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

Bulk density and porosity In pelletizing, there are several important concepts related to minerals, concentrates and pellets - bulk density and porosity. These two concepts are indicators of the degree of completion of the pellet structure formation and the intensity of firing (time and temperature). Bulk density is a property related to a solid material in its granular state. It is the ratio of the mass of a set of particles to the volume occupied by a certain container, determined according to ISO 3852. Bulk density can be determined with or without compaction of the material. Higher values are obtained with compaction. The bulk density of concentrate is 2.6-3.2kg/dm3, bentonite 0.8-1.1kg/dm3, limestone 1.1-1.4kg/dm3, anthracite 0.8-1.1kg/dm3, slaked powdered lime 0.4-0.6kg/dm3, pellets 1.85-2.2kg/dm3. Porosity is the proportion of pore volume in the total volume of a porous body. Total porosity of pellets takes into account two types of pores (closed and open communicating with the external environment) and is defined as a percentage ratio between the total pore volume and the granule volume according to ISO 15901. Iron ore charge used for the production of green pellets consists of the iron ore part (concentrate, roasted crushed return), fluxing (limestone), binding (bentonite, organic polymers) and others (solid fuel, colemanite) and has a porosity of 40-45%. When pelletizing the charge and obtaining green pellets in granulators, their porosity decreases to 30-32% due to compaction, shrinkage and increased strength. In roasted pellets, due to the formation of liquid phases and a decrease in the number of pores, the porosity decreases to 20-25%. When adding solid fuel, the porosity of the fired pellets increases to 25-35% depending on the type and dosage of this fuel. The increase in porosity occurs due to the formation of additional pores inside the pellet during combustion of the solid fuel. In addition to the porosity of the pellet itself, the porosity of the pellet layer itself is also very important on the firing machine. Good porosity along the layer height ensures the best heat exchange by convection during contact of the pellet surface with the heat carrier passing through this layer. To improve the porosity of the layer, it is important to ensure a minimum presence of substandard small classes in bed and raw pellets in the layer, which reduce the number of pores and to reduce the presence of large classes as much as possible, since their content reduces the porosity of the layer by reducing the surface area of contact of hot gases with the pellet surface. Prevent over-moistening of the lower layers of raw pellets in the drying zone, which leads to plastic deformation of raw pellets in layers and filling of pores. Each type of produced pellets has a certain composition of raw materials, which leads to characteristic values of density and porosity. Such data allow you to set certain firing parameters in the production of a given type of pellets.

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Heat balance 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; 0 0 0 0 0

Methods for determining the quality of pellets All quality indicators of roasted pellets are divided into three categories of properties: chemical, physical and metallurgical. The quality characteristics of pellets obtained after firing in an oxidizing unit relate to the physicochemical properties of pellets - compressive strength, abrasion strength, impact strength, grain size distribution, porosity, chemical composition. Roasted pellets should ideally contain the maximum amount of iron in the form of hematite, the content of iron in the form of magnetite should be virtually absent. The presence of silicon-containing materials and harmful impurities: sulfur, phosphorus, alkaline compounds (Na2O, K2O) and NaCl should be as low as possible. The physicochemical properties of the starting materials, charge, raw and fired pellets are determined in laboratories or research centers at each plant, and the data obtained allow for prompt management of changes in the technological process at all stages of pellet production. All studies of the quality characteristics of pellets during reduction in a blast furnace, which are directly related to it, relate to the determination of metallurgical properties. Almost all determinations of the metallurgical properties of pellets can be performed only in specialized laboratories that have the appropriate equipment and trained personnel. Determination of the values of the metallurgical properties of pellets that best meet the requirements of blast furnace smelting allows for a targeted impact on the elements of pellet production technology in order to improve one or another indicator. To assess the comparability of the properties of various iron ore materials in terms of their suitability for use in further processing, it is necessary to conduct comparative tests according to the same standardized documents (for example, ISO or JIS), otherwise the results obtained are not subject to comparative analysis. In recent decades, the issue of determining the strength of pellets during low-temperature destruction (LTD - Dynamic test for low-temperature reduction-disintegration) has become especially relevant for blast furnace operators, so some enterprises incur costs and purchase installations to determine this indicator. Consumers of products believe that the strength of iron ore pellets after low-temperature reduction is one of the main indicators of pellet quality, which affects the operation of the blast furnace, since it is directly related to the gas permeability of the charge material layer and the degree of destruction of pellets in the upper part of the blast furnace. To assess the quality of pellets, the tumble strength indicator is mainly used - the percentage of class +6.3mm. The higher the value of this indicator, the less susceptible the pellets are to damage. In practice, it is accepted that the minimum value of LTD +6.3mm should be at least 80%. Tests are carried out according to ISO 13930. This standard establishes a method for assessing the destruction of iron ores during reduction in a rotating tube at a temperature of, 500 C. This method is used for large-sized ores and pellets, as well as for blast furnace feedstock materials, and provides an assessment of the degree of destruction and reduction in pellet size during the reduction of hematite to magnetite under conditions close to the upper part of the blast furnace shaft at low reduction temperatures. The following common indicators characterizing the metallurgical properties of pellets in blast furnace processing at higher reduction temperatures are: a. the degree of recovery R, %, which is expressed as a percentage and characterizes the value to which the pellets have been recovered over a certain period of time and temperature. Usually, tests are carried out according to ISO 7215 at a temperature of 900 C for 180 minutes and determine the final degree of recovery of the material sample; b. the recovery rate index dR/dt, %/min, which is expressed as the recovery rate in percent per minute. Tests are carried out according to ISO 4695 at a temperature of, 950 C; To assess the technological process, it is advisable to determine the physical and chemical properties of the fired pellets at least once every 2 hours, and to determine the tumble index (TI), abrasion index (AI) and low-temperature recovery strength (LTD) once a day, If these indicators do not correspond to the specified values, the frequency of determining these indicators must be doubled. In contrast to blast furnace pellets (BF), additional requirements for quality characteristics are imposed on pellets (DR) used in direct reduction units. This is due to the fact that, unlike a blast furnace, these units reduce iron oxides in a solid state without further formation of a liquid phase. In this regard, all harmful impurities pass into the composition of the resulting metallized product (DRI/HBI), and do not go into slag as in blast furnace processing. Increased requirements for iron content, strength characteristics and metallurgical properties are imposed on pellets used in metallization units. To assess the suitability and quality control of these pellets for the direct reduction process, an additional concept has been introduced such as the degree of metallization (ISO 11257), which, in combination with the degree of reduction, characterizes the reducibility of granules and the efficiency of the direct reduction unit. The degree of metallization is expressed by the following ratio: Fem - metallic iron content in the recovered sample, % Fet - total iron content in the sum of oxides (Fe2O3+ Fe3O4+ FeO) in the reduced sample, % These values are determined chemically. The degree of metallization in well-functioning direct reduction plants with high-quality raw materials is at least 94-95%. Also, pellets used in direct reduction units are assessed by the reduction destruction index RDI DR , which is determined as a percentage of the -3.15mm class. m0 - weight of the entire restored load, including material collected on dust collectors, grams m1 - mass of the recovered sample remaining on a 10mm sieve, grams m2 - mass of the recovered sample remaining on the 3.15mm sieve, grams The permissible value of this indicator should be no more than 3.0% of the content of class -3.15mm. 0 0 0

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