Comminution of porous materials

The behaviour of porous materials in comminution processes differs substantially from that of non-porous materials. It is strongly affected by the type of porosity, which may be characterized by different void shapes and interconnection degrees. This paper discusses the comminution of porous materials having voids of compact shape (like spheres) and not interconnected. For such materials different behaviours can be expected for the two comminution ranges above and below the mean voids interspace. In fact, for the coarser sizes the mechanical behaviour of the material is that of the porous material defined by its bulk mechanical properties; below the mean voids interspace down to very fine sizes the mechanical behaviour of the material is that of the non-porous material constituting the matrix. Therefore, a transitional region should exist, with the result of bi- or trimodal size distributions for comminuted porous materials. To verify the above mentioned theory, tests were performed on two types of materials, artificial and natural: (a) perforated bricks with 20 mm voids; (b) natural pumice stone. The resulting size distributions differ in slope above and below the mean voids interspace, and the two parts of the distribution are connected by a transition curve. The results may be of interest in studying the comminution of different types of materials, particularly secondary materials and scraps.