The structure of electric furnace lining

The electric furnace consists of a furnace cover (top furnace wall (furnace wall) and a furnace bottom. The outer shell of the furnace body is steel plate. The lining is made of refractory materials and is divided into the furnace bottom, furnace slope, molten pool and furnace wall. One side of the furnace wall is the furnace door, and the other side is the steel outlet and the steel chute. The furnace cover is removable, the outer ring is made of steel, and it is mostly water-cooled. Except for the water-cooling ring, the top is made of refractory materials.

Electric furnace molybdenum smelting uses electricity as its primary heat source, utilizing three-phase AC or DC power. An arc is generated directly between the charge and electrodes, melting the charge at high temperatures. Oxidants, slag-forming agents, and ferroalloys are then added to remove inclusions. The molten steel’s chemical composition and temperature are adjusted to specified values before being poured into a ladle.

To reduce power consumption and increase production, electric furnaces also employ heavy oil or oxygen injection to accelerate melting. Due to the high furnace temperatures, volatile atmospheres, short smelting cycles, and generally cold charge, the furnace lining is frequently exposed to high temperatures, slag erosion, and rapid cooling and heating.

The furnace wall is the part with the shortest lining life. Due to the harsh operating conditions of high-temperature slag in high-power electric furnaces, the lining surface experiences a heat load of 1000W/m², withstanding heat radiation from the electrode arc reaching temperatures as high as 6000°C. The hotspot molten steel reaches temperatures as high as 2000°C. To this end, foam slag is generally used to alleviate the radiation of arc light on the furnace wall, while adjusting the input power imbalance of the three-phase electrodes and controlling the secondary voltage to improve and increase the life of the ultra-high power electric furnace lining.

The development of electric furnaces towards larger sizes, ultra-high power, and intelligent technology has placed higher demands on refractory materials. The erosion rate of refractory materials is directly related to furnace size and power. In the early 1970s, refractory consumption for electric furnaces was reduced by improving material quality, shape, and construction methods. In the late 1970s, with the advancement of electric furnace smelting technology, water cooling of the furnace roof and walls began and quickly became widespread. Subsequently, external refining units (LFs) were developed for the production of specialty steels. At the same time, the steel tapping method was changed from trough to EBT, increasing the water cooling area and subsequently reducing the unit refractory consumption. By the 1980s, refractory consumption for the furnace roof had reached below 10 kg/t, and by the 1990s, it had fallen to below 0.5 kg/t. Refractory consumption for the furnace walls was 1.0 kg/t, bringing the total refractory consumption for electric furnaces to 2.0 kg/t. While the increase in water cooling for the furnace roof and walls reduced overall refractory consumption, the performance requirements for refractory materials continued to rise.

Refractory materials for furnace cover

The furnace cover is a spherical structure with holes for electrodes and exhaust. The outer ring is called the main cover, and the middle part is called the small cover (also called the triangle area). The cover can be removed during charging.

Electric furnace cover water cooling technology

While high-power operation increases the production capacity of electric furnaces, it also significantly increases the heat load on the furnace roof refractory, making it impossible to extend the life of the roof with improved refractory materials. Consequently, water-cooling technology developed and rapidly became widespread.

Water-cooling technology began in the 1980s with the main furnace roof. The main furnace roof employed a water-cooled structure, while the smaller furnace roofs were constructed with brickwork and prefabricated monolithic refractory blocks. This reduced the refractory consumption of the furnace roof by half, to 1.5 kg/t.

In the mid-to-late 1980s, water cooling was also adopted for smaller furnace roofs with lower secondary voltages, ultimately achieving a fully water-cooled furnace roof. Now, fully water-cooled furnace roofs are widely used in electric furnaces, with only the refractory bricks surrounding the electrodes forming the refractory material. Refractory consumption has dropped to less than 0.1 kg/t.

The water-cooled furnace roof is welded from boiler steel plates, and the entire roof is water-cooled. Only three electrode holes on the water-cooled furnace cover are constructed with refractory material or prefabricated refractory blocks. Unformed refractory materials, such as corundum castables, can also be used to prevent arcing and breakdown caused by electrodes colliding with the water-cooled furnace cover.

Large furnace covers are box-type or tubular structures, while small furnace covers are generally internally divided and water-permeable. The water-cooled furnace cover designed by the Beijing Metallurgical Equipment Research Institute has been used in the 70t UHP electric furnace at Zhangjiagang Yongxin Iron and Steel Plant since 1991, with excellent performance and a significant reduction in cost per ton of steel. The system consists of a water-cooled furnace cover and a water-cooled furnace wall. The water-cooled furnace cover is a radial tubular structure cooled by forced convection water. The center portion of the furnace cover houses the three electrode holes, constructed with refractory bricks. The small center furnace cover has a service life of approximately 300 cycles. The inner and upper surfaces of the furnace cover are covered with a refractory layer with embedded reinforcing ribs to prevent refractory material from falling off. At present, the 30tUHP electric furnace of Great Wall Special Steel Company and the 50tUHP electric furnace of Fujian Zhongsteel Company have used this domestically produced water-cooled furnace cover, achieving good technical and economic benefits.

In the 1990s, the integrated spray water cooling method was developed. The original box-type structure sometimes leaked when pressurized water was passed through, and excessive leakage made it difficult to continue using. Spray water cooling creates turbulent flow with a small water volume, greatly increasing the heat transfer coefficient. Furthermore, it uses water at normal pressure, resulting in minimal leakage and safe operation. Spray water cooling is suitable for ultra-high power electric furnace covers, furnace walls, and flue gas elbows. Spray-cooled furnace covers have excellent slag retention properties. Spray-cooled furnace covers at the Plymenth plant in the United States and the Badishe plant in Germany have a lifespan exceeding 1,000 furnaces, while the highest lifespan at the Daido Steel Co., Ltd. plant in Japan has reached 9,000 furnaces. Spray cooling roughly doubles the lifespan of the refractory material in the triangular area of the furnace cover.

According to statistics from Vcar, a US company, the effects of using a spray cooling system are as follows:
(1) Due to the savings in refractory materials and the reduction in hot downtime and maintenance time, the economic benefit per ton of steel can reach US$35;
(2) Spray water cooling has a high heat transfer coefficient and reduces water consumption by 50%;
(3) Safety is greatly improved due to the use of normal pressure water;
(4) The structure is simple and the weight is reduced.

Refractory materials for electric furnace cover

1.Working conditions of furnace cover refractory materials

The lining of the electric furnace cover is the weak link of the entire furnace body. The refractory material of the furnace cover is affected by operational factors and structural factors. Operational factors include operating rate, maximum power consumption, oxygen volume ratio, deoxidation furnace ratio and slag regeneration furnace ratio. Structural factors mainly include furnace size, furnace cover thickness and the distance between the molten pool and the furnace cover. In summary, the factors affecting the life of the furnace cover are:

(1) Thermal shock. Sudden temperature changes caused by arc heating, steel tapping, and furnace cover movement;

(2) Chemical erosion. Molten slag and molten steel splash onto the brick surface, causing chemical erosion, as well as the effects of CO, CO2, and SO2 in the furnace;

(3) Arc radiation. Melting loss caused by thermal radiation generated by arc heating;

(4) Wear caused by high-speed airflow generated by high-speed dust extraction in the furnace;

(5) Structural loosening caused by the pre-damage of brick joints under the action of the vault structure’s own weight, etc.

The combined effect of these factors significantly impacts the lifespan of furnace roofs. The extremely high temperatures caused by arc radiation exacerbate chemical corrosion; the erosion of molten slag and molten steel leads to thinning of the refractory lining and structural changes, which in turn accelerates spalling and damage due to thermal shock.

Therefore, furnace roof refractory materials must possess excellent thermal shock resistance, resistance to high-temperature slag splashing, and good integrity, ensuring a strong, crack-resistant structure.

The development trend of furnace roof refractory materials is from bricks to precast blocks to integral casting. Specific conditions vary from country to country, and the materials used vary.

2.Brick furnace cover

(1) Silica bricks. In the 1950s, electric furnace covers were built with silica bricks (SiO2 93% to 95%). Their true specific gravity was as low as 2.37 to 2.39, and they were light and inexpensive. They had good high-temperature structural strength and creep resistance. Fully silica bricks were used for 30t electric furnaces, with a maximum lifespan of about 80 times. For 100t large electric furnaces, the lifespan was 25 to 35 times. In the late 1960s, oxygen blowing technology was promoted in electric furnace steelmaking, and steelmaking operations were further strengthened. This acidic silica brick furnace cover was not suitable for alkaline smelting, and the smelting temperature was higher than that of silica bricks. The SiO2 melted at high temperatures produced continuous droplets, which reduced the alkalinity of the slag and affected the smelting operation. Therefore, high-alumina bricks with better performance were developed and used.

(2) High-alumina brick furnace cover. The use of high-alumina bricks to build electric furnace covers began in the late 1950s. At the time, U.S. Steel and Armco Steel used high-alumina bricks (65% to 80% Al2O3) for furnace roofs, which increased their lifespan by four times compared to silica bricks. Fired high-alumina bricks (80% to 85% Al2O3), unfired phosphate-bonded high-alumina bricks (80% to 85% Al2O3), and fired or unfired phosphate-bonded mullite bricks all shared the common characteristic of excellent thermal shock resistance. By the early 1960s, furnace roofs had a lifespan of approximately 200 furnaces.

The 85t UHP electric furnace roof at the French steel plant SAULVE uses standard high-alumina bricks (around 75% Al2O3), with an average service life of 150 cycles. A 130t electric furnace, on the other hand, requires 133 cycles, requiring repairs after approximately 90 cycles. The refractory consumption is 2.75kg per ton of steel. Electrode hole bricks, by adding high-alumina bauxite aggregate, have a service life of over 254 cycles, reducing refractory consumption to less than 1.0kg per ton of steel.

Japanese electric furnace covers generally use high-alumina bricks. Fired high-alumina bricks made primarily from high-purity alumina are used in areas around electrode holes and exhaust ports, where melt loss is severe. Alternatively, unfired bricks, which are resistant to corrosion and spalling, are partially used. These bricks incorporate an appropriate amount of CrO₃, imparting high strength and corrosion resistance. Furthermore, unfired high-alumina bricks are predominantly used around non-water-cooled furnace covers, with a reduced tendency to use magnesia-chrome unfired bricks.

(3) Impregnation of furnace cover. In the 1970s, Japan used the impregnation process to improve the performance of high-aluminum products for furnace cover bricks, achieving good results. Oxides such as MgO, Cr2O3, CaO, and SiO2 were used as impregnants to impregnate into the pores of the product to compensate for the defect of the matrix being easily eroded, thereby improving the product structure and enhancing its durability. Using Cr2O3 as an impregnant can reduce the intrusion of low-melting materials into furnace cover bricks and improve creep resistance. The high-temperature strength of corundum bricks impregnated with Cr2O3 is more than doubled, and the deformation at high temperature is reduced by 35%.

(4) Alkaline furnace cover. Magnesia bricks are used for electric furnace covers. To ensure the stability of the furnace cover structure and reduce the deformation and peeling of magnesia bricks, iron sheet magnesia bricks are used or iron sheets are inserted during masonry. Since the iron sheet generates induced current (especially for high-power electric furnace covers), local overheating occurs, so non-magnetic steel is used as the brick shell. On the other hand, the linear expansion coefficient of pure magnesia bricks is too large, so the amount used is not large. Other alkaline bricks such as magnesia-chromium, chromium-magnesium, dolomite, and magnesia-aluminum are used successively. There are high-temperature fired bricks or unfired bricks, or tar-impregnated dolomite bricks. They have excellent refractory properties and resistance to corrosion by iron oxide and slag. Under harsh conditions, their performance is better than high-alumina bricks. However, due to the high volume density of alkaline bricks, the furnace cover is heavy (50% heavier than silica bricks and 30% heavier than high-alumina bricks). Therefore, the construction method of the furnace cover structure is improved, and a combination of different materials is used to improve the performance of the electric furnace cover.

(5) Integrated furnace cover. When Japan Steel Pipe Company used a combination of high-alumina bricks and alkaline bricks for its 30t electric furnace cover, the service life reached 200-300 furnaces, with a refractory material consumption of 2-3 kg per ton of steel. When the water-cooling surface of the main furnace cover reached 70%-80%, the furnace life reached 300 furnaces. Another company used 16 water-cooling box plates on the outer ring of the 40t electric furnace cover, and the small furnace cover was built with chemically bonded magnesia-chrome bricks. High-alumina refractory ramming material was used around the electrode holes. The service life of the furnace was increased from 190 furnaces to about 600 furnaces.

3.Prefabricated furnace roof

One disadvantage of brick-built furnace roofs is the extremely strict dimensional tolerances required for the formed bricks. For fully water-cooled furnace roofs, precast blocks are used around the center electrode hole. Compared to brickwork, precast blocks offer advantages: simplified construction, convenient transportation, no drying required after construction, only a short preheating time, and a longer service life.

Germany has made several improvements to precast blocks for furnace roofs:

(1) Structural design: To obtain good mechanical properties, circular and triangular precast blocks are selected. The corners of the precast blocks are changed to rounded to improve adaptability to expansion and contraction. The appearance of the precast blocks is shown in Figure 2-5.

(2) Material selection: The corrosion of general furnace cover precast blocks is mainly caused by horizontal cracks, penetration and shedding of the dense layer. High-aluminum precast blocks have a high porosity and are prone to corrosion. Corundum precast blocks are used, which have a dense structure, low porosity, low air permeability, high strength, and good thermal shock resistance, which can improve the use effect.

(3) Masonry construction: The gaps between the precast blocks and the triangular area between the side wall and the precast block are filled with refractory ramming materials of the same material as the precast blocks; 2) To compensate for the expansion between the precast blocks and between the water-cooling area, a refractory fiber blanket is used.

The number of times the precast furnace cover is used has increased from 150 furnaces with traditional brick lining to 160 furnaces.

The small furnace cover of Osaka Ceramics’ ultra-high-power electric furnace, originally constructed from ramming material (Al2O3 >85%, SiO2 9%, P2O3 3.4%) and bricks, had an average service life of 453 furnaces. After switching to high-strength castable material (Al2O3 >85%, CaO 0.8%, SiO2 9%), the service life of the precast blocks gradually increased from 506 furnaces to 1029 furnaces.

After the Wakayama Steel Works adopted water-cooling for its 80t UHP electric furnace cover, the entire cover was constructed entirely from precast blocks, with three corundum-mullite precast blocks used in the triangular area and ten magnesia-aluminum spinel precast blocks used in the electrode ring. The small furnace cover was constructed on a prefabricated mold. The electrode holes were first paved with mud, then circumferentially laid from the edges toward the center. Finally, a corundum castable was poured around the triangular area and the electrode holes, then demolded and cured for use. The precast furnace cover has a service life of 600 furnaces, reducing the refractory material content to 0.4 kg per ton of steel.

4.Integrally cast furnace cover

The use of an integrally cast furnace roof has further extended its lifespan. A 130t UHP electric furnace at the Sheffield (SMACC) steelworks in the UK originally had a high-alumina brick roof with an average service life of 133 furnaces. The roof was water-cooled, including around the electrodes. The material used was a high-alumina, ultra-low cement castable containing 33% Cr2O3 and incorporating stainless steel fiber. With minimal repairs, the roof lasted for 594 furnaces, increasing its lifespan by approximately four times.

5.Refractory materials for electric furnace covers in China

In the early 1950s, my country’s electric furnace cover used large pieces of special-shaped silica composite bricks, which were easy to build and tight, and the stress distribution of the entire furnace cover was relatively uniform. However, this type of brick is difficult to shape, and because silica bricks are difficult to adapt to the needs of intensified smelting operations, they are gradually no longer used. my country was the first to study the use of high-alumina electric furnace cover bricks. The high-alumina bricks produced by Tangshan Iron and Steel Refractory Plant were used on 150 to 200 furnaces of Fushan Iron and Steel’s 15t electric furnace. Later, unfired high-alumina bricks prepared with various binders (water glass, phosphoric acid, aluminum dihydrogen phosphate, etc.) were developed, or sillimanite mineral raw materials were introduced into the ingredients, and micropowder, spinel, silicon carbide, etc. were added. In recent years, various types of refractory castables have been developed to meet the needs of the development of ultra-high power electric furnaces.

(1) High-alumina brick furnace cover. my country has rich high-alumina bauxite resources and a full range of varieties, which provides favorable conditions for the development of advanced high-alumina refractory materials. In the mid-1950s, high-alumina bricks based on DK-type diaspore-kaolinite and grade I bauxite clinker (Al2O3>80%) were developed and used. These bricks are made from bauxite clinker with a uniform structure, low impurities, and good sintering (bulk density 3.1-3.2 g/cm3). Clay with good plasticity is used as a binder. The addition of fine clinker powder (<0.088 mm2 >90%) is controlled (40-50%) to buffer the volume expansion caused by secondary mullite formation during firing and prevent the product from becoming loose. In 1987, Tangshan Iron and Steel’s Refractory Plant began producing high-alumina bricks with Al2O3>85%, successfully using 193 furnaces in Wutong’s 75t electric furnace. The refractory material consumption was 1.34 kg per ton of steel. The fired high-alumina bricks with corundum as the main crystal phase are used for ultra-high power electric furnace covers, with a service life of about 150 furnaces. The 40t UHP electric furnace cover of Guangzhou Steel Plant uses water-cooled small furnace covers with a service life of 300~400 furnaces.

Analysis of the microstructure of post-use furnace cover bricks indicates that the damage mechanism is chemical erosion and flaking. Slag (primarily composed of CaO and FeO) splashed onto the furnace cover, along with the melted and volatilized MgO and other alkaline components from the filler material and magnesia refractory materials in the furnace walls, chemically reacts with the high-alumina bricks, causing chemical erosion. Chemical erosion at high temperatures forms a layered structure of slag, permeation layer, and original brick layer. Under periodic temperature fluctuations, transverse cracks form between the permeation layer and the original brick layer, leading to flaking. The thickness of the flaking can sometimes reach 50-100 mm, or as little as 10-30 mm. Flaking occurring in the early stages of a furnace’s service life can sometimes be thicker and larger in area. Preventing flaking and improving erosion resistance are key to delaying refractory brick failure.

(2) Furnace cover bricks containing “three stones”. The “three stones” include sillimanite, kyanite and andalusite, which belong to the sillimanite family of minerals and are allomorphs. Their theoretical composition is A12O363.1%, SiO236.9%, and their chemical formula is A12O3.SiO2 or Al2SiO3. Due to the different generation conditions and structures, the differences in cation coordination make the physical and chemical properties have both similarities and differences. The three mineral raw materials are all high-aluminum minerals and have the properties of high-aluminum minerals, but their unique properties are not possessed by ordinary high-aluminum raw materials. When heated to a certain temperature, the “three stones” are irreversibly transformed into mullite crystals (short columnar and needle-shaped network structure, generally about 3 microns long, up to 20 microns long), and SiO2 is precipitated. During the heating reaction, the volume expands due to mullitization, among which kyanite has the largest expansion rate, and andalusite is in the middle.