blasting technology - chemistry of explosives(3)
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Blasting Technology Chemistry of Explosives
Energy and Performance of Explosives
BlasTinfo
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Index
Definitions
Types of Explosives
Explosive Composition
Explosive Reaction
Oxygen Balance
Physical Properties of Explosives
Energy of Explosive
Energy Separation
Safety, Environment and Detonation Reliability
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Definitions
Explosion
This occurs when large amount of energy is suddenly released. Inorder for an explosion to occur there must be a local accumulationof energy at the site of the explosion.
Combustion
A chemical reaction between a substance and oxygen. It is anexothermic reaction and is usually accompanied by a flame.
Deflagration
A fast chemical reaction between oxidizer and fuel. This process
propagates large amounts of gaseous reaction products, whichraises the pressure and thus accelerates the reaction rate. It is asubsonic process that can accelerate to such an extent that itmakes transition into an explosion.
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Definitions
Detonation
Explosives that on detonation decompose via the passage of a
shockwave (not a thermal mechanism) are called detonating
explosives.
The VOD of explosives range between 1 500 m/s to 9 000 m/s.
The rate at which the material decomposes is governed by the rate
at which the material will transmit the shockwave and not by the
heat it generates.
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Types of Explosives I
Chemical Substances
These explosives contain usually contain oxygen, nitrogen (both
called oxidizer compound) and oxidizable elements such as
carbon and hydrogen.
Molecular Substances
These are fixed molecular substances with certain combinations for
the required applications
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Types of Explosives II
Commercial Explosives
These are explosives where the reactive ingredients are in separate
molecules. A simple example is ANFO where the oxidizer is the
prill and the fuel is the diesel that is added.
Military Explosives
These are explosive where all the reactive elements are present
within single molecules. Examples are TNT and PETN.
Boosters are normally made from PETN that is melted andpoured into moulds.
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Explosive Composition
Explosive IngredientsExplosive is made from a number of ingredients that can be classed
either as oxidizers or fuels. A small portion is made up of inert
ingredients:
ANFO
94% AN
6 % Fuel
Emulsion Explosive
6 % Fuel
76 % ANRemainder = Water, Stabilising,
Gassing Agents
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Explosive Reaction
When an explosive reacts, the Oxygen in the oxidizercombines with the carbon in the fuel to form carbon-dioxide
gas (CO2). The oxygen also reacts with the hydrogen in the
fuel to form water vapor (H2O). Nitrogen gas is also released.
Ammonium Nitrate + Fuel Gas + Water Vapour
Or in more detail:
3N2H4O3+ CH2 3N2+ CO2+ 7H2O
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Oxygen Balance
For maximum energy, there must be the correct quantity of fuel (C
and H molecules) to combine completely with the Oxygen in the
reaction.
If there is too much fuel, the mixture is said to be Oxygen
Negative. In other words, there is not enough Oxygen in theexplosive to combine with all the Carbon and Hydrogen.
If there is too little fuel, the mixture is said to be Oxygen Positive.
This means there is not enough Carbon and Hydrogen to combine
with all the oxygen.
When an explosive is oxygen balanced, the maximum energy isdelivered from the explosive. Here is an example calculated for
ANFO.
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Oxygen Balance
1500
1600
1700
1800
1900
2000
2100
2200
2300
0 2 4 6 8 10
Percent Fuel Oil
Energy
(kJ/k
g)
O -O +
Underfueled Overfueled
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Oxygen Balance
If the mixture is oxygen positive (too little fuel) the excess oxygen
combines with nitrogen to for orange Nitrous fumes. Nitrous
fumes are very poisonous.
If the mixture is oxygen negative (too much fuel) carbon
monoxideforms instead of carbon dioxide (CO instead of CO2). Normally, explosives are formulated to be slightly oxygen
negative. This is to limit the risk of nitrous fumes forming. Also,
when slightly oxygen negative, the energy loss is minimal. Any
contaminants or field variations that might influence the reaction
slightly should shift the oxygen balance leftwards where there is
still leeway for high energy.
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Explosive Physical Properties
The important physical properties of explosive are:
Water resistance
Some explosives are more water resistant than other explosive.
This list of BME products provides a scale.(pg3 Blasting Guide)
Particle size
Particle size of the ingredients affects the speed of reaction. Large
particles, such as PPAN prills have less surface area for
reaction than smaller particles such as emulsion droplets. The
increased surface area results in more intimate contact betweenoxidizer and fuel and therefore a more rapid reaction.
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Explosive Physical Properties
Viscosity
Viscosity is applicable mainly to emulsion explosive. Viscosity is a
measure of how thick an emulsion is.
If the emulsion is too runny, (low viscosity) it will not be able to hold
the sensitizing air bubbles and the explosive will not detonate.
If the emulsion is too thick, (high viscosity) it will be more difficult to
pump it into the blast holes.
Normally the viscosity for emulsion explosives is between 30 000
and 50 000centipoise.
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Explosive Physical Properties
Density of Explosive
The density of an explosive is simply the mass of explosive in a
fixed volume. Density is normally expressed as g/cm3.
The density is a very useful number for calculating the mass of
explosive that goes into a hole. The density value also gives youa general idea as to which explosives will have more energy and
which will have less energy per fixed volume.
Volume
massDensity
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Explosive Physical Properties
The influence of hydrostatic pressure on density
The density of emulsion explosives and Heavy ANFOs having
high emulsion content can be adjusted by increasing or reducing
the number of small sensitizing bubbles. This is referred to
technically as increasing or decreasing the void volume. Airbubbles in emulsion explosives are important for:
Adjusting explosive density
Sensitizing explosive so that it will detonate
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Explosive Physical Properties
In a column of explosive, the weight of the explosive at the top of
the hole places pressure on the explosive at the bottom of thehole. This pressure causes the little air bubbles to be compressedso that there is less void volume. This means that the explosivedensity at the bottom of a hole will be higher than the density atthe top of the hole.If a hole is v ery deep or th e ini t ia l density istoo h igh , there is a r isk that the explo sive at the bottom o f the
ho le wil l be above the cr i t ical density (wi l l not have enoughvoid v olum e) and w i l l not d etonate.
The final cup density
The final cup densityof a gassed explosive is an importantmeasurement. It is measured in a cup after the explosive has
been given time to gas properly (in other words, about 30 minutes(temperature depending) after it has been collected in the cup).The final cup density is the lowest density of the explosive and isuseful for calculating the average density in a blast hole. It alsoindicates to the blaster whether the explosive is being properlygassed.
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Explosive Physical Properties
Sensitivity
Commercial explosives are sensitized by small air voids.Voids
essent ial to sens it ise the explo sive
Pores in Porous Pri l l
Loo se pack ing o f pr i l ls
Gas bub bles in Emuls ion s
Micro bal loons in car t r idge explos ive
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Explosive Physical Properties
Sympathetic Detonation
Sympathetic detonation occurs when the shockwave from a nearbydetonation is sufficient to initiate an explosive without a booster. Thisnormally occurs when explosives are too sensitive or when timingproblems occur in a blast.
Dynamic Desensitization
Sometimes an explosive may be desensitized by the shockwave from anearby detonation. This happens when the air bubbles are compressedand the void volume is temporarily reduced below critical levels by theshock wave. At this time a booster will be unable to initiate the explosive.
The events that can desensitize explosive dynamically:
Shock wave from detonating cord down lines
Detonations of explosive charges that are too close
Channel effect.
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Explosive Energy
Energy Comparison
When you choose an explosive you will consider its physical
properties (water resistance, particle size, viscosity, density,
sensitivity), and also its energy output. There are two energy
considerations: How much energy the explosive will release
How quickly that energy will be released
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Explosive Energy
Relative weight strength (RWS)
The RWS is the energy per unit of weightfor the explosive
compared to the energy of ANFO for the same weight.
In other words, the RWS is the energy in 1 kg of explosive
compared to the energy of 1 kg of ANFO.
Relative bulk strength (RBS)
The RBS is the energy per unit of volumefor the explosive
compared to the energy of ANFO for the same volume.
In other words, the RBS is the energy in 1 litre of explosivecompared to the energy of 1 litre of ANFO.
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Velocity of Detonation and Energy Partitioning
Velocity of detonation (VOD) is one of the least understood but
most wanted information in the mining industry. It is important to
understand that VOD does nottell you how much energy is
delivered, but it doestell you how the energy is delivered.
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Factors governing the VOD of Explosive
Density of the explosive. The more dense the explosive thehigher the VOD. There is a point known as the critical density.Above this density, the explosive will not detonate.
Particle size of the oxidizer. The smaller the particle size, themore intimate the mix with oil and the higher the VOD.
Formulation. Explosives that contains more inert material suchas water will detonate slower than a similar explosive with lesswater.
Confinement of the explosive. The harder the rock is, the higherthe VOD will be. Soft weathered rock will result in lower VODs.
Charge diameter. As the charge diameter increases, so will theVOD. As the charge diameter decreases, the VOD will drop off.There is a point at which the charge diameter becomes toosmall for reliable detonation. This is known as the criticaldiameter.
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Energy Separation
Boreholepressu
re
Gas Volume
Shock
Energy
Gas Energy
Wasted Energy
High initial peak pressure
High VOD Explosive
Boreholepressu
re
Gas Volume
Shock
Energy
Gas Energy
Wasted Energy
Lower initial peak pressure
Low VOD Explosive
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Safety, Environment and Sensitivity to DetonationFumes
Explosive fumes are caused by incomplete reaction of the explosive ingredients.
This can happen when the oxygen balance is too low or too high in the explosiveformulation.
When water is mixed in with the explosive during charging and causes the explosiveto partially break up.
When the booster is not in intimate contact with the explosive due to contaminants inthe hole bottom or if the booster is too small.
There are two types of fumes that are of concern to you.
Nitrous fumes:
These are red-orange colored nitrous oxide gasses that are extremely poisonous to
human beings. If anyone breathes Nitrous fumes by accident, they should be sent tohospital immediately for observation.
This then becomes a reportable accident.
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Safety, Environment and Sensitivity to Detonation
CO fumes:
Carbon monoxide fumes are colorless and cannot be smelled or tasted. They are notas dangerous as NITROUS fumes, unless they occur in confined places such asunderground workings. Carbon monoxide is absorbed by the blood streampreferentially to oxygen. This can result in asphyxiation and death.
CO is lighter than air and will rise up above a blast so it is not usually a threat insurface blasting.
Sensitivity
Safety in bulk explosive
Generally, most modern bulk explosives are quite insensitive to impact and heat.This means that it is unlikely to detonate when it is being handled.
Emulsion based explosives are only sensitized when they are charged into a hole.The gassing agent is added just before the explosive leaves the truck into thedischarge system. Only once the explosive has been gassed is it properly
sensitized. The gassing reaction normally takes between 10 and 45 minutes tocomplete. During this time, holes should not be stemmed. Explosivessuch asANFO and cartridge explosives (Emgel or Megamite) are sensitized before theygo down the hole. These explosives are more dangerous and should behandled with extreme care.
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Safety, Environment and Sensitivity to Detonation
Spillage
BME emulsion explosives contain recycled old oil. However,
emulsion break down is very slow. The ammonium salts that are
released with time pollute ground water and create very corrosive
conditions that are not environment friendly.
Spillage during charging should be kept to a very bare minimum.
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