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Autogenous mill and semi-autogenous mill


View into a SAG mill for wet grinding


Autogenous (AG) and semi-autogenous (SAG) mills have been used for over 20 years for the coarse grinding of diverse ores (including gold, copper and iron ores). They offer advantages with regard to capital costs and operating expense compared to the secondary and tertiary crushers used in conventional size-reduction systems.

The installation of POLYCOM® high-pressure grinding rolls offers additional plant optimisation possibilities.


While AG mills operate without a ball charge, SAG mills use a grinding media charge of normally 3 % - 12 % for comminuting the feed material. The maximum feed size is between 200 mm and 500 mm.


The AG/SAG mill is normally installed upstream of a tube mill operating in closed circuit with hydrocyclones. The material discharged from the AG/SAG mill is either fed directly to the tube mill or first screened. If a screen is used, the screen overflow of the AG/SAG mill is returned for further grinding - either directly or via a crusher. The screen underflow is fed into the pump sump together with the material discharged from the tube mill.

The coarse material discharged from the hydrocyclones is normally returned to the tube mill, or in special cases to the SAG mill. The fine material discharged from the hydrocyclones (product) is fed to the downstream dressing process.

Polysius uses a design of circumferential bearing arrangement for AG/SAG and tube mills with straight end walls that provides clear advantages compared to the conventional mill design with trunnion bearing and conical end walls, such as:

  • elimination of critically large castings
  • lower space requirement and
  • process technological advantages

AG/SAG mill in conjunction with a tube mill and a crusher.



Grinding plant with SAG and tube mill for Gold/copper ore grinding in Australia. This is the world's biggest SAG mill equipped with sliding shoe bearing arrangement and COMBIFLEX® drive system (a favourably-priced alternative to the ring motors normally used with SAG mills of this size).


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