Method for smelting common molten iron to form molten iron for nodular cast iron

Abstract

The invention relates to the technical field of nodular cast iron production, in particular to a method for smelting common molten iron into molten iron for nodular cast iron. According to the method, original ordinary molten iron is smelted into low-carbon, low-phosphorus, low-sulfur and high-temperature casting molten iron or pig iron by utilizing an original steelmaking process and a solvent for reducing the content of anti-graphitization elements. According to the method, after common iron ore is used for smelting to generate common blast furnace molten iron, the common blast furnace molten iron is loaded into a converter for steelmaking, dephosphorization, desulfurization and carbon reduction are conducted in the converter through the steelmaking technology, meanwhile, a special solvent is added into the converter to form a composite slag former, and the content of anti-graphitization elements is reduced; the phosphorus, sulfur and carbon contained in the molten iron smelted in the converter and the content of graphitized elements meet the chemical component requirements of the spheroidal graphite casting molten iron.

Description

Technical Field

[0001] The present invention relates to the technical field of ductile iron production, and more specifically, to a method for smelting ordinary molten iron into molten iron for ductile iron. Background Art

[0002] Ductile iron is a high-strength cast iron material developed in the 1950s of the 20th century. Its comprehensive performance is close to that of steel. Based on its excellent performance, it has been successfully used in casting some parts with complex stress, high requirements for strength, toughness, and wear resistance.

[0003] Molten iron is a key link in ductile iron production. With the development of the manufacturing industry, the demand for ductile cast iron pig iron or molten iron is increasing. The traditional production methods are as follows: The first method is to sinter iron ore fines into pellets, then charge them into a blast furnace for ironmaking. Through the reduction chemical reaction in the blast furnace, the iron ore fines containing iron oxide are reduced to molten iron with a relatively high carbon content. Or, as described in a method for obtaining high-quality molten iron in ductile iron production in CN104726631A, after obtaining high-quality molten iron, the molten iron is discharged from the blast furnace and cast into iron blocks called pig iron. Its chemical composition meets the element control requirements of ductile iron. The pig iron is called ductile cast iron pig iron, and then the molten iron with a chemical composition meeting the requirements of ductile iron is smelted by adding scrap steel in an intermediate frequency furnace or adding scrap steel in a cupola furnace, which is called ductile cast iron molten iron.

[0004] The second method is to smelt ductile cast iron molten iron by adding a carburizer to scrap steel that meets the chemical composition requirements in an intermediate frequency furnace.

[0005] In the above two methods, in the first method, the requirements for using iron ore raw materials are relatively high. It is difficult to use only iron ore raw materials with low sulfur, low phosphorus, and low content of anti-graphitization elements in ductile iron production, resulting in high production costs. In the second method, the requirements for scrap steel in the intermediate frequency furnace smelting of scrap steel are also high, and the cost of adding a carburizer is relatively high, which also leads to high production costs. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problem of high production costs caused by high requirements for iron ore raw materials or scrap steel raw materials in the existing ductile iron production methods.

[0007] The purpose of the present invention is to provide a method for smelting ordinary molten iron into molten iron for ductile iron. By using the original steelmaking process and performing a treatment to reduce the content of anti-graphitization elements, the original ordinary molten iron is smelted into cast iron or pig iron with low carbon, low phosphorus, low sulfur, and high temperature, thereby reducing production costs.

[0008] To achieve the above purpose, the present invention provides a method for smelting ordinary molten iron into molten iron for ductile iron, including the following steps: Step S1: Load ordinary iron ore (ferric oxide / ferroferric oxide-based, TFe≥55%) and metallurgical coke into the blast furnace in layers, and then inject oxygen-enriched blast preheated by the hot blast stove from the tuyere for smelting; After smelting until density stratification occurs in the materials in the blast furnace, obtain ordinary molten iron from the bottom of the blast furnace hearth; Step S2: Perform impurity removal pretreatment on the ordinary molten iron, then after temperature measurement and sampling, quantitatively charge it into the top-blown basic oxygen converter according to the process ratio to complete the raw material loading for the steelmaking process, and then initiate the oxidation reaction of the ordinary molten iron by top-blowing oxygen into the converter; When top-blowing oxygen into the converter, simultaneously add a composite slag-forming agent into the converter to react with the ordinary molten iron, and after generating slag, perform slag separation; Step S3: Charge scrap steel and iron oxide scale into the above-mentioned converter, stir evenly with a rotating paddle, and then blow oxygen and passivated magnesium particles into the converter; After the blowing is completed, immediately measure the temperature and take samples of the molten iron in the converter, quickly detect the sulfur and phosphorus contents. If the sulfur and phosphorus contents meet the standards, that is, the sulfur content is less than 0.06% and the phosphorus content is less than 0.06%, then proceed to the next process. If not, continue blowing; Step S4: When tapping the converter, synchronously add ferrosilicon with a particle size range of 3 - 8 mm to the iron flow. After transferring the molten iron into the ladle, it is cast into bread-shaped iron block products by the casting machine, or transferred to an intermediate frequency electric furnace for chemical composition adjustment, or transferred to the casting / pipe casting production line as a spare raw material.

[0009] As a further improvement of this technical solution, in step S1, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%.

[0010] As a further improvement of this technical solution, in step S1, the temperature range of the oxygen-enriched blast is 1150 - 1250 °C, and the oxygen concentration range in the oxygen-enriched blast is 20 - 25%.

[0011] As a further improvement of this technical solution, in step S2, the impurity removal pretreatment is to add a desulfurizing agent to the ordinary molten iron, and the dosage range of the desulfurizing agent per ton of molten iron is 9 - 11 kg.

[0012] As a further improvement of this technical solution, in step S2, the oxygen supply intensity range when top-blowing oxygen into the converter is 2.5 - 4.0 Nm³ / (min·t).

[0013] As a further improvement of this technical solution, the desulfurizing agent includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:1.

[0014] As a further improvement of the present technical solution, in step S2, after the slag is separated by removing the floating slag on the surface of the molten iron by a slag skimmer, the suspended impurities in the molten iron are removed by an electromagnetic separation device.

[0015] As a further improvement of the present technical solution, in step S3, the injection rate range of the passivated magnesium particles is 5-12 kg / min.

[0016] As a further improvement of the present technical solution, in step S3, when measuring the temperature and sampling, the composition of the molten iron is detected in stages by a direct-reading spectrometer, where: When measuring the temperature and sampling in the tapping stage, the temperature range of the molten iron is 1280-1320 °C; When measuring the temperature and sampling in the refining stage, the temperature range of the molten iron is 1400-1450 °C; When measuring the temperature and sampling in the casting stage, the temperature range of the molten iron is 1450-1500 °C.

[0017] As a further improvement of the present technical solution, in step S4, the mass of the ferrosilicon added is 0.3-1.2% of the mass of the molten iron in the converter.

[0018] In the present invention, after ordinary blast furnace molten iron is produced by smelting ordinary iron ore, the ordinary blast furnace molten iron is charged into a converter for steelmaking. The phosphorus, sulfur and carbon are removed in the converter by using the steelmaking process. At the same time, special solvents are added to the converter to reduce the content of anti-graphitization elements. The molten iron smelted in the converter meets the chemical composition requirements of ductile cast iron molten iron in terms of phosphorus, sulfur, carbon and graphitization element content.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: In the method for smelting ordinary molten iron into ductile cast iron molten iron, the ordinary blast furnace molten iron is pretreated to remove impurities. First, the sulfur and phosphorus in the molten iron are transferred to the slag phase through a slag-making reaction. Subsequently, the ordinary molten iron reacts with a composite slag-making agent to generate slag. The slag and sulfur-containing slag phase are separated by a slag skimmer and an electromagnetic separation device. Then, by adding scrap steel, iron oxide scale and injecting magnesium particles, the sulfur is further removed and the carbon content in the ordinary molten iron is reduced by blowing oxygen. Finally, the silicon content in the molten iron is adjusted by adding ferrosilicon, and the ordinary molten iron is converted into molten iron meeting the requirements of ductile cast iron, thus effectively reducing the production cost of ductile cast iron. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flowchart of the present invention; Figure 2 is a schematic diagram for measuring the sulfur element content of the present invention; Figure 3 is a schematic diagram for measuring the silicon element content of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] With the development of the manufacturing industry, the demand for ductile cast iron pig iron or molten iron is increasing. There are the following two traditional production methods: The first method is to sinter iron ore concentrate into pellets, then proportion the materials and load them into the blast furnace for ironmaking. Through the reduction chemical reaction in the blast furnace, the iron ore concentrate containing iron oxide is reduced to molten iron with a relatively high carbon content. The molten iron is discharged from the blast furnace and cast into iron blocks, which are called pig iron. Its chemical composition meets the element control requirements of ductile cast iron. The pig iron made is called ductile cast iron pig iron. Then, the molten iron that meets the chemical composition requirements of ductile cast iron is smelted by adding scrap steel in an intermediate frequency furnace or adding scrap steel in a cupola furnace, which is called ductile cast iron molten iron.

[0023] The second method is to use high-quality scrap steel that meets the chemical composition requirements and add a carburizer to smelt ductile cast iron molten iron in an intermediate frequency furnace.

[0024] Since ductile cast iron requires low sulfur, low phosphorus, and low content of anti-graphitizing elements (anti-graphitizing elements refer to elements that inhibit or change the graphite morphology in cast iron), and in the above two production methods, only iron ore raw materials or scrap steel raw materials that meet the requirements can be used, the requirements for raw materials are high. Moreover, the method of using iron ore raw materials or scrap steel raw materials that meet the requirements and adding a carburizer will lead to high production costs.

[0025] Therefore, as shown in Figure 1 The purpose of the present invention is to provide a method for smelting ordinary molten iron into molten iron for ductile cast iron, including the following steps: S1. Layer ordinary iron ore (iron oxide-based, TFe≥55%) and metallurgical coke into the blast furnace, and then inject oxygen-enriched air preheated by a hot blast stove with a temperature range of 1150 - 1250°C and an oxygen concentration range of 20 - 25% from the tuyere for smelting. Among them, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%. The metallurgical coke forms a porous skeleton structure in the blast furnace, providing channels for the rising reduction gas (such as carbon monoxide) and the descending liquid iron and molten slag, avoiding the collapse and blockage of the furnace charge. In the high-temperature softening zone (from the furnace waist to the furnace belly), coke is the only solid material, supporting the weight of the upper furnace charge and maintaining the stable operation of the blast furnace; After smelting until density stratification occurs in the materials in the blast furnace, obtain ordinary molten iron from the bottom of the blast furnace hearth; S2. Perform impurity pre-treatment on ordinary molten iron, that is, add desulfurizing agent to ordinary molten iron. The dosage range of desulfurizing agent per ton of molten iron is 9 - 11 kg. Then, after temperature measurement and sampling, quantitatively add it into the top-blown basic oxygen converter according to the process ratio to complete the raw material loading of the steelmaking process. Then, initiate the oxidation reaction of ordinary molten iron by top-blowing oxygen into the converter. Among them, the desulfurizing agent includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:1. The combined use of calcium oxide and calcium fluoride transfers the sulfur in the molten iron to the slag phase for removal through slag-making reaction. In addition, the oxygen supply intensity range when top-blowing oxygen into the converter is 2.5 - 4.0 Nm³ / (min·t). Through the physical and chemical actions of the high-speed oxygen flow and molten iron, it can play the roles of rapid decarburization, heating up, removing impurities, and adjusting the composition; When top-blowing oxygen into the converter, simultaneously add composite slag-making agents such as calcium silicate - calcium bicarbonate, calcium oxide - magnesium oxide, etc. into the converter, mix and react with ordinary molten iron, and perform slag separation after the slag is formed. The slag separation is to remove the floating slag on the surface of the molten iron by a slag skimmer, and then remove the suspended impurities in the molten iron such as micron-sized non-metallic inclusions through an electromagnetic separation device. For example, by applying an alternating magnetic field (frequency 1 - 5 Hz), the Lorentz force is used to drive the inclusions to migrate to the slag layer, which is suitable for removing micron-sized particles such as aluminum oxide and silicon dioxide; S3. Add scrap steel and iron oxide scale into the above-mentioned converter, stir evenly with a rotating paddle, and then blow oxygen and passivated magnesium particles into the converter. It should be noted that the scrap steel added in this step is ordinary scrap steel. The main difference between ordinary scrap steel and high-quality scrap steel lies in the composition. The carbon content of ordinary scrap steel is generally less than 2.0%, while the carbon content of high-quality scrap steel is lower, usually below 0.05%. In addition, the blowing rate range of passivated magnesium particles is 5 - 12 kg / min; After the blowing is completed, immediately measure the temperature and take samples of the molten iron in the converter to quickly detect the sulfur and phosphorus contents. If the sulfur and phosphorus contents meet the standards, that is, the sulfur content is lower than 0.06% and the phosphorus content is lower than 0.06%, then proceed to the next process. If not, continue blowing. Specifically, when measuring the temperature and taking samples, the composition of the molten iron is detected in stages by a direct-reading spectrometer, where: When measuring the temperature and taking samples at the tapping stage, the molten iron temperature range is 1280 - 1320 °C; When measuring the temperature and taking samples at the refining stage, the molten iron temperature range is 1400 - 1450 °C; When measuring the temperature and taking samples at the casting stage, the molten iron temperature range is 1450 - 1500 °C; S4. When tapping molten iron from the converter, ferrosilicon with a particle size range of 3 - 8 mm is added synchronously to the molten iron stream, and the mass of the added ferrosilicon is 0.3 - 1.2% of the mass of the molten iron in the converter. After transferring the molten iron into the ladle, it is cast into bread-shaped iron blocks by the casting machine, or transferred to the intermediate frequency electric furnace for chemical composition adjustment, or transferred to the casting / pipe production line as standby raw materials.

[0026] If ductile cast iron pig iron is to be produced, a certain amount of ferrosilicon is added before tapping in the converter, and it can be cast into ductile cast iron pig iron blocks; if ductile cast iron parts or ductile cast iron pipes are to be produced directly, then no ferrosilicon is added, and the qualified molten iron in the converter is directly transferred into the intermediate frequency electric furnace, and ferrosilicon is added in the intermediate frequency electric furnace. At the same time, the intermediate frequency furnace serves as a conditioning and buffer equipment for ductile cast iron production.

[0027] The method for smelting ordinary molten iron into molten iron for ductile cast iron provided by the present invention is further described through the following specific embodiments.

[0028] Example 1 S1. Ordinary iron ore (iron oxide-based, TFe≥55%) and metallurgical coke are loaded into the blast furnace in layers, and then oxygen-enriched blast with a temperature of 1250 °C and an oxygen concentration of 20% preheated by the hot blast stove is injected from the tuyere for smelting. Among them, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%; After smelting until density stratification occurs in the materials in the blast furnace, ordinary molten iron is obtained from the bottom of the blast furnace hearth; S2. The ordinary molten iron is subjected to impurity removal pretreatment, that is, a desulfurizing agent is added to the ordinary molten iron, and the dosage of the desulfurizing agent per ton of molten iron is 11 kg. Then, after temperature measurement and sampling, it is quantitatively charged into the top-blown basic oxygen converter according to the process ratio to complete the raw material loading of the steelmaking process. Then, the oxidation reaction of the ordinary molten iron is initiated by blowing oxygen into the converter. Among them, the desulfurizing agent includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:1. In addition, the oxygen supply intensity when blowing oxygen into the converter is 2.5 Nm³ / (min·t); When blowing oxygen into the converter, a composite slag former is added to the converter synchronously to react with the ordinary molten iron to generate slag, and then slag separation is carried out. The slag separation is to remove the floating slag on the surface of the molten iron by the slag skimmer, and then remove the suspended impurities in the molten iron by the electromagnetic separation device; S3. Scrap steel and scale are charged into the above-mentioned converter, and after being stirred evenly by the rotating paddle, oxygen and passivated magnesium particles are blown into the converter. It should be noted that the blowing rate of the passivated magnesium particles is 12 kg / min; After the blowing is completed, the molten iron in the converter is immediately subjected to temperature measurement and sampling to detect the sulfur and phosphorus contents. When measuring the temperature and sampling, the composition of the molten iron is detected in stages by the direct-reading spectrometer, where: During the tapping stage, when taking temperature measurement and sampling, the molten iron temperature is 1320 °C; During the refining stage, when taking temperature measurement and sampling, the molten iron temperature is 1400 °C; During the casting stage, when taking temperature measurement and sampling, the molten iron temperature is 1500 °C; S4. When tapping the converter, ferrosilicon with a particle size range of 8 mm is synchronously added to the molten iron flow, and the mass of the added ferrosilicon is 0.3% of the mass of the molten iron in the converter. After transferring the molten iron into the ladle, it is cast into bread-shaped iron block products by the casting machine, or transferred to the intermediate frequency electric furnace for chemical composition adjustment, or transferred to the casting / pipe production line as spare raw materials.

[0029] Example 2 S1. Load ordinary iron ore (ferric oxide / ferroferric oxide-based, TFe≥55%) and metallurgical coke into the blast furnace in layers, and then inject oxygen-enriched air preheated by the hot blast stove at a temperature of 1200 °C and an oxygen concentration of 22% from the tuyere for smelting. Among them, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%; After smelting until density stratification occurs in the materials in the blast furnace, obtain ordinary molten iron from the bottom of the blast furnace hearth; S2. Perform impurity removal pretreatment on the ordinary molten iron, that is, add a desulfurizing agent to the ordinary molten iron, and the dosage of the desulfurizing agent per ton of molten iron is 10 kg. Then, after temperature measurement and sampling, quantitatively add it to the top-blown basic oxygen converter according to the process ratio to complete the loading of the raw materials for the steelmaking process. Then, initiate the oxidation reaction of the ordinary molten iron by top-blowing oxygen into the converter. Among them, the desulfurizing agent includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:1. In addition, the oxygen supply intensity when top-blowing oxygen into the converter is 3.5 Nm³ / (min·t); When top-blowing oxygen into the converter, synchronously add a composite slag-making agent to the converter to mix and react with the ordinary molten iron. After generating slag, perform slag separation. The slag separation is to remove the floating slag on the surface of the molten iron by a slag skimmer, and then remove the suspended impurities in the molten iron through an electromagnetic separation device; S3. Charge scrap steel and scale into the above-mentioned converter, stir evenly with a rotating paddle, and then blow oxygen and passivated magnesium particles into the converter. It should be noted that the blowing rate of the passivated magnesium particles is 8 kg / min; After the blowing is completed, immediately measure the temperature and sample the molten iron in the converter to detect the sulfur and phosphorus contents. When measuring the temperature and sampling, the composition of the molten iron is detected in stages by a direct-reading spectrometer. Among them: During the tapping stage, when taking temperature measurement and sampling, the molten iron temperature is 1300 °C; During the refining stage, when taking temperature measurement and sampling, the molten iron temperature is 1400 °C; During the casting stage, when taking temperature measurement and sampling, the molten iron temperature is 1450 °C; S4. When tapping molten iron from the converter, ferrosilicon with a particle size range of 6 mm is synchronously added to the molten iron stream, and the mass of the added ferrosilicon is 0.8% of the mass of the molten iron in the converter. After transferring the molten iron into the ladle, it is cast into bread-shaped iron block products by the casting machine, or transferred to an intermediate frequency electric furnace for chemical composition adjustment, or transferred to the casting / pipe production line as backup raw materials.

[0030] Example 3 S1. Ordinary iron ore (ferric oxide / ferroferric oxide-based, TFe≥55%) and metallurgical coke are layered and charged into the blast furnace. Subsequently, oxygen-enriched blast preheated by the hot blast stove with a temperature of 1150°C and an oxygen concentration of 25% is injected from the tuyere for smelting. Among them, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%; After smelting until density stratification occurs in the materials in the blast furnace, ordinary molten iron is obtained from the bottom of the blast furnace hearth; S2. Perform impurity removal pretreatment on the ordinary molten iron, that is, add a desulfurizing agent to the ordinary molten iron, and the dosage of the desulfurizing agent per ton of molten iron is 9 kg. Then, after temperature measurement and sampling, it is quantitatively added to the top-blown basic oxygen converter according to the process ratio to complete the raw material loading of the steelmaking process. Then, the oxidation reaction of the ordinary molten iron is initiated by top-blowing oxygen into the converter. Among them, the desulfurizing agent includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:1. In addition, the oxygen supply intensity when top-blowing oxygen into the converter is 4.0 Nm³ / (min·t); When top-blowing oxygen into the converter, a composite slag-making agent is synchronously added to the converter and mixed with the ordinary molten iron for reaction. After generating slag, slag separation is carried out. Slag separation is to remove the floating slag on the surface of the molten iron by a slag skimmer, and then remove the suspended impurities in the molten iron through an electromagnetic separation device; S3. Scrap steel and iron oxide scale are added to the above-mentioned converter, and after being stirred evenly by a rotating paddle, oxygen and passivated magnesium particles are blown into the converter. It should be noted that the blowing rate of the passivated magnesium particles is 5 kg / min; After the blowing is completed, the molten iron in the converter is immediately subjected to temperature measurement and sampling to detect the sulfur and phosphorus contents. When performing temperature measurement and sampling, the composition of the molten iron is detected in stages by a direct-reading spectrometer, where: When performing temperature measurement and sampling in the tapping stage, the temperature of the molten iron is 1280°C; When performing temperature measurement and sampling in the refining stage, the temperature of the molten iron is 1450°C; When performing temperature measurement and sampling in the casting stage, the temperature of the molten iron is 1500°C; S4. When tapping molten iron from the converter, ferrosilicon with a particle size range of 3 mm is synchronously added to the molten iron stream, and the mass of the added ferrosilicon is 1.2% of the mass of the molten iron in the converter. After transferring the molten iron into the ladle, it is cast into bread-shaped iron block products by the casting machine, or transferred to an intermediate frequency electric furnace for chemical composition adjustment, or transferred to the casting / pipe production line as backup raw materials.

[0031] The method provided in the above-mentioned Embodiments 1-3 is used for molten iron production, and the produced molten iron is cast into test bars with a diameter of 20-25 mm. After polishing, the contents of carbon, silicon, sulfur, and phosphorus elements are quickly measured by a spectrometer, and the test results are shown in Table 1.

[0032] Table 1 Comparison of element contents in the molten iron produced in Embodiments 1-3 In the chemical composition requirements of the molten iron for ductile iron, the content ratio of carbon element is 3.2-3.6%, the content ratio of silicon element is 1.8-2.5%, and the content ratio of sulfur element is not more than 0.04%, and the content ratio of phosphorus element is not more than 0.06%.

[0033] It can be seen from Table 1 that the content ratios of carbon, silicon, sulfur, and phosphorus elements in the molten iron produced by the method provided in Embodiments 1-3 all meet the chemical composition requirements of the molten iron for ductile iron. Therefore, the molten iron produced by the method provided in Embodiments 1-3 can be used for the production of ductile iron.

[0034] In the present invention, ordinary iron ore and metallurgical coke are combined for smelting in a blast furnace to obtain ordinary blast furnace molten iron, and then the ordinary blast furnace molten iron is subjected to impurity removal pretreatment. First, sulfur and phosphorus in the molten iron are transferred to the slag phase through a slag-making reaction, and then the ordinary molten iron reacts with a composite slag-making agent to generate slag. The slag and sulfur-containing slag phase are separated by a slag skimmer and an electromagnetic separation device. Then, by adding scrap steel, iron oxide scale, and injecting magnesium particles, further desulfurization and dephosphorization can be achieved, and the carbon content in the ordinary molten iron can be reduced, which can reduce the decarbonization process, and further reduce the energy consumption of the decarbonization process. Finally, by adding ferrosilicon, the ordinary molten iron is converted into molten iron meeting the requirements of ductile iron, thereby effectively reducing the production cost of ductile iron.

[0035] Embodiment 4 During the impurity removal pretreatment of ordinary molten iron, a desulfurizing agent is mixed with the ordinary molten iron, and then under the condition of continuously top-blowing oxygen in the subsequent process, it can react with sulfur, phosphorus, and carbon to generate corresponding slag products, and then float to the slag layer, thereby preliminarily reducing the contents of elements such as sulfur, phosphorus, and carbon in the molten iron. The specific process is as follows: In the formula, is phosphorus, is oxygen, is calcium oxide, is calcium fluoride, is a low-melting eutectic compound formed by calcium oxide and calcium fluoride at high temperature, is the product of fixing phosphorus in slag, is sulfur, is carbon, is calcium sulfide, is carbon monoxide, is carbon dioxide.

[0036] Due to the high temperature in the converter, phosphorus in ordinary molten iron reacts with the introduced oxygen and calcium oxide to form slag, while sulfur reacts with calcium oxide and carbon to form calcium sulfide and carbon monoxide, and carbon reacts with oxygen to form carbon monoxide or carbon dioxide. During the above reaction process, calcium oxide and calcium fluoride form a low-melting eutectic compound at high temperature, which can significantly reduce the viscosity of the slag, enhance the fluidity of the slag, increase the contact area between the slag phase and the molten iron, promote the diffusion of sulfur from the molten iron to the slag, and calcium fluoride can destroy the crystal structure of calcium oxide, increase its surface active sites, thereby enhancing the reaction activity of calcium oxide with sulfur and improving the desulfurization rate.

[0037] In order to verify that the addition of the desulfurizer in the impurity removal pretreatment step is one of the important factors for the molten iron produced by the method provided by the present invention to meet the chemical composition requirements of the molten iron for ductile iron casting, in this example, on the basis of the above Example 2, only the addition amount of the desulfurizer is changed, and the addition amount of the desulfurizer per ton of molten iron is set to 0, 5 kg, 7 kg, 9 kg, 10 kg, 11 kg, 14 kg, and then the molten iron is produced, and according to the element content determination method provided in the above example, the sulfur element content in the molten iron is measured, and the measurement results are as Figure 2 shown.

[0038] According to Figure 2 it can be known that when the addition amount of the desulfurizer per ton of molten iron is 0, that is, when no desulfurizer is added, the sulfur element content in the molten iron is significantly higher and does not meet the chemical composition requirements of the molten iron for ductile iron casting in the material. Therefore, it can be shown that the addition of the desulfurizer in the impurity removal pretreatment step is one of the important factors for the molten iron produced by the method provided by the present invention to meet the chemical composition requirements of the molten iron for ductile iron casting; Moreover, compared with the addition amount of the desulfurizer per ton of molten iron being 5 kg, 7 kg or 14 kg, when the addition amount of the desulfurizer per ton of molten iron is 9 kg, 10 kg or 11 kg, the sulfur element content in the molten iron is lower and meets the chemical composition requirements of the molten iron for ductile iron casting in the material. Therefore, it can be shown that when the addition amount of the desulfurizer per ton of molten iron is 9 - 11 kg, the desulfurization effect on the molten iron is better.

[0039] Example 5 Silicon in ferrosilicon can reduce the oxygen activity of hot metal through deoxidation reaction, promote the floating of non-metallic inclusions, and adjust the carbon equivalent to improve the purity of hot metal. The specific reaction process is as follows: In the formula, is the silicon in ferrosilicon, is oxygen, is silicon dioxide, is calcium oxide, is calcium silicate. When ferrosilicon is added during oxygen blowing, a violent deoxidation reaction will be triggered, causing silicon to combine with oxygen to form silicon dioxide. Combined with the enhanced stirring of blown argon, it promotes the combination of silicon dioxide with the slag phase (calcium oxide-aluminum oxide-silicon dioxide), thereby promoting the floating of non-metallic inclusions. Moreover, when adding ferrosilicon, the particle size of ferrosilicon will affect the dissolution rate, and thus affect the purity of hot metal. When the particle size of ferrosilicon is small, due to its large specific surface area and fast dissolution speed, silicon elements are released rapidly, promoting uniform diffusion, but it is easily oxidized by hot metal to form silicon dioxide, increasing oxide inclusions; if not completely dissolved, the residual particles become the inclusion source. When the particle size of ferrosilicon is large, the oxidation probability is low, the dissolution process is controllable, and it can reduce oxidation inclusions, but the dissolution is slow, the treatment time needs to be extended, and it may cause composition segregation due to too high local silicon concentration.

[0040] Therefore, in order to verify that the particle size of 3 - 8 mm of ferrosilicon is one of the important factors for the hot metal produced by the method provided by the present invention to meet the chemical composition requirements of hot metal for ductile iron casting, based on the above Embodiment 2, only the particle size of ferrosilicon is changed, and the particle size of ferrosilicon is set to 1 mm, 2 mm, 3 mm, 6 mm, 8 mm, 9 mm or 10 mm. Then, hot metal is produced, and according to the element content determination method provided in the above embodiment, the silicon element content in the hot metal is measured. The measurement results are as Figure 3 shown.

[0041] According to Figure 3 it can be known that when the particle size of ferrosilicon is 1 mm, 2 mm, 9 mm or 10 mm, that is, when it is not 3 - 8 mm, the silicon element content in the hot metal increases significantly. Therefore, it can be shown that the particle size of 3 - 8 mm of ferrosilicon is one of the important factors for the hot metal produced by the method provided by the present invention to meet the chemical composition requirements of hot metal for ductile iron casting.

[0042] Example 6 Produce hot metal according to the method provided in the above examples, and conduct chemical composition detection of the hot metal according to the element content determination method provided in the above examples; Tests have shown that under the working conditions of this embodiment, the contents of carbon, silicon, sulfur, and phosphorus elements in the produced molten iron meet the chemical composition requirements of molten iron for ductile iron production, indicating that the method provided by the present invention can effectively convert ordinary molten iron into molten iron for ductile iron.

[0043] In summary, the present invention adopts an oxygen-blowing smelting process, utilizes the heat effect of the oxidation reaction to increase the temperature of the molten iron, and replaces the traditional electrothermal heating method. Taking the production of ductile iron pipes as an example, the heat energy required for the molten iron to rise from 1280 °C to 1500 °C can be completely supplied by chemical reactions, thereby reducing energy consumption. In addition, the scrap steel used in the present invention is ordinary scrap steel, without the need to add high-quality scrap steel, which can save materials and electricity consumption.

[0044] In view of the serious overcapacity of low-end steelmaking production capacity in China, some backward equipment even needs to be disassembled and recycled. If it is converted to ductile cast iron pig iron, there is no need to add high-quality scrap steel after the transformation, avoiding double waste of materials and electricity, simultaneously alleviating the pressure of the shortage of scrap steel resources, and can not only expand the global market but also achieve significant energy conservation.

[0045] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification are only preferred examples of the present invention and do not limit the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for smelting ordinary molten iron to form molten iron for ductile iron, characterized in that: The following steps are involved: Step S1: ordinary iron ore and metallurgical coke are loaded into a blast furnace in layers, and then oxygen-enriched blast air preheated by a hot blast stove is injected from the tuyere for smelting; After the materials in the blast furnace are smelted to form density stratification, ordinary molten iron is obtained from the bottom of the blast furnace; Step S2: pre-treating the common molten iron to remove impurities, then adding the molten iron into a top-blown basic oxygen converter in a quantitative manner according to a process ratio after temperature measurement and sampling, and then initiating an oxidation reaction of the common molten iron by top-blowing oxygen into the converter; When oxygen is blown into the converter, a composite slag-making agent is added into the converter to react with ordinary molten iron, and slag is separated after slag is generated. Step S3: adding scrap steel and iron oxide scale into the converter, stirring them evenly with a rotating paddle, and then spraying oxygen and passivated magnesium particles into the converter; After the injection is completed, the molten iron in the converter is immediately temperature-measured and sampled to detect sulfur and phosphorus content; Step S4: When the converter is tapping iron, ferrosilicon with a particle size range of 3-8 mm is added to the molten iron flow simultaneously. After the molten iron is transferred into the ladle, it is cast into finished bread iron blocks by a cast iron machine, or transferred to a medium frequency electric furnace for adjustment of the chemical composition, or transferred to a casting / cast pipe production line as a backup raw material.

2. The method for smelting ordinary molten iron to form molten iron for ductile iron according to claim 1, characterized in that: In the step S1, the fixed carbon content in the metallurgical coke is ≥85%, and the ash content is ≤12%.

3. The method for smelting ordinary molten iron to form molten iron for ductile iron according to claim 1, characterized in that: In the step S1, the air temperature of the oxygen-enriched blast is in the range of 1150-1250° C., and the oxygen concentration in the oxygen-enriched blast is in the range of 20-25%.

4. The method for smelting ordinary molten iron to form molten iron for ductile iron according to claim 1, characterized in that: In the step S2, the impurity removal pretreatment is to add a desulfurizer to ordinary molten iron, and the dosage of the desulfurizer per ton of molten iron is in the range of 9-11 kg.

5. The method for smelting ordinary molten iron to form molten iron for ductile iron according to claim 1, characterized in that: In the step S2, the oxygen supply intensity range when top blowing oxygen into the converter is 2.5-4.0 Nm³ / (min·t).

6. The method for smelting ordinary molten iron to form molten iron for ductile iron according to claim 4, characterized in that: The desulfurizer includes calcium oxide and calcium fluoride, and the mass ratio of calcium oxide to calcium fluoride is 4:

1.

7. The method for smelting ordinary molten iron into molten iron for ductile iron according to claim 1, characterized in that: In step S2, slag separation is to remove slag on the surface of molten iron by a slag scraper, and then remove suspended impurities in the molten iron by an electromagnetic separation device.

8. The method for smelting ordinary molten iron into molten iron for ductile iron according to claim 1, characterized in that: In the step S3, the spraying rate of the passivated magnesium particles is in the range of 5-12 kg / min.

9. The method for smelting ordinary molten iron into molten iron for ductile iron according to claim 1, characterized in that: In step S3, when measuring temperature and taking samples, the composition of the molten iron is detected in stages by a direct reading spectrometer, wherein: When taking temperature samples during the tapping stage, the molten iron temperature ranges from 1280-1320°C; When taking temperature samples during the refining stage, the molten iron temperature range is 1400-1450°C; When measuring temperature and taking samples during the casting stage, the molten iron temperature ranged from 1450-1500℃.

10. The method for smelting ordinary molten iron into molten iron for ductile iron according to claim 1, characterized in that: In step S4, the mass of the added ferrosilicon is 0.3-1.2% of the mass of the molten iron in the converter.

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