98 lines
5.8 KiB
Plaintext
98 lines
5.8 KiB
Plaintext
ANFOS
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ANFO is an acronym for Ammonium Nitrate - Fuel Oil Solution. An ANFO
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solves the only other major problem with ammonium nitrate: its tendency to
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pick up water vapor from the air. This results in the explosive failing to
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detonate when such an attempt is made. This is rectified by mixing 94% (by
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weight) ammonium nitrate with 6% fuel oil, or kerosene. The kerosene keeps
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the ammonium nitrate from absorbing moisture from the air. An ANFO also
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requires a large shockwave to set it off.
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About ANFO (From Dean S.)
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Lately there was been a lot said about various ANFO mixtures. These are
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mixtures of Ammonium Nitrate with Fuel Oil. This forms a reasonably powerful
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commercial explosive, with its primary benifit being the fact that it is
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cheap. Bulk ANFO should run somewhere around 9-12 cents the pound. This is
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dirt cheap compared to 40% nitro gel dynamites at 1 to 2 dollars the pound. To
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keep the cost down, it is frequently mixed at the borehole by a bulk truck,
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which has a pneumatic delivery hopper of AN prills (thats pellets to most of
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the world) and a tank of fuel oil. It is strongly recommended that a dye of
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some sort, preferably red be added to the fuel oil to make it easier to
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distinguish treated AN explosive from untreated oxidizer.
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ANFO is not without its problems. To begin with, it is not that sensitive
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to detonation. Number eight caps are not reliable when used with ANFO.
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Booster charges must be used to avoid dud blast holes. Common boosters
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include sticks of various dynamites, small pours of water gel explosives,
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dupont's detaprime cast boosters, and Atlas's power primer cast explosive. The
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need to use boosters raises the cost. Secondly, ANFO is very water
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susceptable. It dissolves in it, or absorbes it from the atmosphere, and
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becomes quite worthless real quick. It must be protected from water with
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borehole liners, and still must be shot real quick. Third, ANFO has a low
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density, somewhere around .85. This means ANFO sacks float, which is no good,
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and additionally, the low density means the power is somewhat low. Generally,
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the more weight of explosive one can place in a hole, the more effective.
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ANFO blown into the hole with a pneumatic system fractures as it is places,
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raising the density to about .9 or .92. The delivery system adds to the cost,
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and must be anti static in nature. Aluminum is added to some commercial,
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cartridge packaged ANFOs to raise the density---this also raises power
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considerable, and a few of these mixtures are reliablly cap sensitive.
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Now than, for formulations. An earlier article mentioned 2.5 kilos of
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ammonium nitrate, and I believe 5 to 6 liters of diesel. This mixture is
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extremely over fueled, and I'd be surprised if it worked. Dupont recommends a
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AN to FO ratio of 93% AN to 7% FO by weight. Hardly any oil at all. More oil
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makes the mixture less explosive by absorbing detonation energy, and excess
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fuel makes detonation byproducts health hazzards as the mixture is oxygen
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poor. Note that commercial fertilizer products do not work as well as the
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porous AN prills dupont sells, because fertilizers are coated with various
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materials meant to seal them from moisture, which keep the oil from being
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absorbed.
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Another problem with ANFO: for reliable detonation, it needs confinement,
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either from a casing, borehole, etc, or from the mass of the charge. Thus, a
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pile of the stuff with a booster in it is likely to scatter and burn rather
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than explode when the booster is shot. In boreholes, or reasonable strong
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casings (cardboard, or heavy plastic film sacks) the stuff detonated quite
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well. So will big piles. Thats how the explosive potential was discovered: a
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small oil freighter rammed a bulk chemical ship. Over several hours the
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cargoes intermixed to some degree, and reached critical mass. Real big bang.
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A useful way to obtain the containment needed is to replace the fuel oil with
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a wax fuel. Mix the AN with just enough melted wax to form a cohesive
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mixture, mold into shape. The wax fuels, and retains the mixture. This is
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what the US military uses as a man placed cratering charge. The military
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literature states this can be set off by a blasting cap, but it is important
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to remember the military blasting caps are considerable more powerful than
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commercial ones. The military rightly insists on reliability, and thus a
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strong cap (maybe 70-80 percent stronger than commercial). They also tend to
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go overboard when calculating demolition charges...., but then hey, who
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doesn't....
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Two manuals of interest: Duponts "Blaster's Handbook", a $20 manual mainly
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useful for rock and seismographic operations. Atlas's "Powder Manual" or
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"Manual of Rock Blasting" (I forget the title, its in the office). This is a
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$60 book, well worth the cash, dealing with the above two topics, plus
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demolitions, and non-quarry blasting.
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Incidently, combining fuel oil and ammonium nitrate constitutes the
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manufacture of a high explosive, and requires a federal permit to manufacture
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and store. Even the mines that mix it on site require the permit to
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manufacture. Those who don't manufacture only need permits to store. Those
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who don't store need no permits, which includes most of us: anyone, at least
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in the US may purchase explosives, provided they are 21 or older, and have no
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criminal record. Note they ought to be used immediately, because you do need
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a liscence to store. Note also that commercial explosives contain quantities
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of tracing agents, which make it real easy for the FBI to trace the explosion
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to the purchaser, so please, nobody blow up any banks, orphanages, or old
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folks homes, okay.
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D. S.- Civil Engineer at large.
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Brought to you in the CookBook IV..
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-= Exodus =-
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