What are sag mills used for?
A semi-autogenous grinding, or SAG, mill uses ore together with a charge of steel balls to break coarse feed. It is a circuit-level selection that requires ore-competency and variability testwork.
How it works
The rotating shell lifts ore and balls so impact and abrasion reduce particle size. The ore itself forms part of the grinding media. Critical-size accumulation, pebble handling and downstream classification can materially affect the complete circuit.
When to consider it
Consider SAG milling when the project study supports coarse-feed grinding at the required scale. Compare suitable circuits across the ore body's expected hardness range rather than selecting against one favorable sample.
Application limits to check
SAG response is sensitive to ore competency and variability. It is not justified by throughput alone; power supply, liners, drive support and the downstream grinding requirement need a coordinated design.
Selection data for a useful quotation
These are enquiry inputs, not specifications of a single model. Include measured values, units and the expected operating range; flag anything still to be tested.
| Input | What to provide |
|---|---|
| Ore characterization | Competency, grindability and representative variability samples |
| Feed | Crusher product size, hardness distribution and moisture |
| Production target | Annual tonnage, operating hours and design availability basis |
| Circuit | Pebble discharge, pebble crushing and downstream ball milling |
| Drive | Power, speed arrangement, electrical supply and protection |
| Maintenance strategy | Liner handling, critical spares, lifting and service support |
Compare the complete duty
Compare a SAG-based circuit with staged crushing and ball milling or HPGR-based alternatives using the same testwork, availability assumptions and final grind. Capital cost alone does not settle the choice.
Before accepting a supplier proposal
- Obtain a written testwork-to-sizing basis across the ore variability range.
- Review drive, lubrication, condition monitoring and liner-change provisions.
- Define commissioning and performance tests for the complete grinding circuit.
Buyer questions about sag mills
What does semi-autogenous mean?
The ore participates in grinding, with steel balls added to support breakage. This differs from a ball mill in which added media dominate the grinding action.
Can a SAG mill be selected without testwork?
A reliable project selection needs appropriate ore testing and circuit evaluation. A generic capacity table cannot represent all ore bodies.
Are pebble crushers always included?
No. The need depends on the circuit and ore response. Ask how critical-size material is handled in the proposed design.
Technical reference
Metso: official equipment and technology description ↗
The source provides technology context. The selection checklist is KMS Industrial editorial guidance; manufacturer models and performance figures are not KMS specifications.