The longevity design of blast furnaces is a systematic project. The goal of blast furnace longevity cannot be achieved by any single technology. All aspects of blast furnace design, masonry, maintenance and operation must be considered comprehensively. The erosion of the hearth and furnace bottom seriously endangers the life of the blast furnace, because only the furnace bottom cannot be replaced within one generation of furnace service. Although the burnout accidents have specific causes that vary from furnace to furnace, they can still be summarized into the following reasons. The following is an analysis of the common influencing factors of blast furnaces with safety hazards or even burnouts in the hearth.
A: Blast furnace design defects
01 Structural problems of the hearth
Many blast furnaces use a hearth structure of small carbon bricks and ceramic cups. If this type of hearth is used, once the ceramic cup is eroded or cracks appear on the ceramic cup wall, the molten iron will inevitably come into direct contact with the carbon bricks, so that the carbon ramming layer with relatively low thermal conductivity and the cooling wall with weak cooling capacity will form an obvious "thermal resistance layer". For example, the furnace of a 3200m³ blast furnace uses two sections of cast iron cooling staves. The thermal conductivity of the cast iron cooling staves is 34W/m·K, and the cooling water volume is between 960~1248m³/h. The design of segmented cooling leads to insufficient cooling water for the furnace. The hot surface temperature of the two carbon bricks is equivalent to the temperature of the molten iron, and it is difficult to form a fixed slag iron protective layer. In particular, NMD carbon bricks, whose main component is electrode graphite, are easy to penetrate into carbon-containing unsaturated iron aqueous solution. On the other hand, graphite carbon bricks are not easy to form a stable slag iron protective layer in the furnace, and cannot directly block the penetration and erosion of molten iron, which makes it easy to burn through a certain part of the furnace. At the same time, the mud used with NMA and NMD carbon bricks contains more volatiles, and the minimum brick joints of small bricks can only reach about 1.5~2.0mm. With the disappearance of volatiles, iron infiltration in the gaps and carbon brick dissolution will be more significant.
02 Cooling capacity does not match smelting intensity
With the continuous progress of blast furnace ironmaking technology and the irrational expansion of national steel production capacity, my country's blast furnaces have made significant progress in the two indicators of smelting intensity and utilization coefficient compared with blast furnaces in the 20th century, but at the same time, the heat load per unit furnace wall area and per unit time of blast furnaces will inevitably increase greatly. Therefore, our longevity concept must not remain in the past low cooling water volume or water spray cooling of the furnace shell. Newly designed and built blast furnaces must not choose low water volume, small diameter cooling walls, and low cooling specific surface area. The smelting intensity of today's blast furnaces has more than doubled compared to the 1980s. How to match high smelting intensity and high utilization coefficient with high cooling intensity remains to be studied. The investigation found that the utilization coefficient of blast furnaces with burn-through accidents is generally greater than 2.5, so how to be the most economical for high output and longevity should be calculated comprehensively.
03 Improper selection of carbon bricks
A 1250m³ blast furnace in a certain ironmaking plant had a local ring carbon temperature soaring to over 600℃ 15 days after it was put into operation. It barely maintained production for 8 months and produced more than 70 tons of iron infiltration. The accident of hearth burnout was avoided only by taking timely remedial and preventive measures. After cutting the cooling wall and measuring the carbon bricks in the hearth, it was found that the maximum gap between carbon bricks was 30~70mm, indicating that the carbon bricks were of poor quality and deformed under the high temperature and high pressure environment in the furnace after it was put into operation. The baking temperature of the carbon bricks was not enough, or even they were not baked at all, which caused the carbon bricks to deform after being heated. The accumulated deformation and the poor masonry quality will lead to large gaps in the carbon bricks. Therefore, it is very important to select suitable carbon bricks for the key parts of the hearth and furnace bottom. When designing and selecting carbon bricks for blast furnaces, the following aspects should be considered: (1) The parts of the hearth carbon bricks that are in direct contact with the molten iron, or the parts of the hearth that can directly contact the molten iron after erosion at the end of the furnace life, should not be graphite or semi-graphite carbon bricks. (2) Graphite carbon bricks are not used in the furnace, because graphite carbon bricks have poor affinity with slag iron, and it is not easy to form a slag iron protective layer to protect the furnace. Foreign experience is that if graphite carbon bricks are used in the lower part of the furnace body or the furnace, silicon carbide masonry is usually selected to improve the slag iron protective layer of the furnace. (3) In order to pursue high thermal conductivity, some carbon brick manufacturers add a large amount of graphite to carbon bricks, which greatly reduces the resistance of carbon bricks to molten iron corrosion, which poses a great threat to the safety of the furnace.
04 The depth of the dead iron layer is unreasonable
In recent years, blast furnaces designed in China have all selected a relatively deep dead iron layer. However, after investigating the furnaces that have been burned through, it was found that the elephant foot erosion was at a higher position. Although the cause of this phenomenon needs further study, it is definitely related to the higher slag iron surface. At present, it is generally believed that deepening the dead iron layer can alleviate the erosion of the furnace by the molten iron circulation, but it cannot be deepened blindly. As the depth increases, the static pressure of the molten iron also increases accordingly, and the impact on the furnace also increases. Therefore, the depth of 20% of the cylinder diameter, which is widely used at present, needs further practical demonstration.
05 Improper iron mouth setting angle
The two iron mouths of some domestic blast furnaces are arranged at a 90° right angle. If this arrangement is used, it is not only easy to cause deviation during blast furnace production, but also to strengthen the circulation erosion of molten iron in the furnace, posing a serious threat to the safety of the furnace. The length of the slag groove of some blast furnaces varies greatly. When the production is resumed under abnormal furnace conditions such as furnace opening, air supply, wind stop, and furnace shutdown, iron is often tapped from the iron mouth corresponding to the short slag groove, which makes the iron flow in this iron mouth area seriously eroded and prone to burn-through.
06 Lack of monitoring means
There is a common direct cause of burn-through accidents in blast furnaces, that is, there are few temperature measurement points for the furnace brick lining in the burn-through area, and the temperature of the carbon bricks in the furnace cannot be intuitively found to rise, and preventive measures cannot be taken. In the normal production process, the importance of detecting parameters such as the water temperature difference, water flow, and heat flux intensity of the cooling wall was not realized, and the signs of hidden dangers were not discovered as early as possible, and corresponding preventive measures were not taken. For example, in blast furnaces with better detection methods such as Anshan Iron and Steel Co., Ltd., the furnace temperature had risen significantly before the accident. The blast furnace strengthened the monitoring of key areas, and ultimately did not develop into burn-through, but only iron seepage occurred, and the impact of the accident did not expand further.
B: Defects in manufacturing and installation of cooling walls
The manufacturing and installation quality of cooling walls is very important to the life of the furnace. Once the cooling wall leaks into the furnace, it is very likely to cause a major accident if it is not effectively controlled for a long time.
(1) Some blast furnaces use rolled copper cooling walls produced by drilling holes in rolled steel plates. Due to the manufacturing process, this type of cooling wall has many welding points. The inlet and outlet pipes must be welded to the cooling wall body, and finally the processing holes must be welded and blocked. With so many welding holes, it is easy to leak during transportation, installation, and even production. Once the furnace leaks, it will accelerate the oxidation and damage of carbon bricks, causing major accidents. Therefore, this type of cooling wall should be avoided.
(2) New blast furnaces should not use the blow-by structure in the iron mouth area. The filling material between the furnace cooling wall and the furnace shell should be carefully selected to ensure the safety of the furnace in the iron mouth area.
(3) The carbon ramming material between the carbon brick and the cooling wall should be selected with a thermal conductivity equivalent to that of the carbon brick, reaching 15~20W/mK.
(4) Select a cooling structure with sufficient cooling capacity. The cooling water volume of a 3200m3 blast furnace hearth is 1250m3/h, and the cooling surface area of the cooling wall is only about 0.6. Burn-through occurred more than two years after it was put into operation. Although the cooling water volume of a 4350m3 blast furnace hearth using the same carbon brick is only 1700m3/h, the blast furnace has been in operation for 18 years, and its cooling surface area is about 1.3. Therefore, the cooling surface area of the hearth should be given enough attention and should be above 1.0. The water spray cooling structure and sandwich cooling structure used in some blast furnace hearths have relatively large cooling capacity.
C: Insufficient operation and maintenance after commissioning
01 Adverse effects of harmful elements
In recent years, a large amount of residual alkali metal harmful elements have been found in some damage investigations of blast furnaces that have been burned through. This shows that harmful elements such as potassium, sodium, lead, and zinc have serious damage to the service life of furnace carbon bricks. These harmful elements cannot be discharged from the furnace with other furnace materials, but can only be continuously circulated and accumulated in the furnace, which not only reduces the strength of coke and affects the smooth operation of the blast furnace, but more seriously, forms compounds with a volume expansion rate of up to 50% with refractory materials, accelerating the damage to the furnace brick lining.
02 Water leakage of cooling equipment
A blast furnace in normal production, whether it is water leakage in the furnace body, furnace cooling wall or high-pressure water leakage at the tuyere, as long as water enters the blast furnace, it will eventually seep into the furnace. Therefore, in daily production, individual coolers should be replaced in time if they are damaged, and they should not be replaced together, so as to reduce the damage of water leakage to the carbon bricks in the furnace.
03 Inadequate daily maintenance of the iron mouth
Most of the furnace cylinders are burned through at the iron mouth or near the iron mouth area, which is mainly related to the inadequate daily maintenance of the iron mouth. The iron mouth area has a complex environment and is severely eroded. If the iron mouth depth is insufficient for a long time or the iron mouth splashes frequently, it is easy to cause molten iron to enter the brick joints from the iron mouth channel, accelerating the erosion of carbon bricks.
04 Excessive smelting intensity
In order to seize the market, some steel mills recklessly pursue the smelting intensity of blast furnaces, which has brought great loads to the entire blast furnace and its ancillary systems, including the longevity system. This production and operation concept of killing the chicken to get the eggs is not desirable.
05 Failure to protect the furnace with vanadium-titanium ore
The furnace protection effect is obvious through the appropriate method of vanadium-titanium ore protection. However, most blast furnaces currently use vanadium-titanium ore protection only after the carbon brick temperature has increased significantly. Like painkillers, it is recommended that "it is better to take it often than to take it when it occurs". After the blast furnace is put into production, a part of vanadium-titanium ore should be regularly added to the furnace for protection to eliminate the potential safety hazards in the bud.
06 Improper grouting of the furnace hearth
In recent years, when dealing with abnormally high temperature of carbon bricks in the furnace hearth, it is common to open holes in the gap between the two cooling walls of the furnace skin. This grouting method is particularly suitable for blast furnaces with problems in construction quality, substandard ramming layer, or shrinkage of ramming material after heating, etc., which form a thermal resistance layer. However, special attention must be paid to the grouting method. Once the pressure during the grouting process is too high or the grouting quality is average, it is easy to crush the already weak brick lining, so that the mud enters the furnace hearth directly from the brick joints and contacts with the high-temperature molten iron, which makes the safety of the furnace hearth worse.
07 Smooth running of blast furnace
Both theory and production practice have confirmed that only a stable and smooth blast furnace can achieve the goal of high production and low consumption. The state of the furnace hearth of a blast furnace with frequent fluctuations in furnace conditions will definitely be affected, and the longevity of the furnace hearth and the blast furnace will be out of the question. Because in the smelting process, various abnormal furnace conditions will cause large fluctuations in the heat load of the furnace hearth and furnace bottom. Some treatment measures such as adding furnace cleaning agents to wash the furnace directly cause damage to the furnace hearth and furnace bottom. Therefore, if you want the longevity of the furnace hearth, you must keep the blast furnace running smoothly for a long time and avoid or reduce any operation that is not conducive to the longevity of the furnace hearth.
08 Control the composition of molten iron and physical heat
The silicon and sulfur content and physical heat in the molten iron directly affect the fluidity of slag iron: according to the running condition of the blast furnace, the silicon content should be controlled at about 0.5% (w), and the sulfur content should be controlled at about 0.02% (w), and timely adjustments should be made according to the running condition of the blast furnace, the erosion state of the furnace hearth, or whether vanadium-titanium ore is added to protect the furnace.
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