***Ball Mill Explosion!***

[schroedinger]

Did anyone saw the experiment on how to remote empty a ball mill? What do you think about it?

[oldspark]

Interesting that this subject has gone this way, follow all the safety rules we already know about and you will be much safer then driving your car.

[DavidF]

"Some atoms hold on to their electrons more tightly than others do. How strongly matter holds on to its electrons determines its place in the triboelectric series. If a material is more apt to give up electrons when in contact with another material, it is more positive in the triboelectric series. If a material is more apt to "capture" electrons when in contact with another material, it is more negative in the triboelectric series.

The following list describes the triboelectric series for many materials you find around the house. Positive items in the series are at the top, and negative items are at the bottom:

• Human hands (usually too moist, though) Very positive
• Rabbit fur
• Glass
• Human hair
• Nylon
• Wool
• Fur
• Lead
• Silk
• Aluminum
• Paper
• Cotton
• Steel Neutral
• Wood
• Amber
• Hard rubber
• Nickel, Copper
• Brass, Silver
• Gold, Platinum
• Polyester
• Styrene (Styrofoam)
• Saran Wrap
• Polyurethane
• Polyethylene (like Scotch Tape)
• Polypropylene
• Vinyl (PVC)
• Silicon
• Teflon Very negative

(The above list is adapted from the book Nature's Electricity by Charles K. Adams.)

The relative position of two substances in the triboelectric series tells you how they will act when brought into contact. Glass rubbed by silk causes a charge separation because they are several positions apart in the table. The same applies for amber and wool. The farther the separation in the table, the greater the effect."

________________________________________________________________________________________

 

This is the table they were discussing on Fireworking.com. Glass and hard rubber are 13 positions apart, and so would be quite prone to building up charges. But, BB's rubber jar is only know to be "black rubber".

Lead and vinyl (PVC) are 18 positions apart. It would SEEM from this chart that this is a more dangerous combination to use for milling BP, if one only considered the chart. Yet this is the combination suggested in Lloyd's book. Thousands of mills with this combination are running out there. It is the most common combination in use for the purpose. Of course one might ask: when they say vinyl, do they mean PVC with plasticizers, opacifiers, and coloring agents like in vinyl pool toys? Or do they mean plain hard PVC like we use? Who knows? And why aren't more mills blowing up?

Brass and PVC are only 8 positions apart. Does this mean brass is safer in a PVC jar than lead? Maybe, according to the chart. But what if we consider hardness as a factor, like some of us did with the glass?

My Rebel 17 jars have liners that 'appear' to be black rubber. I use brass media. They are only 2 positions apart on the chart! I must be golden. I know one thing: it works great for hot BP and is practically noiseless. For that reason alone I use it.

 

I did a rough experiment myself after reading this thread. I took my 32oz. glass mortar and pestle and started grinding a bit of BP in it. The BP was well-milled beforehand. I went from rubbing to hard rubbing to pounding- nothing. I took some glass and ground it to a coarse powder in another mortar, made of ceramic. I added this to the BP, and went through the steps again. Nothing. After about 20 minutes of this, I finally shattered the mortar. There was not even the most tiny indication of ignition.

 

The most likely time for an accidental ignition with BP seems to be generally accepted as during milling or emptying the mill jar. That's interesting when we consider that BP is like the flour to a bakery and is handled MANY MANY other times without incident during the natural course of making fireworks. It is stored in many different kinds of containers and manipulated with hands, tools, and machines such as star rollers. The only safety concern that I recall hearing of is that somewhere (maybe Spain) fiberglass star-roller drums are banned.

 

I guess my point is that as long as we are all discussing or even bickering about milling practices, we are thinking about safety. And the people reading our opinions, experiences, and theories are reading about safety. So that's a good thing right there. We will never know for sure what happened to Dagabu or Burrito Bandito. Somebody somewhere mentioned that a complete overhaul of some of our methods might introduce a new hazard that we hadn't considered. That's a possibility too.

 

I would like to clarify that my comments about the tribo-electric series chart do not mean that I have a definite position- I don't. Also, my experiment with the glass and the mortar do not prove anything either. I just failed to produce an ignition THAT DAY.

 

As far as the remote dumping idea goes, that is a good common sense way of reducing risk to person. If a danger cannot be engineered out, it should be moved away from personnel. If these things cannot be done, PPE is the last line of defense. I am going to try to rig up something that separates my BP from media and rubber-like jar, like Ned did on Fireworking.com. His method uses a lot of plastic though. It's a good starting point but I'm leaning towards wood.

[Mumbles]

I've been thinking a lot about this off an on. The obvious starting point is the media. This very well could just be a 1 in 1,000,000 freak accident and the media is just a coincidence of course.

 

I've been talking with BurritoBandito privately about trying to estimate the impact area of two balls to try to estimate the force able to be produced. Reading DavidF's post about triboelectric effect flipped a light on in my head. What about triboluminescence or fractoluminescence (synonyms). It's light produced when a material is broken, fractured, rubbed, scratched, etc. Basically when you break a material you can cause a charge separation, which then can recombine in a spark or flash of light. It's not unreasonable to imagine a situation that when the ball mill jar was being shaken, two marbles collided just right and caused a chip or crack to form. This has always been my biggest concern, though more so from addition friction generating glass particles to the mixture.

 

A lot of materials will demonstrate this effect. Sugar, table salt, quartz, etc. These of course are all crystalline pure materials. Glass is, or should be, amorphous. It appears that glass still demonstrates a similar effect. I found some cool pictures of this: http://www.njnoordhoek.com/?p=974 I'm not going to pretend to be an expert on this, and am just going off of some general reading. Anyway, the above link mentions that the photons emitted are of a high temperature. I was able to read the paper (linked to below), and it basically says the same thing in more words. When glass is broken a high temperature region is created which propagates with the crack. You can probably read in the abstract the temperatures produced. They're in the vicinity of 2000+K, which is extremely hot.

 

http://scitation.aip.org/content/aip/journal/jap/54/10/10.1063/1.331773

 

I was thinking that a crack or fissure in the glass material of the marbles could possibly generate either a spark capable of ignition, or at least a small high temperature zone which may also be capable of ignition.

 

Do you guys find this at all reasonable? I think it makes sense as a possibility, but certainly difficult to prove or definitively call this the cause.

[BurritoBandito]

I think that is a possibility or even pressure induced heat (like in a fire piston). Out of all of the marbles I've found there was one that was split in half. I suspect it may have broken on impact with something after being thrown by the explosion. Both halves were found about five feet from each other and probably 30 feet from where I was standing when the jar blew up, I didn't measure.

[oldspark]

I would agree all these discussions are good but lets not throw the baby out with the bath water, not following safety rules ALREADY in place more then likely caused the accident. Lloyd had safety concerns in his book that was written over 10 years ago that were not being followed.

I use to see this at work all the time, people not following common sense and safety rules in place and then some one gets hurt and more rules were put into place.

One thing I have learned from all this is I was not as careful unloading my ball mill as I should be, pretty sure the reason was because of video on You Tube of a well known pyro tech unloading his ball mill like it was a bowl of corn flakes.

[dagabu]

Loving Lloyd as I do, I cannot find anybody in industry to back up the theory that glass is capable of making a spark and several EE's that say that static can indeed be built up inside a rubber jar regardless of the outside condition or ground as rubber is a known insulator.

 

I still feel that glass is not the answer but I can only speculate that the actual cause was "pressure induced heat" as BB says.

 

Bottom line for me is to not use ball mills at all, you all carry on with doing what you are doing, I have already been there, done that, burnt the T-shirt!

 

(WHAT?!? Too soon?)

 

[oldspark]

Along those lines dagabu (and believe me I am not being a smart ass) I would recommend staying away from fire works all together.

I was an electrician and delt with ESD in my old job and I tend to agree with Lloyd on that issue but give me facts and I am open minded.

Lloyd never said it was a spark.

Now I remember that Lloyd said that IF a charge had been built up it would have discharged when he picked up the jar.

Edited August 7, 2014 by oldspark

[BurritoBandito]

DavidF, IIRC I read (somewhere?) that the HF jar is made from neoprene.

[dagabu]

Along those lines dagabu (and believe me I am not being a smart ass) I would recommend staying away from fire works all together.

I was an electrician and delt with ESD in my old job and I tend to agree with Lloyd on that issue but give me facts and I am open minded.

Lloyd never said it was a spark.

Now I remember that Lloyd said that IF a charge had been built up it would have discharged when he picked up the jar.

 

Oldspark,

 

I understand your sentiments but I see a clear tainting in the disagreement we have with Lloyds view on static and I agree, he never did say it was a spark, he said it was a pressure induced heat that caused the explosion... Interesting that we are still in agreement? The point I was making is that the people I know as electrical engineers, disagree with Lloyd on the charge mysteriously dissipating when it was picked up.

 

I am wondering why you chose this late date to say you think it best for me "stay away from fireworks altogether"? Do you know that my accident was not pyro related? Did you know that it BP for a canon and that it was a BP related injury only? Thats a pretty wide blanket you cast my friend, "stay away from fireworks altogether" sure seems harsh to me.

[oldspark]

Dagabu I believe you misunderstood me, my statement saying to stay away from fireworks all together was based on you saying "for me is not to use ball mills at all" period, I don't know any thing about your accident at all other then you had one.

" disagree with Lloyd on the charge mysteriously dissipating when it was picked up"

Mysterious, interesting you say that, IF the jar builds a charge and then you ground it when you touch the conductive lid unless he picked it up very carefully and did not touch the lid. Correct me if I'm wrong but I thought the lid was conductive.

Edited August 7, 2014 by oldspark

[dagabu]

Nope, the unit was sealed with no way for the inserted aluminum lid plate (completely surrounded by rubber) to pick up, carry or dissipate the charge internally. I don't think it was an electrical spark but according to people with a lot of letters after their names, there is no reason to discount a charge built up inside a closed container.

 

I can make rockets without a ball mill, stars, comets etc as well. But I can also volunteer and not touch pyro, I just thought it was a rather rash statement I assumed was based on a lame joke I told. I'm sorry, that was my fault.

[oldspark]

Sorry for the confusion, don't know what lame joke you told.

I know Lloyd gave a good explanation of the static charge, I will go back and read it, he has knowledge in both ball mills and ESD so he might have an edge on the subject even though there are no letters behind his name.

Maybe I will meet you at PGI, I look forward to it, do you still make tooling for sale (I might have asked you that before)?

Now that I think about it the jar would not have to be conductive to dissipate the charge.

Edited August 7, 2014 by oldspark

[Shadowcat1969]

Lloyd was indeed leaning toward an impact between the glass marbles being the cause.

 

As far as the buildup of charge, it may well be possible to do so in that environment. HOWEVER, in this case, BB OPENED the jar, scooped some powder out for testing, re-closed the jar and shook it, at which point the ignition occurred. I would submit (as Lloyd pointed out) that once opened and handled/contacted, it is unlikely to the point of vanishing improbability, that a static charge remained in the jar.

 

That of course is applicable to this specific accident and not designed to imply that ESD could not happen. I certainly do not have the overall experience or knowledge to lean one way or another on that part of the debate.

[BurritoBandito]

Could anyone help me get in touch with Lloyd? I would very much like to ask him a few questions regarding my accident. Thanks.

[Bobosan]

BB, did you get a hold of Lloyd?

[BurritoBandito]

No sir, I have not.

 

Edit: Arthur brought up the idea that adding very fine glass could sensitize BP. Needless to say using marbles as a milling media there is a very high probability of introducing said contaminant. Add that to the idea of triboluminescence, and it is easy to see how a very small particle of glass could be shattered by the impact of two marbles over a very small surface area, and ignite the powder.

 

Mumbles was so clever as to identify that the area of impact is a function of particle size of the substance being milled. He (Mumbles) estimates a home mill could process the powder down to the 1-5 micron range. Assuming it was a 5 micron powder the surface area would be 0.09973 mm^2. That is an extremely small area. Right now I'm trying to solve for the impact force exerted on that area (Force=1/2*mass*velocity^2/slow down distance). The issue I'm having is figuring out the slow down distance for two 3 gram marbles colliding in the air. Im willing to fudge the velocity for now (5 m/s?) until I can work the math out. If anyone has any advise here I'm all ears.

Edited August 9, 2014 by BurritoBandito

[Bobosan]

Glass pieces were mentioned briefly on fireworking.

 

One of Lloyds earlier posts on impact in that thread................

 

"

I'll have to disagree with you on your last point. In a 4" jar, the furthest any marble could fall (worst case) would be 4".

Velocity after falling a certain distance is http://upload.wikimedia.org/math/1/a/6/1a6e5238d2ab226044c4f776d6ee7e97.png

Say g is roughly 32ft/s/s. Distance is 4", or 0.33333ft. Solving, Vi = 4.62 ft/sec.

Throwing a container side-to-side, one has to double the velocity, since the container will be moving BACK the other direction when the particles make impact on the opposite side.

I can easily shake a 2lb weight at a rate of 5 ft/s. The impact speed would be 10ft/s.

Kinetic energy is E=1/2mv^2. So the energy of the shaken marbles at impact is 4.68 times that of the energy when they impact (worst case) when falling.

In fact, the marbles seldom fall more than 1/2 the distance of the diameter of the jar (even clumped). So then, the velocity would be only 3.26 ft/s. That would make the energy 9.375 times when shaken at 5fps.

And, I hate to say this, but I can also shake a 2lb weight at about 10ft/s with little extra effort. So quadruple the energy to 38 times.

Lloyd"

 

Further down another post................

 

"

We've already seen that with sufficient impact, black powder can ignite. It doesn't require any electrical effects.

The "point" people forget about is that the impacts are point impacts; essentially zero area of contact with large forces.

If the impact 'force were 50 pounds (only), and the point of impact had an area of 1/1000th of a square inch (actually a large impact area between two smooth glass balls), then the 'pressure' at the point of impact would be 50,000 pounds per square inch.

I could run the math; my gut says that the force would be several times that when a jar was shaken vigorously.

Lloyd"

 

And later after he ran the numbers..............

 

"BTW, I ran the numbers.

Two ten gram marbles, one still, one impacting the other at 10fps.An assumed total deformation (stop distance) of .002" when they hit each other. (generous, but I'm assuming some tiny bit of powder cushioning them during deceleration. Only a little though, the jar was 'clumped'. If none, it would be more like 0.0005".
An assumed impact area of three circular mils under deformation (huge; it would be more like one or two, but we'll pretend the powder is spreading the load)

Anybody want to guess what the psi at the contact area is, before you scroll down below to see it?

 

It's about 68,522 psi! It would be four TIMES THAT, if you just shook it twice as fast. I'm 65. I can move a 2lb weight at 20fps. (it's called a hammer!)

Lloyd"

 

also..............

 

"

You're mixing units again, Mixer. In this calculation, Newtons is force, not pressure. It's just Newtons, not Newton/meters^2

915.556 Newtons is a force of 205.05 lb. If it were spread over an area of 1 square inch, that would be a pressure of 205.05 psi. But - in my example - it was spread over 3 thousandths of one square inch. (not one thousandth, which would triple the pressure)

205.05/.003 = 68350 psi.

I used the "average force at impact over a stopping distance" formula.

Just to ugly things up a little more, that's the average force during the entire stop. The instantaneous force at the moment of impact, before the marble begins to decelerate is higher! (velocity squared, and all that) But you've got to use something, and there's no clear way to define exactly how elastic the marble is when the point of impact is only 1/10th of one millionth of a square inch -- or even if that could be the case.

That's OK, Mixer. It hurts my brain, too. I have more trouble keeping units right than probably any other thing in math. <G>

Lloyd"

Edited August 9, 2014 by Bobosan

[BurritoBandito]

Bobosan, thank you for that post. It answers a lot of my questions, and saves me a lot of work.

[Bobosan]

No problem, BB. Might consider joining fireworking. Well worth the $$$.

[Niladmirari]

Take two pieces of quartz (SiO2). In the dark, bang (friction) one piece over another. You will see a flash of light ...

Edited August 11, 2014 by Niladmirari

[BurritoBandito]

Yes, I have discussed that idea with several other people as well, and I think that is a very possible explanation.

[Mumbles]

Yes, only glass is not quartz. Quartz has a very regular crystalline structure, which is required for a piezoelectric spark. Glass is an amorphous solid. If it's making a spark, it's via a different mechanism, like the triboluminescent possibility I mentioned earlier.

[BurritoBandito]

Right. I failed to differentiate between the two (triboluminescence vs piezoelectric). They both seem to be very similar phenomena. I do understand that photons are not electrons, but for some reason I guess I just mentally lumped them into one category.

[BurritoBandito]

Yesterday I had the pleasure of allowing Dr Robert Webber to remove the two k-wires from my left hand. My next appointment is on 09/11/14, and he will be removing the last k-wire and cast from my right hand. Then I can begin physical therapy. The left hand looks pretty good. Though my thumb, ring finger, and pinky will require some work to get back to full functionality. Doc says everything is healing well.

I'll post updated pics after my next appointment. Right now there's not much to see. I have a few scars on my left palm and around the base of my thumb, and some fingers don't open and close completely. The right palm is covered by a cast, because the explosion broke my wrist. The fingers and a thumb look a bit splotchy from dead skin. My ring finger nail is purple and being forced off by the growth of a new nail. This finger is also still pinned, but I can wiggle it a little.