Application of Refractory Materials in Different Furnace Types in Ironmaking Production

1. Blast Furnace and Iron Ladle for Blast Furnace Iron

(1) Blast Furnace

The blast furnace is a thermal equipment used for accumulating, storing, and maintaining a constant temperature (1250–1300°C) while uniformly mixing iron components. The blast furnace is lined with refractory materials. The primary cause of damage to the refractory lining is the infiltration of molten iron into the cracks between bricks, along with brick lifting, slag erosion, and spalling. Depending on the slag’s alkali content, magnesia bricks, magnesite olivine bricks, or high-alumina bricks with high alumina content are used. When the slag has a low alkali content, aluminosilicate products are used. When the slag contains Na₂O > 2%–3%, aluminosilicate products become porous and are damaged. In such cases, magnesia bricks or magnesite olivine refractory materials are used. Special attention must be given to the life of fire clay, which should have the ability to make brick joints dense. Acidic blast furnace linings are generally made of silica bricks.

(2) Iron Ladle for Blast Furnace

With the increase in steel production, the blast furnace, as a storage vessel for molten iron, has lost its specific function and is replaced by the blast furnace type iron ladle, as shown in Figure 11-2. The operating conditions for refractory materials in the iron ladle are similar to those in the blast furnace. From the perspective of stress on the ladle material, it involves the even distribution of load across two supporting beams. Even a slight bending of these beams can cause mechanical load stress of 0.2 MPa or higher on certain areas of the lining. This condition results in creep limitations for the applied refractory materials. It is suggested that under a 0.2 MPa load at 1300°C, the creep rate should not exceed ≥0.03%/h. When using tar-impregnated refractories, the lifespan of the ladle lining is significantly improved. The ladle lining generally consists of three layers: working layer, protective layer, and thermal insulation layer. With the development of molten iron pretreatment technology, the blast furnace-type ladle is used not only as a tool for transporting liquid iron from the blast furnace to steel-making equipment but also as a container for out-of-furnace refining, including desulfurization of molten iron. Lime is used as the desulfurizing agent, which is blown into the molten iron and mixes with the subsequent gas in the nitrogen stream. In such cases, magnesia or alumina-silicon carbide refractories are used for the lining of the iron ladle. When using calcium carbide for desulfurization of molten iron, a tar-bonded, non-burning magnesia-calcium product lining is used, yielding good results. In Japan, desulfurization, dephosphorization, and desiliconization are performed inside the ladle using Al₂O₃-SiC-C products with good results. In China, high-alumina bricks impregnated with pitch, or those with added SiC or C, or both, are used to enhance resistance to erosion and thermal shock. Some use dolomitic refractories.

2. Coke Oven

The coke oven has a complex refractory masonry structure. The most critical part of the masonry is the combustion chamber wall. It operates under the following conditions: during coking, the temperature is about 1300°C with minimal variation; the coking temperature starts at 500–600°C during the coking cycle and rises to 1200–1250°C at the end of coking. At the same time, the temperature at the center of the coke cake reaches 1100°C. The chamber width is 400–450 mm, and the coking period lasts for 14–17 hours. For normal operation of the coke oven, it is essential to maintain high gas tightness of the walls and furnace masonry. The refractory masonry also bears compressive stress due to both the mass of the masonry itself and the weight of the coal charging cars on top of the furnace. Only silica bricks may meet these conditions. Coke ovens that use silica bricks may last up to 40 years. However, silica bricks have relatively low thermal conductivity under coking temperatures, approximately 1.9 W/(m·K), and due to their high volatility, the temperature should not exceed 1250°C. More efficient refractory materials to replace silica bricks are currently in the development stage. For example, there are proposals for magnesia-silica products for wall structures, and experiments with silicon carbide bricks, corundum refractories, and iron-containing silica bricks are underway. Refractory castables (large wall panels) are also used to replace complex-shaped small block products.

3. Direct Ironmaking Refractories

The process of directly producing iron from ore results in sponge iron, granular iron, or liquid iron, which is heated in a reducing gas medium (H₂, CO), mainly to reduce iron oxide to metallic iron. The production of sponge iron is carried out at temperatures below 1000°C in a vertical furnace. Ordinary clay refractories can be used in these furnaces. The reduction gas is produced using natural gas (CH₄) and converted in a specialized gas heater based on a regenerative principle. The gas heater, structurally similar to a blast furnace hot blast stove, uses nickel as a catalyst for natural gas conversion. The grid of the gas heater plays a key role as a catalyst and operates under variable temperatures and gas media, working under conditions that transition from oxidation to reduction. The issues that arise here involve the thermal shock resistance of refractories and their chemical stability with respect to the catalyst. According to relevant data, Al₂O₃-C bricks and MgO-Cr₂O₃ products show better performance. Since the molten iron smelting process is a new technology still under development, the refractory materials used are also being explored.

4. Iron Ore Sintering Furnace Refractories

In order to enhance metallurgical production, various methods have been adopted, including the configuration of equipment for the pre-heat treatment of iron ore raw materials: conveyor belt-type sintering machines and roasting machines; grate plate-tube furnaces-ring coolers. Combined equipment includes vertical furnaces, fluidized bed furnaces, and other thermal equipment. Furnace linings are primarily made of high-alumina products with an Al₂O₃ content of 85%, various compositions of refractory castables, and insulating materials such as mullite, silica boards, perlite bricks, or refractory fibers. Damaged refractory linings are repaired and patched using spray methods to extend their lifespan.