ANFO is an explosive mixture of Ammonium Nitrate (AN) and Fuel Oil (FO). These two constituents act as oxidiser and fuel respectively, in the reaction that takes place during detonation. The normal fuel oil used is diesoline, and the trade name for ANFO made at AEL Intelligent Blasting is ‘ANFEX®’. Owing to its safety, low cost and ease-of-use ANFO is still widely used for underground blasting operations.
This issue of Explosives Today deals with the use of ANFO pneumatically loaded into blastholes not exceeding 50mm in diameter.
Technical and packaging Specification
ANFEX is supplied in 25kg bags or cartons (with convenient 6.25kg inner bags) from the AEL Modderfontein manufacturing facility.
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.
Factors affecting performance of ANFEX
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)
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:
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.
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.
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
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 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:
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.
Pressure/Venturi loader (Lategan© Loader)
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 chamber or measuring flask
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 in- hole 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 ‘extra’ explosives being loaded into the hole. The net effect is over-charged holes, wastage of explosives and potential damage/overbreak.
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:
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 re- circulated. 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.
Static Electricity Hazard
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.
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 anti- static 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.
Advantages of ANFEX
Disadvantages of ANFEX