Erosion damage of refractory materials in the furnace belly area
The blast furnace belly is located above the revolving area of the hearth tuyere. High-temperature furnace gas rises upward through this area. At the same time, the root of the reflow zone is usually located in the furnace belly. Therefore, the working conditions of the refractory lining used in this area are very harsh and it is also a blast furnace. The refractory lining and cooling stave are prone to damage.
In the furnace belly area, it is subject to strong thermal shock from high-temperature gas, especially when the edge gas flow develops. The hot surface temperature of the furnace lining or cooler is usually as high as 1300~1600°C, and due to the unstable air flow distribution, huge temperature fluctuations occur. The brick lining and cooler are subject to thermal shock caused by huge temperature changes, and the thermal damage effect is very prominent. At the same time, the geometric structure of the furnace belly "big at the top and small at the bottom" enables the brick lining and cooler to withstand the erosion, chemical erosion and oxidation of liquid slag iron with molten droplets and high-speed rising high-temperature coal gas flow. The lateral pressure and friction caused by the furnace charge, as well as the huge impact force caused by the blast furnace material collapsing and sitting materials also mainly act on the furnace bosh area.
For blast furnaces with cooling stave structures, no matter what kind of refractory brick lining is used in the furnace belly area, generally within 1 to 3 years after the furnace is opened, the refractory brick lining in the furnace belly area will almost completely disappear, mainly relying on the cooling effect of the cooler. Its hot surface is attached with a layer of a mixture of solid iron, coke and solid slag, which is a protective "slag skin".
Practice has proved that during the first generation of blast furnace operation, the furnace belly area mainly relies on this protective slag skin to maintain its work. The thickness of the slag skin is generally 20~100mm, and the thickness of the slag skin is also constantly changing according to the blast furnace conditions. At the edge of the gas flow, When the furnace wall heat load increases and the charge impact decreases, the slag skin will fall off. Under the cooling effect of the cooler, when the thermal balance is reached, new slag skin will form. This process starts over and over again. It can be seen that it is very important to achieve efficient cooling of the bosh area. It is necessary to improve the water quality, improve the heat transfer performance of the cooler, and form a protective slag skin with a smooth and uniform surface. This is the most important link to achieve longevity in the bosh area.
Large blast furnaces at home and abroad commonly use copper cooling staves in the furnace belly area. The main purpose is to take advantage of the high thermal conductivity of the copper cooling staves to quickly form a protective slag skin. After the copper cooling staves are used in the furnace belly area, its thermal conductivity can even be eliminated. The brick lining on the surface is replaced with a layer of spray paint of about 100mm to protect the copper cooling stave from various physical damages in the early stages of furnace opening. When using cast iron cooling staves and copper cooling plates in the furnace belly area, refractory materials with excellent thermal conductivity should be given priority, and the thickness of the brick lining should be reduced to increase the comprehensive heat transfer performance of the cooler/brick lining so that the protective slag skin can Stable formation.
Damage mechanism of the lining of the furnace waist and lower part of the furnace body
The blast furnace waist and lower area of the furnace body are the areas where the blast furnace soft melting zone is located. Like the furnace belly, they are the most severely damaged parts of the blast furnace body. Due to the differences in the raw fuel conditions and operating conditions of the blast furnace, the damage mechanisms of the furnace waist and the lower part of the furnace body are also different, but they are all the result of the comprehensive action of various damage factors, but the order of the damage causes is different in different blast furnaces. Just a difference.
For most blast furnaces, thermal damage is still an important cause. During the rising process of coal gas, it has to penetrate the coke window of the soft melt zone and move upward, and the gas flow is distributed for the second time in this interval. When high-temperature gas penetrates the coke window, the movement direction of the gas flow is not vertically upward, and part of the gas flows upward along the edge of the furnace wall, causing thermal damage to the furnace lining and cooler. This destructive effect is more serious when the gas flow at the edge of the blast furnace develops excessively.
Actual measurements show that the area with the greatest heat flow intensity in the blast furnace is in the lower part of the furnace body. The heat flow intensity depends on factors such as the degree of smelting intensification of the blast furnace and the distribution of coal gas flow. For the same blast furnace, the heat flow intensity in the lower part of the furnace shaft will also change greatly with the erosion state of the furnace lining. After the furnace lining is completely eroded and disappears, the hot surface of the cooler is completely exposed to the blast furnace, and the blast furnace will rely on cooling The slag skin formed on the hot surface of the device can maintain long-term operation. If the operating conditions change and the edge gas flow develops, the slag skin will fall off, and the heat flow intensity will fluctuate violently.
The heat flow intensity from the furnace waist to the lower part of the furnace body can generally reach 30,000~50,000W/m2 or even higher, which has the most prominent destructive effect on the blast furnace lining and cooler. Thermal destruction behavior is as follows: on the one hand, when high-temperature gas flows through the hot surface of the furnace lining or cooling stave, the slag skin on the hot surface of the furnace lining or cooling stave will fall off and the temperature will rise, causing a large temperature difference heat to be generated inside the furnace lining or cooling stave. stress, leading to thermal stress damage; on the other hand, the thermal shock of high-temperature gas flow causes large temperature fluctuations in the furnace lining or cooling stave, resulting in higher thermal shock stress or thermal shock stress. Both aspects are important causes of thermal damage to the furnace lining or cooling stave, and the thermal shock stress caused by temperature fluctuations is more serious.
When the temperature of the hot surface of the furnace lining increases, various chemical corrosion reactions also intensify accordingly, resulting in comprehensive damage caused by thermal damage and chemical corrosion at the same time.
Chemical corrosion in the blast furnace mainly includes: chemical corrosion of alkali metals and zinc, carbon deposition produced by CO, oxidation of CO2 and H2O, etc. With the changes in blast furnace raw fuel conditions and the recycling of steel plant dust, the alkali metal and zinc enrichment circulation in the blast furnace not only easily produces blast furnace nodules, nodules, and suspended materials in the blast furnace, but also causes chemical erosion of the furnace lining. , shortening the blast furnace life. The destructive behavior of alkali metals and zinc is not only reflected in the carbon bricks at the bottom of the hearth, but also has a destructive effect on the refractory lining of the furnace belly, waist and furnace body that cannot be ignored.
The destructive effects of alkali metals and zinc in blast furnaces cannot be ignored. In recent years, due to changes in the raw and fuel conditions of blast furnaces, the content of harmful elements in the charge has increased. Moreover, due to the extensive use of zinc-containing scrap steel in converters, the zinc content in converter flue gas dust has increased. Increase, and then recycled through the sintering process, increase the zinc load and alkali metal load in the blast furnace charge. Moreover, alkali metals and zinc will be continuously circulated and enriched in the blast furnace. If a special removal process is not adopted, it will be difficult to get rid of the harm caused by this harmful element. Therefore, using a rotary hearth furnace to treat steel plant dust can not only recover the iron element in the dust removal ash, but also remove zinc from the process flow, recover secondary resources, and achieve efficient resource utilization of solid waste. Alkali metals and zinc in the blast furnace will cause accumulation of the hearth, thickening of the furnace lining, and nodulation of the blast furnace, which will damage the smooth operation of the blast furnace. They will also damage the brick lining in the tuyere area, causing upwarping and distortion of the tuyere, which will have a negative impact on the hearth and bottom carbon bricks. The damage is more prominent, and it also reacts chemically with the coke, destroying the strength of the coke, causing the coke to pulverize, and destroying the stability of the blast furnace.
The oxides of alkali metals such as K and Na and Zn react with Al2O3 and SiO2 in the furnace lining to generate low melting point silicate substances, causing damage to the furnace lining. K and Na vapor react with Al2O3 and SiO2 in the furnace lining to form potashite (K2O·Al2O3·2SiO2) and leucite (K2O·Al2O3·2SiO2). The volume expansion of potashite is 49%~50%, and the volume expansion of leucite is 49%~50%. The volume expands by 30%. The formation of these two types of low melting point substances causes the furnace lining to expand abnormally, causing the furnace lining to remelt, become loose, crack, peel off, and cause damage. This destructive effect is even more severe in high temperature areas. The boiling points of K and Na are 799°C and 882°C respectively. K2O and Na2O are reduced to gaseous elements in the medium temperature zone in the middle of the blast furnace shaft. As the coal gas flow rises, they combine with the minerals in the charge to form silicate and carbonic acid. Salt and cyanide enter the high-temperature zone with the charge, and then are reduced to alkali metal vapor and rise with the gas. During the blast furnace smelting process, under the condition that the gas and the charge move in opposite directions, this process goes back and forth. A part of the alkali metal is cyclically transferred between the upper and lower parts of the blast furnace, and a small part is discharged with the gas or slag, causing the alkali metal to remain in the blast furnace. cycle enrichment.
The hazards of zinc in blast furnaces have become more serious in recent years. Studies have shown that furnace condition fluctuations, furnace lining damage, tuyere damage, and cooler damage in many blast furnaces are directly related to the hazards of zinc. Zinc oxide is reduced to elemental zinc vapor in the high temperature zone of 1000°C. The boiling point of zinc is 907°C. It moves upward with the rising gas flow. When it reaches the medium temperature zone, it is oxidized by the CO2 in the gas and condensed to form a powder. ZnO, part of the ZnO is attached to the gas dust and discharged from the blast furnace, and part of the ZnO is attached to the charge and enters the high temperature zone again as the charge drops. Like alkali metals, zinc also has a cyclic enrichment phenomenon inside the blast furnace. The zinc vapor formed in the blast furnace penetrates into the gaps or pores of the furnace lining, causing the volume of the furnace lining to expand and cause embrittlement and damage. For blast furnaces that use cooling staves, zinc will cause the slag skin formed on the furnace lining or the hot surface of the cooling stave to frequently fall off. The falling slag skin will often damage the tuyere, distort it or even break it; for blast furnaces that use cooling plates, zinc will cause The slag skin in the furnace belly and waist area is firmly bonded, and the furnace wall may even be thickened, and hanging materials often appear, destroying the stability of the blast furnace. Zinc vapor in the high-temperature area of the tuyere penetrates into the gaps between the tuyere composite bricks or the cooler. After the temperature drops, it condenses to form a liquid. The zinc that penetrates into the tuyere composite bricks will cause the brick lining to expand, become brittle, and be damaged, resulting in the tuyere. The equipment was squeezed, deformed, twisted and turned up. The destructive effect of zinc on hearth bottom charcoal bricks and ceramic materials is as harmful as alkali metals.
Blast furnace primary slag is generally formed in the furnace waist and the lower part of the furnace body. The primary slag has a high FeO content and is highly corrosive. The refractory brick lining used in the furnace belly, furnace waist and lower part of the furnace body will be corroded by the blast furnace slag, especially For aluminum silicate refractory materials, due to their poor resistance to slag erosion, the erosion effect of slag is more significant. After using Si3N4-SiC bricks, Sialon-SiC bricks and semi-graphite carbon bricks with excellent thermal conductivity, flooding resistance, alkali resistance, wear resistance and oxidation resistance, the slag erosion damage in this area has been better contained. .
HOME