There are no equations to directly determine agitator power for several reasons.  First, the shape of the impeller is important to performance, shape and power characteristics are unique to each mixer type.  Second, the power requirement in a powder application depends on the bulk density and the flow characteristics of the aggregate in the bed.
About the only "equation" that does apply is that power will be roughly proportional to the bulk density of the aggregate, the rotational speed of the mixer cubed, and the diameter of the mixer impeller raised to the fifth power.  The base line characteristics must be determined by experimental measurement.  Actual power requirements may also change with the shape, size, and flow characteristics of the aggregate.
No simple answer to this question - too many variables and unknowns.