The next heat engineering operation after pellet drying is their heating. The preheating zone is the link between pellet drying and firing. The temperature range of the heating zone is from 300-400 C to 1100-1200 C. When the temperature increases and the pellets are heated, the following reactions occur in the heating zone: evaporation of crystal water, decomposition of hydrates, carbonates, sulfates, firing of sulfur from sulfide components, oxidation of magnetite to hematite. Almost all reactions occur in parallel and their components depend to some extent on the composition of the charge components. Hydrate water may be present in iron ore concentrate and its insignificant amount may be contained in inclusions of waste rock of the charge. Hydrate moisture is removed at a temperature of 300-600 C, some components of the charge can retain hydrate moisture up to a temperature of 800-1000 C (for example, thuringite). Since the reaction of hydrate moisture removal is associated with heat costs, with its significant content in the charge (the presence of difficult-to-decompose hydrates), it is necessary to take into account heat costs in the heat balance. Decarbonization processes occur in fluxed pellets. Fluxes play a major role in the processes of solid-phase strengthening, sulfur removal, and liquid phase formation. Ordinary limestone Ca(CO3) or dolomitized Ca,Mg(CO3)2 is used in the charge for pellet production. The decomposition of carbonates is an endothermic process (with heat consumption) and is completely completed at a temperature above 1000 C. 

                                                                                            

                                                                                           
The main factor influencing the process of decarbonization and oxidation of magnetite is the heating rate of the pellet layer: an increase in the heating rate from 25 to 100 C/min leads to an intensification of the decarbonization and oxidation process. With slow heating (below 100 C/min), the oxidation and decarbonization processes occur simultaneously, while the released carbon dioxide (CO2) reduces the oxygen diffusion coefficient and thereby slows down the oxidation process. With rapid heating, the intensive decarbonization process outpaces the oxidation of pellet magnetite, increases porosity and thereby improves the conditions for the oxidation process, allowing it to be completed at higher temperatures and at a higher rate. Consequently, the pellets must be fired so that the decarbonization process occurs first, and then the oxidation process at maximum speed. This is facilitated by rapid heating (over 100 C/min), up to a firing temperature of 1100-1200 C for fluxed pellets and 1000-1100 C for non-fluxed pellets.

       a. oxidation of pyrite at temperatures above 300 C with the formation of iron sulfate (1):
                                                                                            
       b. dissociation of pyrite at temperatures above 600 C to form iron sulfide and sulfur (2), which are oxidized to form iron                 oxide (3):
                                                                                            
                                                                                            
        c. formation of calcium sulfate (4) as a result of the onset of limestone decomposition at temperatures above 600 C.                        Sulfur removal slows down. When the temperature rises to 900 C, freshly formed lime actively absorbs sulfur (5) and                   sulfur removal from pellets does not occur:
                                                                                                

                                                                                             
        d. dissociation of calcium sulfate in pellets at temperatures above 1150 C as a result of the influence of silicon and iron oxides, as well as the reduced oxygen content in the firing zone, with the formation of iron and sulfur oxides (6):
                                                                                             
The degree of desulphurization is calculated using the formula:
                                                                                                              
                                                                              Ƞ – degree of desulphurization of pellets, %
                                                                              Su – sulfur content in unfired pellets, %
                                                                              Sf – sulfur content in fired pellets, %
The physical picture of the desulfurization process is as follows:
      When heating pellets, sulfur removal begins at temperatures of 300-400 C and increases to a maximum at temperatures of 500-600 C regardless of basicity. In the drying zone, wet pellets absorb sulfur from the coolant gas, worsening the desulfurization  process. At  temperatures  of  700-900 C,   the rate of sulfur removal decreases, and at temperatures of  900-     1000 C, noticeable sulfur removal does not occur, since the rate of CaSO4 formation exceeds the rate of sulfate oxidation, absorbing sulfur oxide. Before the firing zone, no more than 10-15% of sulfur is removed and most of it, approximately 80-85%, by this time passes into the sulfate form. With an increase in temperature, desulfurization accelerates due to the dissociation of sulfate in the presence of oxides Fe2O3, SiO2. Replacing limestone with dolomite in the pellet charge reduces the temperature of the onset of active sulfur removal from 1200 C to 1100 C. The sulfur desulfurization process is most complete in unfluxed pellets. At temperatures close to 1300 C, the degree of desulfurization is about 95%. Increasing the amount of limestone introduced into the charge leads to a more complete development of the process of calcium sulfate formation and an increase in the amount of the resulting melt, which blocks the primary grains of iron sulfides and hinders the diffusion of oxygen to the reaction surfaces and gaseous sulfur oxides from the center of the pellets. With an increase in the basicity of the pellets, the temperature of the onset of melt formation and its viscosity decrease, which, other things being equal, also hinders the desulfurization process. When firing pellets with a basicity modulus of 0.7-1.2units, the sulfur removal process is complicated due to the higher chemical strength of sulfates. The highest degree of sulfur removal is achieved when it is contained in the form of magnesium sulfate MgSO4. This is due to the fact that magnesium sulfite is a less stable compound and completely decomposes at a temperature of 1200 C according to the reaction:
                                                                                                 
   Sulfur removal during firing occurs in three stages. The first stage involves the decomposition of sulfur-containing compounds and their oxidation. The second stage involves reactions of sulfur dioxide with alkaline components of the waste rock to form stable sulfate compounds. The third stage is the main process of sulfur removal during thermal dissociation of the original sulfates and those formed in the second stage.

      Basic recommendations for removing sulfur:
      a. the use of flux in the batch in the form of CaCO3 and CaO reduces the degree of sulfur removal;
      b. the use of flux in the batch in the form of MgCO3 increases the degree of sulfur removal by 15-25% compared to the use              of CaCO3;
      c. the addition of carbon to the batch reduces the temperature of sulfur decomposition and intensifies the process of sulfur             removal;
      d. an increase in the grain size of ground limestone in the batch reduces the degree of sulfur removal due to incomplete                     decomposition of the flux, and additionally reduces the strength of the pellets due to the presence of residual inclusions               of unreacted flux;
      e. intensification of the process of sulfur removal by burning natural gas in the furnace of the firing machine with an excess            fuel coefficient α=1.1. The high degree of sulfur removal is explained by the small amount of calcium sulfate formed and              the increased amount of unoxidized magnetite, which ensures more complete sulfur removal at a temperature of 700-                  1000 C;
      The heat treatment mode for sulfur pellets sets the temperature at the layer-bed boundary at 1200-1230 C. To maintain this temperature, it is necessary to reduce the productivity of the firing machine or intensify the drying and heating process of the layer by increasing the filtration rate.
    The process of oxidation of magnetite to hematite is one of the main processes accompanying the heat treatment of magnetite pellets. The transformation of magnetite into hematite is an exothermic reaction with the release of heat.
                                                                                                    
    The oxidation reaction of magnetite begins at a temperature of about 300 C and ends at a temperature of 1200 C. Magnetite is oxidized by oxygen in the coolant gas. The transfer of oxygen into the pellet occurs through various mechanisms and depends on the diameter of the pores, their configuration and the overall porosity of the pellet structure.
       The main factors influencing the oxidation process of magnetite:
       a. heating rate - increasing the heating rate of pellets from 25 to 100 C/min causes a significant increase in the oxidation                  rate;
       b. firing temperature - with an increase in temperature, the degree of oxidation of magnetite increases;
       c. gas phase composition - a change in the composition of the gas phase is mainly due to a change in the oxygen content in              the fuel combustion products. With an increase in the oxygen content in the gas phase, the oxidation rate of magnetite                 increases.This effect is due to an increase in the rate of diffusion flow of oxygen in the pores of the pellets;
       d. pellets porosity - an increase in the porosity of the pellets due to an increase in the pore size and total porosity increases                 the degree of oxidation of the pellets;

      Small particles <0.01mm in size of iron-containing materials oxidize faster in time and at lower temperatures, particles 0.05-0.1mm in size oxidize much longer and at temperatures of 1150-1200 C. In fluxed pellets, an increase in basicity slows down the oxidation process due to the formation of a significant amount of melt, blocking the access of oxygen to magnetite and contributing to the formation of a zonal pellet structure. In practice, under production conditions, it is objectively almost impossible to determine the sufficiency of the oxygen content in the gas phase of the coolant for the oxidation of magnetite. It is necessary to control the operation of air blowers by the pressure and amount of primary and secondary air supplied to the combustion zone and the operation of gas burner devices.
          Migration of gases in the pellet layer occurs in two directions:
          a. gases - penetrating into the granules in proportion to those existing in the surrounding air - oxygen for oxidation of the                iron ore part of the charge and combustion of coal (when added to the charge), nitrogen - an inert gas not participating                in oxidation and combustion reactions;
           b. gases - escaping from inside the granules through pores, obtained as a result of physicochemical reactions in the pellet                   volume: H2O - obtained as a result of moisture evaporation, CO2 - obtained as a result of limestone decomposition                     and coal combustion, SO2 - obtained as a result of S removal from sulfur-containing components, N2 - an inert gas                       initially entering the pellet from the air; 

          The physical and metallurgical properties of magnetite pellets depend significantly on the degree and nature of oxidation of iron ore materials. The structure of oxidized ore grains largely determines the strength of roasted pellets, including strength during reduction. Completion of oxidation processes means obtaining a uniform pellet structure. The presence of an unoxidized core is usually accompanied by the appearance of concentric cracks between the hematite periphery and the magnetite core, which are the cause of the destruction of pellets during their transportation and reduction in the blast furnace.
Modern requirements for the quality of iron ore pellets, especially for the purpose of their subsequent metallization, define the dissociation of hematite as an undesirable process, firstly increasing the content of divalent iron FeO, and secondly accompanied by the expenditure of thermal energy. The dependence of the rate of oxidation and dissociation of magnetite on temperature is shown in diagram. The oxidation of magnetite and the dissociation of hematite is a reaction that can proceed
in the forward and, accordingly, reverse direction.
                                                                                                       
     Under certain pellet firing conditions (temperature over 1300 C), along with the oxidation process, dissociation of the formed hematite occurs, resulting in an increase in the FeO content of the fired pellets. The hematite dissociation process has a significant impact on the heat consumption of the bed and the firing mode, since the fuel heat is spent not only on heating the material, but also on the dissociation reaction. The FeO content in the pellets at the outlet of the firing machine and the total thermal effect are the result of two opposite processes: oxidation and dissociation.