Production of complex Fe-Si-Mn-Cr ferroalloy using high-ash coal: a sustainable metallurgical approach

The article presents the results of comprehensive thermodynamic modeling and laboratory tests conducted for smelting a complex ferroalloy of silicon, manganese, and chromium (Fe-Si-Mn-Cr) from chromium, medium-grade manganese ores, and high-ash coals from Kazakhstan. Thermodynamic analysis was performed using HSC Chemistry software to model the Fe-Si-Mn-Cr smelting process over a temperature range of 900 °C–1800 °C. This analysis involved six actual charge compositions with solid reductant (Csolid) consumption ranging from 5 to 20 kg per 100 kg of Cr and Mn ore mixture. The mechanism of the combined carbothermic reduction of Cr, Mn, Si, and Fe was investigated using the Cr-Si-Al-Ca-Mn-Mg-O-C system. According to thermodynamic data, the optimal consumption of Csolid per 100 kg of ore mixture is 17 kg, and the optimal temperature range for smelting ferroalloys is between 1600 and 1700 °C. Laboratory tests were conducted in a high-temperature Tamman furnace at 1700 °C, resulting in experimental samples of the new complex ferroalloy with an average composition of 14.85% Fe, 14.05% Si, 7.55% Mn, 57.54% Cr, and 6.01% C, with P < 0.03% and S < 0.02%. The phase composition included (Cr, Fe, Mn)3Si and carbides Cr23C6 and (Fe, Mn)3C. The resulting alloy is suitable for alloying high-carbon and tool steels.


Introduction
Producing complex ferroalloys of Fe-Si-Mn-Cr composition is a challenging endeavor, as evidenced by the substantial body of research on this topic in the scientific literature [1][2][3][4][5].The primary difficulty lies in managing the smelting process of the multi-component mixture, which has limited the industrial prevalence of such ferroalloys.Various approaches to charge composition include using: (1) separate ferroalloys for each component; (2) MnO-enriched slag, limestone, chromite ore, and FeSiCr ferroalloy; (3) individual compounds of each element [5,6].Traditionally, carbon, typically sourced from coke in the industry, is used as the reductant [7][8][9][10].
With increasing demands for steel and alloy quality, the need to develop advanced production technologies for complex ferroalloys becomes apparent [5,11,12].One way to optimize these technological processes is by utilizing lower-grade raw materials, including high-ash coals as an alternative to traditional coke, which can lead to cost reduction and increased economic efficiency [8,9,[13][14][15].
The unique composition of Fe-Si-Mn-Cr ferroalloys is crucial, embodying essential elements prevalent in the industry's most common ferroalloys.Integrating these elements into a single alloy significantly enhances their absorption, optimizes deoxidation, and allows for the adjustment of melting temperature and density, thereby facilitating more effective steel and metal alloying.This represents a significant advantage over using individual ferroalloys of these elements, as it ensures a more uniform distribution of alloying elements and improves the quality of the final product [5,6,[14][15][16].
The primary goal of this work is to develop a smelting technology for the complex Fe-Si-Mn-Cr ferroalloy, based on the use of ores and high-ash coal.Through thermodynamic modeling and subsequent experimental melts, it is anticipated that not only can the optimal alloy composition be achieved, but the process efficiency and cost-effectiveness can also be ensured.The research aims to enhance the understanding of component interactions within the alloy and develop technological solutions for stable and controlled production of highquality ferroalloy.

Materials and methods
The initial charge materials utilized were Cr ore from the Don Mining and Processing Plant (Khromtau, Kazakhstan), medium-grade Mn ore from the Zhairem deposit (Karaganda, Kazakhstan), and high-ash coal from the Saryadyr deposit (Ereymentau, Kazakhstan).Table 1 presents the chemical and technical composition of these charge materials.
For the thermodynamic analysis, coexisting phases in the Cr-Si-Al-Ca-Mn-Mg-O-C system were calculated using data from the HSC Chemistry 6 database, which is based on and continually updated by SGTE.The principles underpinning the thermodynamic modeling of the Cr-Si-Al-Ca-Mn-Mg-O-C system were articulated as follows: (1) Temperature Range: The analysis covered a range from 900 to 1800 °C, delineating the initial and final equilibrium states of the system.The lower threshold of 900 °C represents the standard state, below which changes are negligible, while the upper limit of 1800 °C corresponds to the melting points of the components and the emergence of final reaction products.
(2) Pressure Setting: A constant pressure of 0.1 MPa was maintained throughout the calculations, reflecting the approximate atmospheric pressure (1 atm) common in typical metallurgical processes, including those involving solid-phase carbon thermal interactions.
(3) Volume Consideration: The volume of the system was directly dictated by its thermodynamic state.
(4) System Isolation: The system was considered closed, precluding any interaction with the external environment.
A comprehensive thermodynamic analysis was conducted across six different charge compositions, varying the solid reducing agent input from 5 to 20 kg per 100 kg of the ore mixture.This analysis aimed to identify the optimal conditions for the carbothermic smelting of the complex alloy, as detailed in table 2.
The following elements and compounds were selected as associates for calculations: -metal phase: MnC  Utilizing the theoretical data and the physicochemical properties of the charge materials, laboratory experiments were conducted in a Tamman furnace to establish a controlled temperature regime for the smelting of a prototype ferroalloy.
The ore mixture was comminuted to a particle size range of 1-3 mm to increase the specific surface area, thereby facilitating a study on the influence of the reducing agent's chemical activity on the slag reduction process.The prepared charge was then transferred into a mullite-corundum crucible and positioned within the Tamman furnace as depicted in figure 1, ensuring optimal conditions for achieving the experimental objectives.
This high-temperature furnace features a heating mechanism consisting of a graphite tube as the active workspace.Temperature control within the furnace is managed through a thyristor voltage regulator integrated into the primary winding of the power transformer.This setup allows for the application of several thousand amperes of current to the output buses at a low voltage range of 0.5 to 15.0 V. Temperature measurements were taken using a tungsten-rhenium thermocouple, model TR-5/20, positioned at the base of the crucible within a reinforced corundum enclosure.
Over the course of the investigation, a total of five smelting trials were conducted.For material characterization, the microstructure of the synthesized alloy was examined using a JEOL JXA-8230 scanning electron microscope equipped with an energy dispersive x-ray spectroscopy (EDS) detector.Phase analysis was performed using x-ray diffraction (XRD) on a Bruker D8 Advance diffractometer, utilizing Cu Kα radiation with a wavelength of 1.5406 Å.The analysis and interpretation of the diffractogram data, including the calculation of interplanar distances, were facilitated using EVA software.

Thermodynamic modelling
The primary objective of thermodynamic modeling in this study is to delineate the distribution patterns of chemical elements and compounds across the metal, slag, and gas phases during the carbothermic reduction of the ore mixture.Analysis of the data indicates that the initial composition of the charge undergoes significant transformations during reduction.The modeling outcomes reveal distinct trends in the migration of elements and compounds into the metal and slag phases, especially noted during the carbothermic smelting of Fe-Si-Mn-Cr at temperatures up to 1800 °C.These patterns are systematically illustrated in figures 2-7, highlighting the dynamic interactions and phase transitions within the system.
These figures elucidate the variations in the constituent elements of both the metal and slag phases as a function of temperature, offering insights into the thermodynamic interactions within the system under the tested conditions.The graphical analysis of the depicted curves reveals several key findings: -charge mixtures № 1-3 (figures 2-4), metal formation reduction processes begin at 1200 °C.By the benchmark temperature of 1300 °C, the metal phase contains between 4.5 to 10 kg of Fe and a consistent 9 kg of Mn, figures that remain largely unchanged with further increases in temperature or variations in C solid addition from 5 to 11 kg.Notably, at the lower end of C solid input (5 kg), the Cr 2 FeO 4 phase transitions to Cr 2 O 3 at 1300 °C, accumulating 20 kg within the slag.However, as both temperature and C solid inputs  -for charge mixture № 4 (figure 5), metal reduction also begins at 1200 °C, with the Fe and Mn content stabilizing at 8-9 kg by 1300 °C, and this consistency is unaffected by further thermal exposure.When C solid input reaches 14 kg, a marked decrease in Cr 2 O 3 content within the slag is observed when temperatures exceed 1100 °C, while Cr content in the metal phase climbs to 18 kg at around 1500 °C.Yet, the substantial SiO 2 concentration in the slag suggests an ongoing requirement for a reducing agent.-for charge mixtures № 5 and 6 (figures 6 and 7), the metal formation reduction similarly starts at 1200 °C, with the Fe and Mn content in the metal phase maintaining at 8-10 kg at 1300 °C, irrespective of further temperature increases.At a C solid intake of 17 kg per ore mix, Cr achieves full reduction by 1400 °C, resulting in the formation of multiple Cr and Si compounds in the metal phase.Concurrently, the decrease in SiO 2 to 7 kg in the slag indicates a surplus of the reducing agent (table 3).The observations presented are derived from thermodynamic modeling, which quantified the compositional changes within the metal and slag phases for each charge mixture across a broad temperature spectrum, ranging from 900 to 1800 °C.The thermodynamic data facilitated a detailed calculation of how the composition of both metal and slag varied, considering each specific charge mixture within this extensive temperature interval.This comprehensive analysis is in optimizing the smelting process to achieve desired phase equilibria and elemental distributions.
In table 3, the average compositions of the metal and slag at temperatures of 900 °C-1200 °C are presented, as significant metallization only begins at 1300 °C.The calculations allow us to consider the physicochemical processes occurring during the smelting of Fe-Si-Mn-Cr by the carbothermic method.It is important to note that the system under consideration is closed, with no interaction with the environment, and that the consumption of C solid is practically stoichiometric.The high content of Cr 2 O 3 (up to 13%) and SiO 2 (up to 47%) in the slag phase, with a consumption of C solid ranging from 5 to 14 kg, indicates a clear lack of reducing agent.The optimal consumption of C solid is determined to be 17 kg per 100 kg of ore mixture, and a further increase in the reducing agent (as shown in figure 8) corrects the composition of Si in the metal (increasing extraction).
The slower reduction of Si can be attributed to the elevated temperatures required for its reduction from SiO 2 .The reduction of metallic Fe and Mn promotes the reduction of Si, because these metals reduce the activity and activity coefficient of Si within the alloy and facilitate the formation of silicides, thereby impacting the reduction process.The affinity of Si for forming silicides with metals in the ferroalloy follows the sequence Mn > Fe > Cr, based on the standard change in Gibbs free energy associated with the formation of the respective silicides [17,18].Cr, by dissolving up to 70% Mn in the solid state, reduces Mn's activity and enhances the reduction process [2].
It is imperative to acknowledge that the reduction of ferroalloy components under actual conditions may deviate due to the characteristics of the coal used.In study [19], the authors determined that the volatile   compounds released during coal heating shift the solid-phase reduction of higher Mn oxides to lower temperature zones, and the structure of the coal's pyrolytic residue aids in Si reduction.
The basicity of the slag appears to have a substantial impact on the extraction of Cr.Specifically, the extraction rate of Cr increases with increasing basicity, suggesting that a more basic slag promotes the reduction and extraction of Cr.Conversely, the Mn extraction rate remains constant across the tested basicity range, indicating that basicity has less impact on Mn extraction under these conditions.This may be due to Mn oxides being relatively easy to reduce, or it could be influenced more significantly by other factors such as the partial pressure of oxygen and the activity of MnO in the slag.Si exhibits a gradual increase in extraction rate with increasing basicity, although not as pronounced as Cr, implying that while Si extraction benefits from higher basicity, it is less sensitive to these changes than Cr.Fe maintains a consistently high extraction rate across the range, suggesting that its extraction is not significantly influenced by basicity under the tested conditions.
A higher basicity often enhances the reduction of Cr and Si, as the basic oxides in the slag can bind oxygen more effectively than the metal oxides, thus facilitating their reduction.
Based on the thermodynamic data obtained, the optimal composition of the charge mixture (consumption of C solid -17 kg) and a temperature range of 1600 °C-1700 °C for smelting Fe-Si-Mn-Cr in a high-temperature resistance furnace were determined.The chemical composition of the metal at 1700 °C was as follows: 43.25% Cr, 21.18% Mn, 9.90% Si, 24.46% Fe, and 1.21% C. The composition of the slag included 0.04% Cr 2 O 3 , 0.08% Fe 2 O 3 , 45.60% SiO 2 , 28.61% Al 2 O 3 , 6.88% CaO, and 18.79% MgO.

Smelting process
In this series of smelting experiments, a mixture consisting of 31% Cr ore (46.1 g), 31% Mn ore (46.1 g), and 38% high-ash coal (58.05 g) was prepared for the smelting process.This charge mixture was thoroughly homogenized prior to being placed in a mullite-corundum crucible.The crucible, containing the charge mixture, was then subjected to a holding temperature of 1700 °C for a duration of 20 min, which is sufficient to reach equilibrium conditions for the given charge size.Figure 9 illustrates the resultant Fe-Si-Mn-Cr alloy post-melting, capturing the morphology of the metal and slag phases.
Figure 9 provides a visual depiction of a typical outcome from one of the high-temperature smelting tests conducted on the Fe-Si-Mn-Cr alloy.The image distinctly showcases the bifurcation of the smelting products into metal and slag phases.Notably, the process was characterized by an absence of active gas release, indicating a controlled smelting environment.The resultant slag exhibits a solid, stone-like consistency.Quantitatively, the slag-to-metal ratio, based on the weights of the formed slag and metal (m slag /m metal ), is approximately 0.90-0.93.

Research of smelting products
A study of the microstructure of the Fe-Si-Mn-Cr alloy sample and an EDS analysis of the microstructure components were carried out.Figure 10 shows images obtained in the back-reflected electron detection mode and a map of the distribution of elements.
EDS analysis of the sections and points marked in figures 10(a), (b) was carried out.The re-sults are presented in table 4.
Figure 10 presents SEM images elucidating the microstructure of the Fe-Si-Mn-Cr ferroalloy and maps detailing the elemental distribution.The images reveal a markedly heterogeneous structure composed of two primary phases.EDS analysis yields an average composition for these phases: the lighter phase contains approximately 30% Fe, 9% Mn, 14% Si, and 46% Cr, while the darker phase is composed of 6% Fe, 5% Mn, 63% Cr, and 25% C.
Distinct contrasts between phases are evident in the concentrations of Fe, Si, and Cr, as well as in the presence or absence C. The lighter phase is identified as a silicide with the generalized formula (Cr, Fe, Mn) 3 Si,  while the darker phase is composed of Cr 23 C 6 carbides.These identifications are consistent with confirming data from XRD analysis, presented in figure 11, where the carbide (Fe, Mn) 3 C is also identified.
Mn appears to be uniformly distributed throughout the microstructure of both phases.Additionally, porosity is noted within the structure, which is a characteristic often observed in ferroalloys.Overall, the obtained microstructure can be regarded as similar to that of high carbon ferrochromes (HC FeCr) [20].Such microstructural attributes are in-dicative of the high-temperature processes and compositional complexities inherent to these materials.
The average chemical composition of the metal was determined, %: 14.85 Fe, 14.05 Si, 7.55 Mn, 57.54 Cr, and 6.01 C. Such a high content of Cr and C brings the composition of the alloy close to that of HC FeCr, with an increased presence of Si and Mn [21,22].The composition is distinct from the Fe-Si-Mn-Cr ferroalloys reported in the literature, notably due to its higher chromium and carbon content [23,24].
Figure 10 presents SEM images elucidating the microstructure of the Fe-Si-Mn-Cr ferroalloy, alongside maps detailing the elemental distribution.The images reveal a markedly heterogeneous structure composed of two primary phases.EDS analysis provides an average composition for these phases: the lighter phase contains approximately 30% Fe, 9% Mn, 14% Si, and 46% Cr, while the darker phase comprises 6% Fe, 5% Mn, 63% Cr, and 25% C.
There are distinct contrasts between the phases in terms of concentrations of Fe, Si, Cr, as well as the presence or absence of C. The lighter phase is identified as a silicide with the generalized formula (Cr, Fe, Mn)3Si, while the darker phase consists of Cr 23 C 6 carbides.These identifications are corroborated by XRD analysis presented in figure 11, where the carbide (Fe, Mn) 3 C is also identified.Mn appears to be uniformly distributed throughout the microstructure of both phases.Notably, porosity is observed within the structure, a characteristic often seen in ferroalloys.Overall, the obtained microstructure resembles that of high carbon ferrochromes (HC FeCr) [20], indicative of the high-temperature processes and compositional complexities inherent to these materials.
The presence of ferroalloy elements in the form of silicides and carbides plays a crucial role in the subsequent alloying of steel, as their dissolved state reduces the degree of oxidation and minimizes evaporative losses.Increased Si content contributes to a reduction in density, melting temperature, and melting time of the ferroalloy in liquid metal, decreases oxidizability, and enhances the assimilation of alloying elements [23,25].Mn also reduces the melting temperature, albeit to a lesser extent [25], and aids in S removal, acts as a deoxidizer and reductant along with Si during steel alloying, thereby improving the incorporation of alloying elements by the melt [26,27].A complex ferroalloy may find broad application in alloying high carbon stainless steels and tool steels (D2, A2, AISI-5117, AISI-5135, AISI-5145, AISI-5147 etc), containing Cr, Mn and Si.Due to the high C content, decarburization using oxygen may be required before or during the alloying process.It should also be noted that decarburization occurs during the melting process itself as the melt interacts with the air.The ferroalloy also can serve as an alternative to the more expensive HC FeCr.
The slag composition is as follows, %: 43.62 SiO , typical for this composition, are not formed in the slag due to the relatively low smelting temperature of the alloy and the cooling rate [7].The amorphous background observed is clearly indicative of an SiO2-based phase [28].
The use of a charge consisting of only three components significantly simplifies the smelting of the complex Fe-Si-Mn-Cr ferroalloy, compared to previously used and proposed technologies [5,6].The employment of unenriched raw ores and high-ash coal also results in a lower cost of production for the ferroalloy [29,30].
Given that the traditional method for producing such ferroalloys involves smelting in electric arc furnaces, further research in this direction is necessary using data from this work.

Conclusion
(1) Thermodynamic modeling was conducted for the smelting of the complex ferroalloy Fe-Si-Mn-Cr, determining the optimal consumption of the reducing agent C solid to be 17 kg, with the theoretical smelting temperature range established at 1600 °C-1700 °C.Analysis of the alloy and slag composition's dependence on temperature and reducing agent consumption indicates that the dissolution of reduced Fe and the mutual fluxing of slag-forming rocks during the reduction of Cr and Mn ore mixtures with C solid create favorable conditions for reduction reactions at these temperatures.The chemical composition of the metal at 1700 °C was found to be: 43.25% Cr, 21.18% Mn, 9.90% Si, 24.46% Fe, and 1.21% C; and for the slag: 0.04% Cr 2 O 3 , 0.08% Fe 2 O 3 , 45.60% SiO 2 , 28.61% Al 2 O 3 , 6.88% CaO, and 18.79% MgO.
(2) A series of crucible melts of the Fe-Si-Mn-Cr ferroalloy were conducted at 1700 °C for 20 min.These laboratory melts demonstrated the feasibility of using high-ash coal as a reducing agent in the smelting of Fe-Si-Mn-Cr ferroalloys and the potential to produce a new complex ferroalloy.Research on industrial conditions and the organization of mass production of this new complex ferroalloy Fe-Si-Mn-Cr could enable the utilization of lower-quality Kazakhstani Mn ores in production, replacing equivalent amounts of Cr, Mn, and Si from conventional ferroalloys with less expensive and higher-quality options, especially HC FeCr.The resulting slag was hard and stone-like, with a slag-to-metal ratio of 0.90-0.93.

increase to 11
kg, the quantity of Cr 2 O 3 decreases to 8 kg.The persistently high levels of Cr 2 O 3 and SiO 2 indicate a deficit in the reducing agent, corroborated by the scant presence of Si in the metal phase.

Figure 2 .
Figure 2. Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 5 kg C solid per 100 kg of ore mixture.

Figure 3 .
Figure 3. Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 8 kg C solid per 100 kg of ore mixture.

Figure 4 .
Figure 4. Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 11 kg C solid per 100 kg of ore mixture.

Figure 5 .
Figure 5. Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 14 kg C solid per 100 kg of ore mixture.

Figure 6 .
Figure 6.Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 17 kg C solid per 100 kg of ore mixture.

Figure 7 .
Figure 7. Dependence of the metal (a) and slag (b) composition on temperature at a consumption rate of 20 kg C solid per 100 kg of ore mixture.

Figure 8 .
Figure 8. Dependence of the extraction of Fe, Cr, Mn and Si on the consumption of C solid (a) and on the basicity of slag (b) at a temperature of 1700 °C.

Figure 9 .
Figure 9. Products of high-temperature melting of Fe-Si-Mn-Cr.

Figure 10 .
Figure 10.SEM images of the microstructure and a map of the distribution of elements.

Table 1 .
Chemical and technical composition of the charge materials, %.

Table 2 .
Chemical composition of the charge mixture, kg.

Table 3 .
Chemical composition of the metal and slag in the temperature range of 900 °C-1800 °C, %.