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.

Causes and Preventive Measures for Cracking in Glass Kiln Lip Bricks

Glass Kiln Lip Bricks are a relatively special type of irregularly shaped refractory material. Their service life is affected by every aspect, including the raw material composition, forming and manufacturing process, and assembly. Especially during the hammering and ignition processes, the bricks must withstand temperature differences of hundreds of degrees Celsius. Therefore, whether using a single brick or combining several lip bricks, cracking is a potential problem. The causes of cracking in lip bricks and preventative measures are listed below.

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• ① The brick material itself has low compressive strength, poor thermal stability, and a high coefficient of thermal expansion. When subjected to thermal shock, the tensile strength of the brick is less than the expansion thermal stress, causing it to break. To eliminate this factor, in addition to designing a good formula and selecting good materials, the contact surfaces between the fixing screws and the retaining iron used to fix the lip brick and the lip brick should be padded with flexible material, and the iron parts should not directly contact the brick body.
• ② The firing temperature is low, and the dehydration stage of crystal water is short. During the high-temperature dehydration process of the crystal water inside the brick body, the original structure is destroyed and new minerals are generated. To avoid this situation, in addition to avoiding components with a large amount of mineral structural water when determining the lip brick formula, the brick blank must be fully dried after casting before being fired in the kiln. Furthermore, the heat preservation time should be increased in the dehydration temperature range of crystal water according to the mineral composition.
• ③ The lip brick is backed by the tail brick, and there are fixing screws on the front and sides acting on the brick body. When heated, forces from four directions act on the local area of ​​the brick body. The fixing screws restrict the expansion and movement of the brick body, but they can also easily cause the brick body to crack under external forces. Preventive Measures: After the lip brick is fixed to the overflow port support, secure the support with jacking screws and bolts, but do not tighten it completely; leave an expansion gap. Then, heat it with fire, slowly raising the temperature to above 700℃ to allow the lip brick to fully expand. Tighten the jacking screws again before the lead-in.
• ④ Insufficient pre-use baking time prevents the removal of free water from the brick. To eliminate this factor, place the lip brick in a high-temperature environment before use to fully remove free water, or preheat the kiln during on-site construction.
• ⑤ Using deformed lip brick supports may cause cracks or even breakage of the lip brick. Therefore, deformed lip brick supports, especially those with deformed contact surfaces with the lip brick, should not be used.

Lip Brick Replacement

After a period of operation in a rolled glass production line, if defects appear in the glass due to erosion or wear of the lip brick, it needs to be replaced.

Before replacing the lip brick, it must be baked at a high temperature for at least 72 hours to remove free water remaining in the brick due to processing, transportation, or other reasons. Baking can be done using a natural drying method: the brick is placed next to the kiln, relying on the heat emitted by the kiln for baking. Because it is natural baking, it requires a long time and can only remove some free water, so the baking is not thorough. Alternatively, a preheating furnace baking method can be used. A kiln is built with refractory materials, and the lip brick is heated according to a heating curve, baked at 200-300℃ for 24 hours, and then assembled to the overflow port. After the calender is positioned and installed, the temperature is further increased to 1100℃ for the lead-in operation. This method requires specialized hoisting and installation tools. It is also more difficult to operate at high temperatures, but it ensures that the lip brick will not crack. Online baking can also be used: after the lip brick and calender are positioned and installed at the forming port, they are baked using a spray gun. This method uses gradual heating, allowing sufficient time for the free water and crystal water in the lip brick to be fully drained. This method reduces the probability of the lip brick cracking or even shattering.

Before replacing the lip brick, prepare the following tools: pipe wrench, Allen wrench, wrench, pliers, level, measuring tape, 1-3mm sheet metal, square timber, mullite fiber paper, etc.

When removing the lip brick from the calender, first use a sledgehammer and pneumatic hammer to remove the old lip brick. Then, use an electric scraper to clean the tail brick thoroughly, ensuring there is no residual glass or unevenness on the surface. After applying a 3-5mm thick layer of high-temperature mullite fiber paper to the contact surface of the tail brick, assemble the lip brick on the lip brick support as required, or install the lip brick already assembled on the support at the tail brick location. Finally, push the calender into position, check for any problems, and then slowly heat the lip brick to the guide plate with a spray gun to complete the replacement operation.