Silica Mullite Bricks are Mainly Composed of These Four Raw Materials

As an alumina-silica refractory material, silica mullite bricks possess superior thermal shock resistance, a high load softening temperature, and excellent erosion resistance, which are key reasons why they can be used in all sections of cement kilns except the firing zone.

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Of course, compared to high-alumina bricks, the superior properties of silica-mullite bricks are due to the presence of silicon carbide components. Silicon carbide, as a non-oxide material, has excellent high-temperature mechanical properties, combining erosion resistance and thermal shock resistance. The main raw materials used in silica mullite bricks are as follows:

High-Alumina Bauxite

High-alumina bauxite is an important mineral resource with significant strategic and practical value worldwide. China has bauxite reserves of 2.5 billion tons, accounting for 2.4% of the world’s total, indicating abundant reserves. China’s bauxite is mainly of the gibbsite-kaolinite type. The ore has high Al₂O₃ and SiO₂ content and low Fe₂O₃ content, making it suitable as a refractory raw material. The mineral composition of high-alumina bauxite is mainly gibbsite and kaolinite. Gibbsite has the chemical formula AlO(OH), belongs to the orthorhombic crystal system, has a density of 3.3~3.5 g/cm³, and a Mohs hardness of 6~7. Kaolinite has the chemical formula Al₂O₃·2SiO₂·2H₂O, is a triclinic silicate mineral, and is also a type of clay mineral. It has strong water absorption, good plasticity, a density of 2.60~2.63 g/cm³, and a hardness of 2~2.5. The structural characteristics of high-alumina bauxite are described below.

Currently, approximately 65% of refractory materials in my country are alumina-silicon-based products, and about 65% of these products use high-alumina bauxite as raw material. Therefore, high-alumina bauxite is an extremely important raw material for the production of Al2O3-SiO2-based refractory materials. The phase composition of high-alumina bauxite clinker consists of corundum, mullite, and cristobalite. Its chemical composition, bulk density, water absorption rate, and mineral uniformity have a decisive influence on the performance of the high-alumina refractory materials produced. High-alumina bauxite clinker is classified into nine grades according to its different physicochemical properties.

Silicon Carbide

Silicon carbide (SiC) is a covalent compound with atoms bonded by covalent bonds. Due to its strong covalent bonds, this material possesses excellent physical properties at both room temperature and high temperatures. Silicon carbide has high refractoriness, reaching 2600℃ in a reducing atmosphere. It has high hardness (Mohs hardness 9~9.5), high thermal conductivity (64.4 W/(m·K) at 500℃), and a low coefficient of thermal expansion of 5.68×10⁻⁶/℃ (1000~2400℃). It also possesses excellent resistance to corrosion, impact, and oxidation.

Although silicon carbide has very good performance at both room temperature and high temperatures, it is extremely rare in nature. Industrially required silicon carbide is mainly synthesized artificially. There are five main methods for artificially preparing silicon carbide:

• 1) Carbothermic reduction of silicon dioxide.
• 2) Elemental synthesis of silicon carbide.
• 3) Molten salt method for producing silicon carbide.
• 4) Preparation of silicon carbide from gaseous compounds.
• 5) Preparation using steam-liquid-solid phase methods, etc.

Among these, silicon carbide used in the industrial production of refractory materials is often prepared using the SiO2 carbon reduction method. When reducing SiO2 with carbon in an electric resistance furnace, below 1250℃, the generated CO cannot oxidize silicon carbide. At higher temperatures, the generated CO then reacts with SiO2, further reducing SiO2 and precipitating free carbon on the crystal surface.

When silicon carbide is introduced into aluminosilicate refractories, it partially oxidizes during firing to form SiO2, creating a glassy phase of a certain thickness on the surface of the product. Due to the oxidation of the surface silicon carbide, a weak reducing atmosphere is created inside the brick. The internal silicon carbide is activated and oxidized, generating SiO gas. When SiO diffuses to the interface, it is re-oxidized back to SiO2. This SiO2, along with other impurities, forms a glassy phase that blocks pores, inhibits further oxidation, and slows down erosion. In addition, some of the SiO2 generated by the oxidation of silicon carbide will also react with the excess Al2O3 in the material at a certain temperature to form mullite.

Silicon carbide also possesses high thermal conductivity, which, when added, improves the material’s thermal conductivity. Furthermore, silicon carbide has a low coefficient of thermal expansion, very close to that of mullite, significantly reducing internal stress generated during rapid temperature changes. This greatly promotes the improvement of the material’s thermal shock resistance. On the other hand, excessive SiC oxidation leads to an excessively thick oxide layer. The oxidation process also generates SiO gas; increased internal gas production weakens the material’s internal structure, potentially damaging it. Excessive silicon carbide oxidation not only reduces the material’s density but also produces more SiO2, whose crystal transformation is highly temperature-sensitive, negatively impacting thermal shock resistance. Therefore, in certain operating environments, adding an appropriate amount of antioxidant is essential to prevent excessive SiC oxidation. One commonly used antioxidant is Si powder. Si, as an element, is highly reactive and oxidizes before SiC to form SiO2. Therefore, as the amount of Si added increases, the oxide layer thickness tends to decrease. However, when the amount of Si added continuously increases beyond 2%, the amount of SiO2 generated by its oxidation increases, leading to excessive secondary mullitization in the material, excessive volume expansion, resulting in a loose material structure, reduced bulk density, and increased porosity. Another commonly used antioxidant is metallic Al powder. Similarly, metallic Al powder will be oxidized to Al2O3 earlier than SiC, inhibiting silicon carbide oxidation. The Al2O3 and SiO2 generated by the oxidation of metallic Al and SiC respectively will begin the mullitization reaction at around 1400℃.

Furthermore, adding metallic aluminum and elemental silicon simultaneously as antioxidants to the product is extremely effective in inhibiting the over-oxidation of SiC. Although elemental Si has a high melting point of 1414℃, its eutectic temperature with metallic Al is only 577℃. Therefore, when both are added as antioxidants in combination, a liquid phase is generated at a lower temperature, which is conducive to ion diffusion and transfer, thereby reducing the reaction formation temperature.

Mullite-High-Silica Glass Multiphase Material

Due to the low sintering temperature of clay clinker, its phase composition generally contains 10%–25% cristobalite. In the last century, foreign countries began to explore ways to dissolve cristobalite from aluminosilicate refractory clinker into the glass phase, resulting in mullite-high-silica glass multiphase materials with a mullite network structure, by focusing on preparation processes and the introduction of additives.

Mullite-high-silica glass multiphase materials are a new type of refractory material composed of crystalline mullite and an amorphous silica-rich glass phase. They possess excellent properties such as a high load softening temperature, a low coefficient of thermal expansion, and excellent thermal shock resistance. Based on the content of alkali metal oxide impurities, mullite-high-silica glass multiphase materials can be divided into two main categories: high impurity content (R₂O 0.2%–2.0%) and low impurity content (R₂O < 0.2%). As a high-SiO₂ content mullite composite, its phase does not contain cristobalite. Furthermore, its tightly bound mullite network structure and high-viscosity silica-rich glass phase significantly improve the material’s performance at high temperatures, particularly its thermal shock resistance.

A mullite-high silica glass multiphase material was prepared using natural kyanite mineral. The sintering temperature was 1500–1600℃, resulting in large mullite grains and a high mullite content. In this material, the mullite content was 75%–80%, and the glass phase was 20%–25%, with relatively uniform phase distribution. The prepared kyanite-based mullite had a bulk density of 1.99–2.11 g/cm³. However, its room-temperature compressive strength reached 163–264 MPa, significantly outperforming similar materials made from kaolin or high-alumina bauxite.

By introducing appropriate additives, clay was calcined to produce a quartz-free mullite-high silica glass phase material. Experiments have shown that using potassium- and sodium-rich natural minerals like feldspar as additives is more beneficial for the synthesis of mullite-high silica glass phase materials. This is because these minerals enable clay to sinter at lower temperatures, are inexpensive and readily available, and help maintain the stability of the mixture’s composition. However, the amount added should not be too large, otherwise it will reduce the material’s refractory properties.

Kyanite-based mullite (KBM) is a type of mullite-high silica glass multiphase material. In China, it is produced by using kyanite ore as the starting material, adding additives, wet grinding, pressure filtration, vacuum extrusion molding, and finally calcination at 1500℃~1650℃. Based on the different Al2O3 contents of the synthesized kyanite-based mullite, it can be divided into series such as KBM45, KBM50, KBM55, and KBM60.

Among them, the basic raw material used for synthesizing KBM45 and KBM50 is low-grade kyanite ore with an Al2O3 content of about 45%. KBM50 incorporates a small amount of industrial alumina to adjust its composition, and its synthesis temperatures are 1500℃ and 1600℃, respectively. KBM55 and KBM60 use medium-grade kyanite ore with an Al2O3 content of approximately 50% as raw material. Different amounts of industrial alumina are added to adjust the Al2O3 content, along with different amounts of composite sintering aids. Their synthesis temperatures are 1600℃ and 1650℃, respectively. Partial physical properties of the four kyanite-based mullite raw materials are also described.

Guangxi White Clay

Guangxi white clay is a type of soft kaolin clay. Its main chemical components are: SiO2 content 45.3%~51.6%, Al2O3 content 26.0%~36.8%, Fe2O3 content 0.65%~2.20%, and K2O+Na2O <1.50%. The main mineral composition of Guangxi white clay is disordered kaolin, sometimes reaching 90%, followed by quartz, generally below 30%. It also contains trace amounts of ilmenite and other minerals. As a binder in refractory materials, Guangxi white clay possesses excellent fluidity, plasticity, and binding properties, making it the highest quality soft refractory clay discovered in my country to date. Guangxi white clay is widely used in the refractory materials industry, including in the production of clay-bonded castables, sprayed refractory materials, refractory bonding clays, and gunning compound additives.

Refractory Materials for Aacid Slag-Based Electric Arc Furnace Linings

In the high-temperature smelting process of an electric arc furnace with intense charge movement, the refractory lining of the furnace is an essential guarantee for the normal operation of the process. Based on the type of slag and the properties of the refractory materials used, electric arc furnaces can be divided into acidic slag electric arc furnaces and basic slag electric arc furnaces. Most of the aforementioned electric arc furnaces used in steelmaking are basic slag electric arc furnaces, with a slag basicity greater than 1. Since acidic slag does not have desulfurization and dephosphorization effects, the application of acidic slag electric arc furnaces is currently limited in my country. Currently, the furnace linings mostly use basic refractories with good resistance to basic slag, such as magnesia-carbon bricks and magnesia-chrome bricks. Therefore, my country produces a very large amount of basic slag as a byproduct each year. It is estimated that in 2019, the slag produced in my country’s electric arc furnace steelmaking production was approximately 0.2 billion tons. Such a large accumulation of slag, if not properly handled, could cause environmental pollution due to the heavy metals such as chromium, lead, and cadmium it contains, posing a significant potential threat to biological health and the balance of the ecological environment.

Refractory Materials for Lining Acidic Slag-Based Electric Arc Furnaces

The slag in acidic slag-based electric arc furnaces is acidic, and the lining uses acidic or neutral refractory materials. Compared to basic slag, acidic slag is more environmentally friendly. When the SiO2 content in acidic slag is high, it forms acidic silicate slag with a microstructure similar to glass. This is an amorphous, non-crystallizing material that cools slowly, i.e., glassy slag (hereinafter referred to as “glass slag”). Some studies suggest that when amorphous silicate materials are corroded in an acidic environment, the inconsistent dissolution of oxides causes the silicate surface to form a silicon-rich protective layer. This fixes harmful metals in the slag, and the higher the degree of vitrification (amorphous material content), the better the fixation effect. This significantly reduces the risk of environmental pollution and harm to animal and plant health. Based on this characteristic, many industrial waste treatment processes, such as leather industry waste treatment and waste incineration fly ash treatment, are suitable for this high-temperature vitrification process, which is an effective method for fixing metal pollutants. Meanwhile, this glass slag can also be reused as a raw material for preparing microcrystalline glass. The vitrification process of acidic slag has been proven to be a successful example of the harmless treatment and reuse of waste resources.

Acidic slag-based electric arc furnace smelting can be used for the recovery and harmless treatment of valuable metals in secondary resources. One very important smelting process is the electric arc furnace smelting of waste automotive exhaust purification catalysts. Waste automotive exhaust purification catalysts mainly consist of a carrier component (composed of oxides such as γ-Al₂O₃ or cordierite) and an active component (composed of three precious metals: platinum, palladium, and rhodium). After crushing and finely grinding the catalyst, it is mixed with trapping metals, flux, and a small amount of coke, granulated, and then smelted in an electric arc furnace. This utilizes the principle that trapping metals such as iron or copper have a strong affinity for platinum group metals in the catalyst at high temperatures (above 1420 °C). This process enriches the precious metal active components in the catalyst within the molten metal, while the catalyst support components, along with added fluxes such as SiO2 and CaO, form slag that enters the glass slag, achieving efficient and harmless recovery of valuable metal resources. Depending on the attractant and flux composition, the electric arc furnace operates at varying temperatures, reaching up to 1600 °C. At this temperature, the scouring motion of the furnace charge on the refractory material is quite intense. Furthermore, carbon is added during smelting to reduce the oxides of the target metal, creating a reducing atmosphere in the furnace, thus placing high performance requirements on the furnace lining refractory material of the electric arc furnace. Existing refractory materials have a short service life in the aforementioned electric arc furnaces; in pilot-scale small electric arc furnaces, the lining needs to be replaced approximately every two months, limiting the industrial application of this process. This paper addresses the problem of short lining service life in electric arc furnace smelting of acidic slag-based refractory materials, analyzing the mechanism of acidic glass slag corrosion of refractory materials. It also reviews the development and research of refractory materials suitable for high-temperature acidic slag-based electric arc furnaces both domestically and internationally in recent years. The feasibility of these materials in high-temperature acidic slag-based electric arc furnaces was analyzed, and the future development trend of refractory materials for acidic slag-based smelting furnaces was prospected.

The Corrosion Mechanism of Acidic Glass Slag on Refractory Materials

The corrosion of refractory materials by molten slag is a complex process involving many influencing factors. These include the chemical composition and pH of the molten slag and refractory materials, the viscosity and surface properties of the molten slag, the melting atmosphere, the physical properties of the refractory materials (porosity, high-temperature flexural strength, etc.), and the solubility limits of their components in the molten slag (i.e., the concentration of refractory components in the molten slag at saturation). Furthermore, unlike other melting furnaces, electric arc furnaces use a high-temperature electric arc to heat the furnace charge, and the resulting electromagnetic field significantly affects the properties of the molten slag. For example, it alters the wettability and penetration kinetics of the molten slag on refractory materials, influencing the formation and distribution of different phases, thus affecting the erosion of refractory materials by the molten slag. Currently, there is little research, both domestically and internationally, on the influence of the electromagnetic field of electric arc furnaces on the erosion of refractory materials by acidic glassy molten slag. The erosion of refractory materials by glass slag can be divided into two types: penetration erosion and chemical corrosion.

Current Status of Research on Refractory Materials for Acidic Slag-Based Electric Arc Furnaces

Currently, research on the slag erosion resistance of refractory materials largely relies on high-temperature erosion resistance tests to simulate the erosion of refractory materials by molten slag during industrial production, in addition to using refractory materials that have failed in actual industrial production. The quality of refractory material’s slag resistance is evaluated using indicators such as the rate of mass change before and after the test, the erosion rate of the molten slag, and the penetration rate. High-temperature erosion resistance tests can be divided into two types: static and dynamic methods. The difference lies in whether an external force is applied to keep the molten slag flowing relative to the refractory material. The most widely used static method is the static crucible method. This method requires simpler equipment and operation, and the molten slag only statically penetrates and dissolves the refractory material. It is suitable for refractory materials whose erosion is mainly caused by the dissolution of components in the molten slag. Dynamic erosion resistance tests are more suitable for refractory materials with more complex erosion processes.

Application of Refractory Materials in High-Temperature Acidic Slag-Based Electric Arc Furnaces (1600 ℃)

Based on the corrosion mechanism of acidic glassy molten slag (glass slag) on ​​refractory materials, and assuming their potential application in acidic slag-based electric arc furnaces, this paper reviews the research results on the resistance of several refractory materials (Al2O3-SiO2 materials, Al2O3-SiO2-ZrO2 composite materials, chromium-containing materials, densified zirconium (chromium)-containing materials, carbonaceous and carbide materials) to glass slag corrosion, summarizing their advantages and disadvantages. The feasibility of applying these refractory materials in high-temperature acidic slag-based electric arc furnaces (1600 ℃) was discussed, and the following conclusions were drawn:

• (1) Al2O3-SiO2 refractory materials have poor high-temperature corrosion resistance and are not suitable for high-temperature acidic slag-based electric arc furnaces.
• (2) Chromium-containing refractory materials have excellent resistance to glass slag corrosion, but due to the toxicity of Cr, which can easily cause environmental hazards, their use should not be expanded.
• (3) Carbonaceous and carbide refractories have high thermal conductivity and poor insulation performance, which can lead to a significant increase in energy consumption of electric arc furnaces.
• (4) Al2O3-SiO2-ZrO2 composite refractories have good resistance to glass slag corrosion, and the densification process further enhances their corrosion resistance. In addition, the feasibility of using 41# (containing 41% ZrO2) fused cast zirconia-corundum bricks in high-temperature acidic slag-based electric arc furnaces (1600 ℃) has been preliminarily demonstrated.

Currently, with the gradual increase in public awareness of environmental protection and the strict implementation of national environmental protection policies, researchers will focus on improving and developing environmentally friendly and energy-saving refractory materials without affecting service life. The structure and properties of the slag in high-temperature acidic slag-based electric arc furnaces are similar to those of the glass melt in glass melting furnaces and other vitrification furnaces. The development of refractory materials for high-temperature acidic slag-based electric arc furnaces can fully draw on the industrial application experience of both. It is foreseeable that, represented by 41# fused cast zirconium corundum refractory, densified zirconium-containing materials with excellent resistance to glass melt corrosion will become a widely used furnace lining material for high-temperature acidic slag-based electric arc furnaces.

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Rongsheng Well Block Refractory Material – Quality Guaranteed

A seat brick is a refractory brick installed at the bottom of a steel ladle to fix the nozzle brick. The well block refractory brick is square in shape, hence also called a square brick. Its functions are to fix the nozzle position, facilitate nozzle removal and installation, support the lower end of the stopper rod during pouring, and ensure that the stopper rod slides along the curved surface towards the nozzle after pouring to cut off the flow.

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Application and Classification of Well Block Refractory Bricks

Well block refractory bricks are mainly used in conjunction with nozzles in continuous casting tundishes. They are mostly used at the bottom of the ladle and tundish to protect the internal nozzles and permeable bricks. Well block refractory bricks have high corrosion resistance and are used in conjunction with zirconium sizing nozzles.

Initially, well block refractory bricks could be made of clay, high-alumina, or unfired high-alumina materials. Later, well block refractory bricks were classified according to changes in the usage environment and the materials used:

• (1) Al2O3-Cr2O3 well block refractory bricks. Original high-alumina well block refractory bricks had poor heat spalling resistance. Using corundum as the main raw material, adding appropriate amounts of alumina, spinel, chromium oxide, etc. to the matrix can produce Al2O3-Cr2O3 well block refractory bricks with good heat spalling resistance and strong corrosion resistance. This material has a long service life and can be used synchronously with the ladle. Currently, most well block refractory bricks used are Al2O3-Cr2O3 well block refractory bricks.
• (2) Magnesia well block refractory bricks. Precast well block refractory bricks were prepared by adding fused corundum powder (below 1μm) and magnesium oxide to the original alumina base material. Because the material is an alumina castable containing magnesium oxide, the cement content is extremely low, increasing the service life by 40%. The main reasons are: the fine corundum powder containing magnesium oxide densifies the matrix, improving erosion resistance; the matrix density and low cement content increase the material’s high-temperature strength, enhancing its resistance to thermal shock and mechanical spalling; and the magnesium oxide and alumina in the matrix largely form spinel at high temperatures, thus inhibiting slag penetration and improving erosion resistance. The magnesia base bricks are precast, reducing brick joints and simplifying construction.

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Characteristics of Nozzle Well Block Refractory Bricks

1. High-Temperature Corrosion Resistance: Refining steel ladles requires extremely strict temperature and time control, often exceeding 1750℃.
2. High-Temperature Abrasion Resistance: Forced stirring is used in various ladle refining methods, which severely impacts the high-temperature abrasion resistance of the well block refractory bricks.

Well Block Refractory Bricks Installation

Before installing the well block refractory bricks at the taphole, the bottom should be leveled with ramming material to ensure the taphole cylinder and the masonry layer are on the same plane. When laying separate well block refractory bricks, the mating surfaces of each brick must be evenly coated with mortar. The inner holes of the well block refractory bricks must be uniform; on-site correction is necessary if required to ensure proper installation of the taphole tube bricks. Considering that the well block refractory bricks consist of multiple pieces, ramming material should be filled from both sides simultaneously around the taphole well block refractory bricks to prevent rotation.

Main Components of  Permeable Bricks, Nozzle Well Block Refractory Bricks, and Castables

In the steelmaking industry, steelmakers commonly use refractory materials such as permeable bricks, nozzle well block refractory bricks, electric furnace covers, castables, guide sand, and magnesia-carbon bricks in their ladles and refining furnaces. These refractory materials differ significantly in their main components and additives. Chemically, refractory materials are composed of minerals, such as corundum, mullite, and magnesia. Their main components include alumina and magnesia.

The main components of refractory materials are the matrix components that constitute their refractory properties and form the basis of their characteristics, directly determining the properties of the finished refractory product. For example, permeable bricks require high-quality ore and are manufactured through strict and reasonable processes to ensure that the service life of the permeable bricks used by steelmakers meets requirements. The main components of refractory materials can be oxides (alumina and magnesia, etc.) or elements or non-oxide compounds (carbon, silicon carbide, etc.).

Based on the properties of their main components, refractory materials can be classified into three categories: acidic, neutral, and basic. Acidic refractories mainly contain acidic oxides such as silicon dioxide, with silicic acid or aluminum silicate as the primary components. They will form salts under high temperatures and alkaline conditions. Basic refractories mainly contain magnesium oxide and calcium oxide. Common refractory products include guide sand and ladle slide plates. Neutral refractories are strictly speaking carbonaceous and chromium-based refractories. High-alumina refractories (alumina content greater than 45%) tend towards acidic neutrality. Chromium-based refractories are alkaline but tend towards neutrality. Common high-alumina refractories include permeable bricks, nozzle well block refractory bricks, and electric furnace covers.

Rongsheng Refractory Materials Factory researches, develops, produces, and sells permeable bricks, nozzle well block refractory bricks, castables, and other refractory materials. With patented formulas, unique designs, and strict adherence to every process standard, we provide high-quality refractory lining materials for high-temperature industrial furnaces. Contact Rongsheng to get free samples and quotes.