14 replies to this topic

[phildunford]

I've had a couple of reports of ball mill explosions recently - fortunately no one hurt. We had a conversation about this at a recent management meeting & there is some information I thought I'd pass on.

Purity of chemicals:

It's pretty obvious that charcoal can easily be contaminated with metal particles. There can be nails/staples etc in trees & the sieves used & general processing can introduce metal. The producers will generally not care about this, as it does not affect the products primary purpose - ie to burn.

Less obvious is that fertiliser grade Potassium Nitrate can also be contaminated. Pyromanic303 reported extracting several grams of iron filings from a batch.

Now we can reduce risk by trying to remove or avoid such contaminants, but the bottom line is that ball milling can never be 100% safe. Therefore it is imperative that ball mills are run in a safe place where the consequences of an accident will not be too serious.

If time permits, we are hoping to run an article in the next issue of Spark showing the consequences of a ball mill explosion.

Please people stay safe!

[pyrotrev]

Well said Phil. I've never seen the effect of a couple of Kilos of BP going off in a tightly sealed jar with loads of lead balls, but I wouldn't like to be close when it happens: obviously an activity to be carried out outdoors well away from anything that could be harmed. It's struck me for a while that larger diameter and shallow mill jars (like some of the Italian/Maltese factories use) with a lid that can easily "pop off" might be safer than long small diameter jars with screw-on lids, though if it's good BP I guess it could make quite a bang anyhow.

Edited by pyrotrev, 09 September 2010 - 01:02 PM.

[dr thrust]

ekk!,so.. worth running a magnet over your green mix, ill try it next time just out of interest

[Vic]

I have said before a push fit bung is safer than screw type if there was a accident at least it's not confined like a screw top jar;
The flame will be going one way and the jar the other which is better than an all round blast

But as Phil has said, have a safe place to do it and a remote switch, sand bag it but allow for heat build up by having some ventilation gaps

Oh and make your own charcoal using nice clean timber and quality chemicals with the least amount of possible contamination.

[crystal palace fireworks]

I have said before a push fit bung is safer than screw type if there was a accident at least it's not confined like a screw top jar;
The flame will be going one way and the jar the other which is better than an all round blast

But as Phil has said, have a safe place to do it and a remote switch, sand bag it but allow for heat build up by having some ventilation gaps

Oh and make your own charcoal using nice clean timber and quality chemicals with the least amount of possible contamination.

I almost totally agree with everything you said, but would there be issues with leakage with just a push fit bung? or should some strong gaffer tape be used?

[phildunford]

I have said before a push fit bung is safer than screw type if there was a accident at least it's not confined like a screw top jar;
The flame will be going one way and the jar the other which is better than an all round blast

It's possible this might help, but I suspect, given the speed and power of a couple of pounds of BP, it would probably explode even if the lid was open...

[Vic]

I almost totally agree with everything you said, but would there be issues with leakage with just a push fit bung? or should some strong gaffer tape be used?

Yes do use tape. A reasonable interference fit bung made from 3/4 MDF with a jigsaw and a bit of sanding will do. Another piece glued and screwed on top to the outer diameter of your jar
and then once round with insulation tape will fully seal the opening end.

As Pyrotrev suggested keep the diameter of your jar the same as the the length. A 4”x4” mill will yield 100g of BP efficiently using lead media.

[Vic]

It's possible this might help, but I suspect, given the speed and power of a couple of pounds of BP, it would probably explode even if the lid was open...

Maybe some tests could be done to see.

[helix]

Not that long ago someone on Passfire had a ball mill explosion. From memory he was using an 8" plastic pipe with bungs for a jar and was milling 1kg of BP. The pipe seemed to have remained intact and the bungs blew out however his polytunnel (where he had the mill operating ) took a bit of a beating.

There was some discussion as to the cause it was thought to be due to impact between the powder and the media as the jar had pretty large lift bars, which resulted in the media falling onto the comp. I think the majority of the media remained fairly local to the blast.

It was mentioned that bungs with a tape seal over were thought to be the safest option to seal the mill jar. The other issue with threaded lids is that apart from the containment aspect, the friction between the threads when the time comes to open the jar could result in a dangerous situation arising.

I've been using bought hardwood charcoal recently (where I could get away without having to use the faster burning stuff I'd made myself) and was surprised with the quantity of bits of metal that I found - typically staples and nails/ tacks. As a result I now mill a batch of charcoal on its own and run a strong magnet through it to pick up any metal.

[dr thrust]

care to elaborate on the media type?

[helix]

care to elaborate on the media type?

He was using ceramic media, which I have to be honest I would not be comfortable using for milling BP.

the photos of the mill were posted on pyrobin so I dont suppose there's a problem posting a link:

(hope thats ok)

http://www.pyrobin.c...es/img_4006.jpg

http://www.pyrobin.com/files/img_4007.jpg

http://www.pyrobin.com/files/img_4008.jpg

http://www.pyrobin.c...es/img_4012.jpg

[dr thrust]

thanks helix,certainly food for thought, i'll be fitting a bung and checking for foreign metallic particles from now on, and wont be milling with lifter-bars and sparking media!

[helix]

I knew someone had already simulated a ball mill explosion, it was Lloyd Sponenburgh:

http://www.pyrobin.c...milling jar.pdf

[phildunford]

Quite a difference between Lloydes experiment and the poly tunnel explosion - that didn't even spoil his tomatoes!

[helix]

Quite a difference between Lloydes experiment and the poly tunnel explosion - that didn't even spoil his tomatoes!

I did wonder what the relevance of the tomato plant photo was but I left it there anyway!

Secondary confinement plays a big part in the overall result, the gasses from Lloyd's test are largely directed upwards (as the enclosure is contained within a pit) sending the witness box lid to a considerable height considering its lack of aerodynamics. That test used only 600g of powder!.

I suppose it comes down to avoiding risks where possible ie mill jars probably should have push on end caps, the jar body might be better made from MDPE pipe such as large bore gas or water pipe as opposed to PVC, you should be comfortable with your choice of milling media. It would then be sensible to take steps to reduce the risk of other factors causing an explosion such as powder escaping from the jar - could it be ignited from the motor due to a poor choice of motor i.e. commutator motor (either AC or DC) or an induction motor where mill dust could enter.

Also the mill really needs to be outside, it doesn't take a very large explosion within the confines of a room/ small space to cause considerable damage.