Key Factors in Selecting Neutral Reactor Lining Materials:
In the actual production process of medium-frequency furnaces, the thickness of the refractory materials used for the furnace lining is typically only 70 mm to 110 mm. The inner side of the lining comes into contact with high-temperature molten metal, while the outer side is in close contact with water-cooled coils. There is a significant temperature difference between the inner and outer sides of the refractory lining, especially in relatively thin cross-sections and under the highly corrosive conditions of many smelting operations.
With an understanding of these harsh operating conditions unique to medium-frequency furnaces, we’d like to share some key considerations for selecting neutral furnace lining materials.
(1) The furnace lining material must have sufficient refractoriness
The fundamental factors determining the refractoriness of refractory materials are their chemical and mineral composition and the distribution of these components. Various impurities, particularly those with strong fluxing properties, can significantly reduce the refractoriness of the product. The refractoriness of a refractory material refers to the temperature in degrees Celsius at which a conical test specimen of the material, under no load, resists the effects of high temperature without softening or melting. Simply put, refractoriness refers to the property of a refractory material to resist high temperatures without melting when unloaded. Refractoriness serves as the basis for determining whether a material can be used as a refractory. The International Organization for Standardization (ISO) defines refractory materials as inorganic non-metallic materials with a refractoriness of 1500°C or higher.
Therefore, the primary means of improving the refractoriness of refractory materials is to take appropriate measures to ensure and enhance the purity of the raw materials.
During smelting in a medium-frequency furnace, the maximum temperature of molten steel can reach 1700°C, while that of molten iron can exceed 1500°C. Therefore, furnace lining materials must be selected based on the smelting temperature of the metal being processed to ensure they can withstand these temperatures.
(2) The furnace lining material must have good chemical stability
Slag erosion is the most common form of damage to refractory materials during use. Damage to the furnace lining of medium-frequency furnaces, the working lining of ladles, and the furnace lining of cupola furnaces—from the lower part of the furnace body to the hearth—is often caused by this phenomenon. In actual use, approximately 59% of damage is attributed to slag erosion.
Slag resistance refers to the ability of refractory materials to withstand slag erosion and scouring at high temperatures.
When selecting refractory materials, it is advisable to choose those with a chemical composition similar to that of the slag to reduce the intensity of reactions at their interfaces. For example, when smelting high-manganese steel, the furnace lining should consist of alkaline refractories. Alternatively, adjusting the slag composition to align more closely with that of the refractory material is another effective method for improving the refractory’s slag resistance.
(3) The furnace lining material has good thermal shock resistance
Refractory materials are frequently subjected to sudden changes in ambient temperature during use. For example, during the smelting process in a medium-frequency furnace, temperature fluctuations occur during charging, tapping, or other operations; similarly, during the pouring process, the temperature and pressure in the pouring ladle can change abruptly, causing cracks, spalling, or even collapse of the furnace lining and ladle lining. This destructive effect limits the heating and cooling rates of furnace linings, ladle linings, and the furnaces themselves, thereby restricting the intensification of furnace operations. It is one of the primary causes of rapid damage to furnace and ladle linings.
(4) The furnace lining material should have a certain level of high-temperature strength
The high-temperature compressive strength of refractory materials refers to the maximum pressure per unit area that the material can withstand under high-temperature conditions ranging from 1000°C to 2000°C, expressed in N/m².
The compressive strength of refractory materials generally changes significantly as the temperature rises. Starting from room temperature, it initially decreases linearly with increasing temperature. Thereafter, some materials continue to decrease as the temperature rises, while others, once the temperature reaches a certain range, begin to increase with rising temperature, reaching a maximum value at a specific temperature before dropping sharply.
During the smelting process in a medium-frequency furnace, the furnace lining walls are subjected to the impact force of metal charge, the static pressure of molten metal, the forces generated by electromagnetic stirring of the molten metal, and thermal stress caused by temperature differences. Therefore, the selected furnace lining material must possess a certain level of high-temperature compressive strength, resist cracking under the combined action of various forces, and withstand erosion.
The high-temperature strength of the furnace lining material is directly related to the hardness of the particles comprising the refractory material; the bond strength between particles and the porosity of the refractory matrix also affect its wear resistance. For the same material, wear resistance decreases as temperature increases.
(5) The thermal conductivity of the furnace lining material should be low, and its electrical conductivity should be low
During the smelting process in a medium-frequency furnace, there is a significant temperature difference between the inside and outside of the furnace wall, and approximately 10% to 15% of the heat is lost through the furnace wall. To reduce heat loss and improve thermal efficiency, the furnace lining material must have low thermal conductivity.
The furnace lining material must have low electrical conductivity and must not contain any magnetically inductive substances. In particular, it is absolutely impermissible for metal or highly conductive substances to be trapped within the lining during the lining installation process.
