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.