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.

HANDLING OF MISFIRES IN MINES

HANDLING OF MISFIRES IN MINES: Dealing with it is potentially most dangerous activity.

1. INTRODUCTION – Misfire means the complete or partial failure of a blasting charge to explode as planned. The explosive or pyrotechnical products that remain in the ground or in the muckpile might be triggered by any mechanical effect during the digging, milling or crushing stages of the mining process, causing injuries or fatalities to blasters or operators.

The potential consequences of a misfire are such that every reasonably practicable means available to site managers should be taken to avoid misfires. The emphasis should be made on prevention rather than cure.

Dealing with a misfire is potentially the most dangerous activity that site managers and Shotfirers will be involved in during blasting operations. In the event of a misfire, it is likely that unexploded charges and detonators will be left in the face or in the muckpile. These charges could be detonated if drilled into, if struck by an excavator bucket, wheels or tracks, or if inadvertently fed through a crushing plant. Unexploded charges may also be loaded out accidentally and taken either off site in road vehicles or to site tips. In any of these circumstances there may be a risk of danger to the operator or to the public, particularly from fly-rock in the event of a detonation.

Unexploded charges may need to be recovered by hand. All those likely to be involved must realise that this is a potentially dangerous operation. Great care and attention to detail is required to ensure that this is carried out safely.

2. RECOGNITION OF MISFIRES – After firing, a proper examination must be carried out to check the state of the face, that all the charges have fired and that there is no indication of a misfire. However, explosives can still be discovered at the face, in the muckpile or at the processing plant.

Any discovery of undetonated explosives or detonating cord must be reported since their presence constitutes a misfire. Indications of a misfire can include noxious fumes, inadequate ground movement, poor fragmentation, unusual blast sound or vibration trace, flyrock or evidence of undetonated explosives

3. POST BLAST INSPECTION – Post blast inspection is a hazardous task and in all circumstances must be carried out in accordance with the site rules.

Hazards exist not only from the existence of undetonated explosives but also from the post blast environment.

There is the possibility of a misfire remaining undetected even after inspection. It is therefore essential that adequately trained personnel regularly check the muckpile and face throughout the loading operation.

All personnel but especially those operating loading equipment, hauling equipment and crushers should be aware of this possibility and must be instructed to report abnormalities.

The extent and nature of the misfire must be determined as soon as possible after the misfire has been detected.

An exclusion zone must be established and secured until such time as any readily retrievable explosive has been collected and removed.

4. IN THE EVENT OF A MISFIRE – If a misfire is discovered during the post blast inspection then the “all clear” signal should not be given until a new exclusion zone has been established and secured.

The exclusion zone must be established by the responsible person who could be either the site manager or the blasting engineer or the shotfirer.

The immediate priority must be to ensure that arrangements to safeguard personnel in the event of a misfire are adequate and complied with.

Only those personnel directly concerned with the misfire should be within the exclusion zone.

5. DEALING WITH MISFIRES – The following procedures should be considered as possible courses of action in dealing with misfires.

* Removing stemming and re-priming

There may be circumstances in which it is possible to remove stemming in order to gain access and to reprime the charge. This is a potentially hazardous operation, which requires great care. It should only be attempted after detailed consideration.

When a hole contains detonators and it is anticipated that excessive force will be required to remove the stemming then the operation must not be attempted. This could result in premature initiation of the charge, particularly if the detonator is close to the top of the main charge and is immediately below the stemming.

If the hole contains an electric detonator the use of high velocity air to remove the stemming should not be attempted. Static charges sufficient to initiate electric detonators can be created.

Bulk explosive can be washed out of misfired shotholes but the utmost care must be taken in removing cartridges, particularly where detonators are involved. Under no circumstances must explosives or detonators be removed from a borehole by pulling on the detonator leads. Suitable extraction tools are available to enable cartridges to be removed. These usually take the form of a corkscrew or barb of nonferrous material which can be connected to stemming rods.

The following factors must be considered:

The use of high pressure water is unlikely to overcome the mechanical lock of stemming comprising chippings; The use of large quantities of water could desensitise any non waterproof explosive and dissolve any explosive with a high concentration of water soluble ingredients;

In situations where multiple decks of explosives are employed, all the above considerations magnify the difficulty of gaining access to the lower decks of explosives. Irrespective of the number of explosive decks, removal of the stemming in order to gain access to the charge as to re-prime is a technique, which ranges from unattractive at best to extremely difficult.

Any tools used inside the borehole to remove stemming must be non-ferrous.

If all the stemming can be removed and access to the top of the charge is achieved, the charge may be reprimed and refired. However it should be noted that in the event of a partial misfire the burden on the misfired shothole can often be reduced or fractured and a careful assessment of the situation must be made before any decision is reached.

* Drilling and firing relieving holes

The hazards in drilling relieving holes are:-

a) intersecting an explosives column, with a high risk of detonation

b) operating a drill in unstable rock conditions

The object of such holes when fired is:

  •  To disturb and displace the adjacent explosive column so that any primers and detonators remaining unfired are not located within an undisturbed explosive column after this blasting;
  •  To break up the rock mass in the region of the misfired hole in order to facilitate the search for and retrieval of any unexploded charges, primers and detonators.

One or more relieving holes may be drilled behind the misfired hole. The separation distance between the holes depends on the diameter, the inclination and the type of drilling equipment and the sensitiveness of the explosives. Any relieving hole must be drilled parallel to the misfired hole and to the same depth. To ensure that the holes are parallel it is essential that the information relating to the inclination and azimuth of the misfired hole is accurate. Care must be taken to drill the relieving hole at the same angle. The precise location of the relieving hole can only be established after careful assessment of the local conditions. Consideration should also be given to operating the drill rig remotely. It may be necessary to seek expert advice.

A further option is to drill small diameter relieving holes around the collar of the misfired hole. These are systematically fired to work off the rock and expose the charge. There may be adjacent charged holes, which must be considered and their location confirmed before any action is taken.

* Discovery and retrieval of explosives

It may be necessary to move rock from the immediate vicinity of the misfire before access to the charge can be gained. The remaining rock next to any misfired charge is likely to be solid and any attempt to remove this can be fraught with danger. Remedial action can only be decided after careful inspection and appraisal of the misfire site.

It may be possible to remove part of a misfired charge by hand from the socket of a hole but this should only be attempted by experienced personnel after due consultation with, and the approval of site management.

Removal of some charge from a hole will allow the introduction of a primer and detonators. Some stemming may then be added to the hole to create additional confinement, before firing.

If a misfired hole contains more than one deck of explosive, it may be necessary to deal with each deck in turn as a separate misfire, with either full retrieval of charges from each deck or re- priming. Sufficient confinement must be provided before refiring each deck.

Explosive which is recovered should be placed in containers for storage or disposal. Detonators should be separated from the explosives and primers carefully and stored separately from explosives. Explosives should be placed in plastic bags and placed in clearly labelled boxes.

The process of searching for explosive material in the heap may involve the use of loading equipment. Note that it is possible to utilise specially protective devices in order to protect the operators during this process.

Material picked up by the bucket should be taken to a leve1 area, carefully deposited on the floor and searched thoroughly. The minimum number of people should be exposed during this process.

Before excavation commences precise instructions should be given to the machine operator as to how to proceed.

This procedure should help minimise the risk of the impact of the bucket or falling rocks detonating unexploded charges. This work must only be done under direct supervision. From the location of misfired material and information from the blast design it may be possible to determine the quantities and types of explosive involved. A search should continue until, as far as possible, all explosive material has been accounted for. Be aware that explosive material may be concealed below the floor where sub-drilling is used.

A more serious situation occurs when explosive material is found when loading out or processing. It must be assumed that some has made its way into the product, stocking area or tip. It may even have been taken off site. An assessment must be made of the dangers and risks likely to be involved should the explosive be inadvertently detonated. Steps must be taken to arrange for the search and inspection of any location where undetonated explosives have found their way. All personnel must be instructed to report the finding of any explosive material to the shotfirer, the face foreman or the manager as soon as possible.

6. MISFIRE INVESTIGATION – After a misfire has occurred it is important that the “lessons learned” are recorded in order to attempt to prevent a repetition of the event.

Reporting is an important part of this procedure and records must be maintained. This is particularly important if it is suspected that all of the misfired material has not been recovered.

Factors in Controlling Drilling and Blasting Costs in Mines – Discussion

Factors in Controlling Drilling and Blasting Costs in Mines – Discussion

The recent increase in ammonium nitrate  and diesel oil prices have invited many operations in mine to re-examine their drilling and blasting methods to find ways of reducing costs. Under the changed scenario, drilling and blasting companies require to optimize their blasting and costs through a blasting optimization review. Some of the questions faced during blast optimization are discussed here.

Many operations only look for less costly explosives, which company feel would do the job. What can be done to reduce costs if we are only looking for less expensive methods for firing the same blasting pattern? Let’s examine the blasting pattern and supplies used and consider the following questions and how they relate to the specification prevailing in the field.

Do you really know what is your exact drilling and blasting cost actually should be? Countless times when we work with operations we find that their actual costs are much higher than their calculated ideal cost. Some times the reason for this is as simple as, for example, they are assuming a 4 by 4 m drill pattern while in actual fact the drill pattern on average is 3 by 3.5 m. This small difference in average pattern dimensions would already increase cost per ton by over 11%. There is Blasting Cost software available, which will accurately determine calculated ideal costs.

What types of initiators are you using in the blast? Are you using a redundant path system for a shock tube initiation system? Is the redundant path really needed or are you paying double for the cost of the initiators to protect the manufacturer for product defects? Are you paying double for initiators to cover mistakes made by the blasting crew? A single path shock tube system is used by many operations worldwide. Why would it not work in your operation?

One of the lowest cost methods to produce better fragmentation is to use the proper delay times in the blastholes. How do you know if the delay times used produce the best fragmentation, back wall control, and lowest vibration in your particular operation. Selection of proper delay times are site specific and depend on the local geology. Do your blasters know how to select the proper delay times? Using the wrong delay times can greatly increase your production costs.

How many cast primers are used routinely in each blasthole? Is more then one really needed in all holes? What size cast primer are you using: 500 gm, 400 gm, 250 gm, 100 gm? What size and how many primers are really needed from a technical standpoint?

What explosive are you using as the main explosive charge? Many operations are using more expensive, more energetic explosives than needed with the same drill pattern they would use for ANFO. All that results is additional throw of the broken for the additional cost.

Are you using cartridged explosives rather than bulk explosives? The additional explosive that you can place into the blasthole because you are filling the annular space that you would not fill with cartridged explosives will allow you to reduce the drilling cost by expanding the drill pattern.

What do you do with the used motor oil that you generate from your equipment? Some companies use the old motor oil as part of the fuel for their ANFO or in the manufacture of their emulsions rather than paying to have the used oil taken away. Research has shown that used oil diluted with diesel oil produces as much or more energy than pure diesel oil in ANFO.

Deck loading is used in many operations to reduce the quantity of explosives per delay in order to reduce vibration. Deck loading increases the time it takes to load a blast, increases the number of initiators and primers needed, and often produces less efficient fragmentation than when using a full column of explosives. In my experience most blasts are not efficient when it comes to minimizing vibration. If the blast efficiency is improved then deck loading may not be necessary.  By considering these factors and others, many operations have saved as much as 40% of their explosive costs without effecting their blast performance.

A bigger picture must be considered if we are truly concerned about reducing production costs. Blasting is only the first step in the production process for mines and quarries and the costs of this first step is normally only 8% to 12% of the total costs. The total product costs are composed of: drilling, blasting, secondary breakage of oversize, digging, haulage, crushing. Blasting affects every step in the production cycle. What is important is to reduce total costs. If you try to reduce explosive costs alone you may raise drilling cost per ton, secondary breakage costs, digging costs, hauling costs, and crushing costs. If explosives cost would increase but produce better breakage and cost reductions could made in the other production costs then total production cost may radically decrease.

If savings on drilling and blasting costs are desired it is important to determine the actual costs of the production process. What are the drilling and blasting costs per ton? What percent of the blast is oversized and what does it cost to break up the oversized boulders? What is the average cost per ton to dig and load out the blast? What does it cost to haul the material to the crusher? How many trucks dump at the crusher per hour? With good blasting you may be able to greatly increase the truck count per hour. What are the average crushing costs. How many tons per hour go through the primary crusher? Most operations either have the raw data needed or can easily get this data to be able to determine a total production cost.

The major problem we find is that many operations do not have a good understanding and easily obtainable data on of the costs associated with secondary breakage, digging and haulage costs and how this would be affected by better drilling and blasting procedures. It is much easier to look at explosive costs. To totally optimize costs and to make informed decisions, all production costs must be considered when selecting drilling and blasting methods.

Drilling and blasting is only the first phase of the production cycle but influences all costs for all the other activities.

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