Pre-treatment method for molten iron

Abstract

To provide a preliminary treatment method for molten iron which promotes dephosphorization treatment by securing a heat quantity necessary for melting a lime-based solvent and a heat quantity necessary for melting a cold iron source in the dephosphorization treatment, and prevents the ejection of dephosphorization slag from a converter type vessel.SOLUTION: A preliminary treatment method for molten iron comprises repeatedly performing the following steps in order: a molten iron charging step in which a first cold iron source having molten iron and FeO is charged into a converter type vessel, a desiliconization step in which the molten iron is de-siliconized, a dephosphorization step in which a second cold iron source having FeO is charged into the converter type vessel and the molten iron is dephosphorized, and a tapping step in which the molten iron subjected to the dephosphorization step is tapped out of the converter type vessel. The dephosphorized slag produced in the previous preliminary treatment for molten iron is left in the converter type vessel, and the first cold iron source and the second cold iron source are charged into the converter type vessel so that the FeO concentration of the dephosphorized slag in the dephosphorization treatment step is 20 mass% or more and 30 mass% or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for preliminary treatment of hot metal, in which hot metal is subjected to dephosphorization treatment using a converter-type vessel.

Background Art

[0002] In the production of molten steel by decarburizing and refining hot metal discharged from a blast furnace in a converter, conventionally, a steelmaking method has been developed in which preliminary dephosphorization treatment is performed at the hot metal stage to remove phosphorus in the hot metal to a certain extent and then decarburizing and refining is carried out in a converter. In this preliminary dephosphorization treatment, an oxygen source (cooled iron source) such as oxygen gas and solid iron oxide is added together with a refining agent for dephosphorization mainly composed of lime (CaO) for the treatment. That is, the phosphorus in the hot metal is oxidized by the added oxygen source to form phosphates, and the formed phosphates are absorbed and fixed by the CaO-based refining agent for dephosphorization to advance the dephosphorization reaction. The added oxygen source reacts with carbon and silicon in addition to reacting with phosphorus in the hot metal, and although the temperature of the hot metal rises due to this reaction, the dephosphorization reaction proceeds more favorably at a lower temperature thermodynamically. Therefore, a coolant is added to control the temperature of the hot metal after treatment to around 1300°C.

[0003] Here, several methods have been proposed for performing dephosphorization treatment by adding an oxygen source (cooled iron source) during preliminary dephosphorization treatment using a converter-type refining vessel. Patent Document 1 discloses, as a method for preliminary treatment of hot metal, a desiliconization treatment step of charging hot metal and a cooled iron source into a converter-type vessel and performing desiliconization treatment of the hot metal, an intermediate slag discharge step of leaving the hot metal subjected to desiliconization treatment in the converter-type vessel and discharging a part of the desiliconization slag generated in the desiliconization treatment step from the converter-type vessel, a dephosphorization treatment step of performing dephosphorization treatment of the hot metal left in the converter-type vessel in the intermediate slag discharge step, and a tapping step of tapping the hot metal subjected to dephosphorization treatment from the converter-type vessel.

[0004] Furthermore, Patent Document 2 discloses a method for charging a cold iron source into a converter-type vessel in a pre-treatment method for molten iron, based on the weight relationship with the molten iron being charged together. Patent Document 3 discloses a method for charging a cold iron source at any time between 30% and 90% of the dephosphorization refining time in a pre-dephosphorization treatment of molten iron. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2013-167015 [Patent Document 2] Japanese Patent Publication No. 2013-189714 [Patent Document 3] Japanese Patent Publication No. 2007-270238 [Patent Document 4] Japanese Patent Publication No. 2014-159629 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In the pretreatment of molten iron, the FeO concentration of the dephosphorization slag generated during the dephosphorization process is a crucial factor that affects the progress of the dephosphorization. Therefore, it is necessary to properly control the FeO concentration of the dephosphorization slag during the dephosphorization process in the pretreatment of molten iron.

[0007] However, while the technologies disclosed in Patent Documents 1 and 2 disclose the charging of cold iron sources in desiliconization and dephosphorization treatments, they do not offer any suggestions regarding the control of the FeO concentration in the dephosphorization slag. Similarly, while the technology disclosed in Patent Document 3 discloses the timing of charging of cold iron sources in dephosphorization treatments, it does not offer any suggestions regarding the control of the FeO concentration in the dephosphorization slag.

[0008] Furthermore, in conventional pretreatment of molten iron, the molten iron and cold iron source are charged at the same time before the desiliconization treatment. As a result, it has been difficult to simultaneously secure the heat required to melt the lime-based flux and the heat required to melt the cold iron source during the dephosphorization treatment.

[0009] The present invention has been made in view of the above circumstances, and its object is to provide a pretreatment method for molten iron that promotes the dephosphorization process and prevents slag from ejecting from the converter-type container during the dephosphorization process. [Means for solving the problem]

[0010] [1] A method for pre-treating molten iron, comprising: a molten iron charging step of charging a first cold iron source containing molten iron and FeO into a converter-type vessel; a desiliconization step of performing a desiliconization treatment on the molten iron; a dephosphorization step of charging a second cold iron source containing FeO into the converter-type vessel and performing a dephosphorization treatment on the molten iron; and a tapping step of tapping the molten iron that has undergone the dephosphorization treatment from the converter-type vessel, wherein the dephosphorization slag generated during the previous molten iron pre-treatment is left in the converter-type vessel, and the first cold iron source and the second cold iron source are charged into the converter-type vessel such that the FeO concentration of the dephosphorization slag in the dephosphorization treatment step is 20% by mass or more and 30% by mass or less. [2] The pretreatment method for molten iron according to [1], wherein, prior to the dephosphorization treatment step, an intermediate slag discharge step is performed in which the molten iron that has undergone the desiliconization treatment is left in the converter-type vessel and at least a portion of the desiliconized slag generated in the desiliconization treatment step is discharged from the converter-type vessel. [3] The amount of the first cold iron source and the second cold iron source to be charged is determined based on the FeO concentration of the slag after dephosphorization treatment, the FeO content of the first cold iron source and the FeO content of the second cold iron source, such that the ratio of the amount of the second cold iron source to the total amount of cold iron source charged, which is the total amount of the first and second cold iron sources to be charged, is 5% by mass or more and 30% by mass or less, as described in [1] or [2]. [4] The method for pre-treating molten iron according to any one of [1] to [3], wherein the first cold iron source and the second cold iron source have an FeO content of 1% by mass or more. [5] The method for pre-treating molten iron according to any one of [1] to [4], wherein the first cold iron source and the second cold iron source have an FeO content of 1% by mass or more and are recovered products produced and recovered in the ironmaking process. [Effects of the Invention]

[0011] According to the present invention, in the dephosphorization process, it is possible to accelerate the dephosphorization process and prevent the slag from being ejected from the converter-type container during the dephosphorization process. [Brief explanation of the drawing]

[0012] [Figure 1] This diagram shows a schematic configuration of an example of a converter-type vessel. [Figure 2] This diagram shows a schematic configuration for explaining the pretreatment method of molten iron in order of steps. [Figure 3] This figure shows the relationship between the FeO concentration in dephosphorized slag and the phosphorus concentration in molten iron after dephosphorization. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 shows a schematic configuration diagram of an example of a converter-type vessel used in a molten iron pretreatment method. Figure 2 shows a schematic configuration diagram for explaining the molten iron pretreatment method in order of steps.

[0014] In the method for preliminary treatment of hot metal according to the present invention, as an example of a converter-type refining furnace, a converter-type vessel 1 capable of top-bottom blowing as shown in FIG. 1 is used. In the converter-type vessel 1, the top blowing is performed by blowing oxygen gas 12 toward the hot metal 9 from the tip of a top blowing lance 2 that can move up and down. Here, industrial pure oxygen is used as the oxygen gas 12. Further, the bottom blowing is performed using a bottom blowing tuyere 3 provided at the bottom of the converter-type vessel 1. As the bottom blowing gas 13, a gas containing oxygen gas, or an inert gas such as argon gas or nitrogen gas is common, but it may also be a gas having a function of strengthening the stirring of the hot metal 9 by blowing it into the hot metal and promoting the dissolution of the cold iron source, and further a gas having a function of blowing a fluxing agent into the hot metal together with a conveying gas. The hot metal 9 after refining is discharged from the tapping port 4.

[0015] In the present invention, for the refining of the hot metal 9, two or more converter-type vessels 1 are used, and at least one of the converter-type vessels 1 is used for the preliminary treatment method of the hot metal according to the present invention, and the remaining one is used for the decarburization refining of the hot metal that has been preliminarily treated by applying the method of the present invention. For example, it is preferable to perform the preliminary treatment in the converter-type vessel 1 for hot metal pretreatment, and transfer the hot metal after the preliminary treatment to the converter-type vessel for decarburization refining to perform the decarburization treatment.

[0016] The method for preliminary treatment of hot metal according to the present invention using the converter-type vessel 1, as shown in the flow of FIG. 2, implements each process in the order of the hot metal charging process (A), the desiliconization treatment process (B), the dephosphorization treatment process (C), and the tapping process (D), and by repeatedly implementing these processes in the same converter-type vessel 1, efficient preliminary treatment of hot metal becomes possible.

[0017] [[ID=##**WARN**## 11]] Here, for one implementation of the processes (A) to (D), the charging amount of the cold iron source charged into the converter-type vessel 1 (hereinafter referred to as the "total cold iron source charging amount") has conventionally been assumed to be charged at one time at the stage of the hot metal charging process (A), and is determined based on the results of a predetermined calculation or the like targeting the slag after the dephosphorization treatment (hereinafter simply referred to as the "slag after dephosphorization treatment") generated during the previous hot metal preliminary treatment.

[0018] In addition, when charging all the cold pig iron sources at once before the desiliconization treatment step (B) based on the total amount of cold pig iron sources charged, although both the amount of heat required for melting the lime-based flux 16 charged during the desiliconization treatment step (B) and the amount of heat required for melting all the cold pig iron sources are needed, it becomes difficult to secure such amount of heat, and thus a limit is imposed on the supply amount of the cold pig iron sources. As a result, there is also a problem that insufficient charging of the cold pig iron sources occurs, and as a result, the phosphorus concentration of the hot metal 9 that has undergone the dephosphorization treatment step (C) also increases.

[0019] In contrast, in the present invention, with respect to the total amount of cold pig iron sources calculated based on the slag after the dephosphorization treatment, it is divided into the charging amount of the first cold pig iron source 11a charged into the converter-type vessel 1 during the hot metal charging step (A) and the charging amount of the second cold pig iron source 11b charged into the converter-type vessel 1 during the dephosphorization treatment step (C), and the first cold pig iron source 11a and the second cold pig iron source 11b are characterized in that they contain FeO. That is, in the present invention, among the respective steps (A) to (D) (see FIG. 2) repeatedly performed, the first cold pig iron source 11a containing FeO is charged during the hot metal charging step (A), and the second cold pig iron source 11b containing FeO is charged during the dephosphorization treatment step (C).

[0020] And in the present invention, for the purpose of adjusting the FeO concentration of the slag during the dephosphorization treatment in the dephosphorization treatment step (C), before performing the hot metal charging step (A), the charging amounts of the first cold pig iron source 11a and the second cold pig iron source 11b are calculated. Specifically, the charging amounts of the first cold pig iron source 11a and the second cold pig iron source 11b are determined such that the FeO concentration of the slag during the dephosphorization treatment in the dephosphorization treatment step (C) is 20% by mass or more and 30% by mass or less. Also, the "slag during the dephosphorization treatment" here means the slag at the end point of the dephosphorization treatment step (C). That is, the "FeO concentration of the slag during the dephosphorization treatment" means the FeO concentration of the slag at the end point of the dephosphorization treatment step (C). And the calculation of the FeO concentration of the slag during the dephosphorization treatment may be calculated by a conventionally used calculation formula (such as formula (6) in Patent Document 4).

[0021] In this case, the respective amounts charged to the first cold iron source 11a and the second cold iron source 11b may be determined based on the FeO concentration of the slag 17 after dephosphorization treatment, the FeO content of the first cold iron source 11a, and the FeO content of the second cold iron source 11b, such that the ratio of the amount charged to the second cold iron source 11b to the total amount of cold iron sources charged, which is the total amount of the first cold iron source 11a and the second cold iron source 11b, is 5% by mass or more and 30% by mass or less.

[0022] The molten iron charging process (A) shown in Figure 2 will be explained. In the molten iron charging process (A), new molten iron 9 is charged from the charging ladle 14 while the dephosphorized slag 17 generated in the pretreatment of the previous molten iron remains in the converter-type vessel 1, or a first cold iron source 11a such as iron scrap is charged before the molten iron 9 is charged. The first cold iron source 11a to be charged in advance may be iron scrap as specified in the "Unified Standards for Inspection and Acceptance of Iron Scrap" of the Japan Iron Source Association, or it may be iron-based, such as directly reduced iron or cold iron.

[0023] The amount of the first cold iron source 11a charged in the molten iron charging process (A) (hereinafter referred to as "amount of the first cold iron source charged") may be calculated before the molten iron charging process (A) based on the calculated value of the FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, and the amount and FeO content of the second cold iron source 11b charged into the converter-type vessel 1 during the dephosphorization process (C).

[0024] The desiliconization process (B) shown in Figure 2 will be described below. In the desiliconization process (B), the converter-type vessel 1 is placed upright, and oxygen gas 12 is supplied to the molten iron 9 via the top-blowing lance 2 to perform desiliconization. In this desiliconization process, the silicon source 15 contained in the hopper 5 and the lime-based fluxing agent 16 contained in the hopper 7 are charged into the converter-type vessel 1 via the chute 6 and chute 8, respectively. In addition, carbon material which serves as a heat source, and iron oxide which serves as a silicon source or oxygen source are also charged in the same manner. As an oxygen source for the desiliconization process, from the viewpoint of dissolving a large amount of the first cold iron source 11a, it is preferable to use only oxygen gas 12 without using iron oxide which has a large heat absorption capacity.

[0025] In this desiliconization process, silicon contained in the molten iron 9, or silicon contained in the silicon source 15 and the first cold iron source 11a that migrates into the molten iron upon dissolution, is desiliconized by reacting with the oxygen source (Si + O2 → SiO2), thus helping to improve the reaction efficiency in the subsequent dephosphorization process. Oxidation heat is generated during this desiliconization reaction, and this oxidation heat raises the molten iron temperature, promoting the dissolution of the cold iron source 11 in the molten iron.

[0026] The dephosphorization process (C) shown in Figure 2 will be explained. After the desiliconization process (B), a lime-based flux is added to the molten iron remaining in the converter-type vessel 1, and oxygen blowing, which serves as an oxygen source, is performed to dephosphorize the molten iron. The phosphorus in the molten iron is oxidized by the oxygen in the supplied oxygen source to become phosphorus oxide (P2O5), and this phosphorus oxide is stably incorporated into the slag produced by the lamination of the lime-based flux, thereby promoting the dephosphorization of the molten iron.

[0027] In the present invention, in the dephosphorization process (C), a second cold iron source 11b containing FeO is charged into the converter-type vessel 1. The amount of the second cold iron source 11b charged (hereinafter referred to as "amount of second cold iron source charged") may be calculated before the molten iron charging process (A) based on the calculated value of the FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the amount and FeO content of the first cold iron source 11a charged into the converter-type vessel 1 during the molten iron charging process (A), and the FeO content of the second cold iron source 11b.

[0028] Figure 3 shows the relationship between the FeO concentration of the slag during dephosphorization and the phosphorus concentration of the molten iron after dephosphorization. As shown in Figure 3, if the FeO concentration of the slag during dephosphorization is low, the dephosphorization of the molten iron becomes difficult to proceed, and the phosphorus concentration of the molten iron after dephosphorization becomes high. On the other hand, if the FeO concentration of the slag during dephosphorization is high, the slag becomes peroxidized, causing the slag to erupt from the converter-type vessel and resulting in a shutdown of operations.

[0029] In this invention, the FeO concentration of the slag 20 during the dephosphorization process (C) is adjusted to an appropriate value by charging the first cold iron source 11a in the molten iron charging process (A) and charging the second cold iron source 11b into the converter-type vessel 1 in the dephosphorization process (C). Specifically, the FeO concentration of the slag 20 during the dephosphorization process (C) is adjusted to 20% by mass or more and 30% by mass or less by charging the first cold iron source 11a and the second cold iron source 11b into the converter-type vessel 1. The amounts of the first cold iron source 11a and the second cold iron source 11b to be charged may be determined such that the ratio of the amount of the second cold iron source charged to the total amount of cold iron source charged is 5% by mass or more and 30% by mass or less.

[0030] Specifically, the amount of the first cold iron source charged may be determined based on the calculated FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the FeO content of the second cold iron source 11b, and the amount of the second cold iron source 11b charged in the dephosphorization process (C). In this case, the amount of the first cold iron source charged may be determined such that the ratio of the amount of the second cold iron source charged to the total amount of cold iron source charged is 5% by mass or more and 30% by mass or less.

[0031] In this case, the amount of the first cold iron source charged may be determined by adjusting the calculated value of the FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the FeO content of the second cold iron source 11b, and the amount of the second cold iron source 11b charged in the dephosphorization process (C), so that the FeO concentration of the dephosphorized slag 20 during the dephosphorization process (C) is 20% by mass or more and 30% by mass or less.

[0032] The amount of the second cold iron source charged may be determined based on the calculated FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the amount of the first cold iron source 11a charged in the molten iron charging process (A), and the FeO content of the second cold iron source 11b. In this case, the amount of the second cold iron source charged may be determined such that the ratio of the amount of the second cold iron source charged to the total amount of cold iron source charged is between 5% by mass and 30% by mass.

[0033] In this case, the amount of the second cold iron source charged may be determined by adjusting the calculated value of the FeO concentration in the dephosphorized slag 17 generated during the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the amount of the first cold iron source 11a charged in the molten iron charging process (A), and the FeO content of the second cold iron source 11b so that the amount of the second cold iron source 11b charged becomes the optimal value, so that the FeO concentration of the slag 20 during the dephosphorization process (C) is 20% by mass or more and 30% by mass or less.

[0034] Furthermore, after determining the first and second cold iron source charges, the amount of oxygen gas 12 added may be adjusted to precisely adjust the FeO concentration of the slag 20 during the dephosphorization process (C) to 20% by mass or more and 30% by mass or less. In this case, during the desiliconization process (B) and the dephosphorization process (C), oxygen gas 12 with an adjusted amount added may be added via the top-blowing lance 2 along with a dephosphorization refining agent mainly composed of lime (CaO). At this time, the FeO concentration of the slag 20 during the dephosphorization process may be further adjusted by adjusting the height of the top-blowing lance 2 and the injection speed of the oxygen gas 12.

[0035] Therefore, in the dephosphorization process (C), the FeO concentration of the slag 20 used for dephosphorization of molten iron can be ensured, and the dephosphorization of molten iron can be efficiently promoted. Furthermore, by adjusting the FeO concentration of the slag 20 used for dephosphorization, it is possible to prevent deterioration of the phosphorus concentration of the molten iron in the dephosphorization process (C) and to prevent problems such as the slag being ejected from the converter-type container during dephosphorization.

[0036] Furthermore, in order to accurately adjust the FeO concentration of the slag 20 during the dephosphorization process (C), the FeO concentration of the slag 20 during the dephosphorization process (C) may be measured, and the additional amount of the third cold iron source 11c containing FeO to be charged into the converter-type container 1 may be calculated based on the measured FeO concentration of the slag 20 during the dephosphorization process. Then, the third cold iron source 11c may be charged into the converter-type container 1 based on the calculated additional amount of the third cold iron source 11c (hereinafter referred to as the "amount of third cold iron source charged").

[0037] The FeO content of the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c, which contain FeO, is preferably 1% by mass or more. This is because a FeO content of at least 1% by mass or more promotes the dephosphorization reaction of molten iron. Furthermore, by individually recovering the by-products containing FeO from among the by-products generated in the ironmaking process and using them as the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c, it becomes possible to supply the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c at a low cost.

[0038] The tapping process (D) shown in Figure 2 will now be explained. In the tapping process (D), when the phosphorus concentration in the molten iron has decreased to a predetermined value after the dephosphorization process (C), the converter-type container 1 is tilted toward the side where the tapping port 4 is installed, and the molten iron in the converter-type container 1 is tapped into a molten iron holding container (not shown).

[0039] As described above, the pretreatment method for molten iron according to the present invention can accelerate the dephosphorization of molten iron in the dephosphorization process. Furthermore, since the FeO concentration of the slag during the dephosphorization process can be adjusted, it is possible to prevent deterioration of the phosphorus concentration of the molten iron in the dephosphorization process (C) and to prevent problems such as the ejection of slag from the converter-type container during the dephosphorization process.

[0040] Herein, the molten iron pretreatment method according to the present invention can be applied to various smelting processes as long as a converter-type vessel is used for the molten iron pretreatment. For example, the molten iron pretreatment method shown in this embodiment may also include an intermediate slag removal step.

[0041] In this case, specifically, an intermediate slag discharge step may be performed before the dephosphorization step (C), in which the molten iron 9 that has undergone desilicate treatment in the desilicate treatment step (B) is left in the converter-type vessel 1, and at least a portion of the desilicated slag generated in the desilicate treatment step (B) is discharged from the converter-type vessel 1.

[0042] By performing an intermediate slag discharge step after the desiliconization process (B), the low-basicity desiliconized slag containing a large amount of SiO2 generated during the desiliconization process is discharged from the converter-type container 1. Intermediate slag discharge of the desiliconized slag is effective in reducing the amount of lime-based flux used to adjust the slag basicity to an appropriate level in the subsequent dephosphorization process. Furthermore, in the case of a pre-treatment method for molten iron in which a large amount of dephosphorized slag generated during the previous molten iron pre-treatment remains in the furnace while the new molten iron is desiliconized, the phosphoric acid concentration in the desiliconized slag becomes higher than conventional methods because the desiliconization process is performed to prevent rephosphorization from the slag to the molten iron. Therefore, by performing an intermediate slag discharge step, the amount of phosphoric acid contained in the desiliconized slag can be reduced, and the dephosphorization process in the next dephosphorization process (C) can be further accelerated.

[0043] Furthermore, cold iron sources containing FeO and by-products (recovered materials) recovered in the ironmaking process may be pre-sorted based on their FeO content and applied to the pre-treatment method for molten iron according to the present invention. Examples of by-products (recovered materials) recovered in the ironmaking process that also contain FeO include sintered powder and the like.

[0044] In the molten iron pretreatment method according to the present invention, it is preferable that the charging amounts of the first cold iron source 11a and the second cold iron source 11b are determined such that the ratio of the charging amount of the second cold iron source 11b to the total charging amount of the first cold iron source 11a and the second cold iron source 11b is 5% by mass or more and 30% by mass or less.

[0045] In other words, by adjusting the amount of the first cold iron source 11a charged to be greater than the amount of the second cold iron source 11b charged, it becomes possible to smoothly charge these cold iron sources in both the molten iron charging process (A) and the dephosphorization process (C). Specifically, in the molten iron charging process (A), in which the converter-type vessel 1 can be tilted, a large amount of the first cold iron source 11a can be charged, while in the dephosphorization process (C), in which the upright state is maintained, an appropriate amount of the second cold iron source 11b can be charged, considering charging only from above.

[0046] Furthermore, by charging a large amount of the first cold iron source 11a during the molten iron charging process (A), the dissolution of the first cold iron source 11a can be promoted in the subsequent desiliconization process (B) by supplying a large amount of oxygen gas 12 through the upper blowing lance 2. Therefore, in the subsequent dephosphorization process (C), the dissolution of a small amount of the second cold iron source 11b can be carried out smoothly, further promoting the dephosphorization process.

[0047] Therefore, by adjusting the amount of the first cold iron source 11a charged to be greater than the amount of the second cold iron source 11b charged, the charging of the cold iron sources can be carried out smoothly in both the molten iron charging process (A) and the dephosphorization process (C), and the melting of all the cold iron sources (first cold iron source 11a and second cold iron source 11b) can be further accelerated at the stage of the dephosphorization process (C). This further accelerates the dephosphorization process.

[0048] Here, the term "cold iron source" in this invention may be interpreted in a broad sense to include iron scrap, reduced iron, and iron oxide, etc. [Examples]

[0049] Next, the results of an implementation of the molten iron pretreatment method according to the present invention, performed by charging molten iron into a converter-type vessel, will be described. In the example, the molten iron pretreatment method was implemented by changing the ratio of the amount of the first cold iron source charged and the amount of the second cold iron source charged to the total amount of cold iron source charged, which is calculated based on the dephosphorized slag generated during the previous molten iron pretreatment. The first and second cold iron sources used were cold iron sources with an FeO content of 1% by mass or more. The results of the implementation of the example are shown in Table 1.

[0050] [Table 1]

[0051] As shown in Table 1, in the examples, the FeO concentration (mass%) of the slag during the dephosphorization process, the phosphorus concentration (mass%) of the molten iron after dephosphorization, and the incidence rate (%) of problems such as ejection from the converter-type vessel were confirmed.

[0052] Comparative Example 1 shows a conventional example where the cold iron source is charged all at once during the molten iron charging process. That is, in Comparative Example 1, the amount of cold iron source charged during the molten iron charging process (shown as "100" in Table 1) corresponds to the total amount of cold iron source charged. It also shows that the FeO concentration of the slag during the dephosphorization treatment was 15% by mass. The results of the following examples (Inventive Examples 1-2, Comparative Examples 2-5) were evaluated based on the results of Comparative Example 1.

[0053] In Comparative Example 2, by setting the charging ratio of the second cold iron source to the total cold iron source charging to more than 30% by mass and less than or equal to 40% by mass, the FeO concentration of the slag during dephosphorization treatment was more than 30% by mass and less than or equal to 40% by mass, and the phosphorus concentration of the molten iron after the dephosphorization treatment process was reduced by 0.003% by mass. However, because the FeO concentration of the slag during dephosphorization treatment increased compared to the conventional example (Comparative Example 1), the incidence rate of problems such as spurting from the converter-type vessel was 30%.

[0054] In Comparative Example 3, by setting the charge ratio of the second cold iron source to the total cold iron source charge to less than 5% by mass, the FeO concentration of the slag during dephosphorization treatment became 18% by mass, which was a slight increase in FeO concentration compared to the conventional example (Comparative Example 1). However, the phosphorus concentration of the molten iron after the dephosphorization treatment process was only slightly reduced by 0.001% by mass.

[0055] Comparative Examples 4 and 5 show the results of charging the cold iron source in a single step during the molten iron charging process, similar to the conventional example (Comparative Example 1). As shown in Table 1, the FeO concentration of the slag during the dephosphorization treatment was 5% by mass in Comparative Example 4 and 10% by mass in Comparative Example 5, which are lower values ​​than in the conventional example (Comparative Example 1). Consequently, the phosphorus concentration of the molten iron after the dephosphorization treatment increased by 0.015% by mass in Comparative Example 4 and by 0.010% by mass in Comparative Example 5.

[0056] In Invention Example 1, by setting the charging ratio of the second cold iron source to the total cold iron source charging to 10% by mass or more and 30% by mass or less, the FeO concentration of the slag during dephosphorization treatment could be adjusted to 20% by mass or more and 30% by mass or less. As a result, the phosphorus concentration of the molten iron after the dephosphorization treatment process could be reduced by 0.003% by mass, and problems during the dephosphorization treatment process could be prevented.

[0057] In Invention Example 2, the FeO concentration of the slag during dephosphorization was adjusted to 25% by mass by setting the charging ratio of the second cold iron source to the total cold iron source charging to 5% by mass or more and 20% by mass or less. As a result, the phosphorus concentration of the molten iron after the dephosphorization process could be reduced by 0.003% by mass, and problems during the dephosphorization process could be prevented.

[0058] In Invention Example 3, by setting the charging ratio of the second cold iron source to the total cold iron source charging to 15% by mass or more and 30% by mass or less, the FeO concentration of the slag during dephosphorization treatment could be adjusted to 22% by mass or more and 30% by mass or less. As a result, the phosphorus concentration of the molten iron after the dephosphorization treatment process could be reduced by 0.003% by mass, and problems during the dephosphorization treatment process could be prevented. [Explanation of symbols]

[0059] 1. Converter-type container 2. Upward-blowing lance 3 Bottom-blown tuyere 4. Hot water outlet 5, 7 Hopper 6, 8 shots 9. Molten iron 11a 1st cold iron source 11b 2nd cold iron source 11c Third cold iron source 12. Oxygen gas 13 Bottom-blowing gas 14 Charging pot 15 Silicon source 16 Lime-based media 17. Slag after dephosphorization treatment 20 Slag during dephosphorization treatment

Claims

1. A molten iron charging process involves charging a first cold iron source containing molten iron and FeO into a converter-type vessel, A desiliconization process is performed to desiliconize the molten iron, A dephosphorization process involves charging a second cold iron source containing FeO into the converter-type vessel and performing a dephosphorization treatment on the molten iron, A step of discharging the molten iron that has undergone the dephosphorization treatment from the converter-type vessel, A pretreatment method for molten iron, which involves repeatedly performing the following steps in sequence: The slag generated during the previous molten iron pretreatment is left in the aforementioned converter-type vessel after dephosphorization. The first cold iron source and the second cold iron source are charged into the converter-type container such that the FeO concentration of the slag during the dephosphorization process is 20% by mass or more and 30% by mass or less. The respective amounts of the first cold iron source and the second cold iron source charged are determined based on the FeO concentration of the slag after dephosphorization treatment, the FeO content of the first cold iron source, and the FeO content of the second cold iron source, such that the ratio of the amount charged to the second cold iron source to the total amount of cold iron source charged, which is the total amount of the first and second cold iron sources charged, is 5% by mass or more and 30% by mass or less. Pre-treatment method for molten iron.

2. The method for pre-treating molten iron according to claim 1, wherein, prior to the dephosphorization treatment step, an intermediate slag discharge step is performed in which the molten iron that has undergone the desiliconization treatment is left in the converter-type vessel and at least a portion of the desiliconized slag generated in the desiliconization treatment step is discharged from the converter-type vessel.

3. The method for pre-treating molten iron according to claim 1 or 2, wherein the first cold iron source and the second cold iron source have an FeO content of 1% by mass or more.

4. The method for pre-treating molten iron according to claim 1 or 2, wherein the first cold iron source and the second cold iron source are recovered materials produced and recovered in the ironmaking process, having an FeO content of 1% by mass or more.

5. The method for pre-treating molten iron according to claim 3, wherein the first cold iron source and the second cold iron source are recovered materials produced and recovered in the ironmaking process, having an FeO content of 1% by mass or more.

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