What caused a ball of pure black powder to form?

[TranslucentDragon]

So I'm just minding my own business on a Saturday and get in the mood to make some BP. I'm having fun, as entralling as that is, measuring out my materials, putting them in the jar, recording what I did, closing the jar, and turning on the ball mill. I think to myself, "I wonder what this will be like if I mill it for 24 hours?" and decide I'm going to do a 24 hour batch. I wait, and every four hours I shake the jar to try to disturb the clumps that always form on the bottom of the jar. After 8 hours it looks like it's gonna rain, so I turn off the mill and take everything inside. The next day I take it back out and let it run for 10 hours, again, shaking it roughly every 4 hours. After the 10 hours is up, I take everything inside, again. It's been a long Sunday, so I decide I'll just sift it through a screen later. Well... I left it sitting undisturbed for about 3 weeks (as I was on vacation for about 2 of those 3 weeks) and I open the top after all this time and I find a ball of pure BP (no lead balls in it or anything) just sitting on top of everything. And my question: what the heck did I do (have I broken physics)?

[LambentPyro]

I've gotten this a couple of times. It's usually from a high humidity in the air. However other factors such as not fully dried KNO3, or even milling with Dextrin binds the BP into a ball.

 

The only times I got this is when it was super humid out and it was down pouring rain.

[Bobosan]

Moisture.

[TranslucentDragon]

I live in South Florida... I'm assuming that means I have an excellent amount of humidity for this to occur. How would I stop this? I heard sun drying but I'm not really sure how to. Are there any other ways?

[Bobosan]

If you can use the kitchen oven you might pre-dry the individual chems seperately at a low temp before adding into mill jar and make sure the jar is sealed well. A wrap of masking tape around the drum where the metal lid joins should help reduce moisture pickup as well as keeping the contents from possibly leaking out. Once seperate chemicals are oven dried, store the chems well sealed with dessicant.

[leedrill]

i will say yes humidity is a factor but even when i dry my chems i can get this to happen im led to beleive than it will happen no matter what when milled for long enough so i usually put the cause down to overmilling

 

there is a point where it is serviceable a point where it is as hot as you need it and from then on your just running your mill for no reason

[pyroman2498]

ive never had this happen to me with BP, but its caused by humidity in the air and none dry chemicals. ive only had this happen with my sugar fuel used in my rockets , that stuff it hydroscopic , even more so than BP, and its a pain to keep it from not sticking.

[jessoman]

I have also had it happen if I overloaded the mill (only slightly) I live in a humid area also. When I was troubleshooting I simply tried slightly under fill my mill to standard/optimum 'specifications' try that? Edited February 9, 2014 by jessoman

[leedrill]

also as i said try optimum conditions with oven dried chems and leave in the mill for 24 hours + it just seems to happen milled kno3 cakes very badly and once everything is in my opinion finer than necessary it just seems to do it even though the char and sulphur are in there

[Coulterbart]

Most likely moisture in something. That plus the dextrin did the job.

 

I will add my dextrin in after I mill and just shake it into the mix. Will avoid water in other Chems activating it plus you will get a little smoother burn.

[leedrill]

id really like to get to the bottom of this phenomenon im still left saying as ive said in my previous posts that you can get this to happen even when every thing has been oven dried

 

id expect that it is still possible to happen in larger mill jars but id say a lot less likley for the way when it is optimised the angle that the media is falling isnt that much different but is has more momentum

 

id say masser is right but id see this as the main cause {overfilling the jar with comp allowing the snowball effect to happen the same way that happens when rolling stars but instead of it happening the media grinds itself clean and the small clump that forms manages to stay on top of the media because it is a lot lighter

 

so my thought still remains that it is from milling for longer than necessary and overfilling the jar

 

ive had it happen to me and was told it was from moisture so i dried the hell out of my chems and it happened again so id really like to raise the argument that this is not the cause

 

please prove me wrong but id say that if there is enough moisture to be the cause the dextrin would some what activate but when screening one of these balls it returns to what i assume much less than 10 microns in size { i dont have fine enough screens to test this though } but it does not clump again when stored after screened

[marks265]

Two other possibilities to keep in mind would be:

 

That your mill jar wasn't sealed when not in use. Moisture would be wicked into the media and residual amount of powder still left in the jar.

 

Possibly an excess amount of resin (or sap) in the charcoal.

 

Do you get the same results when milling back to back batches? If the 2nd and 3rd batches don't do this, then the media collected the moisture.

 

Have you tried a different charcoal or batch of charcoal? This might help to prove the theory of an excess resin in the charcoal possibly from not being cooked quite enough. Does this happen when milling pine only or does it do it with willow too?

[leedrill]

too me it has only happened with my paulownia and at 8 hours it is so damn fine its just noy funny as it should be but if i go over to the girls house and the mill runs for up to 24 hours it just seems to happen from time

 

but very good thought marks these could be contributing factors keep em coming

[Bobosan]

I use a 15lb jar with optimum media a chem charge and mill Paulownia for 4 hours. In the past when I milled using the all rubber HF jar, I would begin to see clumping form at the jar bottom and also on the lead media. Same 4 hour mill time, same chems. The 15lb drum is a rubber lined all metal clad (except where lid seal is exposed). I've seen the clumping begin to form on the lid seal where it contacts the chems but nowhere else nor on the media.

 

My conclusion is that the HF rubber mill jar is somewhat porous and allows moisture to weep inside during extended mill run time.

Edited February 10, 2014 by Bobosan

[leedrill]

i think i might have to oven dry the chems and charcoal, and put a desiccant in the mill jar with the media for a few days with a humidity test strip and mill for a couple of days to see if it can be replicated

 

ive done this without a desiccant in my mill jar and i never have my jars open unless loading or unloading but it might be a contributing factor

 

will be interesting to see a test like this with an optimised jar an over filled jar and an under filled jar all milled for the necessary time and then milled for way longer than necessary

 

having a listen too the boys playing with unmilled methods in can shells the effects of milling are obvious but the point made in ballmilling threads about milling to the point bp is serviceable and milling for longer than necessary becomes clear the point that confined milled bp's performance tapers off is a lot less time in the mill than i originally thought

[Wolverine]

I had this happen to me when I had that batch of moist sulphur last summer/fall. First when milling it alone it turned into a solid shell around the inside of my jar. Then when used in BP it made a ball just like that. (The fact that my jars were spun too fast may or may not have contributed to that too) A little oven drying of the offending chemical fixed it right up.
That said I vote moisture either in the chemicals or from the humidity in the air.
As directed by the folks here, I now check the weight of new chemicals I purchase to that of a sample known to be thoroughly dry and dry them if necessary.
In industrial ball milling it seems to be accepted that the finer a powder is milled the more prone to clumping it becomes so that could be a likely culprit or contributing factor as well.

[leedrill]

i do digress from my attack on the over milling or overcharging of the jar if there is moisture the it will happen for sure but id really like to see if it can happen with everything dried as much as possible cause from what i have done it still will with very dry chems if over milled

[Bobosan]

leedrill, what type mill jar do you use?

 

During pre-milling of prilled KNO3, it started caking and balling up after an hour mill run in HF jar. Milled for half an hour, no problems. I did this in my basement before the dehumidifier was setup.

[Wolverine]

i do digress from my attack on the over milling or overcharging of the jar if there is moisture the it will happen for sure but id really like to see if it can happen with everything dried as much as possible cause from what i have done it still will with very dry chems if over milled

I'm pretty sure no matter how dry the chemicals are eventually the particle size becomes so small that Van Der Waals force applies. If I remember correctly that is why even the driest powders clump. If that is the case there is plausibility in the over milling hypothesis as well.

I'm not sure where I remember that from or even if it's entirely correct but I leave it as food for thought.

Edited February 10, 2014 by Wolverine

[schroedinger]

Yeah your right, but van der Waal forces are very weak, and the particel sizes discussed here are around 10^-2 to 10^-3 to big for the van der Waals forces to work proper.

[Bobosan]

Ok, you guys got me researching and for what its worth from Wiki comes this;

 

"From the expression above, it is seen that the van der Waals force decreases with decreasing particle size ®. Nevertheless, the strength of inertial forces, such as gravity and drag/lift, decrease to a greater extent. Consequently, the van der Waals forces become dominant for collections of very small particles such as very fine-grained dry powders (where there are no capillary forces present) even though the force of attraction is smaller in magnitude than it is for larger particles of the same substance. Such powders are said to be cohesive, meaning they are not as easily fluidized or pneumatically conveyed as easily as their more coarse-grained counterparts. Generally, free-flow occurs with particles greater than about 250 μm.

 

The van der Waals' force of adhesion is also dependent on the surface topography. If there are surface asperities, or protuberances, that result in a greater total area of contact between two particles or between a particle and a wall, this increases the van der Waals force of attraction as well as the tendency for mechanical interlocking."

 

I am definitely not a scientist but the above description certainly sounds like it may account for the formation of balled BP occurring during an extended mill run.

 

Still, high humidity environments will also cause problems.

[Mumbles]

Van der waals forces are not going to adhere the BP into a ball. Basically all they'd be capable of is making the powder unable to flow smoothly, or appear a little static prone. If you pour granulated sugar from a cup, it will probably flow quite nicely. If you try to pour powdered sugar from that same cup, it'd be a no go. You have to tilt the container more, and eventually it will come out as a mass of powder at once. This is probably not attributable to van der waals forces, but the same concept. The rough surface is probably much more to blame.

 

This is probably most noticeable in atomized metal powders. Coarse atomized Ti will flow like water. This should be very obvious to anyone who has ever dropped any on the ground. Atomized Al for glitters will generally flow, but might stick up a little bit. Trying to pour very fine atomized Al, it's like trying to pour wet sand at times. It's much easier to just scoop it from place to place.

 

The clumping of BP is almost certainly due to moisture from some source. On the plus side, when my BP clumps, it's often some of the hottest BP I can make.

[Wolverine]

I knew Mumbles would know and would have an informative answer for the van dur bit . Thanks, Mumbles, I learn a lot from your posts.

Edited February 10, 2014 by Wolverine