It’s important to point out that in a SAG mill, the crushing load is composed of two components; the mineral rocks and the iron balls. Thus the mineral type affects SAG mills more than ball mills.

Due to the heterogeneous nature of the foundations, the minerals don’t possess a single hardness.  The existence of this variance is what causes fluctuations in SAG mills, moving from a high soft mineral treatment capacity to a low treatment capacity for harder minerals. Also, even if the material hardness is high, inadequate granulometry feeding occurs (for example without wide content) you will obtain a low production scale even with softer minerals.

In ball mills when the feeding flow increases, the mineral is discharged faster. However, given the fact that the energy requirements needed  depend on the ball charge, the values remain the same even with a change in flow.  Consequently, the same amount of energy is used to move a large flow of minerals and the distribution of the product becomes thicker when the flow increases.  On the contrary, if the feeding flow decreases, you will have the same energy in order to crush a small number of minerals. This leads to a finer crush. 

For SAG mills, evidence shows that that feeding rate has a small effect on the final product size and that a maximum flow exists in order to generate a stable operation. This is due to the fact that when you increase the feeding flow an increase in the crushing rate is produced. This eventually generates a steady operation. 

The increase in the crushing rate is due to the accumulation of thick material inside the mill but this does not have a negative effect on the product size due to the action of the discharge grille.  The accumulation of material inside the mill elevates the filling level and consequently produces an increase in motor energy. The final outcome is that the overall energy available for each ton of processed material remains constant without affecting the distribution of product sizes. 

Disadvantages of SAG  Mills:

Less disposal time in crushing plants (90% vs 95%)

More instability for operations (high fluctuations)

Higher energy consumption

Requires high-skilled professionals for operation. 

Source:

Juan Rivera Zeballos, “Compendio de Conminución”