Causes of ball mill accidents?

[Mumbles]

I guess I left out the jar, which is probably equally important as the media itself. I was using a 6" ID sponnen type jar, spinning at approximately 70 RPM. It was one of the Hobby Firework mills.

 

I was just curious. I know others can toss in charcoal chunks without a problem, where as I could not. I was mainly asking if you pre-grind the charcoal, and if so to what degree. For me a meat grinder through the coarse plate (~1/4" I think) was still a bit too big. I would have to run it through the coarse plate, screen out the chunks through a window screen, and then run what left either through a blender, coffee grinder, or the finer plate. The charcoal was willow from CustomCharcoal, but it seemed much harder than anything I had previously cooked myself.

 

If you get stuff fine enough, the smaller media is a beast at milling though. I have no measurables, but my powders seemed finer and my BP seemed faster once I made the switch.

[DavidF]

Oops, brain fart! I use ground charcoal passing 20-40 mesh. If it's oak, it's -40. If it's something soft like sumac, I use -20. I have used larger stuff before and found some chunks too.

[DavidF]

My experiments are ongoing, but just a quick update: I have exceeded the performance of Goex with screen-mixed powders, pucked and corned. I have achieved flight times of 10-11.6 seconds with baseball tests. Each test used .4oz (10 grams) of powder, contained in a cup in the bottom of the mortar. All charcoals were milled with stainless steel media the size of peas. The potassium nitrate was also pre-milled, and the sulfur (H10) used straight out of the bag.

 

OK, so the next step was to see if I could make screen-mixed powder on a hobbyist scale that could approach the above levels of performance. A modified Lortone rock tumbler was used to grind 50 grams of -40 mesh willow charcoal at 75 RPM for 6 hours. The media was the same pea-sized stuff I used in the larger milling setup. With the larger setup, the mill time was 4 hours. This time, the screen-mixed (2X through 40 mesh) powder was riced through 10 mesh with 50% ethyl alcohol and dried in the sun. A single test with 10.00 grams gave a flight time of 11.2 seconds. Next tests will be with BP riced with water only, and with MCRH.

[DavidF]

Here is an update on the screen-mixed BP made with the supermilled charcoal. I used balsa-based 75-15-10 and 5% additional dextrin, pasted 6:1 on rice hulls. I used it to burst a 6" ball shell made my usual way. I used no booster.

 

https://www.youtube.com/watch?v=cmEyEpcVOLI

 

It was a heckuva nice burst, IMHO. So, the search continues, to find an application where screen-mixed BP made with supermillled charcoal is lacking in any way, compared to 'regular' BP.

 

P.S. How the heck does a guy make the Youtube still picture with the 'play' arrow show up instead of just a link?

[memo]

that was a nice break, the shell was pretty good too

[Mumbles]

It does it automatically for me up to about 3 or 4 videos per post. To the best of my knowledge the actual code is: URL here but replace the greater and less than signs with brackets. In the little toolbar above the posting window, in the top row 3rd from the left there's an icon that looks sort of like a photograph with a green line through it. It's sort of hard to make out the details. It's next to the lightswitch, which turns the hidden formatting on an off, and an eraser which removes all formatting. If you mouse over the correct one it should say "special BBcode". If you click on it, it brings up a little macro. Select "media", and copy and paste the URL in.

https://www.youtube.com/watch?v=cmEyEpcVOLI

 

Anyway, I agree, great break.

[ClayDog]

Ex missile maker onboard, making first post...

 

In our MIL STD 1686 © compliant production facility, it was dissapative work surfaces / flooring for areas of electronics assembly as a means of protecting sensitive electronics from ESD. It was grounded conductive (read metal) work surfaces and flooring in areas of ordnance & rocket motor assembly. The intent of the latter was to insure things remain at ground potential, vs given a resistive path for a "slower" disharge to protect electronics. Once loading up a missile with the fun stuff, the attitude shifted to protecting the worker. Protection of electronics became a distant second at that point.

 

My practice is to employ a grounded, conductive work station in my shop while observing other ESD protocols for storage, etc.

 

C.

[ClayDog]

And now last night's event that brought me to this forum topic.

 

Set up: Skylighter 6lb Ball Mill; modified with larger diameter rubber sleeved drive roller; 2 x neoprene cannisters, 200g BP each; using .50'cal lead mini's for media; 3 prong power plug; Atlanta GA, Temp = 40F, RH = 80%. I have run volumes using this set up.

 

At end of a 4 hour run, I approached the mill and as always, first touched bare metal of chassis. Now we are both at the same potential, ground. I then reach for a cannister and *BAP*, the dreaded Blue Arc. Damn......I truly forgot my name for a moment. Pretty startling stuff. Oddly, the arc was from cannister to chassis once I started lifting it. I was not shocked (electrically). I immediatly placed it back onto the rollers and walked away, still a bit freaked out. So there it sits this morning, still plugged in, still loaded with BP. I will do a remote discharge of the other cannister prior to removal from the mill today.

 

Anyone heard of this? The upside is the spark was external, but that aint helping my nerves much. I am considering another modification, to install a chassis grounded carbon brush to remain in contact with neoprene exterior of cannisters as they rotate. But my thinking is, if this is a legitmate approach to neoprene cannister discharge, why dont the mill manufactures do this already? Think about those black brushes that sweep against your shooes, ankles on escalators. Conductive carbon to ground you on the machine.

 

I guess the take away is, that with current configuration and proper grounding methods, the cannisters remain isolated from ground. They are rubber containers, rolling on rubber rollers. They can and do build up a Hell of a charge. Going forward, I will always discharge the cannisters to chassis remotely prior to approaching the ball mill. I will update the carbon brush mod here.

 

C.

[lloyd]

C,

A paper discharge brush from a xerographic copying machine works very well for that. It can be placed unobtrusively just behind the rear-most roller, and will have the desired effect. The nice thing is that most of them are long enough to cover the entire length of the straight walls on a 1-gallon jar.

 

IF you're milling BP (or anything high in carbon content), you can be pretty comfortable that all of the jar's contents have equalized to one-another, and (ostensibly) to the jar's wall. But that insulative tub rolling on insulating rollers is certainly an issue.

 

We're lucky that the humidity in Central Florida is seldom low enough for a mill's jar to accumulate much of a charge. When it is, we just don't mill that day.

 

LLoyd

[ClayDog]

Thanks for validating this discharge brush idea. Seems sensible, ya just dont hear much about it.

 

Keep Blastin,

 

C.

[DavidF]

Claydog, the mill manufacturers don't make the machines for milling black powder. Commercial black powder manufacturers don't use ball mills for black powder. Ball milling black powder is the fastest, cheapest, easiest way to make it. With practically the entire amateur pyro community milling complete black powder, there's bound to be the occasional accident.

 

For those not willing to accept the unlikely (but very real) risks of explosion or fire while milling or when separating media, single component milling is an effective alternative. Perfectly good black powder can be made with a modified rock tumbler and inexpensive media. I personally feel that is safer all the way around than milling complete black powder.

[lloyd]

And lest anyone "Pooh-Pooh" Dave's ideas, I've seen his testing figures. He gets VERY serviceable BP from single-ingredient milling.

 

If you distrust it, or wish not to try that method, the only other way to mill complete BP mixtures safely is to provide enough distance (and perhaps barricades/shielding) between your mill and any people or structures. That's what I do. But I have space to do it.

 

But, what can I say... I am sort-of 'accustomed' to milling BP as a mixture! <grin>

 

LLoyd

[OldMarine]

I figure shaking my media around in my separator screen is more dangerous than milling powder. I just make sure I tell Mama I love her regularly and get on with it.

[lloyd]

Patrick,

Just to be clear, this last matter isn't a "milling" issue. It's an ESD issue. <shrug>

 

Lloyd

Edited January 6, 2017 by lloyd

[Ubehage]

I figure shaking my media around in my separator screen is more dangerous than milling powder. I just make sure I tell Mama I love her regularly and get on with it.

I was under the impression that this was generally a safe way to mix in chemicals that you, for different reasons, won't put through your screen or in the ballmill.

That would mostly be Titanium/BP and MgAl color compositions in my case.

 

I also think about static build-up when doing so, and I make sure to touch the ground after shaking. Not sure if it matters, though.

Edited January 6, 2017 by Ubehage

[Sulphurstan]

Just want to share what happened to me today. I made my first ball milled BP today (pine charcoaled from a commercial supplier), no issues, everything OK, but still frighten when opening the jar at the end! Then, I was about to mix 75,10,15 into the jar, with my "just finished" willow charcoal which I made in a steel retort. I dont know why, I looked in my retort (an old paint bucket made of steel), just in case of... and saw some very fine shiny flakes sticking on the inside walls (???). Steel retort, heated.... Put a magnet on these flakes and wtf! they were in fact steel particles that flaked off during "retorting", so my willow charcoal was potentially contaminated with it. Needless to say that my charcoal batch just went directly to the trashcan, and ballmixing postponed.

 

My question is: does anybody uses a steel retort for charcoal? And if yes, how to make sure that any little steel flake is eliminated from the charcoal? I now think to go through my charcoal chunks with a strong neodymium magnet, to take out these flakes... Or washing thoroughly the charcoal chunks before mixing and milling?

[OldMarine]

The only retort I used was a roofing tar bucket and I had to burn it out before use. I suspect I burned off whatever lining the bucket had in the process as well. I never noticed any flaking except for oxidized metal that is harmless as far as I've read. Is there a chance it could have been a galvanizing layer you were seeing? How many times have you used the retort before?

 

Edit: I just reread and see that you stated the particles were magnetic. I don't know how steel could have flaked without oxidizing but I think I'd find a new bucket!

Edited January 29, 2017 by OldMarine

[Sulphurstan]

Yes, OM, you're right, heated, oxidized should give non magnetic particles... I'll run the retort more and more and investigate each time! I wish I could find and earthware or ceramic bucket... For my retort now, it was the fourth time I retorted charcoal in it...

In the past ten minutes, just after my previous post, I've been through other threads (BB's accident in 2014), where this contamination of retorted charcoal issue was discussed, the magnet solution was mentionned and said pretty functional.

 

Re-edit: WRONG: oxydized, the steel gives Fe304 which IS magnetic, and static electricity conductive!

Edited January 31, 2017 by Sulphurstan

[Boophoenix]

Sulph, what about a stainless steel stock pot? This is just a sample photo of a design I've been working on. That's one of my mortar tubes atop the reducers is why the flat plate is there.

 

[OldMarine]

I wonder if the fact you are using a retort rather than a TLUD is not allowing the metal schluffed off during heating to oxidize? I've used paint cans for TLUD processing and have yet to see any flaking. I like Brad's stainless rig idea because even if it flaked it would be a non-sparking metal but pretty sure it wouldn't flake.

[Boophoenix]

If that getup flakes I need to start from scratch. I visited Lloyd and talked him into some lathe work on the bells so they'd slip together. I want it to be able to dismantle it when not in use easily. Don't tell Caleb he's on the visit list for the remained of the stack, lol. The pipe is in the bed of the truck should I make a wrong turn towards MO.

[Mumbles]

One oxide of iron, magnetite or Fe3O4, is magnetic for what its worth. I've read of this issue before but am unsure if anyone ever figured out if it was iron or a magnetic oxide. This seems to mostly be an issue with paint cans from what I've heard, but they're also the most popular smaller retort.

 

I've experienced this before myself. I generally discarded the powder and flakes and stuff at the bottom in favor of larger wood-like chunks for this reason.

[NeighborJ]

A simple way to test for Fe3O4 is with a torch. The Sparks it makes are branching, regular steel doesn't branch. On forged steel products it is usually called mill slag I often need to grind it off of steel before welding and if the grinder Sparks are still branching then I didn't grind enough. It will continue to form on hot steel surface every time it is reheated but only flakes off when the the steel is thin enough to experience rapid cooling.

The only two ways I know of to prevent these flakes is to use thick steel or to keep the retort can just cool enough that it doesn't glow. It forms because the oxygen and CO2 in the air is burning on the hot steel surface.

[Boophoenix]

Interesting. neighbor. Any idea what that temperature might be to be glowing and causing the problem reported?

[Sulphurstan]

Yes, I've to look closer to the tlud method, but still, as you said OM, flakes do appeapr.. or go for a stainless steel thing, Boophoenix, that one looks really nice.