Explosive and blasting accessories ~ Mining solutions

Explosive

explosive are used to break coal and other ores in underground or opencast mining.
an explosive is a solid or liquid or a mixture of a substance which change itself instantaneously into a large volume of gases at high temperature and pressure when a flame heat or a sudden shock is applied to it.

Constituents of explosive

Combustible material such as sulphur, fibre,and charcoal.
oxidation agent such as sodium nitrate, potassium nitrate, aluminium nitrate.
Stabilizer such as magnesium and calcium carbonate.
Anti setting agent to prevent caking of explosive.
Sensitizer like metallic powder to prevent sensitiveness.

Properties of explosive

Strength
Velocity of detonation
Density
Water resistance
Sensitivity
Fume characteristics
Legal permission

Strength

This is the amount of energy released by the explosive during blasting.
It is also the ability of explosive to do useful work.

velocity of detonation

it is the rate at which the destination wave passes through a column of explosive.
example -slurry explosive used in mines have VOD ranging between 2500 to 5000m/s. For high explosive VOD carries between 6000 to 7000m/s

Density

density is a deciding factor for the explosive regarding the concentration of energy of a shot.
for blasting hard ground high-density explosive is used where as for soft ground low-density explosive is used.

Water resistance

it is the capability of an explosive without losing any sensitivity and efficiency to withstand exposure to water.
Gilatineous type explosive are better water resistance and ANFO has very poor water resistance property.

Sensitivity

Explosive should be insensitivity during normal handling and function.
But it should be sufficiently sensitive while it is detonated and should propagate VOD nicely from one catridge to another.

Fume characteristics

explosives which are to be used where ventilation is restricted most produce a minimum of harmful gases.
Slurry explosives and ammonium nitrate base explosive are preferable.

Legal permission

Only permitted explosive of property types should be used in underground coal mines.

Types of explosive

Low explosive
High explosive

Low explosive

The effect of explosion energy and velocity of propagation of flame is low in this case.
Explosives are generally fire by ignition.
The chemical reaction inside the explosives is done by the rate of combustion.
Example of lower explosive is gunpowder.

High explosive

The effect of explosion energy and velocity of detonation of this type explosive is very high.
Explosives are fire by detonation.
the chemical reaction inside the explosive is done with high velocity and detonation wave which propagate instantaneously.
At high temperature and high pressure a large volume of a gas release instantaneously.
Nitroglycerine base explosive are the example of high explosive.

Permitted explosives

All explosives create heat and some flame when fire , To avoid explosion of gas or coal dust in underground coal mine it is essential that the heat and flame produced by the explosive should be incapable of igniting gas or coal dust.
for this reason explosives which are to be used in underground coal mines should be approved by DGMS , and such type of explosives are called permitted explosive.

Types of permitted explosive

P1-Unsheathed explosives.Ex-Ajax G, Viking G etc.
P2-Sheathed explosives.
P3-Equivalent sheathed explosives.Ex-Unisax Gpermadyne.
P4-Ultra safe explosives.
P5-For solid blasting.Ex- soligex,slurry explosives.

P1(Unsheathed explosive)

This type of explosives comes under the permitted explosives group P1.
This type of explosives are without coating of any substance.
Ex-Ajax G,Viking G etc.

P2(Sheathed explosive)

This type of explosives comes under the permitted explosive group P2.
A seathed of sodium bicarbonate about 3 mm thickness is coated along the length of explosive catridge.
as a result of blasting sodium bicarbonate decomposes and form carbon dioxide and water which suppress the heat and frame produced after blasting.

P3(Equivalent Sheathed explosive)

This type of explosives comes under the permitted explosive group P3.
Here ineart material like mixture of ammonium chloride and soda nitrate is coated uniformly throughout the composition of the explosive inside the cartridge.
The result of blasting as safe as sheathed type blasting.

P4(ultrasafe explosive)

This type of explosives comes under permitted explosives group P4.
This explosives are design to give greater measure of safety during blasting in underground coal mines.
This type of explosives contain very high percentage of colling agent like finely grounded sodium chloride and nitro chalk.

Detonator

A detonator is a small copper or aluminium tube containing some special explosive.
a chemical reaction initiated by a flame or electric current in the special explosive can project a detonation wave through out the column of explosive cartridge.
The bese charge of the detonator content PETN(pentaerythritol tetranitrate).And the priming charge contain ASA(azide stphynate aluminum).

Types of detonator

Plain detonator
Electric detonator

Low tension
High tension
Delay detonator
Half second delay
Milli second delay

Plain detonator

This are fired by safety fuse,spark causing the detonator to explode.
These are some type called as ordinary detonator.

Electric detonator

Electric detonator are fired by passage of electric current through the detonator.These are further divided into low tension, High tension and delay detonator.
High tension detonators are not generally used in mines.

Delay detonator

These are electric detonators with a delay elements.
Delay elements is present in between fuse head and priming charge .
Generally copper or brass sleep is used as delay elements.
The use of delay elements is to delay the blasting time of various detonators.
in various detonators blasting time is delayed by the use of barring length of delay element.
the delay detonators of varying length of the elements are named as delay no- 1,delay no-2,delay no -3 etc.
The detonator and non delay detonators are distinguished by the colour of lead wire.
The delay no is Mark on a tag attached to the lead wire of delay donator.
permission from the DGMS is required to use delay detonator in underground coal mines.
In underground coal mine aluminium detonator are not permitted.But only copper detonator should be used.
Delay detonators and non delay detonators should not be kept in the same box.
Delay detonators is divided into two types-half second delay and millisecond delay.
In short delay the delay interval carries up to 25 milliseconds ,where as long delay the delay interval varries up to 300 millisecond.

Advantages of delay detonator

blasting is more efficient due to availability of free space for each round of short.
Example-blasting due to delay number-one detonators gives free space for blasting of delay number 2 detonator.
The fragmentation of rock is good which helps smooth loading of Rock and coal.
The Mili second delay detonators have been observed to produce less ground vibration in compare to other detonators.

Safety fuse

A safety fuse which looks like a cord consists of a core of fine-grained gun powder and wrapped with layer of textile yarn and waterproof coating.
The burning speed is usually 100 to 120 metre/ second.
one end of the safety fuse when ignited it carries the flame at a uniform rate to ignite or to detonate a plain detonator.

Detonating fuse(DF)

For large number of holes blasted at a time in mechanised mine electric detonator are used.For deep hole that is more than 3 metre long length of detonating fuse is used.
The detonating fuse consist of a core of PETN which is rapped with cloth.
The detonating fuse is then completely enclosed in a cover of plastic material which is white for Cortdex and orange for Geocard detonating fuse.
The detonating fuse looks like a plastic card.
It's has a VOD about 6500m/s.
It's external diameter is about 5 mm and weight about 20gm per meter of length.
A large no of holes connected with detonating fuse can be blasted by a single detonator.

Circuit Tester

If electric shot firing before blasting the holes, the circuit is sometime tested to make sure that there is no open or short circuit. Such testing should be done approved apparatus and it is important that the current passed during the testing should be limited so that there is no possibility of accidental explosion of the detonators.
All testing most be done from a safe place with safe distance.
The instrument used for testing it is known as blastometer.

Crimper

A crimper is used to crimp or press the end of plain detonator tube on a safety fuse inserted into it ,so that the safety fuse cannot come out of the detonator.

Shot firing cable

In electric shot firing the lead wire of detonators are connected to a long shot firing cable which is not less than 30 metres in length to fire the holes form Safe distance.
The cables are twin core and insulated to withstand at least 250volt.

Nonel

Nonel means non electric delay detonate.
this is a flexible plastic tools whose external diameter is 3 mm and internal diameter is 1.5 mm.
One end of the tube is fitted with non electric delay detonators and other end is seal. The end having the detonator is inserted into explosive catridge while the seal end is projected outside of the hole.
The sealed end is initiated by a detonator or a detonating cord.
The nonel system of detonation is developed by NOBEL AB of sweden.
After initiation the shock wave passage through the plastic tube, the inside of which coated with reactive substance to maintain the shock wave at the rate of approximately 2000m/s
The advantages of the nonel is its extream resistance to flame ,frinction static electricity etc.

Exploder

In electric shot firing system exploder are used to supply the necessary current for firing the electric detonators.
Electric shot firing is safer ,quicker and more convenient .
There are two types of exploder generally used in india.

Magneto/Dynamo Exploder

In this type exploder electric energy is generated by the use of dynamo.
An armature revolves between the poles of a magnet by a rotary handle and the voltage produced depends upon the speed of revolving armature and magnetic flux created by the magnets.
when the Rake bar is pushed down with the help of a key, it produce direct current to fire electric detonator.
A high and low tension exploder produces a voltage of 125volt and 15 volt respectively.
Ex- little demon,schaffler-750.

Battery operated condenser discharge type exploder

In this type exploder a capacitor is changed by a battery present inside the exploder and then current is discharge through the shot fairing cable during blasting.
The capacitor is charged by placing charge button.
When the signal 🚥 light glows,the exploder is charge and ready to fire.
Fairing is done by pressing the fire button along with pressing down the charge button.
Here current is generator from a battery of 4 to 12 volt dry primary cells connected in series.

References

G.k pradhan, explosive and blasting accessories.
D.J Deshmukh, elements of mining technology vol-1.
B.A.kENADY.,
Alan Bauer william A.crosby BLASTING.

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