G_priming of Explosives

8/3/2019 G_priming of Explosives

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PRI MI NG OF EXPLOSIVES

If an explosives column is not initiated properly, its optimumenergy cannot be generated.

A change in the configuration or type of initiation, priming or

boosting can lead to a significant increase in blasting efficiency.

The terms primer and booster are often confused.

The Mine Safety and Health Administration USA (MSHA) defines

primer as a unit of cap-sensitive explosive used to initiate other

explosives or blasting agents. A primer contains a detonator or

other initiating device such as detonating cord.

The primer cartridge should be assembled at the work-site.

The transport of cap primers is a hazard and is against the

regulation of most countries.

Priming should be done correctly as in Figure 1. The primer

cartridge must not be tamped nor dropped into the blasthole.

When priming blasting agents, the primer should have a

diameter which is close to the diameter of the blasthole.


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Figure 1: Correctly assembled primer cartridge

A booster is a cap-sensitive explosive but does not contain a

detonator. Its purpose is to maintain or intensify the explosive

reaction at a specific point in the explosive charge along a

blasthole.

It is a specially manufactured explosive that can produce a high

velocity of detonation (VOD) such as cast boosters that have

VOD of 7,600 m/s. The most common used boosters in

Malaysia are the pentolite boosters.

A pentolite booster is made up of a mixture pentaerythritol

tetranitrate (PETN) and TNT.


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Figure 2: Cast boosters

Priming

When an explosive column is initiated at a point, the full

steady-state VOD is generally attained some distance awayfrom that point. This distance is called the run-up distance.

The run-up distance varies between explosives. ANFO has the

maximum (about six charge diameters) and PETN/TNT

explosives have the least (about one charge diameter) as in

Figure 3.


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Figure 3: Run-up distance of two types of explosives

A VOD less than 2,000 m/s is not considered stable. Tests

carried out by Swedish Detonic Research Foundation

(SVEDEFO) showed that a Dynamex M (NG based explosives)

primer cartridge initiates ANFO directly to its full velocity.

The same result will be obtained with an Emulite 100 or now

renamed as Emulex 100 (AN based emulsion explosive) primer,

provided that its diameter is close to the blasthole diameter.

Figure 4 shows a primer that has a stable detonation velocity

greater than the ANFO stable detonation. This will ensure that

ANFO will reach its stable velocity in a shorter time and the

blasting agent will explode efficiently.


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Figure 4: Effect of primer on velocity of detonation (VOD) - Olofsson

Priming of ANFO

When ANFO is efficiently primed it rapidly reaches its steady

state velocity of detonation and maintains it.

The steady state velocity depends on the density, the

confinement and particle size of ANFO as well as the blasthole

diameter.

The VOD increases as the blasthole diameter increases and

reaches its highest value at a blasthole diameter of 300 mm as

shown in the table below.


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The steady state velocity of ANFO for different blasthole diameters

Blasthole diametermm

VODm/s

89

102

152

270

3,700

3,800

4,200

4,400

The purpose of a primer is to initiate the ANFO so that it rapidly

reaches its steady state velocity.

The primer may initiate the ANFO with low order velocity (VOD

lower than the steady state VOD) or overdrive velocity (VODhigher than the steady state VOD).

Low order initiation is caused by a primer being too small

(Figure 5) or too low detonation pressure.


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Figure 5: Effect of inefficient primer. Most of the energy is wasted, as ittakes more explosive in reaching the steady state VOD.

The velocity distance curve (Figure 5) shows that it takes

approximately the length of four blasthole diameters. The low

energy initiation in the bottom of the blasthole may have

serious effect on the blasting result.

Figure 6 shows how various types and sizes of primers affect

the distance from the primer at which ANFO reaches steady

state VOD.

In general, the closer the primer diameter is to the borehole

diameter, the more effective a primer will be in initiating ANFO.


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Figure 6: Effect of primer diameter on ANFO VOD.

Top versus bottom initiation

In large diameter blastholes in bench mining, an ANFO charge

may have a 10 m column, and its VOD of 4 000 m/s. If this

charge is bottom primed, the stemming and the top part of the

burden are not affected by the detonation until 2.5 ms after

initiation.

Thus the bubble or the gas energy has more time to work near

the bottom to move the toe before explosion gases escape

through the fractured rock.


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The practice of bottom priming provides a much lower

probability of cut-offs, and hence greatly reduce incidence of

misfires.

Qualities of a primer

Four properties of primer have a significant influence on its

performance, particularly in blastholes 150 mm in diameter.

i) Detonation pressure : An effective primer should have a

minimum detonation pressure of 5 000 MPa (Figure 7).

Figure 7: Effect of primer quality on ANFO VOD (After ICI)


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ii) Diameter : The primer should match the hole diameter

as closely as possible; however, its diameter should not

be less than 0.67 times the blasthole diameter (Figure6).

iii) Length : It should be sufficiently long for maximum

VOD to be reached (that is, run-up distance shorter

than the primer length).

iv) Shape : The importance of shape can be seen in Figure

8, which shows the results of a double-pipe tests.

In this test a low sensitivity explosive column, together

with the primer, is loaded into a 80 mm diameter steel

or plastic pipe. The charged pipe is then placed on top

of the heavy duty steel witness pipe. After the

explosive is detonated, the depth of the dent produced

along the witness pipe is measured and plotted (as in

Figure 8). It can be concluded from this graph that

blast results can be improved without any increase in

primer mass.


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Figure 8: Effect of size and shape on primer performance (after AECI)

Booster

Sometimes, after detonation, a low sensitivity explosive may

show signs of losing the VOD progressively along its column.

This may arise when an ANFO charge is contaminated with

water.

The boosters can be placed at appropriate intervals (about 30

times the blasthole diameter) to increase the VOD along the

explosives column.

Boosters can be placed at appropriate spots where the ground

is especially hard and requires extra pressure for satisfactory

breakage.


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Figure 9: Charging of explosives into the blastholes

G_priming of Explosives

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