Which is the suitable process for ball milling?
Wet ball milling is much more effective due to Rehbinder effect producing a disjoining pressure.  Adsorption layers from surfactants that arise in microcracks significantly reduce the energy expenditure necessary to crush solid materials as a result of a decrease in their free surface energy upon contact with a medium containing substances capable of adsorption. Effect is observed in solids and porous materials, e.g. metals, ionic, covalent and molecular mono- and polycrystalline substances, glasses, polymers, both partially crystallized and amorphous.

This reduces milling time and wear of the working surface, at the same time improving homogeneity of the grinding fraction. The same effect is also used at the lab scale when, for example, metal oxides are dispersed in a mortar much more effectively when acetone is added. Sometimes after such treatment the reaction even takes place in a solid phase.