Abnormal furnace condition and its causes
1.1 Abnormal furnace condition
(1) Furnace wall adhesion. Since the opening of No. 2 blast furnace, the No. 1, No. 12, and No. 19 tuyere operations have been blocked for a long time. The furnace condition has remained in a good forward state, and various technical and economic indicators have remained at a good level (see Table 1).
Starting from March 23, 2023, the temperature variation range of the 9th and 12th cooling walls of the No. 2 blast furnace body has narrowed. Although measures to reduce the cooling intensity were taken in time, the effect was minimal, and eventually developed into furnace wall adhesion.
(2) Abnormal gas flow. After 22:00 on April 7, 2023, the blast furnace began to have obvious air blocking, which was mainly manifested by the tension between the pressure and volume in the furnace, the shrinkage of the air volume, and the air volume entering the furnace was reduced from the normal 4700m³/min to 4400m³/min. High-pressure spikes frequently appeared in the furnace, and the probe was horizontal.
At 03:00 on the 8th, the first material collapse occurred in the furnace, and the material line slid from 1.1m to 5.8m. At 04:25, the material collapsed again, and the material line slid from 2m to 5.9m. At 05:20, the adhesive in the middle and upper part of the furnace body frequently fell off. At 10:06, the third material collapse occurred, and the material line slid from 2.1m to 6m.
At 10:56 on the 9th, the fourth material collapse occurred, and the material line slid from 1.4m to 5.6m. Due to the fall of deep space material and furnace body adhesive to form local pipelines, the temperature change range of the 12th section cooling wall of the furnace body was reduced to within 5°C, the material surface was tilted, and the gas utilization rate was sharply reduced. From the furnace top camera, the furnace top material surface was blown up and scattered during the pipeline travel, and the gas flow in the furnace was turbulent and irregular. The gas flow distribution and material surface picture of the No. 2 blast furnace on April 8 are shown in Figure 1.
(3) Abnormal operation of the furnace. Due to abnormal gas flow in the furnace, a large amount of slag skin fell off the furnace body, long-term deep empty material in the furnace, large amount of water leakage in the tuyere sleeve, concentrated coking and other factors, the thermal system in the furnace changed drastically, causing slag phase fluctuations [2]. The difference in iron content and molten iron temperature at the two iron mouths increased, and finally the furnace hearth malfunctioned. On April 8, the molten iron [Si] fluctuated by 0.12%~1.28%, the molten iron temperature fluctuated by 1424~1497℃, and the slag binary basicity R₂ fluctuated by 1.14~1.19. On April 9, the molten iron [Si] fluctuated by 0.26%~1.12%, the molten iron temperature fluctuated by 1367~1498℃, and the slag binary basicity R₂ fluctuated by 1.12~1.18.
(4) A large number of tuyere sleeves were damaged. Due to the malfunction of the furnace, a large amount of slag skin fell off the furnace body, and the fluidity of the slag iron in the furnace became poor[3]. By 20:06 on April 8, it was confirmed that 6 small sleeves were damaged, and the tuyere was forced to be shut down and replaced urgently.
The abnormal condition of No. 2 blast furnace resulted in a reduction of about 9,800t of molten iron output, 539.4t of empty materials and concentrated coking, 3h50min of wind stop treatment, and direct economic losses of about RMB 3.9405 million.
1.2 Causes of abnormal furnace condition
(1) Direct causes: There are two main reasons.
First, the small tuyere sleeves have been leaking for a long time. As of January 30, 2023, it was confirmed that the No. 16 and No. 18 small sleeves were leaking, and industrial water was switched in time to maintain production, and the water replenishment of the soft water system was normal. Starting from March 20, the water replenishment of the soft water system increased, and the leakage of small and medium sleeves in the tuyere was investigated. As of April 7, a total of 4 small sleeves were confirmed to be leaking, and all of them were switched to industrial water to maintain production. Long-term leakage of multiple small tuyere sleeves, on the one hand, caused the furnace to absorb a lot of heat and increase consumption; on the other hand, it caused the furnace activity to decrease and the furnace to form a “local dead zone”.
The second is the adhesion of the furnace wall. Starting from March 23, the temperature of the 12th cooling wall of the furnace body dropped to about 55°C, and the range of change was reduced to within 2°C, indicating that a large area of adhesion had occurred in the middle and upper parts of the furnace body. From March 23 to April 8, the blast furnace repeatedly experienced a vicious cycle of adhesion falling off-adhesion-falling off again. The fallen slag blocked the gas passage, causing the pipeline travel and material surface to deviate, and developed into abnormal gas flow.
(2) Indirect causes: There are four main reasons.
First, the planned maintenance cycle of the blast furnace is too long. After the No. 2 blast furnace was overhauled on November 15, 2022, it was originally planned to be overhauled in March 2023, but in order to cooperate with the company’s overall production plan, the overhaul was postponed until April 18. Among the leaking small sleeves replaced on April 8, 8 small sleeves have been used since the date of commissioning and have been in service for 405 days, and 1 small sleeve has been in service for 358 days. The inner wall of the overdue tuyere small sleeve is severely worn, resulting in water leakage. The service life of the No. 15 tuyere small sleeve is only 145 days. The damage of the small sleeve is mainly due to the front-end melting loss caused by the shedding of slag skin and inactivity of the furnace.
Second, the metallurgical properties of sintered ore deteriorated. Starting from March 20, the proportion of Brazilian powder was reduced from 9% to 2%, and the remaining part was replaced by Mac powder. Due to the change in the material structure, the low-temperature reduction pulverization strength RDI+6.3mm of the sintered ore was reduced to below 70%, with the lowest being only 37.55%. The low-temperature reduction pulverization index RDI+3.15mm of the sintered ore was reduced to below 80%, with the lowest being only 54.02%. The wear index RDI₀.5m of the sintered ore increased to more than 10%, with the highest rising to 21.08%. The softening range of the sintered ore widened by 20°C. The change in the soft melting zone caused the high-temperature zone to fluctuate and become unstable, and the low-temperature reduction pulverization index deteriorated [4], resulting in the deterioration of the permeability of the block-shaped material column. The superposition of multiple factors eventually led to the adhesion of the furnace wall on the upper part of the furnace body.
Third, the quality of flux fluctuates, causing fluctuations in the blast furnace slag phase. On the one hand, the production line that ensures the flux required for sintering and pelletizing production was shut down on March 15, 2023, and the stability of the quantity and quality of the newly supplied flux decreased; on the other hand, starting from March, the mixing and stacking process of dolomite powder was optimized and switched to direct distribution in the sintering batching bin. Since the weighing error of the electronic scale in the second sintering batching room could not meet the process requirements, the chemical composition of the sintered ore and pelletized ore fluctuated greatly, which in turn caused changes in the blast furnace slag phase [S]. The binary basicity of the No. 2 blast furnace slag fluctuated between 1.10 and 1.27, and the (MgO) in the slag fluctuated between 7.58% and 8.73%. The continuous changes in the slag phase caused the slag skin to fall off frequently, which in turn caused fluctuations in the blast furnace gas flow.
Fourth, the effect of the furnace top watering is not good. The furnace top watering system of No. 2 blast furnace does not have a separate water supply system, but adopts the furnace top booster pump to directly supply water. The water pressure of the furnace top watering is only 0.5MPa, while the furnace top pressure is as high as about 0.2MPa during normal production of the blast furnace. When the pipeline travel occurs, the furnace top pressure is even higher. In the process of handling abnormal furnace conditions, especially when encountering pipeline travel, the effect of uniform atomization watering and cooling cannot be achieved. In the process of handling abnormal furnace conditions this time, the phenomenon of being forced to significantly reduce the wind due to the excessive furnace top temperature occurred many times, which is not conducive to the recovery of special furnace conditions.
Treatment measures
In combination with the characteristics of abnormal furnace conditions and their development, systematic adjustments were made to the charging system, air supply system, slag making system, thermal system, etc. Main measures taken included reducing ore batches and reducing loads, centralized coking, adjusting the charge matrix, and adjusting cooling intensity.。
2.1 Adjustment of charging system
(1) Reduce the ore batch and reduce the load. The primary task of handling special furnace conditions is to improve the permeability of the material column and ensure sufficient heat. The main means is to reduce the ore batch to reduce the thickness of the ore layer; reduce the coke load and increase the thickness of the coke layer to achieve the purpose of improving the permeability of the material column or supplementing heat.
The first material collapse occurred at 03:00 on April 8. At 03:45, the ore batch was reduced from the original 54t to 52t, and the coke load was reduced from 4.54 to 4.37. The oxygen and coal were temporarily stopped for adjustment. At 04:25, the material line collapsed again to 5.9m. At 05:20, the adhesive in the middle and upper part of the furnace body fell off partially. At 06:28, the ore batch was reduced to 50t for the second time, and the load was reduced to 4.00. At 07:30, the adhesive in the middle and upper part of the furnace body fell off on a large scale, and the material surface was seriously tilted. At 08:16, the ore batch was reduced to 40t for the third time, and the coke load was reduced to 2.90.
(2) Adjust the charge matrix. As the ore batch and coke load change, the upper charge system must be adjusted as necessary. As the ore batch continues to shrink and the ore layer becomes thinner, at 10:40 on April 8, the number of charge rings for the original ore block 8 was reduced from 2 to 1.5, that is, C211.502322 was adjusted to C22211.50i.5322, in order to reduce the edge coke load and ensure the stability of the two airflows. At 13:30 on April 9, the number of charge rings for the ore block 8 was further reduced from 1.5 to 1, and the number of charge rings for the coke block 1 was increased from 1.5 to 2, that is, C221208322. On April 10, as the furnace conditions gradually recovered, the charge matrix was also restored, and the number of charge rings for the coke block 1 was reduced from 2 to 1.5, and the number of charge rings for the ore block 8 was increased from 1 to 1.5.
(3) Empty material and centralized coking. Frequent collapse of materials and abnormal gas flow in the furnace cause the high-temperature gas flow to overflow directly without normal heat exchange, which will inevitably increase heat loss. At the same time, the slag skin frequently falls off, causing a large amount of low-temperature slag and iron mixture to directly enter the furnace, increasing the heat absorption of the furnace. In terms of operation, centralized coking must be adopted to improve the permeability of the material column and compensate for the heat loss and increase the heat.
Due to the pipeline travel, frequent collapse and sliding of materials and slag skin falling off, the gas utilization rate decreased. On April 8, 21.8t of net coke was added at 06:00, 32.7t of net coke was added at 07:00, 57.5t of net coke was added at 08:00, and 38.4t of net coke was added at 09:00. During the remaining empty materials, net coke was added in the manner of “every nine plus ten” until the materials were full. The total empty materials and centralized coke addition for the whole day was 310.2t. On April 9, 54t of empty materials and centralized coke addition were added at 11:00, 54t of empty materials and centralized coke addition were added at 12:50, 51.2t of empty materials and centralized coke addition were added at 14:00. During the remaining empty materials, net coke was added in the manner of “every nine plus ten” until the materials were full. The total empty materials and centralized coke addition for the whole day was 229.2t.
2.2 Adjustment of air supply system
(1) Replace the leaking small sleeves and add more blocked tuyere. Compared with normal furnace conditions, when the furnace condition is abnormal, the air volume entering the furnace is reduced, and the kinetic energy of the blast in front of the tuyere is reduced. In order to quickly restore the furnace condition, the air volume entering the furnace can basically only be maintained at 3800~4400m³/min after 05:00 on April 8, and the standard wind speed is reduced from about 255m/s in normal times to 210~230m/s. At 20:06, 10 leaking tuyere small sleeves were replaced during the wind stop. On the one hand, the water leakage was eliminated in time. On the other hand, in order to implement the plugging of tuyere No. 7, No. 16, and No. 24, a total of 6 tuyere were blocked to adjust the furnace condition at 23:56 when the air was restored to ensure that the standard wind speed in front of the tuyere can be maintained above 250m/s.
(2) Scientifically grasp the progress of furnace condition recovery. The adjustment of special furnace conditions is a dynamic process. Whether the timing and rhythm of each adjustment action are accurate largely determines the progress of furnace condition recovery. In the actual process of furnace condition recovery, timely adjustments are made based on whether the furnace temperature is sufficient, the air volume and the material speed are matched. The main contents of the adjustment are to poke the tuyere, expand the ore batch, and increase the load. When the pressure-volume relationship in the furnace matches, the material is unloaded smoothly, the furnace temperature is sufficient, and when concentrated coking is required, it is possible to consider poke the tuyere to expand the air inlet area and speed up the smelting process. Otherwise, it is not advisable to poke the tuyere. At 02:30 on April 9, tuyere No. 16 was opened, at 16:55 on April 10, tuyere No. 24 was opened, and at 06:26 on April 11, tuyere No. 7 was opened. At the same time, combined with the size of the air volume entering the furnace, the ore batch was expanded and the coke load was increased when the material speed obviously turned faster.
2.3 Adjustment of slag making system
(1) Slag phase adjustment. The fluctuation of chemical composition of sintered ore and pelletized ore, as well as the fluctuation of furnace temperature, lead to the change of SiO₂ reduction amount, which in turn leads to the fluctuation of slag phase. In the process of furnace condition adjustment, the main measures to adjust are acid-base material substitution or direct addition of flux. According to Kunming Steel’s production experience, 0.8-1.2t of silica is required for each 10t of concentrated coking. For this special furnace condition adjustment, the binary basicity of the slag was adjusted once at 10:00 on April 8 by using the material substitution method of pelletized ore with 800kg/batch of sintered ore. A total of 25t of silica was brought in during empty material and concentrated coking; 28t of silica was brought in during concentrated coking on April 9, and 3.2t of fluorite was added to wash the furnace and adjust the slag phase.
(2) Timely discharge of slag. When dealing with special furnace conditions, especially when the furnace is malfunctioning, timely discharge of slag in front of the furnace is the key to smoothly recovering the furnace condition. First, timely discharge of slag provides necessary space for the charge to descend, which is beneficial to improving the air permeability of the charge column; second, a large amount of slag skin falls off and enters the furnace, which increases the heat absorption of the furnace. Timely discharge of low-temperature slag in the furnace is beneficial to quickly increase the temperature of the furnace; third, timely discharge of slag is also beneficial to activate the furnace and improve the working condition of the furnace. In the actual production process, it is mainly organized and implemented in the form of zero interval or parallel iron discharge.
From the first material collapse at 03:00 on April 8 to the wind stop at 20:06, there were 8 tappings, with a total tapping time of 938 minutes and a slag tapping time of 812 minutes. There were 3 parallel tappings, including the 3203620th tapping, from opening the tap hole at 07:32 to blocking the tap hole at 09:00, only 113.55 tons of iron were tapped, but the slag tapping time was as long as 70 minutes, and the slag discharge was about 250 tons. On April 9, there were 10 tappings throughout the day, with a total tapping time of 1232 minutes and a slag tapping time of 1041 minutes. There were 4 parallel tappings.
2.4 Heat system adjustment
(1) Control the reasonable theoretical combustion temperature. In the process of handling special furnace conditions, it is often necessary to reduce the load to full coke smelting, reduce the furnace air temperature, stop the oxygen-enriched coal injection operation, and in order to increase the furnace temperature or improve the permeability of the material column, it is inevitable to take centralized coking. Therefore, it is particularly important to control the reasonable theoretical combustion temperature. Usually, it is adjusted by increasing or decreasing the furnace air temperature and using steam humidification.
According to the needs of smelting cycle and furnace condition adjustment, oxygen was stopped at 05:00 on April 8, coal was stopped at 12:00, and the furnace air temperature was reduced from 1200℃ to 1100℃, and then reduced to 1050℃ again at 13:00. Due to the low furnace air volume, the mixed air regulating valve could not quickly reduce the furnace air temperature to below 1000℃ even if it was fully opened. In order to maintain the theoretical combustion temperature at 2350~2450℃8], steam humidification of 5g/m³ was used from 14:00, and the maximum was 13g/m³. From April 9 to 10, the steam humidification amount was maintained at 3~20g/m³, and the furnace air temperature was regulated between 950～1000℃. Humidification was stopped at 02:00 on April 11.
(2) Increase the amount of water to improve the cooling intensity. Controlling the operating furnace type reasonably is essentially controlling the distribution of the longitudinal and circumferential temperature field of the blast furnace. During production, the heat load of the furnace, the water temperature difference, and the circumferential uniformity of the furnace temperature should be ensured [9]. The maximum water supply capacity of the No. 2 blast furnace combined with soft water closed circulation cooling system can reach 5100m³/h. The total cooling water flow rate during normal production of the blast furnace is 4600~4900m³/h, and the water temperature difference of the whole furnace is controlled at 6~7℃. The purpose of increasing the cooling intensity is mainly achieved by increasing the cooling water volume or reducing the inlet water temperature.
Starting from March 17, 2023, the temperature difference of the whole furnace water of No. 2 blast furnace has been continuously reduced. On March 25, the average temperature difference of the whole furnace water was reduced to 4.4℃. From March 21, 9 water reduction operations were carried out successively. By April 5, the cooling water volume was reduced to 4100m³/h at the lowest. At the same time, the inlet water temperature increased from 39±1℃ to 40±1℃. On April 7, the average temperature difference of the whole furnace water was reduced to 4.9℃. On April 8, the average temperature difference of the whole furnace water was reduced to 4.3℃ again (as shown in Figure 2).
Afterwards, with the shedding of the furnace wall adhesive and the long-term deep empty material, the furnace wall temperature gradually increased. The operation took measures to increase the water volume to restore the cooling intensity. Starting from 02:00 on April 8, the water volume was increased at a rate of no more than 100m³/h each time. By 17:00 on April 9, the cooling water volume was restored to 4600m³/h, and the inlet water temperature was restored to 39±1℃. On April 12, as the furnace condition gradually recovered, the cooling water volume was increased to 4700m³/h, and the temperature difference of the whole furnace water recovered to about 6.5℃.
Treatment Effect
With the complete fall of the adhesive on the furnace wall, the gas flow distribution in the furnace is stable, the pressure-volume relationship is significantly improved, the iron out of the furnace is normal, and the slag iron temperature is sufficient. On April 9, at 19:16, the material was full and turned smooth. At 05:52 on April 10, the ore batch was expanded from 40t to 46t, the coke load was increased to 3.71, and coal injection of 15t/h was started at 10:00. At 14:25, the ore batch was expanded to 48t again, the coke load was increased to 4.00, and oxygen enrichment of 5000m³/h was started at 15:50. At 16:55, the No. 24 air vent was opened, and the oxygen enrichment was increased to 10000m³/h at 18:00. The daily output of molten iron on the 10th was 4667tV/d. The gas flow distribution and material surface in the furnace on April 10 are shown in Figure 3.
At 02:15 on April 11, the ore batch was expanded to 50t, and the coke load was increased to 4.17. At 06:26, the No. 7 tuyere was opened, and the 27 tuyere operations were restored to normal levels. At 11:03, the ore batch was expanded to 52t, and the coke load was increased to 4.33. At 13:00, the oxygen enrichment was restored to 18,000m³/h. At 20:19, the ore batch was expanded to 54t, and the coke load was increased to 4.50. On the 11th, the daily output of molten iron reached 6568.947t/d, and the fuel ratio was reduced to 516kg/t.
At 07:46 on April 12, the ore batch was expanded to 55t, and the coke load was increased to 4.54. At this point, the furnace conditions were fully restored. On the 12th, the daily output of molten iron was restored to 7058.789t/d, and the fuel ratio was stabilized at 516kg/t.
The blast furnace experienced the first material collapse at 03:00 on April 8, and coal injection and oxygen enrichment began at 10:00 on April 10, which took 55 hours. It took 89 hours and 19 minutes for the ore batch to be expanded to 54 tons and the coke load to be increased to 4.50 at 20:19 on April 11, and the furnace condition to be fully restored to normal. The following are the main experiences in summarizing the treatment process:
(1) Strengthening the supply quality and quantity of flux for sintering and pelletizing and ensuring the stable quality of sintered ore and pelletized ore can create basic conditions for efficient and low-consumption production of blast furnaces.
(2) Formulate a scientific and reasonable regular maintenance plan, pay close attention to the service life of key equipment such as the tuyere sleeve, and strictly implement the regular maintenance system. Upgrade the furnace top sprinkler system to make the sprinkler pressure reach above 1.2MPa to ensure that the furnace top sprinkler can achieve a uniform atomization effect.
(3) The treatment of special furnace conditions is a systematic project, which requires scientific and reasonable regulation of the charging system, air supply system, slag making system, thermal system, etc., and requires cooperation between the inside and outside of the furnace to achieve the desired effect.
