The refractory material for ferroalloy electric furnaces comprises three parts: furnace roof refractory material, furnace wall refractory material, and molten pool refractory material (furnace slope and furnace bottom). During ferroalloy smelting, the refractory materials in these different parts are under different working conditions. To ensure the normal operation of the electric furnace, refractory materials with high refractoriness and softening temperature under load, good resistance to rapid heating and cooling, good slag resistance, large heat capacity, and certain thermal conductivity must be selected for the furnace lining.
Refractory Materials for Ferroalloy Furnace Linings
The properties and characteristics of refractory materials commonly used in ferroalloy production furnace linings are as follows:
Clay Bricks
The main raw material for manufacturing clay bricks is refractory clay with good plasticity and bonding strength.
The main performance characteristics of clay bricks are: strong resistance to acidic slags, good resistance to rapid heating and cooling, good heat insulation capacity, and certain insulation properties. However, they have relatively low refractoriness and softening temperature under load. Clay bricks are not suitable for direct use under high-temperature conditions or special requirements.
In ferroalloy production, clay bricks are mainly used for lining the exposed parts of submerged arc furnaces, serving as the outer lining of the furnace walls and bottom for heat insulation and insulation, or for lining the inner layer of molten iron ladles.
The main raw material for manufacturing high-alumina bricks is high-alumina bauxite, and the binder is refractory clay.
Compared to clay bricks, the biggest advantages of high-alumina bricks are high refractoriness, high softening temperature under load, good slag resistance, and high mechanical strength. The disadvantage is poor resistance to rapid heating and cooling. In ferroalloy production, high-alumina bricks can be used for lining the taphole of submerged arc furnaces, the top of refining electric furnaces, and for lining the molten iron ladle.
The main raw material for manufacturing magnesia bricks is magnesite, and the binder is water and brine or sulfite pulp waste liquid.
The main performance characteristics of magnesia bricks are: high refractoriness and excellent resistance to alkaline slag. However, they have a relatively high thermal conductivity and electrical conductivity at high temperatures, a low softening temperature under load, and poor resistance to rapid heating and cooling. They pulverize when exposed to water or steam at high temperatures.
In ferroalloy production, magnesia bricks are used to construct the furnace walls and bottoms of high-carbon ferrochrome reduction electric furnaces, medium- and low-carbon ferrochrome converters, shaking furnaces, and refining electric furnaces, as well as the linings of ladles containing ferrochrome and medium- and low-carbon ferromanganese. Magnesia-alumina bricks are used instead of magnesia bricks for furnace roof construction. Magnesia sand has a high refractoriness and is commonly used in ferroalloy production for bridging furnace bottoms, making and repairing furnace walls and bottoms, and as a plugging material or for making bridging ingot molds.
The main raw materials for manufacturing carbon bricks are crushed coke and anthracite, with coal tar or pitch as the binder.
Compared to other common refractory materials, coal bricks not only have high compressive strength, a low coefficient of thermal expansion, and good wear resistance, but also high refractoriness and softening temperature under load, good resistance to rapid heating and cooling, and particularly good slag resistance. Therefore, coal bricks can be used as lining materials for submerged arc furnaces for smelting ferroalloys that are not susceptible to carburization.
However, coal bricks are highly susceptible to oxidation under high-temperature conditions, and their thermal and electrical conductivity are relatively high. In ferroalloy production, coal bricks are mainly used for lining the furnace walls and bottom of submerged arc furnaces in areas not exposed to air.
Analysis of the Causes of Damage to Ferromanganese Alloy Electric Furnace Linings and Effective Solutions
Currently, ferromanganese alloy electric furnace linings are mainly divided into two types: “insulating” and “cooling.” The “insulating” lining materials are mainly carbonaceous materials, clay refractory bricks, and refractory mortar. Their low thermal conductivity reduces the heat transfer from the furnace chamber through the lining and shell, resulting in higher thermal efficiency. However, due to factors such as the lining and taphole construction method, and the physicochemical properties of the carbonaceous materials, there are many “weak points” in daily use, which can easily cause localized overheating and burn-through, affecting service life. Based on the distribution of “weak points” in the ferromanganese electric furnace lining, a “turtle-back” lining repair method was designed, which effectively extends the service life of the lining and has achieved significant results.
Structure and Damage of “Insulating” Carbonaceous Lining
The carbonaceous lining material mainly consists of carbon bricks, clay bricks, a filling layer, asbestos board, and refractory mortar. A schematic diagram of the specific construction is shown on the right (Figure 1). The red line in the figure represents the erosion line caused by molten iron during daily use. The main area affected by the oxidation and erosion is the carbon bricks around the furnace eye, particularly the furnace eye itself and two surrounding carbon bricks.
Based on an analysis of the lining damage in a high-carbon ferromanganese electric furnace at a certain plant, the main causes of lining burn-out are twofold: ① carbon brick oxidation, and ② erosion and scouring by high-temperature molten iron. The analysis reveals that the damage to the high-carbon ferromanganese electric furnace lining is primarily centered around the furnace eye, with the furnace eye carbon bricks, surrounding lining, and bottom carbon bricks suffering from oxidation due to high-temperature slag, iron, air, and moisture.
“Turtle-back” Furnace Shell Repair Method and Its Application
The “turtle-back” furnace shell repair method mainly involves extending the furnace shell outwards by 35-45cm within a radius of 2-4 carbon bricks around the furnace eye, filling this area with electrode paste and refractory bricks.
Advantages of the “Turtle-back” Furnace Shell Repair Method
The “turtle-back” furnace shell repair method effectively avoids problems such as insufficient electrode paste filling and incomplete melting that occur during conventional repairs.
The “turtle-back” furnace shell repair method, combined with the furnace eye heat sealing operation, allows the electrode paste in the furnace eye area to fully bond with the electrode paste in the furnace lining area, forming a unified whole. This results in better structural integrity and extended service life.
The outward extension of the “turtle-back” furnace shell increases the furnace lining thickness and enhances the air-cooling effect of the extended section, leading to even better results when used in conjunction with a blower.
Precautions for Repairing “Turtle-Back” Furnace Shells
When performing “turtle-back” repairs, slag and iron deposits near the furnace bore must be thoroughly cleaned to avoid creating “void areas.”
Planned “turtle-back” repairs can effectively shorten hot shutdown time and improve repair results.
This method must be used in conjunction with furnace bore heat sealing.
Strictly control the quality of the filler paste, using molten paste whenever possible. Electrode paste fragments can be used to fill the sealing area.
By observing the burn-through of the furnace bore (turtle-back furnace shell), the main cause is uneven sintering of the electrode paste within the “turtle-back,” resulting in stratification. When the “turtle-back” thickness is too large (approximately 400mm), the sintering of the electrode paste within the “turtle-back” is poor, requiring wood-fired drying on the outside to ensure even sintering. Simultaneously, strictly control the initial furnace load and electrode insertion depth.
This method has been widely used in the actual production of high-carbon ferromanganese electric furnace plants, effectively extending the service life of the furnace bore and furnace lining, and greatly reducing the maintenance cost of the furnace lining.
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