Ball Mill

A ball mill is device that can be used to grind chemicals much more easily
and to a finer consistency than can possibly be done by hand with a mortar and
pestle. It is often usefull to grind chemicals in order to increase their
surface area. This will generally increase the rate at which they react in a
pyrotechnic composition and make mixtures more homogenous which results in a
steady burn rate. For making good quality black powder at home a ball mill is
essential, and black powder is needed in very many pyrotechnic devices.

Note: ball milling inevitably causes friction and shocks and
possibly leads to static electricity buildup as well depending on the design.
Ball milling of mixtures should only be attempted when an appropiate place is
available to operate the mill (ie one where it causes no damage or injury in
case of explosion) and it must be turned on and off remotely. Most chemicals can
be milled more safely but several sets of balls and containers must be available
to prevent contamination and milling of metal powders can be dangerous as they
may become pyrophoric.

Materials
I used whatever I could get cheaply to construct my mill, so
you will probably not be able to get the exact same materials and have to
improvice somewhat.

Motor: I used to use a electrically powered drill to drive my mill,
but several people have advised against it as these are not designed to operate
for extended periods of time. They tend to get excessively hot and eventually
fail. Old motors can be found in many places for little or no money. Washing
machines for example contain durable motors and are often plentifull at scrap
yards. Mine came from a pump from an old coffee machine.

Container: A piece of PVC sewer tubing works well. I use a 34 cm long
11 cm diameter piece, with two fitting end caps. PVC is hard and will not suffer
from erosion as much as most other materials I've tried. This container has
lasted about 150 hours of operation thus far and shows no visible signs of wear.
One point of discussion is that PVC may accumulate significant amounts of static
electricity during operation. I've tested if the container is able to attract
small chips of polyethylene right after use, which it doesn't. I'm not entirely
convinced PVC is harmless though. For some more ideas on this matter, check out
this
page on the charging of powders in a rotating drum. Strips of rubber from a
bicycle tire are glued around the casing with hot melt glue to improve its grip
on the roller.

Roller: The roller is conveniently made of PVC tubing as well. I used
2.5 cm outer diameter sewer tubing, but the diameter may be chosen depending on
the speed of the motor availalable. The ratio of 2.5 cm (roller) to 11 cm
(container) yields a factor 4.4. reduction in speed. As the motor operates at
about 250 rpm, the container would theoratically rotate at a rate of about 60
rpm. In practice it reaches 50 rpm due to slipping. Somewhat faster would be
more efficient. It used to operate at 80 rpm when I was still using the drill,
and black powder would be ready in 3 hours. Now, it takes 5 hours to obtain a
similar quality, but the time gain is not worth an expensive tool. The roller is
coated with rubber to improve grip on the container. I did this by sliding the
ends of cut inner tube from a bike tire on the ends of a piece of 2.5 cm PVC
tubing. Inflating the tube allows it to slide over the PVC as the air escapes.

Media: Most of the money was spend on media. I bought 150 lead balls
of 1.78 cm diameter in a hunting supplies store. They are used for reloading
rifles. Lead and brass media are preferred as they don't spark. Sparks could
accidentially ignite the mixture you are milling, with disastreous results.
Other media such as glass, steel or ceramic can be used for pure chemicals and
non explosive mixtures but not for ignitable substances. How much media will you
need? I recall that it is most efficient to fill exactly half of your container
with balls. My container is 11 cm diameter and 34 cm length, which took 150 lead
balls of 1.78 cm diameter.

Assembling

Constructing the mill is fairly simple. The pictures
below illustrate the principle. The container rests on the roller and two small
wheels. A copper rod lengthwise through the roller is driven by the motor on the
right.

Using the mill

Ball milling can be dangerous. It inevitably causes friction and shocks that
could ignite a mixture. Depending on your container and media material and the
substance you are milling the process can generate static electricity as well.
Milling metals can make them pyroforic. When a mixture ignites inside the mill
the results can obviously be disastrous. The explosion will produce large
amounts of shrapnell from the media and the milling container that can be hurled
away hundreds of meters. Under no circumstances should you attemp to mill
explosive mixtures such as black powder inside or near a building. Operate it on
a piece of open terrain and turn the ball mill on and off remotely. Barricade it
with sand bags. You can often safely mill pure compounds, but be aware that
metals (which can be milled with steel media) can become pyroforic and ignite
when you open the milling container (thereby letting oxygen in). Pyroforic metal
powders can be very treacherous. They do not necessarily ignite immediately
(although it could happen), but after some time when you think they are safe
since there has been no reaction upon contact with air. When ignition occurs
expect a large, extremely hot ball of flame, the burns of which can easily cause
a painfull death. Use your common sense when ball milling and be prepared for
the worst.

I do not know wheter the materials I chose for my ball mill (PVC and lead)
generate static electricity. I have prepared many batches of black powder in my
mill (using a wet process) and have not had any accidents so far. Any comments
on the above design are very welcome ([email-masked]).

Pictures

This shows the basic construction. The black horizontal bar is the
rubber-covered roller, driven by the motor on the right. The two small
white structures are small plastic wheels, such as are used under
furniture.

The container consists of a section of PVC tubing with fitting end
caps. The picture shows some of the lead media (1.78 cm diameter lead
balls), and a few chunks of charcoal. These will be broken up in the mill
in a matter of minutes. Strips of rubber are glued around the container as
shown to improve its grip on the roller.

The container where it belongs.

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