Causes of ball mill accidents?

[Norwest]

Hi,

 

I have been reading many posts and other references in preparation of making my first batch of black powder including accident reports. I have a 15 lb capacity rubber lined rock tumbler (Thumbler's Tumblers) and 200 brass 1/2" beads. If I am making standard black powder with this setup what is the likelihood and possible causes of unintended ignition? Would static be most likely culprit? Does the biggest risk occur when opening and dumping out BP and beads? Seems like using sparking milling media and milling more reactive mixtures other than BP are the most common causes of accidents. Is this so or have I missed something? I have a back yard but can only get about 50 ft away from house and neighbors. Digging a pit will not be well received by wife but do have some tires I could place around the tumbler. Going to try to convince a friend who has a farm to do it on his property by fear his wife will nix it. Going to try to join local pyro club so perhaps through them I can find a safer place to mill.

 

Thanks,

 

Norwest

[fckiamdead]

Hi,

 

I have been reading many posts and other references in preparation of making my first batch of black powder including accident reports. I have a 15 lb capacity rubber lined rock tumbler (Thumbler's Tumblers) and 200 brass 1/2" beads. If I am making standard black powder with this setup what is the likelihood and possible causes of unintended ignition? Would static be most likely culprit? Does the biggest risk occur when opening and dumping out BP and beads? Seems like using sparking milling media and milling more reactive mixtures other than BP are the most common causes of accidents. Is this so or have I missed something? I have a back yard but can only get about 50 ft away from house and neighbors. Digging a pit will not be well received by wife but do have some tires I could place around the tumbler. Going to try to convince a friend who has a farm to do it on his property by fear his wife will nix it. Going to try to join local pyro club so perhaps through them I can find a safer place to mill.

 

Thanks,

 

Norwest

 

I never heard of any accident's during milling, but som few when they opened the drum and pored out the BP. Could have bin static or sparks from metall to metall ect. I think it's rather "safe" to mill.

 

BUT STILL always think "murphy's law" and use ppe and a safe location. Tires and no one else around the place (outside the house) sounds like a good plan.

 

Regards

[SideGlance]

Glass marbles was the cause of one explosion that I know of.. You should always baracade the mill with heavy sand bags. If it were to blow you want the stuff to go up not sideways.. But yeah pretty darn safe using proper media and rubber jar

[taiwanluthiers]

Seems accidents are happening when the person is emptying the jar, so barricading the mill won't solve that. Perhaps some remote way of emptying the mill jar is in order?

[Ubehage]

Seems accidents are happening when the person is emptying the jar, so barricading the mill won't solve that. Perhaps some remote way of emptying the mill jar is in order?

https://www.youtube.com/watch?v=8xX5Wgo4o6Q

[mikeee]

Another viable option is to sandwich a stainless steel screen or plastic sorting screen between two mill jars the same size.

Slowly reverse the positions of the mill jars and tap and shake the jars to sift the comp through the screen and contain

the milling media in the other jar. The main issues are heavy hard impact of the media on sparking surfaces and static

discharges to the chemical compound while emptying the mill jar. Minimizing static potential is something everyone talks about

but very few people do anything to reduce the risk. Dry environments multiply the risk of static sparks higher levels of humidity

and damp surfaces lower the risk. Bonding yourself to a grounded metal surface will discharge static potential of your person,

touching the exterior of the mill jar while grounded will help lower the risk of a static spark reaching the compound when you open and empty the jar.

[dagabu]

You can make sure to avoid an electrical spark (non-static) by installing a single pole switch (light switch) on your extension cord that allows the ground to remain in place by disconnecting the hot wire from making a circuit. DO NOT! Unplug the ball mill and remove the ground, if by whatever there is an electrical charge built up, the energy will go to ground and not create a spark (hopefully).

 

One crispy critter is enough around here.

[mikeee]

Maintaining a ground to all of your electrical equipment is necessary to say the least.

An external clip on ground wire is the next level of protection used on process equipment.

ESD (electrostatic discharge) grounding mats and straps are the third level of protection for preventing static spark potential.

The ESD mats and straps normally use a 1 megohm resistor in series to protect the user.

ESD paints can also be used to coat working surfaces and equipment.

There are several resources on the internet that go into detail on ESD grounding protection principals.

[taiwanluthiers]

I never gotten any static shocks in Taiwan (humidity is way too high for that) but I'm getting a LOT of them in Texas. Doesn't help that I'm wearing a polyester vest at work that is just a magnet for static. This is another reason why you must wear all cotton clothing when working with explosives, because polyester can easily build up a static charge.

 

If you must humidify the heck out of your pyro workspace, will probably make things very uncomfortable if you are in a hot climate though...

[SmokinJoe]

My background is in semiconductor, where we employ static dissipation (conductive clothing, wrist strapes, conductive tables, mats, etc.) and a continuous path from the device to ground. I find it odd to see suggestions on the use of static charge building materials such as rubber and non-conductive plastics (e.g. plastic paddles in mixers) in making pyro mixtures. I prepare my mixtures on a conductive table which is is connected to ground, and while I don't always where a conductive labcoat and my wriststrap, always touch my hand to the table before contacting the mixture/its holder. My 2 cents.

[dagabu]

My background is in semiconductor, where we employ static dissipation (conductive clothing, wrist strapes, conductive tables, mats, etc.) and a continuous path from the device to ground. I find it odd to see suggestions on the use of static charge building materials such as rubber and non-conductive plastics (e.g. plastic paddles in mixers) in making pyro mixtures. I prepare my mixtures on a conductive table which is is connected to ground, and while I don't always where a conductive labcoat and my wriststrap, always touch my hand to the table before contacting the mixture/its holder. My 2 cents.

 

And you are probably 100% right in that statement. The problem for me and others (if they are honest) is that many of the processes we use are out of necessity because we are limited in the materials, components or knowledge.

[lloyd]

Dagabu, no: he's NOT 100% right. He's on the right track, but it's an "excessive remedy for a mild malady". Kind of like treating a common cold with thoracic surgery!

 

Mil-spec calls out certain specifications for humidity and conductivity of flooring and footwear. When the 'general conditions' are inside about 35%-60%R.H. and floor and footing conductivity is around <100Kohms, everything is 'pretty safe', and extreme measures such as working on conductive worktops (which present a WHOLE SET OF THEIR OWN DANGERS) become unnecessary.

 

"Conductive" worktops are not the same as "static dissipative" worktops. Static-dissipative surfaces are to be desired. Conductive ones present a whole 'nuther bunch of control issues you don't want to have to deal with!

 

Lloyd

Edited May 25, 2016 by lloyd

[Nessalco]

Lloyd, a question came up a while back, relating to absolute vs relative humidity and static discharge. I'd be interested in your take on that question, especially since you're involved in the manufacturing end and know how the 'big boys' handle things.

 

http://www.amateurpyro.com/forums/topic/9422-preventing-electrostatic-discharge/

 

Kevin

[lloyd]

Kevin,

YOUR analysis of it, and the ASHRAE evaluation are more accurate than what is considered "standard practice". In fact, RH readings have proven sufficient over DECADES of practical use making very-sensitive explosives for military goods.

 

It's completely true that the absolute humidity is a better indicator, but RH has 'worked'. Oh, well! What works, works! What doesn't kills folks.

 

My leaning is to think that the military will be slow to adopt any new standards, given the success of the old ones... no matter HOW valid the new ones might be.

 

Lloyd

[dagabu]

Dagabu, no: he's NOT 100% right. He's on the right track, but it's an "excessive remedy for a mild malady". Kind of like treating a common cold with thoracic surgery!

 

Mil-spec calls out certain specifications for humidity and conductivity of flooring and footwear. When the 'general conditions' are inside about 35%-60%R.H. and floor and footing conductivity is around <100Kohms, everything is 'pretty safe', and extreme measures such as working on conductive worktops (which present a WHOLE SET OF THEIR OWN DANGERS) become unnecessary.

 

"Conductive" worktops are not the same as "static dissipative" worktops. Static-dissipative surfaces are to be desired. Conductive ones present a whole 'nuther bunch of control issues you don't want to have to deal with!

 

Lloyd

 

Dang, I forgot to use the "smarky" icon when posting again...

[lloyd]

<G>

L

[Nessalco]

Kevin,

YOUR analysis of it, and the ASHRAE evaluation are more accurate than what is considered "standard practice". In fact, RH readings have proven sufficient over DECADES of practical use making very-sensitive explosives for military goods.

 

It's completely true that the absolute humidity is a better indicator, but RH has 'worked'. Oh, well! What works, works! What doesn't kills folks.

 

My leaning is to think that the military will be slow to adopt any new standards, given the success of the old ones... no matter HOW valid the new ones might be.

 

Lloyd

 

Thanks, Lloyd. I appreciate the insight.

 

Kevin

[Arthur]

The most likely cause of unintended ignition inside a closed drum is included grit especially hard grit -probably especially glass hard grit.this forms pressure concentrations between milling media.

 

Other than that take effective measures to minimise static and wear clothes at all times two layers of full body cover may be "too hot" but the searing flame of burning BP is much hotter. I've seen vids of people making fireworks in T shirt and shorts, that really isnt sensible.

 

Someone from the UKPS once filmed ball mills being ignited. Small mill 100g BP almost no issue, just the end popped off. Big mill 5kilos BP -lead balls over 20m radius and big fireball. Make moderate batches.

[DavidF]

One guy had an explosion while milling when he used large ceramic media and large lifter bars in his jar. Another guy used a steel chain (I think) in a steel drum. He was warned of the danger. He pointed it out the barn door in case it blew up while milling, and it did.

 

Greenhouse grade saltpetre can have hard gritty pieces of stone in it- at least mine does. I screen all my saltpetre through 40 mesh before milling now. I had some old milled stuff around and went to make up some scratch mix. There was a rock the size of a pea in it.

[Ubehage]

One guy had an explosion while milling when he used large ceramic media and large lifter bars in his jar. Another guy used a steel chain (I think) in a steel drum. He was warned of the danger. He pointed it out the barn door in case it blew up while milling, and it did.

 

Greenhouse grade saltpetre can have hard gritty pieces of stone in it- at least mine does. I screen all my saltpetre through 40 mesh before milling now. I had some old milled stuff around and went to make up some scratch mix. There was a rock the size of a pea in it.

Wow, thank you for sharing that information.

 

I have 2 drums: One is with ceramic media, about 8-9mm in diameter. And the other is with different sized lead media.

The ceramic drum is used only for single chemicals, where I used the other for BP (and BP-like compositions, like FairyFountains).

[Mumbles]

The accidents I'm aware of using ceramic media all involved large media (1" or larger), and it was all second hand. It's difficult to pin point the cause in this case. You have contamination of the media, increased impact force due to size, and some uncertainty in the ceramic composition of the media it self to contend with.

 

There are hundreds of people in both the hobbyist and professional levels using ceramic media without any issue. I would err on the side of caution and only trust media I obtained directly from a supplier however. High alumina ceramics (90+%) are the preferred material Regardless, milling in a remote location is really the most important part.

[DavidF]

I'm one of the hundreds that has used ceramic media without incident. I used mine for hundreds of hours. It was apparently second-rate material from a cheap pyro (now passed on) supplier. The media eventually developed craters in the surface, and I just didn't feel safe using it any more.

 

Now that I have used various types and sizes of media, I can say this: the ceramic was the least efficient of the media I used. I can also say that the media that I found to be by far the most efficient was 5/16" 304 stainless steel spheres. It is so efficient at milling charcoal that I have been able to completely forego the BP milling process for my nozzleless rockets- and they are still super fast! I'm currently working on proving the value of efficiently milling charcoal to be used for BP for lift and burst as well. My first pucks are drying now. I plan to match Goex performance with screen-mixed powder.

 

After I succeed with this project I will scale it down to rock tumbler size and see how well the same media does. I have been working on this idea for some time now. I should be able to demonstrate my screen-mixed black powder within a week or two. The intention is to use a 3lb nozzleless rocket to lift a 6" ball shell. The burst will be the same powder. More of the same powder will be used to lift and burst shells.

[MadMat]

I built my own ball mill and here are some of my ideas. Feel free to constructively criticize as I am no expert. My mill is belt driven and the drum is well grounded (look at how a Van De Graff generator is constructed and you will immediately see why I made sure of grounding.) My idea on the belt drive mill was that it puts the electric motor directly underneath the drum as opposed to the side of the drum. That way, if there is accidental ignition, the motor doesn't become a low flying projectile, but rather is driven straight into the ground. This part may draw some criticism, but, I used a piece of 4" PVC pipe for the drum. I capped off the ends with what are called test plugs. One end is glued and the other I wrap teflon tape to seal it and secure it with strips of tape. Test plugs are fairly thin material and simply securing the top cap with just tape would allow the ends to "blow out", directing the blast wave to the sides, and possibly saving the drum from exploding. As far as shielding my mill while it is in operation; one cannot contain a blast. It is foolhardy to try. The best one can hope for is to direct the blast wave in a safe direction. With this in mind, I have two walls of cinder block around my mill. On the inside wall the blocks are on there sides, their holes forming a baffle. The outer wall. spaced about 12" away from the inner wall is set with the holes in the block vertical and they are filled with fine sand. These walls are three courses of block high and there is no top on them. My setup is out in a field well away from anything. I use 30' wires to run power to the motor (a 12 volt motor). Well that's about it.

Edited May 27, 2016 by MadMat

[Mumbles]

I will tell you from experience that you're on the right track. Myself and others have been experimenting with basically the same thing, but mostly on the cylinder shell side of things. This branches off of some of the techniques used in Malta. Screen mixed BP using a hot charcoal actually works quite well, as long as all components are milled to impalpable powders before hand. You can get to commercial BP levels of performance without a whole lot of trouble. For burst if things are not hot enough, you can dust it with slow flash. This even works for polverone made with poorer charcoals. I would have to retry with just a black aluminum to see if that would be enough. That'd be safe enough that it'd work for lift too.

 

The biggest area of trouble is that it doesn't really work well for small shells. The burning speed isn't important with multi pound shells, but becomes imperative for small devices. You can get around this with miniature lift maroons, but that gets old very quickly.

 

How fine are you grinding the charcoal before milling it? I used to use 000 buckshot (~3/8"), and had trouble with particles if they were larger than a certain point. This was mostly KNO3, but I also experienced some issues with charcoal as well. I usually ended up having to reduce it to -30 mesh or so before it would mill efficiently. I'm just trying to determine if this was a size issue, or if the additional hardness imparted by stainless steel might have provided an additional assistance.

[DavidF]

Mumbles, before I comment: I'm not an expert on theory. I know what I have seen though. I use the Rebel 17 rubber-booted jars to do my milling. I used to use 6" ID PVC Sponenjars. The Sponenjars ran at 68 RPM, which I think is about right. The rubber booted jars have a much larger diameter and they run at 58 RPM on the same setup.

 

If you are saying 3/8" media is not milling very well, the first thing I would wonder about is the jar diameter. Are you talking about the rubber rock-tumbling jars? My jars have +30# each of media. I think the total weight of the load is an important factor in milling.

 

My main area of milling interest has been with charcoal. My singular and unpopular opinion is that with that substance, the fine particles are created more by the rubbing action than with impacts. If we can agree that rubbing is just a bazillion tiny impacts, then OK, it's impacts.

 

Charcoal is a difficult thing to mill by itself efficiently. It's fluffy generally, and if the media is large, the large impacts are fewer in number than if the media were smaller (with smaller impacts). I believe the size of the spaces that the charcoal can hide from the media in is a huge issue. Anyhow, I don't want to be purposely contrary, but that's what I think.

 

I don't- yet- know the size of my charcoal particles. Somebody else is classifying a bit for me to give me a better idea.

 

Madmat you sound like you put a lot of thought into the mill setup. When I first started milling BP I did it in my shed. Not my brightest moment!