The purpose of milling is to ensure that the ore is finely dispersed  so as to liberate the valuable from the unwanted minerals (gangue  minerals). The grain size of the valuable material is very important, as  it determines the liberation size. Mills are operated in closed circuit  with hydro cyclones that separate the finely ground material from  the course material. The course material goes back into the mill for  regrinding, while the fine material goes for the separation process