PRIMING OF EXPLOSIVES FOR EFFICIENT BLASTING

PRIMING OF EXPLOSIVES FOR EFFICIENT BLASTING

By: Partha Das Sharma

1. INTRODUCTION:

The massive use of blasting agents such as ANFO, Heavy ANFO etc., in rock breakage has brought about an important development of initiation and priming techniques. This is due to, on one hand, the relative insensitivity of these compounds and, on the other hand, a desire to obtain maximum performance from the energy released by the explosives used in the process.

The detonation process requires initiation energy so that it can develop and majntain stable conditions. The most frequent terminology used in initiation is:

Primer: High strength, sensitive explosive used to initiate the main column in the blasthole. They are cap and detonating cord sensitive, including ones of low core load.

Booster. Powerful explosive charge with no initiation accessory that has two functions:

I. Complete the initiation work of the primer in the explosive column, and

2. Create zones of high energy release along the length of the column.

In the following paragraphs present day knowledge is discussed in order to obtain maximum yield from the explosives.

Detonation pressure is the pressure in the reaction zone as an explosive detonates.It is a significant indicator of the ability of an explosive to produce good fragmentation.A high detonation pressure is one of the desirable characteristics in a primer

 

A blasting agent is an explosive that:Comprises ingredients that by themselves are non-explosive; can only be detonated by a high explosive charge placed within it and not by a detonator.All blasting agents contain the following essential components :

  • Oxidiser – A chemical that provides oxygen for the reaction. Typical oxidisers are ammonium nitrate and calcium nitrate.
  • Fuel – A chemical that reacts with oxygen to produce heat. Common fuels include fuel oil and aluminum.
  • Sensitiser – Provides the heat source (‘hot spot’) to drive the chemical reaction of oxidiser and fuel. Sensitisers are generally small air bubbles or pockets within the explosive.

2. PRIMER AND BOOSTER:

  • Priming a charge is simply positioning a suitable primer within a charge or column of explosives.
  • The object is to provide the primary-initiating explosion needed to detonate the main charge efficiently.
  • If an explosives column is not initiated properly, its optimum energy 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.
  • Primer is 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 hazardous and is against the regulation of most countries.
  • Priming should be done correctly by experienced shot-firers.
  • The primer cartridge must not be tamped nor dropped into the blasthole.
  • When priming blasting agents such as ANFO, the primer should have a diameter which is close to the diameter of the blasthole.
  • 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 are the pentolite boosters.
  • A pentolite booster is made up of a mixture pentaerythritol tetranitrate (PETN) and TNT.
ANFO generates a relatively low detonation pressure, but provides very good heave performance. The steady state VOD of ANFO is approximately 4200ms in 310mm diameter blast holes.The steady state detonating velocity is also a function of loading density. Poured ANFO densities range between 0.78 and 0.85 g/cc while pneumatically loaded ANFO can reach densities up to 0.95 g/cc, consequently achieving higher detonation velocities.ANFO is highly insensitive to mechanical actions (shock, friction, impact).

ANFO should not be placed in conditions where heavy impact or excessive heating may occur as detonation is possible especially if under confinement.

ANFO is desensitised by absorbing moisture.

Every explosive has a certain critical diameter below which detonation will not propagate beyond the primer point.

Confined, ANFO’s critical diameter is approximately 1 1/4 inches.

That is, a borehole or column of ANFO less than two inches in diameter will detonate in the immediate area of the primer, but cannot reliably carry the detonation process much beyond that point.

When ANFO reaches its full VOD the strength is given as:

  • The weight strength of ANFO (94.5%/5.5%) is 912 Kcal/Kg and
  • Bulk strength is 730 Kcal/Cum

3. PRIMING:

  • When an explosive column is initiated at a point, the full steady-state VOD is generally attained some distance away from 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 fig – 1.

1

Fig – 1

  • A VOD less than 2,000 m/s is not considered stable.
  • Tests carried out by Swedish Detonic Research Foundation (SVEDEFO) showed that a NG based explosives primer cartridge initiates ANFO directly to its full velocity.
  • The same result will be obtained with an AN based emulsion explosive primer, provided that its diameter is close to the blasthole diameter.
  • Figure 2 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.

 2

Fig-2

4. 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.

0

  • 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 (VOD higher than the steady state VOD).
  • Low order initiation is caused by a primer being too small or too low detonation pressure.

 3

Fig – 3

  • The velocity distance curve (Figure 3) 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 4 shows how various types and sizes of primers affect the distance from the primer at which ANFO reaches steady state VOD.

 4

Fig – 4

  • In general, the closer the primer diameter is to the borehole diameter, the more effective a primer will be in initiating ANFO.

5. 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.
  • The practice of bottom priming provides a much lower probability of cut-offs, and hence greatly reduce incidence of misfires.

6. QUALITIES OF A PRIMER

  • Four properties of primer have a significant influence on its performance.
  • Detonation pressure: An effective primer should have a minimum detonation pressure of 5 000 MPa.

 5

Fig – 5

  • 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.
  • Length: It should be sufficiently long for maximum VOD to be reached (that is, run-up distance shorter than the primer length).
  • Shape: The importance of shape can be seen in Figure 6, which shows the results of a ‘double-pipe tests’.

 6

Fig – 6

7. 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.

8. SUMMARY

In the priming of ANFO, the efficiency of a primer is defined by its detonation pressure, dimensions and shape. The higher the detonation pressure, the greater its initiating ability.

When priming blasting agents with holes up to 2 1/2 inches in diameter, a full cartridge of high velocity explosives like 60 percent ammonia gelatin, gels, slurries, or cast primers with a blasting cap, is a sufficient charge.

For larger holes, the priming requires much more care, especially if the hole is wet or decked charges are used. A small quantity of a high-velocity primer is better than a large amount of a lower velocity primer. The detonating velocity of the primer must be greater than or equal to the detonating velocity of the agent for efficient detonation.

The best location for priming a charge is at either end of the charge. The placement of primers anywhere else within the powder column shall never be done if there is not also a bottom primer.

With large diameter holes, the shape of the primers, as well as the strength, is important. The diameter of such primers should approach the diameter of the borehole so that the major portion of the available energy is released to propagate a strong detonation wave along the column.

Therefore, the conditions that a primer should comply with in order to eliminate low detonation velocity zones in the ANFO are: the highest possible detonation pressure and a diameter above 213 that of the charge, approximately. The length of the primer is also important, as the primer itself is initiated by a blasting cap or detonating cord and they have a run-up distance in the detonation velocity zone.

The use of detonator cord as a sole detonant is not recommended, since it could cause deflagration rather than detonation of the charge.

The objective of the primer is to achieve a stable detonation. Neither over-priming or underpriming the agent is desirable. The diameter of the primer must be larger than the critical diameter of the explosive.

Every explosive has a certain critical diameter below which detonation will not propagate beyond the primer point. Confined, ANFO’s critical diameter is approximately 1 1/4 inches. That is, a borehole or column of ANFO less than two inches in diameter will detonate in the immediate area of the primer, but cannot reliably carry the detonation process much beyond that point.

The problem of determining how many primers to use and where to locate primers in an explosive column is a difficult one. Too many unnecessary primers add to the cost of blasting, while too few primers rob the blast’s efficiency. Basically, the primers must be located so that the detonation travels through the entire powder column before any of the gas and pressure is vented.

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Effect of double-primer placement on fragmentation and rock fracture: The double-primer placement is based on the principle of shock wave collision. When two shock waves meet each other, the final pressure is greater than the sum of the initial two pressures. Stress analysis indicates that this should be favorable to rock fracture and fragmentation in blasting. Double-primer placement was tested successfully in various mines by using electronic detonators, aiming to improve rock fragmentation.

It has been experienced, when two primers are placed at different positions in a blasthole and they are initiated simultaneously (with the same timing), shock-wave collision takes place. In other words, the double-primer placement is based on the principle of shock wave collision. When two shock waves collide each other (head on), the final pressure is greater than the sum of the initial two pressures. Stress analysis indicates that this should be favorable to rock fracture and fragmentation in blasting.

Theory on shock wave collision – According to one-dimensional shock wave theory, when one shock wave with pressure P1 meets another shock with pressure P2, the final shock pressure P3 produced is greater than the sum of the pressures of the initial two shock waves, i.e. P3 > P1+P2. This case is called shock wave collision.

A shock wave collision is different from an elastic wave collision. In one-dimensional condition, as an elastic stress wave with stress σ1 meets with another elastic wave with stress σ2, the final stress σ3 produced is equal to the sum of the stresses of the initial two elastic waves, i.e. σ3 = σ1+σ2. In fact, shock wave is not elastic; thus the resultant intensity of pressure is more than double, hence the benefit of fragmentation.

Generally, in the case of double-primer placement, one primer is placed at the bottom (or slightly above bottom) of the borehole and other placed at the middle (not at the collar) of the borehole.

Experiments showed that, the amplitude of stress waves in rock mass due to two-primer placement in a blasthole was much greater than the double of the amplitude of the waves caused by one single primer in a similar blasthole. These experiments indicate potential applications of a two-primer placement in rock blasting.

When electronic detonators came into being, shock collision theory was used to improve fragmentation more precisely.

As fragmentation is improved greatly by placement of double primer in borehole, for side-cast blast and Ring-blast this method of double-priming is very advantageous to get higher percentage of cast (throw) and ore recovery respectively.

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Cost effective new formulation of ANFO by using Biomass Briquette as Additive

Cost effective new formulation of ANFO by using Biomass Briquette as Additive

By: Partha Das Sharma

Endeavour is to promote cost effective composition of ANFO, which do not compromise with the energy output (VOD) very much.

1.       Introduction:

ANFO (or Ammonium Nitrate/Fuel Oil) is a widely used bulk industrial explosive mixture in mines and quarry operation. It consists of 94 % porous  Ammonium Nitrate (NH4NO3), (AN) that acts as the oxidizing agent and absorbent for the fuel – 6 % number 2 fuel oil (FO), popularly known as High Speed Diesel (HSD). This forms a reasonably powerful commercial explosive. ANFO is non cap-sensitive explosives and requires a large shockwave to set it off.

ANFO has found wide use in coal mining, quarrying, metal mining, and civil construction in undemanding applications where the advantages of ANFO’s low cost and ease of use matter more than the benefits offered by conventional industrial explosives, such as water resistance, high detonation velocity, and performance in small diameters. It is estimated, use of this product accounts for an estimated 80% of explosives used annually, globally. To keep the cost down, bulk ANFO is used, i.e., mixed at the mines / quarry near the borehole by a bulk truck or stationary mixer.

2.     Industrial Ammonium Nitrate and its use:

Ammonium nitrate is widely used as a fertilizer in the agricultural industry. In many countries its purchase and use is restricted to buyers who have obtained the proper licence. This restriction is primarily because it is an attractive and simple component used in the production of fertiliser bombs.

In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and heating oil. In this form, it has a bulk density of approximately 840 kg/m3. The density of individual prills is about 1300 kg/m3, while the density of pure crystalline ammonium nitrate is 1700 kg/m3. AN prills used for explosive applications are physically different from fertiliser prills; the former contain approximately 20% air. These versions of ANFO which use prills are generally called explosives grade, low density, or industrial grade ammonium nitrate. These voids are necessary to sensitize ANFO: they create so-called “hot spots”. Finely powdered aluminum can be added to ANFO to increase both sensitivity and energy; however, this has fallen out of favour due to cost. Other additions include perlite,  chemical gassing agents, or glass air bubbles to create these voids.

AN is highly hygroscopic, readily absorbing water from air. It is dangerous when stored in humid environments, as any absorbed water interferes with its explosive function. AN is also water soluble. When used in wet mining conditions, considerable effort must be taken to dewater boreholes.

Other explosives based on the ANFO chemistry exist; the most commonly used are Emulsions. They differ from ANFO in the physical form the reactants take. The most notable properties of emulsions are water resistance and higher bulk density.

AN is a substance with relatively weak explosive properties and is not considered an explosive in many classifications. Adding some combustible material, such as nitro compounds, liquid hydrocarbons, solid Hydrocarbons or metal powders to AN increases significantly its explosibility and Energy. Uniform mixing of oil and ammonium nitrate is essential to development of full explosive energy. High explosives boosters are sometime spaced along the borehole to assure propagation throughout the column.

The popularity of ANFO is largely attributable to its low cost and high stability. In most jurisdictions, ammonium nitrate need not be classified as an explosive for transport purposes; it is merely an oxidizer. Many mines prepare ANFO on-site using the same diesel fuel that powers their vehicles, although heating oil, which is nearly identical, may cost less than diesel fuel due to lower fuel tax. Many fuels can theoretically be used; the low volatility and cost of fuel oil makes it ideal.

ANFO has a wide variety of applications in dry hole blasting conditions. It is one of the most cost effective blasting agents available for use in large hole diameter mining through to small hole diameter quarrying.  Pneumatically loaded, ANFO is also effective in underground development and tunneling applications.

Features and benefits of ANFO are, (a) it is a dry and free flowing product, allowing delivery by loose pour or pneumatic loading, (b) its low bulk density provides excellent charge distribution in the blasthole, (c) it provides excellent heave energy.

3.       Effort in reduction of Cost of ANFO by using Biomass briquette or White Coal as additive:

With recent escalations in Ammonium Nitrate and fuel prices, there have several attempts to reduce the cost of ANFO, without sacrificing the blast performance.

In order to reduce the cost of ANFO, green fuel – Biomass Briquette or White Coal – additive is used.

Nowadays white coal is made from Groundnut shells, Cotton hulls and salks, Castor seed shells, Forest leaves; wood chips and shavings, Sugarcane bagasse, Rice husk and paddy straw, Mustard waste, Coir dust, Coffee husk, Sunflower waste, Maize stalks, Bajra cobs, Sesame seeds oil cake, Wheat straw etc.

Benefits of white coal are (a) White coal is cheaper than coal and fire wood, (b) There is no sulphur in the white coal, therefore no toxic gases, (c) Moisture content is nil, (d) Biomass briquettes have a higher practical thermal value, (e) Briquettes have consistent quality, have high burning efficiency, and are ideally sized for complete combustion, (f) Combustion is more uniform compared to coal due to higher quantity of volatile matter in briquettes, (g) Low ash contents and (h) The calorific value of the finished briquettes is approximately 4000 to 5000 kcal/kg.

Environmentally, the use of biomass briquettes produces much fewer greenhouse gases, specifically, 13.8% to 41.7% CO2 and NOX.

In this system it has been tried to apply combination of low-cost liquid Hydro-carbon fuel (used Diesel) and Biomass Briquette or White Coal  i.e., Solid green Fuel (crushed) with porous Ammonium Nitrate, in order to make the blend cost effective, at the same time, without compromising the Energy output of such blend during the blast.

4.       Development of the new Formulation:

  • As uniform mixing of fuel and ammonium nitrate is essential to obtain maximum explosive energy, several methods of mixing in the field and ingredients used can be employed to achieve optimum result.
  • It has been experimented, Ammonium Nitrate blended with solid green fuel, in the form of crushed Biomass Briquette or White Coal and liquid Hydro-carbon fuel (in the form of Diesel oil and used oil) in various proportions to enhance blasting efficiencies.
  • The hydrocarbon required for combustion in ANFO is provided by Fuel Oil / Used oil to the extent of 4%.

Therefore, in effect, the blend consists of following four components: (a) Free-flowing Ammonium Nitrate, (b) Liquid fuel oil / Used Oil, (c) Solid green crushed Biomass Briquette or White Coal.

ANFO / Biomass Briq Cost Calculation:
Qty Unit Rate Cost (Rs)
ANFO conventional 1000 KG
AN 1000 KG 32.00 32000
HSD 70 Lit 51.00 3570
Total 1000 KG   35570
Qty Unit Rate Cost (Rs)
ANFO/Briq. 1000 KG
AN 910 KG 32.00 29120
Briquette 90 KG 5.50 495
HSD 40 Lit 51.00 2040
Total 1000 KG   31655
Saving in cost of ANFO / Briq 3915.00
%age saving of ANFO / Briq 11.01

Cost Effectiveness – Reduces overall cost by about 11%

Note:

  1. Maximum possible size reduction of Biomass Briquette should be done before use.
  2. Replacing some portion of Diesel with used oil may reduce further cost.
  3. First Ammonium Nitrate is to be mixed with required percentage of Diesel or oil (as mentioned), then crushed / grind Biomass Briquette is to be mixed with above ANFO.

5.       Blast Performance:

  • The new blends comprising of ANFO-waste oil-Solid green fuel (Biomass Briquette) blend has been tested.
  • Besides lower the blasting costs, the formulation is comparative in terms of rock fragmentation to standard ANFO (94% AN with 6 % FO).
  • The blast output results into low flyrock and consequently effective casting of the blasted rock.
  • This helps in safer and secured mucking & loading of the blasted material as well as lower mucking cost since the entire blast was well-fragmented.
  • This results in optimum energy utilization of the blast energy and that too for fragmentation and prevents loss of energy resulting from following unsafe and wasteful means: (a) Fly Rock, (b) Vibrations, (c) Noise (d) better heave and (e) preventing of unwanted spreading of blasted material.
  • VOD of the new formulation found to be 3600 ± 500 m/sec.

 6.       Conclusion:

Effectiveness is the most advantageous factor Reduces overall cost keeping blast performance at par / better than conventional ANFO.

Lessened tendency to have fly rock, lessened amount of back break outside drill pattern was observed. Moreover, less “orange smoke” observed during side-by-side comparisons of ANFO and ANFO-Briquette blasts, suggesting less NOx (orange smoke) formation.