SAG Mill Product

Overview

A semi-autogenous mill (SAG mill) is a large rotating cylindrical drum that grinds ore using a combination of ore-on-ore impact (autogenous grinding) and impact from steel balls. Unlike ball mills, which rely entirely on steel balls for comminution, a SAG mill carries fewer balls—typically 6–10% of the mill volume—and allows large ore pieces to tumble and fracture against each other. This hybrid approach makes SAG mills economical for grinding coarse primary mill products (−200 to +10 mm) to intermediate sizes (−100 to −75 μm), preparing the ore for flotation, leaching, or final ball mill grinding.

SAG mills dominate the primary grinding stage in modern metal mining. A typical installation handles 50–500 t/h, with the largest units exceeding 10 MW of installed power. The technology is applied to copper, gold, iron, molybdenum, and nickel ores, as well as industrial minerals.

How it works

Crushed ore is delivered from a primary crushes (jaw or gyratory) into a [[sag-mill-feed-hopper|feed hopper]] holding 50–200 tonnes. A [[sag-mill-feed-gate|feed gate]] meters the ore into the [[sag-mill-mill-shell|rotating mill shell]] at a controlled rate. Inside the drum, the ore tumbles in a slurry of water, smaller ore particles, and steel balls. As the shell rotates at 10–14 rpm, the ore near the periphery is carried upward by the [[sag-mill-lifter-bars|lifter bars]]—steel or composite bars molded to the shell liners. When the ore reaches a critical angle (typically 50–60° from vertical), it cascades down through the mill interior, striking other ore particles, the lifter bars, and steel balls, causing fracture and size reduction through impact and abrasion.

The more frequently a particle is lifted and dropped, the more energy it absorbs. A particle may make hundreds of passes through the mill before exiting; smaller pieces exit faster. This selective action—large pieces circulate many times, fines exit on their first pass—creates a natural size distribution optimized for downstream flotation or leaching circuits.

As grinding progresses, the ore and water form a slurry. The mill is typically operated at 65–75% of its volume filled with solids and water (''mill load''). The slurry flows to the [[sag-mill-discharge-system|discharge system]], where an internal [[sag-mill-grate-bars|grate]] partitions the mill. Only particles small enough to fall through the grate apertures (typically 6–12 mm) exit; larger particles continue to tumble in the mill, recirculating.

Ground slurry overflows a [[sag-mill-overflow-weir|weir]] lip at the discharge end and spills out through a [[sag-mill-discharge-chute|chute]] to a [[sag-mill-pump-feeder|pump]] that delivers the slurry to a hydroclassifier or thickener for separation.

Mill Shell and Liners

The [[sag-mill-mill-shell|mill shell]] is a welded mild steel cylinder, 6–12 m in diameter and 2.5–4.5 m long, with walls 30–50 mm thick. The shell must withstand impact loads from tumbling ore and centrifugal forces at rotation. Large [[sag-mill-shell-flanges|end flanges]] at each end are reinforced to carry the [[sag-mill-ring-gear|ring gear]] and to support the [[sag-mill-trunnion-pins|trunnion pins]] that hold the mill aloft.

Inside, the shell is protected by replaceable [[sag-mill-liners|liners]]—rubber or composite segments 50–150 mm thick, bolted in circumferential rings. These liners absorb impact and are worn away by ore abrasion; they are replaced every 12–24 months depending on ore type and throughput. The [[sag-mill-lifter-bars|lifter bars]] are molded to or bolted on the liners and shape the cascade path. At the discharge end, [[sag-mill-grate-liners|grate liners]] protect the [[sag-mill-grate-bars|grate bars]] from erosion.

Drive and Rotation

The mill shell is rotated by a [[sag-mill-mill-drive|drive system]] comprising a large AC [[sag-mill-drive-motor|motor]] (2–10 MW), a [[sag-mill-main-gearbox|multi-stage helical gearbox]], and a [[sag-mill-pinion|pinion gear]]. The motor runs at 1500–1800 rpm; the gearbox reduces this to 60–120 rpm, which is then reduced further by the pinion meshing with the [[sag-mill-ring-gear|ring gear]] to achieve 10–14 rpm at the mill shell.

Modern mills use soft-starters or variable frequency drives (VFDs) to ramp the motor up smoothly, reducing inrush current and mechanical shock during start-up. Without soft-starting, the inertia of a 500-tonne drum accelerating from rest can cause damage to the gearbox or motor.

The [[sag-mill-ring-gear|ring gear]] is typically forged or fabricated as segments and bolted around the mill shell flange. Teeth engage the pinion continuously, transmitting torque to rotate the shell.

Bearings and Support

The rotating mill shell is supported by large [[sag-mill-trunnion-bearings|trunnion bearings]] at the front and rear. These may be rolling-element bearings (tapered or cylindrical rollers) or hydrostatic (oil-film) bearings. The [[sag-mill-trunnion-pins|trunnion pins]] are conical hardened steel pins projecting from the stationary mill frame; they sit in the rotating bearing assemblies. A centralized [[sag-mill-bearing-lubrication|grease pump]] supplies the bearings continuously, essential for managing the intense pressure and heat from a 500-tonne rotating load.

Discharge and Control

Product exits the mill through a [[sag-mill-grate-bars|grate]] partition inside the shell. Steel grate bars are parallel and spaced 6–12 mm apart; particles larger than this aperture size remain in the mill and continue circulating until they are small enough to pass. An [[sag-mill-overflow-weir|overflow weir]]—a rubber lip—controls the static slurry level inside the mill. A [[sag-mill-discharge-chute|chute]] conveys the discharged slurry to a [[sag-mill-pump-feeder|pump]] that delivers it to a [[sag-mill-ball-mill|secondary ball mill]] or to separation plant (cyclones, flotation cells, gravity concentrators).

Operational Control

Modern SAG mills are highly instrumented. A [[sag-mill-level-sensor|mill level sensor]] (radar or capacitive) continuously measures the internal ore level. If the level is too high, the mill is overloaded, risking stalling or grinding to a halt—called "mill jam.'' If too low, the balls and ore cannot cascade effectively, and grinding power is wasted. A [[sag-mill-motor-current-sensor|motor current sensor]] measures real-time power draw, which correlates to mill load and grinding efficiency.

A [[sag-mill-plc|programmable logic controller]] reads these sensors and automatically adjusts the [[sag-mill-feed-gate|feed gate]] and water addition to maintain optimal conditions. If mill current spikes (indicating jam), the PLC may trigger an alarm, reduce feed rate, or activate a mill ''unload'' procedure to temporarily increase water and reduce solids, allowing ore to re-flow.

SAG Mill vs. Ball Mill

A SAG mill is more energy-efficient than a ball mill at grinding coarse ore but requires larger capital investment and more sophisticated automation. Ball mills are simpler and more robust, but cannot economically process coarse (+10 mm) material. In modern circuits, a SAG mill typically precedes one or more ball mills in a ''grinding train,'' each stage progressively fine-tuning product size.

Typical Applications

SAG mills are the workhorse of hard-rock mining. In copper flotation, a SAG mill feeds a pebble mill and regrind ball mill, producing −75 μm slurry for flotation cells. In gold leaching, SAG mill product feeds agitated tanks. In iron ore concentrates, SAG mills prepare feed for magnetic separation or flotation. The flexibility to vary ball load (6–10%) allows operators to tune the mill for soft ores (higher autogenous content) or hard ores (higher ball load).