# Ball mill ball sizes



## Dwf (Oct 5, 2020)

I have a 2’ diameter by 3’ long ball mill. It’s running pretty smoothly, and I currently have it running at 70% of critical speed. 
I got the balls used, and they vary from 2”-3”. I’ve noticed my production is pretty slow when taking it to -100 mesh. Even when feeding 30 mesh material, I’m only getting maybe 300lbs an hour. 
Is this normal, or would getting some smaller balls help with the output for a finer grind?

Any and all advice is appreciated!


----------



## Johnny5 (Oct 5, 2020)

I went back and looked through some of your other posts, and saw that you work with hard rock ore, is that what you're grinding? If so I think the size you are using is sufficient. However I would push the speed to full critical speed, to get the highest drop you can. Or you could swap to 4"+ rods and slow your speed way down. But I don't believe the rod method is going to increase your output.
I think you are getting a reasonable output for that small of a mill. I'm sure it can be tweaked to get some more, but I personally don't think you are going to get a substantial amount more, unless you increase the total size. 
My mill was 22.5" diameter, and about 3' long, but I never produced nearly the output you are getting, and I was only running ceramic cpus. I would have loved to have gotten the output you are.


----------



## Dwf (Oct 5, 2020)

I was running at just under critical speed, and then watched a video showing the different actions happening at different speeds. It looked like a better grind at 70%, so I slowed it down. I’m definitely getting more output at 70%. I was getting very little material at all coming out under 100 mesh at near critical speed.


----------



## snoman701 (Oct 5, 2020)

smaller media will be more efficient to produce fine particles as the packing efficiency is greater, increasing effective surface area of contacts that do the grinding 

larger material will be more efficient for bulk breakdown from large size, as it's mostly a matter of momentum causing fragmentation during impact

your speed may be a little fast at 70% of critical, for most efficient grinding. 70 is top end of grinding range (50-70) and approaching fragmentation/impact where balls are traveling maximum distance to impact.


----------



## Dwf (Oct 6, 2020)

Thanks for that advice. 
That makes sense 
I’ll try adding some smaller balls first, then slowing it down a bit.


----------

