Aluminum Milling Media

[Canadian_Pyro]

(No, I am not attempting to collect opinions on the process of

breaking large aluminum particles up into smaller ones...)

 

Well, since it seems the people over at rec.pyrotechnics are a bunch of

close minded lead lovers, I have come here in hopes of finding some

opinions.

 

A friend who is just starting out in pyrotechnics recently inquired to

me about media for a 6" mill jar that he was constructing for the

purpose of creating lift powder. I told him that he must use a

material that is guaranteed not to spark, and proceeded to list off

potential candidates such as lead and brass. I also mentioned the fact

that lead is highly toxic in quantities as small as tens of micrograms

per deciliter of blood, and can cause neurological problem, etc. You

can probably guess his reaction.

 

I use brass media for milling BP, which works very well, making

excellent lift powder in 3 hours. He however, is significantly

deterred by the substantial cost (Around $100) of filling his jar with

brass media, and asked if aluminum could be used instead, as he can

obtain several pounds of small ingots for free. I was unable to

provide what I would consider to be an accurate answer, which is why I

have decided to ask here.

 

Aluminum is quite lightweight, so milling efficiency would be

relatively low, and the powder would probably require a few hours of

additional milling time to match the quality of my powder. This I am

sure he can deal with, but aluminum's high reactivity had me pondering

potential safety issues.

 

The first issue is the introduction of aluminum powder to the BP mix.

As I understand, the majority of the milling action occurs due to

crushing impacts rather than the grinding of surfaces. This leads me

to believe that very little powder would be introduced, especially

considering that aluminum is of moderate hardness, and would not

likely chip as would a harder alloy, and would be less susceptible to

frictional wear than lead. However, this small amount of powder might

be capable of potentially increasing the ignition sensitivity of the

powder. Any opinions on whether or not this could be an issue?

 

The second issue involves the reaction of the Potassium Nitrate in the

BP with the aluminum media. Chemistry tells us that in the presence of

damp and basic conditions, aluminum may reduce Nitrate compounds, with the eventual product being ammonia. Does anyone know if media

corrosion would be an issue, or if the nitrate/aluminum powder (if it

is produced) reaction could cause accidental ignition of the powder,

considering the reaction is quite exothermic?

 

My friend also stated that he plans to cut the ingots into 0.5" - 1"

or so diameter cubes using a bandsaw, as this is most convenient. It

occurred to me that cubes could potentially be more effective than

spheres due to their greater surface area to volume ratio, and their

tendency to lift and throw each other as opposed to having a natural

tendency to roll. Thoughts?

 

I also considered that using larger than typical media could help to

compensate for aluminum's low density, and that the cubic shape and

numerous "catch points" would still allow for the necessary cascading

action that occurs most effectively when spherical media is 1/8 - 1/10

of the jar's diameter. This is speculation of course, and it would be

great to hear from someone who has actually used the type of media in

question.

 

Thanks for reading.

 

PS - Please don't tell me to use lead media. I've had enough people completely ignore my questions and instead suggest the use of lead, which is an evil, toxic metal that I would not willingly allow my friend to expose himself to. Also, telling me that lead "isn't that bad" will be roughly equivalent to talking to a wall, so don't bother.

[tentacles]

I would not personally use aluminum (and don't use lead) for milling media, but I can see a few potential problems..

 

Sharp points provide places that have far less strength, and will wear away faster, contaminating your product. Oh, and it would be hell on your milling jar, those sharp points impacting with that soft plastic...

 

Aluminum spheres or cylinders (with rounded edges) could last quite a long time, no idea on the efficiency though. It's very light and probably would never make great BP, though it could definitely make serviceable BP.

 

What's wrong with brass or stainless, other than cost? If he's not willing to put down for the proper tools, yada yada. I HAVE heard of using river stones to mill BP - like the rounded smooth ones. This is apparently an old school method. Just don't use anything like flint, or iron ores. Find some nice granite or something.

 

Your buddy is far better off finding some Mg to melt that aluminum with and selling off some MgAl to pay for some real media, stopgaps and half measures are just not appropriate for this particular area of pyro.

 

I do have a question - what is your basis for believing that lead is evil? All the lead I've met have been all right sorts, except for that one with the hat. Damn that bastard. But he wasn't evil.. just an asshole.

[mike_au]

I'm not a chemist so I can't comment on the issues you mention, but I can suggest another one:

 

When ever anyone talks about milling Al powder, they are warned that they need to let oxygen in regularly so Al-oxide will form a protective layer, otherwise opening the jar at the end could result in a lot of Al oxidizing all at once.

 

Since you won't have anything harder than Al in the jar it shouldn't make *much* powder, but I imagine corners scraping against sides and the like could expose a reasonable amount of new Al to the air.

 

If enough of it was exposed (since he will probably be running the mill for a while to make up for the low density, a lot of scratches could build up) it might be enough to ignite the mixture.

 

I'm very much a newb at all of this, so maybe there is some fundamental flaw in this thinking, but I would be careful.

 

How about brass coated lead?

[hst45]

don't tell me to use lead media. I've had enough people completely ignore my questions and instead suggest the use of lead, which is an evil, toxic metal that I would not willingly allow my friend to expose himself to. Also, telling me that lead "isn't that bad" will be roughly equivalent to talking to a wall, so don't bother.

..nice open mind...I've been loading shotshells, rifle and pistol round for over a quarter century with no ill effects......anyway.....

 

 

You seem to have already thought through, or even over-thought the issues, so I have no great bolt of knowledge to add. The bottom line is that aluminum is too light to be an effective medium, so just go with brass and be done with it. 1/2" brass rod can be had for a heck of a lot less them $100 for the pound or two that it will take to fill the mill jar; keep looking for cheaper sources. You shouldn't have to spend more than $20 max.

[tentacles]

Lead filled brass tubing is a good call, I forget about that option too often.. Just be sure to use a little flux so it sticks to the brass.

 

Water pipes used to be brass rather than pure(ish) copper. You might try a scrapyard for some, it should look a bit more yellowish than the usual pipes and it will be marked as brass. Just to note, we're buying brass at $1.70/lb right now.. Copper is going for $3/lb. (Winnipeg, $CDN)

 

edit:

hst: a 6" mill jar needs FAR more than a pound or two! More like 30-50.

[Canadian_Pyro]

What's wrong with brass or stainless, other than cost?

Cost is the only issue. We can't find a scrap metal dealer that sells "scrap" brass round stock at any less than $5/lb. Could you tell me where you purchase brass for $1.70/lb?

[Mumbles]

It only takes 30lb of lead to fill a 6" jar. I'd estimate that it would take around 20ish pounds, but that is a total guess.

 

They are probably getting such a good price on the brass due to quantity. It's unlikely they would be willing to sell you 20lb of brass at $1.70.

 

My personal opinion is that it is way too light to be suitible. I don't think reactivity would be an issue, but the density sure would. It is even lighter than ceramic, and that only gets away with it because it's so hard.

 

I can also believe the price. I just had a look, and from the few suppliers I was looking at, it'd be well over $100 for a 6" jar worth of media, and close to $90 just for the tubing to then fill with lead.

[tentacles]

What I meant is that the scrapyard I work at buys brass @ $1.70/lb. We don't sell anything, unless you're looking for a 40ft container full... And frankly, we don't get much roundstock of *anything*. We had a bunch of brass pin sort of things, 1.25" diameter and real shitty condition. But way too big for anything but a corning machine.

 

I probably have 25lbs of stainless so far, and it doesn't even fill my 6" jar halfway. There might even be more than that in there. But my 6" jar is a bit long, 11" I think?

 

My 8" PVC jars are 7.5" ID and will be ~9" long. 1.7 gallons! Should be big enough to do 2 kilos at a go.

 

edit: Yes, I AM gloating. I've put a LOT of work into this project. I still have to go buy some seals for the jars but I know a place that will have what I want. I think that given the amount of time I've put in so far, that I could have reasonably purchased media if I'd just put in the same time as OT at work.

[Canadian_Pyro]

Well, about the density, a 0.75" diameter aluminum cube has a roughly similar mass to a 0.5" diameter lead ball, so although milling would be slower, I don't think it would be a massive difference. I guess the only way to find out is to try it. I'll get some of the aluminum from my friend, and test it in my mill.

[asilentbob]

With all those crystals in granite I'd worry about the piezoelectric effect. I would not recommend milling with stones either.

 

A good solution is brass rod.. 1/2" to 3/4" in diameter rod can be bought in 3+ foot lengths at some home improvement stores and cut up into little cylinders. It would likely need to be de-burred a bit, but it should be pretty efficient.

 

And I feel his pain. I have a 15lb thumlers and I know I have put waaaay too much money into milling media... and I STILL don't have it up to optimum media fill... I use .54 caliber lead musket balls. I work with a respirator and nitrile gloves anyways so I'm not too worried about lead dust.

 

On a side note in the hopefully near future I'll have an even bigger mill completed... so yeah... I'll need yet more media again. I'll be going for 3/4" Brass barstock for BP milling. 1/2" through 1" steel barstock for magnalium milling. And some extra steel and maybe ceramic media for sulfur, charcoal, benzoates, etc...

 

I just hope the price of copper goes back down... Though I think theres about a 1 in 1,000,000 chance of that... Perhaps some gargantuan vein will be found...

[Frozentech]

I just hope the price of copper goes back down... Though I think theres about a 1 in 1,000,000 chance of that... Perhaps some gargantuan vein will be found...

Oh, a gargantuan vein has been found already. Here in Alaska, we are embroiled in controversy over development plans for "Pebble" mine. Estimates run up to 18.8 billion pounds of copper, 31.3 million ounces of gold, and 265 million pounds of molybdenum.

 

http://en.wikipedia.org/wiki/Pebble_Mine

 

OK, OT, I know, but it's a major issue up here. It is enough copper to notably effect world commodity prices for Cu.

[Mumbles]

I don't know what your fascination with the cube shape is, but I have a feeling it would just compound the inefficiency of the low density of the aluminum by further increasing the surface area. Less blow per impact type of thing.

 

Anyway, we're not saying that it wont work at all, perhaps just take 5 hours where lead can do it in 4. I do however feel you'd be much better off going with cylinders. The price for a 6" jar would run around $25 to $30, so it is not a devastating invenstment if it turns out poorly. You can always make MgAl with it like already suggested.

[jacob]

My local scrapyard is selling yellow Brass for about $2 per lb in small quantities, is this a good price?

[tentacles]

For buying it from them, yes, definitely. I know Bonny on here uses a lot of randomly shaped brass for milling media. Some of it is just melted down blobs, some is chunks cut from blocks, etc. But it does make good BP!

 

I'd suggest looking at their selection of stainless, but unless you've got a bandsaw and a bi-metal blade, don't bother. Brass works just fine anyways.

[Bonny]

If you can find any brass at all, it will work. My media is a mix of brass chunks mostly from old bushings broken up with an iron worker (sheared). I also have some cut up flat bar and even some high power rifle casings that I brazed full of brass. Might not work as well as rods cut down, but I have good results.

[jacob]

Ok thanks guys, that's what I was going to do, get a bunch of random Brass pieces and cut them up to my likeing, thanks for the idea Bonny.

 

I would love some SS but this is a very small scrapyard and they said they didn't have much, if any, I don't have a bandsaw either so maybe I'll try Ceramic for individual chems.

[Canadian_Pyro]

Anyway, we're not saying that it wont work at all, perhaps just take 5 hours where lead can do it in 4.  I do however feel you'd be much better off going with cylinders.

I considered that, but I find it likely that most impacts would occur at a single point on each cube, because it is unlikely that the cubes would align themselves perfectly parallel. But, if this proves to be wrong and milling efficiency is poor, we could probably melt the aluminum and cast it into cylinders, as it has a pretty low melting point.

 

As a side note, has anyone ever considered the process of high energy ball milling? Seems like it could help efficiency when using lightweight media.

 

http://upload.wikimedia.org/wikipedia/commons/4/4f/High-energy_ball_milling.gif

[Frozentech]

As a side note, has anyone ever considered the process of high energy ball milling? Seems like it could help efficiency when using lightweight media.

Read Von Maltitz - Blackpowder Manufacturing, Optimizing, and Testing. He goes over the pro's and cons of a dozen types of milling. It comes back to a trade off of safety and efficiency, which heavy media at optimum speed to get a good cascade provides. I.E., the basic lead media "Sponen-mill" ala Lloyd Sponenburgh. (Lloyd's book on Balling Milling Theory and Practice for Amateur Pyrotechnicians is an excellent read )

 

It only took one person blowing his mill up with a load of high-alumina ceramic media to make me a believer, so I only use that for pure chems, I use brass for day to day BP milling, but I do have lead media as well.

 

BTW, the person who's mill exploded was Steve Baron, aka "the charcoal guy" so I consider him fairly authoritative.

[Swede]

Is steel other than stainless simply "verboten?" There are very soft leaded steels (12L14) that I simply cannot see sparking, but I will definitely heed the accumulated wisdom of the hobby and industry and avoid it.

 

How about sintered tungsten carbide? There are very few easily obtainable materials with a density as high... it should make a killer media if it is non-sparking. The reason I bring it up is that I have seen BUCKETS if not 55 gallon drums of used tungsten carbide inserts that get used up by big machine shops and industry giants like Boeing and other aerospace firms. Could these be used? Talk about a grinding action - picture a jar full of mixed carbide lathe inserts!

[tentacles]

I've heard of carbide being used.. a bit expensive, we buy it for $8/lb... But the shapes would collect an awful lot of whatever you're milling. Although I've heard of using tungsten carbide dust as a fuel, I think? But also, stainless sawchips are sometimes used as a replacement for steel filings in fountains and whatnot.

[Canadian_Pyro]

Well, yesterday I cut up some of the aluminum ingots. Using a hacksaw, I created about 60 1" diameter cubes, a sufficient quantity to fill a 4.5" long, 5.75" diameter plastic jar to around 50% capacity. Each cube has a mass of ~40 grams, similar to that of 0.75" diameter lead balls. Even though this is satisfactory, I had my doubts, so I swapped pulleys and geared my mill up to run at 90% of the calculated critical speed (previously 65% for my brass media). This is pushing the line at which centrifugal force will prevent cascading of the media, but upon starting the mill, it was evident that cascading was still occurring, and much more violently so.

 

The edges were sharp, so I filled the jar to 75% capacity with these cubes and some powdered flour. I figured the flour would help to protect the jar as the sharp corners were rounded off. I was wrong. After an hour, the cubes were smooth, but the jar was completely destroyed; cracked in several places and leaking clouds of flour. The flour had also acquired a gray tint. I threw out both the jar and the flour.

 

After finding another identical jar, I decided to attempt to mill some prilled KNO₃. I let the mill run for an hour, then stopped it. To my surprise, a cloud of white dust floated up from the jar when I removed the lid. The KNO₃ was as fine as flour. No ammonia smell was present in the jar, and the power was still white.

 

I have yet to attempt to mill BP with this media, but it seems to work well. To the people with lead and brass media, how much time is generally required to mill prilled crystalline substances to fine powders?

[Frozentech]

10 minutes or so, using 3/4" brass hex stock cut to 3/4" lengths,

 

Can you try with some lump wood charcoal, starting with about 1/2" x 1/2" chunks ?

[Canadian_Pyro]

Interesting, I will try milling some more KNO3 for shorter time periods to see where the threshold is. Your media is similar in mass to my aluminum cubes, but you probably have a larger amount of surface area in total, so I imagine it would take my aluminum longer than 10 minutes. I have never milled anything with brass or aluminum for less than an hour, so I assumed both took somewhere around there. Guess I am over milling with at least brass.

 

I'll try some charcoal tomorrow, and maybe some BP, but I need to pick up more sulfur.

[Mumbles]

My severely undercharged mill with lead took lump charcoal from a mixture of 1/2" chunks and -6 mesh granules to airfloat in a period of about 10 min. I was very suprised actually. This makes preparing burst MUCH easier. You have no idea how much time I have wasted trying to grind charcoal in blenders, coffee grinders, and grating it through screens in order to make the 200g I needed for burst. Well over 2 or 3 hours per batch. I will now just coarsely grind the charcoal, and toss them in the mill. The mill was even over charged, about 1/2 full of charcoal, although it is pretty light weight. My fingers are happy they no longer have to grate charcoal, it really stars to hurt.

 

I'm starting to think that aluminum might be a viable option for larger mills, where they would start to have the neccesary mass to break up the KNO3, charcoal, etc I am still rather skeptical about smaller ones though. It would certainly be more attractive if they didn't tear up the jar on the first run. Then again 15 minutes with a grinding wheel should probably take care of it. Using a more sensible shape would also help. Cube has too many sharp corners, and poor grinding geometry.

[Canadian_Pyro]

Mumbles, what dimensions would fit within your definition of a "larger mill"? I think aluminum would be quite ineffective in mill jars of diameters around 4", aka the rock tumbler mills that so many people use, but in ~6" or larger diameter jars where 7/8" - 1" media will still provide good cascading action, it seems like a decent alternative.

 

I realized afterwards that I could have simply used my belt sander to remove the sharp edges and make the cubes more resemble balls rather than taking the lazy way out and letting the mill do the work. Oh well, a 1 gallon jar still needs about double the amount of aluminum as this test jar for a proper charge, so it won't happen again.

 

Cubes are a convenient shape - it took me about half an hour with a hacksaw to cut 60 cubes out of 5 1 pound aluminum ingots. Once the corners are rounded, they seem effective. Possibly more effective than spheres due to a higher surface area to volume ratio, but I don't have empirical evidence to prove the validity of this speculation. Spheres are guaranteed to impact each other at a single point, thus creating maximum pressure given a fixed applied force, but I would expect cubes, even with their flat faces, to behave similarly. Due to the tumbling nature of the media, perfectly parallel impacts of media faces seems unlikely.