# the ball mill



## danec (Apr 21, 2012)

so from my post continuos ore roaster comes this question since a ball mill came up.heres a couple pics of a ball mill im working on.the idea is the ore and water come through the small hole visible through the pic of the big hole.rods will be inside crushing ore.im hopeing the water thats coming in will wash out the crushed ore of a certain size that will be determined by the size of the screen that sits at the end of the big hole.theres a 3 inch drop on the inside of the big hole to the drum surface.it will be filled with rods up to the level of the hole.my worry is that the ore will not be able to exit easily because of this wall even though there will be rods, ore and water filling it up.how do the bigger rod and ball mills get the ore up and out of the center.has anyone built something similar and have some opinions.i want this to run continuos also not a batch type.it will be drivin by an electric motor and gear reducer with a chain and a sprocket mounted on the face of the drum.any idea on turning speeds with it being a 12 inch drum.thanks.


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## AztekShine (Apr 21, 2012)

No pics bud...


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## Harold_V (Apr 22, 2012)

A few things aren't necessary. The huge discharge hole, for one. It can even be the same size as the intake hole, although having is slightly larger ensures that the intake doesn't get backed up. 

However, the large hole may be the only way you can access the interior. I'd suggest an insert (door) in the body instead. 

Discharge will be controlled by the input. You need not worry about anything making its way out of the ball mill. Simply keep the discharge side slightly lower than the intake, and don't use a classifying screen inside. There is no need. Do install something that will prevent large pieces from being discharged, or any of the balls or rods. I used an insert with ¼" diameter rods spaced on 3/8" centers, so there was a gap of 1/8" to permit the slurry to evacuate. You control how long you keep material inside by the feed rate of both water and product. 

I classified at the bottom of the long chute that showed in the picture of my ball mill, along with a tiny (coarse) screen directly under the discharge. It's safe to say that only about 5% (maximum) had to be fed back to the ball mill. I did not use rods, I used balls, sized from about 2" down to about 3/4". I was completely satisfied with the performance. It was *VERY* loud to operate. Hearing protection is mandatory.

Know that the speed of the drum is critical to good performance. If the rods/balls do not drop from the apex, you are not running at the proper speed. When you do, you do not need lifters, which are not exactly in your best interest because they can interfere with proper crushing action, and the same can be said for running the wrong speed, even with lifters. 

Do not over fill the mill with rods/balls. Keep the level such that when they drop, there is drum below, not more rods/balls. That way the crushing medium falls the greatest distance, imparting the most crushing action. Do not allow the rods/balls to roll instead of drop. That creates undue wear on the ball mill and yields poor crushing as well. Instead of crushing, the material is abraded, which is not the action you should achieve. 

I do not have the formula at hand to calculate the proper speed for a ball mill. I found the information in an old book, which I believe to be Rose's The Metallurgy of Gold. I may be wrong, as it was a long time ago (about 20 years). 

Harold


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## danec (Apr 22, 2012)

thanks harold.actually the end with the bolts is 2 pieces.i can take off the the discharge part and put in a screen or i can take off the the big ring and sweep everything out the end.or if i wanted a plate on the discharge and run it as a batch type.


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## rookieminer (Apr 29, 2012)

My brother just finished building a ball mill, It has a 24" diameter drum. He had rigged up a screen on the discharge end as well, but an old miner he had met told him the screen would only cause problems (it does have a rebar grate welded on to keep the large pieces, and balls from coming out). He told him to weld in a spiral, that would feed the larger pieces back to the ball mill, and the fines would flow out over the top of the spiral. We have'nt ran any ore through it yet, but it looks like it will work fine. I am a little concerned that it's not turning fast enough though, I will have to double check.


rookieminer


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## Harold_V (Apr 29, 2012)

rookieminer said:


> I am a little concerned that it's not turning fast enough though, I will have to double check.


That's something that warrants investigation. Proper speed is essential for efficient crushing. It will reduce ore at any speed, however, so long as the balls can't hitch a ride around the drum. As I've stated on more than one occasion, ore will be reduced by attrition, but it's a slow process, and does a lot of damage to the ball mill via wear. The balls really need to be dropped in order for them to work efficiently. With a 24" drum, it should work very well, assuming the drum has an acceptable wall thickness. 

Harold


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## rookieminer (Apr 30, 2012)

Harold_V said:


> rookieminer said:
> 
> 
> > I am a little concerned that it's not turning fast enough though, I will have to double check.
> ...



Thanks again Harold,

The ball mill is powered by a hydraulic motor. It has plenty of power so increasing the speed will just be a mater of changing the sprocket size. The drum has a 3/4" thick wall, and is lined with ruber. I would think that this is plenty thick, but I would like to hear your opinion.

rookieminer


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## rookieminer (Apr 30, 2012)

Hey danec, 

I don't mean to take over your thread, if I am bugging you just let me know, and I will move my questions to another thread.

Nice looking ball mill by the way, I can post you some pics of ours if you want to compare, or critisize it. 
I always view critisizm as help, so I am never offended.


rookieminer


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## Harold_V (Apr 30, 2012)

rookieminer said:


> The drum has a 3/4" thick wall, and is lined with ruber.


Lose the rubber! That's exactly what you don't want. It's important that the drum behave as an anvil. You do NOT want to cushion the falling balls, which will crush ore against the drum. It WILL BE NOISY! 

My ball mill had only a 3/8" thick wall and worked perfectly well. Yours should do a terrific job if you get it running the proper speed (and remove the rubber). 

Harold


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## rookieminer (Apr 30, 2012)

Thanks again Harold,

It was my understanding that the rubber would slightly decrees the output, but would increase the life of the drum. We got our rough plan from an article in Popular Mining. The article stated that a 3/8 thick wall would wear through after running 100 tons through it.

But if the rubber cuts down production by a large margin, it may be an acceptable expense. Since putting the drum together was the easier part of the build, and replacing it is just a matter of lifting the drum off, and setting the new one in place.


rookieminer


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## butcher (May 1, 2012)

Information is from Norbert Adolph Lange handbook of chemistry 1946:

Ball mill speeds

Diameters being equal the efficient speed of the ball mill depends upon the size and amount of balls or pebbles, slippage between the lining of the mill and the charge (consistency), and the size of charge.
In general practice the ball mill is first half filled with balls or pebbles, and then half as much enamel or glaze is added, and the required water if it is to be wet ground.
The speeds (below) are an average covering a large variety of materials and conditions 

Inside diameter of the ball mill in feet--- speed of ball mill in RPM 
(For dry mills use next higher value)

1'---42 to 55 RPM
1.5'---40 to 53 RPM
2'---35 to 40 RPM
2.5'--- 30 to 38 RPM
3' --- 26 to 30 RPM
3.5' --- 25 to 28 RPM
4' --- 23 to 27 RPM
4.5' --- 22 to 24 RPM
5' --- 20 to 23 RPM
6' --- 17 to 20 RPM
7.5' ---16 to 18 RPM
8' --- 12 to 14 RPM


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## nickvc (May 1, 2012)

I'm in agreement with Harold, lose the idea of rubber inside the drum, use it to coat some boards to insulate the mill when working and put some under the mill, not too much but it helps cushion the vibrations and reduce the noise level.


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## drifter (May 7, 2012)

What size ore are you feeding into the mill? 

only asking as i reckon 12" would be a bit small to do any primary crushing (worthwhile throughput anyway). if you are going 1/4 minus-ish feed then id certainly keep the rubber and lifters. You'd want the mill to grind not crush, just keep the charge level up to avoid excess rod wear. 

pick up a small 3 phase mtr (3/4 KW ? guessing) from the wreckers and a cheap single phase to three phase inverter drive to control the speed. $300 will get the lot, cheaper if you shop around.


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## qst42know (May 7, 2012)

Rubber on the outside might be a plus for the noise level.


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## Harold_V (May 9, 2012)

drifter said:


> What size ore are you feeding into the mill?
> 
> only asking as i reckon 12" would be a bit small to do any primary crushing (worthwhile throughput anyway). if you are going 1/4 minus-ish feed then id certainly keep the rubber and lifters. You'd want the mill to grind not crush, just keep the charge level up to avoid excess rod wear.


I'm not sure I follow your logic. Ball mills crush-----they do not "grind". If you allow the balls to roll and not drop, they don't work worth a damn. There's about as much wear on the ball mill as there is on the material being processed. Lifters, too, are not in anyone's best interest. Running the correct speed is always desirable, so the balls drop and crush---which is what ball mills are intended to do. 

My ball mill was about 18" in diameter and was fed ½"- material. Largest balls were about 2", with various sizes down to about 3/4", plus a variety of steel rollers that ranged from 3/4" diameter X about an inch long, and up to about 1" diameter. A ball mill with similar descriptions can easily turn out fine powder. 

Harold


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## Oz (May 9, 2012)

I think we are talking apples and oranges here (two different things). Drifter wrote “You'd want the mill to grind not crush, just keep the charge level up to avoid excess rod wear”. That sounds like a comment about a rod mill instead of a ball mill. They are two very different machines, that can give you a similar end product, but by a different mechanical action.


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## Harold_V (May 9, 2012)

Oz said:


> I think we are talking apples and oranges here (two different things). Drifter wrote “You'd want the mill to grind not crush, just keep the charge level up to avoid excess rod wear”. That sounds like a comment about a rod mill instead of a ball mill. They are two very different machines, that can give you a similar end product, but by a different mechanical action.


Good catch, Oz. 
Yep, caught me sleeping, it appears. 

Harold


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## Oz (May 9, 2012)

Ha! I should be sleeping, but am instead reading posts on the good ol Gold Refining Forum.


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## drifter (May 9, 2012)

Harold_V said:


> drifter said:
> 
> 
> > What size ore are you feeding into the mill?
> ...



ball mills can be designed to either crush or grind :mrgreen: 

by grinding I mean the sliding action of ball against ball with product caught in between, crushing would be as you describe - ball falling onto ball or the mill case. The lifters would get a larger proportion of the ball / rod mass moving against each other earlier rather that relying just on the surface balls cascading down the charge face. I could be wrong about it, but im not about to rip out the lifters in mine :lol: saying that, with your setup (crushing) I could see that lifters would be a waste of time - you need the balls to perform a very different function.


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## turtlesteve (May 13, 2012)

rookieminer,

I'm in the ceramic processing field and use ball milling frequenly. First I must say I agree with drifter's comment that ball mills can be designed either to grind or crush. However the mills are designed very differently for these two applications. 

For crushing I agree completely with Harold's comments and you should follow his advice. All of his suggestions pertain to mills designed for crushing large chunks of ore into a coarse powder.

A different approach is used to reduce a coarse powder to a fine powder. When the mill is designed for grinding or attrition, you want to maximize shear when the balls rub past each other. In this design the milling balls are often hard - alumina or zirconia ceramic - and are smaller (in my case, 1/8" to 1/4"). The container holding the media may be hard ceramic, rubber, or plastic. These changes minimize the wear problems that Harold commented on. Similar designs may be used for abrasive grinding, polishing, etc (rock tumblers, for example).

So hope this clears up some of the confusion. Don't mix design aspects used for crushing with those used for grinding, or you will have a mill that performs poorly at both tasks.

Steve


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## butcher (May 13, 2012)

turtlesteve, thanks good post, I would like to hear more from you about the differences in the types of ball mills.


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## turtlesteve (May 14, 2012)

Butcher,

Unfortunately for crushing I don't know much more than has been stated by Harold. For grinding I might be able to help, but I'm not really sure what utility this would have for this forum. In my case the milling is used prior to green forming of ceramic parts. 

One other thing I might mention is that for grinding, the size of the grinding media (balls) should scale with your powder - large media for coarser powders, smaller media for finer powders. It's kind of obvious that small balls are useless at breaking up large particles, but the opposite is also bad - large balls are ineffective if you want fine powders. In extreme cases I am aware of attrition mills (which use a stirrer in a pot, kind of like a food processor) that use 0.2 mm balls to produce powders down to 40 nanometers in size. 

I would assume ball size is similarly important for crushing, at least to some extent. Perhaps Harold or someone else can comment more?

Steve


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## Harold_V (May 14, 2012)

turtlesteve said:


> I would assume ball size is similarly important for crushing, at least to some extent. Perhaps Harold or someone else can comment more?
> 
> Steve


The experience I had was feeding ½"- to my ball mill. The largest balls were about 2" in diameter, with the smallest about 3/4" (I'm trusting to memory--it was long ago). 

It the process of fine crushing the ore, I had a somewhat larger piece that hadn't been crushed, but it had to be included in the lot. I would guess its size about about 1½". I inserted it in the ball mill, and allowed it to run while continuously feeding the crushed material. It became well rounded, but did not crush, instead being worn down slowly. When it was reduced to less than an inch, it quickly vanished, as it was then small enough to be crushed by the balls in use. I concluded that ball size was, indeed, very important. I also assume that because I had a mixture of ball sizes, along with rollers, that I was successful in reducing the ore to a fine powder. It was fine enough for the cyanide to extract virtually 100% of the assayed content, so I considered the entire operation to be a success. I concluded that one must use reasonably large balls if the material is too coarse, so, yes, in my opinion, size is critical. 

Thanks for the comments on fine grinding. I have no experience that parallels yours, but found your comments quite interesting, in particular, the idea that the mill is lined to protect against the wear I spoke of. I confess, that does make sense. 

Harold


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## drifter (May 14, 2012)

The next mill I build will be ball-less. Chuck some big chunks of rock in with the stuff straight out of the ground, maybe a few big balls but thats it. It will take longer, but will use less diesel and be alot easier to clean up. :mrgreen:


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