The SAG (Semi-Autogenous Grinding) mill is an extensive tumbling mill, which employs some portion of steel balls (usually 2-22% volumetric filling) alongside the ore as grinding media, whereas Bond Work Index (Wi) represents hardness by the amount of energy (kWh/t) consumed for reducing the size of material from infinite to 80% passing 100 µm. In case of hard and competent ores (Wi > 18 kWh/t), due to inherent resistance of such ore against breakage, the accumulation of “critical size” pebbles (13-75 mm) occurs, hindering productivity unless dealt with.
In case of Wi > 18 kWh/t, a better choice would be to adopt the SABC circuit (SAG mill + ball mill + pebble crusher) compared to a single stage SAG circuit. The need for employing pebble crushing in this case arises because of its ability to deal with critical size accumulation, otherwise a major hindrance to the efficiency of the SAG mill.
Hardness testing must be done accurately: SAG Design hardness testing (JK Axb, SMC testing) must be performed with specific energy determined from F80 = 152 mm to T80 = 1.7 mm, and Bond Ball Work Index testing conducted for SAG-milled ore. If Wi is greater than 12 kWh/t, then dry locked-cycle tests are suitable, and pilot-plant testing on 50-100 t of ore may also be required to determine accurate SAG sizing.
The size of the SAG mill must be based on unit power × desired output. For hard ores, the SAG mill requires a power of 15-25 MW using gearless drives (wrap-around motors) when the diameter of the mill is equal to or exceeds 11.6 m. Sizing of the SAG mill must consider the ratio of D to L ≥2, and ball charge volumetric filling is usually around 10% but may go up to 15% for hard ores.
In case there is a pebble crusher, use vibrating screens instead of trommel screens for SAG discharge classification since screens have high efficiency and high pebble recovery rate. The pebble crushing machine should accept feed size range of 13 – 75 mm, have metal handling system (metal detector + bypass), have storage capacity (bin or stockpile), and send crushed pebbles back into SAG feed or back directly into the ball mill circuits to relieve the SAG mill.
Pebble crushing machines are to be chosen with open grate and large aperture design in order to fully utilize the pebble crushing machine circuit and maximize the capacity of mills. Optimal ratio between shell lifters spacing and height is 2.5:1 – 5.0:1. Work the machine at 75 – 77% critical speed; for extra hard ores, higher speed (up to 77% CS) and higher steel charge (15%) may be necessary. Control mill density and load automatically.
As SAG mills are to shut down the entire concentrator during outages, it is important to design the circuit in such a way that downtime due to relining is kept to a minimum by using large-sized liners. Abrasive ores are tough on liners, and high alloy steel liners can be considered for them. There should be provision for a maintenance bypass on the pebble crusher, and consideration should be taken for any possible future retrofits on the machine.
