# Grinding & Classification Circuits

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## Rod Mill Tangling and Minimum Rod Makeup Diameter (7 replies)

B
Brendan158
1 year ago
Brendan158 1 year ago

Hello Mets,

I'm looking for information around rod mill media size selection to prevent rod tangling. There is the rule that the length of the mill/rod should be 1.4 to 1.6 times the diameter but there is very little information on if there is a minimum diameter of the rod makeup size to use to prevent tangling.

My understanding is that makeup rods need to be a sufficient size to break the worn rods into small pieces so that they won't tangle. What would be this minimum size to ensure breakage? Does it depend on rod mill length/diameter? i.e. is the minimum diameter rod the same for a 2m long mill as a 5m long mill?

This isn't an issue generally come across as rod media size is usually estimated from the Allis-Chalmers equation, however, it is not able to be used in this case as the rod mill is being used as a secondary grinding mill (after a primary rod mill, F80~1mm), and the estimated rod size from the equation is much smaller than can be practically made.

I know secondary rod milling is inefficient grinding but there are product size distribution reasons downstream for it. Hence, looking to improve efficiency as much as I can by decreasing current rod makeup size but need to know what the limits are and avoid tangling.

Any help would be greatly appreciated.

Brendan

David
1 year ago
David 1 year ago

Brendan,

What is your rod charge level? How low are you operating at?

B
Brendan158
1 year ago
Brendan158 1 year ago

Fairly standard 30% with design maximum 40%

SmartDog
1 year ago
SmartDog 1 year ago

A rod mill length that is only 1.4 times the diameter is pretty short.  The diagonal is 20% longer than the rod length which can lead to tangling fairly easily.  My general recommendation is to have the mill 2 times the diameter.  This can mean for a given feed rate the diameter can be less allowing for fewer, but longer rods.

The size of the make-up rods is dependent on the feed size of the material being ground, and the desired product size.

B
Brendan158
1 year ago
Brendan158 1 year ago

The 1.4 to 1.6 length to diameter ratio was what was recommended by Rowland and Kjos in 1980. A L/D of 2 would probably be safer but not possible for us as the length of our mill is already 20ft and they won't make straight 30ft rods. Not that I can change the design now.

If I use the suggested make-up rod size equation from Allis-Chalmers, our secondary mills require 12mm rods. Obviously this isn't possible but I would like to decrease the rod diameter from our current 90mm if I can avoid rod tangling.

The question is what minimum diameter rod size could we safely go down to? Considering the feed F80 is 1mm, the mill length is 20ft, and mill diameter is 15ft.

Thanks

David
1 year ago
David 1 year ago

You are feeding 1 mm to a Rod Mill?

I keep my rod charge high (higher) to eliminate rod tangling.

SmartDog
1 year ago
SmartDog 1 year ago

With a L/D of 1.333 (15/20) you are already below the 1.4 you mentioned.

B
Brendan158
1 year ago
Brendan158 1 year ago

How does make-up rod diameter affect the risk of tangling?