The basic ideology of the pellet production process is to obtain a material with high mechanical strength and good reduction ability, which are among the most important requirements for pellets in a blast furnace. Insufficiently roasted pellets with low initial mechanical strength swell and disintegrate during reduction, regardless of the type of concentrate and additives used. Pellets with a high gangue content are less prone to swelling and disintegration. Pellets with a low gangue content, and especially in the absence of low-melting components, swell intensively and are subject to destruction unless certain countermeasures are taken. This must be taken into account in the technological process for the production of a certain type of pellets.

    The decrease in pellet strength during reduction occurs for two reasons. The first is intensive swelling with an increase in volume during the transition of hematite to magnetite (the magnetite layer becomes thicker than the hematite layer by this amount) due to a change in the crystalline structure. During the reduction process, a sequence of crystalline changes occurs with the displacement and rearrangement of iron and oxygen atoms. During the transition from phase to phase, oxygen atoms are gradually released as a result of the reaction with the reducing gas CO, with a concentration of iron atoms:
                                                                                     
                                                                         
                                                                           
    Carbon - C and hydrogen - H2 are also used as a reducing agent along with CO in reduction reactions. The removal of oxygen atoms from the crystal system and the rearrangement of atoms in the unit cells contribute to the occurrence of internal stresses in the crystalline structure of the pellet, leading to the occurrence of cracks in the crystals and porosity in the grains. A change in the unit cells occurs with a change in their volume. Starting with the hematite cell, a large expansion of the volume (+95%) is observed in the transition to the magnetite phase and, starting from it, a large reduction in the transitions to wustite (-74%) and iron (-92%). The bulk of structural destruction, directly affecting the operation of the reduction units, is formed in the first phase of the hematite → magnetite transition at reduction temperatures of 400-600 C. In such a temperature range, the waste rock phases are a supporting framework, since they are in a solid state and the degree of destruction of pellets depends on their properties (glassy or plastic). An example of hematite transformation during recovery is shown in figure.The second is the anisotropy of the reduction rate, i.e. the reduction rate is different in different directions and, as a consequence, the thickness of the formed magnetite layer is different, which creates additional destruction forces and structural damage. The transformation of the crystalline structure from hematite to wustite leads to its weakening and destruction, as indicated by the disintegration of pellets into fragments and an increase in the proportion of the class less than 0.5mm.

Practice has shown that an increase in the pellet volume to 20% during reduction does not destabilize the operation of blast furnaces. However, higher values are considered unacceptable and require a comprehensive analysis of the causes of such behavior of pellets during reduction. To reduce the swelling value, a certain selection of the basicity value is required in relation to the chemical composition of the concentrate and the amount of gangue, i.e. by changing the basicity, it is possible to produce pellets from various types of iron ore raw materials that meet the requirements of both the blast furnace processing stage and metallization plants in relation to changing the swelling index. Thus, the adaptation of the chemical composition of the acidic components of gangue contained in the ore or concentrate by adding CaO is an effective method for pellet production. Swelling measurement has long been the main method for determining the quality of pellets during reduction, but it only indirectly allows one to estimate all the necessary quality characteristics and indicates the presence of structural changes at a temperature of 900-1000 C. At lower reduction temperatures of 400-600 C and in a gas atmosphere with a low reduction potential - as is the case in the upper part of a blast furnace - there is insufficient information on structural changes and it is necessary to use other methods (LTD and RDI) determining the quality of pellets in this temperature range. Summarizing the above, it can be said that for normal operation of reduction units it is necessary to select technological modes for obtaining iron ore materials in such a way that their reducibility, destructibility at low temperatures and swelling at the stage of the appearance of metallic iron do not go beyond the permissible limits.