在炼铁生产中，如何选择高炉用耐火材料
高铝耐火材料对高炉渣的作用，铁水冲蚀，高温下的磨损，具有高的稳定性，并对一氧化碳有惰性。可是在碱介质中，在高温条件下工作时，发现刚玉转化为β-氧化铝。这个转化伴随体积增大20%，而导致碎裂。基质首先与碱蒸气相互作用。刚玉砖用莫来石或铬铝的基质代替刚玉结合，提高制品对碱作用的稳定性。
进一步提高高炉耐火材料效率是采用碳化硅制品；特别是用于炉身下部和炉腹内衬。碳化硅——化学惰性，对磨损，碳化硅耐火材料有非常高的稳定性，与碳质制品比较，碳化硅对氧化的稳定性较好。高炉应用碳化硅制品的主要问题是开发对碱具有足够高稳定性的结合剂。从试验的焦油结合碳化硅石墨制品，陶瓷（黏土）结合的，氧氮化物和氮化物结合的碳化硅制品，氮化物物Si₃N4和氧氮化物Si₃ON₂结合的制品有比较好的稳定性。可是发现自结合碳化硅制品更抗碱。既然自结合碳化硅制品制造困难，研究碳化硅含量各种组合的耐火材料：高铝碳化硅的，石墨碳化硅的等。
镁质耐火材料在炉身下部，炉腰和炉腹经过试验，它的寿命指标比高质量的黏土砖高些，然而低于碳化硅。炉身下部和炉腹选择合适的耐火材料，现在还在继续进行。
炉身下部和炉腹经过连续工作时间到5~8年，个别情况到10~12年。炉身和炉腹的中间修理（Ⅱ级检修）①，通常要12~18d完成。它是停炉，吹掉内衬、冷却，检查，内衬更换，检修金属构件和设备以及熔炉。个别场合的修理主要进行喷补。它由生产致密砖时应用的同样原材料构成喷补料，利用氧化铁含量低的矾土水泥或高铝水泥做结合剂。已知是不停炉，修理体积不大的方法，在这种情况下，专用成分的耐火泥料通过炉壳的孔，小于1.5MPa的压力下在炉中完成。这种泥料使用的黏结物质或软化温度超过200℃的煤焦油，冲淡的液体油，沥青或水玻璃结合的高铝水泥。
①检修高炉内衬分级：Ⅰ级：炉身和炉床检修；Ⅱ级：炉身检修；Ⅲ级：炉喉检修。
炉身上部修理（Ⅱ级修理）可以不冷炉进行喷补。
炉床和炉缸内衬基本上由碳质砌筑。碳质制品的优越性表现它在还原或惰性气氛中耐火度（超过3000℃）高，随温度提高，强度增大，在很宽的温度范围内体积不变，抗热震性好，对金属和渣的不润湿性以及能够制取公差要求严格的大块砖。炉床砌上3~4列有褶皱的碳质大砖，长到1.5~3.5m，断面积（0.5~0.65）m×（0.5~0.65）m。炉床上面的墙砌上碳质大砖。炉床中心上部系列用高铝褶皱（预防浮起）砖完成。为了预防砖浮起还用所谓顶砌的方法砌砖。耐火材料与生铁相互作用的最低温度为1100~1150℃。如果炉床上部应用高铝制品，沿砖表层通过1100~1150℃等温线，引起出铁温度为 1450℃左右。高铝砖在1100~1150℃等温线层变薄时，砖外层温度提高，出铁温度降低。
这样一来，出铁温度成为高炉炉床内衬状态的指示器。比较大的炉子由炉床下面配备调节冷却。炉床损毁的主要原因（砖浮起）——在铁静压力作用下，铁钻进砖之间的锯齿缝里。所以炉床砌体要在精心准备好的大砖试验台上预先装配。较大炉子的炉床耐火材料砌体的总厚度达到5m。必须预防炉子底座由于加热（特别是不均匀）和炉床破坏时产生铁的意外破口。
用碳质和硅酸铝质耐火材料砌筑炉缸。炉缸墙的碳质砌体用标准冷却装置或水套（二层炉壳）帮助冷却。炉缸上部（风口带）适合砌筑碳化硅耐火材料。
历史上曾选择有效耐火材料。认为碳的氧化物分解时析出的碳储存在耐火制品的气孔和裂隙里，以至造成耐火材料破坏。耐火材料组成中的氧化镁和沉积的锌，被认为是这个反应的催化剂。为了减少钢的增碳，采用氧化铁含量最小的致密制品，然而重要的是提高高炉内衬寿命。
后来注意力转向碱蒸气的破坏作用。高炉炉料中含的碱在炉身下部与内衬接触（周边区域）达到10%~15%，同时炉子中心部分含有1%~2%。耐火材料内衬与碱相互作用包括如下阶段：形成含碱化合物蒸气，往耐火材料内部渗透，凝结，熔化，充满气孔，熔体与炉子气体介质反应。耐火材料中存在 SiO₂时（莫来石颗粒或基质中），形成碱的化合物：K2O·Al2O3·4SiO2白榴石，K2O·Al2O3·2SiO2钾霞石，Na2O·Al2O3·2SiO2钠霞石，K2O·6Al2O3和Na2O·6Al2O3·β-氧化铝，Na2O·Fe2O3·4SiO2钠辉石；K2O·Al2O3·6SiO2正长石。形成碱的硅酸铝，伴随体积增大到45%，造成应力产生和形成新断裂。硅酸铝耐火材料被碱饱和部分开口气孔率降低到2.5%。莫来石在碱的作用下被溶解，碱的化合物在碳的作用下可能被还原和分解。1500℃时形成气体状钾和金属硅。在炉身下部形成氰化物KCN和 NaCN。氰化物一部分随气体急速离去，而其他的与耐火材料反应形成熔体。
研究碱与耐火材料相互作用，无疑证明在高炉结构的下部不能接受含SiO2的耐火材料。
已知刚玉制品（Al2O395%~98%）对碱稳定。然而，这又不是最合适的判断。像刚玉制品已经表现出形成β-氧化铝，此外还具有高的线膨胀系数，使它裂开。
耐火材料本身与铁相互作用，甚至全部具备热力学可能的反应，在铁的作用下，耐火材料损毁很小，因为相互作用的速度不大，而反应产物形成硬的惰性保护层。1500℃时，铁对大多数耐火材料的润湿角大于 100°。
近年来，高炉内衬成功地使用碳质，特别是碳化硅耐火材料。碳化硅耐火材料与其他高炉耐火材料比较有以下优势：对碱有高的稳定性，抗热震性好和热导率高，气孔尺寸最小。这样一来，碳化硅制品从高炉下部结构挤出其他所有耐火材料。碳化硅耐火材料与石墨相配合被认为是有前途的。
后来开发的 Sialon结合碳化硅制品与氮化硅结合的相比，结晶较大，气孔率低，抗氧化性好，在一些国家的高炉中段试用。
高炉耐火材料的化学矿物组成，按气孔率对它提出要求，认为制品气孔率小于 12%，明显减小它被渣浸润，可是浸润不带闸性质。气孔率小于12%的制品，在室温下的磨损比气孔率17%制品低5~6倍，而980℃时低10~20倍。
冷却是高炉内衬稳定的重要因素。
炉缸和炉床内衬连续使用到15~20年。这期间鼓风风口，铁和渣出口按综合进程检修。风口采用特殊质量的耐火材料，并根据磨损情况替换它。每个周期放出铁和渣时，用专用炮泥打入出铁口。炮泥应该具有可塑性和对铁水和渣液的稳定性，高的硬化速度，它不应该分泌出烟或污染环境的有害气体。
现在广泛使用焦油结合的无水炮泥，它含氧化硅，高铝材料，碳化硅，不大数量的碳和黏土，并有约15%的煤焦油或沥青。可是这种泥硬化慢，形成烟，造成笨重的工作条件。用苯酚甲醛的焦油作为结合剂可以排除不足，它具有热反应性，即加热时转为固体状态。焦油的溶剂使用乙醇，Al2O3-SiC-C系统和MgO-C系统泥料对本酚甲醛的焦油有高的稳定性。采用这种泥，出铁口补修时间减短，因而有利于增加炉子产量。
高炉出铁沟耐火材料应该有高的化学稳定性，提高温度时耐磨损和对温度变换的稳定性。提高沟衬寿命采取：加入碳化硅的高铝料；碳化硅的、碳化硅氧化铝的和氮化硅结合的碳化硅。氮化硅结合的碳化硅泥料有比较好的寿命。氮化硅的特点是：在使用温度下有高的强度与低的热膨胀相配合和最大的热导率。所以这种结合剂具有高的抗热震性。此外，提高温度时，氮化硅晶体表面上形成氧化物薄膜，预防结合剂磨损。
出铁沟内衬的寿命不仅取决于耐火材料的性质，而且又与炉子工作制度，实现内衬的方法和沟本身结构有关。我国大型高炉的出铁沟已从捣打料发展成浇注料，主要材质为电熔刚玉，碳化硅，少量金属硅与金属铝粉，适量促凝剂，解胶剂，超细粉等高级原料配制。浇注料在一些高炉使用，获得效果较好。
How to choose refractories for blast furnaces in ironmaking production
The effect of high aluminum refractory on blast furnace slag, hot metal erosion, wear at high temperature, has high stability, and is inert to carbon monoxide. However, in alkali medium, when working under high temperature conditions, it is found that corundum is converted into β-alumina. This transformation is accompanied by a 20% increase in volume, which leads to fragmentation. The matrix first interacts with the alkali vapor. Corundum brick with mullite or chrome-aluminum matrix instead of corundum bonding, improve the stability of the product against alkali action.
Silicon carbide products are used to further improve the efficiency of blast furnace refractories. Especially for the furnace body and furnace belly lining. Silicon carbide – chemically inert, wear, silicon carbide refractories have very high stability, compared with carbon products, silicon carbide oxidation stability is better. The main problem in the application of SIC products in blast furnaces is to develop a binder with sufficient high stability to alkali. From the test tar combined with silicon carbide graphite products, ceramic (clay) combined with oxygen nitride and nitride combined with silicon carbide products, nitride Si₃N4 and oxygen nitride Si₃ON₂ combined products have better stability. However, self-bonded silicon carbide products were found to be more alkali resistant. Since the manufacture of self-bonded silicon carbide products is difficult, the study of various combinations of refractory materials with silicon carbide content: high aluminum silicon carbide, graphite silicon carbide, etc.
Magnesia refractories were tested in the furnace body, waist and belly, and its life index was higher than that of high quality clay bricks, but lower than that of silicon carbide. The selection of suitable refractories for the furnace body and belly is still in progress.
The lower part of the furnace body and the furnace belly after continuous working time to 5~8 years, in individual cases to 10~12 years. The middle repair of the furnace body and the furnace belly (Class II maintenance) is usually completed in 12~18d. It is to shut down the furnace, blow off the lining, cool, inspect, replace the lining, overhaul the metal components and equipment as well as the furnace. The repair of individual occasions is mainly carried out by spraying. It consists of the same raw materials used in the production of dense bricks, using bauxite cement with low iron oxide content or high aluminum cement as a bond. It is known that the repair volume is not large, in this case, the special composition of the refractory mud through the hole of the furnace shell, less than 1.5MPa pressure in the furnace to complete. This slurry uses a bonding substance or a high alumina cement combined with coal tar, diluted liquid oil, asphalt or water glass at a softening temperature of more than 200 ° C.
① Classification of blast furnace lining for maintenance: Grade I: furnace body and hearth maintenance; Class II: furnace body repair; Level III: furnace throat repair.
Furnace body repair (Class II repair) can not be cold furnace spray repair.
The hearth and hearth liner are basically made of carbon masonry. The advantages of carbon products show that it is high in the reduction or inert atmosphere refractoriness (more than 3000℃), with the increase of temperature, strength increases, the volume is unchanged in a wide temperature range, good thermal shock resistance, non-wettability of metal and slag, and can make large bricks with strict tolerances. The furnace bed is built with 3~4 rows of large carbon bricks with folds, the length is 1.5~3.5m, and the fault area is (0.5~0.65) m× (0.5~0.65) m. The wall above the hearth was made of large carbon bricks. The central upper series of the hearth is finished with high aluminum pleated (anti-float) bricks. In order to prevent the bricks from floating, the so-called roofing method is also used to lay bricks. The lowest temperature of interaction between refractory and pig iron is 1100~1150℃. If the upper part of the furnace is applied with high aluminum products, the iron extraction temperature is about 1450℃ through the isotherm of 1100~1150℃ along the brick surface. When the isotherm layer of high alumina brick becomes thinner at 1100~1150℃, the temperature of the outer layer of brick increases and the temperature of iron discharge decreases.
In this way, the iron discharge temperature becomes an indicator of the state of the furnace bed lining. Larger furnaces are equipped with regulated cooling under the hearth. The main cause of hearth damage (brick float) – under the action of iron static pressure, iron gets into the jagged cracks between the bricks. Therefore, the hearth masonry is pre-assembled on a large, carefully prepared brick test bench. The total thickness of the hearth refractory masonry of the larger furnace reaches 5m. Accidental breaks of iron in the furnace base due to heating (especially uneven) and hearth damage must be prevented.
The hearth is constructed with carbon and aluminum silicate refractory materials. The carbonaceous masonry of the hearth wall is cooled by standard cooling devices or water jackets (two-layer shells). The upper part of the furnace cylinder (tuyere belt) is suitable for laying silicon carbide refractories.
Historically, effective refractories have been selected. It is believed that the carbon precipitated during the decomposition of carbon oxides is stored in the pores and cracks of refractory products, and even cause the damage of refractory materials. The magnesium oxide and deposited zinc in the refractory composition are considered to be the catalysts for this reaction. In order to reduce the carburization of steel, dense products with minimal iron oxide content are used, but it is important to increase the life of the blast furnace lining.
Later attention turned to the destructive effects of alkali vapors. The alkali contained in the blast furnace charge reaches 10%~15% in the lower part of the furnace body and the lining (the surrounding area), while the central part of the furnace contains 1%~2%. The interaction between the refractory lining and alkali includes the following stages: forming the alkali compound vapor, penetrating into the refractory, condensing, melting, filling with pores, and the melt reacts with the furnace gas medium. In the presence of SiO₂ in the refractory (in mullite particles or substrates), a compound that forms a base: K2O·Al2O3·4SiO2 leucite, K2O·Al2O3·2SiO2 potassium nepheline, Na2O·Al2O3·2SiO2 sodium nepheline, K2O·6Al2O3 and Na2O·6Al2O3· β-alumina, Na2O·Fe2O3·4SiO2 sodium pyroxene; K2O·Al2O3·6SiO2 orthoclase. Aluminum silicate, which forms a base, increases in volume to 45%, causing stress and forming new fractures. The porosity of the aluminum silicate refractory in the alkali-saturated part is reduced to 2.5%. Mullite is dissolved under the action of alkali, and alkali compounds may be reduced and decomposed under the action of carbon. At 1500℃, gaseous potassium and metallic silicon are formed. Cyanide KCN and NaCN are formed under the furnace body. Some of the cyanide rushes away with the gas, while others react with the refractory to form a melt.
The study on the interaction between alkali and refractories undoubtedly proves that the refractories containing SiO2 cannot be accepted in the lower part of blast furnace structure.
Corundum products (Al2O395%~98%) are known to be alkali stable. However, this is not the most appropriate judgment. Products like corundum have shown the formation of beta-alumina, in addition to having a high coefficient of linear expansion that causes it to crack.
The refractory itself interacts with iron, and even all have thermodynamic possible reactions, under the action of iron, the refractory material damage is very small, because the interaction speed is not large, and the reaction product forms a hard inert protective layer. At 1500 ° C, the wetting Angle of iron for most refractories is greater than 100°.
In recent years, the furnace lining has been successfully used with carbon, especially silicon carbide refractories. Compared with other blast furnace refractories, SIC refractories have the following advantages: high stability to alkali, good thermal shock resistance and high thermal conductivity, and the smallest porosity. In this way, the silicon carbide product extruded all other refractory materials from the lower structure of the blast furnace. The combination of silicon carbide refractories with graphite is considered promising.
Later developed Sialon combined silicon carbide products and silicon nitride combined, compared with larger crystallization, low porosity, good oxidation resistance, in some countries in the middle of the blast furnace trial.
The chemical mineral composition of blast furnace refractory material is required according to its porosity, and it is considered that the porosity of the product is less than 12%, which significantly reduces the slag infiltration, but the infiltration does not have gate properties. The wear of products with porosity less than 12% is 5 to 6 times lower than that of 17% products at room temperature, and 10 to 20 times lower at 980 ° C.
Cooling is an important factor for the stability of blast furnace lining.
The hearth and hearth lining can be used continuously for 15 to 20 years. During this period, the air blast outlet, iron and slag outlet are repaired according to the comprehensive process. The tuyere uses a special quality refractory and replaces it according to wear. When the iron and slag are released each cycle, the special shot mud is driven into the iron opening. The mortar should have plasticity and stability to hot metal and slag, high hardening rate, and it should not secrete smoke or harmful gases that pollute the environment.
Tar-bonded anhydrous mud is now widely used, which contains silicon oxide, high aluminum material, silicon carbide, a small amount of carbon and clay, and about 15% coal tar or bitumen. But the mud hardens slowly, forming smoke and creating heavy working conditions. Deficiencies can be eliminated by using phenol formaldehyde tar as a binder, which has thermal reactivity, that is, it turns into a solid state when heated. Ethanol, Al2O3-SiC-C system and MgO-C system slurry have high stability for tar with benzol-formaldehyde. Using this kind of mud, the repair time of the iron outlet is shortened, which is conducive to increasing the output of the furnace.
Blast furnace discharge ditch refractories should have high chemical stability, wear resistance and stability to temperature change when increasing the temperature. To improve the lining life of the ditch by: adding silicon carbide high aluminum material; Silicon carbide, silicon carbide alumina and silicon nitride combined with silicon carbide. The silicon carbide mud combined with silicon nitride has a better life. Silicon nitride is characterized by high strength combined with low thermal expansion and maximum thermal conductivity at service temperatures. So this bond has a high thermal shock resistance. In addition, when the temperature is increased, an oxide film is formed on the surface of the silicon nitride crystal to prevent the wear of the bond.
The life of trench lining not only depends on the nature of the refractory material, but also depends on the working system of the furnace, the method of realizing the lining and the structure of the trench itself. The iron channel of China’s large blast furnace has been developed from ramming material to castable, the main material is fused corundum, silicon carbide, a small amount of metal silicon and metal aluminum powder, an appropriate amount of coagulant, degumming agent, ultra-fine powder and other advanced raw materials preparation. The castable is used in some blast furnaces with good results.
