The firing machine is a heat unit, the heat scheme of which must ensure balanced productivity of each technological zone and is determined by the conditions of heat and mass transfer processes. As a heat unit, the firing machine has a heat balance for pellet firing. An example is given in table.

       An important parameter that affects the thermal efficiency of the process is the temperature of the cooled pellets coming off the firing machine. The higher it is, the greater the heat loss, and the lower the cooling efficiency of the thermal unit (diagram). This parameter should be constantly monitored, and its value should be minimized. The generally accepted temperature of pellets after cooling is considered to be 100-120 C (maximum acceptable, no more than 160 C).

      The heat consumption of units processing hematite concentrates is significantly higher than the heat consumption of units processing magnetite concentrates. Since the oxidation of magnetite to hematite releases additional heat, which is used in the heat balance of the process. To ensure the required heat consumption for firing hematite ores, solid fuel is added to the batch for pellet production. The combustion of this fuel inside the pellets directly provides part of the heat required for the process, with a higher heat output. Also, increased heat consumption is observed when using leaner hematite and magnetite concentrates with a low iron content and an increased amount of gangue (in particular, silica).

      There are a certain number of factors that directly affect heat loss and which must be eliminated or their impact minimized. Unfortunately, heat units contain a large number of flues equipped with compensators and moving elements, the presence of which complicates the sealing processes and often leads to a significant amount of heat loss.

    Particular attention should be paid to the condition of the heat-insulating lining of flues, collectors, vacuum chambers, process fan volutes and the serviceability of all types of compensators. Sealing of the moving parts of the firing machine with stationary parts (longitudinal and curtain seals), eliminating the suction of ambient air into the unit and emissions of high-temperature coolant to the outside. The condition of the grate field, firing trolleys (burnout, cracks, burnouts), refractory lining, losses with exhaust gases. The technological process and operation of the equipment must be configured in such a way as to exclude as much as possible the ingress of cold air into the furnace of the heating unit through various seals and leaks, that is, the vacuum in the furnace must be maintained at values close to the minimum. All these items are mandatory conditions for the normal operation of the heating unit.

     It is necessary to pay close attention to the reduction of heat losses from the bottom and side beds. The bed is calcined pellets of class 10-16mm, reused as a recirculation load for the protective layer on the firing trolleys. Repeated firing of these pellets worsens their physical and metallurgical properties. Their total amount is 25-35% of the total pellet layer (bed+raw pellet) placed on the firing trolley. On the one hand, reducing the height of the bottom bed (the recommended height by the equipment manufacturer is 80-100mm) and reducing the width of the side bed affect the increase in the productivity of the firing machine and a decrease in the amount of heat loss, on the other hand, this leads to an increase in temperature and additional heat loads on the grates, middle parts and sides of the firing trolleys, which significantly reduces their service life. In addition, the bed is necessary for uniform distribution of the gas flow over the height and area of the firing trolley, which leads to improved gas permeability of the pellet layer as a whole. Reducing the height of the bottom bed has a greater effect on reducing the recirculation load than reducing the width of the side bed, so to increase the productivity of the firing machine, the height of the bottom bed is usually reduced.

     From the technological point of view, to optimize the thermal efficiency of firing machines, pellets should have optimal moisture, good porosity, be uniform in granulometry composition and have an optimal size that ensures their most complete contact in the layer with the hot coolant at the drying-firing stages and cold air at the cooling stage. There should be as few non-uniform pellets and pellets of small and large class as possible. Differences in the technological process of firing hematite and magnetite pellets affect the productivity of firing machines and, as a result, the specific costs of heat and electricity. The difference in specific productivity when working with magnetite concentrate is plus 20-25% than when working with hematite raw materials, the same applies to the costs of heat and electricity.

     The firing machine has five process zones: drying, heating, firing, recovery and cooling. The operating parameters in these zones are adjusted by changing the vacuum and pressure in the corresponding sections of the gas path and the temperature of the gas and air entering the pellet bed. To change the temperature of the coolant passing through the pellet bed, the amount of natural gas and air on the burners of the heating and firing zone is changed, and the change in the speed and volume of the gas-air flow is regulated by the position of the blades and throttle devices on the valves of the gas ducts and process fans. Since pellet firing is an oxidizing process, the gaseous coolant must be enriched with oxygen. This problem is solved by using high-temperature air coming from the cooling zone and entering as oxidation and dilution air to the gas burner devices of the mixing chambers of the furnace through the direct recovery manifold.

       All phases of heat treatment are consecutive regardless of the firing system used. In this regard, various factors influencing the processes at each stage should be maximally coordinated with each other (temperatures for each reaction, the time of each reaction, heat supply to the layer to provide the process stages with the necessary thermal energy). The following parameters are controlled during the firing process: temperature, pressure/vacuum in the furnace and vacuum chambers, speed of the firing trolleys, machine load on green pellets, layer height, humidity, granulometric, chemical composition, strength characteristics of green and fired pellets. The main thermal engineering parameter of firing is temperature. The maximum value of the firing temperature is determined by the melting temperature of the pellets and the possibility of sintering them together into conglomerates. The minimum firing temperature for almost all batches is 1200-1230 C. A certain minimum temperature must be achieved to ensure the necessary crystalline transitions and oxidation reactions of the charge components. As noted earlier, it is higher the less slag-forming components are in the charge (e.g. deep enrichment concentrates). The optimal firing temperature and firing cycle in combination with the appropriate firing time are the main criteria for obtaining strong pellets. 

        An example of the temperature profile for pellet firing is shown in diagram.

     Other influencing factors (e.g. granulometry composition, use of various additives) become fully effective only if these requirements are met, although their indirect influence on pellet characteristics can be traced throughout the entire production stage. The smaller the number of thermal factors influencing each other, the easier it is to control the heating and firing temperature and thermal processes. Optimal use is a single source of external heat (natural gas, fuel oil). Introduction of an additional source of intra-layer heat - the use of various types of solid fuel or biofuel in the charge or the use of combined gas burner devices operating on natural gas and gaseous or solid biofuel complicates the control of the thermal firing process and requires additional parameters for monitoring and controlling the technology. Before switching to the use of several types of fuel, it is also necessary to assess the change in the impact on emissions of harmful components into the environment (COx, NOx, SOx), the concentrations of which will definitely increase due to the presence of a significant amount of sulfur compounds, carbon and nitrogen in their composition. This is relevant when using solid fuel (coal, anthracite, coke) or biofuel (especially unprepared - for example, sunflower seed husks, sawdust, nutshells, etc.). For each firing machine that is a source of emissions of harmful substances during combustion or addition to the batch of various types of fuel, at the state level for a certain period, the norms of permissible emissions into the environment are established, exceeding which is unacceptable. These standards are periodically revised towards tightening. The introduction of new types of fuel requires an unambiguous calculated assessment of the change in the concentrations of emissions of harmful substances during their use and further practical confirmation of the values of actual concentrations, which often leads to the need to revise the established environmental standards for emissions upwards with justification for this decision or the need to install additional gas cleaning devices.

      Along with the firing temperature parameters, another important process parameter is the residence time of pellets on the firing machine, starting from the drying zone and ending with the cooling zone. Each type of pellets has a certain processing time in the heating unit, like any other parameter of the technological process, it should not be subject to significant deviations in the time interval. Both underburning and overburning of pellets leads to deterioration in the quality of pellets. The average statistical duration of pellet firing is 20-25 minutes and is regulated by the speed of movement of the firing trolleys. Heat supply should be proportional so that the firing temperature is constantly increasing. Too slow heating does not have a negative effect on the quality of pellets, but the firing time increases and, as a result, the productivity of the firing machine decreases. Too fast heating can lead to heat accumulation and overheating of the pellet surface, which will lead to a decrease in the quality characteristics of the pellets.

      To obtain optimum strength, non-fluxed pellets produced from magnetite concentrates with a low SIO2 content of 1-5% should have a higher firing temperature of 1300-1350 C than pellets obtained from magnetite concentrates with a high SIO2 content of 5-10% at a temperature of 1250-1280 C. That is, the higher the SIO2 content, the lower the optimum firing temperature should be. The introduction of basic additives into the charge maintains this tendency, but reduces the optimum firing temperature with an increase in the basicity of pellets for both types of concentrates. It should also be taken into account that the dissociation of hematite also begins at lower temperatures if the pellets contain basic additives and increases with an increase in the proportion of magnetite as the temperature rises. One of the main reasons is the release of an additional amount of heat inside the pellet during the formation of calcium ferrites. Therefore, it is necessary to pay great attention to the upper limit of the firing temperature and the firing time interval when conducting the technological process. If the strength characteristics of the pelletsat the outlet of the firing machine have decreased, it is necessary, as the first measure of the stabilization process, to reduce the speed of the trolleys and then, along the entire technological line, to analyze the causes that influenced these changes, starting from the charge materials and ending with the operation of the firing machine itself and the technological fans.

       If we do not consider the reasons related to the shutdown or malfunction of the main equipment or incorrect actions of the process personnel, which in principle are almost always obvious and understandable, then often the main reason for the disruption of the firing process is a change in the moisture content of the charge, which subsequently entails a change in the moisture content, grain size distribution and strength characteristics of the raw pellets. Feeding raw pellets with increased moisture leads to a disruption of the drying mode, over-wetting of the layer, incomplete removal of moisture, additional heat consumption and a decrease in temperatures in the drying and heating zone. Pellets with residual moisture that has not been removed are destroyed during heating due to intense vaporization, clog the layer with small particles, worsen the gas permeability of the layer and at the subsequent firing stage do not allow obtaining pellets of a given quality along the layer height. The temperature in the vacuum chambers of the drying and heating zone decreases, therefore it is necessary to reduce the speed of the firing trolleys, which will increase the time interval of pellets in the drying zone in order to remove residual moisture and restore the optimal temperature regime. Simultaneously with the change in the speed of the firing trolleys, the load of raw pellets on the firing machine decreases and the layer height decreases, which will additionally avoid the effect of overmoistening.

       Since iron ore concentrate is the main component of the charge, the increase in the moisture content of the charge and then the raw pellets is associated with the constantly changing moisture content of the concentrate. If the moisture content of the concentrate is within a certain established range of deviations (for example, ± 0.2%) from the optimum value established for a given type of concentrate, taking into account its mineralogy, granulometry composition, specific surface area and particle shape, the influence of such deviations is minimal or absent. Each industrial production facility establishes its own requirements for the value of the required moisture content of the concentrate and the deviation range. The values of the deviation ranges may be different for positive and negative deviations. The optimum moisture content of the concentrate may change for each specific type of pellets. For unfluxed pellets, the optimum moisture content of the concentrate is lower than in the production of highly fluxed pellets with a basicity modulus of 0.6-1.0units. The higher the pellet basicity value, the higher the optimum concentrate moisture content. In principle, both in the case of fluxed and non-fluxed pellet production, we should reach an equivalent charge moisture content that is optimum for pelletizing for a given pellet type. In practice, the optimum moisture content for different pellet types should be determined experimentally. Such concentrate parameters as specific surface area, particle shape, and grain size distribution do not change as dynamically during the technological process and have a much smaller effect on the quality characteristics of green pellets. However, their effect should be taken into account in the context of changes in concentrate moisture content or their separate effect on the pelletizing process (for example, with a significant increase or decrease in the specific surface area or grinding fineness of the concentrate).

      Three coupling mechanisms are responsible for obtaining strong pellets at all stages of the technological process from raw materials to finished pellets:

        a. liquid bridges between individual grains of granulated material particles are responsible for the strength of green pellets                during their formation; 

        b. solid bridges in the form of "soluble bonds" and skeletal base, arising due to the presence of gels (bentonite), various                      hydrates and interfacial forces are responsible for the strength of dry pellets;

        c. solid bridges form the final strength of fired pellets due to crystal changes, crystal growth and sintering of intergranular                melting phases;