Explosive is a solid or liquid substance or a mixture of substances which on application of a suitable stimulus is converted in a very short time interval into other more stable substances, largely or entirely gaseous, with the development of heat and high pressure.
Classification of Explosives
The explosives can be classified into the following types:
- Low (or deflagrating) explosives
- High explosives (a) Primary and (b) Secondary.
Low explosives were the earliest to be developed. These lead to an explosion which is really a rapid form of combustion in which the particles burn at their surfaces and expose more and more of the bulk until all has been consumed. Such an explosion is called deflagration and the reaction in this case moves slower than the speed of sound. Typical examples of this category are the blasting powder or gun powder, propellants in ammunition, rocket propellants and pyrotechnics.
High explosives, depending on their composition, explode at velocities of 1500-8000 m/s and produce large volumes of gases at considerable heat at extremely high pressures. High explosives themselves may be further subdivided into primary explosives and secondary explosives.
Primary explosives are characterised by their sensitiveness to stimuli like weak mechanical shock, spark or flame, the application of which will take explosive compounds from state of deflagration to detonation easily. Examples of these explosives are Mercury Fulminate, Lead Azide and other mixtures. These explosives are used as initiating charges in the initiating devices such as detonators.
Secondary explosives are capable of detonation only under the influence of a shock-wave, normally generated by the detonation of primary explosives. Secondary explosives of this type are military explosives like TNT, RDX, PETN, Tetryl and other combinations of these and industrial explosives like nitro-glycerine, emulsion, slurries, watergels, ANFO and other powder explosives. These explosives are normally set off with suitable initiating devices like detonators or detonating cords and in some cases there is need of initiation by another high explosive. The explosive needing another high explosive is called Blasting Agent such as ANFO, some slurries, some emulsions and mixtures of emulsions and ANFO.
Nitro-glycerine (N.G.)
It is an oily fluid with sp. gr. of 1.6 and freezing point at 13°C. It is insoluble in water and is very sensitive to explosion by shock of any nature. The sensitivity to shock increases when nitroglycerine freezes. "lb render it suitable for industrial use, it must either be absorbed by some inert material or be gelatinised. NG based explosives are available in 3 consistencies; gelatinous, semi gelatinous and powdery.
All explosives containing NG have a highly shattering effect and they produce fumes which cause headache after long exposure. Explosives containing NG are liable to freeze when the temperature falls to 8° or less and are then more sensitive to detonation by friction and impact. To avoid this, a low freezing agent, usually di-nitro-glycol which is itself as powerful an explosive as NG, is used. Low freezing explosives are designated by such prefix as "Polar", e.g. Polar Viking. Polar special gelatine, etc.
Ammonium Nitrate (NH₄NO₃)
It is a white hygroscopic salt, very soluble in water and is comparatively very safe to handle. When it is detonated it is, however, a powerful explosive. Though ammonium nitrate is more powerful than the low explosives it is not as powerful as N.G. and it is difficult to denote it by itself with the help of a detonator, but it can be detonated by a booster of high explosive.
To use ammonium nitrate as an explosive, it should be mixed with diesel oil, N.G. or TNT. Ammonium nitrate is an interesting compound, . In that it is a high explosive, an oxidising agent, and a cooling agent at one and the same time. Prilled (a pellet or solid globule) ammonium nitrate of fertiliser grade mixed with diesel oil is used for large dia. holes in quarries.
Ammonium nitrate does not occur in nature and it is prepared by reacting ammonia gas with nitric acid. When detonated by extreme shock NH4N03 decomposes according to the equation.
2NH₄NO₃ = 4H₂O + O₂ + 2N₂ + 346 kcal/gm
This property comes into play in the utilisation of NH₄NO₃ as an explosive.
Ammonium Nitrate Dry Mixes (ANFO)
Prilled (a pellet or solid globule) ammonium nitrate (AN) and fuel oil (FO) mixtures, known as ANFO, were introduced for blasting operations in mid 1950s. Ammonium nitrate in a proper form when mixed with carbonaceous or combustible material in appropriate proportion forms a blasting agent. Although, many forms of AN could be used with a solid or liquid fuel to form a blasting agent, the porous prilled forms are preferred for ANFO. Through the years, fuel oil has proved itself to be an almost ideal fuel for AN.
It is readily available, is inexpensive and easily mixed with AN to produce a uniform mix commonly known as ANFO. Small porous spherical prills absorb oil readily giving a dry mix which has good pouring properties. The microprills in each prill allow the AN to absorb and retain the optimum amount of fuel oil. The prills are hard enough to withstand transport shocks without breaking down, yet soft enough to breakdown and give a high loading density when blow loaded into blast holes with equipment such as pneumatic blast hole charger.
AN is stable at ambient temperatures but can absorb moisture from the atmosphere if the humidity is above about 60%. To minimise moisture absorption and caking, the prills are lightly coated with anti-caking agents. While it is strong supporter of combustion, AN is not flammable. However it is an oxidiser.
Proper mixing of AN and FO is important for predictable explosive performance.
The most common method of mixing small quantities of ANFO is to pour the correct amounts of both ingredients into a hand-operated mixer (concrete type) and after thorough mixing, tip the ANFO back into the empty bags, and from there into blastholes. Such mixing care is essential for small diameter blastholes and for maximum explosive energy output. Commonly, the bowl is lined with plastic or epoxy resin to prevent corrosion. Aluminium bowl is also used.
Where several users require relatively large amounts of ANFO, a central mixing plant is sometimes installed to meet this need. Mixing of AN and FO constitutes manufacture of an explosive. For this reason, a license or permit to mix ANFO must be obtained from the statutory authority.
Although ANFO mixtures are explosives, they are relatively insensitive and unless suitably primed, reliable detonation in large blastholes will not occur. The size and type of primer required depends upon blasthole diameter, degree of confinement, dryness of blastholes, etc. In general, the primer should have a high VOD and the maximum possible diameter. Needless to mention, the primer should be in intimate contact with the ANFO.
Lack of water resistance is the major limitation and disadvantage of ANFO. AN is readily dissolved by water, and, unfortunately the addition of 5.64% FO does little to reduce the rate of dissolution. Both die strength and VOD of ANFO are reduced by the addition of water. ANFO which contains more than about 10% water usually fails to detonate. As one would expect, the longer ANFO is exposed to water, the greater is the damage, and the lower the blast effect. Wherever there is a chance of blasthole water desensitising ANFO and make it less effective, efforts should be made to minimise the period between charging and firing the blast.
The experience of thousands of the mine operators throughout the world provide ample warning that the blasting performance of the ANFO cannot be relied upon in the presence of water in blasthole. The efficient use of ANFO in wet blasthole depends almost entirely upon the protection given to it by polyethylene liners or by the packaging materials used to make up cartridges of ANFO. Dewatering of blastholes is also carried out. The use of Heavy ANFO has allowed operators to overcome this important shortcoming.
Slurry Explosives
Slurry explosives were first developed as a result of attempts to waterproof, improve density and strength of ammonium nitrate.
A slurry is a mixture of nitrates such as ammonium nitrate and sodium nitrate, a fuel sensitiser, either explosive or non-explosive, and varying amounts of water. Although they contain large amounts of ammonium nitrate, slurries are made water resistant through the use of gums, waxes, and cross linking agents.
Emulsion Explosives
An emulsion is a two phased system in which an inner or dispersed phase is distributed in an outer or continuous phase. In simpler terms an emulsion is a mixture of two liquids that do not dissolve in one another. This unique feature coupled with the fact that minute size of the nitrate solution droplets are tightly compacted within the continuous fuel phase results in good intimacy between the oxidiser and fuel and increased reaction efficiency compared to other systems.
Three products are available in emulsion category. These products are as given below:
- Straight emulsions;
- Doped emulsions;
- Repumpables
Straight emulsions
Basic formulation of an emulsion explosive can be represented as follows:
Ammonium nitrate = 60 - 70%
Calcium nitrate/Sodium nitrate = 0 - 20%
Fuel oil = 2 - 6%
Aluminium = 1 - 3
TNT & Water = Varies
Straight emulsions are normally hot mixed at temperature over 50°C and a suitable chemical gassing agent is incorporated to control density. Since the final product is straight emulsion, the explosive displays exceptionally high water resistance, stability and explosive reaction efficiency leading to superior velocity of detonation (VOD) characteristics.
Doped emulsions
This is a generic term for emulsion explosive to which varying percentage of ammonium nitrate are added to achieve a wide range of strengths. Pumpable characteristics as in the case of emulsions are maintained at all stages and so also the superior VOD and water resistance properties of emulsions.
Repumpable emulsions
These formulations are designed for small and intermediate diameter and apart from the normal advantages of emulsions are designed for pumpability at low temperatures.
These are low cost units for fast and accurate loading of small quantities per borehole. Like all bulk explosive all the components are non-explosive and are mixed on the mine bench and loaded directly into the borehole.
Heavy ANFO
None of the above slurry and emulsion systems have cost competitiveness with ANFO as such their usage has been limited. The exception to this is densified or Heavy ANFO. The aim in Heavy ANFO is to have the advantages of both, the high density and water resistance of slurries and emulsions plus the low cost of ANFO. In the mix of Ammonium Nitrate and Fuel Oil about 50% of air in total volume exists, roughly 30% in the prills and 70% between the prills. This volume is filled with a high density, waterproof product such as emulsion (15-40%), the resultant mix is denser and more water resistant than ANFO and with very little added cost.
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