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 7
The use of ANFO in Surface Blasting Ken Meiring - Senior Explosives Engineer
ANFO is an explosive mixture of Ammonium Nitrate (AN) and Fuel Oil (FO). These two constituents act as oxidiser and fuel in the reaction that takes place during detonation. The normal fuel oil used is dieseline, and the trade name for ANFO made at AEL Mining Services is ‘ANFEX®’.
with the use of ANFO mixed on bench, i.e. in blastholes exceeding 50mm in diameter.
Owing to its effectiveness and low cost, ANFO gained wide acceptance in all types of blasting operations, since its introduction in the 1950’s. This issue of Explosives Today deals
Storage of AN
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typically also covers an allocated area for parked transport trucks.
Ammonium Nitrate - Definition “Ammonium Nitrate emulsions and gels, intermediate for blasting explosives” – means a mixture of ammonium nitrate with one or more other oxidisers and one or more fuels, with or without the addition of any other substance which is intended for use in the manufacture of explosives or for on-site manufacturing.
AN must be stored in a suitable magazine constructed and licensed as set out in the appropriate Explosives and Mining laws and regulations. This
Figure 1 Bulk storage of Ammonium Nitrate Prills
Explosives Today - Series 4, No 7
Figure 2 Storage of bagged AN prill and Handling equipment
Conveyance of AN AN may be transported by box truck, tanker on road, by rail and by ship in a variety of containers and bags as specified in the appropriate chapters of the Explosives and Mining laws and regulations.
Properties of AN Ammonium Nitrate is hygroscopic and therefore very soluble in water. As a result it is quickly rendered inert in wet holes and this is its most serious disadvantage.
Figure 3 Transport of AN by “Rapid Reloader” - Tanker
The dark grey areas indicate the fuel oil within the prill cavities. AN-E is used in the manufacture of AN based bulk explosives where AN solution is not readily available. Figure 4 shows that about 6% by mass is required to obtain an oxygen balanced mix which produces maximum power and minimum blasting fumes. With too much or too little fuel, energy yield declines. Too much fuel (insufficient oxygen in mixture) also causes excess CO to be liberated in the detonation gases, whereas insufficient
Property
Anfex® - Poured
Density (g/cc) Velocity of Detonation (m/s) Water resistance Fragmentation Heave Ideal Delivered Energy (MJ/kg) at 20MPa Bulk delivered Energy (MJ/kg) at 20MPa Relative Effective Energy (REE) at 20MPa Relative Bulk Strength (RBS) at 20MPa Ideal Delivered Energy (MJ/kg) at 100MPa Bulk delivered Energy (MJ/kg) at 100MPa Relative Effective Energy (REE) at 100MPa Relative Bulk Strength (RBS) at 100MPa
0.78-0.8 2000 – 3000 Nil Good Excellent 2.84 2.27 100 100 2.26 1.81 100 100
NOTE – The energies and strengths have been determined using the AEL Vixen_i detonation code. Density (pneumatically loaded Anfex (g/cc) 0.90 to 1.05
AEL manufactures ammonium nitrate prill in two forms; Ammonium Nitrate Emulsion grade (AN-E) or dense grade; and Ammonium Nitrate ANFO grade AN-A. AN-A prill is porous to allow for the absorption of fuel oil as can be seen in figure 4.
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fuel (excess oxygen in mixture) encourages the generation of greater volumes of nitrous fumes. Because it is much more serious to have too little fuel than too much, and because the exact mix can never be achieved, AEL specifies 6% fuel, with a maximum range of 0.5% either side.
Figure 4 Prill showing diesel absorption
ANFO Mixing Ratio Maximum power during detonation is achieved by mixing the AN and FO in the approximate ratio of 94:6 by mass i.e. or
94kg AN with 6kg FO 100kg AN with (100/94) x 6 = 6.4kg FO
If the density of the FO is p g/cc Then volume of FO/100kg AN = (6.4/p) = 7.5 litres per 100kg Where p = 0.85 g/cc or 3.75 litres/50kg bag
On Site mixing of ANFO The manufacture of ANFO is normally regulated by the appropriate Mining and Explosives laws and regulations.
Ammonium Nitrate ANFO grade (ANA) The porous prilled AN manufactured by AEL Mining Services is treated with special anti-caking and crystalhabit modifying agents that ensures
Explosives Today - Series 4, No 7
AEL Mobile Manufacturing Units (MMU)
ANFO output kg/ min
AN hopper capacity (tons)
Capability
A Series A12T
200/500/750
12
Capable of auguring ANFO
H Series H20T
250/500/750
7
Heavy ANFO MMU capable of auguring the ANFO and the full range of AEL’s doped1 or blended2 explosives products
E Series E15T and E20T
750
5 and 7
MMU capable of pumping or auguring the full range of AEL’s doped1 or blended2 explosives products
Doped 0 – 35% ANFO 2Blended – 54 – 74% ANFO
1
Doped – Emulsion & additives have been added 2. Blended – AN or ANFO, emulsion added 1.
Figure 5 Effect of fuel oil content on strength of ANFO
low friability (less than 2%), i.e. its formation of dust. The anti-caking agent counters adherence between prills and promotes free flow during handling, while the crystal-habit modifier raises the temperature at which a phase change of the AN crystals occurs. This phase change is an important cause of prill deterioration and setting up (caking) in AN transport and storage at temperatures in excess of 32°C. ANA remains stable to prill deterioration even under hostile environments of up to 40°C.
Use of ANFO
Fuel Oil and Substitutes
For example, where p = 0.8g/cc and D = 250mm
Various types of fuel may be used because solid fuels are not as effective as liquid fuels in ANFO. Black or recycled oil is increasingly being considered as a replacement but they are still being researched with the absorption of waste oil by the prill the main focus. The absorption takes longer due to the higher viscosity of the waste oil, resulting in the outer bag being coated in a thin film of oil, which in turn interferes with the heat sealing of the bags.
ANFO is especially effective in dry blast holes exceeding 100mm in diameter. The mass per metre run of blasthole (MC) is given by the equation MC = 0.785 x p x D2 OR MC= p x D2/1273 Where D = blast hole diameter (mm) p = density of explosives (g/cc)
MC = 0.785 x 0.8 x 2502 MC = 39250 g/m The density of ANFO made with ANA has a nominal density of 0.78g/cc but can be
as high as 0.82g/cc, depending on the size and depth of the hole. Also, blasthole diameters can vary significantly from their nominal size and these two factors must be taken into account when accurately predicting the mass of ANFO per unit length contained in a blasthole. If this is not of critical importance, controlled loading experiments will determine the correct factors to apply. The blasting effect of ANFO relative to other explosives is assessed on the basis of their relative energies per metre run of blasthole.
Heavy ANFO (Doped and Blended bulk emulsion) ANFO can be combined with emulsion to create customised bulk explosives products. These are referred to as doped or blended emulsions with the generic term for blended emulsion being “Heavy ANFO”. This product is an excellent replacement for ANFO in damp conditions where the blast is fired immediately after charging is completed. The added emulsion helps to “waterproof” and protect the prill from water which allows for the heave characteristics of ANFO to be harnessed. Due to the thickness of the explosive as a result of the prill content, this range of products can only be augured.
ANFO Mobile Manufacturing Units (MMU) AEL manufactures 3 model series, the choice of which depends on the scale of the mining operation and the diameter of the blast holes. Figure 6 MMU in action 3
Table 1. AEL MMU
Explosives Today - Series 4, No 7
Typical velocity of detonation, VoD, range between 4000 to 6000m/s depending on hole diameter and confinement, (Rock properties.)
Product
Emulsion
ANFO
Fuel Oil
Straight emulsion
100%
0%
0%
Doped emulsion
65 – 90%
35 - 10%
0%
Initiation and Boostering
Blended emulsion
46 – 26%
54 – 74%
3 – 3.5%
Typical VOD (m/s)
4500
3500
NA
A booster is a charge of high energy explosives which is reliably initiated by a detonator or by detonating cord. AEL manufactures Pentolite boosters in three sizes of which two can be used with ANFEX in surface applications. For best performance, the booster selected should be the largest that will easily fit the blasthole. All Pentolite boosters have one or two axial holes large enough to accommodate both shock tube or electronic detonators or 10g/m detonating cord. They are waterproof, have low impact sensitivity and possess the advantage over cartridged explosives of being more convenient to store and use. Pentolite boosters are preferable for use in bulk explosives usage due to their high VoD characteristic. As blasthole diameter increases, the ability (sensitivity) of ANFO to initiation decreases, but the reliability of propagation increases. Thus a charge as small as an 8D detonator, can initiate ANFO (having been loaded pneumatically resulting in sufficient fine material to assist with propagation) in blasthole diameters below 51mm. As blasthole diameter increases, so the size of the initiator required also increases. Historically, poured ANFO was used in conjunction with detonating cord in blast holes. This practice led to desensitisation or side initiation of the ANFO in the hole with the result of poor blasting.
Table 2. Comparison of products
Acceptor Diameter (mm)
Bulk Anfex® 76 - 127
140 - 215
Bulk Anfex® Eco
250 - 311
76 - 127
140 - 215
250 - 311
X
X
Donors 60g 150g 400g
For use in holes less than 76mm in diameter X
X X
X
Table 3. Booster compatibility
of between 50 to 102mm around the detonating cord. This equated to a 10% loss in the explosive mass/hole and in theory an increase to approximately 38% loss in explosive mass/hole in 165mm holes.
Small diameter holes (<80mm) using poured ANFO and detonating cord In smaller hole diameters (typically around 80mm and less) the detonating cord causes density to exceed the critical density at which ANFO can detonate, (known as “dead pressing”) and leads to poor or incomplete detonation. This required that additional boosters be placed in the explosives column to maintain propagation of the detonation wave. As shock tube and electronic initiating systems replaced detonating cord, it has become necessary to double prime only in deeper holes of where
geological conditions exist. e.g.when a particularly hard band of rock within the softer material requires an additional booster for a second deck of explosives (See Figure 7). Additionally, a 10 – 20% improvement in the mean and distribution sizes of the fragmentation and increased burdens and spacings became possible.
Blasting with ANFO in wet hole conditions Good blasting results can be achieved with ANFO in wet conditions but only if one of these two methods are considered: Dewatering and sleeving Dewatering equipment, in our experience, always leaves some sludge or water at the toe of wet holes. This will dissolve the ANFO at the toe and render it ineffective and as the toe is where the most difficult breaking conditions
Large diameter holes (>80mm) using ANFO and detonating cord Research has proven that side initiation due to the use of detonating cord with ANFO in large diameter holes negatively affects the fragmentation and thus loading rates and causes poor floor conditions due to ineffective breaking. Where side initiation does not occur and the product is desensitised, then based on work with ANFO in pipes, “dead pressing” occurred within a radius 4
Figure 7. Use of extra primer to improve explosives distribution in hard rock bands
Explosives Today - Series 4, No 7
Figure 8 - Use of ANFO with water resistant toe charge in wet conditions
are found. A plastic sheath may be used to encase the ANFO to protect it against this moisture. The insertion of “lay-flat’ sleeving can be a slow and onerous procedure and where holes are making water, this practice has two inherent risks. 1. The ANFO column may be ‘pinched’ out by water pressure around the sleeving. This will decrease the breaking effect of the ANFO and may even result in detonation failure in these holes. 2. Despite every precaution taken, the ‘lay-flat’ sleeving may be torn by projections in the side of the blasthole, allowing water to penetrate the sleeving and destroy the ANFO. Water resistant toe charges In this method the wet portion of the hole is charged with cartridged water resistant explosives such as Magnum® Buster Gel. To preserve good breaking, this decoupled explosive should have at least the same relative energy of the fully coupled ANFO. Sealing of the holes above the water level is achieved by slitting the cartridges and then 5
dropping these onto the toe charge. ANFO is then loaded until the required column rise has been achieved. It has been shown that, even in dry blastholes, ANFO does not always have the ability to break out toes. In such cases a more powerful explosive such as Magnum Buster Gel should be used as a bottom charge. This has the advantage of maintaining or even increasing burdens and spacings, while a water resistant explosive is immediately available with the advent of wet holes. Additional options to consider when using ANFO in wet conditions include: • Using a water resistant ANFO based product such as Heavy ANFO where the hole toe is not completely flooded • Using underground bulk emulsion from a Smart Trailer instead of cartridges as the base charge • Bulk emulsion NOTE: Before adopting the practice of slitting cartridges, permission must be obtained from the CIE or DMR depending on which inspector has jurisdiction over the operation. The advantages of these methods are:
a) Simple and reliable a) Fragmentation can be maintained b) Little time is lost in charging up c) The toe charge will serve as an effective booster for the ANFO d) Holes full of water may be completely charged with the water proof explosive and fragmentation maintained with the same burdens and spacings.
Conclusion ANFO is usually a cost effective explosive, especially in softer rock formations. Its disadvantages are: a) Low relative energy b) Lack of water resistance Its advantages are: a) Lower price b) Low transportation and storage costs for AN c) AN transport not restricted to scheduled explosives trains d) Simple to use e) High loading rates f) Gives good blasting results under the right conditions
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