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®’.
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 with the use of ANFO mixed on bench, i.e. in blastholes exceeding 50mm in diameter.
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.
Storage of AN
AN must be stored in a suitable magazine constructed and licensed as set out in the appropriate explosives and Mining laws and regulations. This typically also covers an allocated area for parked transport trucks.
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.
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 5.
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 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.
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. 94kg AN with 6kg FO or 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 crystal- habit modifying agents that ensures 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 AN excess of 32˚C