Coke substitution with anthracite in sintering production

Citation: Pustejovska, P.; Sikora, M.; Jursova, S.; Pustejovska, K.; Kardas, E. Coke Substitution with Anthracite in Sintering Production. Processes 2024, 12, 440. https://doi.org/10.3390/ pr12030440 Academic Editor: Blaž Likozar Received: 31 January 2024 Revised: 13 February 2024 Accepted: 17 February 2024 Published: 22 February 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Article Coke Substitution with Anthracite in Sintering Production Pavlina Pustejovska 1,* , Michal Sikora 1, Simona Jursova 2, Kristyna Pustejovska 3and Edyta Kardas 4 1Department of Metallurgical Technologies, Faculty of Materials Science and Technology, VSB—Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic 2Department of Computer Science, Faculty of Electrical Engineering and Computer Science, VSB—Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] 3 Department of Industrial Systems Management, Faculty of Materials Science and Technology, VSB—Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic 4Department of Production Management, Faculty of Production Engineering and Materials Technology, Czestochowa University of Technology, J.H. D ˛abrowskiego 69, 42-201 Czestochowa, Poland; [email protected] *Correspondence: [email protected] Abstract: This article summarises the possibility of replacing the coke breeze sintering fuel with an economically and ecologically more suitable fuel, anthracite. The main focus is on the possibility of replacing coke breeze with anthracite, during which, the replacement process is accelerated and the other properties are also affected. The analyses performed showed that the replacement of coke breeze with different amounts of anthracite does not have a negative effect if the initial permeability of the sintering bed is the same. Keywords: sintering; coke breeze; anthracite 1. Introduction The metallurgical industry is increasingly seeking substitutes for commonly used raw materials and fuels [ 1 ]. In the iron and steel production processes, it is very important to reduce the amount of fuel used [ 2 ]. Most of the atmospheric pollutants released come from the iron and steel industry, and more than 42% of the dust, 65% of the SO 2 , and 55% of the NO x are emitted as exhaust gas pollutants during the sintering process. The S and N content in fossil fuels is high and a large amount of SO 2 and NO x is released during iron ore sintering [3,4]. Metallurgical research seeks to reduce the energy intensity of processes and minimise environmental impacts. The blast furnace charge consists of three main raw materials: fuel, iron ore (sinter, pellets), and additives. The efficiency of the blast furnace process is affected by both the quality of the blast furnace coke and the quality of the ferrous burden materials [5–7]. Metallurgical firms consume significant energy in the steel production process, and the iron and steel industry is characterised by a high energy intensity and material usage. Almost half of the input material was found to be off-gases, process gases, and solid production residues [8]. Between 2010 and 2022, pig iron production in the European Union (EU) saw a rise from around 94.05 million metric tons to a peak of approximately 95.19 million metric tons. Pig iron production decreased in the following years. In 2022, it was about 73.7 million metric tons. The production of pig iron within the EU is shown in Figure 1. This statistic illustrates the total pig iron production in the EU from 2010 to 2022 [ 9 ]. According to the International Energy Agency (IEA), steel production is projected to grow by 30% by 2050 [10]. Processes 2024,12, 440. https://doi.org/10.3390/pr12030440 https://www.mdpi.com/journal/processes Processes 2024,12, 440 2 of 13 Processes 2024, 12, x FOR PEER REVIEW 2 of 13 The type of fuel used has an effect on the economy and ecology of production, as well as the properties of the sinter. The price of anthracite is, on average, lower than the price of producing coke (coke breeze). Coke breeze has a non-volatile carbon content of 91%, a volatile content of 4.5%, an ash content of 17%, and a sulphur content of 2%. Sulphur constitutes a significant portion (approximately 65–80%) of the organic component, the remainder being composed of sulphur in various forms such as FeS2, FeSO4, CaSO4, and elemental sulphur. The concentration of SO2 emissions is 800–2000 g per ton of sinter. The sulphur content of the fuels influences both the quality of the sinter and the amount of SO2 emissions. The suitable substitute fuel is anthracite according to its chemical composition. The amount of sulphur in anthracite is in the range of 0.25 to 0.48%. The faster combustion reactivity of anthracite leads to a higher maximum temperature and a shorter sintering time. When using anthracite, the strength of the sinter increased in the 0.5–6.3% fraction [4,8]. Figure 1. Total pig iron production in the European Union (EU) from 2010 to 2022 in million metric tons [9]. 1.1. Sintering Process In ferrous metallurgy, the process of sintering fine ferrous compounds, such as fine ores, blast furnace residues, steelwork waste, mill scale, and electrostatic precipitator dust, is known as the sintering process (Babich et al., 2008) [11]. This sinter production is a thermotechnical procedure in which the materials are heated using carbonaceous fuel. Typically, the main carbonaceous fuel used is coke breeze, a by-product of metallurgical coke sorting [12]. Coke breeze is the main fuel used in the sintering process, with a particle size of 1 to 3 mm, and is considered the most favourable for effective sintering [13]. Alkaline additives should have a particle size below 3 mm. During the initial phase, individual grains of alkaline additives may undergo dissociation, and in the subsequent phase, the reaction typically occurs between the CaO decomposition product and SiO2 from the surface of the molten ore grain [14]. Chemical Reactions during Sintering During the sintering of the agglomeration mixture, a large number of physicochemical events take place in the layer, which affect the sintering process and the quality of the produced agglomerate in different ways and to varying degrees. Knowledge of the course of individual events and processes is a prerequisite for the economic development of the agglomeration and to increase its quality [2,11]. 0 10 20 30 40 50 60 70 80 90 100 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 Milion metric tons Year Figure 1. Total pig iron production in the European Union (EU) from 2010 to 2022 in million metric tons [9]. The type of fuel used has an effect on the economy and ecology of production, as well as the properties of the sinter. The price of anthracite is, on average, lower than the price of producing coke (coke breeze). Coke breeze has a non-volatile carbon content of 91%, a volatile content of 4.5%, an ash content of 17%, and a sulphur content of 2%. Sulphur constitutes a significant portion (approximately 65–80%) of the organic component, the remainder being composed of sulphur in various forms such as FeS 2 , FeSO 4 , CaSO 4 , and elemental sulphur. The concentration of SO 2 emissions is 800–2000 g per ton of sinter. The sulphur content of the fuels influences both the quality of the sinter and the amount of SO 2 emissions. The suitable substitute fuel is anthracite according to its chemical composition. The amount of sulphur in anthracite is in the range of 0.25 to 0.48%. The faster combustion reactivity of anthracite leads to a higher maximum temperature and a shorter sintering time. When using anthracite, the strength of the sinter increased in the 0.5–6.3% fraction [4,8]. 1.1. Sintering Process In ferrous metallurgy, the process of sintering fine ferrous compounds, such as fine ores, blast furnace residues, steelwork waste, mill scale, and electrostatic precipitator dust, is known as the sintering process (Babich et al., 2008) [ 11 ]. This sinter production is a thermotechnical procedure in which the materials are heated using carbonaceous fuel. Typically, the main carbonaceous fuel used is coke breeze, a by-product of metallurgical coke sorting [ 12 ]. Coke breeze is the main fuel used in the sintering process, with a particle size of 1 to 3 mm, and is considered the most favourable for effective sintering [13]. Alkaline additives should have a particle size below 3 mm. During the initial phase, individual grains of alkaline additives may undergo dissociation, and in the subsequent phase, the reaction typically occurs between the CaO decomposition product and SiO 2 from the surface of the molten ore grain [14]. Chemical Reactions during Sintering During the sintering of the agglomeration mixture, a large number of physicochemical events take place in the layer, which affect the sintering process and the quality of the produced agglomerate in different ways and to varying degrees. Knowledge of the course of individual events and processes is a prerequisite for the economic development of the agglomeration and to increase its quality [2,11]. Processes 2024,12, 440 3 of 13 1. Moisture Removal Hygroscopic moisture already begins to evaporate intensively at a temperature of around 55 ◦ C. The evaporated water passes from the charge into the gas phase, which ensures its further transport. 2. Dissociation of hydrate and carbonate The bound water in the form of hydrates begins to be released only at temperatures above 320 ◦ C. The dehydration process only takes place intensively at higher temperatures, and the complete removal of bound water can occur even at temperatures around 1300 ◦ C. Carbonates enter the agglomeration batch through the ore part of the mixture, which may contain FeCO3 a MnCO3 , and also through basic additives that adjust the basicity of the mixture. The dissociation of siderite requires a temperature of 400 ◦ C to 550 ◦ C and proceeds according to the following equation: FeCO3=FeO +CO2(1) part of the FeO reoxidises according to the equation FeO +1 3CO2=1 1Fe3O4+2 3CO2+1 3CO (2) Rhodochrosite dissociates according to the equation MnCO3=MnO +CO2(3) To ensure the necessary basicity of the agglomerate, limestone ( CaCO3 ), magnezit (MgCO3), or dolomite (CaMg(CO3)2) is added to the sintering mixture. The highest temperature for the dissociation of all carbonates is required for the dissociation of limestone. It decomposes at a temperature of about 900 ◦ C according to the following equation: CaCO3=CaO +CO2(4) Magnesite ( MgCO3 decomposes at a temperature of 620 ◦ C–680 ◦ C according to the reaction MgCO3=MgO +CO2(5) The dissociation of dolomite takes place in two stages. In the first stage, MgCO3 dissociates at a temperature of around 700 ◦C: CaMg(CO3)2=CaCO3+MgO +CO2(6) and in the second stage at 900 ◦C, the resulting limestone decomposes. The solid products of the dissociation of CaO and MgO participate in the formation of calcium and magnesium ferrites, respectively, which are a welcome form of agglomerate. 3. Reduction and oxidation processes in the sintering layer Simultaneously, with the dehydration of the charge with the dissociation of carbonates, a partial reduction of oxides begins to occur in the sintering layer. Reduction processes take place mainly in the immediate vicinity of fuel grains, where there is a significant concentration of CO. Among the reduction processes, the following can take place mainly in the sintering layer: - Reduction of Fe2O3to Fe3O4by carbon monoxide and solid carbon; - Reduction of Fe3O to FeO by carbon monoxide and carbon; - Reduction of Mn3O4by carbon monoxide. Of the oxidation processes that take place in the layer, the most significant are - Oxidation of fuel carbon to CO2or CO; Processes 2024,12, 440 4 of 13 - Oxidation of CO by oxygen to CO2; - Oxidation of FeO to Fe3O4a Fe2O3; - Oxidation of sulphur-containing compounds [2,11]. Coke dust is thoroughly mixed with wetted ore mix, limestone, or dolomite, as well as industrial waste such as the sinter recovery fraction, slag, metallurgical waste sludge, etc. In the first stage of the process, the materials are mixed and carefully layered on a sintering belt by adding waste materials from other ironmaking operations. The last layer is usually composed of coarse material. Coke burning on the sintering belt takes place according to Equations (7)–(10): C+O2→CO2(7) and (in part) C+1 2O2→CO (8) As the temperature increases, the subsequent gasification of carbons takes place: C+CO2→2CO (9) as well as CO oxidation: CO +1 2O2→CO2(10) Unlike Equations (7) and (8), the other equations are exothermic, which then leads to an increase in the temperature on the sintering belt. This temperature rise can be successfully reduced by the optimal distribution of the fine coke particles. The air intake causes the sintering zone, which has formed at the top, to move downward, and the agglomerate is gradually formed. Cold air drawn through the bed cools the already baked layer, heats, and preheats the layer below the combustion zone. The process is complete when the sintering zone reaches the lowest layer. This is followed by an oxidation zone and a cooling zone. The resulting sinter is further crushed and sieved [11]. The raw material for the sintering process is fine-grained iron ore and iron ore concentrates, which are imported from abroad. Their composition is shown in Tables 1and 2. The process of sintering iron ores involves the heating of the dust agglomeration mixture (ore, fuel, additives) to such a temperature that the surface of the individual grains of the charge melts. The resulting melt will form liquid bridges between the grains, which after solidification will ensure the formation of a solid porous material, sinter. Table 1. Chemical composition and grain size of fine iron ore samples. Composition of Fine Iron Ores (FIOx) (w.%) Grain Size (mm) Raw Material Fe SiO2CaO MgO Al2O3P Mn S Na2O K2O Zn FeO H2O<0.5 mm >10 mm FIO1 56.6–64.0 6.2–17.8 0.02–0.10 0.05–0.34 0.59–0.88 0.015–0.35 0.01–0.04 0.012–0.016 0.090–0.201 0.033–0.053 0.002–0.007 0.14–0.68 3.5–4.5 20–24 <12 FIO2 59.8 12.1 0.06 0.14 0.77 0.026 0.02 0.011 0.148 0.043 0.004 0.37 3.8 35–37 <15 FIO3 62.7 7.3 0.52 0.23 0.84 0.023 0.06 0.020 0.055 0.027 0.004 0.90 4.6 20–23 <10 FIO4 63.1 2.4 0.15 0.04 1.38 0.047 0.27 0.016 0.016 0.011 0.003 0.14 7.0 21–24 <11 FIO5 65.0 3.5 0.08 0.06 0.90 0.042 0.08 0.005 0.005 0.008 0.003 2.02 7.4 30–36 <14 FIO6 65.6 1.1 0.10 0.05 0.89 0.074 0.31 0.014 0.013 0.024 0.003 0.18 6.8 19–23 <16 FIO7 64.5 6.7 0.10 0.05 0.96 0.047 0.16 0.004 0.013 0.015 0.003 3.92 7.5 33–36 <12 FIO8 67.05 1.4 0.70 0.52 0.47 0.001 0.02 0.003 0.001 0.001 0.001 8.04 4.0 20–26 <11 FIO9 64.7 1.3 0.03 0.07 0.77 0.057 0.02 0.007 0.016 0.000 0.006 0.14 5.5 30–34 <13 Table 2. Chemical composition and grain size of iron ore concentrates. Composition of Iron Ore Concentrates (IOCx) (w.%) Grain Size (mm) Raw Material Fe SiO2CaO MgO Al2O3P Mn S Na2O K2O Zn FeO H2O <0.5 mm >10 mm IOC1 65.54–67.50 5.90–7.83 0.12–0.14 0.35 0.15 0.007–0.008 0.016–0.023 0.022–0.027 0.052–0.059 0.014–0.030 0.003 27.16–27.55 10.1–10.2 8 8 IOC2 64.82 8.01 0.25 0.36 0.1 0.007 0.02 0.035 0.069 0.058 0.003 26.61 9.4 13 4 IOC3 64.90–67.5 5.9–8.49 0.05–0.19 0.32–0.35 0.25–0.31 0.011–0.015 0.032 0.012–0.073 0.025–0.060 0.030–0.042 0.003 26.96–28.0 9.3–10.5 9 11 IOC4 66.36 6.78 0.21 0.44 0.18 0.014 0.041 0.035 0.045 0.049 0.003 28.07 8.8 47 10 IOC5 65.68 7.28 0.23 0.29 0.25 0.013 0.027 0.066 0.032 0.042 0.003 27.05 9.4 62 10 IOC6 67.79 4.92 0.16 0.33 14 0.011 0.025 0.044 0.062 0.035 0.002 28.22 9.5 51 9 IOC7 65.61 7.59 0.17 0.28 0.06 0.011 0.015 0.018 0.06 0.181 0.003 26.08 9.5 21 6 IOC8 64.74 0.42 0.23 5.49 1.78 0.038 0.43 0.269 0.026 0.032 0.034 23.75 2.7 15 48 IOC9 69.65 0.97 0.24 0.13 0.78 0.075 0.056 0.02 0.011 0.022 0.005 37.43 7 28 12 Processes 2024,12, 440 5 of 13 Key steps of the sintering process: The thermal processes in the sintered layer affect mainly the quality of the agglomerate produced. Ignition of the mixture The charge is ignited with a mixed gas. A certain amount of heat (about 45 MJ/m 2 ), a certain time (40 s to 90 s), and a certain negative pressure are needed. Fuel burning After ignition, a 10–30 mm high combustion front is formed. The speed of the combustion front is 0.03–0.04 cm/s. There is excess air in the bed, and coke carbon burns to produce about 80% CO2and 20% CO. Heat transfer In the upper zone of heat transfer, the hot sinter preheats the passing air, whereas in the lower zone, the hot flue gases preheat the charge. At the beginning of the process, neither the air nor the charge is preheated, and at the end of the process above the grate, the preheating is at the maximum. Therefore, there is significantly more heat in the lower part of the layer than in the upper part. This is reflected in the mineralogy composition and properties of the sinter produced [2,11]. The absence of coking coal and the environmental considerations for decarbonisation create opportunities for substituting coke with alternative fuels. Anthracite, charcoal, and other types of biomass are used, as the appropriate char obtained from car tyres could be a potential substitute for some of the coke breeze used in the iron ore sintering process [ 15 ]. The replacement of coke dust in the production of sinters with alternative fuels has been tested since the 1970s. In Sumitoma’s study [ 16 ], tests were carried out with different types of substitute fuel and anthracite coal, and the results showed that up to 45% of the coke dust could be substituted. BlueScope Steel, CSIRO, and OneSteel [ 17 ] are investigating the replacement of dust coke with biomass-derived charcoal. Research has shown that specially treated charcoal is a potential substitute for coke dust or anthracite. As a result, the sinter made from a 50% charcoal replacement had a lower strength. The reason for using substitute fuels is to reduce the emission burden of the sintering process. The fuel for sintering must have a high calorific value and, if possible, a low content of ash and volatile substances. During sintering, the fuel burns only in a narrow area with a thickness of approx. 10 to 30 mm. This region moves toward the grid at a rate of 0.03 to 0.04 cm/s. The highly flammable fuel will burn in a given layer before the filtered air gives up the heat obtained in the cooling zone of the agglomerate (in the upper zone of heat transfer) at this level. The consequence is a reduction in the maximum temperature and a prolongation of the course of temperature waves. The specific fuel consumption increases and equipment productivity decreases. If the fuel is not very flammable, it will burn at the given level of the layer at the time when the most preheated air flows over it. In this case, the maximum temperature will decrease and the sintering zone will expand. The optimal fuel flammability lies in the region between these extreme cases. With a low fuel content, the necessary strength of the agglomerate is not achieved, while a fuel content that is too high leads to the formation of an excessive amount of melt, to a decrease in productivity, and to a deterioration of the reducibility of the agglomerate. Charcoal naturally contains a higher moisture content. An increasing proportion of charcoal increases the vertical sintering rate, while the yield and strength index decrease [8,14,17]. Currently, efforts are underway to enhance the quality of the sinter by reducing the fuel consumption. Fine-grained coke dust or coke pulp is considered a premium fuel for sinter production. A recent trend involves substituting coke dust with anthracite. The fuel used for sintering typically has a particle size of less than 3 mm. The sintering of iron ores is characterised by significant thermal, mineral, and chemical variations in height and volume. The grain size of the sintering components plays a crucial Processes 2024,12, 440 6 of 13 role, as it influences the chemical composition of the pellets and the distribution of the solid fuel within the sintering layer, leading to uneven temperature-kinetic sintering conditions. The amount of return sinter fines serves as an indicator of the strength and reducibility of the finished sinter. An increase in the quantity of resulting return sinter fines suggests a decrease in strength and an increase in reducibility and vice versa. Consequently, the sinter mixture exhibits an uneven distribution throughout its volume. The research emphasis has been placed on obtaining more detailed information on the mineralogical composition and mechanical properties of the sinter as a result of the significant importance of its structure. Some researchers [ 18 ] have focused on minimising the coke breeze in the sintering process, exploring its impact on the yield and productivity of the final sinter. Using a basicity of 2.5% and 1.5%, along with a gradual 50% reduction in coke, the yield demonstrated an improvement of approximately 8%. This led to a better production productivity of about 30 t·m−2per day. Scientists from Australia [ 19 ] have examined the correlation between the structure and chemical composition, including basicity, and the quality of sinters. As the basicity of the sinter increased, more binding phases were formed, resulting in an enhancement of the pore structure. The strength of the sinter saw an increase with an increase in basicity up to 3. However, when the basicity of the sinter reached 3.5, there was a marked decrease in strength. 2. Materials and Methods The experiments were performed in a sinter plant in the Czech Republic, where the sintering fuel (coke) was replaced by anthracite. The replacement coefficient ranged from 0–100%. Raw Materials The raw materials comprised iron-containing substances (iron concentrates, iron ore fines), fluxes (limestone, dolomite), solid fuels (coke breeze and anthracite), and return fines. In the fine iron ores used in the Czech Republic, the presence of an undesirable coarse fraction exceeding 10 mm typically amounts to approximately 7%. The chemical compositions of the raw materials are presented in Tables 1and 2[20]. The choice of fuel for the sintering charge is of significant importance, as it supplies the necessary heat for both the physical and chemical processes and contributes to the formation of sinters with desired properties. Approximately 65% of the iron ore enters the blast furnace in the form of sinter, while around 30% is in the form of pellets [ 21 ]. In the Czech Republic, the fuel used for the sintering process includes coke breeze and anthracite. The composition and parameters of these fuels undergo change over the years, and a comparison of their average parameters is presented in Table 3. Table 3. Comparison of the average parameters of anthracite and coke breeze. Anthracite Coke Breeze C (%) 85.30 77.20 N (%) 0.71 1.05 H (%) 1.36 0.23 Zn (%) 0.004 0.004 H2O (%) 10.40 15.40 Ad(%) 13.00 16.30 Vdaf (%) 1.33 4.12 S (%) 0.30 0.53 Qir (MJ/kg)23.90 23.99 Qsa (MJ/kg)27.01 28.74 Processes 2024,12, 440 7 of 13 As is clear from the table, the difference in calorific value between coke dust and anthracite is very small (approx. 1.7 MJ/kg) compared to, for example, the calorific value of biomass (Q = 15.1 MJ/kg) [15]. Typically, coke breeze serves as the primary fuel in the sintering process due to its high calorific value, low reactivity, and diminished volatile content. However, in recent years, alternative fuels such as brown coal coke, anthracite, and biomass have also been used in the sintering process [22,23]. As part of the technological flow of the material, the ore components and basic correction additives are transported to the dosing tanks by a conveyor belt, and the pulverised fuel is transported through a vibrating screen to the raw fuel tank above two two-roll mills, which grind the fuel to the required grain size (minimum of 85% of grain should be 0–3 mm). Ground fuel is transported by conveyor belts and a bucket elevator to technological fuel storage tanks, where the ore components are dosed using proportional dosing, the basic ingredients are dosed using dosing scales, and ground fuel is dosed by dosing scales onto the conveyor belt, where the warm return sinter is also dosed through the steel passport. Wet anthracite is not suitable for grinding and subsequent uniform dosing; due to its lower roughness, it sticks to the walls of the containers and sticks to the feeding screens. The preparation of the sintering mixture (charge) begins with the packaging technology. It is produced using a combination of firstand second-stage mixing drums. In the first-stage mixing drum, the mixture is mixed and sprinkled with a set of nozzles. The parameters are checked using an automatic multifunctional sensing device. The monitored parameters include the weight of the dosed raw materials, the opening position of the cap of the added return sinter, the temperature of the return sinter, the amount of water added, the humidity of the mixture, the temperature of the mixture, and its permeability. In practice, the machinist must take into account three factors that determine the optimal structure of the mixture: 1. The amount of material entering the mixing drum and its properties, mainly with respect to fraction and moisture content. The material then does not have time to fully thaw because of its flow rate. Larger grains formed by contact with frozen water enter the packing drum and, in the mixer (under the influence of heat), they disintegrate during the packing process or in already-formed bales. When the mixture is thawed, the bound water is released and its participation is influenced by the packability of the mixture. Here, it depends on the storage method, the previous condition, and the climatic conditions at the time of the addition of the raw materials. 2. The amount and temperature of the incoming return sinter. The standard cycle of movement of the return sinter is such that, as the quality of the sinter increases, the amount of the returnable fraction decreases. With a relatively constant withdrawal, the level in the reservoir drops. Subsequently, an increasingly hot sinter enters the batch, bringing more heat into the packaging process. 3. Hysteresis of ongoing regulation. The effect of changing the amount of water in the batch mixture is shifted by the sum of two influences. The first is the time required to achieve a change in the mixture in the packing drum. The second is the time delay caused by the distance between the measurement point and the operator’s evaluation of the state of the mixture from the ongoing changes in the packing drum. After reaching the optimal state of the chargémixture, the operator has the option of switching the dosing system to the automatic position. The dosing control system then evaluates the read parameters in real time and makes corrections to maintain the set limits in which the measured parameters of mixture moisture, permeability of the mixture, and its temperature were moving at the time of switching to the automatic mode. This control system is applicable when there are no sudden changes in the monitored system. In general, it can be said that the worse the packing properties of the mixture are, the less realistic the possibility is to keep the automatic water dosage control system at the optimal values of the mixture. After leaving the first-stage mixing drum, the sintering mixture travels through Processes 2024,12, 440 8 of 13 conveyor belts to a two-part distribution hopper with a distribution valve. From this flap, the material is divided into separate paths for each sintering belt. Behind the hopper, the mixture enters a pair of second-stage mixers. In them, the mixing and packaging process is completed. Fine dusting absorbs the moisture available from the mixture and is wrapped in bales. The packing process is completed when the mixture is placed into containers. The accumulation of the mixture in the reservoir causes its further compaction. In addition to maintaining a sufficient supply for the sintering process, the reservoirs also serve as a capacity buffer in the event of unforeseeable shutdowns of the sintering belts for the smooth shutdown of the mixing equipment; the quality of the mixture produced decreases during immediate shutdown. The mixture is then dosed onto the sintering belt using a delivery drum. Its amount is controlled by the regulating trowel so that a sufficient discharge of the mixture is achieved throughout the width of the sintering belt. The sintering of the sinter mixture in the monitored sinter plant was carried out using a Dwight–Lloyd machine. The mixture must form an even homogeneous layer with a thickness of 25–50 mm. The upper layer of the sinter mixture on the sintering belt is ignited by the ignition head, and because of the negative pressure created by the turbo exhauster, the mixture layer gradually burns through and the dust ores themselves sinter into a piece of sinter. The individual bands that connect to each other move towards the grid surface at the so-called vertical sintering speed. After that, the layer above the grate surface is sintered and the sintering ends, which occurs above the penultimate 15th suction chamber. The sinter pieces that fall at the discharge end are crushed by rotary hedgehog crushers to a size of up to approximately 200 mm. This is followed by transport by a steel conveyor to the sorter. Here, the return sinter with a grain size of 0–6 mm is sorted, which is returned to the batch by the conveyor. The finished sinter is transported by conveyors to storage tanks as a charge for the blast furnaces. The main prerequisite for the correct course of the process is the adjustment of the amount of gas supplied to the ignition nozzles in such a way as to achieve the optimal sintering temperature and the uniformity of the ignition of the upper layer of the sintered charge. The inspection is carried out visually when the charge exits under the ignition unit; at the same time, the condition of the carriage and the sealing of the spaces between the carriages and the entrances to the extraction chambers are checked. Furthermore, the sintering process is monitored according to the temperature value of the outgoing flue gases. An ideal combustion process is characterised by an increase in the temperature of the exhaust chamber and the ability to reach maximum temperatures in the chamber. The temperature values in the extraction chamber signal the ongoing cooling of the sintered layer of the charge. The course of negative pressure values is the main indicator for the breathability of the mixture, that is, the course of changes in the thickness of the burning zone along the length of the sintered strip. The heat generated by burning fuel increases the temperature of the sintered mixture to 1380 ◦ C–1450 ◦ C. There is gradual burning through the mixture layer and the actual sintering of the dust ores into lumpy sinters [ 20 ]. In the sintering process, coke breeze and anthracite with a particle size of less than 3 mm are used as fuel. The sintering belts have 80 bulk sintering carts with a side height of 40 cm. The extracted area is 75 m 2 , with a cart width of 2.5 m. The technical data of the sintering carts are provided in Table 4. Processes 2024,12, 440 9 of 13 Table 4. Sintering belt parameters [20]. Length of the working section (m) 30 Width (m) 2.5 Extracted area (m2)75 Number of sintering trolley/sintering belt (pcs) 80 Max. layer height (mm) 400 In an effort to intensify the sintering process as much as possible, it was necessary to accelerate the burning in the layer of the sintering mass and thereby ensure the formation of liquid phases. This ensures the fulfilment of one of the main parameters of the sintering process, the compliance with a sufficient vertical sintering speed. This, in turn, is somewhat dependent on the optimal permeability of the sintering mass. Furthermore, the sintering process is monitored according to the temperature value of the outgoing flue gases. An ideal combustion process is characterised by an increase in the temperatures of the exhaust chamber and the ability to reach maximum temperatures in the chamber. The temperature values in the extraction chamber signal the ongoing cooling of the sintered layer of the charge. The course of negative pressure values is the major indicator for the permeability of the mixture, i.e., the course of changes in the thickness of the burning zone along the length of the sintered strip. 3. Results In the monitored operation of the company, the sintering charge uses coke breeze as the primary fuel, and since 2013, anthracite has been introduced as an additional component. At first, it was only a partial substitute for the fuel, but its share gradually increased, similar to that of the world [ 7 , 15 , 23 , 24 ]. During the years 2020–2022, anthracite was added as part of the sintering charge in different proportions. Table 5provides a summary. The advantage of replacing coke with anthracite depends on the current purchase prices of these commodities. As is clear from the table, in some (especially winter) months, anthracite was not added at all. On the other hand, in the summer months, the replacement of coke dust with anthracite reached 100%. As part of the technical control, sinter samples are taken regularly. The content of Fe, FeO, and other elements and oxides in the sinter produced is determined. Granulometry, strength, and abrasion resistance are determined according to the requirements of the technologists; the evaluation is shown in Table 6. Table 5. Summary of effects of adding anthracite to the sintering charge in different proportions. Production (kt) Specific Power (t/m2/d) Total Fuel Consumption (kg/t) Anthracite (t) Coke Breeze (t) 2020 1 128,000 28.99 45.70 - 5850 2 117,900 28.60 44.36 - 5230 3 128,300 28.99 46.69 2483 3507 4 129,600 30.73 43.90 3395 2295 5 140,600 31.96 45.59 6410 - 6 127,500 30.71 51.92 3862 2758 7 129,100 29.88 48.64 2933 3347 8 129,700 29.90 45.88 2692 3258 9 121,800 28.86 46.63 1929 3751 10 81,600 27.51 53.86 1194 3201 11 121,300 28.43 44.27 390 4980 12 123,100 28.01 47.03 - 5790