According to Dyno Nobel, ANFO Prilled Ammonium Nitrate (AN) is a nominal blend of porous Ammonium Nitrate prill and fuel oil. It is a dry, free flowing explosive; formulated to ensure the appropriate oxygen balance providing optimal energy and sensitivity. There has always been a debate about the power of ANFO. Retired explosives expert John Dasef has the answer.
“In general, it is on the lower end of the scale of commonly used commercial explosives, but when used properly, it is nonetheless quite effective.
Let’s get one bit of definition out of the way first. ANFO stands for Ammonium Nitrate Fuel Oil. The standard commercial formulation is 94% ammonium nitrate and 6% diesel fuel by weight. The percentage mix can and does vary but this is the most common formulation – at least it was when I was active in the field. Diesel fuel isn’t the only thing that can mix with ammonium nitrate to form an explosive, but if any other material is used, it isn’t ANFO anymore, so I’m not considering any of those.
ANFO has some characteristics not shared by other commercial explosives – its critical diameter and detonation velocity can both be varied depending on how it is used.
Critical diameter first – All explosives have a critical diameter, which is the amount of explosive material necessary to sustain the detonation past the point of initiation. For commercial prilled (think little white BBs) ANFO that is between 2 and 2 1/2 inches, depending on how it is loaded. ANFO in a 3-inch diameter borehole will reliably detonate if properly initiated, below that, not so much. (My apologies, I’m using U.S. measurements because that is what I’m most familiar with) However, density is also a function of critical diameter, so if the ammonium nitrate is ground finer than prills, the crititical diameter can be somewhat reduced. In general, though, ANFO is not considered a suitable small-diameter borehole explosive.
Now, about detonation velocity – In a 3-inch borehole, ANFO will detonate at between 9,000 and 10,000 feet per second. As the borehole diameter increases, so does the detonation velocity. In a 6-inch borehole, that same ANFO will be detonating at 15,000 fps or higher with a corresponding increase in detonation pressure. This gets us into the area of how much “work” an explosive will do, which depends on matching the type of explosive with the kind of work you want to do.
If we use TNT as a base one, ANFO comes in – generally – at about 3/4 as powerful overall, but with a large number of variables attached.
For example, using other explosives for comparison, you might think that C-4, with a detonation velocity of 27,000 + fps, is more powerful, and therefore ‘better’ than ANFO, but that depends entirely on the kind of work you want the explosive to do.
I could spend another hour listing comparisons of the task to be performed and the best explosive for that particular job, but I don’t have the time or the energy for that, so I’ll leave it at this. Of the explosives generally available commercially, ANFO is on the lower end of the scale as far as “power” is concerned, but it is ideally suited for certain tasks that other, more powerful explosives will not do as well.“
It is important to add that ANFO is very sensitive to water. So it’s important to make cartridges in case the mine holes are full of water.
