Rock and Roll: a simple ball mill

Jeff Davis June 18, 2006

In order to make my first batch of fireballs I needed some charcoal fines or dust. I charred some switchgrass and leaves to start with. That would be easy to reduce, but was unsure how to go about this task at hand. Having already removed the paddles from the cement mixer, I got the bright idea to place the switchgrass and leave char in the cement mixer plus one large rock, see picture #1. So I gave it a try, not knowing that there is such a device as a ball mill. I learned fast that a lid was in order! Dust cloud anyone? This worked well for the charred switchgrass.

Well, I did not have enough dust for my experiment so I robed my heathut, wood furnace, for some charcoal. I though that one could just crush it up with a hammer and save some time. I was more than surprised to realize how tough wood charcoal is! I learn fast that another method was needed. So I set the mixer up one more time, but now with lumps of wood charcoal in the mixer with my large rock, see picture #2.

It started to reduce the wood charcoal kind of fast but as some dust was made the dust production slowed. In other words the presents of dust seemed to slow down the reduction of the charcoal. Next I added one more large rock. This helped somewhat. I let it run for about one hour and decided to shut it off. Picture #3 shows what the end product looked like. Simply too many small lumps mixed up with the fines. Next I decided to screen the charcoal with a kitty litter scooper. Not a speedy operation but it got the job done and you can see in picture #4 what the lumps looked like. Finally I had my charcoal fines and they can be seen in picture #5.

After I reported my findings to the gasification list I learned that there is such a machine as the ball mill. Below you can read what Kevin had to say.

Kevin Chisholm wrote:
The problem with your charcoal crushing phase of the operation is that the speed of the concrete mixer is appropriate for mixing concrete, and not for operation as a ball mill. Mixing mills operate in the "Cascading Mode", where the materials tumble over themselves. Grinding mills operate in the "Cataracting Mode." If you speed up the mixer so that it is running about 75% of Critical Speed, it will operate in the cataracting mode, and the grinding behavior will radically improve. Basically, the rocks will get lifted by centrifugal force to about the 10:30 or 11 o'clock position, then fall away from the wall of the mill and come crashing down on the toe of the pile. The Critical Speed is Ncrit = 42.3/(D)^.5 where D is mill diameter in meters. Given that your mill is variable diameter, you might wish to use the largest inside diameter in the above calculation. You will probably find that it will work better with more rocks of smaller diameter.
Best wishes,

End of Kevin's quote.

Just a small note here: From the pictures the cement mixer looks like it has a variable diameter but in reality a large portion is constant.

Art wrote:
Ball mills, which is what you are duplicating with your rock in a cement mixer, function at higher size reduction rates when there are many more balls in them rather than just the one rock you have in yours. They grind best when allowed to rub against each other with your product between them.

I would predict that you could easily increase the throughput of your size reduction equipment by a factor of four or five just using a dozen or so 2 1/2 - 3 inch round rocks as balls in your ball mill.

You would need LARGE rocks to increase the grinding pressures between surfaces. It will take a long time and lots of power to grind plastic since it is flexible rather than rigid like cellulose. Ball and rod mills are used to grind solid products like rocks to the consistency of rouge.

Plastics are more difficult because they do not shatter under pressure. If you ground them in a bed of liquid nitrogen it would occur faster but at a higher cost.

End of Art's quote.

One more small note here: I tried to reduce wrapping plastic in this machine and it seemed to do a poor job.

A New Species of Fuel?

About six months later I got the silly notion to try to reduce retted switchgrass in my poor excuse for a ball mill. As you can see in picture #6 I found some smaller rocks for this experiment and more of them. Picture #7 shows the switchgrass particles flying out of the cement mixer. It seemed to work but I was loosing most of my switchgrass particle out of the opening of the cement mixer. Lacking the necessary time to make a temporary cover I decided to add some water in order to settle the dust as you could say. Also this would work in nicely with hydrogen bonding, but thats for another article.

Of course to start with I added too much water and the switchgrass and rocks just stayed put, lost traction, as the cement mixer turned. So most of the water was drained out. Picture #8 shows the operation in motion. It seemed to be too noisy so more switchgrass was added and that can be observed in picture #9. One could also observe some fibers being produced but the amount seemed low for the amount of time and energy being exerted.

I noticed some objects flying out of the cement mixer during this phase of the experiment. After closer observation they appeared to be small wafers of switchgrass. Picture #10 shows the first two wafers gleaned. They were still wet at this time. Please excuse my dirty hand that appears in this photo!

In picture #11 one can see them in reference to the size of a penny. The penny is in the center of the picture. The wafers are dry in this and the remaining pictures. Picture #12 shows a closer view. Picture #13 shows a close up of just one of the wafers. Picture #14 is just one more shot of the wafers in reference to the size of a US penny and picture #15 shows a group of wafers.

I do not know if there is a name for these objects made in a ball mill. If not I have thought of a few possible names: ball mill wafers (BMW), ball wafers, firewafers (after fireballs) or biowafers.

Testing these wafers will have to be left for another article.

Jeff Davis

Picture #1: Carbonized switchgrass is  the ball mill.Picture #1: Carbonized switchgrass is the ball mill.

Picture #2: See the article "Rock & Roll; a simple ball mill.Picture #2: See the article "Rock & Roll; a simple ball mill.

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