Iron ore pellets consist of an ore component represented by magnetite or hematite or their mixture with a predominance of one of the minerals and the inclusion of another mineral and a non-metallic part represented by silicates. The silicates predominantly include quartz and other silicate-containing minerals, which are present in smaller quantities in ore rocks (bionite, chlorite, cummingtonite, rhodusite, stilpnomelane, green mica and others). During the enrichment process, these minerals in varying quantities pass into the iron ore concentrate depending on the degree of enrichment of the ore part and directly participate in the physicochemical processes of strengthening firing along with iron-containing components. Silicate-containing minerals present in the concentrate have different melting points, and as a result, they affect the formation of a liquid-phase melt in different ways. The characteristics of silicates of the non-metallic part of concentrates are reflected in annex2 . 

   The composition of the silicate phase of the concentrate has a significant effect on the strengthening firing of unfluxed pellets, since it is the source of the primary strengthening melt. In fluxed pellets, the ferrous ferrite melt has a more significant effect on pellet strengthening, since its volume significantly exceeds the amount of high-silicon silicate melt. The best characteristics in terms of strength properties during strengthening firing and subsequent reduction are found in pellets, in the composition of which low-melting silicates are present. Pellets in the composition  of which  high-melting silicates predominate have significantly lower strength characteristics, this is especially true for unfluxed magnetite pellets with a silicon oxide content of less than 3.0%. The main reason for the difference in strength properties is the different microstructure of the roasted pellets. In pellets with the presence of refractory silicates in the charge, the ore binder prevails due to solid-phase sintering, in pellets with the presence of low-melting silicates in the charge, along with the ore binder, there is a silicate binder. The degree of reduction has an inverse relationship and is higher in pellets with a charge with refractory silicates due to the presence of a larger number of pores. With an increase in the basicity of pellets, the amount of silicate binder increases due to the formation and melting of  ferrites and  the interaction of the ferrite melt with  the non-metallic part of the charge, which reduces the effect of the composition of the non-metallic part of the charge on the structure and composition of the silicate binder of fluxed pellets. Therefore, one of the recommendations for increasing the strength characteristics of non-fluxed pellets in the production of pellets from high-grade magnetite concentrate (SIO2 content less than 3.0%) is the addition of up to 1.0% limestone to the batch for pellet production.

  The mineralogical composition of non-fluxed pellets is mainly determined by the material composition of the charge concentrate. In fluxed pellets, the following factors influencing the mineral composition of pellets are added to the material composition of the charge - this is the flux composition and the basicity value (CaO/SIO2). Before the appearance of the liquid phase, quartz and iron ore components are an inert part of the process. Two-phase non-fluxed pellets (type I) consist of hematite and silicate binder and are formed in the presence of a primary melt that appeared due to the dissociation and melting of low-melting minerals of the host rocks with the absorption of all refractory minerals of the charge in the molten state (figure). If the amount of the primary melt is insufficient to absorb the refractory silicate components of the batch, in addition to hematite and silicate binder, the pellets contain residues of refractory minerals of the host rocks (for example quartz). Unfluxed pellets (type II) are an option if all silica minerals of the batch are refractory compounds. Such pellets are strengthened only due to solid-phase sintering reactions and hematite recrystallization and do not contain a silicate binder (figure). Unfluxed pellets that do not contain a silicate binder or if its amount is very small are prone to significant destruction during the reduction process, although they have good cold strength indicators. With rare exceptions, almost all magnetite concentrates contain a certain amount of low-melting minerals, and to obtain a primary silicate melt, it is necessary to select a temperature-time firing mode. Hardening of non-fluxed pellets occurs due to solid-phase sintering, the basis of which is the process of oxidation of magnetite grains of the ore fraction and minerals of the non-metallic part to hematite. If the composition of the charge is represented mainly by refractory silicates, then the hematite framework acquires a fine-grained structure. With the predominance of low-melting silicates, a large-crystalline hematite framework is formed due to the increased mobility of iron ions.

    The final stage of the transformation of the charge and strengthening of the pellets is the formation of a liquid phase in the pellets, which is determined by the physicochemical properties of the silicate phases of the concentrate. This must be taken into account when selecting the composition of the iron ore charge, taking into account the composition and amount of minerals in the non-metallic part.

  Hardening of fluxed pellets differs significantly from hardening of non-fluxed pellets and is a complex system of transformations in the process of solid-phase and liquid-phase sintering. Oxidation of magnetite to hematite and the additional heat released in this case intensify the reaction of limestone decomposition and initiate ferrite formation processes starting from a temperature of 700 C. A further increase in temperature to 900 C and then to 1200 C leads to the involvement of ore particles of 0.002-0.01mm in size and larger particles of up to 0.2mm in size in the ferrite formation process, while ferrites CaO∙Fe2O3 (monocalcium, dicalcium, semicalcium) and wollastonite (CaSiO3) are formed.

   The variety of mineral formation processes occurring at the stage of low-temperature strengthening represents a complex multiphase system representing a ferrite-hematite framework at the final stage of solid-phase sintering, in the volume of which there are iron ions. grains of hematite of a large fraction, quartz and inclusions of unreacted flux. Further strengthening of pellets during the firing process occurs due to the melting of ferrite phases and the formation of a liquid silicate binder in the temperature range of 1200-1280 C, significantly exceeding the primary melt formed by melting silicates of the nonore part of the concentrate by volume. The process of formation of the silicate binder occurs throughout the volume and leads to shrinkage of the pellet. The composition and structure of pellets are formed at a temperature of 1250-1280 C and represent a twophase system consisting of hematite in a solid or partially plastic state and a silicate melt in a liquid state (a three-component slag consisting of silicon oxide of silicate phases, calcium oxide and a small amount of hematite of small fractions). In the temperature range of 1250-1350 C, reverse processes of hematite dissociation with the formation of magnetite occur. The process of hematite dissociation to magnetite increases with an increase in the firing temperature and an increase in the basicity modulus of pellets. Therefore, it is necessary to maintain a balance between the firing temperature and the basicity modulus of fluxed pellets, practically minimizing the processes of dissociation and formation of secondary magnetite. The optimal hardening temperature for different charge compositions and types of heat treatment is different.

  A mandatory condition for pellet strengthening throughout the entire volume is the complete interaction and homogenization of all components of the charge, which is achieved by holding at an optimal firing temperature that allows for complete melting of the silicate and ferrite phases. The general direction of mineral formation processes at the stages of solid-phase and liquid-phase strengthening for fluxed pellets from magnetite concentrates is shown in figure.

    The diagram shows two directions of phase transformations. The first reflects the process of transformation of the large ore fraction, which during the entire firing process is only oxidized from magnetite to hematite. This ore phase is a carrier of iron. It does not enter into a mineral formation reaction with other oxides of the system and does not dissolve foreign impurities in its lattice. The second direction of phase transformations refers to the sequence of mineral formation during the formation of a silicate binder. Silicate binders of fluxed pellets mainly consist of three oxide systems of silicon, calcium and iron oxides, which form different phase systems with different basicity. In pellets with a basicity of 0.3-0.8units, a glass-like siliceous iron-silicate binder is formed, consisting of tridymite-wollastonite-hematite, solidifying at a temperature of 1204 C.

    In pellets with a basicity of 1.0units, a glass-like calcium iron-silicate binder is formed, consisting of wollastonite-rankinite-hematite, solidifying at a temperature of 1214 C, with a predominance of wollastonite and hematite. In pellets with a basicity of 1.2units, a calcium iron-silicate binder is formed in the form of glass, consisting mainly of rankinite and solidifying at a temperature of 1230 C.

   The main iron ore phase of pellets consists of hematite and has the same composition. The silicate binder has a different phase composition and microstructure. This causes different properties of pellets in the cold state and during heat-reducing treatment. Pellets obtained in the basicity range of 0.3-1.5units with different volume combinations of hematite and silicate binders have high cold compressive strength in the range of 200-400kg/pellet.

    The main condition for such strength is the absence of solid-phase particles and unreacted residual components of the batch. However, such pellets have significant differences in strength properties during the reduction process, which depend on the concentrate enrichment depth and the pellet basicity modulus. Pellets obtained in the basicity range of 0.3-0.8units have the highest strength during reduction (LTD+6.3mm - ISO 13930).

   They have a binder of high-silica glass of the tridymite-wollastonite composition. Such a binder is stable during reduction and heat treatment. The reducing gas does not penetrate into the glass and does not destroy the binder. The stability of such a binder explains the high strength of pellets with low basicity. Pellets are reduced from the periphery to the center without disturbing the shape and without the appearance of cracks. The glass binder goes into a plastic state and compensates for the stresses that appear during the reduction of hematite to magnetite. Increasing the pellet basicity to 1.0units leads to a decrease in the strength during reduction due to the formation of silicate bonds represented mainly by wollastonite and partially by wustite. The process of crystallization and reduction of the silicate bond occurs throughout the pellet volume, with cracks forming across the pellet cross-section with an outlet to the surface leading to an intensification of the reduction processes due to a more complete penetration of the reducing gas into the pellet. The crystalline wollastonite bond, which is in a solid state, does not compensate for the stresses of the system during the structural transition of hematite to magnetite.

   Different mechanisms of pellet reduction with different basicity also determine different reducibility. Pellets of natural basicity, mainly on an ore binder, have a higher degree of reduction due to an insignificant amount of silicate binder and the presence of many open pores for the access of reducing gas. High-siliceous difficult-to-reduce silicate binders with a basicity of 0.3-0.8units block the ore phases of pellets without being reduced themselves, they prevent the reduction of hematite. Binders of high basicity, containing wollastonite crystals in their composition, the faces of which are conductors of the reducing gas, lead to the reduction of hematite in the entire volume of the pellet without the formation of a metal frame. Therefore, the high crystallinity of silicate binders leads to low pellet strength during reduction.

    For pellets from ferruginous quartzite concentrates, there is a dependence of their strength during reduction on the silica content in the concentrate and, accordingly, the amount and composition of the silicate binder formed in them (figures). Pellets with a basicity of 0.5units, regardless of the silica content and, as a consequence, the amount of silicate binder, have high strength indicators during reduction. Pellets with a basicity of 1.0units show a deterioration in the strength properties during reduction with an increase in the amount of silica in the concentrate, which is associated with an increase in the silicate binder, which has a high crystallization capacity. Therefore, it is advisable to obtain pellets with high basicity from deep enrichment concentrates. Binders that make it possible to obtain fluxed pellets with high strength during reduction are in the range of pellet basicity of 0.3-0.7units. Binders with low strength during reduction lie in the range of basicities of 1.0-1.5units. When using deep enrichment concentrates, the range of basicities that allow obtaining high strength of pellets during reduction is expanded due to a decrease in the amount of silicate binder with high crystallization, and is 0.3-1.5units. With complete interaction of all components of the initial charge, the pellets consist of hematite and silicate binder. Since hematite in  pellets of  different  basicities  has  the  same composition,  the difference  lies only in the composition of  thesilicate   binder. Silicate binders of pellets with a basicity of 0.3-0.8units are represented by high-silica glass with an iron content of up to 12-16%. Pellets with a basicity of 1.0-1.5units form bonds from high-calcium glass containing wollastonite at a basicity of 1.0units, and dicalcium silicate with an iron content of up to 12% at a basicity of 1.2-1.5units.

   Pellets with a basicity of 1.0-1.5units form bonds from high-calcium glass, which contains wollastonite at a basicity of 1.0units, and dicalcium silicate with an iron content of up to 12% at a basicity of 1.2-1.5units. During oxidative firing, the amount of strengthening iron-silicate melt in the pellets varies from 1.5 to 21.7%, depending on the enrichment depth of the original concentrate and the basicity modulus (table). The lower the SiO2 content in the iron ore component of the charge, the lower the content of iron-silicate strengthening melt, which leads to additional requirements for the technological process and the addition of components (blend concentrates with different properties, adding a fluxing additive) that increase the amount of melt to ensure pellet strength.

    It has been established that pellets of the same basicity, obtained from concentrates of the same enrichment depth, but of different origin, have different phase composition, microstructure and, accordingly, different physical and mechanical properties of the finished product. Oxidative firing of pellets maintains the iron-containing phase unchanged.

     During liquid-phase strengthening at the contact boundary of the solid (plate phase) hematite and iron-silicate melt, there is virtually no diffusion of non-metallic components of the charge into the hematite lattice. As a result, the iron-containing phase is preserved in pure form in the roasted pellets. The second important component of the ore is   siliconcontaining phases. It has been established that pellets with different quality characteristics are obtained from concentrates of different genesis, provided that they have the same Fe and SiO2 content and similar heat treatment modes. The main reason is the difference in the mineral forms of SiO2 existence in the original concentrate. The mineral form of silica existence determines the initial hardening temperature, the depth of interaction of the charge components, the composition and properties of the resulting phase bonds - determining the strength properties of the pellets. Depending on the melting point, silicon-containing  materials are divided into low- and high-temperature minerals. Low-temperature phases melt at a temperature of 1000-1250 C and are a source of melt during pellet firing. Hightemperature materials include silicon-containing minerals with a melting point exceeding  the firing temperature of the pelletized product. In the process of firing they are inert phases and participate in strengthening of pellets only in case of their absorption by the primary melt formed by melting of low-temperature mineral phases or calcium ferrites formed during firing at the stage of solid-phase sintering of the charge. Such an approach requires determining the optimal basicity, temperature-time and gas-dynamic firing modes for each type of concentrate. In concentrates produced from ferruginous quartzites the predominant silicon component is refractory quartz with a melting point of 1750 C. Such pellets have high strength properties during reduction in the range of basicities of 0.5-0.8units due to the formation of silicate binders containing high-silica glass, which does not collapse during repeated heating in a blast furnace almost until the complete reduction of hematite and the formation of a metal frame. After that, the silicate binder melts with separation into metal and slag. When the basicity decreases to the natural 0.05-0.12units during the production of unfluxed pellets or increases to above 1.0-1.2units during the production of highly fluxed pellets, pellets are destroyed in the upper part of the blast furnace and the strength during recovery decreases. During the production of unfluxed pellets, this is due to an insufficient amount of silicate binder, and in highly fluxed pellets, this is due to a change in the mineralogical composition of the binder. The silicate binder of pellets with a basicity of 1.0-1.2units is represented by the wollastonite phase and upon repeated heating in a blast furnace, it recrystallizes, which leads to  the destruction of pellets until a metal framework is formed. Strength can be increased during reduction in two ways: without changing the chemical composition of the batch by reducing the oxidation potential of the gas phase during liquid-phase strengthening of the raw material and the transition of the wollastonite binder CaO-Fe2O3-SIO2 into a strong ferromonicellite binder CaO-FeO-SIO2 due to the accumulation of FeO in the liquid iron silicate melt. The second way is to change the chemical composition of the batch by adding alumina- and magnesium-containing materials and obtaining a melilite binder CaO-Fe2O3-MgO-AL2O3-SIO2. For example, adding aluminacontaining iron ore concentrate and fluxing the batch with dolomitized limestone.

   The introduction of magnesium-containing additives (dolomite or dolomitized limestone) into the charge changes the mechanism of mineral formation. Magnesium oxide introduced into the charge instead of calcium oxide limits the process of ferrite formation  and,  as a consequence, the formation of  a  melt,  since it  leads  to the formation  of magnesia spinel MgFe2O4 with a melting point of 1750 C, which does not enter into the reaction of silicate bond formation up to a temperature of 1300 C. Magnesia spinel dissolves in the iron-silicate melt only in the temperature range of 1300-1380 C, leading to the accumulation of FeO in the system and the replacement of hematite with a solid magnetite solution and the development of silicate formation processes (figure).

      Magnesium oxide increases the amount of refractory ore phases (magnesia spinel) in pellets due to a decrease in the amount of low-melting phases (silicoferrite), which reduces the strength of pellets in the lower layers, where firing is performed at lower temperatures. Magnesium oxide at the stage of liquid-phase strengthening is the cause of the formation of a magnetite solid solution in place of hematite, which reduces the pellet reducibility, but increases the strength of pellets during reduction. A decrease in the amount of melt already at the initial stages of liquid-phase strengthening allows for higher-temperature firing of magnesia pellets compared to pellets fluxed only with limestone.

    The increase in the proportion of refractory phases in the magnesium-containing pellets makes them essentially similar to pellets obtained from concentrates of deep enrichment. The introduction of dolomitized limestone into the pellet charge affects the change in the phase composition of the pellets at different basicities and, as a consequence, leads to a change in the physical and mechanical properties of the pellets themselves.

      In pellets with a basicity modulus of 0.5units, when magnesium-containing additives are added to their phase composition, along with hematite and glass, a magnetite solid solution and a crystalline silicate phase of the pyroxene composition CaO-MgO-FeO-SiO2 appear. Changes in the phase composition and microstructure do not lead to significant changes in the physical and mechanical properties of the pellets. Recrystallization of the silicate binder leads to an insignificant increase in open porosity, which affects the decrease in strength during recovery. The absence of noticeable deviations in the properties of magnesite pellets with a basicity modulus of 0.5units compared to ordinary pellets of the same basicity, but fluxed with ordinary limestone, is explained by the fact that the composition of magnesite pellets contains a silicate phase in the form of glass, which is a strong bond between all other phases of the pellet: hematite, magnetite solid solution and pyroxene. Conventional pellets with a basicity modulus of 1.0units (CaO/SiO2) have a three-phase composition. The ore phase is represented by hematite, the binder is wollastonite and glass. The addition of dolomitized limestone leads mainly to a change in the ore component: a magnetite solid solution is formed instead of part of the hematite. With an increase in the amount of magnesium oxide in the charge, the amount of magnetite increases. The addition of MgO does not affect the phase composition of the pellet binder, which consists of wollastonite and glass. However, with an increase in the magnesium content in the pellets, the binder composition shifts towards an increase in glass and a decrease in the amount of wollastonite (with a simultaneous increase in the crystal size), cracks and concentric circles appear in the pellets, which lead to a decrease in the strength of the pellets.

      Conventional pellets with a basicity modulus of 1.2units (CaO/SiO2) have a two-phase structure of hematite and glass with rare inclusion of the β-modification of Ca2SIO4 - larnite. Addition of magnesium oxide leads to the formation of a solid solution of magnetite and an increase in the amount of glass. With an increase in the magnesium content in pellets to 4%, the silicate formation reaction is activated, leading to the appearance of cracks during cooling of the pellets. The decrease in strength is greater, the smaller the concentration depth of the concentrate and the greater the addition of magnesium oxide. The silicate binder in such pellets does not undergo qualitative changes, since even at maximum firing temperatures the amount of hematite FeO formed during dissociation is not sufficient for the transition of the wollastonite melt CaSIO3 to the olivine field (Mg,Fe)2SIO4 of the MgO-Fe2O3-FeO-SiO2 system, since in highly fluxed pellets the MgO introduced into the charge and most of the FeO formed in the melt are spent on building the magnetite lattice. MgO introduced into the charge at different basicities affects the strength characteristics of the pellets differently. In pellets with a basicity of 0.5units, MgO is mainly a silicate-forming component and, to a lesser extent, a spinel-forming one. With an increase in basicity to 1.0-1.2units, is already a spinel-forming component. Having passed to the stage of liquid-phase strengthening from magnesium spinel, MgO promotes the dissociation of iron oxide and the formation of FeO in the silicate melt, which leads to a change in the phase composition and microstructure of the pellets. The formation of a magnetite solid solution instead of hematite and an increase in the amount of silicate binder with an increase in the basicity and amount of MgO leads to a decrease in the porosity of the pellets and, as a consequence, a decrease in the recoverability and an increase in the compressive strength of the pellets. When talking about the quality characteristics of pellets, we consider the combination of three main components, their characteristics and influence on technological processes during firing and reduction: the ore phase, which includes the bulk of the iron, the silicate binder, as a binding element and carrier of pellet strength, and pores as elements of the pellet microstructure that determine the number of contacts between mineral phases and the gas environment and, to a certain extent, influence the strength of the pellet.

    During reduction, hematite gives up oxygen and turns into an iron sponge with a change in the original volume and crystalline structure, the silicate binder, depending on the morphology and composition, to one degree or another resists these transformations, preventing the destruction of pellets at the reduction stage. The pores are conductors of the reducing gas and try to reduce the role of the binder in maintaining the strength properties of the pellets during reduction. In pellets of natural basicity, a small amount of 1.5-4.0% high-siliceous, viscous and poorly wettable melt is formed. Porosity is formed due to local shrinkage of substances, the pellets retain the original microstructure of the pelletized charge. The pores are predominantly open and have the shape of lenses elongated along the circumference. In the entire volume of pellets with a basicity of 0.5-1.0units. During liquid-phase strengthening, a silicate melt of the corresponding basicity is formed, which, when the temperature increases, fills the gaps between the grains, moves and compacts the pellet ore body, forming its porosity. Such a melt has a lower viscosity and better wettability. An increase in the amount of liquid-mobile melt with an increase in basicity, capable of partially moving and compacting particles, leads to an increase in pores and an increase in porosity.

      During the process of strengthening oxidative firing, the magnetite component of the pellets undergoes oxidation, sintering and recrystallization stages. The resulting hematite fraction does not react with the non-metallic components of the charge and is not a source of new phases. The liquid phase in the pellet volume is formed as a result of melting low-temperature minerals of the non-metallic part of the concentrate in unfluxed pellets and ferrite phases during the production of fluxed pellets and further interaction of the resulting melt with quartz of the charge. This process leads to the formation of an iron-silicate melt, which, when the pellets are cooled, turns into a silicate binder of the corresponding composition, depending on the basicity of the pellets produced. As noted above, the amount of silicate binder depends on the concentration depth of the concentrate and the basicity of the pellets produced and varies from 1.5% in unfluxed pellets obtained in concentrates with a high concentration depth to 19% in concentrates with a minimum concentration depth and a basicity of 1.2units.