Crushing, Screening & Conveying

Crushing, Screening & Conveying

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SAG mill VS Ball & Rod Mill (2 replies and 2 comments)

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Nikita Gupta
4 years ago
Nikita Gupta 4 years ago

I would like to understand why do companies use SAG mills instead of Rod mills for grinding these days? Apart from the alleviation of need for secondary and tertiary crushers, is there any other reason for inclination towards the installation of SAG mills. 

Mike
4 years ago
Mike 4 years ago
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2 likes by Eder Figueiredo and Nikita Gupta

The main reason is capacity versus plant area.  Rod mills have a major restriction in length to diameter, they have to be significantly longer than their diameter to prevent the rods from tumbling and turning into a twisted knot.  SAG mills are the opposite, they generally have a larger diameter than their length.  This gives them an inherent higher capacity for a given length.  Rods are also generally more expensive than balls and harder to handle.  This means a rod mill equal in capacity to a ball or SAG mill would take up more plant floor area requiring a larger building.

Also since SAG (semi-autogenous grinding) uses less media than a comparable rod or even ball mill.  While less efficient in actual grinding SAG mills are lower capital and operating cost per unit ground, especially at large particle sizes.

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Nikita Gupta
4 years ago

I actually did a cost benefit analysis and found that for use, the per unit cost was lower in the stream that used Rod Mills as compared to SAG mill. Is this an anamoly?

Alex Doll
4 years ago
Alex Doll 4 years ago
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1 like by Nikita Gupta

In addition to issues described in the earlier comment, you can make SAG mills bigger than rod mills.  The largest SAG mills consume around 25,000 kW whilst the largest rod mills consume less then 2,500 kW.  It is better to install one large SAG mill than 10 small rod mills.

https://www.sagmilling.com/

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Peter Murphy
4 years ago

Surely its got to do with the feed size and product reduction size required?
SAG mills are used predominantly with feed material with a larger max lump sizes (from primary crushing plant typically in the max lump size range 150 mm to 300mm and a psd to follow) and the ball or rod mills are for smaller feed lump sizes. The final product size depends on the retention time and grinding capability of the type of mill. The finer the grind size equates to the decision of a Ball, Vertimill, SMD (e.g. ISAmill).

The pairing mill combination of a SAG/Ball mill is common in copper/gold mines for that reason. This combination can aim for a fine product sizes are around typically 100-200 micron whereas a Vertimill or ISAMill are finer at 20-40 micron or smaller.

ISAMill was developed to achieve P80 of 7 micron. Depends on the downstream processing requirements and the P80 particle size required.

Some processing plants need less fine grind size than others. Usually determined by testwork. Every ore commodity has different requirements. Copper sulphites versus oxide as example.

In one particular process plant I know about, the Bauxite milling plant has feed sizes of 25mm and therefore Rod mills are used. Its an older plant but still operating.

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