Coles Associates offer a selection of different sized and powered SAG mills suitable for a range of applications including;
- Single stage SAG milling
- SAG milling before a pebble crusher in closed circuit
- SAG milling combined with secondary ball milling Read more
OUR EXPERIENCE
We have over 40 years experience designing, manufacturing, installing and commissioning SAG and Ball Mill grinding equipment. We have 25 years experience procuring and re-applying used grinding equipment to new mining projects, often with significant lead time and capital cost savings over the purchase of new equipment.
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The objective of grinding material is to achieve a size reduction necessary for liberating valuable minerals that can be extracted in downstream processes.
SAG and AG mills (Semi Autogenous Grinding) and (Autogenous Grinding) are often operated as single stage primary grinding mills or are configured with secondary ball milling, usually operated in closed circuit with hydrocyclones.
In the case of Autogenous grinding, the ore itself is used as the grinding media, while SAG milling will use the ore plus additional grinding media in the form of steel balls.
One advantage of AG or SAG milling over a traditional rod / ball mill circuit is the elimination of two of the stages in a three stage crushing circuit thereby saving the capital and operating costs, including the screen maintenance.
AG / SAG mills may be run using run of mine ore screened through a grizzley, but more typically use primary crushing. AG / SAM mills are able to accept much larger feed sizes than ball mills and are often only limited to the practical limits of the SAG mill feed chute opening.
AG / SAG mills can have a particular advantage when used on ores that are extremely abrasive. In this case the AG / SAG mills replace traditional circuits employing three stage crushing and ball milling because the traditional circuits experience wear rates that are excessive in terms of the crusher and mill liners and also the grinding media employed by the ball mill leading to high operating and maintenance costs.
AG / SAG mills also have an advantage when treating sticky, clay ore types which would prevent the operation of traditional three stage crushing and screening circuits.
In metal mining, the majority of grinding is conducted on a wet basis meaning that water is added along with the mill meed material . The AG / SAG mill will be fitted with discharge grates and in the case of trunnion discharge mills they use pulp lifters behind the grates to remove and discharge a slurry product from the mill. In South Africa end discharge mills are sometimes used where the mill is only fitted with a discharge grate and no pulp lifters.
In some sulphide roasting applications, and some whole of ore roast plants AG / SAG mills are operated on a dry basis. The mills are air swept to remove the ground products from the mill which are classified in a cylone classifier. Oversize particles are returned to the mill. The liner design in dry AG / SAG mills are very important.
High Aspect Ratio AG / SAG Mills
US and Canadian operators have tended to select autogenous mills that have a diameter that is 1.5 to 3 times the length of the AG / SAG mill. These are mills are termed high aspect ratio mills.
Square Mills
Square mills are when the diameter of the AG / SAG mill is roughly equal to the length.
Low Aspect Ratio AG / SAG Mills
Many European mill practices have operated low aspect ratio AG / SAG and pebble mills that have a length that is 1.5 to 3 times their diameter. Low aspect ratio mills are also common in South Africa, and in some operations in Australia.
The method of size reduction in the SAG mill can be characterised by three main breakage mechanisms: abrasion, attrition and impact.
As material enters the mill by the feed chute it is lifted by the rotation of the shell of the mill as the mill charge is lifted by the lifter bars. Motion within layers in the mill charge will cause abrasion and attrition interactions, while at the toe of the charge impact breakage can be found.
Abrasion in SAG / AG Mills
Abrasion is caused by inter particle contact between two particles leading to small pieces of their surface being removed. The contact between particles breaks pieces off, leaving a diminishing core of the two larger rock particles and the smaller products. The interaction occurs in a parallel plane.
Attrition in SAG / AG Mills
Attrition is caused by the interaction between three particles, two larger particles and one small particle trapped between the two larger particles. The smaller particle is broken as a result of this interaction. As with abrasion the interaction occurs in a parallel plane.
Impact in SAG / AG Mills
Impact is caused by an one particle impacting another in a direction perpendicular to one another. Impact breakage is dependent on the amount of energy that the target particle experiences in this interaction. The JK Tech drop weight test is one key method used to quantify impact breakage in tumbling mills, particularly autogenous mills and SAG mills.
Ore Characterisation Testwork for AG / SAG Mills
Clients will generally conduct their own ore characterisation tests, standard bond work index, crusher work index and abrasion index tests. They will also have impact and other testwork conducted in a licensed JK Tech laboratory ready for equipment selection and sizing to be conducted by their process engineers.
Ball Charge Volume and Ball Size
Operating practice and theory suggests that the higher specific gravity of steel balls (S.G. = 7.85 ) will facilitate an increase in impact breakage within a SAG mill than if the SAG mill was operated autogenously. Using steel grinding media will cause a shift to more impact breakage and less abrasion and attrition breakage. The existence of middle size rocks can play an important role in developing a fine product, but the ball charge will remove this material and as a result may cause the mill to produce a coarser product. At high ball charges > 15% the ball charge may reverse this effect.
Please contact Richard Coles on
to discuss your SAG milling or other process equipment requirements including used SAG mills, cone crushers, rod mills We have over 40 years experience procuring, designing, installing and commissioning SAG and Ball Mills and Crushers and over 25 years experience acquiring and supplying pre-owned mills for installation into new mining projects.