Three major causes and analysis of blast furnace hearth burn through accident

　　Blast furnace longevity design is a systematic project. It is impossible to achieve the goal of blast furnace longevity only by any single technology. All links such as blast furnace design, masonry, maintenance and operation must be considered as a whole. The erosion of hearth and bottom seriously endangers the service life of blast furnace, because only the bottom can not be replaced in a generation of furnace service. Although burn through accidents have specific causes, which vary from furnace to furnace, they can be summarized as the following causes. The following is an analysis of the common influencing factors of the blast furnace with potential safety hazards or even burn through in the hearth.
 

　　1、 Design defects of blast furnace

　　1. Hearth structural problems

　　Many blast furnaces in Anshan Iron and steel group adopt the hearth structure of small carbon bricks and ceramic cups. Once the ceramic cup is eroded or cracks appear on the wall of the ceramic cup, the molten iron will inevitably directly contact the carbon bricks. In this way, the carbon tamping layer with relatively low thermal conductivity and the cooling wall with weak cooling capacity will form an obvious "thermal resistance layer". Anshan Iron and steel headquarters new No. 3 3200m ³ Two sections of cast iron cooling staves are selected for the hearth of blast furnace. The thermal conductivity of cast iron cooling staves is 34w/m · K, and the cooling water volume is 960~1248m ³/ H, the subsection cooling mode is designed for cooling, resulting in insufficient cooling water for the hearth.

　　The hot surface temperature of both carbon bricks is equal to that of molten iron, so it is difficult to form a fixed slag iron protective layer. In particular, the main component of NMD carbon brick is electrode graphite, which is easy to penetrate into the unsaturated molten iron solution containing carbon. On the other hand, the graphite carbon brick is not easy to form a stable slag iron protective layer in the hearth, and can not directly prevent the penetration and erosion of molten iron, so it is easy to form burn through in a certain part of the hearth. At the same time, the slurry used together with NMA and NMD carbon bricks contains more volatiles, and the minimum masonry joint of small bricks can only reach about 1.5~2.0mm. With the disappearance of volatiles, the iron infiltration in the joint and the dissolution loss of carbon bricks will be more significant.

　　2. Mismatch between cooling capacity and smelting intensity

　　With the continuous progress of intensified blast furnace ironmaking technology and the irrational expansion of national iron and steel production capacity, China's blast furnaces have made significant progress compared with those of the 20th century in terms of smelting intensity and utilization coefficient of blast furnaces, but at the same time, the thermal load per unit furnace wall area and per unit time of blast furnaces will inevitably increase greatly. Therefore, our concept of longevity must not remain in the way of low cooling water volume or water spray cooling of furnace shell in the past. For the newly designed blast furnace, the cooling staves with low water volume and small diameter and low cooling specific surface area must not be selected. Nowadays, the smelting intensity of blast furnace has more than doubled compared with that in 1980s. How to match the high smelting intensity, high utilization coefficient and high cooling intensity remains to be studied. It is found that the utilization coefficient of blast furnaces with burn through accidents is generally greater than 2.5, so how to calculate the most economical comprehensive account for high production and long life.

　　3. Improper selection of carbon brick form Yangchun Iron Making Plant in China

　　One 1250m ³ After the blast furnace was put into operation for 15 days, the local carbon ring temperature once soared to more than 600 ℃, and the production of iron infiltration reached more than 70 tons for 8 months. The hearth burn through accident was avoided only by taking remedial and preventive measures in time. After cutting the cooling stave and measuring the carbon bricks in the hearth, it is found that the maximum gap between the carbon bricks is 30~70mm, indicating that the quality of the carbon bricks is insufficient in the furnace under high temperature and high pressure after production, resulting in deformation. The insufficient calcination temperature or even no calcination of carbon bricks will cause deformation of carbon bricks after heating. The accumulation of deformation and poor masonry quality will lead to large gaps in carbon bricks. Therefore, it is very important to select appropriate carbon bricks at the key parts of the hearth and bottom.

　　The following aspects shall be considered in the design of blast furnace and the selection of carbon bricks:

　　(1) Graphite or semi graphite carbon bricks shall not be selected for the parts where the hearth carbon bricks are in direct contact with molten iron, or the hearth parts that can be in direct contact with molten iron after erosion at the end of furnace life.

　　(2) Graphite carbon brick is not selected for the hearth, because the affinity between graphite carbon brick and slag iron is poor, it is not easy to form slag iron protective layer to protect the hearth. Foreign experience is that if graphite carbon bricks are used in the lower part of the furnace body or hearth, silicon carbide masonry is usually selected at intervals to improve the slag iron protective layer on the hearth.

　　(3) In order to pursue high thermal conductivity, some carbon brick manufacturers add a large amount of graphite into 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. Unreasonable depth of dead iron layer

　　The domestic blast furnaces designed in recent years have chosen relatively deep dead iron layer. However, after investigating and recording the hearth where the burn through occurred, it is found that the elephant foot erosion is at a higher position. Although the cause of this phenomenon needs further study, it must be related to the higher slag iron surface. At present, it is generally believed that deepening the depth of dead iron layer can alleviate the erosion of molten iron circulation on the hearth, but it can not be deepened blindly. With the increase of depth, the static pressure of molten iron will increase correspondingly, and the impact on the hearth will also increase. Therefore, the depth of 20% of the cylinder diameter widely used at present needs further practice and demonstration.

　　5. Improper iron port setting angle

　　The two tapholes of some domestic blast furnaces are arranged at 90 ° right angles. If this arrangement is adopted, it is not only easy to produce deviation during blast furnace production, but also strengthen the circulation erosion of molten iron in the hearth, which poses a serious threat to the safety of the hearth. The length of some blast furnace slag ditches varies greatly. When the production is resumed under abnormal furnace conditions such as furnace startup, air supply, air shutdown and shutdown, the iron is often drawn from the iron port corresponding to the short slag ditch, which makes the iron flow erosion in this iron port area serious and easy to burn through.

　　6. Lack of monitoring means

　　There is a common direct reason for the blast furnace burn through accident, that is, there are few temperature measurement points in the hearth brick lining in the burn through area, so the temperature rise of the hearth carbon brick can not be found intuitively, and preventive measures are taken. During the normal production process, they did not realize the importance of testing the cooling stave water temperature difference, water flow, heat flow intensity and other parameters, and failed to find hidden dangers as soon as possible and take corresponding preventive measures. For example, for the new No. 1 blast furnace of Angang and other blast furnaces with better detection methods, the hearth temperature has increased significantly before the accident. The blast furnace has strengthened the monitoring of key areas. In the end, there is no burn through, but iron infiltration, and the impact of the accident has not been further expanded.
 

　　2、 Manufacturing and installation defects of cooling stave

　　The manufacturing and installation quality of the cooling stave is very important to the life of the hearth. Once the cooling stave leaks into the hearth, it will not be effectively controlled for a long time, which is likely to cause major accidents.

　　(1) Some domestic blast furnaces use the method of drilling rolled steel plates to produce and process copper cooling staves. Due to the manufacturing process, there are many welding points of the cooling staves. The inlet and outlet pipes must be welded on the cooling stave body, and finally the processing holes need to be welded and blocked. With so many welding holes, it is easy to leak during transportation, installation and even production. Once there is water leakage in the furnace, it will accelerate the oxidation and damage of carbon bricks and cause major accidents. Therefore, such cooling staves should be avoided.

　　(2) The new blast furnace should not choose the blow by structure in the iron port area, and the filler between the hearth cooling stave and the furnace skin should be carefully selected to ensure the safety of the hearth in the iron port area.

　　(3) The carbon ramming material between the carbon brick and the cooling stave shall be the ramming material equivalent to the thermal conductivity of the carbon brick, up to 15~20w/mk.

　　(4) Select a cooling structure with sufficient cooling capacity. Angang New 3 3200m ³ Cooling water volume of blast furnace hearth is 1250m ³/ h. The cooling specific surface area of the cooling stave is only about 0.6, and burn through occurred after more than two years of operation. Baosteel 4350m with the same carbon brick ³ Although the cooling water volume of blast furnace hearth is only 1700m ³/ h. However, the blast furnace has been operating for 18 years, and its cooling ratio surface product is about 1.3. Therefore, enough attention should be paid to the specific surface area of hearth cooling, which should be more than 1.0. The cooling capacity of the water spray cooling structure and the jacket cooling structure used in the blast furnace hearth abroad is greater than that of the current cooling structure in China.
 

　　3、 Insufficient operation and maintenance after commissioning

　　1. Adverse effects of harmful elements

　　In recent years, a large number of harmful alkali metal elements have been found in the damage investigation of some burned through blast furnaces, which indicates 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 out of the furnace along with other furnace materials, but can only be continuously recycled and accumulated in the furnace, which not only reduces the coke strength and affects the smooth operation of the blast furnace, but also forms a compound with a volume expansion rate of up to 50% with the refractory, accelerating the damage to the hearth brick lining.

　　2. Water leakage of cooling equipment

　　For a blast furnace in normal production, water will eventually seep into the hearth as long as it enters the blast furnace, whether it is the leakage of the cooling stave of the hearth and hearth or the leakage of high-pressure water at the tuyere. Therefore, in daily production, individual coolers damaged should be replaced in time, and should not be replaced together, so as to reduce the damage of water leakage to carbon bricks in the hearth.

　　3. Inadequate daily maintenance of iron port

　　Most of the burnthrough parts of the hearth are near the iron mouth or the iron mouth area, which is mainly related to the inadequate daily maintenance of the iron mouth. The environment of the taphole area is complex and severely eroded. If the taphole depth is insufficient for a long time or the taphole splashes frequently, it is easy to cause molten iron to enter the brick joint from the taphole channel and accelerate the erosion of carbon bricks.

　　4. Excessive smelting intensity

　　In order to seize the market, some iron and steel plants recklessly pursue blast furnace smelting intensity, which has brought great load to the whole blast furnace and its auxiliary systems, including the longevity system. This kind of production and management concept of killing the goose that lays the golden egg is not desirable.

　　5. No vanadium titanium ore furnace protection

　　The furnace protection effect of vanadium titanium ore is obvious through appropriate ways. However, at present, most blast furnaces use vanadium titanium ore to protect the furnace only after the carbon brick temperature has increased significantly. Like pain relievers, it is recommended to "eat it again if you have made it. It is better not to eat it often". After the blast furnace is put into operation, a part of vanadium titanium ore should be regularly used to protect the furnace to eliminate the potential accident in the bud.

　　6. Improper grouting of hearth

　　In recent years, when the temperature of hearth carbon brick is abnormally high in China, it is commonly used to open holes in the gap between the two cooling staves of the furnace skin. This grouting method is especially suitable for blast furnaces with problems in construction quality, substandard ramming layer or shrinkage of ramming material after heating. However, special attention must be paid to the grouting method. 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 slurry directly enters the hearth from the brick joint to contact with the high-temperature molten iron, which will add insult to injury to the hearth safety.

　　7. Running condition of blast furnace

　　Both theory and production practice have proved that only a stable and smooth running blast furnace can achieve the goal of high production and low consumption. The hearth state of blast furnace with frequent fluctuations in furnace conditions must be affected, and the longevity of hearth and blast furnace is impossible. Because in the smelting process, various abnormal furnace conditions will lead to large fluctuations in the hearth and bottom heat load. For example, some treatment measures such as adding furnace cleaning agent to wash the furnace directly cause damage to the hearth and bottom. Therefore, in order to prolong the life of the hearth, it is necessary to keep the long-term stable and smooth operation of the blast furnace and avoid or reduce any operation detrimental to the longevity of the hearth.

　　8. Controlling hot metal composition and physical heat

　　The silicon and sulfur content in molten iron and physical heat directly affect the fluidity of slag iron: the silicon content should be controlled at about 0.5% (W) and the sulfur content should be controlled at about 0.02% (W) according to the forward running condition of the blast furnace, and timely adjustment should be made according to the forward running condition of the blast furnace, the erosion state of the hearth or whether the vanadium titanium ore is used to protect the furnace.