The origins of “Explosives Today” Claude Cunningham
Then... The 1960’s was a time of rapid evolution for blasting technology in South Africa. The sole explosives supplier, African Explosives and Chemical Industries Limited (AE&CI), operated the world’s two biggest explosives factories (at Modderfontein and Somerset West) and thrived chiefly on delivering dynamite and capped fuse systems to the nation’s huge underground gold, platinum and coal mines. Working methods were mature, but there was burgeoning pressure to increase productivity, whether by harnessing the ability to drill large diameter holes or by increasing the yield per blast from limited face length. Explosives technologies were also developing and offered great promise for moving toward these goals. The marketing group of the Explosives Division, under the General Manager Peter Lambooy, ran a team of explosives service engineers, mainly drawn from the mining industry, which worked with the company and its customers to implement efficient and safe blasting both in the country and in Africa. The demand for help in applying the new technologies and implementing safe and efficient blasting was more than the service engineers could handle; the result was “Explosives Today”, a technical bulletin written by these engineers, which would become a globally recognized source of useful blasting information. The first issue, “ANBA and Inclined Drilling”, was published in August 1965. Series Two started in September 1976 with “Selection of Explosives for Narrow Reef Blasting”, and finished with “Safety in Surface Blasting” in 1988. The first issue of the third series, launched in September 1988, was “The Historical Development of Commercial Explosives”, and the series ended with the tenth issue, “Destruction of Explosives Accessories” in December 1990.
The series was such an acclaimed resource for blasting information because it was entirely written by the engineers engaged in real blasting activities, subjected to intense peer review, with the meticulous attention of senior engineers. For most of this time, AECI Explosives (which changed its name progressively) was either the only explosives supplier, or by far the majority supplier, and had not yet encountered the disciplines and pressures of working in a highly competitive market. Long after the series was discontinued, old copies have been treasured and used as vital reference by personnel concerned with safe and efficient blasting around the world. And now... In line with our value proposition of Thought Leadership and a move to revive the publication, AEL’s Mining Optimisation Team has replaced these treasured copies with a new series. The first two issues were produced in the first quarter. This technically driven customer publication authored and tailored by our Mining Optimisation team is now available to customers in the form of an A4 folder equipped with a CD and flash stick containing all issues of the publication.
For more information and to order your copy, contact the Mining Optimisation Team c/o Simon Tose, Tel: +27 11 606 3960 Email: simon.tose@aelms.com
Explosives Today Series 4 I No 8
The use of ANFO in Underground Ken Meiring - Senior Explosives Engineer
ANFO is an explosive mixture of
Technical and packaging Specification
Ammonium Nitrate (AN) and Fuel
ANFEX is supplied in 25kg bags or cartons (with convenient 6.25kg inner bags) from the AEL Modderfontein manufacturing facility.
Oil (FO). These two constituents act as oxidiser and fuel respectively, in the reaction that takes place during detonation. The normal fuel oil used
Property
Anfex® - Poured
Anfex® - Pneumatically loaded
is diesoline, and the trade name for
Density (g/cc)
0.78 – 0.82
0.9 – 1.05
ANFO made at AEL Mining Services is
Velocity of Detonation (m/s)
2000 - 3000
2500 -3500
‘ANFEX®’. Owing to its safety, low cost
Water resistance
and ease-of-use ANFO is still widely used
Fragmentation
for underground blasting operations.
Heave
This issue of Explosives Today deals with the use of ANFO pneumatically loaded into blastholes not exceeding 50mm in diameter. 1
Nil Good Excellent
Ideal Delivered Energy
2.84
3.14
Bulk delivered Energy
2.27
3.30
RWS (to 20MPa)
100
111
RBS (to 20MPa)
100
145
Explosives Today - Series 4, No 8
Transport and storage underground In underground workings, ANFEX can be transported together with cartridged explosives and detonating cord. It may be stored for up to 72 hours and in quantities not exceeding 150kg per receptacle and separate from any other explosives products. Shelf life is dependent on the conditions under which ANFEX is stored. Under dry conditions at an ambient temperature of <31°C, indefinite shelf life is possible. Humid conditions typically experienced underground, could result in moisture penetrating through the holes punched in the bag to allow easier stacking, thus causing the prill to solidify into a solid mass inside the bag.
2. Loaded density The loaded density of ANFEX is dependent on the air pressure at the loader and on the bulk density of the porous prills, and varies as follows: oo Pour loaded – Densities are close to the range of densities ex-plant, i.e. about 0.78 – 0.82 g/ cc oo Soft loaded – With an air pressure of about 140 kPA, having a total density of about 0.85g/cc oo Normal loaded – with an air pressure of about 500 kPa, having a typical density of about 0.90 – 1.05 g/cc
Factors affecting performance of ANFEX 1. Moisture
Because ANFEX has no water resistance, its use is restricted to dry conditions. Even small amounts of water will adversely affect both sensitivity to initiation, and fume production.(Figure 1)
Figure 2 - Anfex
The technique of soft loading results in less explosive per metre of charge. This minimises the shattering effect of ANFEX, which is desirable in holes adjacent to a weak hangingwall. Owing to the reduced power, however, extra holes may be needed to achieve effective breaking. By contrast, ANFEX loaded under high pressure will have more explosive energy per unit length of hole which promotes good breaking, but increases the possibility of overbreak. Loading is also quicker. 3. Confinement Sensitivity to both the initiation and propagation is markedly affected by the confinement of the charge and is optimised only when the cross section of the hole is completely filled (i.e. 100% coupled), which is the normal loading practice. In order to reduce the shattering effect, ANFEX is sometimes poured into a plastic sleeve, which is then inserted into the hole. (It should be borne in mind that if it is done outside the blasthole, the sleeving of ANFEX is a manufacturing process in terms of the Explosives Act.) These decoupled charges have been successfully used in such applications as smooth blasting but because of the reduced confinement a powerful initiator in the form of a cartridge primer (or booster) is necessary. This is loaded into the hole first and is followed by the sleeve of ANFEX.
Figure 1 - Moisture and Anfex
When sleeving ANFEX, a detonator must not be placed into the sleeve and the ANFEX poured or blow loaded into it because of the static electricity hazard. Smaller diameter cartridges, train-loaded, are preferred to sleeved ANFEX for smooth blasting due to cost effectiveness and safety requirements. 4. Strength of Initiator
Figure 3 - Reverse Priming
2
Previously, capped fuse was the initiation system of choice, available in underground blasting. Both No. 6 strength (6D) and No. 8 strength (8D) detonators were available, with 8D
Explosives Today - Series 4, No 8
strength detonators recommended as the minimum initiator for ANFEX. ANFEX is less sensitive to initiation than cartridge explosives and in the case of jackhammer holes, a 6D detonator is a marginal initiator, thus the recommendation that a primer cartridge be used with 6D capped fuse with ANFEX. Currently, with shock tube (and to a lesser extent electronic initiators) replacing capped fuse due to safety concerns, the size of the detonator has standardised at a longer length and larger diameter due to the inclusion of a delay element. A potential hazard when using these longer detonators is when reverse priming is practiced. (Reverse priming - Figure 3 - is the practice of reversing the detonator direction so that the end of the detonator faces the collar of the hole while being charged. This has the effect of directing the detonator energy towards the explosive column, preventing the detonator from lying in dissolved ANFEX at the toe of a wet hole, preventing the detonator pulling out of the booster, or preventing the initiator from falling out of the hole when charging vertical holes). In some instances, the detonator is inserted into the charging lance (blowpipe) which is inserted into the blasthole. The detonator is then ejected from the lance tip by the force of the ANFEX being blown through the lance. The hazards that arise from this practice include:
New design incorporates fitting for blowing out of holes during charging
Stoping loader
Anfex Loaders Pressure/Venturi Loader)
loader
(LateganŠ
This loader is mobile and enables charges of pre-determined mass to be loaded through hoses of up to 30m long. ANFEX contained in the hopper is admitted through a gate valve into the measuring flask/pressure chamber. With the gate valve closed, air pressure is applied to the chamber and the venturi allowing dynamic transfer of the ANFEX through the hose into the blasthole. To reduce ANFEX wastage the pressure chamber or measuring flask should be such that only 2/3 of the length of the hole is filled by one charge. In mines where drill steel gauge loss occurs, the loader capacity should be such to fill the smallest diameter hole.
Size of pressure measuring flask
3
chamber
or
When calculating the size, allowance must be made for the increase in density of ANFEX (about 10%) when pneumatically loaded. The air pressure used when charging with ANFEX plays a major role in determining the final inhole density. It is important to ensure that the compressed air pressure be limited to ensure the prills are not damaged by excessive pressure, resulting in breakdown of the prill which in turn causes a higher in-hole density. This translates to a shorter charge length which in turn leads to â&#x20AC;&#x2DC;extraâ&#x20AC;&#x2122; explosives being loaded into the hole. The net effect is over-charged holes, wastage of explosives and potential damage/overbreak.
oo Damage to the tubing or lead wire of the initiator by the charging lance, leading to misfire oo The initiator becoming stuck in the hole after misfiring oo Premature detonation of the detonator, either by the force of the ANFEX leaving the lance or due to being struck by the lance when attempting to remove a misfired detonator. It is recommended that reverse priming only be practiced when there is a mine standard (risk analysis and procedure to deal with misfired detonators) in place.
Development loader
Figure 4 - Increase in density with air pressure
Explosives Today - Series 4, No 8
Figure 5 - Increase in air pressure
Figure 7 - Relationship between mass/metre and hole diameter for ANFEX and un-compacted Magnum cartridged explosives
Figure 6 - Higher air pressure blow loading effect
A study carried out underground at a platinum mine showed that the ANFEX density increased significantly toward the end of shift, due to the increase of air pressure as drilling was completed (see Figures 4 and 5). Due to the increase in explosives density, column rise decreased resulting in operators “double charging” the holes. This increase in mass per hole is illustrated in Figure 6. Excessive air pressure has the added disadvantage of causing ANFEX to be blown out of the hole during charging, causing the stope panel to appear to be “snowed in”. This results in additional wastage and impacts negatively on explosives efficiencies. The effect of gauge loss of the drill 4
steel on ANFEX consumption can be considerable. For example, a 41mm hole holds twice as much ANFEX as a 29mm hole; a standard charge would thus be half as long in the 41mm hole. In order to counter this problem the distribution of drill steels should be controlled so that only a small range of hole sizes is drilled in any one area. Figure 5 shows the mass of explosives per metre run for various hole sizes at different loaded densities. For stoping holes it is recommended that an aluminium tube longer than the depth of hole be inserted into the end of the charging hose, with the following objectives: oo To increase the life of the loading hose
oo To increase the annular space over that obtained between the loading hose and the wall of the hole. This reduces back pressure through better venting of the discharged air oo To use the loading pipe as a blowpipe for blowing out holes prior to the loading, where the necessary permission has been granted by an Chief Inspector of Occupational Health and Safety. However, the pipe must be maintained regularly i.e. sawn off square to prevent the burring (shown in the photograph) from damaging the prill during charging.
Explosives Today - Series 4, No 8
Photograph 4 - Required condition of blowpipe end
oo The internal diameter of the aluminium tube usually varies between 13 and 25mm to cater for the range of hole sizes in common use. The supplier of these ANFO loaders has recently released an updated version, which includes a separate fitting for blowing out holes during charging (See Photograph 4) All blast holes are legally required to be stemmed except when Anfex® is used (an exception to this is when Anfex® is collar primed). However routine use of stemming will lead to improved blasting results due to improved confinement of the explosives gases, which is also true when using Anfex®. Commercially formulated clay capsules are the best tamping material due to their ease of use and availability. Destruction of Anfex and treatment of misfires ANFEX is destroyed by dissolving in water. It should be remembered that the result of this practice is a corrosive, nitrogen rich solution which should be treated if the water is to be recirculated. Strong ammonium nitrate solution is poisonous and must not be discharged into the drinking water. Where reverse priming is practiced, a safe method/procedure of removing the ANFEX and the unexploded detonator must be in place. This procedure will be different from mine to mine with the chief aim being to prevent the detonator being struck by the charging lance when removing misfired ANFEX.
5
Photograph 5 - Aluminium blowpipe showing burred edge
Photograph 6 - Anti-static hose
Static Electricity Hazard
To summarise:
No special precautions need to be taken to prevent the generation of static electricity where the compressed air or ambient air in contact with the ANFEX, either prior to or during loading has a relative humidity of more than 60% and a temperature in excess of 20°C.
Advantages of ANFEX
However, since the relative humidity of the compressed air in pressure pot load loaders is usually less than 30%, an electrical path to the earth must be provided by the use of metal legs and semi-conductive hose. The Lategan© Loader is equipped with an antistatic strap connecting the measuring flask to the legs, which together with the anti-static hose provides good protection from static electricity. NO Detonator of any kind is to be placed inside plastic sleeves with ANFEX. This practice has resulted in a number of accidents through static electricity causing premature initiation of the detonator. Where ANFEX is to be pneumatically loaded over the lead wires of electric detonators, only static safe electric detonators (StatSafe Type 1) which require a higher firing energy may be used. Collar priming of holes using standard Type 0 delay detonators secured to detonating cord tails is permitted, provided that there are no detonators present during pneumatic loading operations. Shock tube detonators are less susceptible to static electricity and can be safely used with pneumatically loaded ANFEX.
oo Rapid and efficient loading of holes oo Very safe under recommended conditions of use oo Misfires easily treated oo Usually cost effective under dry conditions oo No stemming required by law oo Longer underground storage permitted Disadvantages of ANFEX oo Cannot be used in wet conditions oo Dependent on compressed air to effect loading oo Wastage often difficult to control, susceptible to overcharging if excessive air pressure is applied to the loader oo Overbreak is difficult to control, especially where overcharging occurs oo Where drill gauge loss occurs overcharging of the holes can result in poor blasting efficiencies oo Static electricity generation can pose safety hazards, when used with electric detonators
DISCLAIMER AND INDEMNITY Any recommendations given by AEL Mining Services (AEL) in respect of this document are given in good faith based on information provided. AEL does not however warrant that particular results or effects will be achieved if the recommendations are implemented, due to potentially unknown aspects and/or conditions. AEL further does not accept liability for any losses or damages that may be suffered, as a result of the customer acting, or failing to act, on the recommendations given. Explosives Today, Series 2 No 30 Surface and 31 Underground by M Beattie December 1982 and KS Ireland March 1983 respectively are replaced by this series. COPYRIGHT All copyright that subsists in this publication together with any and all diagrams and annexures contained herein, which shall include all and/or any ideas, plans, models and/or intellectual property contained in this document vests in AEL. Any unauthorised reproduction, adaptation, alteration, translation, publication, distribution or dissemination (including, but not limited to, broadcasting and causing the work to be transmitted in adiffusion service) of the whole or any part of this document in any manner, form or medium (including, but not limited to, electronic, oral, aural, visual and tactile media) whatsoever, will constitute an act of copyright infringement interms of the Copyright Act No.98 of 1978 and will render the transgressor liable to civil action and may in certain circumstances render the transgressor liable to criminal prosecution. This document remains the intellectual property of AEL. Intellectual Property: All ideas, concepts, know-how and designs forming part of this publicationbelong to AEL, save for where it is clearly indicated to the contrary.
AEL Mining Services Limited (PTY) Ltd 1 Platinum Drive, Longmeadow Business Estate North Modderfontein, 1645 Tel: +27 11 606 0000 www.aelminingservices.com 6