Blast furnace longevity design is a systematic project. No single technology can achieve the longevity goal of blast furnace. It is necessary to consider all aspects of blast furnace design, masonry, maintenance and operation. The corrosion of the hearth and bottom seriously endangers the life of the blast furnace, because only the bottom of the furnace cannot be replaced within a generation of furnace. Although burn-through accidents have specific causes and 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 the blast furnace with hidden safety hazards and even burn-through in the hearth.
One, blast furnace design defects
1. Hearth structure problem
Many blast furnaces in Anshan Iron and Steel use a small carbon brick and ceramic cup hearth structure. If the ceramic cup is corroded or cracks appear on the ceramic cup wall, the molten iron will inevitably directly contact the carbon brick. The relatively low carbon ramming layer and the cooling stave with weak cooling capacity will constitute an obvious "thermal resistance layer". The hearth of the new No. 3 3200m³ blast furnace of Anshan Iron and Steel Co., Ltd. selects two-stage cast iron stave form. The thermal conductivity of the cast iron stave is 34W/m·K, and the cooling water volume is between 960~1248m³/h. The segmented cooling method is designed. Perform cooling, resulting in insufficient cooling water in the hearth.
The hot surface temperature of the two types of carbon bricks is equivalent to that of the molten iron, and it is difficult to form a fixed slag iron protective layer. Especially NMD carbon brick, its main component is electrode graphite, electrode graphite is easy to penetrate into the carbon-containing unsaturated iron aqueous solution. On the other hand, graphite carbon bricks are not easy to form a solid slag-iron protective layer on the hearth, and cannot directly block the penetration and erosion of molten iron, which makes it easy to burn through at a certain part of the hearth.
At the same time, the mud used with NMA and NMD carbon bricks contains a lot of volatiles, and the minimum gap between the bricks of small bricks can only reach about 1.5~2.0mm. With the disappearance of volatiles, iron and carbon bricks are infiltrated in the gaps. Dissolution loss will be more significant.
2, the cooling capacity does not match the smelting intensity
With the continuous progress of strengthening blast furnace ironmaking technology and the irrational expansion of national steel production capacity, my country’s blast furnaces have made significant progress compared with the 20th century blast furnaces in terms of blast furnace smelting strength and utilization coefficient. However, at the same time, the blast furnace unit wall The area and heat load per unit time will inevitably increase tremendously. Therefore, our longevity concept must not remain at the low cooling water volume in the past or the cooling method of spraying water to the furnace shell. Newly designed and constructed blast furnaces must not choose cooling staves with low water volume, small pipe diameters, and low cooling specific surface area.
The smelting intensity of today's blast furnace has more than doubled compared with that in the 1980s. How to match the high smelting intensity and high utilization factor with the high cooling intensity remains to be studied. The investigation found that the utilization factor of blast furnaces with burn-through accidents is generally greater than 2.5, so how the high output and longevity are the most economical should be accounted for in a comprehensive account.
3. Improper use of carbon bricks
A 1250m³ blast furnace in my country’s Yangchun Ironmaking Plant. 15 days after it was put into operation, the local ring carbon temperature once soared to above 600℃. It barely maintained the production for 8 months and reached more than 70 tons of iron infiltration. The hearth burning was avoided due to timely remedial and preventive measures. Wear accident. After measuring the hearth carbon bricks by cutting the cooling stave, it was found that the maximum gap between the carbon bricks was 30~70mm, indicating that the quality of the carbon bricks was insufficient under the high temperature and high pressure environment in the furnace after production, which led to deformation. The baking temperature of the carbon bricks is not enough, or even no baking at all, so that the carbon bricks will be deformed after being heated. The accumulation of deformation and poor masonry quality will result in large gaps in the carbon bricks.
Therefore, it is very important to select suitable carbon bricks for the key parts of the hearth and bottom of the furnace. The following aspects should be considered when designing blast furnaces and selecting carbon bricks:
(1) The part of the hearth where the carbon bricks are in direct contact with the molten iron, or the part of the hearth that can directly contact the molten iron after corrosion at the end of the furnace age, should not choose graphite or semi-graphite carbon bricks.
(2) Graphite carbon bricks are not selected for the hearth part, because the affinity of graphite carbon bricks and slag iron is poor, and it is not easy to form a slag iron protective layer to protect the hearth. Foreign experience is that if you choose to use graphite carbon bricks in the lower part of the furnace body or hearth, you usually choose silicon carbide masonry at intervals to improve the slag and iron protective layer of the hearth.
(3) In order to pursue high thermal conductivity, some carbon brick manufacturers add a large amount of graphite to the carbon brick, which greatly reduces the molten iron corrosion resistance of the carbon brick, which poses a great threat to the safety of the hearth.
4. The depth of the dead iron layer is unreasonable
The blast furnaces designed in recent years in China have chosen a relatively deep dead iron layer, but after investigating and recording the hearth that burned through, it was found that the erosion of elephant feet was at a higher position. Although the reason for this phenomenon needs further study, But it is definitely related to the higher slag iron surface. At present, it is generally believed that deepening the depth of the dead iron layer can alleviate the corrosion of the molten iron circulation on the hearth, but it cannot be deepened blindly. The increase in the depth will increase the static pressure of the molten iron correspondingly, and the impact on the hearth will also increase. Therefore, the depth of 20% of the widely used cylinder diameter needs further practical demonstration.
5. Improper angle of iron port setting
In some domestic blast furnaces, the two iron holes are arranged at a right angle of 90°. If this arrangement is not only easy to produce deviations during blast furnace production, but it will also strengthen the circulation of molten iron in the hearth, posing a serious threat to the safety of the hearth. The length of some blast furnace slag ditch varies greatly. When production is resumed under abnormal furnace conditions such as furnace start-up, air supply, shutdown, and shutdown, the iron tapping corresponding to the short slag ditch is often used, which makes the iron flow in this tap area severely corroded. Burn through is easy to occur.
6. Lack of monitoring methods
There is a common direct cause of burn-through accidents in the blast furnace, which is that there are few measurement points for the temperature of the hearth brick lining in the burn-through area, and the temperature increase of the hearth carbon bricks cannot be found intuitively, and preventive measures have been taken. In the normal production process, they did not realize the importance of detecting the temperature difference, water flow, heat flow intensity and other parameters of the cooling stave, failed to detect hidden dangers as soon as possible, and took corresponding preventive measures. For example, Anshan Iron and Steel No. 1 blast furnace with better detection methods, the hearth temperature has risen significantly before the accident, and the blast furnace has strengthened the monitoring of key areas. In the end, it did not develop to burn through, but iron infiltration occurred. The accident has no impact. Expand further.
2. Defects in the manufacture and installation of cooling staves
The quality of the manufacturing and installation of the cooling stave is very important to the life of the hearth. Once the cooling stave leaks water into the hearth, it will not be effectively controlled for a long time, which is likely to cause a major accident.
(1) Some domestic blast furnaces use rolled steel plate drilling to produce and process rolled copper staves. Due to the manufacturing process, there are many welding points for this stave. The inlet and outlet water pipes must be welded to the stave body, and finally welding is required. Craft hole. With so many welding holes, it is easy to leak during transportation, installation, and even production. Once water leaks in the furnace, it will accelerate the oxidation and damage of carbon bricks and cause major accidents. Therefore, this type of cooling stave should be avoided.
(2) The blow-by structure in the taphole area should not be selected for the newly built blast furnace, and the filler between the hearth cooling stave and the furnace skin should be selected with special care to ensure the safety of the hearth area in the taphole area.
(3) The carbon ramming material between the carbon brick and the cooling stave should be selected with a ramming material equivalent to the thermal conductivity of the carbon brick, reaching 15-20W/mK.
(4) Choose a cooling structure with sufficient cooling capacity. The cooling water volume of the hearth of Angang's new No. 3 3200m3 blast furnace is 1250m3/h, and the cooling specific surface area of the stave is only about 0.6. Burn through occurred after more than two years of operation. Although the cooling water volume in the hearth of Baosteel's 4350m3 blast furnace using the same carbon bricks is only 1700m3/h, the blast furnace has been in operation for 18 years, and its cooling ratio is about 1.3. Therefore, the cooling specific surface area of the hearth should be paid more attention to, and it should be above 1.0. The cooling capacity of the water spray cooling structure and the sandwich-type cooling structure used in the hearth of foreign blast furnaces is greater than the current cooling structure in my country.
3. Insufficient operation and maintenance after production
1. The adverse effects of harmful elements
In recent years, a large amount of alkali metal harmful elements have been found in the damage investigation of some burn-through blast furnaces, which shows that potassium, sodium, lead, zinc and other harmful elements have serious damage to the service life of the furnace body carbon bricks. These harmful elements cannot be discharged out of the furnace along with other charge materials, but can only be continuously circulated and accumulated in the furnace. This not only reduces the strength of coke, but also affects the blast furnace forward movement. What is more serious is that it forms a volume expansion rate of up to 50% with refractories. The compound accelerates the damage to the hearth brick lining.
2, the cooling equipment leaks
A blast furnace in normal production, whether it is a leakage of the furnace body, the cooling wall of the hearth or the high pressure water leakage of the tuyere, as long as water enters the blast furnace, it will eventually seep into the hearth. Therefore, in daily production, individual coolers should be replaced in time if they are damaged, and should not be replaced together, so as to reduce the damage of water leakage to the carbon bricks in the hearth.
3, the daily maintenance of the iron gate is not in place
Most of the burn-through parts of the hearth are near the taphole or taphole area, which is mainly related to the inadequate daily maintenance of the taphole. The environment of the taphole area is complex and severely corroded. If the taphole depth is insufficient for a long time or frequent taphole splashing, it is easy to cause molten iron to enter the brick joints from the taphole channel and accelerate the erosion of carbon bricks.
4. Excessive smelting intensity
In order to seize the market, some steel plants recklessly pursue the smelting strength of blast furnaces. This puts a huge load on the entire blast furnace and its auxiliary systems, including the longevity system. This kind of production and management philosophy is not desirable.
5. No vanadium-titanium ore furnace protection
The vanadium-titanium ore furnace is protected by an appropriate method, and the furnace protection effect is obvious. However, most blast furnaces currently use vanadium-titanium ore to protect the furnace after the temperature of the carbon bricks has increased significantly. The vanadium-titanium ore protects the furnace to eliminate the hidden dangers of the accident in the budding state.
6. Improper grouting of the hearth
In recent years, when the temperature of carbon bricks in the domestic treatment hearth rises abnormally, 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 where there are problems with the construction quality, the ramming layer does not meet the standard, or the ramming material shrinks after being heated, and other reasons that form a thermal resistance layer. But be sure to pay special attention to the method of grouting. Once the pressure in 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 hearth directly from the brick gap and the high temperature The contact with molten iron will add to the safety of the hearth.
7, the blast furnace forward situation
Both theory and production practice have confirmed that only a stable blast furnace can achieve the goal of high production and low consumption. The condition of the blast furnace hearth that fluctuates frequently must be affected, and the longevity of the hearth and the long life of the blast furnace cannot be discussed. Because in the smelting process, various abnormal furnace conditions will cause large fluctuations in the heat load of the hearth and bottom of the furnace. Some treatment measures such as adding a furnace cleaning agent to the furnace directly cause damage to the hearth and bottom of the furnace. Therefore, in order to have a long life of the hearth, it is necessary to maintain the long-term stable traverse of the blast furnace and avoid or reduce any operations that are detrimental to the long life of the hearth.
8. Control the composition and physical heat of molten iron
The level of silicon and sulfur content in the molten iron and physical heat directly affect the fluidity of the slag iron: the silicon content should be controlled at about 0.5% (w) and the sulfur content at about 0.02% (w) according to the anterograde situation of the blast furnace. Adjust in time according to the situation of the blast furnace, the corrosion state of the hearth, or whether the vanadium-titanium ore is installed to protect the furnace.
Post time: Aug-09-2021