Manufacturing method of nuggets

Abstract

Provided is a technology that enables efficient production of grained iron having a low P concentration. The present invention comprises: a first step for melting reduced iron to obtain primary molten iron; a second step for separating the primary molten iron from slag; a third step for subjecting the primary molten iron separated from the slag to dephosphorization to obtain secondary molten iron; and a fourth step for solidifying the secondary molten iron into grains to obtain grained iron. In the third step, an oxygen source and a CaO source are supplied to the primary molten iron for dephosphorization, and the temperature of the secondary molten iron when the dephosphorization is finished is set to be equal to or lower than the temperature of the secondary molten iron when the dephosphorization is started.

Description

[Technical field]

[0001] The present invention relates to nuggets having a reduced P concentration and a method for producing the same. [Background technology]

[0002] In recent years, there has been an increasing demand for the expanded use of cold iron ore (scrap) in the steelmaking industry. Recycling of iron ore is essential to building a recycling-oriented society. Furthermore, from the perspective of preventing global warming, increasing scrap usage is essential to meet the demand for reducing CO2 emissions. Unlike iron ore, which is iron oxide (Fe2O3), scrap does not require a reduction process in the smelting process, making it possible to reduce CO2 emissions. As a result, the usage of cold iron ore is steadily increasing.

[0003] The blast furnace-converter process is a steelmaking process in which iron ore (Fe2O3) is charged into a blast furnace together with coke (carbon source) as a reducing agent to produce molten pig iron with a carbon concentration of about 4.5-5%, and the molten pig iron is then charged into a converter to oxidize and remove impurities such as C, Si, and P. When producing molten pig iron in a blast furnace, about 500 kg of carbon source is required per ton of molten pig iron for the reduction of iron ore. About 2 tons of CO2 gas is generated. On the other hand, when producing molten steel using scrap iron as a raw material, the carbon source required for the reduction of iron ore is not required. Therefore, even when considering the energy required to melt scrap iron, replacing 1 ton of molten pig iron with 1 ton of scrap iron leads to a reduction in CO2 emissions of about 1.5 tons. From the above, it is necessary to increase the amount of scrap used in order to reduce greenhouse gas emissions and maintain production activities at the same time.

[0004] However, due to the tight supply and demand of iron scrap, especially high-quality iron scrap that is essential for the production of high-grade steel, there is a growing need for reduced iron instead of scrap. Reduced iron is produced by reducing iron ore, but there is no need to make the carbon concentration in the produced iron high as in the blast furnace-converter process, and since no excessive carbon source is used, this leads to a reduction in CO2 emissions of about 0.2 t per ton of iron. In addition, by using hydrocarbon gases such as hydrogen or natural gas as the reducing agent instead of a carbon source, it is possible to further reduce CO2 emissions.

[0005] One of the issues when using reduced iron is the phosphorus contained in it. Phosphorus in steel products reduces quality by causing hot brittleness and other problems, so it is necessary to reduce the P concentration to a level appropriate for the required quality. However, when reduced iron is melted to produce molten iron using an electric furnace, most of the phosphorus in the reduced iron ends up in the molten iron (also known as rephosphorization). For this reason, currently, reduced iron is produced from high-grade iron ore with a low P concentration (P concentration of approximately 0.01% by mass), and the P concentration of the reduced iron is approximately 0.02% by mass.

[0006] On the other hand, it is predicted that high-grade iron ore with a low P concentration will be depleted, and in the future, molten steel production using reduced iron made from low-grade iron ore with a high P concentration will be required. The P concentration in iron ore used in the current blast furnace process is 0.05 to 0.10 mass% (0.10 to 0.15 mass% when converted to the P concentration as reduced iron), and it is predicted that the P concentration will further increase in the future. This P concentration is 5 to 10 times or more than the P concentration of reduced iron produced from the high-grade iron ore with a low P concentration. In order to prevent the quality degradation due to phosphorus in steel products, it is necessary to remove phosphorus when melting reduced iron with a high P concentration to produce molten steel, or to remove phosphorus when producing reduced iron from iron ore with a high P concentration. Several phosphorus removal technologies have been proposed.

[0007] Patent Document 1 proposes a flux for dephosphorization refining in an electric arc furnace, which is composed mainly of calcium oxide, 5 to 15 mass % of aluminum oxide, 25 to 35 mass % of iron oxide, and the balance being unavoidable impurities, in order to remove phosphorus from molten steel to a low concentration in a relatively short time using an electric arc furnace alone.

[0008] Patent Document 2 also describes a method for producing iron ore, titanium-containing ore, nickel-containing ore, chromium-containing ore, or a mixture mainly composed of these ores, with a CaO content of 25 mass% or less and a CaO / (SiO2+Al2O3) ratio of 5 or less, and a mixture of these ores with Ar, He, N 2、 CO, H 2、 A method has been proposed for removing phosphorus by contacting a hydrocarbon gas or a mixture of these gases at temperatures above 1600°C.

[0009] Patent Document 3 proposes a method in which iron ore with a high P concentration is crushed to 0.5 mm or less, water is added to this to make the pulp concentration about 35 mass%, H2SO4 or HCl is added as a solvent and reacted at pH 2.0 or less to decompose and dissolve the phosphate minerals, and then magnetized materials such as magnetite are collected by magnetic separation, and non-magnetic materials such as SiO2 and Al2O3 are precipitated and separated as slime, and the P dissolved in the liquid at this time is neutralized to a pH range of 5.0 to 10.0 by adding slaked lime or quicklime, and then separated and recovered as calcium phosphate. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 8-120322 [Patent Document 2] Japanese Patent Application Publication No. 54-83603 [Patent Document 3] Japanese Patent Application Publication No. 60-261501 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the above-mentioned conventional techniques have the following problems to be solved. That is, the technology disclosed in Patent Document 1 assumes that an iron source with a low P concentration, such as scrap, is used. Specifically, in order to reduce the P concentration in molten steel from 0.020 mass% to 0.005 mass%, 350 g of flux is added per 7000 g of molten steel. Assuming that the P concentration in reduced iron is 0.15 mass%, the amount of flux required to reduce the P concentration to 0.01 mass%, which is the same as that of steel products, is 230 kg per ton of molten steel. This causes a problem that the volume ratio of the flux in the arc furnace increases, reducing the amount of molten steel processing and deteriorating production efficiency.

[0012] The method disclosed in Patent Document 2 has a treatment temperature of 1600°C or higher, and the specification states that "for more effective dephosphorization, it is preferable to use a temperature of 1800°C or higher. Although it is difficult to achieve such a high temperature range using normal heating methods, it can be achieved, for example, by using plasma arc or high-frequency induction plasma." This poses the problem that it requires a lot of energy and is not suitable for large-scale treatment.

[0013] The method disclosed in Patent Document 3 is a wet treatment using an acid, and has the problem that it takes time and costs to dry the recovered magnetized material in order to use it as a main raw material. Furthermore, this method also has the problem that it takes time and costs to crush the material to 0.5 mm or less in advance.

[0014] The present invention has been made in consideration of the above circumstances, and aims to provide a technique that enables efficient production of nuggets of iron with a low P concentration, even when the raw material is reduced iron obtained from low-grade iron ore with a high P concentration. [Means for solving the problem]

[0015] The method for producing granular iron of the present invention, which advantageously solves the above-mentioned problems, includes a first step of melting reduced iron to obtain primary molten iron, a second step of separating the primary molten iron from slag, a third step of dephosphorizing the primary molten iron separated from the slag to obtain secondary molten iron, and a fourth step of solidifying the secondary molten iron into granules to obtain granular iron, characterized in that in the third step, an oxygen source and a CaO source are supplied to the primary molten iron to perform dephosphorization, and the temperature of the secondary molten iron at the end of the dephosphorization is set to be lower than the temperature of the primary molten iron at the start of the dephosphorization.

[0016] The method for producing nuggets according to the present invention is as follows: (a) The temperature T of the secondary molten iron at the end of the dephosphorization treatment f is the solidification temperature T of the secondary molten iron at the end of the dephosphorization process. m 20℃ or more higher than the (b) The slag composition at the end of the dephosphorization treatment has a basicity, which is the ratio of the CaO concentration (%CaO) to the SiO2 concentration (%SiO2) by mass, in the range of 1.0 to 4.0; (c) in the fourth step, a granulated iron manufacturing apparatus is used, the granulated iron manufacturing apparatus comprising: a granulation device for granulating the secondary molten iron into droplets; a water flow control vessel disposed at a position for receiving the droplets and accommodating cooling water; and at least one cooling water pipe connected to the water flow control vessel and supplying cooling water to the water flow control vessel, the water flow control vessel having an inclined surface that is inclined downward so that the horizontal cross-sectional area of ​​the water flow control vessel becomes narrower, and an outlet is provided below the inclined surface. This is thought to be a more preferable solution.

[0017] The nugget iron of the present invention, which advantageously solves the above problem, is characterized in that the raw material is reduced iron having a P concentration of 0.050 mass% or more, the P concentration is 0.030 mass% or less, and the particle size is 1 mm or more and 50 mm or less. Effect of the Invention

[0018] According to the method for producing nuggets of the present invention, it is possible to efficiently produce nuggets of low P concentration from reduced iron obtained from low-grade iron ore with high P concentration. Furthermore, the nuggets of the present invention satisfy the P concentration required for many steel products, that is, 0.030 mass% or less. Therefore, molten iron with a P concentration equivalent to that of steel products can be obtained simply by remelting the nuggets of the present invention.

[0019] Furthermore, since the dephosphorization process is followed by solidifying the raw materials into granules to produce iron nuggets, it becomes possible to produce steel at a separate site, rather than at a large-scale steelmaking plant, by separating the iron source manufacturing plant from the demand site. For example, it is conceivable that the raw material producing country could produce dephosphorized iron nuggets, and the steel producing country could use these dephosphorized iron nuggets to produce steel.

[0020] In addition, by carrying out the production of reduced iron and the production of nuggets according to the present invention in an iron ore producing country, it is possible to separate gangue contained in the raw material iron ore as slag. Therefore, by transporting only nuggets, the transportation amount per unit of Fe is reduced, and the transportation cost and energy consumption to the demand site can be suppressed. In addition, if the particle size of the nuggets is 1 to 50 mm, the degree of freedom of the facilities used for transportation, storage, and supply is increased when considering the transportation and storage of nuggets to the demand site and the supply to the facilities at the demand site. In addition, there is an effect of suppressing the risk of hanging in the supply hopper. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, the embodiments of the present invention will be described in detail. Note that the following embodiments are merely examples of devices and methods for embodying the technical ideas of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

[0022] The inventors considered the following when conceiving the present invention. The properties of reduced iron produced from iron ore as a raw material, such as the metallization rate and composition, vary depending on the brand of iron ore used, the type and unit of raw material component adjuster mixed, the type and unit of reducing agent, the reduction temperature, and the type of reduced iron production equipment. Table 1 shows examples of the components of reduced iron. In the example in Table 1, the P concentration in molten iron is 0.057 to 0.152 mass%, calculated by dividing the P concentration by the T.Fe (total iron) concentration. Therefore, it is difficult to achieve a P concentration at the level of steel products (0.030 mass% or less) by melting this reduced iron as it is. In addition, when the reduced iron is simply melted and dephosphorized, there are problems such as the amount of slag generated being huge due to gangue such as SiO2 contained in the reduced iron, which increases the ratio of the slag to the volume of the treatment equipment, reducing productivity, and the amount of CaO source required to ensure the amount of phosphorus removed from the molten iron, which increases the treatment cost.

[0023] [Table 1]

[0024] Therefore, the inventors have devised a process for producing nuggets of iron, in which reduced iron is first melted to obtain molten iron and at the same time at least a portion of the slag resulting from gangue is removed, an oxygen source and a lime source are supplied to the obtained molten iron to carry out dephosphorization, and then the dephosphorized molten iron is solidified into granules.

[0025] Hereinafter, an embodiment of the present invention will be described in detail. In the first step, the reduced iron is heated and melted in an electric furnace to obtain primary molten iron. The reduced iron may be, for example, one produced in an adjacent reduced iron production plant, which is held horizontally at a high temperature and used. Of course, the reduced iron may be cooled to room temperature before use. The electric furnace may be any of an arc furnace, a submerged arc furnace, and an induction melting furnace. In addition, the thermal energy supplied in the first step for melting the reduced iron, which is the solid iron source, and for heating the iron source may be not only electrical energy, but also a gaseous fuel such as natural gas or propane gas, a liquid fuel such as heavy oil, or the combustion heat of a combustible solid such as coal, metallic Al, or Si. It is preferable that these energies are renewable energies from the viewpoint of reducing CO2 emissions.

[0026] In the second step, the slag, which is the gangue of the reduced iron, is separated from the primary molten iron. For example, the molten iron is tapped into a transport container and transported to a facility for dephosphorization. When performing dephosphorization in the subsequent step, a CaO source is added to generate slag for dephosphorization, so at least a portion of the slag, which is generated with the melting of the reduced iron and contains a lot of SiO2, may be carried over in order to ensure the amount of slag and adjust the composition. Slag may be removed from the melting and heating container used in the first step using a slag dragger or the like.

[0027] In the third process, the molten iron is dephosphorized to obtain secondary molten iron. The dephosphorization reaction requires an oxygen source and a CaO source, as shown in the following formula (1). 2[P]+5 / 2·O2(g)+3CaO(s)=3CaO·P2O5(s) ···(1) Here, [P] represents phosphorus in molten iron.

[0028] Pure oxygen gas is generally used as the oxygen source for the dephosphorization process. Since the dephosphorization reaction is an exothermic reaction, it is advantageous to carry out the dephosphorization process at a low temperature. Considering that the molten iron will be solidified and turned into granular iron in the next process, it was concluded that it is advantageous to lower the molten iron temperature within a range that does not affect the process.

[0029] As a result of the investigation, it was found that sufficient dephosphorization is possible while cooling the molten iron by supplying air or an iron oxide source such as iron ore or mill scale as the oxygen source. When air is used, the heat is removed as sensible heat of the nitrogen gas contained in the air, providing a cooling effect that is greater than that when pure oxygen gas is used. When an iron oxide source is used, the heat is absorbed during the endothermic reaction in which the iron oxide source is reduced to metallic Fe, or when molten slag is formed in the form of iron oxide, providing a cooling effect that is greater than that when pure oxygen gas is used.

[0030] Secondly, calcium carbonate contained in limestone absorbs heat when it decomposes into CaO and CO2, so by using limestone as a CaO source, it becomes possible to cool molten iron. A similar cooling effect can be obtained by supplying carbonates such as raw dolomite, but if the CaO ratio in the auxiliary raw materials is low, the amount of auxiliary raw materials to be added increases, which can lead to operational issues such as an increase in the amount of slag produced and a longer time required for addition, so it is preferable to adjust the type and amount of auxiliary raw materials to be added in consideration of the desired cooling effect and stable operation.

[0031] The occurrence of spitting varies depending on the freeboard (height from the molten iron surface to the top of the vessel) of the dephosphorization vessel and the shape of the nozzle of the top blowing lance, so it is preferable to adjust the supply rate of pure oxygen or air and the height of the lance according to the operation status of the dephosphorization treatment. In addition, inert gas is preferably blown in to impart agitation to the molten iron. The inert gas is preferably blown through a porous plug installed at the bottom of the furnace or by immersing an injection lance in the molten iron. The slag composition at the end of the dephosphorization treatment is preferably in the range of 1.0 to 4.0 in terms of slag basicity, which is the ratio of CaO concentration (%CaO) to SiO2 concentration (%SiO2) by mass. The amount of SiO2-rich slag carried over to the second step and the type and amount of CaO source to be added are adjusted. If necessary, a SiO2 source such as silica stone or ferrosilicon or a CaO source such as quicklime may be added.

[0032] If the slag basicity is low, the amount of phosphorus removed during the dephosphorization process is small. If the slag basicity is high, some of the slag solidifies and adheres to the refractory material when the molten iron temperature drops, making it difficult to remove the slag after the dephosphorization process. This can lead to problems such as abnormal reactions when molten iron is charged in the next process and residual slag being mixed into the produced slag and causing the components to be out of regulated.

[0033] In addition, since a large amount of high-temperature exhaust gas is generated in such a dephosphorization process using air, exhaust heat may be recovered using a boiler or the like.

[0034] In the fourth step, the secondary molten iron after the dephosphorization is solidified into granules to obtain granular iron. Examples of methods for producing granular iron include a method in which the molten iron after the dephosphorization is allowed to flow down and collide with a refractory plate, or a method in which the molten iron that has flowed out is allowed to collide with water to form granular iron, which then falls into a water flow control vessel to obtain solidified granular iron. In this case, the diameter of the granular iron changes depending on the flow rate of the molten iron, so it is preferable to transfer the molten iron after the dephosphorization to a tundish that can keep the flow rate constant.

[0035] At this time, the temperature of the molten iron drops while the dephosphorization-treated molten iron is transported and supplied to the granulated iron manufacturing equipment. If the temperature of the molten iron after the dephosphorization is too low, some of the molten iron will solidify before all of the molten iron in the container is supplied to the granulated iron manufacturing equipment, resulting in a decrease in production yield. On the other hand, if the temperature of the molten iron after the dephosphorization is high, the thermal load when solidifying in the granulated iron manufacturing equipment will be large, and the amount of cooling water used will increase, resulting in a bottleneck in the cooling rate and a decrease in productivity, or the waiting time before granulation will be long due to the drop in the temperature of the molten iron. Thus, assuming granulation after the dephosphorization, there is a suitable range for the temperature of the molten iron after the dephosphorization. Specifically, from the viewpoint of productivity, the molten iron temperature T f The molten iron temperature T i The temperature at the end of the dephosphorization process, T f is the solidification temperature T of the secondary molten iron at the end of the dephosphorization process. m It is preferable to increase the temperature by 20° C. or more from the above temperature because this makes it possible to supply molten iron to the granulated iron manufacturing equipment with a high yield.

[0036] Here, the solidification temperature T m (℃) may be determined by either directly measuring the solidification temperature of a sample, or by estimating the C concentration of molten iron after dephosphorization based on past operational records (C concentration before dephosphorization, temperature, type of oxygen source and supply conditions, etc.) and reading it from the liquidus temperature of the Fe-C phase diagram.

[0037] The granulated iron manufacturing apparatus includes a granulation device that turns molten iron into droplets, and a water flow control vessel that is provided at a position to receive the droplets and contains cooling water. At least one cooling water pipe that supplies cooling water is connected to the water flow control vessel, which drops and solidifies the molten iron. The cooling water is discharged from the cooling water pipe to generate a water flow, thereby suppressing the generation of stagnant areas of the cooling water in the vessel. Therefore, a local temperature rise in the cooling water is suppressed, and the granulated iron can be cooled efficiently, and the granulated iron is prevented from fusing together due to insufficient cooling. In addition, the water flow control vessel has an inclined surface that is inclined downward so that the horizontal cross-sectional area of ​​the vessel becomes narrower, and the lower part of the inclined surface is the discharge outlet. By setting the inclination angle of this inclined surface to be equal to or greater than the angle of repose of the granulated iron in water, the granulated iron can be guided to the discharge outlet without being retained on the inclined surface.

[0038] When the nuggets obtained as described above are used as part of the iron source in a blast furnace or converter, the P concentration can be diluted according to the proportion used. This reduces the load in the dephosphorization process and alleviates the restrictions on the raw materials used in the blast furnace and converter.

[0039] In addition, when the granular iron obtained in this embodiment is used as an iron source in an electric furnace, a blast furnace, or a converter, there is a particle size range that is easy to use. Therefore, it is good to adjust the flow rate in the tundish to obtain a desired particle size. In addition, it is good to classify it as necessary. In general, it is easy to use those with a particle size in the range of 1 to 50 mm. If the granular iron contains granular iron with a particle size smaller than 1 mm, there is a high possibility that the conveyor for transportation will be clogged or the hopper will hang, so it is good to classify it to 1 mm or more. In addition, if the particle size exceeds 50 mm, there is a high risk of damaging equipment such as the conveyor for transportation and the hopper when a collision occurs due to falling. Therefore, it is good to reduce the flow rate in the tundish so that the particle size is 50 mm or less. In addition, it is good to classify it as necessary and remove those over 50 mm. Here, the particle size range of 1 to 50 mm can be the oversieve of a sieve with a mesh size of 1 mm and the undersieve of a sieve with a mesh size of 50 mm. EXAMPLES

[0040] Example 1 Reduced iron A shown in Table 1 was melted in a 250 ton electric furnace, the temperature was adjusted, and the slag was transferred to a ladle-shaped container. At this time, of the slag generated during melting in the electric furnace due to gangue contained in the reduced iron, approximately 10 kg / t of molten iron was transferred to the ladle-shaped container, and the remainder was transferred to a separate slag container. The ladle-shaped container was moved to the dephosphorization treatment equipment, and dephosphorization treatment was carried out by changing the types and amounts of the oxygen source and lime source supplied. The dephosphorization treatment equipment is equipped with a top gas blowing lance, a hopper for cutting out auxiliary materials, and a bottom blowing porous plug. Approximately 1 Nm of slag was blown from the top gas blowing lance. 3 Gas containing pure oxygen or air can be supplied at a rate of 10 kg / min / (min·t-molten iron). There are three auxiliary raw material discharge hoppers, each filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO3), and each can be supplied at a rate of approximately 10 kg / min. Gas can be supplied from the bottom-blowing porous plug, and in this embodiment, the gas can be supplied at a rate of approximately 0.1 Nm 3 Pure Ar gas was supplied at a rate of 1000 ml / min·t of molten iron.

[0041] The melting temperature in the electric furnace was adjusted so that the temperature of the molten iron before the dephosphorization treatment was approximately 1590°C. The timing before and after the dephosphorization treatment were defined as before and after the top gas blowing lance was lowered and when the top gas blowing lance finished rising after the treatment, respectively. Temperature measurement and sampling were carried out using a sublance for each. The sampled samples were cut and polished, and the C concentration [C] and P concentration [P] in the molten iron were evaluated using a calibration curve prepared in advance by optical emission spectroscopy. It was also possible to measure the solidification temperature of the molten iron at the timing of the sublance temperature measurement and sampling, and the solidification temperature T of the molten iron after the dephosphorization treatment was determined. m was actually measured.

[0042] The start of the dephosphorization process was determined as the start of the downward movement of the top gas blowing lance, and after the top gas blowing lance reached a predetermined height, the supply of the oxygen gas source and the addition of the auxiliary materials were started. The end of the dephosphorization process was determined as the time when the top gas blowing lance had risen to the standby position after the supply of the predetermined amount of the oxygen gas source and the auxiliary materials was completed. The period during which the dephosphorization process was completed was determined as the treatment time t f (minutes).

[0043] After the dephosphorization treatment, the ladle-shaped vessel was tilted, and the slag on the molten iron was removed by a slag dragger. A part of the removed slag was collected and subjected to chemical analysis. Then, the ladle was lifted and tilted by a crane, and the molten iron was transferred to a tundish. The molten iron was allowed to flow down from the tundish and collide with a refractory platen to form droplets. The molten iron was then dropped into a water flow control vessel and solidified to produce granular iron. The particle size of the obtained granular iron was 0.1 to 30 mm. The particle size distribution was +0.1 mm-1 mm: 17.2 mass%, +1 mm-10 mm: 31.3 mass%, +10 mm-20 mm: 38.8 mass%, and +20 mm-30 mm: 12.7 mass%. Here, +NM means that the sieve size is above the N mesh size and the sieve size is below the M mesh size.

[0044] Table 2 shows the temperature T of molten iron before and after dephosphorization. i and T f (℃), C concentration [C] i and [C] f (mass%), P concentration [P] i and [P] f (mass%), the type and amount of oxygen source and CaO source supplied, and treatment time t f The treatment time (min) and the basicity of the slag after treatment (ratio of CaO concentration (%CaO) to SiO2 concentration (%SiO2) by mass (%CaO) / (%SiO2), hereafter referred to as C / S) are shown for Treatment Nos. 1 to 5.

[0045] As shown in Table 2, in all of the invention examples, the post-treatment molten iron temperature T f is the molten iron temperature before treatment T i Lower post-treatment P concentration [P] f The comparative example is at a temperature T f is the temperature before dephosphorization T i As a result, the P concentration after treatment [P] f The temperature of the molten iron after treatment in No. 4 was high, and waiting time occurred in the granulated iron production process, resulting in a decrease in productivity. f As a result, the P concentration [P] fThe temperature of molten iron after treatment, T f is the molten iron temperature before treatment T i Lower the temperature of the molten iron after treatment T f is the solidification temperature T of molten iron m 20℃ higher than the above, P concentration after treatment [P] f The temperature was sufficiently reduced, and all of the steel was produced into nugget iron with good yield, without any decrease in productivity.

[0046] [Table 2]

[0047] Example 2 The dephosphorization treatment and the production of granulated iron were carried out using the same method as in Example 1. Table 3 shows the temperature T of the molten iron before and after the dephosphorization treatment. i and T f (℃), C concentration [C] i and [C] f (mass%), P concentration [P] i and [P] f (mass%), the type and amount of oxygen source and CaO source supplied, and treatment time t f The slag basicity C / S after treatment is shown for treatments No. 6 to 12. As shown in Table 3, treatment No. 11 had a lower slag basicity C / S than treatments No. 6 to 10, so the P concentration after treatment was higher. In treatment No. 12, the slag basicity C / S was high, and slag solidification was confirmed.

[0048] [Table 3]

[0049] Example 3 The reduced iron A shown in Table 1 was melted together with anthracite in a 250 ton electric furnace to produce molten iron containing about 2.0 mass% C, and the temperature was adjusted before transferring to a ladle-shaped container. Thereafter, dephosphorization and granulated iron production were carried out using the same methods as in Examples 1 and 2. Table 4 shows the temperature T of the molten iron before and after the dephosphorization.i and T f (℃), C concentration [C] i and [C] f (mass%), P concentration [P] i and [P] f (mass%), the type and amount of oxygen source and CaO source supplied, and treatment time t f (min), and the basicity C / S of the slag after treatment are shown for treatments No. 13 to 19. As shown in Table 4, the basicity C / S of the slag in treatment No. 18 is lower than that in treatments No. 13 to 17, so the P concentration after treatment [P] f In treatment No. 19, the slag basicity C / S was high and slag solidification was confirmed.

[0050] [Table 4]

[0051] The nuggets produced by processes No. 8-10, 12, 14-17, and 19 had a P concentration of 0.030 mass% or less. When this reduced iron was melted in an electric furnace, the P concentration of the resulting molten iron was 0.030 mass% or less. These achieved the P concentration required for steel products, and no additional dephosphorization treatment was required. In addition, the nuggets produced by processes No. 8-10, 12, 14-17, and 19 were classified to 1 mm or more and then used in electric furnaces, blast furnaces, and converters without any problems.

[0052] In this specification, the unit of mass "t" is 10 3 kg. Also, the unit of volume "Nm 3 " indicates the standard state of the gas, which is defined in this specification as 1 atm (=101325 Pa) and 0°C. In the chemical formula notation, [M] indicates that element M is dissolved in the molten iron or reduced iron. [Industrial Applicability]

[0053] According to the method for producing nuggets and nuggets of the present invention, even if the raw material is reduced iron obtained from low-grade iron ore with a high P concentration, nuggets with a low P concentration can be efficiently produced. This is industrially useful because molten iron with a P concentration equivalent to that of steel products can be obtained simply by remelting the nuggets of the present invention.

Claims

1. A first step of melting reduced iron having a P concentration of 0.050% by mass or more to obtain primary molten iron having a C concentration of 2% by mass or less; a second step of separating the primary molten iron from the slag; a third step of dephosphorizing the primary molten iron separated from the slag to obtain secondary molten iron; A fourth step of solidifying the secondary molten iron into granular iron, In the third step, an oxygen source and a CaO source are supplied to the primary molten iron to perform a dephosphorization treatment, and the temperature of the secondary molten iron at the end of the dephosphorization treatment is set to be equal to or lower than the temperature of the primary molten iron at the start of the dephosphorization treatment.

2. The temperature T of the secondary molten iron at the end of the dephosphorization treatment f is the solidification temperature T of the secondary molten iron at the end of the dephosphorization treatment. ｍ The method for producing nuggets according to claim 1, wherein the temperature is increased by 20°C or more.

3. The slag composition at the end of the dephosphorization treatment is, by mass, SiO ２ Concentration (%SiO ２ The method for producing nuggets according to claim 1 or 2, wherein the basicity, which is the ratio of the CaO concentration (% CaO) to the CaO concentration (% CaO), is in the range of 1.0 to 4.

0.

4. The fourth step uses a granulated iron manufacturing apparatus comprising: a granulation device for converting the secondary molten iron into droplets; a water flow control vessel disposed in a position to receive the droplets and containing cooling water; and at least one cooling water pipe connected to the water flow control vessel and supplying cooling water to the water flow control vessel, wherein the water flow control vessel has an inclined surface that is inclined downward so that the horizontal cross-sectional area of ​​the water flow control vessel becomes narrower, and an outlet is provided below the inclined surface.The fourth step uses a granulated iron manufacturing apparatus comprising: a granulation device for converting the secondary molten iron into droplets; a water flow control vessel disposed in a position to receive the droplets and containing cooling water; and at least one cooling water pipe connected to the water flow control vessel and supplying cooling water to the water flow control vessel, wherein the water flow control vessel has an inclined surface that is inclined downward so that the horizontal cross-sectional area of ​​the water flow control vessel becomes narrower, and an outlet is provided below the inclined surface.

5. A method for producing granular iron as described in claim 4, wherein the granular iron has a P concentration of 0.030 mass% or less and a particle size of 1 mm or more and 50 mm or less.

Images