Applications and Usage Methods of Ferrosilicon Nitride in Refractory Materials
With the development of high-temperature technologies such as iron and steel smelting, the research and development of high-quality, high-efficiency refractory materials and raw materials have become increasingly urgent. As promising refractory raw materials, synthetic materials are being applied more widely. Ferrosilicon nitride (Fe-Si₃N₄) is a new type of synthetic raw material developed in recent years, prepared from FeSi75 ferroalloy through nitriding technology and high-temperature synthesis processes.
Containing the Si₃N₄ phase, ferrosilicon nitride possesses the excellent properties of silicon nitride, including high refractoriness, superior corrosion resistance, high mechanical strength, excellent thermal shock resistance, low thermal expansion coefficient, and outstanding oxidation resistance. At the same time, the iron plastic phase it contains provides favorable sintering performance. Compared with pure silicon nitride, ferrosilicon nitride features lower cost and easier industrial production, facilitating its widespread adoption. Therefore, it is widely used as a refractory raw material, a high-temperature bonding phase, and a high-temperature structural material, and has been extensively applied in blast furnace iron trough castables and tapping clay. In recent years, research on ferrosilicon nitride and its applications in refractories has increased, yielding fruitful results. This paper elaborates on the synthesis and characteristics of ferrosilicon nitride, as well as research progress on its applications in castables, tapping clay, and composite refractories, and discusses its future development.
At present, ferrosilicon nitride is mainly applied in castables, tapping clay, composite refractories, and other refractory products.
01. Castables
The application of ferrosilicon nitride in castables mainly focuses on Al₂O₃-SiC-C iron trough castables, magnesia castables, and high-alumina castables.
1.1 Al₂O₃-SiC-C Iron Trough Castables
Al₂O₃-SiC-C castables feature excellent slag corrosion resistance and scouring resistance and are widely used in the main trough, skimmer, and branch trough of blast furnace tapping channels. With the advancement of smelting technology, continuous improvement of blast furnace utilization efficiency, and the demand for longer blast furnace service life, it has become urgent to further extend the service life of Al₂O₃-SiC-C iron trough castables.
Currently, such castables are prone to peeling under cyclic chemical corrosion from molten slag and hot metal, thermal shock, as well as scouring by slag and molten iron. Meanwhile, high-temperature oxidation of the silicon carbide and carbon components inside the castables damages the material structure, eventually leading to material failure.
The Si₃N₄ phase in ferrosilicon nitride is hardly wetted by molten slag and hot metal, which improves the corrosion resistance of iron trough castables. Its oxidation products form a dense SiO₂ protective film on the material surface, inhibiting further oxidation and enhancing oxidation resistance. In addition, the metallic iron plastic phase exerts a sintering-aiding effect and optimizes the mechanical properties of the castables.
A comparative study was conducted on the anti-oxidation performance of Al₂O₃-SiC-C iron trough castables with 8 wt% silicon nitride and 8 wt% ferrosilicon nitride added, respectively, at 1500℃. The results show that under a high-temperature oxidizing atmosphere, Si₃N₄ in ferrosilicon nitride is first oxidized to form SiO₂, which constitutes the main part of the oxide layer. As the iron phase is oxidized, the generated iron oxides reduce the melting point and viscosity of the oxide layer, improve the wettability and fluidity of the molten phase on the castable surface, and form a complete oxide layer that restrains the oxidation of carbon components. Hence, ferrosilicon nitride delivers better oxidation resistance than pure silicon nitride.
The iron inside the castables does not exist in the form of iron oxides, so it causes no adverse impact on high-temperature performance. Relevant research by Liu Bin drew the same conclusion. It was also found that N₂ produced by the high-temperature oxidation of Si₃N₄ in ferrosilicon nitride and CO generated from the oxidation of carbon materials can block internal pores of the material, effectively preventing further oxidation.
Research shows that adding 5 wt% ferrosilicon nitride can enhance the high-temperature flexural strength and oxidation resistance of Al₂O₃-SiC-C iron trough castables. The slag corrosion resistance is slightly improved with increasing ferrosilicon nitride content.
According to Liu Zhijun's research, optimal slag resistance is achieved when the addition reaches 9 wt%. Excessive addition produces a large amount of free iron during the reaction, forming numerous low-eutectic substances inside the material, which consequently weakens the slag corrosion resistance of the castables.
1.2 Magnesia Castables
Magnesia castables feature high refractoriness and a high load-softening temperature, do not contaminate molten steel, and possess excellent resistance to alkaline slag erosion. Therefore, they are widely used in thermal equipment such as steelmaking furnaces and ladles.
Using silica fume as a binder, studies were conducted on the effects of different dosages of fine ferrosilicon nitride powder on the room-temperature physical properties and high-temperature mechanical properties of magnesia castables. The results show that when the addition of ferrosilicon nitride is 3 wt%, the room-temperature strength after firing at 1200℃ and 1500℃, as well as the high-temperature flexural strength at 1400℃, reach their peak values. During heating, SiO₂ generated by the oxidation of ferrosilicon nitride reacts with magnesia to form forsterite, which enhances the material's strength. Meanwhile, the solid solution formed by the iron phase and magnesium oxide accelerates the sintering process of the castables.
To address the poor sintering of magnesia castables after adding ferrosilicon nitride, the influence of boron carbide (B₄C) dosage on their mechanical properties was investigated. It was found that the addition of B₄C promotes sintering and improves the strength of samples after medium- and high-temperature treatment. Nevertheless, the liquid phase formed by the oxidation of B₄C during sintering weakens the direct bonding between particles, leading to a decline in the high-temperature flexural strength of the specimens.
1.3 High-Alumina Castables
High-alumina castables exhibit superior mechanical properties, good penetration resistance, and excellent erosion and impact resistance. They are widely used in various lining parts of power station furnaces, boilers, melting furnaces, heating furnaces, soaking furnaces, heat treatment furnaces, and induction furnaces.
Using high-alumina bauxite clinker as the main raw material and pure calcium aluminate cement as the binder, the effects of ferrosilicon nitride addition on the properties of high-alumina castables for aluminum melting furnaces were studied. The results reveal that as the ferrosilicon nitride content increases, silica (SiO₂) produced by its oxidation reacts with alumina to form mullite, which improves the room-temperature strength of the fired specimens. However, partial oxidation of ferrosilicon nitride creates pores within the matrix and exacerbates slag penetration into the material. Severe molten aluminum penetration occurs in crucible specimens when the ferrosilicon nitride addition reaches 5 wt%.
02. Tapping Clay
Tapping clay is an essential refractory material used to block the taphole of blast furnaces. During tapping, molten slag and hot metal flow out simultaneously through the taphole. With the enlargement and longer service life of blast furnaces, as well as the continuous optimization of smelting technologies such as high blast temperature, oxygen-enriched injection, and high-pressure operation, higher performance requirements have been placed on taphole tapping clay.
It is required to have strong resistance to chemical corrosion, slag erosion, and molten iron attack; favorable sintering properties and filling performance; excellent thermal shock resistance and high-temperature volume stability; low environmental pollution; easy opening performance; prolonged tapping duration; and effective hearth protection.
Tapping clay has evolved from a simple consumable refractory into a functional refractory material, and its quality directly determines the smooth operation of blast furnace production. Traditional tapping clay can no longer meet the demands of modern smelting, making the development of high-performance tapping clay imperative.
Silicon nitride has a high melting point, high strength, superior thermal shock resistance, and a stable structure. It can improve the high-temperature strength, oxidation resistance, corrosion resistance, and scouring resistance of tapping clay to a certain extent; however, it has little effect on optimizing clay opening performance. In addition, the high cost of silicon nitride limits its widespread application in tapping clay.
Ferrosilicon nitride possesses all the superior properties of silicon nitride. Its internal metallic iron plastic phase facilitates sintering and addresses the poor sintering problem of silicon nitride. Meanwhile, it is much more cost-effective than silicon nitride, which has led to increasing research on the application of ferrosilicon nitride in tapping clay.
During high-temperature heating of ferrosilicon nitride-modified tapping clay, apart from asphalt decomposition, carbonization, and liquid-phase sintering caused by sintering aids, the dominant reaction is between ferrosilicon nitride and carbon-containing materials under a mixed atmosphere of nitrogen, oxygen, carbon dioxide, carbon monoxide, and other gases. The main possible chemical reactions are listed as follows:
It is evident that after adding ferrosilicon nitride, the silicon nitride on the specimen surface oxidizes at high temperature to form a dense silica protective film, which inhibits further oxidation and improves the oxidation resistance of tapping clay. With the participation of the metallic iron plastic phase and carbon as reaction catalysts, ferrosilicon nitride within the tapping clay reacts to form new phases, including silicon oxynitride, silicon carbide, and aluminum nitride, which strengthen the matrix and microstructure and enhance the medium- and high-temperature strength of the material.
Moreover, the silica generated by the oxidation of internal silicon nitride is highly chemically active and can react with alumina to form mullite, further increasing high-temperature strength and scouring resistance and extending tapping duration. The escape of nitrogen, carbon monoxide, and other gases at high temperature creates internal pores, optimizing taphole opening performance in actual service. Meanwhile, the released gases reduce friction at the interface with molten iron, and part of the gases is retained within the pores. These two effects jointly inhibit the penetration and corrosion of molten iron and slag, greatly improving the corrosion resistance and penetration resistance of tapping clay.
Research shows that adding 12 wt% ferrosilicon nitride effectively enhances the high-temperature flexural strength and scouring resistance of tapping clay and prolongs tapping time. Anhydrous Al₂O₃-SiC-C tapping clay containing 5 wt% and 10 wt% ferrosilicon nitride has been successfully applied in medium- and large-scale blast furnaces with volumes of 3200 m³, 580 m³, 260 m³, and 2000 m³. The medium- and high-temperature strength, as well as corrosion and scouring resistance, are significantly improved. This tapping clay exhibits a slow hole expansion rate and excellent opening performance, with tapping time extended to over 120 minutes, reducing tapping frequency and greatly lowering the labor intensity of front-furnace workers.
After the addition of ferrosilicon nitride to the blast furnace tapping clay at Meishan Iron & Steel, slag resistance and scouring resistance were enhanced, and the problem of coke jamming was effectively alleviated. Nevertheless, some studies indicate that the dosage of ferrosilicon nitride has a slight or negligible effect on the slag corrosion resistance of tapping clay.
High-performance anhydrous tapping clay has been developed by simultaneously adding ferrosilicon nitride, metallic aluminum, and metallic silicon. Based on the in-situ reaction principle, which generates nitrides and hydroxides, this approach achieves self-repair and self-strengthening of the damaged tapping clay microstructure. This product has been successfully applied at Shougang Iron & Steel, Qian'an Iron & Steel, and Shouqin Iron & Steel.
Compared with tapping clay doped with silicon carbide, tapping clay containing ferrosilicon nitride achieves higher high-temperature flexural strength across various temperature ranges. This improvement is attributed to ductile iron reinforcing particles and intermetallic iron silicide present in ferrosilicon nitride. Iron silicide accelerates the sintering process and promotes the phase transformation from α-Si₃N₄ to β-Si₃N₄, thereby improving the bonding strength between oxides and non-oxides. However, it has been observed that when the ferrosilicon nitride content exceeds 24 wt%, the porosity of tapping clay increases significantly, leading to a corresponding decrease in flexural strength at all temperature stages.
03. Composite Refractories
Composite refractories are a new type of functional refractory with superior comprehensive properties. They are fabricated by combining two or more kinds of refractory raw materials with different characteristics using physical or chemical methods at both macro and micro levels. The application of ferrosilicon nitride in composite refractories is mainly divided into carbon-containing composite refractories and carbon-free composite refractories.
3.1 Carbon-containing Composite Refractories
When ferrosilicon nitride is added to ASC bricks, the silicon nitride within it converts into silicon oxynitride during high-temperature service. As the ferrosilicon nitride dosage increases, the amount of silicon oxynitride formed rises steadily, which consistently improves the high-temperature flexural strength and slag corrosion resistance of ASC brick specimens.
Alumina-carbon materials are widely used as carbon-containing composites in the metallurgical industry. They possess high mechanical strength, excellent thermal shock resistance, and outstanding slag resistance, and are extensively applied in continuous casting functional components such as sliding gates.
In alumina-carbon systems, ferrosilicon nitride mainly facilitates the phase transformation from silicon nitride to silicon carbide at high temperature. Specifically, α-Si₃N₄ first transforms into β-Si₃N₄ and eventually converts into SiC. During this process, Fe₃Si particles in ferrosilicon nitride gradually refine and disperse uniformly among the newly formed SiC phase and residual untransformed β-Si₃N₄, forming a dense and compact microstructure.
Studies on the high-temperature phase evolution and the role of iron in the Fe-Si₃N₄-C system show that, compared with pure Si₃N₄-C materials, the iron in Fe-Si₃N₄-C can significantly accelerate the transformation from Si₃N₄ to SiC and substantially lower the formation temperature of silicon carbide.
In the presence of carbon, Fe₃Si in ferrosilicon nitride transforms into an Fe-Si-C molten phase and increases the activity of elemental iron. Subsequently, it reacts with silicon nitride and absorbs silicon to form a silicon-rich Fe-Si-C intermediate phase. As this intermediate phase flows and penetrates, it further reacts with carbon to generate SiC or precipitate SiC crystals from the molten phase, thereby realizing the catalytic effect of iron on the phase transformation.
Meanwhile, the formation of SiC refines large iron particles into fine grains and eventually forms a multiphase structure, with fine iron particles uniformly dispersed in the SiC matrix. A small amount of silica impurities in ferrosilicon nitride disappears at high temperature, and partial silicon nitride transforms into silicon oxynitride, which disperses together with Fe₃Si within the newly formed SiC phase.
3.2 Carbon-free Composite Refractories
When ferrosilicon powder (FeSi₂) is incorporated into silicon carbide particles, followed by pressure forming and direct nitriding firing in a nitriding furnace to prepare ferrosilicon nitride–bonded SiC composites, it is found that the content of ferrosilicon powder added should be less than 15 wt%. Furthermore, it is necessary to regulate the equilibrium partial pressure of nitrogen inside the nitriding furnace and reduce the heating rate to control the nitriding reaction, thereby relieving internal stress in the samples during nitriding and avoiding sample damage.
In the temperature range of 1100–1300℃, the oxidation of ferrosilicon nitride–bonded SiC composites mainly originates from SiC and Si₃N₄. The generated SiO₂ oxidation product forms a protective film that inhibits further oxidation. The mass change per unit area follows a linear rule in the early oxidation stage, a quadratic rule in the middle stage, and a parabolic rule in the later stage. Research also shows that, compared with silicon nitride–bonded SiC composites, the iron component of ferrosilicon nitride–bonded SiC composites can further improve the thermal shock resistance of the material.
Using FeSi75 alloy and SiC as the main raw materials, high-performance ferrosilicon nitride–bonded SiC composites were successfully fabricated via direct nitriding sintering at 1450℃. The resulting composite exhibits a compressive strength of 145 MPa and an initial load softening temperature of 1750℃. Its main crystalline phases are SiC, α-Si₃N₄, and Fe₃Si, with α-Si₃N₄ as the dominant nitrided phase and a small amount of β-Si₃N₄. In addition, the iron does not participate in the nitriding reaction but exists stably as the intermetallic compound Fe₃Si, distributed uniformly at grain boundaries.
04. Conclusions
As a new type of synthetic refractory raw material, ferrosilicon nitride is more cost-effective compared with silicon nitride. Its addition to refractories effectively addresses the poor sintering behavior of silicon nitride, and it has been increasingly applied in iron trough castables, tapping clay, and composite refractories. The direct synthesis of ferrosilicon nitride–bonded SiC composites from ferrosilicon alloys provides a new avenue for further applications of ferrosilicon nitride.
Nevertheless, although FeSi75 ferrosilicon alloy-the mainstream silicon source for ferrosilicon nitride production-has replaced expensive metallic silicon, its market price remains relatively high. The carbothermal reduction nitridation method can reduce production costs to some extent; however, vacuum treatment is still required to maintain a high-purity nitriding atmosphere in the furnaces and related equipment. In addition, most nitridation reactions must be carried out under high nitrogen pressure, which complicates the scaling up of high-pressure reaction vessels and limits the mass production of ferrosilicon nitride.
These factors result in the relatively high cost of ferrosilicon nitride powder and its bonded composite refractories, restricting cost control and large-scale adoption in the metallurgical industry.
Therefore, refractory researchers and developers need to focus on addressing several key challenges in future studies: developing cheaper alternative raw materials to replace ferrosilicon alloys; exploring energy-efficient, safe, and industry-compatible equipment and processes for synthesizing ferrosilicon nitride and its bonded refractories; achieving large-scale and continuous production of ferrosilicon nitride; and promoting its application across a broader range of refractory fields.