly and spontaneously by the evolution of huge amounts of heat and gas, under the affect of thermal or mechanical shock. Explosives have a great amount of potential energy which can produce an explosion if discharged suddenly, generally accompanied via the production of heat, light, sound and pressure. Commercial explosives generally encompass detonation velocities. 

Classification of Explosives:

Explosives can be fundamentally categorized according to the velocity and sensitivity.

I) According to Velocity:

A) Low Explosives:

Low explosives burn at a slower rate and form less pressure. They are generally employed as propellants to send a rocket to space or force a bullet out of a gun. Generally, low explosives have combustible substances, an oxidant which burns at speeds ranging from a few cm/sec to 400 m/sec, however generally on the lower end of that scale. Low explosives join altogether a combustible substance and an oxidant at adequate temperature liberating heat and quickly expanding gases. The chemical reaction in low explosives is termed as deflagration that is a fast process of combustion devoid of accompanying any shock wave however provides a heaving effect. The common illustrations of low explosives comprise gum powder, black powder and nitrocellulose.

B) High Explosives:

High explosives are the chemicals which explode faster than the speed of sound. They burn more quickly and generate more pressure compared to low explosives. They detonate instantly, that is, reactions in high explosives are characterized via an associated shock wave initiated through a detonator. There are many reaction sequences comprises in a detonation procedure. It comprises combination of the metal with chlorine liberating surplus energy in the method. Other reactant combinations comprise hydrogen with chlorine, hydrogen with oxygen, metal with oxygen, carbon with oxygen, oxygen with carbon monoxide and nitrogen with hydrogen. Illustrations of high explosives comprise Nitroglycerine, 2, 4, 6,-trinitrotoluene (TNT) and Dynamite.

II) According to Sensitivity:

A) Primary explosives:

The sensitive materials which can be exploded via a relatively small amount of heat or pressure are the primary explosives.

B) Secondary explosives:

Materials which remain relatively insensitive are the secondary explosives.

Characteristics of explosives:

The different characteristics of explosives comprise the following:

1) Velocity of detonation: This is the speed at which detonation wave travels via the media and it based on the explosive type. The average velocity of detonation ranges from 2500 m/s to 5800 m/s.

2) Weight strength: It is the energy produced via an explosive relative to that prepared by an equivalent weight of 94 AN 6 FO (94 %) Fuel oil.

3) Water Resistance: Explosives differ broadly in resistance to water and moisture penetration. Some of the explosives deteriorate fast under wet conditions however others are designed to endure water for long periods.

4) Fume Characteristics: The explosives whenever used under stipulated ventilation conditions must discharge a minimum of injurious gases in the products of detonation.

5) Thermal stability: Explosives compositions must be in such a way that it will be stable under all normal conditions of usage. 

Production of explosives:

1) Nitrocellulose Production Process:

Nitrocellulose is formed by causing cellulose to react by nitrating acid (that is, a mixture of nitric acid and sulphuric acid). After nitration, centrifuges separate the nitrocellulose from the spent nitrating acid employed in surplus quantities. This spent nitrating acid is separated to nitric acid, sulphuric acid and water, and fully recycled back to the process. The subsequent phase is the pre-stabilization phase where the nitrocellulose is washed by water, therefore separating off any acid left on the fibres. Subsequently, the viscosity of the nitrocellulose is adjusted via pressure boiling (that is, heating under pressure to temperatures above 100°C).

The next phase is the post-stabilization phase where the nitrocellulose is washed by water and heated to temperatures beneath 100°C. The water is afterward separated off to leave water content of 35%. This water-wet nitrocellulose can then be packaged.

2) TNT Production Process:

The TNT production procedure is mainly based on the known reaction of toluene direct nitration by nitric acid in the presence of sulphuric acid. The method depends steadily on a sequence of reactors. The TNT plant is provided as a continuous line beginning from the raw materials feeding up to the final drying, flaking and packing. This is highly automated, therefore the process runs automatically. The main process stages are Raw materials feeding; Nitration; Purification; Drying; Flaking; and Packing. 

Applications of Explosives:

The main utilization of explosives has been in warfare. High explosives have been employed in bombs, explosive shells, torpedoes and missile warheads. Non-detonating explosives, example, gunpowder and the smokeless powders, have found extensive use as the propellants for bullets and artillery shells. The most significant peaceful utilization of detonating explosives is to break the rocks in mining. A hole is drilled in the rock and filled by any of a variety of high explosives; the high explosive is then detonated, either electrically or through a special high-explosive cord. Special explosives, known as permissible explosives, should be employed in coal mines. Such explosives create little or no flame and explode at low temperatures to prevent the secondary explosions of mine gases and dust. One significant explosive employed in mining, termed as ANFO, is a mixture of ammonium nitrate and fuel oil. Its use has revolutionized some features of open-pit and underground mining due to its low cost and relative safety.

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