The initial heat engineering stage of obtaining a fired pellet is pellet drying. The basic concept of drying is the removal of internal and external moisture in pellets without destroying them. The moisture contained in raw pellets evaporates under the influence of the heat of gases passing through the layer. Water can be contained in the pellet structure in various forms:

          a. water contained in the pores and capillaries between the particles and grains of the material (capillary);
          b. chemically bound water in minerals accompanying iron oxides (in the form of hydrate);
          c. water included in the composition of binders that tend to form gels, such as bentonite (in the form of hydrate                                 Ca(OH)2);
          d. in the form of crystalline components in existing and added salts;
       Thus, the moisture of the green pellet is formed from the water that is directly in the material and the water added during the pelletizing process. Depending on the form of the content in the pellet structure, water evaporates at different temperatures during drying - capillary at a temperature of 100 C, salts and hydrate components at higher temperatures. Since this is the first stage of the technological process, it is the main link between the green pellet, pellet drying and subsequent stages of the process. Drying of the green pellet is accompanied by three main stages: the first is the evaporation of surface moisture, the second is the evaporation of capillary moisture from the inside of the granules (after this stage, the granules are usually dry), the third is hygroscopic and chemically bound water (hydrated). The main factor in obtaining a strong green pellet is the presence of strong interparticle bonds at the grain boundary due to the presence of a binding liquid in the pellet volume. During the drying process, there is another task – complete removal of the binding liquid (capillary and molecular moisture) from the pellet structure. During pellet drying, two processes of moisture release occur. The first is the movement of moisture in the pores from the center to the surface, the second is from the surface to the surrounding phase of the heat carrier. Naturally, the limiting stage is the internal process of transformation and movement of moisture, which is simultaneously in a liquid, vaporous and vapor-liquid state. Phase transformations of moisture form resistance to moisture diffusion on the pellet surface. During intensive heating of the pellet, excess pressure is formed inside due to the internal resistance of the layers to the movement of steam formed as a result of moisture evaporation.

        Heating of pellets should be carried out in such a way that the amount of steam formed is in balance with its movement and removal to the pellet surface and further into the surrounding gas phase. The higher the temperature gradient, the greater the volume of steam phase formed. An increase in the amount of steam phase inside the pellet and, as a consequence, an increase in pressure leads to damage to the pellet structure (cracks and ruptures) when the "shock" temperature is reached (the temperature at which such an amount of steam phase is formed and such excess steam pressure is created at which the pellet structure begins to collapse). The critical pressure at which pellets collapse depends on the size of the concentrate, the density of the pellets, the type of flux, the binder additive, and the strength of the ore frame.  The cause of destruction is the development of a volumetric stress state above the maximum permissible value, and is determined by the gradient of moisture content and temperature. Explosive shock, leading to a complete loss of pellet shape, is observed in the final period of drying, therefore the critical drying rate is determined by the pellet shock temperature. The limit of the critical drying rate is the absence of cracks and ruptures in the pellet structure. If the strength of green pellets mainly depends on the surface tension and capillary expansion of the wetting agent, then the addition of binders, such as bentonite, has a decisive effect on the strength of a dry pellet. Its use increases the strength of dry pellets to 5-15kg/pellet due to the formation of rigid skeletal bridges in the pellet structure, increasing the adhesive forces. For this reason, there are certain limitations on the use of organic binders in the charge, which cannot create a strengthening bridge structure during drying and heating of pellets, since they burn out at a temperature of 250-300 C. This is especially critical if bentonite is not added. Therefore, in practice, a charge with the addition of both bentonite and an organic binder is often used. It is important to preserve the strength of dry pellets by means of a binder additive until the onset of solid-phase sintering, when other strengthening forces begin to act due to the adhesion of ore grains of charge materials. The addition of bentonite to the charge binds and strengthens the structure of pellets and increases the resistance to destruction, raising the "shock" temperature of pellets. The higher the quality of bentonite (swelling index and water absorption), the higher the destruction temperature of the pellet. Sometimes, calcium hydroxide or slaked lime - Ca(OH)2 is used as a binder. However, its use is limited by the following conditions:
         a. slaked lime should not contain inclusions of unslaked lime, which undergo a slaking process with an increase in volume               during the pelletizing process and at the initial stages of drying, which leads to cracking of green pellets and, as a                             consequence, a violation of the technological mode of heating and firing and a decrease in the strength                                               characteristics of the fired pellets;
         b. the binding capacity of slaked lime is lower than the binding capacity of bentonite (the addition of 0.5-1.0% bentonite                  to the batch as a binding component is equivalent to adding 2-3% Ca(OH)2);
          c. the "shock temperature" of calcium hydroxide is significantly lower than that of bentonite and is about 300-400 C;
      Numerous industrial experiments conducted recently show the inefficiency of using this binder in the production of pellets, due to a decrease in the strength characteristics of the fired pellets and a loss in the productivity of the firing machine. The fired pellets should not contain free CaO, since after some time it hydrates with the moisture of the environment and weakens the structure of the pellets. This is especially true:
          a. during long-term storage in open warehouses;
          b. in the autumn-winter period or in the season of increased precipitation;
         c. for fluxed pellets, the higher the degree of fluxing and, accordingly, the basicity modulus, the more the pellets are                            subject   to destruction;
          d. during numerous reloads during transportation to the consumer;
         Green pellets obtained from finely ground concentrates with a high specific surface area and low porosity (due to a reduction in pore size) dry more slowly and have a lower "shock" temperature. 

        The intensity of pellet drying is higher, the greater the value of the total and, in particular, open porosity and the average pore size. The main criteria leading to a decrease in the intensity of the drying process are the size of the pellets, low pellet strength, the presence of cracks, an over-moistened structure in relation to the type of concentrate used and charge additives and shock destruction during heating. A decrease in the amount of vapor phase from the parity and, as a consequence, the pressure inside the pellet slows down the process of moisture movement to the surface and slows down the pellet drying process, which ultimately affects the productivity of this operation and, in general, all subsequent technological zones of the firing machine.

      In practice, three periods of pellet drying are defined. In the first period (warming up), the pellet moisture changes insignificantly, the surface temperature and evaporation intensity increase. At the end of the period, a constant surface temperature is established. Thermal equilibrium occurs between the amount of heat perceived by the material and the heat consumption for moisture evaporation. As moisture is removed, the iron ore pellet changes from a plastic to a solid elastic-brittle state. The porosity of the pellet increases and the density of the body decreases, nonequilibrium stresses arise within the structure of the material, and the intensity of heat and moisture transfer changes.

       The second period is characterized by drying at a constant rate. The pellet temperature is practically in equilibrium with the temperature of the heat carrier saturated with moisture vapor. This period is accompanied by pellet shrinkage, the unevenness of which can cause cracks. Shrinkage occurs until a strong layer of bound matrix macromolecules and pores is formed. Pellets shrinkage depends on the moisture content, concentrate type and drying intensity. The main shrinkage of pellets occurs at the first and second stages of drying to a temperature of, 200 C; the wetter the pellet, the greater its shrinkage. Therefore, the pellet must be dried at a safe heating rate, depending on its moisture content, size and the temperature of the heat carrier supplied to the drying layer.

        In the third period (with decreasing drying rate), the intensity of the process is determined by the rate of heat and moisture supply to the evaporation surface, moving into the pellet body. A decrease in the evaporation rate is accompanied by a decrease in heat consumption and an increase in the average pellet temperature. When equilibrium moisture is reached on the pellet surface, the drying rate becomes zero, i.e. the process of moisture removal stops and all supplied heat is spent on further physical heating and increasing the pellet temperature. 

      For optimal drying, it is necessary to have a significant component of the temperature difference between the coolant supplied to the vacuum chambers of drying zone 1 and the furnace hood of drying zone 2 and drying zone 3. The higher it is, the shorter the period of time required for drying pellets. The temperature of the coolant supplied to drying zone 1 and drying zones 2 and 3 must be maintained at 270-320 C. The temperature of the coolant supplied to the drying zones is also related to the final temperature of the coolant after drying, which is then released into the atmosphere through a dust cleaning system (electrostatic precipitator, wet and dry scrubber, bag filter). The value of the temperature of the exhaust gases after drying is especially critical for bag filters and electrostatic filters. It must be maintained at 140-190 C, preventing the formation of a "dew point" and condensation of moisture inside the dust cleaning devices. When producing pellets from magnetite concentrates with a high content of sulfur compounds, it is desirable to bring the exhaust gas temperature closer to the maximum permissible values, or to use a wet scrubber with the addition of a lime milk solution to neutralize sulfur. When heating pellets during drying, moisture evaporates and steam diffuses in the pellet volume. If the amount of steam formed is greater than its amount removed during diffusion by the coolant, stress concentration and cracks in the pellets occur. Therefore, the time of water evaporation and steam diffusion are the limiting factors of the drying process. When drying pellets, it is necessary to maintain a balance of three components - the temperature of the coolant, the drying time and the flow rate of the coolant passing through the layer of raw pellets. In the presence of a significant number of fired pellets with cracks, the coolant temperature is usually reduced or the drying time is increased. When drying pellets, due to the evaporation of moisture, the weight of the raw pellets is lost. 

     Drying time takes up to 30% of the total time of pellets on the induration machine, and up to 10-15% of the heat required for pellet heat treatment is spent on moisture removal. The complexity of the moisture removal process is associated with the heterogeneity of moisture across the pellet cross-section and the pore structure (uneven distribution of pores and their shape). The main goal of pellet drying is to minimize the effect of the overmoistening zone in the layer. For this purpose, a two-section drying zone with a coolant reverser is organized on induration machines. In the first drying zone, the coolant is fed from below into the layer of raw pellets through the bed layer, which avoids overmoistening of the lower layer of pellets and eliminates or minimizes swelling, increased plasticity and aerodynamic resistance of the layer and, as a result, the destruction of pellets in this part by subsequent overlying layers of pellets. Overmoistening of the layer always limits the increase in the height of the layer of raw pellets, If it is necessary to increase the height of the layer of raw pellets, pellets with increased strength properties, reduced humidity and an initially elevated temperature - preliminary drying outside the machine, or primary drying by suction at the beginning of the firing machine (zero chamber) must be fed to the firing machine. 

     The option of increasing the height of the raw pellet bed with a corresponding decrease  in the speed of the incinerator allows for better accumulation of the heat of the recirculation gases passing through this bed in the drying, heating and incinerator zones. This solution allows for a decrease in the consumption of process fuel and the filtration rate of the coolant in the incinerator zone. However, any increase in the bed height on the incinerator trolley will lead to an increase in power consumption due to an increase in the resistance of the higher bed and, as a consequence, an increase in the loads on the drives of the main process fans. Therefore, when considering the implementation of this technical solution, it is necessary to understand - the presence of a power reserve for the drives of the process fans and the opening value of the control servo drive dampers in the basic operating mode (the degree of opening should not exceed 75-80%, with a greater degree of opening of the dampers, this option cannot be implemented without replacing the fan drive with a more powerful one).

      The parameters of the heat carrier in drying zones 1 and 2 must be maintained based on the properties of the original pellets in order to achieve maximum drying intensity while maintaining the integral structure of the pellets. The drying process can be improved by reducing the initial moisture content of the raw pellets as much as the technology of producing raw pellets allows and by increasing the heat carrier consumption. Drying of pellets is the main limiting process for increasing the productivity of the indurating machine; until the moisture in the pellets is completely removed, a subsequent increase in the pellet temperature and transition to the next heating stage without loss of quality due to the presence of residual moisture inside the pellet body is impossible. Over the past few decades, attempts have been made to partially remove moisture before the firing unit (heating and partial drying of pellets on a conveyor belt, roller stacker) in order to further eliminate over-moistening of pellets in the layer and accelerate the pellet drying process over time to increase productivity and reduce heat costs for drying pellets. However, to date, the proposed solutions have not been implemented on an industrial scale due to the high costs of heating and implementing heat supply for this operation, the insignificant residence time of pellets and multi-layer conical stacking during transportation from unloading from the pelletizer to loading onto firing trolleys.
     The main directions for optimizing pellet drying on the firing machine were aimed at modernizing the drying zone and partially heating with the organization of a zero additional chamber at the beginning of the firing machine, a three-section drying zone instead of a two-section one, increasing the filtration rate of the heat carrier in the ascending flow in drying zone 1, and the use of three-level roller screens to optimize the layer-bylayer stacking of raw pellets on the firing trolley. The main factors influencing the pellet drying process are divided into two groups. The first group includes external factors (temperature, consumption and moisture content of the heat carrier), the second - internal factors (pellet diameter, its density, composition and structure):
         a. specific surface area of concentrate (cm2/gram) – increasing the specific surface area of concentrate leads to increasing                  the drying time;
         b. pellet porosity (%) – increasing porosity leads to decreasing the drying time;
         c. pellet size (mm) – increasing the average pellet diameter leads to increasing the pellet drying time;
        d. uniformity of particle size distribution – increasing the uniformity of the particle size distribution reduces the drying                     time due to improving  the gas permeability of the layer;
         e. moisture content in pellets – increasing the moisture content in pellets leads to increasing the pellet drying time;
         f. pellet bed height – increasing the height of the pellet bed leads to increasing the pellet drying time;
         g. bentonite, limestone content (kg/ton) – an increase in the addition of these materials to the batch leads to a decrease in               the pellet drying rate and an increase in its duration due to the compaction of the pellet structure and an                                            increase in the amount of bound water;
         h. heat carrier filtration rate (m/s) – an increase in the heat carrier filtration rate reduces the drying time;
         i. heat carrier temperature (degrees) – an increase in the heat carrier temperature reduces the drying time due to improved               heat exchange and an increase in the moisture capacity of the heat carrier;
         j. content of class 0-5 mm in the bottom bed (%) – an increase in this class leads to an increase in the drying time due to                   an  increase in the gas-dynamic resistance of the layer;