In pelletizing, there are several important concepts related to minerals, concentrates and pellets - bulk density and porosity. These two concepts are indicators of the degree of completion of the pellet structure formation and the intensity of firing (time and temperature). Bulk density is a property related to a solid material in its granular state. It is the ratio of the mass of a set of particles to the volume occupied by a certain container, determined according to ISO 3852. Bulk density can be determined with or without compaction of the material. Higher values are obtained with
compaction. The bulk density of concentrate is 2.6-3.2kg/dm3, bentonite 0.8-1.1kg/dm3, limestone 1.1-1.4kg/dm3, anthracite 0.8-1.1kg/dm3, slaked powdered lime 0.4-0.6kg/dm3, pellets 1.85-2.2kg/dm3.

      Porosity is the proportion of pore volume in the total volume of a porous body. Total porosity of pellets takes into account two types of pores (closed and open communicating with the external environment) and is defined as a percentage ratio between the total pore volume and the granule volume according to ISO 15901. Iron ore charge used for the production of green pellets consists of the iron ore part (concentrate, roasted crushed return), fluxing (limestone), binding (bentonite, organic polymers) and others (solid fuel, colemanite) and has a porosity of 40-45%. When pelletizing the charge and obtaining green pellets in granulators, their porosity decreases to 30-32% due to compaction, shrinkage and increased strength. In roasted pellets, due to the formation of liquid phases and a decrease in the number of pores, the porosity decreases to 20-25%. When adding solid fuel, the porosity of the fired pellets increases to 25-35% depending on the type and dosage of this fuel. The increase in porosity occurs due to the formation of additional pores inside the pellet during combustion of the solid fuel.

     In addition to the porosity of the pellet itself, the porosity of the pellet layer itself is also very important on the firing machine. Good porosity along the layer height ensures the best heat exchange by convection during contact of the pellet surface with the heat carrier passing through this layer. To improve the porosity of the layer, it is important to ensure a minimum presence of substandard small classes in bed and raw pellets in the layer, which reduce the number of pores and to reduce the presence of large classes as much as possible, since their content reduces the porosity of the layer by reducing the surface area of contact of hot gases with the pellet surface. Prevent over-moistening of the lower layers of raw pellets in the drying zone, which leads to plastic deformation of raw pellets in layers and filling of pores. Each type of produced pellets has a certain composition of raw materials, which leads to characteristic values of density and porosity. Such data allow you to set certain firing parameters in the production of a given type of pellets.