blasting technology - chemistry of explosives(3)

8/12/2019 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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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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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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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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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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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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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.

blasting technology - chemistry of explosives(3)

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