Refining method of molten iron in converter

Abstract

To provide a method for refining hot metal in a converter that can increase the blending ratio of cold iron sources such as iron scrap.SOLUTION: In the refining method of hot metal in a converter, powdery or granular carbonaceous materials for heat-rising containing volatile components in excess of 20 mass% by dry mass are injected and supplied from a central hole in the center of the shaft of a top blown lance tip, together with carrier gas, toward the bath surface of hot metal in the converter. In addition, oxygen gas is injected and supplied through a peripheral hole provided around the central hole of the top blown lance tip to heat the hot metal between the top blown lance tip and the hot metal bath surface by combustion heat generated by mixing volatile component gas, which is volatilized by heating the carbonaceous materials for heat-rising with the high temperature atmosphere in the converter, with the surrounding oxygen gas, and burning the mixed gas and the hot metal is also supplied with the carbonaceous materials for heat-rising from which volatile components are removed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten pig iron in a converter, and more particularly to a method for refining molten pig iron in a converter, in which operation is carried out while increasing the thermal margin of the molten pig iron by using a carbonaceous material for heating. [Background technology]

[0002] In recent years, from the perspective of environmental protection, reducing CO2 emissions in the steelmaking process has become an important issue, and efforts have been made to increase the blending ratio of cold iron sources such as scrap iron and reduce the blending ratio of molten pig iron as the iron source used in the steelmaking process. This is because, in the production of steel products, the production of molten pig iron in a blast furnace requires a great deal of energy to reduce and melt the iron ore, and also emits a large amount of CO2, whereas cold iron sources only require heat for dissolution, and so using cold iron sources in the steelmaking process can reduce the amount of energy used for the heat of reducing the iron ore, thereby significantly reducing CO2 emissions.

[0003] However, in a molten steel production process using a combination of a blast furnace and a converter, the heat sources for melting the cold iron source are the sensible heat of the molten pig iron and the heat of combustion resulting from the oxidation of carbon and silicon in the molten pig iron, and there is a natural limit to the amount of cold iron source that can be melted. Moreover, dephosphorization treatment has become a pretreatment method for the molten pig iron, and not only does this add a processing step to the decrease in the molten pig iron temperature, but the carbon and silicon in the molten pig iron are oxidized in the dephosphorization treatment, reducing their contents, which is unfavorable for melting the cold iron source.

[0004] Therefore, in the dephosphorization and decarburization refining of molten iron, various methods have been proposed for supplying an additional carbon source to the molten iron in order to increase the thermal margin of the molten iron and increase the blending ratio of the cold iron source, such as supplying a lump-shaped carbonaceous material for heating from the top of the converter, or supplying a powdered carbonaceous material for heating from a bottom nozzle installed at the bottom of the furnace or from a top lance at the top of the converter. For example, Patent Document 1 proposes a method for supplying a heat source from an external source to improve the blowing capacity of a converter during converter steelmaking from molten iron, in which powder or liquid (such as kerosene) of a carbon-containing substance (such as carbon or coke) and oxygen gas are sprayed from the top of the furnace through a lance and blown onto the molten iron bath surface in the furnace as a flame to heat the molten iron bath surface, and the oxygen gas and the carbon-containing substance are supplied to the molten iron bath. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 61-009512 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned conventional techniques have the following problems.

[0007] That is, in Patent Document 1, in order to achieve the purpose of preventing an increase in the sulfur concentration in molten iron in addition to the purpose of providing a heat source, all or most of the added carbon-containing substance is burned on the bath surface as a measure to prevent the sulfur in the carbon-containing substance, which causes an increase in the sulfur concentration in molten iron, from migrating into the molten iron. Therefore, there is a problem in that the original function of the heating carbonaceous material, which is to make up for the carbon content reduced due to the progress of the decarburization reaction in the molten iron and further promote the decarburization reaction in the molten iron, cannot be fully utilized.

[0008] As described above, although various means have been proposed for supplying an additional carbon source to the molten iron in order to increase the thermal margin of the molten iron and increase the blending ratio of the cold iron source in the dephosphorization and decarburization treatment of the molten iron, there is still room for improvement.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for refining molten pig iron in a converter, which is capable of increasing the blending ratio of a cold iron source such as iron scrap by operating the converter while increasing the thermal margin of the molten pig iron. [Means for solving the problem]

[0010] [1] A method for refining molten pig iron in a converter, characterized in that a powdered or granular heating carbonaceous material containing more than 20% by mass of volatile components on a dry basis is supplied by injection from a central hole provided at the axial center of the tip of a top-blowing lance, together with a carrier gas, toward the bath surface of the molten pig iron in the converter, and oxygen gas is supplied by injection from peripheral holes provided around the central hole of the tip of the top-blowing lance, thereby heating the molten pig iron between the tip of the top-blowing lance and the molten pig iron bath surface by the combustion heat generated by mixing and burning the volatile component gas generated by heating the heating carbonaceous material in the high-temperature atmosphere in the converter with the surrounding oxygen gas, and supplying the heating carbonaceous material from which the volatile components have been removed to the molten pig iron. [2] The method for refining molten iron in a converter according to [1], wherein the volatile components of the carbonaceous material for heating are 40 mass % or less on a dry mass basis. [3] A method for refining molten iron in a converter according to [1] or [2], characterized in that the flow rate of the oxygen gas injected from the peripheral holes is equal to or greater than the amount of oxygen that completely burns the volatile components in the carbonaceous material for heating before they reach the surface of the molten iron bath. [4] A method for refining molten iron in a converter according to any one of [1] to [3], wherein the carbonaceous material for heating is a carbonized material obtained by carbonizing at least one selected from the group consisting of coal, plant biomass, and waste plastics. [Effects of the Invention]

[0011] According to the above configuration, a heating carbonaceous material containing more than 20% by mass of volatile components is injected from the tip of the top-blowing lance, and the volatile components are burned on the way to the molten pig iron bath surface, thereby radiating heat from the bath surface to the molten pig iron. In addition, the heating carbonaceous material from which the volatile components have been removed is directly supplied to the molten pig iron (recarburization), thereby further heating the molten pig iron through a decarburization reaction and improving the thermal margin of the molten pig iron, and therefore the blending ratio of a cold iron source such as iron scrap can be increased. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of converter equipment used in carrying out the present invention. [Figure 2] 2A and 2B are diagrams showing the tip of the top blowing lance 3 shown in FIG. 1, where (a) is a schematic vertical cross-sectional view thereof and (b) is an end view thereof. [Figure 3] FIG. 10 is a diagram showing the measurement results of the temperature distribution of the flame of the heat-up carbonaceous material extending in the injection direction from the tip of the top-blowing lance when the amount of volatile components in the heat-up carbonaceous material is changed. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be specifically described below.

[0014] The present invention can be applied to decarburization refining of molten pig iron, which is carried out by supplying oxygen gas from a top-blowing lance to the molten pig iron contained in a converter.

[0015] The molten pig iron used in the present invention is produced in a blast furnace, and this molten pig iron is received in a molten pig iron transport vessel such as a hot metal ladle or a torpedo car and transported to a converter where dephosphorization and decarburization refining are carried out. When dephosphorization is carried out in a converter, it is preferable to discharge the slag generated during the dephosphorization treatment before the decarburization treatment.

[0016] Generally, in the decarburization and refining of molten pig iron, oxygen gas is blown onto the molten pig iron from the top or bottom lance tuyere to remove the approximately 4 mass% of carbon contained in the molten pig iron when it leaves the blast furnace through a decarburization reaction such as C + 1 / 2O2 → CO, thereby adjusting the composition of the steel to a tougher one. The oxygen gas used here can be industrially pure oxygen gas or a mixture of industrially pure oxygen gas and an inert gas, but industrially pure oxygen gas is generally used.

[0017] The temperature of the molten steel after decarburization and refining can be raised to about 1650°C at the end of converter refining using only the components of the molten iron contained when it was tapped from the blast furnace, without the addition of any new components. However, if the temperature of the molten iron drops before decarburization due to dephosphorization or other processes, or if a cold iron source such as iron scrap is being melted, it is necessary to supply a heating carbonaceous material to the molten iron to induce a further decarburization reaction and raise the temperature of the molten iron.

[0018] In view of the above circumstances, the present inventors conducted specific thermal calculations to study the thermal margin of molten iron that would enable an increase in the blending ratio of the cold iron source in the dephosphorization treatment and decarburization refining of molten iron. When producing molten steel from molten pig iron in a converter, the decarburization reaction in which carbon contained in the molten pig iron is removed with oxygen can be expressed by the following formula. C+1 / 2O2→CO-2,200[kcal / kg-C] ···(1) C+O2→CO2-7,840[kcal / kg-C] ···(2) Most of the decarburization reaction remains within the reaction of formula (1), but some progress to the reaction of formula (2) {CO2 / (CO+CO2) ≒ 15%}.

[0019] If we assume that typical molten iron contains about 4% by mass of carbon (about 40 kg per ton of molten iron), then the total carbon content per ton of molten iron is as follows: (2,200[kcal / kg-c]×0.85+7,840[kcal / kg-c]×0.15)×40[kg] =121,840[kcal / hot metal t] ···(3) The heat generated by the decarburization reaction is

[0020] On the other hand, if the temperature of the molten iron charged into the converter is 1200°C and the temperature of the molten steel at the end of the decarburization process in the converter is 1650°C, the specific heat of the molten steel is 0.206 [kcal / kg·°C], {0.206[kcal / kg ℃]×960[kg molten steel / hot metal t]×(1650-1200)[℃]} =88,992[kcal / hot metal t] ···(4) It requires heat. Considering that the thermal efficiency of a typical converter is about 70%, 121,840[kcal / hot metal t]×0.70=85,288[kcal / hot metal t] ···(5) Therefore, the amount of heat required to heat the molten iron from the molten iron temperature before the decarburization reaction to the molten steel temperature after the decarburization reaction (the value of equation (4)) is almost the same as the value of the amount of heat generated by the decarburization reaction (the value of equation (3)).

[0021] Furthermore, if the hot metal charging temperature is high, for example 1300°C, the value equivalent to equation (4) is 69,216 kcal / t of hot metal, which is lower than the value of equation (5), and this difference is the excess heat. In normal operation of a converter, this excess heat is used as a heat source to melt the cold iron source, such as the iron scrap mentioned above.

[0022] Next, the heat required to melt a cold iron source such as iron scrap and heat it up to 1650°C is assumed to be 412 [kcal / t-Fe]. As an example, consider the case where the blow-end temperature is 1650°C, the molten iron charging temperature is 1300°C, the ratio of cold iron source (scrap iron) such as iron scrap (scrap iron) is 3.75%, and the molten iron ratio is 96.25%. From equations (3) and (5), the amount of heat generated by the decarburization reaction per ton of hot metal in this case is 121,840[kcal / hot metal t]×0.9625×0.70=82,089[kcal / molten steel t] ···(6)

[0023] In addition, the heat of melting the cold iron source (scrap iron) is 412[kcal / t-Fe]×0.0375×1000[scrap iron kg / molten iron t] =15,469[kcal / molten steel t] ···(7) In addition, the heat of molten steel rising is 0.206 x (1650 - 1300) x 960 [kg-Fe / t of hot metal] x 0.9625 =66,617[kcal / molten steel t] ···(8)

[0024] In the above-mentioned study example, the value of formula (6) is the heat generated by the decarburization reaction, and the heat of melting the cold iron source (scrap iron) in formula (7) and the heat of heating the molten steel in formula (8) are the heat required for the treatment, and the generated heat and the heat required for the treatment are in balance.

[0025] Here, if we focus on the "heat transfer efficiency of 70%" in the equation for heat generation in equation (6), if this value is large, the amount of surplus heat increases, and more cold iron sources such as scrap iron can be consumed. Therefore, we will first consider the background to the generation of wasted heat.

[0026] The mechanism by which heat is generated by carbon in the molten steel manufacturing process, such as in a converter, is as follows. 1) Pure oxygen gas is supplied from a lance or bottom nozzle to oxidize the molten iron and generate heat. Fe+1 / 2O2→FeO+1150[kcal / kg-Fe] ···(9) 2) The generated FeO is reduced with C. FeO+C→Fe+CO-680[kcal / kg-Fe] ···(10) The difference between equation (9) and equation (10) is the amount of heat generated, which raises the temperature of the molten iron bath.

[0027] However, since CO(g) generated in the reaction of Equation (10) generates sensible heat when it leaves the molten iron bath, it absorbs more heat from the molten iron bath than the heat absorbed by reduction. Furthermore, when a heating carbonaceous material is added, gas is generated during the reaction of Equation (10), and some of the heating carbonaceous material is entrained in the generated gas and leaves the molten iron bath unreacted, thereby absorbing only sensible heat. For these reasons, the amount of heat equivalent to about 30% of the added heating carbonaceous material becomes wasted heat.

[0028] Based on the above findings, the present inventors further investigated a method for refining molten pig iron in a converter, which operates while increasing the thermal margin of the molten pig iron by injecting a heating carbonaceous material from the tip of a top-blowing lance. As a result, the present inventors came up with the idea of ​​dividing the heating function of the added heating carbonaceous material between before and after the molten pig iron is delivered. Before the molten pig iron is delivered, the molten iron surface is heated by the combustion heat of the volatile components of the heating carbonaceous material, and after the molten pig iron is delivered, the molten pig iron is heated by the decarburization reaction in the molten iron, as with the function of conventional heating carbonaceous material. This idea led to the idea that this would halve the problem of wasted heat caused by heat removal by CO gas generated in the molten iron. The present invention was made based on the above findings and further investigation.

[0029] The present invention is based on the premise that, when further decarburization reactions are to occur and the temperature of the molten pig iron is to be increased, a powdery or granular heating carbonaceous material is injected from the tip of a top-blowing lance toward the surface of the molten pig iron bath. In this case, a powdery or granular heating carbonaceous material containing more than 20% by dry mass of volatile components is used as the heating carbonaceous material. This allows the volatile component gases that are heated and volatilized between the tip of the top-blowing lance and the surface of the molten pig iron to be mixed with the surrounding oxygen gas and combusted, thereby additionally heating the molten pig iron. Furthermore, by directly supplying the heating carbonaceous material from which the volatile components have been removed to the molten pig iron, a combined effect can be obtained, including the heating of the molten pig iron by the decarburization reactions in the molten pig iron bath.

[0030] FIG. 1 is a schematic cross-sectional view showing an example of converter equipment used in carrying out the present invention, and FIGS. 2(a) and 2(b) are a schematic vertical cross-sectional view and an end view of the tip of the top-blowing lance 3 shown in FIG. 1.

[0031] As shown in FIG. 1 , a converter furnace 1 used in the present invention for decarburization refining of molten pig iron 26 includes a furnace body 2 having a steel shell 21 as its outer shell and a refractory material 22 inside the steel shell 21, and a top-blowing lance 3 inserted into the furnace body 2 and movable vertically. A tapping port 23 is provided at the top of the furnace body 2 for tapping molten steel 26, which has been refined from molten pig iron to molten steel by decarburization refining. The bottom of the furnace body 2 is provided with multiple bottom-blowing tuyeres 24 for injecting stirring gas 28. The bottom-blowing tuyeres 24 are connected to gas inlet pipes 25. The stirring gas 28 may be oxygen gas, an inert gas, or other gases, depending on the purpose. During converter refining, slag 27 is formed from quicklime, burnt dolomite, iron ore, mill scale, manganese ore, and other materials introduced into the furnace from an auxiliary material introduction device (not shown).

[0032] The top-blowing lance 3 is connected to a heat-up carbonaceous material supply pipe 37 for supplying powdered or granular heat-up carbonaceous material 42 together with a carrier gas, which is a mixture of nitrogen gas and an inert gas such as Ar gas, an oxygen gas supply pipe 34 for supplying a refining oxidizing gas such as oxygen gas, and a cooling water supply pipe and a drain pipe (not shown) for supplying and discharging cooling water for cooling the top-blowing lance 3.

[0033] The other end of the heat-up carbonaceous material supply pipe 37 is connected to a dispenser 40 containing powdered or granular heat-up carbonaceous material 42, which is in turn connected to a heat-up carbonaceous material carrier gas supply pipe 41. The inert gas supplied to the dispenser 40 through the heat-up carbonaceous material carrier gas supply pipe 41 functions as a carrier gas for the powdered or granular heat-up carbonaceous material 42 contained in the dispenser 40, and the powdered or granular heat-up carbonaceous material 42 contained in the dispenser 40 is supplied to the top blowing lance 3 through the heat-up carbonaceous material supply pipe 37 and can be sprayed from the tip of the top blowing lance 3 toward the molten pig iron 26. The flow rate of the inert gas supplied to the heat-up carbonaceous material carrier gas supply pipe 41 can be adjusted by a flow control valve (not shown).

[0034] As shown in the schematic longitudinal cross section of Figure 2(a), the tip of the top-blowing lance 3 mainly comprises a lance body 30, peripheral holes 31 as the outlet for oxygen gas for decarburization, a peripheral hole oxygen gas passage 33, a central hole 35 as the outlet for powdered or granular carbonaceous material for heating accompanied by carrier gas, a carbonaceous material for heating passage 36, and a cooling water passage 38. It is preferable that the central axis (X) of the peripheral holes be inclined to a certain extent (indicated as an inclination angle θ in Figure 2(a)) with respect to the lance central axis (Y). Without this inclination, the oxygen gas flow injected from the peripheral holes 31 would excessively combine with the flame of the volatile component gas volatilized from the powdered or granular carbonaceous material for heating 42 injected from the central hole 35, resulting in a cooling and possible extinguishing.

[0035] Figure 2(b) is an explanatory diagram showing the arrangement of the central hole and peripheral holes on the heat-receiving surface of the tip of a top-blowing lance. 30 denotes the lance body, 31 denotes the peripheral holes, and 35 denotes the central hole. The peripheral holes 31 are arranged on a concentric circle 32 centered on the central axis of the lance, and the central hole is aligned with the central axis of the lance.

[0036] The volatile component content of the heat-up carbonaceous material of the present invention is more than 20% by mass in terms of dry mass. If the volatile component content of the heat-up carbonaceous material is less than 20% by mass in terms of dry mass, as shown in Fig. 3 described later, the flame temperature formed by combustion of the volatile component of the heat-up carbonaceous material at the tip of the top-blowing lance in the converter cannot be made to reliably exceed the ambient temperature in the converter and cannot be made to be a flame temperature that can efficiently heat the molten iron surface by radiant heat.

[0037] Furthermore, the volatile component content of the carbonaceous material for heating according to the present invention is preferably 40% by mass or less in terms of dry mass. This is because even if the volatile component content is increased to about 40%, the jet temperature reaches a plateau as shown in Fig. 3 described later, and the volatile component content exceeding 40% becomes excessive and does not contribute to combustion.

[0038] In the present invention, it is desirable that the flow rate of the oxygen gas injected from the peripheral holes be a flow rate equal to or greater than the amount of oxygen required to completely combust the volatile components in the heating coal before they reach the molten iron bath surface, in order to effectively utilize all of the volatile components in the heating coal and increase thermal efficiency.

[0039] The carbonaceous material for heating of the present invention is a carbonized material obtained by carbonizing at least one material selected from the group consisting of coal, plant biomass, and waste plastics. In particular, when plant biomass or waste plastics is used as the carbonaceous material for heating, this is desirable from the standpoint of environmental protection, as it substantially reduces CO2 emissions in the steelmaking process. [Example]

[0040] A lance with the shape shown in Figure 2 was installed in a vertical combustion test furnace capable of reproducing a furnace atmosphere of 1600°C, simulating the converter equipment shown in Figure 1. The two types of carbonaceous materials listed in Table 1 were blended to prepare four types of powdered carbonaceous materials for the test, with volatile components of 8.1, 18.2, 27.2, and 36.3 mass% on a dry basis. The particle size of these powdered materials ranged from 0.2 mm to 6 mm, with the 1 mm to 2 mm particle size peaking at approximately 25% of the total powdered material, and the 0.2 mm to 1 mm range accounting for approximately 35% of the total powdered material.

[0041] [Table 1]

[0042] After adjusting the temperature inside the vertical combustion test furnace to 1600℃, the four types of powdered carbon materials prepared for heating were injected into the central hole of a lance with a diameter of 51mm at a rate of 2,000Nm. 2 N2 was used as a carrier gas at a flow rate of 340 kg / min (maximum 640 kg / min) at 1000 m / h. Simultaneously with the supply of the carbonaceous material powder for heating, a total of 30,000 Nm was supplied from four peripheral holes with a diameter of 41 mm around the central hole of the lance. 2 / h of industrially pure oxygen gas was injected at an outward inclination of 12 degrees from the axial center. The jet of carbonaceous material transport and the jet of industrially pure oxygen gas combined to form a single jet after being injected from the lance. Figure 3 shows the measurement results of the distribution of the average temperature of the jets at each position in the injection direction. The horizontal axis of Figure 3 is expressed as L / D, where D is the diameter of the jets after combination and L is the jet reach distance.

[0043] As shown in Figure 3, all jets are initially cooler than the furnace atmosphere. However, as the jet advances, the temperature rises. In most cases, the furnace temperature reaches 1600°C or higher. Once the L / D ratio exceeds 80, the fuel burns out and the temperature begins to drop. Among these, the case with a volatile content of 8.1% barely exceeds 1600°C until the end, and the temperature begins to drop as the L / D ratio exceeds 80, just like the other cases. The higher the volatile content, the higher the temperature becomes near the lance nozzle, reaching a maximum temperature around 80. Furthermore, the higher the volatile content, the longer the high-temperature atmosphere can be maintained, and the more heat can be supplied by radiant heat. Therefore, in this invention, when carbonaceous material is injected from a top-blowing lance as a heat source, the volatile content must be at least 20%. However, even if the amount of volatile components is increased to around 40%, as can be seen from Figure 3, the jet temperature reaches a plateau, and volatile components exceeding 40% become excessive and do not contribute much to combustion. Also, considering that the original purpose of supplying carbonaceous material is to supply pure oxygen gas to the molten iron bath to reduce the iron oxide generated, it is clear that the upper limit of volatile components should be set to 40% or less in order to achieve this original purpose as well. [Explanation of symbols]

[0044] 1 Converter equipment 2 Furnace body 3 Top-blowing lance 21 Ironhide 22 Refractories 23 Tap 24 Bottom-blown tuyere 25 Gas inlet pipe 26 Molten metal (molten pig iron, molten steel) 27 Slag 28 Stirring gas 30 Lance body 31 Peripheral hole 32 Concentric circles around the center of the lance 33 Peripheral hole oxygen gas flow path 34 Oxygen gas supply pipe 35 Center hole 36 Carbon material flow path for heating 37 Carbon material supply pipe for heating 38 Cooling water flow path 40 Dispenser 41 Heat-up carbonaceous material carrier gas supply pipe 42 Carbon material for heating (powder)

Claims

1. In a method for refining molten iron in a converter, A powdery or granular carbonaceous material for heating, which contains more than 20% by mass and not more than 40% by mass of volatile components on a dry mass basis and has a particle size range of 0.2 mm to 6 mm and a particle size distribution peak at 1 mm to 2 mm, is supplied by being sprayed together with a carrier gas from a central hole provided in the axial center of the tip of a top-blowing lance toward the bath surface of the molten pig iron in the converter, and By injecting and supplying oxygen gas from peripheral holes provided around the central hole at the tip of the top blowing lance at a flow rate equal to or greater than the amount of oxygen that completely burns the volatile components in the carbonaceous material for heating before the carbonaceous material reaches the molten iron bath surface, Between the tip of the top-blowing lance and the surface of the hot metal bath, the hot metal is heated by the high-temperature atmosphere in the converter to volatilize the carbonaceous material, and the volatile component gas is mixed with the surrounding oxygen gas and combusted, and the hot metal is heated by the combustion heat; A method for refining molten iron in a converter to increase the blending ratio of the cold iron source, characterized by supplying the heat-raising carbonaceous material from which the volatile components have been removed to the molten iron.

2. 2. The method for refining molten iron in a converter according to claim 1, wherein the carbonaceous material for heating is a carbide obtained by carbonizing at least one selected from the group consisting of coal, plant biomass, and waste plastics.

Images