The technology of iron ore pellet production is one of the most complex in metallurgy, which is due to the following circumstances:
         a. the initial materials being processed have a complex physical, chemical, mineralogical and granulometric                                            composition, and the  wet pellet is a porous body, the main carrier of strength in which the molecular and                                         capillary forces that form this strength are interconnected;
         b. in the process of  heat treatment of the initial pellets, it is necessary to obtain an agglomerated product with                                     high mechanical  strength and specified metallurgical properties. To do this, it is necessary to create                                                     conditions for the most complete  development of physicochemical, mass transfer and structural                                                          processes  in an individual pellet;
         c. heat treatment of the initial iron ore pellets is accompanied by a noticeable number of physicochemical,                                              mineral-forming and structural processes, often occurring in the same temperature ranges,                                                                      mutually  influencing and hindering each other;
         d. heat treatment of pellets is carried out in a polydisperse layer due to the cross-flow of a gaseous heat carrier,                                       which creates noticeable temperature, mass transfer and time unevenness, both in the height of  the                                                      layer and in the width of the processed layer;
          e. in a single unit (annealing conveyor machine), several technological operations (drying, heating, annealing,                                      cooling) are carried  out sequentially, each of which has its own characteristics, and the efficiency of                                                     the subsequent operation depends on the previous  one;

       The input parameters of the initial pellets – quantity, humidity, granulometry composition, chemical composition and strength properties affect the output parameters of the fired pellets and determine the operation of the charge preparation, pelletizing sections and the firing machine itself as a whole. To coordinate the input parameters and physical and chemical processes occurring in the thermal unit, automated process control systems (APCS) are used, operating according to certain specified control algorithms. 

          Modern control systems for the operation of a firing machine must contain the following technological subsystems:
          a. automatic control of the operation of smoke exhausters, throttles of gas-air ducts and burners regulating the                                    flow rate of process gases, temperature conditions and flow rate of natural gas (excluding overheating of  the                                      main process equipment and refractory lining);
          b. automatic control of the productivity of the firing machine allowing for prompt assessment and                                                          coordination  of the operation  of the heating unit taking into account the thermal capabilities, quality                                              characteristics of raw (humidity,  grain  size composition and mechanical characteristics) and roasted                                                   pellets (strength, grain size composition);
          c. automatic control of the volume of exhaust gases (process gas flow sensors) and the composition of                                                     emissions  (devices for determining the concentration of harmful substances)

    This chapter analyzes the main factors influencing the production of pellets with specified physicochemical and metallurgical properties. Pellets are produced in two types: pellets for blast furnace processing and direct reduction pellets. The requirements for pellets used in blast furnace processing are less critical, since some of the harmful elements that reduce the quality of the product in subsequent stages leave the process with blast furnace slag. The requirements for direct reduction pellets have strict restrictions on the content of harmful impurities, since all oxides, including harmful impurities, enter the final product and are then used in steel production in electric arc furnaces.

      The main requirement  for the chemical composition of pellets is a maximum iron content of at least 67.5% and a minimum content of SiO2, Al2O3, P, S. The main properties of finished pellets begin to form at the stage of obtaining raw  pellets from the moment of their placement on the firing machine and until the moment of cooling of the already finished roasted pellets. Raw pellets produced on pelletizers and loaded on the firing machine must have the appropriate fractional composition, the content of class 9-16mm of at least 95%, have the specified strength properties - crushing strength of at least 1.0kg/pellet, dropping strength of at least 4.0kg/pellet. 

         It is generally accepted that the strength characteristics of an ideal green pellet should ensure its passage from the pelletizer to loading onto the firing machine and transportation in the firing machine until hardening. At the same time, the pellet should not be subject to destruction during the physical and chemical processes inside it. However, practice has shown that this is often not the case and in fact, in the total mass of fired pellets, there is a certain part of the pellets with strength indicators of less than 150kg/pellet, containing cracks, chips or even pieces of material that disintegrate into several parts, as well as an under-fired structure along the cross-section of the granules. There are several reasons - under-fired lower part of the pellet layer, bed pellets, green pellets, which from the very beginning of the firing process did not meet the requirements of strength characteristics and were destroyed during the thermal exposure. In order to understand the reason for the decrease in the quality characteristics of roasted pellets, it is necessary to analyze all possible factors influencing this process. It is necessary to evaluate the obtained quality characteristics of roasted pellets, starting from the composition  of the charge materials to the operation of the firing machine itself. It is almost impossible to evaluate the influence of the mineralogical and phase composition of the concentrate  and other components of the charge on the quality characteristics of finished pellets, especially in the operational control of the technological process due to the lack of the necessary laboratory equipment and the duration of the studies.

    The influence of various physical factors on the quality characteristics of pellets has been studied most - compressive strength, abrasion strength, impact strength, content of small classes. This assessment is constantly carried out at all factories producing pellets and almost all metallurgical enterprises consuming pellets, and their values serve as a guide for obtaining a pellet with specified properties. If the necessary equipment is available, operational control of the metallurgical indicator of pellet quality - low-temperature reduction strength LTD is also carried out. All other indicators related to the metallurgical properties of pellets are performed periodically in a specialized laboratory.

          Figure shows a generalized structural diagram of pellet production. 

        The main operation following the receipt of raw pellets and their placement on the firing machine is heat treatment, as a result of which the pellets undergo various heat engineering operations: drying, heating, firing, recovery and cooling. As a result of passing these stages, the pellets obtain the necessary physical and metallurgical properties that meet the requirements of various consumers of prepared iron ore raw materials. All operations from drying to cooling are interconnected, and violation of the process parameters at one of the stages makes it practically impossible to correct deviations at subsequent stages without loss of productivity or quality characteristics of the pellets. 

     The physicochemical properties and metallurgical characteristics of the roasted pellets depend on the mineralogical composition of the roasted pellets, which is formed as a result of reactions in the pellet volume between the ore particles, as well as between the ore particles and the gangue or flux particles during solidification. Thus, the size, chemical composition and mineralogical composition as well as the fluxing and firing conditions of the green pellets determine the mineralogical composition of the roasted pellets. Due to the differences in mineralogy, different types of pelletizing raw materials require very different fluxing and firing conditions, such as preheating temperature, preheating duration, oxygen potential, firing temperature and firing duration. Oxygen potential is especially important for the oxidation reactions of green pellets made from magnetite concentrates.

       Complete oxidation of magnetite minerals determines not only the quality of the roasted pellets, but also the energy intensity of the firing process. Since pellets are produced from different types of ore under different conditions, they will have different mineralogical compositions to achieve the required quality. A number of measures have been developed to optimize the effect of mineralogical composition on the properties of the produced pellets, such as blending different types of concentrates, using boron-containing additives and MgO-containing components, optimizing basicity, and pre-treating the concentrate with high-pressure rollers HPGR.