Pelletizing of the charge The main component of the pellet production process is the production of green pellets in the disc or drum pelletizer itself. Drum pelletizers operate in a closed cycle with roller screens for screening of large (+16mm) and small fractions (-8mm). Disc pelletizers can operate both independently and in pairs with roller screens for screening only large class (+16mm) or screening of large (+16mm) and small fractions (-8mm). The main factors influencing the production of green pellets are: a. physical and chemical properties of concentrate; b. physical and chemical properties of additives; c. amount of added substances; d. humidity of batch; e. speed of rotation of tray (drum); f. angle of inclination of tray (drum); g. change in height of tray side; h. number and location of bottom and side scrapers in tray; i. place of loading of batch into tray; j. location of water supply; k. amount of water supplied; Points a, b, c, d were discussed above. The charge must be fed to the pelletizer with a moisture content below the optimum value so that the plant's process personnel have the opportunity to bring the charge moisture to the optimum value by feeding water to the pelletizer. In general, feeding water to the pelletizer is one of the important elements of controlling the rolling of raw pellets, which allows adding moisture to the pelletizer with a dry charge, and reducing the water feed to the pelletizer when the moisture content increases above the optimum. The process of obtaining raw pellets is conventionally divided into stages. The first begins with the formation of pellet embryos, and water serves as the center of their formation. Embryos are dense, over-moistened lumps of the charge onto which the pellet shell rolls. The charge almost always contains areas of material with increased humidity and density, which are formed after filtration, transportation and loading into the pelletizer. If their number is sufficient for pelletizing the charge with optimal humidity, then additional moistening of the charge is not performed. If the humidity of the charge is less than optimal by 0.3-1.5%, then the formation of embryos is formed by drip irrigation in the charge feed zone, which moves along a descending trajectory of the working space of the plate. The formation of the embryo also occurs after the destruction of fragile pellets. At the second and third stages of obtaining raw pellets, there is a rapid growth of the embryo and formation of lumps of the batch, which, under the action of dynamic forces when rolling along the inclined surface of the plate and impact effects between themselves and the wall of the pelletizer, gradually form the structure of the final pellet. The average static indicators of the content of the mass of fractions in the pelletizer batch are as follows: 15-20% of the conditioned class, 10-15% of embryos, 65-70% of the intermediate class. In practice, it often happens that the moisture in the batch is the optimal value or even slightly higher than the optimal value and water is not supplied to the pelletizer. If the moisture of the batch increases above the optimal value, it is necessary to take prompt measures to stabilize it. For this purpose, the consumption of bentonite or crushed return in the batch is increased and the moisture of the concentrate is stabilized by adjusting the filtration mode or changing the concentrate batching (feed from the warehouse/direct feed from the process). Diagrams show some aspects of the influence of concentrate moisture on the performance of the pelletizing process and the operation of the firing machine. The main control lever for the pelletizing process, along with changing the amount of moisture and bentonite consumption (return), is changing the rotation speed of the pelletizer. Changing the pelletizer speed to a smaller or larger value allows for the production of larger or smaller pellets. The pelletizer speed can be adjusted from 0 to 8rpm. The next element of pelletizing process control is changing the pelletizer tilt angle. For disc pelletizers, the working range of tilt angle is from 46 to 50. Adjusting the pelletizer tilt angle is used last, when all other control methods have failed to achieve the desired result. In practice, a control method is often used by decreasing or increasing the load on the pelletizer with simultaneous redistribution of the load on other pelletizers, however, this method is associated with the need to adjust the pelletizing process on all pelletizers with a redistributed feed load of the batch. Recently, some manufacturers of pelletizing equipment (Haver & Boecker Niagara) have added another element of pelletizing process control – a variable height of the pelletizing plate side. This technology works on an industrial scale, but requires further development and a clear control algorithm. Bottom and side scrapers are installed permanently during commissioning of the pelletizer according to the recommendations of the equipment supplier and during operation during operational control of the pelletizing process, their position in the pelletizer is not changed manually, Industrial samples of intelligent scrapers have been developed, which are equipped with drives and allow their position in the pelletizer plate to be quickly adjusted during operation. All criteria for the formation of high-quality green pellets must be optimally selected for each type of unit and production conditions, i.e. there is a certain relationship depending on the type and characteristics of the concentrate used, which must be determined by testing between speed, angle of inclination, side height (for a disc pelletizer), degree of filling and residence time. The green pellet production process is a multifactorial process with many changing variables. The more parameters change, the more often this happens and the greater the range of deviations from the set values, the more difficult it is to regulate the pelletizing process. 0

Solid fuel When producing pellets on a conveyor firing machine, heating is performed due to the heat of combustion of natural gas (fuel oil) above the surface of the layer, which leads to uneven heating of the pellets along the height of the layer. The strongest pellets are formed at the top of the layer, and less strong pellets are at the bottom of the layer. Solid fuel in the composition of pellets performs two functions: an energy carrier for the release of heat and the creation of the necessary temperature conditions for the thermal treatment of pellets (internal heat source) and as a component of the charge, necessary for the occurrence of physicochemical processes in an individual pellet and the processed layer. The main objective of using solid fuel is to stabilize the strength properties of pellets along the layer height due to more complete firing of the bottom and middle of the layer, reducing the consumption of natural gas and increasing the productivity of the firing machine. Therefore, the choice of fuel is determined by a combination of many factors. As an additional additive to the batch for pelletizing, solid fuel (fossil coals - hard and anthracite, peat and coke) is introduced, which is constantly used in the production of pellets from hematite concentrates and serves as a source of additional heat for heating the layer and the physicochemical processes of pellet firing. When using magnetite concentrates, heat sources are practically not used inside the layer, since during the oxidation of iron oxides of magnetite to hematite during heating and firing of pellets, an additional amount of heat is released necessary for the technological process. When adding solid fuel to the charge and firing pellets from magnetite concentrates, cases of sintering are possible due to local excess heat and an increase in the amount of liquid melt. Firing of coal-containing pellets is carried out at reduced firing temperatures (20-50 C) in order to prevent overheating of the middle and lower parts of the layer. It is necessary to find a balance between the consumption of solid fuel and the firing temperature, at which the top of the layer will be fully fired, and the bottom of the layer will not be subject to local melting. To optimize the use of solid fuel when using magnetite concentrates, coke nut was tested as a material with the lowest ash and sulfur content and the highest carbon content. The experiment was based on coke nut with an ash content of no more than 13%, with a moisture content of up to 20.0% and a fraction of 8-25mm. Before feeding into the charge, the solid fuel was crushed together with limestone to a fraction of 86-88% class 0.056mm and a moisture content of 1-3%. Optimum results in compressive strength, drum test and abrasion were obtained by adding 2kg of coke nut per ton of pellets. A slight increase in the productivity of the firing machine by 1-2% was also noted. A further increase in the addition of solid fuel led to a decrease in the strength properties of pellets by 8-15kg/pellet due to an increase in the porosity of the pellet layer and an increase in the FeO content by more than 2.0-5.0% due to the occurrence of a reduction reaction involving coke nut carbon in the process. The use of solid fuel for magnetite concentrates may be appropriate if the pellet strength required is up to 250kg/pellet. The further requirement of the sales market to increase the compressive strength above this value is not commensurate with the use of solid fuel, since an increase in pellet porosity leads to its decrease, which has excluded the widespread use of this technology at present when using magnetite concentrates in the charge. The next additive to the batch, along with the use of coconut nuts, was the use of activated peat, which was also used as an intra-layer heat source to equalize the temperature of the ingredient along the layer height. Also, the addition of crushed peat, due to the high water absorption of the material, was used to reduce the moisture content of the batch, which led to a decrease in bentonite consumption. However, since with the use of coconut nuts, an increase in the porosity of the fired pellets when using peat led to a decrease in the compressive strength of the fired pellets of less than 250kg/pellet. The use of peat also found its limited application due to the high humidity of the source material of about 50-60%, which complicated the process of its preparation before feeding into the batch. A positive feature of using solid fuel is the reduction in natural gas consumption and a slight increase in the productivity of the firing machine by 1-2%. The presence of ash and sulfur in solid fuels must be constantly monitored because they add undesirable elements and oxides to the pellet composition and lead to an increase in the content of sulfur compounds in the exhaust gases emitted into the environment. Therefore, it is desirable to use solid fuels with a high hydrocarbon content, low ash and volatile matter content, as well as sulfur and phosphorus. 0 0

41 Introduction to Mineral Processing Amir Eskanlou, Ph.D. Read more 42 Sticking and Swelling of Iron Ore Pellets: Mechanisms and Controlling Factors S. Purohit, M. I. Pownceby, A. Guiraud Read more 43 Beneficiation of Iron Ore - Existing Best Practices/Technologies and Way Ahead Satyendra Sudershan, Bhawna Khalkho Read more 44 Weathering impacts on iron ore pellets José Aurélio M . da Luz Read more 45 Understanding strength of dried iron ore pellets Victor Claremboux Read more page 1 page 2 46 Hydrogen: The Future of Iron Making Industries Hemant Panchal, Yutik Rana and Vrushita Rana Read more 47 Improving the pelletizing process of hematite-magnetite concentrates to enhance pellet quality Shayan Navaei, Saeed Sheibani, Leila Hosseini, Nader Esmaeili Read more 48 Assessing Iron Ore Pellet Quality Parameters for Optimized Performance Across Pellet‑Based Furnace Operations Leonardo Tomas da Rocha, Seongkyu Cho, Jung‑Ah Kim, Kyoung‑Oh Jang, Sung‑Mo Jung Read more 49 Weathering impacts on iron ore pellets José Hamilton Resende Tavares, José Aurélio Medeiros da Luz, Fábio Ferreira Ribeiro Read more

Firing of pellets On a conveyor firing machine, pellets are heated along the layer height in the heating and firing zone from top to bottom by gas – a heat carrier passing through the pellet layer and giving off part of the heat to heat the pellets. The temperature rise curve in the layer also changes – in the upper part, pellets are heated to 1280-1350 C, in the lower part to 1200 C. Different horizons of the layer along its height are processed with different temperature-time parameters. For example, pellets in the upper part of the layer are at a temperature of 1200 C for about 7-8 minutes, in the middle part of the layer for about 3-4 minutes, the presence of pellets in the lower part at the layer-bed boundary practically tends to zero time values. The strength characteristics of the pellets also change along the layer height. The strongest pellets are in the upper part of the layer, the least strong in the lower part of the layer. This is due to the low firing temperature in the lower part of the bed of about 1180- 1220 C and a short time interval between pellets being at these temperatures. To equalize the temperature characteristics along the bed height, it is necessary to keep the pellets in the lower part of the bed at the firing temperatures for a longer period of time and increase the filtration rate of the coolant in the bed. Temperatures in the furnace of the firing machine are monitored using thermocouples installed on the longitudinal walls of the furnace and determining the temperature of the coolant in the furnace or using pyrometric devices installed outside the furnace and directly determining the surface temperature of the pellet bed. The completion of the pellet firing process is monitored by the temperatures in the last vacuum chamber of the firing zone and the vacuum chamber of the recovery zone. The recommended temperature values in these vacuum chambers should be in the range of 450-600 C. In practice, when assessing the required temperature conditions in the furnace of the firing machine, the values of temperatures in the vacuum chambers, filtration rates and productivity of the firing machine, it is necessary to be guided by the requirements for the main quality characteristics of the roasted pellets shipped to the consumer - compressive strength (CSS), abrasion strength (AI), tumble strength (TI), strength at low-temperature recovery (LTD). When the specified values are met, the operation of the firing machine is optimized empirically, for example, by increasing the productivity of the firing machine or reducing the regulatory temperatures in the furnace of the firing zone or vacuum chambers. 0 0 0 0 0

Heating of pellets 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 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. 0 0

Limestone Limestones are used in various stages of metallurgical production. Depending on the magnesium oxide content, they are divided into ordinary fluxing and dolomitized fluxing. Depending on the purpose, limestones are used for the production of pellets, blast furnace production, open-hearth and converter production. They differ in granulometric and chemical composition, compressive strength and water absorption. In the production of pellets, limestone is a fluxing additive. Ordinary or dolomitized limestone is traditionally used to produce pellets with varying degrees of fluxing. The dissociation of limestone during the firing of pellets is an endothermic reaction and occurs with the expenditure of heat for its decomposition. Limestone also has virtually no binding effect, but in some cases its significant addition to the batch (in the production of highly fluxed pellets) allows for the pelletizing process to be carried out with increased moisture content in the concentrate or to reduce bentonite consumption somewhat. It is supplied raw in wagon loads with a moisture content of no more than 3% and a size of 3-25mm in summer and 20-60mm in winter. The need for pellet fluxing is one of the blast furnace operators' requirements. The addition of CaO corrects the pellet acidity, giving it the necessary physical and metallurgical properties suitable for use in cast iron production. The introduction of calcium and magnesium containing materials into the charge is necessary to obtain a slag of a certain composition and physical properties (melting point, viscosity, fluidity), which is capable of removing sulfur, phosphorus, and other harmful impurities. Feeding raw limestone into the blast furnace leads to overconsumption of coke, which is more expensive than natural gas, for decomposition of limestone. Therefore, highly fluxed agglomerates with a basicity modulus of 1.2-1.8units and fluxed pellets with a basicity modulus of 0.3-1.0units are used as non-metallic blast furnace charge in order to reduce the feed of raw limestone. In principle, blast furnace operators find a balance between the supply of non-metallic materials of varying degrees of fluxing and the required amount of iron introduced into the iron ore charge for optimal operation of the blast furnace. For the production of iron ore pellets of varying degrees of fluxing, various types of limestone are used as a fluxing additive. The formula of ordinary limestone is CaCO3. Dolomitized limestone MgCO3 is characterized by the presence of an increased content of magnesium oxide MgO 5-9.0% in the structure of the material. Less commonly used is dolomite CaCO3·MgCO3 with an MgO content of 9-21%. Magnesite and dunite are also sometimes used as a source of MgO (over 40%). The use of the first additive is limited by its availability and reserves, the second additive contains a significant amount of SIO2. When selecting limestone for pelletizing, the following characteristics are considered - the content of basic oxides (CaO+MgO), moisture content, grain size distribution, content of silicon oxide (SiO2), sulfur (S), phosphorus (P), insoluble residue. In some cases, strength and water absorption criteria are additionally used. These indicators are important, since high water absorption and low strength often lead to the destruction of limestone pieces during transportation, storage and worsen the process of its processing, since the material becomes heterogeneous with an admixture of small wet fractions. The main criterion for choosing limestone for pelletizing is the maximum content of basic oxides (CaO+MgO), this is especially relevant for highly fluxed pellets, since it allows reducing its addition to the batch while maintaining the basicity modulus of the pellets produced. In order to assess the applicability of limestone for each metallurgical process (agglomeration, pelletizing, blast furnace, steelmaking, lime kilns), a rating of limestones is compiled, in which a list of specific quality parameters of limestone required for inclusion in the rating calculation is determined for each metallurgical process. The following parameters are taken into account as key parameters when calculating the limestone rating, depending on its grade and method of use in metallurgical production: a. mass fraction of (CaO+MgO), %; b. mass fraction of insoluble residue (IR), %; for open-hearth and converter production it is taken equal to the mass fraction of (SiO2); c. moisture content (W), %; d. compressive strength (CS), MPa; The remaining limestone parameters (mass fractions: MgO, S, P, water absorption, granulometric composition) must comply with the requirements of the technological process. Before being fed into the batch, the limestone undergoes the technological stages of crushing (when using a large fraction) and grinding with drying in ventilated ball mills or similar units. When dosing into the batch, the prepared limestone has a moisture content of 0.3-0.7%, the content of class 0.056mm is 84-90%. The insignificant moisture content in ground limestone allows using a concentrate with a higher moisture content in the production of fluxed pellets than in the production of unfluxed pellets. The higher the basicity modulus, the higher the moisture content of the concentrate that can be fed into the batch for the production of green pellets. Thus, the difference in moisture content in the concentrate between the production of pellets with a basicity modulus of 0.12units and a basicity modulus of 1.0units is can be 0.4-0.5% (abs.).

Green pellets The quality characteristics of green pellets are characterized by three main parameters – fractional composition, pellet strength on dropping and pellet strength on crushing. The moisture content is also additionally controlled. Green pellets at the outlet of the bowl pelletizer should have a uniform fractional composition. A narrow size distribution of green pellets is an important criterion for the quality of pellets, which characterizes high gas permeability of the layer during oxidative firing on a firing machine and the reduction process in blast furnaces and metallization units. The size of granules is adjusted either mechanically, by changing the gaps of the roller screen, or technologically, by changing the moisture content, load, binder dosage, rotation speed, side height, plate or drum inclination angle. It is known that increasing the dosage of the binder component reduces the growth rate of raw pellets, making them smaller, while increasing the moisture content leads to the opposite effect of enlarging the granules. Stable operation of the granulator requires control of the humidity and grain size distribution of the batch components. During the production of raw pellets, the granulometry composition of suitable pellets fluctuates, which requires constant monitoring of the process by process personnel. To increase the granule size at a constant load, additional moistening of the material, an increase in the rotation speed of the pelletizer and a decrease in the angle of inclination are required. To reduce the pellet size to the standard size, it is necessary to remove or reduce additional moistening of the material if possible, reduce the rotation speed of the plate and increase the angle of inclination of the pelletizer. An increase in the load leads to a decrease in the pellet size, and a decrease in the load makes the pellets larger. The correctness of the pelletizing process is determined by the maximum value of the 8-16mm fraction from the total mass of raw pellets coming out of the plate. The optimal value for the production of DR pellets is 50% of the 8-12.5mm class and 50% of the 12.5-16mm class. When producing BF pellets, it is advisable to shift the granulometric composition of raw pellets towards increasing the content of the 8-12.5mm class. The drop strength must be maintained for at least 4-7drops from a height of 500mm onto a flat steel plate. It is considered that green pellets should have a minimum required strength, sufficient for their transfer to the conveyor belt from the pelletizer and further transportation to the firing machine. If the strength is insufficient, even green pellets that have retained their shape may have cracks, both internal and coming to the surface, which significantly reduce the strength of the roasted pellets, provided that they are not destroyed during the firing process. Ideally, a high-quality green pellet should have high strength and porosity. However, in practice, as the strength of the green pellet increases, the porosity of the pellet decreases and vice versa. The strength of the green and dry pellet is necessary to prevent its destruction before placing it on the firing machine and during the firing process. Porosity is necessary for the processes of moisture removal and oxidation reactions of iron oxides to form a solid single-layer hematite structure of the pellet. In the process of pellet production, it is necessary to select the optimal values for the strength and porosity of the green pellet and ultimately focus on the quality indicators of the fired pellets (compressive strength, abrasion strength, impact strength, granulometric composition). The golden rule says that high-quality raw pellets are the main determinant of obtaining high-quality fired pellets. High-quality raw pellets have a huge potential to increase the quality of fired pellets and the productivity of firing units. The correct required strength value can be determined independently by calculating the number of overloads of raw pellets after the pelletizer plus one additional overload. The lower critical limit of the drop strength is 4 times (optimum 6-7 times), crushing strength is not less than 1kg/pellet (optimum 1.2-1.5kg/pellet). These values are calculated taking into account the load on the raw pellet in the pellet layer on the firing trolley. When designing a section of raw pelletizing at newly constructed pelletizing plants, these requirements for the quality of raw pellets are necessarily taken into account and, if possible, the number of reloads and the height of the pellet fall between reloads are minimized. In practice, it is not always possible to maintain these values for strength, which leads to the formation of an additional amount of raw pelletizing return, which is returned to the pelletizers and serves as a source of destabilization of the pelletizing process. Cracks and chips appear in some pellets, which are subsequently destroyed in the firing machine during drying and heating. Many pelletizing plants install video control systems for pellet grain size distribution to minimize significant deviations in the process of green pellet production. These systems allow for prompt determination of pellet grain size distribution by fraction. Their operation is based on contactless measurement of three main pelletizer performance indicators: the proportion of large class, average diameter, and roughness index of green pellets (size determination and analysis subsystem). Based on the received and processed data, the following subsystem, green pelletizing optimization, generates the main values of the pelletizing line control parameters (rotation speed, batch load, water consumption, bentonite consumption, plate inclination angle, and plate side height) in accordance with the developed action algorithm for each control loop. The system can also solve the problem of optimal load distribution between pelletizers in accordance with the quality characteristics of the pellets they produce. The optimal place for installing a video monitoring system is the tray chute, in this place it is possible to determine with a high degree of probability the entire fractional composition of pellets leaving the pelletizer. It is also common practice to install video surveillance systems on the outfeed conveyors from the pelletizer, but such control does not allow to fully determine the entire fractional composition of the raw pellet, since only the upper part of the material located on the belt is controlled. Based on video monitoring systems for the granulometry composition of raw pellets, many companies have installed automatic control systems for the granulometric composition of raw pellets. The control elements of the system are the rotation speed, the angle of the tray, the height of the side, the amount of moisture, the consumption of bentonite, the load. The most difficult thing in this system was to create an algorithm for controlling the operation of the pelletizer, since it is necessary to determine the sequence of action of each factor or their combined effect on the process. Manual process control algorithms with several professional pelletizers were studied, and a comprehensive pelletizing process control algorithm was created on this basis. In general, the green pellet production control system in automatic mode is operational only with minor deviations in the quality characteristics of the batch or batch materials fed to the pelletizer or changes in the batch characteristics that change gradually without sharp fluctuations. If the moisture content, specific surface, fractional composition of the batch materials often change over time, the system cannot adequately assess the correctness of the control actions, it is necessary to transfer the system to manual control and then everything depends on the professionalism of the process personnel. Over time, when the neural network is implemented and a significant base of deviations and correct decisions is developed, the manual mode of operation will be minimized. Today, this system can only really assess the granulometry composition of green pellets by size classes in the video surveillance mode.

Constancy of composition The main problem of preparing the components of the charge before mixing is maintaining the constancy of the specified characteristics over time. In practice, for high-quality pelletizing, deviations in the moisture content of the concentrate should not exceed 0.2%, specific surface area ±100cm2/gram, fractional composition 0.5% in absolute terms. Two parameters that allow you to quickly monitor and manage the state of the concentrate are the moisture content and fractional composition of the concentrate. Many enterprises install moisture analyzers of the concentrate, which promptly display the moisture content in the concentrate online and allow you to quickly manage the concentrate filtration process. In fact, the process of stabilizing the constancy of moisture is much more complicated than it seems in reality, since this is a multifactorial system and the influence on this criterion begins at the stage of ore mining in the quarry, its crushing and directly the enrichment technology itself. The preparation of other components of the charge (bentonite, limestone, solid fuel, return) does not cause any particular problems since the process of preparation (crushing, grinding with drying) of these materials is carried out directly at the pelletizing plant itself. In practice, two indicators are constantly monitored in the technological process: the fractional composition of the material and the moisture content. The specific surface of non-ferrous materials of the charge is determined periodically and monitored by the research laboratory during experiments, significant violations of technology or commissioning work on the grinding equipment. The preparation of the batch components (limestone, bentonite, return, solid fuel) can be performed either separately for each type of material or jointly for several materials. Solid fuel is ground together with another inert material, such as limestone or bentonite. This is especially important when the gas cleaning units of the mill paths contain electrostatic precipitators. With small volumes of the batch of fluxing and binding additives and in order to optimize costs, three materials are ground simultaneously (limestone, bentonite and return) in one mill cycle. Since the return of fired pellets has strong abrasive properties, it is advisable to grind it with limestone or bentonite in order to reduce the wear of the volutes, fan rotors and gas duct systems of the mill paths. Organic additives are often used to reduce the consumption of bentonite, improve the shape and grain size distribution of raw pellets. They are delivered to the workshop already prepared in big bags and are dosed into the batch using screw feeders. Organic additives are very sensitive to moisture, including hygroscopic, therefore, to avoid the formation of lumps of material, they must be stored in closed polyethylene containers. When storing in intermediate or screw feeder bunkers, heating of these bunkers to a temperature of 25-300 C must be provided. Before the mixing operation, the procedure for dosing organic binders into the batch is as follows: concentrate → limestone (return) → organic binder → bentonite. The chemical composition of materials is determined for all components of the charge to correctly perform the charge and obtain the required quality characteristics of the fired pellets in terms of basicity modulus and iron content. Dosing and mixing of the material must ensure the specified parameters for dosing accuracy (at least ±0.5%) and mixing coefficient (at least 90%). It is recommended to use vortex mixers "Lödige" or intensive mixers "Eirich" with a mixing coefficient of 90-95%. Rotary and drum mixers do not provide the required mixing quality, since the mixing coefficient is 60 and 75%, respectively. 0