Causes of ball mill accidents?

[Sulphurstan]

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.

Yep, the rapid cooling is an interesting point. I usually take my paint can - VERY thin steel indeed - out of the fire, and put it directly outside (right now, we've around 5°C), so the cooling couldn't be quicker !

 

As Boophoenix stated before, stainless steel should meet the goal, just need to figure out, how I can find a stainless steel Lid, that's fit the stock pot airtight (I just punch 2 holes in the lid, to prevent oxygen sneaking in).

[Sulphurstan]

 

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.

First time I retorted my charcoal with wood sawdust, and after having seen these iron or Fe3O4 flakes, I know that I'll always use chunks, nothing else! And sieve the dust out + magnet. Anyway, I'll try to go for stainless if possible.

[NeighborJ]

I cannot find any info to exactly determine the temperature of the Fe oxide formation but from experience the reactiom seems to stop once the metal stops glowing. I often work in steel mills and from watching the giant steel slabs roll off the slab caster i can say that the formation of Fe3O4 can be seen and observed flaking from the steel short after it has been poured and up to the point it no longer glows. At these temperatures 2400-2000f steel will burn off in thin layers much like the reaction of steel wool to form Fe3O4. I say to use thicker steel because it will more easily conduct the heat away from the point of aplication and provide a better control of heat to prevent over heating.

[Mumbles]

I actually can't find any reference to magnetite actually sparking or burning itself. Do you have anything handy? There is plenty of mention of hot metal particles burning up to magnetite in dry air from grinding and welding however.

[NeighborJ]

Mumbles im not certain the magnetite burns, its a product of steel burning thou. I believe it sparks and branches because it is hot, then it breaks apart upon contact with cool air(thermal shock).

It just occured to me that your question reffered to magnetite sparking in the mill. I have no idea but it is ferrous and as such it can transfer static electricity. In this case id rather play it safe. No, I havent found any research or data.

Edited January 30, 2017 by NeighborJ

[Sulphurstan]

Mumbles im not certain the magnetite burns,

IMO, magnetite wouldnt burn, since already strongly oxydized, rather it can be an oxydizer (some thermites are made with Fe304 instead of Fe203) but, as mentionned, it is ferrous, can surely transfer static electricity, that's why it's good to avoid this material at all costs in the charcoal, if ball milled as BP afterwards in a ball mill.

[Mumbles]

Why would being iron (II) i.e. ferrous, introduce static electricity? I think you may be confusing the chemical term ferrous with the more broadly used adjective ferrous meaning iron containing. That's not to say that either has anything to do with static electricity though. Magnetite itself should not really be dangerous or anything more than a contaminant in black powder. It's not at all uncommon to add iron oxides to pyrotechnic compositions like glitters, rocket fuels, smokes, or other things.

 

If stuff is flaking off there is always the chance to bring metallic iron or steel with it. This is why I always discarded it. Passing a magnet through charcoal is always a good idea to remove things like staples, nails, screws, or other metal shards that may have gotten embedded in the wood or charcoal source over time. This would also protect against flakes or what not from the retort or TLUD. When using small retorts like coffee cans, I always lined the inside with aluminum foil. It made cleanup easier as I could just toss the tar soaked foil afterwards. It would also prevent rust or steel from flaking off, though this was not really on my mind when I was cooking such small batches.

[Sulphurstan]

Why would being iron (II) i.e. ferrous, introduce static electricity? I think you may be confusing the chemical term ferrous with the more broadly used adjective ferrous meaning iron containing.

What was on my mind, regarding the static electricity issue: Fe304 is conductive, so I've been thinking that, with the friction of the milling media, it might accumulate some charges. I've read, long time ago an article about" capacitive behaviour of nanocrystallites of magnetite" (was with liquids electrolytes). Static electricity is probably different from the "regular" electricity generated within electrolytic solutions, now, what I miss is if potentially capacitive particles can charge up with static Electricity?

 

Regarding Aluminium foil I was thinking of doing the same (for the same reasons), but thought, that the heat would just melt it down. SO I'll make my next batch with a foil.

[NeighborJ]

Sulfer I've read an article specifically about milling magnetite. It states that it will phase on a nanometre sized scale from Fe3O4 to hexagonal hematite(a-Fe2O3). These phases can be characterized by X-ray diffraction as well as by transmission electron microscopy which will show interfaces in the nanoscale between the magnetite and hematite phases. The conduction of these nano interfaces are described as being a hopping conduction.

Whheww, OK so long story short I believe there is a static discharge possibility if not a likelihood.

Edited January 31, 2017 by NeighborJ

[lloyd]

For those concerned with static accumulating INSIDE a mill jar, I ask you to consider this:

 

Static charges are equalized by providing a conduction path between areas of differing charges. In the case of 'safe' discharges (that is, allowing accumulated charges to equalize without producing sparks), we typically insert a fairly high value carbon resistor between the charged bodies.

 

Thus the incarnation of the 'static leash' protection method.

 

Now... inside a mill jar that's milling black powder, there exists a "carbon resistor", well-distributed among the other ingredients, and touching the jar's walls.

 

The entire jar may charge up relative to its surroundings. The contents will not develop a charge relative to areas in the mass or relative to the jar wall.

 

Lloyd

[Ubehage]

Now... inside a mill jar that's milling black powder, there exists a "carbon resistor", well-distributed among the other ingredients, and touching the jar's walls.

 

The entire jar may charge up relative to its surroundings. The contents will not develop a charge relative to areas in the mass or relative to the jar wall.

 

Lloyd

So.. Let me make sure I understand this correctly:

When milling BP (with "safe" media), there is no way a static charge will make it go boom as long as it's still closed up?

 

Other factors may come in, but the BP and media alone won't cause a risk?

[lloyd]

Charcoal dust is conductive (up in the tens of mega-ohms per square, but still conductive). It is my opinion that no 'localized' charges can build up on the contents, or between the contents and the jar wall.

 

Lloyd

[lloyd]

Ubehage,

A further issue helps with the matter.

 

Let's say for a moment that BP components had NO conductivity... that each particle could develop a charge and hold it.

 

The individual particles are being rather rapidly mixed, dispersed, and distributed (more-or-less evenly, or you wouldn't get a uniform 'grind').

 

So... even IF individual particles could hold charges relative to adjacent ones, they would all be so thoroughly 'mixed up' in the mass so as to produce the same effect of no localized charges. And, the individual particles are so small and close-together, that they would swap and share charges via 'field effect', even if there were no actual conduction going on.

 

Lloyd

[NeighborJ]

Id have to agree with you Lloyd about the possibility of the capacitive charge bleeding thruought the comp thru the carbon. I still have another concern. While reading thru various technical articles about milling magnetite i found one which discussed usind argon in the milling jar to prevent the hematite from forming in the first place. And what they found is that the hematite would form almost imediately when the jar was opened and exposed to oxygen. Now I do not know if this nanostructure phasing is an exothermic reaction but I do have concern that it may be, So if during the milling process all the oxygen inside the jar is used up by this reaction an more is needed to complete the phase conversion. Is there a possibility of creating a pyrophoric reaction once the jar is opened? Would this oxygen starved reaction be able to pull oxygen from the KNO3 in the comp?

Im really not trying to be difficult, I want to try and undrstand all the possible risks.

[lloyd]

Neighbor,

I'm going to pull "empiricism" on you, rather than theory.

 

I've kept as good track as I could of all the various ball-milling accidents in the hobby, because it's a subject close to my heart.

 

I have never heard of such an ignition upon opening a jar, and I'm pretty sure many hobbyists have iron/iron-oxide particulates in their charcoals -- if for no other reason than they did not know that the retort was sloughing-off particles (and never bothered to check).

 

The only accidents I'm acquainted with have had to do with jars opening during milling, and depositing the powder on friction elements like bearings, OR from high-impact of hard media, from over-large jars. (or my own, from trying to mix Mg dust and Cab-O-Sil in an air atmosphere).

 

I've not ever heard of nor had any ignition simply from allowing air into a jar.

 

Of course that does not mean "it can't happen", but I'd think with the numbers of hobbyists who are milling with home-made charcoal, we'd have heard of such, if it had.

 

Lloyd

Edited January 31, 2017 by lloyd

[Mumbles]

Do you have any links or copies of these technical articles? I'd be interested in reading more about this.

[Sulphurstan]

Neighbour, lloyd. these explanations are very interesting, because they rely on physical behavior and logic, I love that... quite technical but this is why Pyro is so fascinating: reading a lot, trying to understand what is happening in our mixtureS, how they may react when "disturbed", and how they could harm us, sharing ideas, and of course trying the practical way also.

 

This forum is definitely a good place I landed on, 3 months ago!

[lloyd]

Mumbles,

No. I always assume an "accident link" will be deleted when the victim (perpetrator!) decides it's not a good form of defense to leave it up.

 

So I take note - sometimes (rarely) make a few notes - and go on with my mental assessment of it.

 

There have been a couple noted on Passfire.com, one or two on Fireworking.com, and a few communicated to me by emails from curious or affected individuals.

 

Overall, it's been of little consequence to the art. There have been few injuries (mine, among them), with most of the damage restricted to the mill and/or its enclosure. There really have not been many -- perhaps six or eight (that I've been made aware of) that were documented since I first published my short treatise in 1995.

 

Lloyd

[Mumbles]

Sorry, I should have been more specific or at least addressed whom I was talking to. I was referring to the technical articles about magnetite that NeighborJ has been talking about. Just personal curiosity more than anything.

 

Anyway, I agree about the accidents. I tend to keep details for myself or at least mental synopses of what happened from the ones I'm familiar with. With risky hobbies there are bound to be mishaps.

[NeighborJ]

I'm sorry Mumbles yesterday when I stumbled across the article I read it and took a few notes and anticipating your interest I attempted to copy the URL but I'm a dumbass when it comes to computers and I lost it somehow. I've been searching for the article but haven't found it as of yet, there are thousands of scientific articles involving milling of magnetite but that one seemed the most credible with all the testing to back up it's claims.

[Guest]

This is static buildup OUTSIDE the jar. I have no idea or opinion on what's going on INSIDE the jar, but the two phenomena should be separated as long as there is no conductive path from the inside to the outside.

This is serious. Maybe industry-disrupting, stickyworthy serious. IDK. You decide.

 

 

The ground prong is connected directly to earth ground right at the ground prong, to a 16 inch steel rod driven into the earth. I know because I was using an ungrounded extension cord and thought that might be the culprit. Nope. The original video shows all this, but the upload limit made me have to edit.

 

The humidity was high, it was muggy and there was dew on the ground. RH or absolute humidity idk, but it was sticky muggy.

 

The media was a mix of 3/8, 5/15, and 1/4, 1:3:9 ratio respectively, total 3 lbs per jar with 100 g COMPLETE BP mix per jar. (that's why i had it outside.)

 

The rubber feet were clean, insulating the frame from the aluminum tool box it was in. The only point of contact between the jar and frame was at the bearing. (rollerskate bearing, bushing and bolt. Super easy mod, I should write up a tek.)

 

MadMat has it right with the VanDeGraff generator.

Edited July 30, 2021 by Guest

[Guest]

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.).

 

^^^This^^^

 

I had an idea for a way to equalize the potential differential that exists between jar and frame. I like the ESD brush idea, but copier brushes may be a little hard for people to get to... TENS electrodes! They are designed for static-like voltages, have a sticky conductive pad, are cheap and easy to obtain, and easy to implement.

 

Scrape off a little paint from the frame where you want to stick your pad, cut the plug off, strip back the insulation, fan out the carbon-type filaments, stick it on, and easy ESD brushes...

Edited July 30, 2021 by Guest

[MadMat]

There is a very inexpensive way to make a decent static electricity discharge "brush" or "comb". Buy some brass or copper shim stock at your local hardware store. A good thickness is .010-.015". You cut out a piece of the desired size and then cut your comb edge with a pair of pinking shears. Pinking shears are available where ever sewing supplies are sold. they cut a triangular saw tooth pattern and I do believe that the local Wally World even carries them in their arts and crafts department. I found this out in a book named "Homemade Lightning", which had plans for building a Wimshurst static generator. There are a number of ways to mount this brush, the simplest of which is to simply solder a wire onto the edge opposite the saw tooth.

Edited August 23, 2021 by MadMat

[Guest]

What do you think about the potential for micro-arcing (for lack of a better phrase) between the discharge brushes and drum, though? At this point, I would have to suggest not doing any of these modifications until further research is done. Also an in-line bleed off resistor? I haven't checked the resistance of the fibers in the tens pads leads, but I know for a fact they aren't copper. Some sort of carbon, as it is IMPOSSIBLE to solder them.

 

Wouldn't it make more sense to just use some conductive rollers, and ground the shaft to the frame? Eliminate the static buildup, makes dissipation unnecessary...

 

 

on a side note, I think it's awesome how the frequency of an electric arc can be modulated so it makes music. Interesting parallel to tuning a comp's combined emission spectra for a desired color.

Edited August 23, 2021 by Guest

[MadMat]

There doesn't need to be any "micro-arcing" for a charge to be bled off of the milling jar. The migration of charged ions in the air will accomplish discharging the milling jar. What is necessary is that collector have sharp edges/points to facilitate the discharge.