Green pellets loaded into the indurating machine must contain at least 95% of the 9-16mm class. In this range of values, it is advisable for blast furnace pellets to have as high a content of the 9-12.5mm class as possible; this class usually ranges from 65 to 80%; for direct reduction pellets, the content of the 9-12.5mm class is regulated by a value of 45-55%. This difference is primarily due to the different behavior of pellets in a blast furnace and in metallization furnaces. In blast furnaces, the pellet in the charge passes into the liquid phase after reduction, and in metallization furnaces, after reduction, the pellet must retain its almost original shape, preventing the formation of conglomerates and small fractions. Larger pellets are less prone to sticking together and forming lumps of material due to a decrease in the contact area of the surfaces, and larger granules have greater gas permeability, an increased yield of the good class due to the formation of a smaller number of small particles after recovery. However, there are also disadvantages of such a distribution by fractional composition - this is a decrease in the efficiency of recovery due to a decrease in heat transfer and strength of granules. If the production of raw pellets of the 9-16mm fraction in the composition of more than 50% of the 12.5-16mm class does not cause any particular difficulties when obtained both on disc and drum pelletizers, then the production of raw pellets with a content of class 9-12.5mm more than 50% is easier to organize on drum pelletizers, which are less demanding on the influence of changing characteristics of the supplied raw material and can operate with a recirculation load of up to 300%.

     In practice, the production of raw pellets of a given class is associated with two moments. Either we produce raw pellets with the maximum required grain size distribution by class and partially improve it on a roller screen by screening out a small amount of substandard large and small classes, or we produce an average product that does not correspond to the specified class in grain size distribution and completely improve it on a roller screen by removing large and small classes as much as possible by setting appropriate gaps between the rollers on the upper and lower decks of the roller  screen. If this situation is still acceptable when using drum pelletizers, then the use of disc pelletizers with a recommended recirculation load of up to 25% will only worsen the quality characteristics of pellets, since the recirculation load will be 25-40%. Therefore, if we want to produce raw pellets with the maximum grain size of 9-12.5mm, it is necessary to minimize deviations in the quality characteristics of the feedstock materials and install modern pelletizing equipment. Some pellet manufacturers, who for one reason or another (high moisture content of the concentrate, use of dehydrating reagents) cannot obtain the optimal grain size of raw pellets after disc pelletizers and still focus on a recirculation load of up to 25%, install gaps on the upper deck of the roller screen to limit the grain size class slightly higher than 16mm, for example 17mm or 18mm.

     Practical results of the work show that with a decrease in the content of the 9-16mm class in raw pellets, the amount of return increases, and, as a consequence, the recirculation load. The most optimal average pellet diameter is in the range of 11.8-12.2mm. The operation of pelletizers in the range above these average values lead to a decrease in the number of suitable pellets in the required range of 9-16mm and, as a consequence, a decrease in the homogeneity of the layer on the firing trolleys, which is critically important for the subsequent stages of the technological process. Also, raw pellets of the 12.5-16mm fraction are more prone to destruction during transportation to the firing machine with the formation of cracks and chips, which further worsens the gas permeability of the layer and during the firing process leads to an increase in small substandard fractions less than 6.3mm, which are sieved at the screening stage and reduce the yield of suitable pellets. Green pellets of class 10-12.5mm are more compacted and homogeneous, less susceptible to damage. In fired pellets, there is practically no stratification into a two-layer structure (core and shell). Diagrams show how the percentage of cracks changes in green and fired pellets of different granulometric composition. 

    The larger the diameter of the raw pellets, the more difficult and time-consuming it is to remove water from the pellet during drying due to the increase in temperature between the core and the surface. The process of removing the final moisture shifts to the firing zone, where additional vaporization pressure is created due to the increase in temperature, at this point the rate of steam formation exceeds the rate of moisture removal in the pellet volume, which leads to a violation of the integrity of the pellet (rupture) destruction of the pellet with the formation of surface and internal cracks.

     An increase in the pellet size leads to the formation of various thermochemical conditions of the firing process across the pellet cross-section (temperature, partial oxygen pressure, oxidation reactions). As a result, its dual structure is formed. In the cross section, hematite is observed mainly in the outer shell, and magnetite in the inner part. During cooling, hematite and magnetite change their volume differently (due to differences in the structure of the crystal lattice), forming concentric cracks and disrupting the integrity of the pellet. In large pellets, oxidation processes do not occur to the full extent, which leads to the presence of residual magnetite across the pellet cross-section.
    The compressive strength of roasted pellets is determined according to ISO 4700. Numerous tests to determine the compressive strength of roasted pellets by size classes show an increase in the compressive strength of pellets of the 12.5-16mm class. This situation occurs because large pellets have a larger average diameter and the applied compression force is distributed over a larger area of the contact point of such a pellet. Diagram shows, as an example, the difference in the strength indicators of non-fluxed pellets of different fractional composition. 

       If these values are converted to the average pellet diameter by size class and pellet surface area, the specific strength value will be higher, on the contrary, for pellets of the 10-12.5mm fraction (table).

       All tests according to ISO methods for determining the physical and metallurgical properties of pellets are carried out on pellet samples with a size class of 10-12.5mm. Statistics of tests carried out to determine the dynamic strength to destruction by class +6.3mm (TI+6.3mm) according to ISO 3271 show the highest strength of fired pellets in the range of 10-12.5mm (diagram).

    The fracture strength of fired pellets during low-temperature reduction performed according to ISO 13930 for class +6.3mm (LTD+6.3mm) also in the range of 10-12.5mm has the most optimal strength values (diagram).

      As was reflected earlier, this test characterizes the tendency of pellets to destruction in the upper part of the blast furnace and reflects the ability to destruction during the reduction of hematite to magnetite with a change in the crystal lattice from hexagonal hematite to cubic magnetite. There is a change in anisotropic dimensions and, as a consequence, the occurrence of additional stresses in the pellet structure, which leads to the occurrence of cracks at the grain boundary. In pellets there are two types of bonds that affect the strength of the pellets - bonds of iron oxides magnetite and iron oxides hematite and silicate slag bond obtained at high firing temperatures as a result of the interaction of silica with iron, calcium and magnesium oxides with the formation of a melt in a volume of 5-10% of the pellet volume with partial dissolution of various minerals in it.

     After pellet cooling, the melt crystallizes as complex silicates and forms a glassy phase. Hematite bonds affect the pellet strength in the cold state, but lose their function when reduced to magnetite in a blast furnace. Slag silicate bonds do not undergo structural changes and are actually the only main source of pellet strength during reduction. Therefore, a sufficient amount of liquid slag must be formed during the firing process, which must be distributed evenly throughout the pellet volume, followed by the formation of solid silicate bonds during cooling. The formation of a liquid silicate phase is especially important for unfluxed pellets produced from low-silicon concentrates with a silicon oxide content of less than 3.0% and an iron content of 70.0% or more. It is recommended to add up to 1% limestone to form a liquid melt and create a slag bond to stabilize and increase low-temperature strength when restoring LTD+6.3mm.

      During these tests it was found that with an increase in the size class, the number of cracked pellets increases. The main reason is the presence of a dual structure in the pellet volume - a hematite base with the inclusion of parts of unoxidized magnetite, and the larger the pellet, the more hematite-magnetite interfaces appear in the pellet volume. With a change in the pellet size, the phase composition of the pellets changes (diagram). An increase in the average pellet diameter leads to an increase in magnetite and a decrease in the amount of hematite. The larger the pellet size, the larger the unoxidized magnetite core remains due to insufficient oxygen supply to the inner part of the pellet, and local zones are also formed along the pellet cross-section, which are the result of an incomplete transformation of magnetite into hematite.

      Hematite predominates in the oxidized shell, and magnetite and slag in the core, due to which the inner part of the pellet sinters more densely than the outer part, which affects the heterogeneity of the pellet structure. An increase in the pellet size leads to a modification of the pore structure inside the pellet. From numerous small and round pores in pellets of the 8-12.5mm class, in larger pellets the pores become larger, there are fewer of them, and they acquire a more elongated shape. This pore shape becomes a source of additional stress concentrations on the pore walls and reduces the cold strength of the pellets. The pellet size also affects the content of divalent iron oxide FeO. The lowest value of this oxide characterizes the degree of completion of the oxidation process. It is necessary to achieve the lowest possible FeO value when implementing the technology. Diagram  shows the change in this oxide by pellet size classes.

     Of great interest are also the results of the test (diagram) of dropping fired pellets from a height of 10 meters onto a flat metal surface (method proposed by the Corem research center).

     The data obtained allow us to estimate the degree of destruction of pellets and the formation of an additional amount of small fractions less than 6.3mm during transportation of pellets from the supplier to the end user, taking into account the initial quality of the pellets and additional destructive mechanical effects acting on the pellets during numerous overloads during delivery of the material.
    The chemical composition and basicity modulus of pellets by size classes do notchange and are within the limits of permissible deviations when determining the chemical composition of substances (table). 

       Pellets swelling is practically independent of granule size and should not exceed 20% for blast furnace pellets (diagram). The swelling index is determined according to ISO 4698.

     The increase in pellet volume during reduction is mainly due to the transformation of hematite into magnetite. High swelling is associated with low mechanical strength of pellets. Determination of this indicator is necessary to assess the effect of increasing pellet volume during reduction on the gas permeability of the column of charge materials in the blast furnace.
The next indicator, which depends on the size of pellets loaded into the blast furnace, is the reduction index – RI. The reduction index is determined according to ISO 7992 (Determination of reduction under load at 1050 C) and ISO 4695 (Determination of reduction by reduction rate index at 950 C). Since the reduction reaction occurs from the surface to the center of the pellet, the inner part of large pellets remains unreduced. The larger the pellet diameter, the more unreduced core remains in the pellet cross-section. The degree of reduction in smaller pellets is much greater than in large pellets (diagram).

        In fact, the higher the cold strength of pellets, the lower the pellet recovery. The main reason is the decrease in gas diffusion of reducing gases due to pellet compaction with a decrease in their open porosity. At temperatures from 1280 C to 1350 C, an additional amount of slag bonds are formed, which fill the voids and intergranular bonds with liquid melt. To obtain high cold strength of pellets, elevated temperatures are required in the furnace of the firing machine. On the other hand, it is necessary to have the highest possible degree of pellet recovery, which means it is necessary to look for an optimal compromise between pellet strength and recoverability. At most pelletizing plants, due to the remoteness from the end user and numerous overloads during delivery of pellets to the customer, preference is still given to the cold strength of pellets in order to reduce the formation of fines.