You're probably looking for the fracture energy -- the energy required to create a fracture with a unit surface area.
Take the grain size you're looking to end up with, compute the surface area and mass, combine it with the fracture energy, and you should have a pretty decent estimate for the energy required to grind a particular substance to a particular size.
Take the grain size you're looking to end up with, compute the surface area and mass, combine it with the fracture energy, and you should have a pretty decent estimate for the energy required to grind a particular substance to a particular size.