From: ahahma@polaris.utu.fi (Arno Hahma)
Subject: Re: Ammonium Nitrate
Organization: University of Turku
X-Newsreader: Tin 1.1 PL5
Date: Wed, 11 Nov 92 21:08:42 GMT

bill nelson (billn@hpcvaac.cv.hp.com) wrote:

: Yes, AN is very hygroscopic. Straight ANFO is not supposed to be used in
: a wet hole. However, there are a number of slurries, some containing AN
: that are used successfully for blasting. Maybe Arno will post a bit more
: on what these contain.

Ok.

 A slurry is a mixture of three or four major components: oxidizer,
water, binder and fuel, the two latter may be combined or not. The
oxidizer is any soluble, oxidizing salt. In practice, it is selected
from the group nitrate, chlorate, perchlorate. Ammonium nitrate is
very good, since it is highly soluble.

The binder/fuel system varies more and it also determines the type of
the slurry. There are emulsion slurries of two kind, homogeneous
slurries and heterogeneous slurries with solid fuels. 

If the fuel is a non-soluble liquid, such as diesel oil, it is mixed
in as an emulsion.  The emulsion can be oil-in-water type, in which
case an additional thickener is required or it can be water-in-oil
type, in which case a thickener can sometimes be left out. The
percentage of the oil is usually adjusted for zero oxygen balance.

The fuel can also be soluble, such as polyvinyl alcohol/acetate,
alcohols in the homogeneous slurries (strictly speaking, no slurry is
homogeneous, as there are some voids. The homogeneous means only the
fuel-oxidiser system). In this case, a binder is also required, but it
can also be the fuel itself. These types of slurries usually include a
solid fuel as well, typically aluminium powder, that increases the
explosive power very effectively. In that case, the slurry is no
longer a homogeneous mixture. High explosives or waste smokeless
powder are frequently used as the solid fuel, too. At the same time,
they make the slurry more sensitive.

All slurries have to be sensitized in some way to make it possible to
detonate them at all. This is done by adjusting their density. The
density is adjusted by introducing gas bubbles (typical for the
emulsion slurries) or other, low density material, such as glass
microballoons or tiny styrofoam balloons (typical for the homogeneous
and solid fuel slurries). However, lowering the density does not make
the slurry too sensitive, a strong detonator and confinement are still
required.  Actually, that is the original idea of slurries: to have a
very safe explosive, that can be handled without any special
precautions and only made explosive in a borehole.

However, adjusting the density brings about another problem. The
detonation velocity drops with the density. That is not always a
desired effect. It can be countered by using chemical sensitization,
that is not free of problems either. If sufficient catalyst for a high
detonation velocity is added, the sensitivity tends to go too high,
even to the level of cap sensitivity, which is highly undesirable. It
is not the sensitivity but INsensitivity with reliability at the same
time, that is hard to achieve.

A slurry is really a versatile explosive, as you can put virtually
everything into them. A common way to destroy waste explosives is to
make slurries of them and use the slurry for blasting. Surplus
fertilizer, that has got wet and caked could be used as a raw material
- whatever oxidizers you happen to have plus some combustible
material.

: Not likely. Black powder does not detonate. As such, it does not produce
: the shock wave that is normally needed to initiate secondary explosives.
: Now, if the ANFO was confined, and heated hot enough to be producing
: some decomposition products, then it might work.

Bill is right, black powder will not bring about a detonation in ANFO.
The pressure it generates is not rapid nor high enough. Still, ANFO
can undergo a deflagration to detonation, if strongly confined and
heated.  Such a situation could be achieved by packing the ANFO into a
strong, tight metal container and then placing that on a fire. In
fact, there used to be a standard test for explosives using this
concept: the critical diameter of a hole in a standard container that
did not promote a transition to detonation. For ANFO the critical hole
diameter is less than 1 mm. As a comparison, for RDX it is 8 mm, for
TNT 3 mm.

: Bill

ArNO
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