In iron smelting, how to select refractory materials for blast furnaces

　　The effect of high-alumina refractory materials on blast furnace slag, iron water erosion, and high-temperature wear is high stability and inertness to carbon monoxide. However, when working under alkaline conditions at high temperatures, it was found that corundum transforms into β-alumina. This transformation is accompanied by a 20% volume increase, leading to fragmentation. The matrix first interacts with alkaline vapor. Replacing the corundum binder with mullite or chromia-alumina matrix in corundum bricks improves the stability of the product against alkaline action.

　　Further improving the efficiency of blast furnace refractory materials is the use of silicon carbide products; especially for the lower part of the furnace body and the furnace belly lining. Silicon carbide—chemically inert, resistant to wear, and silicon carbide refractory materials have very high stability. Compared with carbonaceous products, silicon carbide has better oxidation stability. The main problem with using silicon carbide products in blast furnaces is developing a binder with sufficiently high alkali stability. From the tested tar-bonded silicon carbide graphite products, ceramic (clay)-bonded, oxynitride and nitride-bonded silicon carbide products, nitride Si₃N₄ and oxynitride Si₃ON₂ bonded products have relatively good stability. However, it was found that self-bonded silicon carbide products are more alkali-resistant. Since self-bonded silicon carbide products are difficult to manufacture, refractory materials with various combinations of silicon carbide content are being studied: high-alumina silicon carbide, graphite silicon carbide, etc.

　　Magnesite refractory materials have been tested in the lower part of the furnace body, the furnace waist, and the furnace belly. Their service life is higher than that of high-quality clay bricks, but lower than that of silicon carbide. The selection of suitable refractory materials for the lower part of the furnace body and the furnace belly is still ongoing.

　　The lower part of the furnace body and the furnace belly have been in continuous operation for 5 to 8 years, and in some cases for 10 to 12 years. Intermediate repairs (Class II repairs) of the furnace body and furnace belly usually take 12 to 18 days to complete. This involves shutting down the furnace, blowing off the lining, cooling, inspection, lining replacement, repair of metal components and equipment, and the furnace. Repairs in individual cases mainly involve spraying. The spray material is composed of the same raw materials used in the production of dense bricks, using bauxite cement or high-alumina cement with low iron oxide content as a binder. It is known as a method that does not require shutting down the furnace and repairs small volumes. In this case, refractory mud of special composition is applied through holes in the furnace shell, and completed in the furnace under a pressure of less than 1.5 MPa. The binder used in this mud is a bonding substance or coal tar with a softening temperature exceeding 200℃, diluted liquid oil, asphalt, or water glass-bonded high-alumina cement.

　　① Classification of blast furnace lining repair: Class I: Repair of furnace body and hearth; Class II: Repair of furnace body; Class III: Repair of furnace throat.

　　Repairs to the upper part of the furnace body (Class II repairs) can be carried out by spraying without cooling the furnace.

　　The hearth and furnace bottom lining are basically made of carbonaceous masonry. The advantages of carbonaceous products are their high refractoriness (over 3000℃) in reducing or inert atmospheres, increased strength with increasing temperature, constant volume over a wide temperature range, good thermal shock resistance, non-wettability of metals and slag, and the ability to produce large bricks with strict dimensional tolerances. The hearth is laid with 3 to 4 rows of corrugated carbonaceous large bricks, up to 1.5 to 3.5 m long, with a cross-sectional area of (0.5 to 0.65) m × (0.5 to 0.65) m. The wall above the hearth is laid with carbonaceous large bricks. The upper part of the hearth center is completed with high-alumina corrugated (to prevent floating) bricks. To prevent brick floating, the so-called top-laying method is also used. The lower temperature at which the refractory material interacts with pig iron is 1100 to 1150℃. If high-alumina products are used in the upper part of the hearth, the isothermal line of 1100 to 1150℃ passes through the brick surface layer, causing the tapping temperature to be around 1450℃. When the high-alumina bricks become thinner at the 1100 to 1150℃ isothermal line layer, the temperature of the outer layer of the bricks increases, and the tapping temperature decreases.

　　Therefore, the tapping temperature becomes an indicator of the state of the blast furnace hearth lining. Larger furnaces are equipped with regulated cooling from below the hearth. The main cause of hearth damage (brick floating) is that iron penetrates into the serrated gaps between the bricks under the action of hydrostatic pressure. Therefore, the hearth masonry should be pre-assembled on a well-prepared large brick test bench. The total thickness of the hearth refractory masonry in larger furnaces reaches 5 m. It is necessary to prevent unexpected iron breakthroughs in the furnace base due to heating (especially uneven heating) and hearth damage.

　　The furnace bottom is constructed using carbonaceous and aluminosilicate refractory materials. The carbonaceous masonry of the furnace bottom wall is cooled with standard cooling devices or water jackets (double-shell furnace). The upper part of the furnace bottom (tuyere zone) is suitable for laying silicon carbide refractory materials.

　　Historically, effective refractory materials have been selected. It is believed that the carbon deposited during the decomposition of carbon oxides is stored in the pores and cracks of the refractory products, causing damage to the refractory materials. Magnesium oxide and deposited zinc in the refractory material composition are considered to be catalysts for this reaction. To reduce carbon enrichment in steel, dense products with lower iron oxide content are used, but it is important to increase the service life of the blast furnace lining.

　　Later, attention turned to the destructive effect of alkaline vapor. The alkali content in the blast furnace burden reaches 10% to 15% in the lower part of the furnace body where it contacts the lining (peripheral area), while the central part of the furnace contains 1% to 2%. The interaction between the refractory lining and alkali includes the following stages: formation of alkaline compound vapor, penetration into the refractory material, condensation, melting, filling of pores, and reaction of the melt with the furnace gas medium. When SiO₂ is present in the refractory material (in mullite grains or matrix), alkaline compounds are formed: leucite K₂O·Al₂O₃·4SiO₂, kalsilite K₂O·Al₂O₃·2SiO₂, nepheline Na₂O·Al₂O₃·2SiO₂, K₂O·6Al₂O₃ and Na₂O·6Al₂O₃·β-alumina, Na₂O·Fe₂O₃·4SiO₂ hedenbergite; orthoclase K₂O·Al₂O₃·6SiO₂. The formation of alkaline aluminosilicates is accompanied by a volume increase of up to 45%, causing stress generation and the formation of new fractures. The open porosity of the alkali-saturated part of the aluminosilicate refractory material is reduced to 2.5%. Mullite is dissolved under the action of alkali, and alkaline compounds may be reduced and decomposed under the action of carbon. At 1500℃, gaseous potassium and metallic silicon are formed. Cyanides KCN and NaCN are formed in the lower part of the furnace body. Some of the cyanides rapidly leave with the gas, while others react with the refractory material to form a melt.

　　Studies on the interaction between alkali and refractory materials undoubtedly show that SiO₂-containing refractory materials are unacceptable in the lower part of the blast furnace structure.

　　It is known that alumina products (Al2O3 95%~98%) are alkali-stable. However, this is not a suitable judgment. Alumina products have been shown to form β-alumina, and also have a high coefficient of linear expansion, causing them to crack.

　　Refractory materials themselves interact with iron, even exhibiting all thermodynamically possible reactions. Under the action of iron, the damage to refractory materials is small because the interaction rate is low, and the reaction products form a hard, inert protective layer. At 1500℃, the contact angle of iron on most refractory materials is greater than 100°.

　　In recent years, carbonaceous, especially silicon carbide refractory materials, have been successfully used in blast furnace linings. Compared with other blast furnace refractory materials, silicon carbide refractory materials have the following advantages: high alkali stability, good thermal shock resistance, high thermal conductivity, and small pore size. As a result, silicon carbide products squeeze out all other refractory materials from the lower structure of the blast furnace. The combination of silicon carbide and graphite is considered promising.

　　Later developed Sialon-bonded silicon carbide products, compared to those bonded with silicon nitride, have larger crystals, lower porosity, better oxidation resistance, and have been tested in the middle section of blast furnaces in some countries.

　　The chemical and mineral composition of blast furnace refractory materials, according to porosity, requires that the porosity of the product be less than 12%, significantly reducing its slag penetration, but the penetration is not gate-like. Products with a porosity of less than 12% have 5-6 times lower wear at room temperature than products with a porosity of 17%, and 10-20 times lower at 980℃.

　　Cooling is an important factor in the stability of blast furnace linings.

　　The furnace hearth and furnace bottom lining are continuously used for 15-20 years. During this period, the tuyere, iron, and slag outlets are inspected according to a comprehensive process. The tuyere uses refractory materials of special quality and is replaced according to wear and tear. Each time iron and slag are discharged, special gunning clay is used to fill the taphole. The gunning clay should have plasticity and stability against molten iron and slag, high hardening speed, and should not emit smoke or harmful gases that pollute the environment.

　　Currently, tar-bonded anhydrous gunning clay is widely used. It contains silica, high-alumina materials, silicon carbide, a small amount of carbon and clay, and about 15% coal tar or asphalt. However, this clay hardens slowly, produces smoke, and creates heavy working conditions. Using phenol-formaldehyde tar as a binder can eliminate these shortcomings; it is thermally reactive, turning into a solid state when heated. The solvent for the tar is ethanol. Al2O3-SiC-C system and MgO-C system clays have high stability with phenol-formaldehyde tar. Using this clay, the taphole repair time is shortened, thus contributing to increased furnace output.

　　Refractory materials for blast furnace tapholes should have high chemical stability, wear resistance at high temperatures, and stability against temperature changes. To improve the service life of the taphole lining: adding silicon carbide to high-alumina materials; silicon carbide, silicon carbide-alumina, and silicon nitride-bonded silicon carbide. Silicon nitride-bonded silicon carbide clay has a relatively good service life. The characteristics of silicon nitride are: high strength at operating temperatures combined with low thermal expansion and high thermal conductivity. Therefore, this binder has high thermal shock resistance. In addition, at high temperatures, an oxide film forms on the surface of the silicon nitride crystals, preventing binder wear.

　　The service life of the taphole lining depends not only on the properties of the refractory materials, but also on the furnace operating conditions, the method of lining implementation, and the structure of the taphole itself. The taphole of large blast furnaces in China has developed from ramming materials to castables, mainly composed of electrofused alumina, silicon carbide, a small amount of metallic silicon and aluminum powder, an appropriate amount of accelerators, release agents, ultrafine powders, etc. Castables have been used in some blast furnaces with good results.