Starting as small as 13 feet (4 m) in diameter on 330 HP and as large as 24′ (14.5 m) in diameter on 3,300 HP, the 911MPE Semi & Fully Autogenous Mills tumble crushed ore without iron or steel grinding media. They are used when the crushed ore pieces are hard enough to perform all the grinding. SAG mills tumble mainly ore but they use up to 15% volume of steel balls to assist grinding. Semi-autogenous mills are more common.
The AG Mill or SAG grinds primary crusher product and prepares it for final grinding in a ball mill. Its product is usually passed over a large vibrating screen to separate oversize ‘pebbles’ from correct-size particles. The correct-size material is sent forward to a ball mill for final grinding. The oversize pebbles are recycled through a small eccentric crusher, then back to the SAG or autogenous grinding mill. This procedure maximizes ore throughput and minimizes electrical energy consumption.
Assuming your ore is AG-amenable: 7 TPH to 15 TPH is a comfortably production average for a 13′ x 4.5′ AG Mill.
A 13′ x 6′ SAG Mill has a reasonable range of:
- 20 to 30 TPH for gold ore SAG & Ball Mill duty
- 25 to 40 t/h for copper porphyry SAG & Ball Mill duty
In Single Stage SAG duty, the approximate capacity reduces to only 8 to 13 TPH for gold ore duty.
The autogenous mill itself is a coarse-grinding device, consisting of tumbling drum with a 25% to 40% volume filling of ore. Metallic or manufactured grinding media is not used. Autogenous mills are fed run-of-mine ore or primary crusher product that is <25 cm (10″). Inside the mill, large pieces break into smaller pieces a few inches in size. These natural pebbles act as the grinding media the autogenous mill. The main modes of breakage are thought to be impact breakage and abrasion.
Many circuit configurations are possible, but essentially the autogenous or SAG mill is operated as a single-stage primary mill, or it can be followed by secondary pebble or ball milling. The semi & autogenous mill is often operated in closed circuit with a trommel screen or external vibrating screen classifying the discharge. Circulating loads are low compared with those in ball mill circuits, because autogenous mills do not benefit from high-circulating loads in the same way ball mills do. Intermediate crushers are sometimes used to crush the largest pieces in the recycle stream.
The product size of an AG/SAG mill depends strongly on the nature of the ore being ground so that tests must determine the competency of the ore and its “natural” particle size.