Online MiningExplosives and Blasting-Part 1
An explosive is a substance or mixture of substances, which with the application of a suitable stimulus, such as shock, impact, heat, friction, ignition, spark etc., undergoes an instantaneous chemical transformation into enormous volume of gases having high temperature, heat energy and pressure. This, in turn, causes disturbance in the surroundings which results in shattering and demolition.
Explosives are manufactured using fuels, oxidizers, sensitizers, energizers and few other substances in varying percentage.
DETONATION AND DEFLAGRATION
When an explosive is initiated, it undergoes chemical decomposition. This decomposition is self-propagating exothermic reaction, which is known as an explosion. The gases of this explosion with an elevated temperature are compressed at a high pressure. This sudden rise in temperature and pressure from ambient conditions results into a shock or detonation waves traveling through the unreacted explosive charge.
Thus, detonation is the process of propagation of the shock waves through an explosive charge. The velocity of detonation is in the range of 1500 to 9000 m/sec. well above the speed of sound.
Deflagration is the process of burning with extremely rapid rate the explosive’s ingredients, but this rate or speed of burning, is well below the speed of sound.
Common ingredients of explosives
Items Ingredients
Explosive itself Fuels + Oxidizers + Sensitizers + Energizers + Miscellaneous Agents
Common fuels Fuel oil, carbon, aluminum, TNT
Common oxidizers AN, Sodium nitrate, Calcium carbonate
Common sensitizers NG, TNT, Nitro-starch, aluminium etc
Common energizers Metallic powders
Common miscellaneous agents Water, thickeners, gelatinizers, emulsifiers, stabilizers, flame retarders etc.
Main elements of these ingredients Oxygen, Nitrogen, Hydrogen and Carbon, plus, certain metallic elements such as: aluminum, magnesium, sodium, calcium etc.
Classification of explosives
Permitted explosives
These explosives have been designed to use in u/g coalmines to avoid methane-coal dust explosion. These are available in granular, gelatinous and slurry forms. For wet coal mines gelatinous type is more suited. The V.O.D of these explosives is in the range of 6000 to 16,000 ft/sec.
A cooling agent is incorporated in all permitted explosives. Common amongst them are sodium chloride, potassium chloride and Ammonium chloride. A 3 mm thick cover (sheath) of sodium bicarbonate, when wrapped all along the length of the cartridge, this is known as Sheathed explosive. This is also a permitted type of the explosive, which can be used in coalmines.
BLASTING PROPERTIES OF EXPLOSIVES
a. STRENGTH
It is the energy released/unit weight (known as weight strength); or per unit volume (known as bulk strength) of an explosive. It is now a days expressed relative to ANFO at 100% i.e. taking ANFO as standard. High strength is needed to shatter the hard rocks but use of high strength explosive in the soft; weak and fractured rocks will be wastage of the excessive energy imparted by these explosives.
Explosive strength is measured by the Trauzl lead block test.
Strength of an explosive is measured by Shock generated (VOD and speed of chemical reaction), Gas volume, Energy, Detonation pressure, Explosion temperature.
- Velocity of detonation (VOD) is the measure of the shattering effect of an explosive, an important parameter for hard rock blasting. It changes with change in diameter and density of explosives. ‘Dautriche’, electronic or Hess method or tests can measure VOD.
- Gas volume- Larger the gas volume of an explosive large will be the throw obtained. If throw is to be minimized its ingredients should be adjusted to get minimum volume of gas and maximum heat output. ‘Ballistic Mortar’ test and ‘Trauzl block’ test generally measure it.
- Energy- The oxygen balance and reactive ingredients determines the energy output of an explosive. This energy represents the temperature of explosion and hence the maximum work that can be done by an explosive is indicated by this value.
- Detonation Pressure- Based on detonation velocity and density of explosives a shock wave pressure that is built ahead of reaction zone is known as detonation pressure. Higher the detonation pressure, higher would be the brisance capability (i.e. the ability to break or shatter rock by shock or impact). Its value varies from 5 to 150KB. Due to this property a primer having higher detonation pressure should be selected.
P = 2.5 ρv2 x 10-6
Where, P = detonation pressure in kilobars (KB)
ρ = explosive density in gms/c.c
v = velocity of detonation in m/sec.
Above the critical density, detonation pressure is zero, as the cartridge does not explode.
- Explosion temperature- This parameter is calculated based on the thermodynamic data of the ingredients. In coal mines a balancing of explosion temperature and the gas volume play an important role. If explosion temperature exceeds 1000°C it can make the methane atmosphere incendive i.e. mixture of air & methane can catch fire and explode.
b. DETONATION VELOCITY
It is the velocity with which the detonation waves move through a column of explosives.
Factors that affect the detonation velocity are Explosive type, Diameter, Confinement, Temperature & Priming.
In general, higher the velocity of detonation better will be the shattering effect. The explosive’s detonation velocity ranges from 1500–6700 m/sec. In general, larger the diameter the higher is the velocity of detonation until a steady state velocity is reached. For every explosive there is a minimum critical diameter at which the detonation process once initiated, will support itself in the column.
c. DENSITY
The explosives’ density is in the range of 0.5 to 1.7. A dense explosive release more energy/unit volume, hence it is useful for the hard and denser strata. For any explosive there is a critical density, above which, it cannot reliably detonate.
d. WATER RESISTANT
A practical way to judge the ability of any explosive to resist water is its capability to withstand exposure to water without losing sensitivity or efficiency. ANFO is poor water-resistant. Slurries are good water-resistant, and whereas, the NG based explosives are the best water resistant.
e. FUME CHARACTERISTICS
An explosive after blasting should generate minimum amount of toxic gases such as carbon mono oxide, oxides of nitrogen etc. It varies from 0.023m3/kg (fume volume/unit weight) to as high as 0.094m3/kg. In some of the NG based explosives, the fumes emitting out from it, enters into the blood circulation-causing headache.
f. OXYGEN BALANCE
As stated above that any explosive contains oxidizing and combustible (fuels) ingredients. A proper balance of these ingredients is essential to minimize production of the toxic (poisonous) gases, e.g. an excess of oxygen produces such as nitric oxides, nitrogen peroxide and deficiency of oxygen result in the production of carbon monoxide.
Also such an imbalance effects the energy generation. This can be illustrated by taking example of ANFO explosive, which is mixture of ammonium nitrate and fuel oil. The former acts as an oxidizer and the later a combustible agent.
g. DETONATION PRESSURE
Based on detonation velocity and density of explosives a shock wave pressure, which is built ahead of reaction zone, is known as detonation pressure. Higher the detonation pressure higher would be the brisance capability. Its value varies from 5 to 150KB. Due to this property a primer having higher detonation pressure should be selected.
h. BOREHOLE PRESSURE AND CRITICAL DIAMETER
It is an important parameter, which measures the breaking and displacement property of an explosive. Its value varies from 10–60 KB (1000 to 6000 kpa).
Critical diameter: Sensitivity of an explosive is an important property, which is measured by its ability to propagate the detonation wave. The detonation wave tends to fall or fade when diameter of explosive charge decreases. The minimum diameter of a charge, below which the detonation does not proceed, resulting in misfire, is called ‘Critical Diameter’. At lower diameter even if the explosive is sensitive, the reaction in the cartridge may be incomplete.
i. SENSITIVITY
It is measured as the explosive’s propagation property to bridge a gap between two consecutive cartridges or a column of an explosive charge e.g. if a cartridge is cut into two halves, and the resultant pieces are kept apart. By initiating one of them, with how much gap the other will be able to accept the propagation wave, if blasted unconfined in a paper tube.
j. SAFETY IN HANDLING & STORAGE QUALITIES
ANFO is having poor storage quality being hygroscopic in nature. ANFO if handled without gloves can cause skin irritation. Also salt of some explosives under extreme temperature conditions evaporates, making its cartridges hard and deformed. By proper waxing of the explosive cartridges the effect of moisture on them can be minimized. One of the important requirements of an explosive is that it can be stored, transported and used under the normal conditions without any risk to the persons handling it and carrying out the blasting operations.
In order to have a safe manufacturing, transport, handling and the end use of an explosive, various tests are made on the ingredients and final product. The tests include Impact test (fall hammer test), Friction pendulum test, Torpedo friction test, Projectile impact test and bullet sensitivity test.8 For example, NG powder will explode if a weight of 0.5 kg fall on it from a height of 20–30 cms. Whereas if a weight of 0.5 kg falls from about 8 m on it, a cap-sensitive slurry may explode.
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Keywords- Explosives, detonation, deflagration, permitted explosives, velocity of detonation, Detonation pressure, critical diameter, oxygen balance
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