For calculating a dry SAG mill’s air flow rate your request needs:

Comminution rate you desire
Particle size distribution and shape by size
Particle size selected to be removed
Gas velocity to entrain the particle size to be removed
Gas velocity necessary to overcome the tumbling interference
Mill geometry to comminute to particle set
Breakage properties of material to be comminuted
Percentage surface moisture
Grinding media and properties

We model all types of grinding mill systems using our proprietary DEM code ROCKY. ROCKY can mimic breakage from laboratory tests such as JKMRC T-10 protocol and drop weight program. ROCKY is a program developed in-house for Granular Dynamics, International (GDI).

Based on this and your mill geometry, mill operating parameters, et al. we can generate the gas dynamic solution to render your answer. A good start would be to know what the rock behavior is with known mills.

The solution is multiple stages of first obtaining the rock breakage properties, given the moisture and size distribution entering the mill, creating the equivalent Breakage Function ( one or more rock strengths by size and their size reduction for a given normal and tangential force spectra within the mill geometry), creating the rock Selection Function ( probability of all collision energy spectra by each size fraction) as per a standard Population Balance Model parameter set, except our Selection Function is set by DEM simulation and Breakage Function is set by laboratory testing. The Breakage Function must be determined for each rock group and also determine their stochastic properties.

Pressure drop is the most important parameter in dry SAG mills because it can make material discharge possible. I depend on mill length and the amount of material in front of the max trunnion opening. This parameter can be affected by inner deflector liner design.