The Operation of Contemporary Blast Furnaces

The process of contemporary integrated steelworks consists of raw materials, iron-making, steelmaking, steel rolling and so on as a continuous chain. Every stage of the chain must be operated in order; otherwise, the whole production stream would be out of gear. But iron-making production is different from others for the reason that its importance is not because it lies in front of the production processes, but because it plays a special role in iron and steel enterprises.

In Guxing, Zhengzhou, Henan Province, there are two sets of ancient (in Han Dynasty, 2000 years ago) BF remainder, near which 15 pieces of salamander were discovered. The largest one weighs about 20 t (Fig. 2.1) (Zhang in Metallurgical Iron Ruins of Guying Town in Han Dynasty. Guangling Book Society, Yangzhou, 2009). Because of the low capacity of blast facility, the hearth was not active then the hot metal would freeze easily. The ancient smelting workers had recognized that the blast could not penetrate into the burden of larger hearth, so they changed the hearth into oval shape to promote blast blowing to the center. Actually, this was the earliest adjust for BF blast.

Blast furnace is a countercurrent reactor. The gas rises from the lower part and passes through the charge layers. It interacts with the burden descending from the upper part to complete the smelting process of heating, melting, reducing and carburizing. Smooth burden descend and reasonable gas distribution are essential for stable blast furnace operation. The function of the burden distribution is to change the distribution of gas flow and affect the shape of the cohesive zone through different charging methods. The method of burden distribution with two bells is monotonous. The possibility to change the distribution of burden is limited, and it is quite inconvenient to regulate the gas flow. However, the bell-less top burden distribution method is so flexible that burden can be distributed to any position of the throat. Therefore, it is relatively convenient to regulate the distribution of the gas flow in blast furnace.

Stable furnace temperature is an important condition to ensure the stability of blast furnace. The change of furnace temperature not only affects fuel consumption, but also affects the blast volume and output directly. Stabilizing the furnace temperature and controlling the required furnace temperature level to produce the most economical varieties is the way to maintain and create the best economic benefits of blast furnace production.

In the process of iron-making, the slag and gas with dust are accompanied by the production of hot metal. The slag comes from burden and the ash of the fuel. Slag affects blast furnace production very much, especially the quality controlling of hot metal. The viscosity and stability of slag play significant role in the smooth operation of blast furnace. The impact of slag on the campaign life of blast furnace is not only reflected in the corrosion of the lining, but also in the repair ability of the corroded lining.

In the process of smelting in blast furnace, part of burden descends into the hearth in the form of slag and hot metal; the other part rises in gasification and discharge from the top of the furnace. The slag and hot metal gradually accumulate in the hearth. Figure 6.1 shows an example of the process of iron and slag accumulation in the hearth of No. 3 blast furnace of Capital Steel within interval 2.5 h between two tapping.

Stable and smooth operation is the best status of blast furnace production process and the premise of obtaining the best index of blast furnace. Smooth operation is the best requirement for blast furnace operation. If the burden is the decisive factor (the first factor) for the smooth operation, the effect of operation on the smooth operation of BF is the second one. The wrong operation could destroy the smooth operation of the blast furnace, while the inferior charge would make it difficult to maintain the smooth operation for a long time. In fact, blast furnace smelting process changes constantly, which is accompanied by the production process instead of man-making. Mainly: (1) The burden changes, especially the change of coke quality; (2) Gas distribution changes; (3) The furnace profile changes; (4) Furnace temperature changes; (5) The blast furnace equipment faults in operation. Blast furnace workers should timely detect changes and take measures to keep the process running smoothly and stable. First of all, small changes should be detected in a timely manner, which requires blast furnace workers, especially foremen and furnace managers, to be able to skillfully identify changes in the blast furnace. The main method is knowing the data display provided by various instruments of blast furnace.

The damage of cooling devices is the barometer for people to observe the damage of furnace body. Once the cooling devices are burned, the surface is deformed and cracked, gas and flame will be ejected if it occurs in the stack; if it is in the belly of the furnace, coke and slag will be ejected. At that time, if water leaks into the furnace in an instant, a large amount of water gas would be produced and thus leading to explode, which is enough to destroy the furnace itself. If it occurs in the hearth, the hot metal will outflow from the hearth, leading to destructive, unpredictable explosion when meets with water.

The damage caused by accretion to the blast furnace is very serious. It is because that the accretion on higher part of stack is difficult to remove by smelting and generally it is necessary to stop the smelting to remove it by explosion, which needs to open holes on the sealed furnace body in general situation and sometimes needs to remove the cooling staves and lining. This is not only the loss of production, but also a serious damage to furnace body. Although the accretion in the middle and lower parts of the stack can be flushed away, the loss and destructiveness are also amazing. There was a 300 m³ small blast furnace, for the treatment of removing accretion, more than 1000 t of coke and 200 t fluorite were consumed in 20 days before and after and the output loss was more than 2000 t [1]. There was also a blast furnace, due to the late discovery of the accretion, the blast furnace was abnormal for a long time, and the loss was very much. Therefore, the prevention of accretion is really the primary task of blast furnace operators.

The main target of blowing on (BF on steam/BF starting up) is safe and smooth. Usually, some equipment faulty would happen in the blow-in process, but less human injury. A lot of BFs started up in China frequently to accumulate rich experience to operate smoothly and realize the designed standard very soon. It was indicated that irregular situation generally came from less preparing, not enough burden, wrong operating, especially unqualified united test run for facilities.

The safety of blowing out and blowing on must be guaranteed to strive for shortening the blowing out period to create a condition of production. There are different management modes for blowing out in operation instructions of every plant, in general: (1) The plan of blowing out of BF should be decided by the head of the plant and approved by the general dispatching center of company. Temporary blowing out for renewing some cooling facilities should be decided by the controlling center of plant. If some serious accidents happen, such as tuyere and blowpipe burn out, the foreman could stop blast independently, while report to relative departments without delay. If the period is longer than 4 h, the plant control center should report to the head of plant after blowing out. (2) The blowing out must be after hot metal and slag tapping cleanly and stop slowly in some special situations to prevent blooding of tuyere. The shift foreman reports for the plant control center to make a decision. (3) It is necessary to contact individual units including plant control center, gas washing, blower, hot stove, coal injection, bunker (bin), TRT, dry dust collection, etc. (4) The longtime blowing out with descending charge level, top igniting and driving out the gas for relining or maintaining should be planned in advance to assign technical coordinator to direct. (5) To keep furnace regular performance and enough furnace temperature before a longtime blowing out. (6) If the blowing out is for repairing steam system, gas system, top system or someone working on the top, the top igniting should be necessary to prevent poisoning and explosion in charge of foreman or shift head. (7) When longtime blowing out or blower fault, the cold blast valve and flue valve should be opened to prevent gas back draught explosion. (8) In general, the minimum hot blast pressure sets as 0.05 MPa (0.5 kg/cm²) to permit charging and mixer selector valve opening.

In the theme park of Jiu Steel, there is a cenotaph (Fig. 12.1) to memorialize the martyrs who died in the explosion of BF and to report the story of this accident. It was on March 12, 1990, that the blast furnace (1513 m³) disappeared and submerged in the strong smoke and fire suddenly. The supporting structures collapsed and the broken-up shell flew away (the farthest was more than 200 m). Even seriously, the disaster also spread to the hoisting machine house. Unfortunately, 19 workers died in an unprecedentedly serious accident.