The main ironmaking methods include the blast furnace method, the direct reduction method, and the smelting reduction method. This article focuses on the blast furnace ironmaking process and provides some introductions.
Blast furnace ironmaking is the most important link in an integrated iron and steel enterprise that combines ironmaking, steelmaking, steel rolling, trade, transportation, and power generation, playing a crucial role in connecting upstream and downstream processes.
Brief Analysis of Blast Furnace Process
Basic processes of blast furnace ironmaking
Blast furnace ironmaking is the main method of modern ironmaking and a crucial link in steel production. This method evolved from and improved upon the ancient vertical shaft furnace ironmaking process. Although many new ironmaking methods have been researched and developed worldwide, blast furnace ironmaking still accounts for over 95% of the world’s total iron production due to its favorable technical and economic indicators, simple process, large production capacity, high labor productivity, and low energy consumption.
The ironmaking process involves charging iron-containing raw materials (sintered ore, pellets, or iron ore), fuels (coke, pulverized coal, etc.), and other auxiliary raw materials (limestone, dolomite, manganese ore, etc.) into the blast furnace from the top in a certain proportion. Hot air is then blown into the blast furnace from the bottom through the tuyeres around the furnace to aid coke combustion (some blast furnaces also inject auxiliary fuels such as pulverized coal, heavy oil, and natural gas). At high temperatures, the carbon in the coke burns with the oxygen in the blown air to produce carbon monoxide and hydrogen. As the raw materials and fuels descend during the smelting process in the furnace, they encounter the rising gas, resulting in heat transfer, reduction, melting, and decarburization to produce pig iron. Impurities in the iron ore combine with the flux added to the furnace to form slag. Molten iron is intermittently discharged from the bottom of the furnace into ladles and sent to the steel plant. At the same time, blast furnace gas and slag are produced as two byproducts. Blast furnace slag is formed by the combination of non-reducible impurities in the main ore and fluxes such as limestone. After being discharged from the slag outlet, it is used as a raw material for cement production after water quenching. The gas produced is discharged from the top of the furnace, and after dust removal, it is used as fuel for hot blast stoves, heating furnaces, coke ovens, boilers, etc.
Blast furnace process flow diagram
Brief Introduction to Main Process Equipment of Blast Furnace
The main process equipment system of blast furnace smelting consists of: ① blast furnace body; ② feeding system; ③ air supply system; ④ injection system; ⑤ gas treatment system; ⑥ slag and iron treatment system.
During production, the various systems cooperate and constrain each other, forming a continuous, large-scale high-temperature production process. After the blast furnace is started, the entire system must produce continuously day and night. Except for planned maintenance and temporary shutdowns due to special accidents, it is generally not shut down until the end of its generation life.
Blast furnace body
The blast furnace body is a vertical cylinder composed of a metal furnace shell, cooling equipment, and refractory lining. Its internal structure consists of five sections, from top to bottom: the throat, the body, the waist, the belly, and the hearth. The dimensional variations of this internal structure adapt to the physicochemical changes within the furnace.
feeding system
The main components include silos (used to receive raw materials and coke transported from sintering or pelletizing plants, coking plants, and storage yards), screening equipment, weighing devices, top charging equipment, belt conveyors, and relevant gates. The various pieces of equipment are primarily connected by belt conveyors. Medium and large blast furnaces mostly use belt conveyors for transportation. The core materials fed into the blast furnace by the feeding system are: raw materials and fuel. Raw materials: sinter, pellets, lump ore; fuel: coke and pulverized coal. Coke is added from the top of the furnace, and pulverized coal is fed into the furnace through the tuyeres via the injection system. The “Design Specification for Blast Furnace Ironmaking Engineering” specifies the quality requirements for sinter, pellets, lump ore, coke, and pulverized coal used in blast furnaces of different volumes.
The impact of coke quality changes on ironmaking
Under conditions of high smelting intensity and high pulverized coal injection ratio, the impact of coke quality variations on blast furnace indicators is approximately 35%. Many large blast furnaces experience operational malfunctions primarily due to deteriorating coke quality or large fluctuations in its composition, which are difficult to adjust promptly and appropriately during operation. To stabilize blast furnace operation, relatively uniform and stable coke quality is required. Coke testing data must meet national standards. Furthermore, meticulous control must be implemented starting with coal blending to minimize large fluctuations, avoiding the blending of coals with poor melting properties, low fluidity, or high proportions of inert components.
The role of pulverized coal injection in blast furnaces
(1) Replacing metallurgical coke with pulverized coal reduces the coke ratio in blast furnace ironmaking, thus lowering pig iron costs;
(2) Adjusting furnace conditions and thermal regimes for stable operation;
(3) The gasification and combustion of injected pulverized coal before the blast furnace tuyeres reduces the theoretical calorific value. To maintain the theoretical value, compensation is needed, which creates conditions for using high-pressure blast and oxygen-enriched blast in the blast furnace;
(4) Replacing part of the coke with injected pulverized coal can save on coking investment, reduce the number of coke ovens, and decrease air pollution caused by coking; on the other hand, it can greatly alleviate the tight supply and demand of coking coal.
The impact of changes in pulverized coal quality on blast furnaces
The Influence of Pulverized Coal Quality on Blast Furnace Gas Flow Distribution
- Uneven coal gas flow distribution caused by uneven coal injection
Poor quality pulverized coal, such as high ash content, coarse particle size, numerous impurities, and high moisture content, can easily clog the pulverized coal injection pipeline and lances, causing intermittent pulverized coal injection or forcing some lances to stop injection altogether. Uneven pulverized coal injection at each tuyer results in varying sizes of the combustion zone and swirl zone corresponding to each tuyer. The gas flow on the tuyer side where pulverized coal is injected is relatively strong, while the gas flow on the side where pulverized coal injection is stopped is weaker. In severe cases, this can lead to uneven material distribution and uneven furnace size. Over time, this can even cause uneven erosion of the furnace lining and irregular furnace shape.
- Affecting breathability weakens the central airflow.
When the quality of pulverized coal injection is poor, such as high ash content and large particle size, and the injection rate is not reduced in time, the combustion rate of pulverized coal in front of the tuyeres decreases, and a large amount of unburned pulverized coal fills the space between the stacks. Practice shows that unburned pulverized coal tends to concentrate on the outer layer of the dead stack, which worsens the permeability of the stack and weakens the ability of the blast to penetrate to the center, thus weakening the central airflow. In some blast furnaces, during high-volume pulverized coal injection, the peripheral airflow is relatively developed. This is also because the high-volume pulverized coal injection produces too much unburned pulverized coal, which blocks the path of the blast to penetrate to the center, resulting in a relatively developed peripheral airflow. Of course, unlike the deterioration of pulverized coal quality, high-volume pulverized coal injection also has the problem of insufficient thermal compensation, which lowers the theoretical combustion temperature and causes insufficient blast kinetic energy, leading to the development of the peripheral airflow.
Effect on replacement ratio
- Poor quality pulverized coal results in a lower combustion rate before the tuyere.
On the one hand, a large amount of pulverized coal enters the slag and is discharged outside the furnace with the slag or escapes outside the furnace with the gas and enters the dust collector ash, generating a large amount of waste and reducing the replacement ratio. On the other hand, a large amount of unburned pulverized coal deteriorates the permeability of the pulverized coal column, weakens the central airflow, and develops the peripheral airflow, reducing the utilization rate of blast furnace gas, thus also reducing the replacement ratio. Generally, we consider the replacement ratio of pulverized coal injection to be around 0.8. When the quality of pulverized coal is poor, the heat compensation is insufficient, or the injection volume is too large, its replacement ratio will also decrease, and in severe cases, it may be less than 0.6. Strictly speaking, under different blast furnace production conditions and with different pulverized coal quality, the replacement ratio will vary. Striving to improve the pulverized coal injection replacement ratio is one of the main purposes of studying blast furnace pulverized coal injection.
- The low calorific value of pulverized coal directly leads to heat loss in the furnace hearth and poor liquid permeability, among other adverse effects.
Impact on furnace conditions
- When the pulverized coal has a high impurity content, coarse particle size, and high hardness, it will cause significant wear to the tuyeres.
- Poor quality pulverized coal reduces its combustion rate before the tuyeres. Under high-volume pulverized coal injection, a large amount of unburned coal enters the slag, worsening the slag’s fluidity, hindering its penetration into the hearth, and accumulating at the tuyer front, severely affecting furnace operation and causing frequent tuyer burn-out.
- When the pulverized coal has a thick plastic layer, a large amount of this layer cokes at the tuyer front or the nozzle tip, causing nozzle blockage and affecting injection. Coking at the tuyer not only hinders subsequent pulverized coal from smoothly entering the combustion zone before the tuyer, but also exacerbates wear and burn-out of the tuyeres caused by pulverized coal injection.
Enterprises attach great importance to indicators such as the calorific value of pulverized coal.
Pulverized coal injection replaces part of the coke for heating and has a significant price advantage, making it highly valued by various enterprises.
The impact of pulverized coal injection quality on blast furnace smelting is mainly determined by the combustion rate of the pulverized coal before the tuyeres. A higher combustion rate results in greater gas production, better fuel permeability, less slag, better central airflow development, a higher replacement ratio, and a higher economic injection rate, making it suitable for increasing the coal ratio and facilitating smooth furnace operation.
1. Pulverized coal quality is one of the key factors determining the coal ratio and replacement ratio. The impact of pulverized coal quality on blast furnace smelting is achieved through differences in its combustion rate before the tuyeres.
2. The physicochemical properties and preparation quality of pulverized coal both affect blast furnace smelting. Specific analysis should be conducted during production to identify the main issues.
3. When the quality of injected pulverized coal deteriorates during production, the coal ratio should be appropriately reduced, and the injection rate decreased, to minimize the impact on furnace conditions.
4. With the increase in the injection rate of pulverized coal, the attention paid to the quality of injected pulverized coal in daily production should be no less than the attention paid to the quality of coke.
Factors affecting the coke ratio in blast furnaces
1. Effect of iron grade in blast furnace feed: A 1% increase in ore grade leads to a 1.0-1.5% decrease in coke ratio and a 2-2.5% increase in output.
2. Effect of sinter basicity (CaO/SiO2): A 0.1% decrease in sinter basicity (when CaO/SiO2 < 1.85) leads to a 3-3.5% increase in coke ratio and a 3-3.5% decrease in output.
3. Effect of FeO content in sinter: A 1% increase in FeO content in sinter leads to a 1.0-1.5% increase in blast furnace coke ratio and a 1.0-1.5% decrease in output.
4. Effect of <5mm powder content in sinter: A 1% increase in <5mm powder content leads to a 0.5% increase in coke ratio and a 0.5-1.0% decrease in output.
5. A 1% increase in sinter and pelletizing drum size leads to a 0.5% decrease in blast furnace fuel ratio.
6. A 1% increase in sulfur content in ore leads to a 5% increase in fuel ratio.
7. Effect of RDI (Reduced Dimer Content) of Sintered Ore: When RDI+3.15 ≤ 72%, a 10% increase in RDI+3.15 reduces the coke ratio by 1.655% and increases output by 5.64% (the increase decreases as RDI ≥ 72%).
8. Effect of Reducibility of Iron-Containing Burden on Coke Ratio: A 10% decrease in the reducibility of iron-containing raw materials increases the coke ratio by 8-9 kg/t. A 1% increase in MgO content in the sinter reduces reducibility by 5%.
9. Effect of SiO2 and Slag Amount in Burden Charge on Coke Ratio: A 1% increase in SiO2 in the burden charge increases slag amount by 30-35 kg/t. A 100 kg/t increase in slag amount increases the coke ratio by 3.0-3.5% (correction value 20 kg).
10. Effect of Hot Blast Temperature: Increasing the blast furnace hot blast temperature by 100℃ (within the blast temperature range of 950℃~1300℃) reduces the coke ratio by 8~20 kg/t, decreasing further with increasing blast temperature.
11. Effect of Blast Heat Humidity: Increasing the blast furnace blast humidity by 1 g/m³ reduces the coke ratio by 1 kg/t iron and increases output by 0.1~0.5%.
12. Effect of Oxygen Enrichment: Enriching the blast furnace blast with 1% oxygen decreases the coke ratio by 0.5% and increases output by 2.5~3.0% (decreasing further with increasing oxygen enrichment).
13. Effect of Top Gas Pressure: Increasing the top pressure by 10 kPa reduces the coke ratio by 0.3~0.5%.
14. Increasing the blast furnace top temperature by 100℃ increases the fuel ratio by 30 kg/t.
15. Effect of Blast Furnace Gas Utilization Rate: Increasing the gas utilization rate by 1% reduces the coke ratio by 5 kg/t iron. A 1% increase in CO2 content leads to a 20 kg/t decrease in coke ratio.
16. Effect of fixed carbon content in coke: A 1% decrease in C leads to a 2% increase in coke ratio and a 3% decrease in yield.
17. Effect of moisture content in coke: A 1% increase in H2O content leads to a 1.1-1.3% increase in coke ratio and a 2.0-3.0% decrease in yield.
18. Effect of sulfur content in coke: A 0.1% increase in sulfur content leads to a 1.2-2.0% increase in coke ratio and a 2.0-3.0% decrease in yield.
19. Effect of ash content in coke: A 1% increase in ash content (A) leads to a 1.7-2.3% increase in coke ratio and a 2.0-3.0% decrease in yield.
20. Effect of M40 content in coke: A 1% increase in M40 content leads to a 5.6 kg/t decrease in coke ratio and a 1.6% increase in yield.
21. Impact of Coke M10: A 0.2% decrease in coke M10 leads to a 7 kg/t decrease in coke ratio and a 5.0% increase in output.
22. Impact of Coke Hot Properties: A 1% increase in coke reactivity (CRI) leads to a 3 kg/t increase in coke ratio and a 4.0% decrease in output. A 1% decrease in coke post-reaction strength (CSR) leads to a 3-6 kg/t increase in coke ratio and a 4.5% decrease in output.
23. Impact of Pig Iron Si Content: A 0.1% decrease in pig iron Si content leads to a 4-5 kg/t decrease in the coke ratio at the furnace feed.
24. For every 100 kg/t increase in blast furnace slag content, the blast furnace fuel ratio increases by 40 kg/t.
Air supply system
The blast furnace air supply system includes the blast furnace blower, cold blast duct, hot blast stove, hot blast duct, tuyeres, and various valves on the ducts. Generally, a blast furnace has three to four hot blast stoves.
jet blowing system
The pulverized coal injection system consists of different types of injection tank groups and corresponding bell valves, fluidizing devices, etc. Pulverized coal injection usually involves filling the injection tank group with compressed air, introducing secondary compressed air from the self-mixed gas, and injecting pulverized coal into the blast furnace tuyeres through pipelines and injection guns.
Gas processing system
The raw coal gas (0.1-0.25MPa, 100-250 degrees Celsius) produced by the blast furnace first enters the gravity dust collector, where larger particles settle. The semi-clean coal gas exits the gravity dust collector and enters the bag filter unit through pipelines. When the temperature of the semi-clean coal gas is in the range of 80-260 degrees Celsius, it enters the bag filter. Due to the filtration effect of the filter bags, the coal gas passes through the filter bags and enters the clean coal gas main pipe, while the dust remains outside the filter bags. After dust removal, the blast furnace coal gas enters the clean coal gas main pipe, and after being depressurized to 10-15kPa by the TRT generator or pressure regulating valve group, it enters the clean coal gas pipeline network.
Slag and iron handling system
- Molten iron is transported to the steelmaking plant via special molten iron transport vehicles.
- After the liquid slag is processed into water slag by high-pressure water at the tapping area, most of the water is removed from the mixture of water slag and water. The water slag is then grabbed by grabbing buckets, loaded onto trucks, and transported out of the plant.
