ANFO is the most commonly used industrial blasting agent in the world and is often used as a reference for comparison of other explosives. It is known to work well in large borehole diameters, under dry conditions, whilst in small borehole diameters increasing amounts of toxic fumes (CO, NO and NO2) can be formed. However, ANFO is not one type of blasting agent but a family of products whose performance depends on:
- – Internal structure
– Prill size distribution
– Density
– Oil absorption and retention
- – Composition
– Properties
By selecting the correct Ammonium Nitrate Porous Prill (AN-PP) for the job, the amount of noxious fumes from a detonation can be minimised. Our tests show that it is important to choose the correct AN-PP grade for different applications. A good AN-PP for open pit blasting is not necessarily a good product to use for underground blasting and vice versa. It is also pointless to use ANFO as a reference blast agent for velocity of detonation (VOD) and toxic fumes without referring to the AN prill characteristics and the test conditions.
Background
Ammonium Nitrate (AN) is just one chemical compound, but the name does not say anything about its physical properties and therefore, nothing about its blasting performance. As mentioned, the blasting performance is influenced by several different properties of the AN being used. Most important is the internal structure as this influences:
In addition, the prill size distribution is of importance since this will influence the total outer surface area. Bulk density is also important, but this is determined by the two factors mentioned above.
Also, the use of ANFO can be extended by using special formulations like water resistant ANFO by addition of guar gum; increased energy by addition of aluminium powder; reduced energy by addition of inert materials.
Inner structure
When viewing pictures of four different grades of Ammonium Nitrate prills, taken under an electron microscope, it is apparent that ‘fertiliser’ grade has a massive, non-porous structure but with a few large cracks. A low quality ‘porous’ AN-PP contains a few large inner voids, but is not able to retain oil. A high quality ‘surface’ grade shows evenly distributed pores that allow absorption of oil throughout the prill. In addition there are a certain amount of unfilled pores that will act as ‘hot spots’ and promote propagation of the detonation. High quality ‘underground’ grade has an even finer micro-crystalline structure that will ensure a very intimate contact between fuel and oxidiser and a high amount of voids that will act as hot spots.
If ANFO was made from the four different products above, significant variations in VOD and composition of detonation fumes would result.
The bulk density and oil absorption of the four grades of AN can be seen in Table 1. If oil retention had also been included, it would illustrate that the retention equals the maximum oil absorption for surface and underground AN. For low grade and fertiliser grade the retention is much lower than the oil absorption.
Detonation tests
The tests shown in Figure 1 and Table 2 were performed in a tunnel with a volume of 200m3. ANFO was filled into steel tubes (inner diameter = 54mm, wall thickness = 2mm and length = 950mm) and lightly tapped; a 30g booster was then inserted. The amount of ANFO in each tube was recorded.
VOD was recorded with Proberod/DataTrap from MREL, and fume composition was determined with a Testo gas analyser. After detonating the charge, the doors were closed, and a mixing fan was started. Figure 1 shows the relative amounts of the three different gases where the ANFO testing was carried out using the underground AN-PP as a reference (=1). The values for low grade AN-PP and especially for fertiliser grade are more than likely worse than shown here, as these results are from a partial detonation.
Values from the VOD testing showed a stable VOD for underground and surface grades of AN, measured between 400 and 800mm from initiation. The low grade AN showed a slowly fading detonation and the fertiliser grade stopped after about 500mm.
Measurements from blasting in rock
In the tests summarised in Figure 2, ANFO based on high quality surface and high quality underground AN was used. Neither low quality or fertiliser grade AN was used as they do not propagate a stable detonation in 35mm boreholes.
The steel tube used in the first test had a wall thickness of 17mm. Figure 2 shows the effect of changing from a normal high quality AN to an AN with optimised structure. Overall, NOx reduction from full scale blasting has been found to be about 40%.
In Figure 3 one can see that the VOD in small diameter boreholes (<50mm) is higher in the underground grade of AN than with surface grade. At a diameter above about 125mm, the surface AN detonates with the highest VOD.
Except for the smallest borehole diameters (<48mm), where gentle pressure loading was used, all other holes were loaded with only minimal crushing of the AN.
Conclusion
The tests confirmed that ANFO performance depends on the grade, oil, mixing, loading and blasting. However, the smaller the diameter of the borehole the more important these factors become.
By selecting a porous AN with optimised properties, a significant reduction NOx can be achieved. In small diameter boreholes a low density AN with optimised crystal structure (underground grade) will perform best. However, in large diameter boreholes, medium density, high quality AN (surface grade) will give the highest VOD. In this case the ANFO will also have a higher mechanical strength that is better suited to pneumatical loading. Testing ANFO in small diameter tubes is useless if the product is intended for large diameter blasting.
Related Files
Figure 2
Figure 1
Figure 3
