Ball milling safety

Safety First! Precautions/procedures and equipment.

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Niall
Posts: 527
Joined: Sun Jul 21, 2013 6:45 pm

Ball milling safety

Post by Niall »

Try again.... I wrote this on the train, so I am open to all questions and criticisms that help answer the original question properly. It's up for discussion.

Nobody really wants to give a definitive answer on whether practices are safe or not, simply because we don't know what you do in your own time, what your methods are or the quality of your housekeeping! If we declare something as being acceptable; then what if something goes wrong and we were unable to spot any errors in your practice having given the go-ahead that led you into harm's way?

So I hope I can provide some information as a standard upon which you can make an informed decision. With any luck you might ease your mind. I will be giving out egg-sucking advice, but that caters for people who may know less than you and I wish to be accessible as possible in my writing.

'Safety' is a word that really has no place in pyrotechnics. Be clear from the outset that explosive and pyrotechnic formula mixtures are not safe, no matter who tells you differently. Safe means there is no known harmful outcome, after which it becomes entirely a subjective consideration.

In pyrotechnics there is only risk mitigation, not safety, the more you mitigate your risk, the closer to safety you will be on the spectrum of dangers. For clarity the world ranges from safe to lethal and everything in between has a different risk, it takes little imagination to arrive at where we wish to be on that scale. We want to be safe.

With respect to ball milling there are several factors we need to consider to assess the risks, and then respond to those risks by limiting their likelihood and effect. Mitigating risk in order to stay as close to 'safe' as possible.

First we must consider our risks. What is the risk? I shall deal exclusively with the mechanical process of milling black powder compositions here, it applies to no other milling or mixing of chemicals nor the chemical hazards of the individual ingredients.

Our concern with black powder compositions then, in this thread, are the explosive properties of the intended product. The risk is that the composition will ignite and potentially lead to a catastrophic outcome. So we must consider the ignition elements that black powder relies upon to start the chain reaction.

It's all electrickery! Whether it's heat, a hammer blow or a spark, it's all electricity - arrived at by different energy inputs. I shall quite arbitrarily divide these energies up into percussive, electrical and heat for the purposes of what I am trying to convey. If you want to subdivide these categories it will only run back to electricity in the end. Friction is heat, friction is electrical, percussion is friction, percussion is electrical heat is electrical, etc.!

Heat is the common ignition method we a know as used to start a backyard celebration. Simply, a fire or ember that can transfer enough heat to rapidly excite the composition at a rate that is greater than the heat can be dispersed elsewhere! Typically the heat input required for good black powders is around 250degC upwards, I set the bar here for extra tolerance.

We must consider then, whether we are creating these temperatures in the mill. There are two places the heat an come from, inside and outside of the jar. If you are ball milling in a heated oven and are able to read this, perhaps Darwin was wrong! Another external source of heat beyond environment is the ball mill motor. Does the motor get hot? Do the rollers cause the jar to heat?

The motor may well get warm, depending on your machine. I suspect there should be very little heat from your rollers as the friction is designed to transfer kinetic energy rather than restrain it. A well loaded ball mill should give little heat from the rollers. See my ball milling methods for guidance.

Then there are the heat generation elements of the process, percussion and internal friction will generate heat in the mill, there are other factors that could lead to heat, but reality makes them negligible and I will deal with them below. The friction of the powders and the constant hammering of the media will generate sound and heat.

Experience says that your jar never quite reaches 250degC, for that matter it never reaches a temperature that would lead you to wear a glove in order to handle it.

The powders are getting rolled over and through the media but the transfer of heat is dissipated faster than it can accumulate in a single point because the temperatures of these interactions are exceedingly low.

It would appear that heat is not going to be a self-generated process risk, but the environment could be. To mitigate this risk, an increase in temperature differential would be the sensible approach. The greater the differential, the faster the heat in any one place is diminished to its surroundings. Leave the ball mill to run in a cool dry place.

Unless you are putting electrodes in your jar, electricity is a very low consideration for black powder compositions. If you are putting electrodes in your jar, please make sure you have your organ donor card up to date and permanently in your hand for ease of access.

Electricity is a broad term and covers everything from that which your makes your heart to what makes it beat. Electricity at eye level can do two things that concern us, create a spark or generate heat. There are hairs to be split here, but that conversation leads us no further forward. Sparks and heat.


Inside a ball mill we are concerned with one form of electricity, static. Static potential difference is generated by friction in simple terms, something rubbing against something else! Potential difference is also called charge. It is the charge difference between two objects, everything has a charge and that charge 'wants' to be equal. When you get a zap off your car in the summer, that is the potential difference between you and the car finding a common charge. This is why pyrotechnics places an emphasis on grounding yourself.

Static is everywhere and is an immediate discharge, that is why you can experience a spark in the air but feel little effect. All the difference is negated immediately, unlike a battery which discharges at a rate governed by voltage and current giving persistent power. Sparks are our concern here as we have dealt with heat from friction above.

Inside the jar is our concern with sparks during milling, and outside the jar when opening it. Everything in the jar is in contact with all else inside. The potential for static buildup is constantly being discharged and never gets there.

I will add at this point that you should pick the jar up when your process is complete and transfer it to your work bench and place it down to settle for around 10 minutes. When coming to open it, touch the work-surface and jar together with your hands to eliminate charges. A respectable pyro technician will have a grounded bench.

Again, black powder compositions are relatively insensitive to static discharge and the charges are self-eliminating within the jar, and our precautions are clear above.

The last of my identified sources of ignition is percussion, the collision of two objects transferring energy by heat, sound, deformation and/or momentum. A hammer blow. This covers heat and electricity all together and simple deduction from the above could provide our answers here. Black powder is relatively insensitive to percussive ignition, and we are not seeing any examples of the required forces inside the jar, in fact, friction is our greater risk, which is fairly low in itself.

At this point I'd like to eradicate the myth that bullets are propelled by a black powder composition reaction having been initiated by a firing pin, not true. The firing pin hits a percussion sensitive primer which ignites the propellant composition. Hence flintlocks were the direct ignition in early weapons. A spark energy is required.

So, if the chemicals are pure, your methods are exacting and your environment is appropriate, you have a very low risk. It doesn't end there though, there are other factors to consider, contamination is our most prevalent.

Keep your jar clean. Keep your chemicals sealed. Never leave your jar open, unless you are drying it, in which case dismantle it completely and let it air, then use a clean paper towel to wipe it then reassemble it and close it. Never leave your jar open, just in case you missed it the first time. You cannot account for what may fall into it. Never use your Black powder media for anything other that black powder constituent compositions, ie tiger tail, rocket powders and black powder. Never, never allow metals to be introduced to your jar or media. Lead media is acceptable because it is too soft to spark - that is another subject, take my word for it on this one!

You should have one jar and one set of media that is used for nothing other than BP compositions. You should have a designated tub for the BP media which is clearly marked and the only other place that the media is stored other than the jar.

The caveat on chemicals allowed into the jar is binders. Gum, resin and sugars only. Keep it like that, keep it simple and you'll have little to worry about.

Metals in black powder. Metal is a funny beast, it changes the properties of compositions in the way it stores it's energy, it can create sparks, change charges and ruin your day, without going into detail, never let it near your jar, no exceptions. One example of this is if you allow a composition of black powder to contain 15% titanium powder, you will have a composition with a higher sensitivity than binary flash powders. No metals.

I've hopefully looked at our risks and their individual concern and how to mitigate them. In the event of a catastrophic failure, that is highly unlikely if your process considers the entirety of the text above, what can you do to limit the harm?

Environment. Ball mill outside in an area that is as far away from everything as it can be. Covered over for sun and rain proofing. Keep it cool and allow ventilation. Have a remote power switch that is easy accessible. Never leave a ball mill more than 30 seconds away from your attendance. Never leave it unattended at large.

Have response options. Choose the correct fire precautions, choose the location in such a way that if a fire developed, it would burn itself out rather than spread.

Not all of these scenarios can be matched to perfection. But employ them where you can. Limit the risk, limit the damage. Mitigate.

In summary, given the above, and with diligent husbandry, ball milling BP comps is an extremely low risk process. Carry on, think about the risks and what you should do to negate them and react to them should your defences be breached.

If you want peace of mind, you'll only get it by knowing you've covered the initial risks and have a response option for any secondary effects.
All wretch and no vomit.......
davidg
Posts: 280
Joined: Mon Jul 29, 2013 2:07 pm
Location: Manchester

Re: Ball milling safety

Post by davidg »

All good advice.
Niall
Posts: 527
Joined: Sun Jul 21, 2013 6:45 pm

Re: Ball milling safety

Post by Niall »

To wrote this in response to a question. I need to play with it, but I have transferred it here to get it into visibility.
All wretch and no vomit.......
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