JP6631265B2 - Method for producing derinsed slag - Google Patents

Description

  The present invention relates to a method for producing dephosphorized slag suitable for use as a raw material for phosphoric acid fertilizer and for producing molten iron having a P concentration similar to that of molten iron produced in a normal blast furnace.

  In Japan, the amount of rainfall is high, so minerals flow out of the soil and the soil is easily acidified. For this reason, basic phosphate fertilizers that increase not only the concentration of phosphoric acid in soil but also the pH of soil at the same time are widely used as phosphate fertilizers used for growing plants. At present, fused phosphate fertilizer containing a large amount of alkali is used as a basic phosphate fertilizer. The fused phosphorus fertilizer is manufactured by melting phosphate ore, magnesium oxide, and the like in an electric furnace, mixing them, and quenching them with a jet stream.

  In the past, Thomas Phosphorus fertilizer was present as a basic phosphate fertilizer other than fused Phosphorus fertilizer. Non-Patent Document 1 and the like describe a method for producing Thomas phosphorus fertilizer as follows. High P hot metal (P: 1.7 to 2.1% by mass, C: 3.5 to 3.8% by mass, Si: 0.2 to 0.4% by mass) is charged into a converter, and flux is applied. Liquid steel (P: 0.08% by mass or less, C: 0.10% by mass or less) is produced by adding a certain lime and performing refining by blowing air.

In the method of Non-Patent Document 1, refining is performed by raising the furnace temperature using the oxidation reaction of carbon and phosphorus in the hot metal, and the slag generated after the refining has a basicity (CaO / SiO ₂ mass). Ratio) is about 4 to 6, and the concentration of phosphoric acid is about 15 to 20% by mass. Therefore, this slag has been used as a raw material of a phosphate fertilizer (Thomas phosphorus fertilizer) having a high fertilizer effect.

  Therefore, Thomas Phosphorus fertilizer has been used as a raw material for basic phosphate fertilizer, but currently, a common top-bottom blow converter method as a steelmaking method is advantageous in terms of cost and energy. Since it is possible to obtain molten steel with low impurity concentration, the initial impurity concentration of the hot metal to be refined is high, and the impurity concentration of the molten steel after refining is also high. .

  On the other hand, currently, about 0.1% by mass of phosphorus contained in blast furnace molten iron as impurities is removed by oxidation by adding flux and blowing oxygen in the steelmaking process, and is transferred to steelmaking slag as phosphoric acid and discharged. I have. The phosphoric acid content in the steelmaking slag is considered as a useful phosphate fertilizer resource, and the phosphoric acid content in the steelmaking slag is concentrated to produce slag with a high phosphoric acid concentration (hereinafter, high phosphate slag). Attempts have been made to use the slag as a raw material for phosphate fertilizer.

  For example, Patent Literature 1 discloses a silicate phosphate fertilizer composed of steelmaking slag collected in a hot metal pretreatment step. Since the steelmaking slag is not intentionally concentrated in phosphoric acid concentration, the phosphoric acid concentration is about 1 to 4% by mass, which is not a sufficient concentration as a phosphate fertilizer. Fertilizer effect as fertilizer is low.

In addition, Patent Document 2 discloses a method for reducing the steelmaking slag to produce high-P hot metal (P: 0.5 to 3% by mass, C: approximately 4.5% by mass, Si: approximately 0.05% by mass). In a converter, a flux having a basicity of 2 to 8 was added, an iron oxide source was added, or oxygen was blown in to perform a dephosphorization treatment (temperature after treatment: 1550 ° C. or more), and molten steel (C: 1 mass) % or less, P: 0.1 to 0.2 mass%) and slag (basicity: 5~6, P ₂ O ₅ concentration: describes a method of producing a 10 to 30 mass%).

The method of Patent Document 2, compared to Thomas steelmaking, because of the low Si concentration of the molten iron to be processed, to reduce the generation amount of slag, thereby improving the P ₂ O ₅ concentration of the slag. When the P ₂ O ₅ concentration of the slag increases, the melting point of the slag decreases. Therefore, according to the method of Patent Document 2, it is possible to produce a high-phosphorus slag with good slagging properties without using fluorite. However, the fertilizer effect is unknown because the formation of the phosphate-containing mineral phase is not intentionally controlled.

  In Patent Documents 3 and 4, oxygen gas is injected together with powdered CaO onto high-P hot metal (P: 0.5 to 3% by mass) produced by reducing and melting steelmaking slag to reduce the basicity of the slag to 1. A method is described in which the phosphoric acid concentration is increased to 15% by mass or more by adjusting the slag as a phosphoric acid fertilizer by adjusting the concentration to 5 or more.

  By the way, when using high phosphate slag as a raw material for phosphate fertilizer, it is important that the concentration of phosphoric acid in the slag is high and that the fertilizer effect as a phosphate fertilizer is high. Therefore, when manufacturing phosphate fertilizer raw materials, by modifying the production conditions such as components, compositions, etc., and the melting temperature and cooling method, the crystal state and mineral phase of phosphorus in slag and fertilizers can be reduced. Control to increase the fertilizer effect.

  For example, in the above-mentioned fused phosphorus fertilizer, the fertilizer raw material discharged in a molten state from the electric furnace is quenched by a jet water stream to intentionally convert the phosphorus-containing mineral phase in the raw material into amorphous, that is, glass. Increase the fertilizer effect. Hereinafter, the phosphorus-containing mineral phase refers to a phase in which phosphorus is concentrated in each mineral phase in the fertilizer.

In the above-mentioned Thomas phosphorus fertilizer, the main components are CaO, P ₂ O ₅ , and SiO ₂ , and the phosphorus-containing mineral phase is a Ca ₃ (PO ₄ ) ₂ -Ca ₂ SiO ₄ solid solution phase, 5CaO · SiO 2 _{It is a 2} · P ₂ O ₅ phase (silico carnotite) or a 7CaO · 2SiO ₂ · P ₂ O ₅ phase (Nagelschmittite) (hereinafter may be collectively referred to as a “solid solution phase”). It is generally known that these mineral phases have a high fertilizer effect.

Since the methods described in Patent Documents 1 and 2 do not intentionally control the phosphorus-containing mineral phase, it is unclear about the phosphorus-containing mineral phase and its fertilizer effect. On the other hand, the method described in Patent Document 3 enhances the fertilizer effect by controlling the phosphorus-containing mineral phase to a phase similar to that of Thomas phosphorus fertilizer by controlling the composition and the cooling rate, that is, a solid solution phase. Further, the method described in Patent Document 4 controls the composition and the cooling rate, and intends the phosphorus-containing mineral phase to have an α-Ca ₃ (PO ₄ ) ₂ phase or a Ca ₄ O (PO ₄ ) ₂ crystal structure. The effect of fertilizer is enhanced by making it suitable.

Japanese Patent No. 5105322 JP-A-11-158526 JP 2015-140473 A JP 2015-140294 A

Japanese Journal of Soil Fertilizer Science, Volume 13, p93

  Conventionally, as described above, a phosphoric acid fertilizer has been produced by producing high-P hot metal from a phosphoric acid content in steelmaking slag as a raw material and dephosphorizing it. However, since iron oxide is inevitably generated during the dephosphorization treatment of the high-P hot metal, the incorporation of iron oxide into the phosphate fertilizer is inevitable. Therefore, it is desired to develop a phosphate fertilizer having a high phosphoric acid concentration and a high fertilizer effect even if it contains iron oxide.

  However, stable production of phosphate fertilizer or its raw material at low cost is required. To that end, it is necessary to develop a dephosphorization method for stably producing low-cost dephosphorized slag as a raw material.Furthermore, by appropriately processing the dephosphorized slag, a phosphate fertilizer raw material with a high fertilizer effect is required. There is a need to develop a manufacturing method. Further, as one of the measures, it is necessary that the hot metal after dephosphorization can be used without waste as a raw material in a normal steelmaking process without requiring special treatment. For that purpose, it is necessary to make the hot metal after dephosphorization have a phosphorus concentration (0.1 to 0.15 mass%) of the same level as the hot metal that is tapped from the blast furnace.

For example, the above-mentioned Thomas Phosphorus fertilizer has a high basicity expressed as a mass ratio of CaO and SiO ₂ of 4 or more and has a high fertilizer effect, but the basicity is usually less than 4 in many cases. When steelmaking slag is used as a fertilizer raw material, a solid solution phase does not always precipitate, and it is unknown whether a high fertilizer effect can be obtained.

In order to obtain the desired slag with a high P ₂ O ₅ concentration this time, if a dare to use Thomas Steelmaking for hot metal with a high P concentration, a converter and exhaust gas treatment equipment of a required scale are required. In other words, since a large amount of CO gas is generated by the oxidation reaction of C in the hot metal, a converter having a large furnace volume is required so that the forming and sloping of the generated slag do not hinder the operation, and A CO gas treatment facility corresponding to the converter is also required.

As described above, when the Thomas Steelmaking method is used for hot metal having a high P concentration, an increase in equipment costs cannot be avoided. In addition, in order to obtain a desired slag having a high P ₂ O ₅ concentration, the same problem as in the case of using the Thomas Steelmaking method occurs when the method of Patent Document 2 is used.

Furthermore, in the method of Patent Document 2, a dephosphorization treatment is performed by adding a flux of about 2 to 8 and adding an iron oxide source or blowing gaseous oxygen, thereby obtaining molten steel (C: 1% by mass or less, P: 0.1 to 0.2 mass%) and slag (basicity: 5~6, P ₂ O ₅ concentration: manufactures 10 to 30 mass%). However, since the molten steel is manufactured, the temperature after the treatment needs to be 1550 ° C. or higher, and a high basicity slag must be used in such a dephosphorization treatment. In that case, it is necessary to raise the furnace temperature by utilizing the carbon oxidation reaction. However, if the furnace temperature is increased, refractory wear will be severe, and the life of the furnace will be shortened. Costs rise.

  Further, in order to make hot metal after dephosphorization into hot metal that can be easily reused in a steelmaking process, it is necessary to reduce the phosphorus concentration to about the same level as ordinary hot metal. If hot metal with a high phosphorus concentration in the hot metal is returned to the steelmaking process as it is, an increase in the P concentration of the hot metal used in the steelmaking process is unavoidable, and as a result, the amount of auxiliary materials used in the steelmaking process increases, resulting in cost reduction. Increase. In the steelmaking process, since a large amount of molten steel is constantly produced, it is desired that operating conditions are not changed as much as possible.

Therefore, when returning the hot metal to the steelmaking process, it is necessary to reduce the P concentration in the hot metal after dephosphorization to about the same level as ordinary hot metal. In this case, the amount of slag generated in a large amount during the dephosphorization process from the hot metal having a high phosphorus content to the hot metal discharged from a normal blast furnace poses a problem. The amount of slag generated when dephosphorizing high P hot metal and producing hot metal that can be returned to the steelmaking process as hot metal and dephosphorized slag as a raw material for phosphate fertilizer is calculated by the following formula (1) per t of hot metal. ) Can be calculated.
[P] _i = [P] _f + (W _s / 1000) * (% P ₂ O ₅ ) * 62/142 (1)

At this time, [P] _i , [P] _f , (% P ₂ O ₅ ) and W _s are respectively the P concentration (mass%) in the initial hot metal and the P concentration (mass) in the hot metal after dephosphorization. %), The concentration of phosphoric acid in the dephosphorized slag (mass%), and the amount of slag generated per ton of hot metal (kg / t). At this time, the phosphoric acid concentration (% P ₂ O ₅ ) in the dephosphorized slag was set to 15% by mass, and the P concentration ([P] _f = 0.12% by mass) until the hot metal after dephosphorization could be returned to the steelmaking process. When the amount is reduced, the amount of dephosphorization slag W _s generated can be calculated as in the following equation (2).
W _s = ([P] _i −0.12) * 1000 / (15 * 62/142) (2)

If the P concentration [P] _i in the initial hot metal is 0.5, 1, 2, and 3% by mass, the dephosphorization slag amounts W _s are 58, 134, 286, and 439 kg / t, respectively. If the P concentration is 1% or more, a very large amount of slag will be generated. The amount of slag that can be processed in a normal converter is considered to be at most about 200 kg / t, and a special measure is required for a slag amount larger than that. In the above-mentioned Thomas Steelmaking, the operation was performed at around 200 kg / t. Further, even if the dephosphorization method using a pot as disclosed in Patent Documents 3 and 4 is used, the same problem of the slag amount occurs.

  In addition, in order to use dephosphorized slag as a raw material for phosphate fertilizer, a uniform molten state, homogeneity of slag, uniform temperature distribution, etc. are required. Temperature distribution and agitation conditions are greatly different, making it difficult to guarantee the quality of slag as fertilizer.

  In addition, in order to reduce the amount of slag, it is conceivable to increase the phosphoric acid concentration of the dephosphorized slag, but there is a phenomenon that increasing the phosphoric acid concentration rather decreases the fertilizer effect. Has an upper limit, so it is difficult to reduce the amount of slag.

Accordingly, the present invention, based on the above-mentioned current situation, avoids a large-scale facility and, together with a dephosphorized slag having a high P ₂ O ₅ concentration suitable for a raw material of a phosphate fertilizer, a hot metal that can be added in a steelmaking process. It is an object of the present invention to provide a method for producing derinsed slag that can be produced rationally.

  In order to solve the above-mentioned problems, the present inventors have intensively studied the most rational production of dephosphorized slag, which is a raw material of a phosphate fertilizer having a high fertilizer effect, in view of the composition of the slag and the production conditions. As a result, the following findings were obtained.

  When the distribution ratio was measured in the composition range of the phosphoric acid concentration, the dephosphorization slag having a phosphoric acid concentration of 8% by mass or more and the hot metal having a P concentration of 0.15% by mass or less usable in the steelmaking process were suitable for the phosphate fertilizer raw material. It has been found that it is difficult to produce these by a single dephosphorization treatment.

Therefore, in order to treat a large amount of slag, the dephosphorization treatment is divided into two steps, a second step and a third step, and in the second step, by controlling the amount of the supplied flux and the oxidation source, The phosphoric acid concentration is 8% by mass or more, preferably the total of CaO, SiO ₂ , P ₂ O ₅ and iron oxide is 60% by mass or more, the basicity is 1.5 to 3.0, and the concentration of iron oxide Produces a dephosphorized slag with a high phosphoric acid concentration of 5 to 25 mass in terms of Fe, and in the third step, by performing a dephosphorizing treatment on the hot metal reduced to a certain P concentration in the second step, It was found that it was possible to reduce the phosphorus concentration to about the same level as that of hot metal from a blast furnace.

Further, it was also found that the dephosphorized slag produced in the third step has a low phosphoric acid concentration and cannot be used as it is as a phosphate fertilizer raw material, but can be reused as a flux in the second step.
Furthermore, depending on the P concentration in the hot metal, it was found that by repeating the second step a plurality of times, the scale of the equipment could be suppressed and dephosphorized slag with a high phosphoric acid concentration could be produced.

  The present invention has been made based on the above findings, and the gist is as follows.

(1) a first step in which a steelmaking slag containing P generated in the steelmaking refining process is subjected to a reduction treatment to produce hot metal having a P concentration of 0.5 to 4% by mass;
With the first performs the dephosphorization process, dephosphorylated been hot metal and the concentration of phosphoric acid to produce an 8% by weight or more of dephosphorization slag using a flux and an oxygen source with respect to molten iron prepared in the first step A second step of separately separating the dephosphorized hot metal and the dephosphorized slag ,
Performing a second dephosphorization treatment on the hot metal dephosphorized and separated in the second step using a flux and an oxygen source to produce hot metal having a P concentration of 0.15% by mass or less; ,
Have a,
The hot metal treated in the third step is mixed with hot metal discharged from a blast furnace, and the slag generated in the third step is reused as a flux used in the second step. Manufacturing method of rinse slag.
(2) In the second step, the dephosphorized hot metal and the dephosphorized slag are separately separated by scraping out molten slag using a drakker. Manufacturing method of rinse slag.
(3) When the P concentration of the hot metal dephosphorized in the second step is 0.5% by mass or more, using a flux and an oxygen source for the hot metal dephosphorized in the second step. The method for producing a dephosphorized slag according to (1) or ( 2 ), wherein the first dephosphorization treatment is performed again.
(4) In the second step, the first dephosphorization is performed on the hot metal produced in the first step by subjecting the hot metal subjected to the Cr removal treatment or the Mn removal treatment to a flux and an oxygen source. The method for producing a de-rinsed slag according to any one of (1) to (3) , wherein the treatment is performed.
(5) The dephosphorization slag contains CaO, SiO ₂ , P ₂ O ₅ , and iron oxide (in terms of Fe) in a total of 60% by mass or more, and has a basicity α expressed by a mass ratio of CaO and SiO _2. 1.5 to 3.0, containing P ₂ O ₅ in an amount of 8% by mass or more (−4α ² + 23α−4)% by mass or less, and iron oxide containing 5% by mass or more and 25% by mass or less in terms of Fe. The method for producing a dephosphorized slag according to any one of (1) to (4) .
(6) The dephosphorization slag contains CaO, SiO ₂ , P ₂ O ₅ , MgO, and iron oxide (in terms of Fe) in a total of 70% by mass or more, and has a basicity α expressed by the mass ratio of CaO and SiO _2. Is not less than 1.5 and not more than 3.0, P ₂ O ₅ is not less than 8% by mass (−4α ² + 23α−4)% by mass, iron oxide is 5% by mass to 25% by mass in terms of Fe, and MgO is The method for producing a derinsed slag according to any one of claims 1 to 4, wherein the content is 8% by mass or more and 23% by mass or less.

According to the present invention, as a scale of the equipment is not increased, along with the raw material to a suitable P ₂ O ₅ higher concentrations dephosphorization slag phosphate fertilizer, rationally manufacture a molten iron which can be added in the steel making process A method for producing a derinsed slag can be provided.

It is a figure for demonstrating the process of manufacturing the dephosphorization slag containing phosphoric acid, and the process of dephosphorizing the hot metal manufactured at that time. It is a diagram showing a relationship between the basicity α to be displayed in the preferred phosphate concentration and CaO and SiO ₂ weight ratio of dephosphorization slag.

The method for producing a de-rinsed slag of the present invention (hereinafter sometimes referred to as “the production method of the present invention”) is described below.
(1) As a first step, a phosphorus-containing hot metal having a P concentration of 0.5 to 4% by mass is produced by reducing steelmaking slag,
(2) As a second step, the phosphorus-containing hot metal obtained in the first step is subjected to a dephosphorization treatment using a flux and an oxygen source to produce a dephosphorized slag having a phosphoric acid concentration of 8% by mass or more,
(3) As a third step, the dephosphorized hot metal obtained in the second step is subjected to dephosphorization again using a flux and an oxygen source, so that the P concentration in the hot metal is 0.15% by mass. The following hot metal is manufactured.

Further, from the dephosphorized slag having a phosphoric acid concentration of 8% by mass or more produced according to the present invention, a phosphate fertilizer raw material can be produced, for example, by the following procedure.
(i) The totality of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, and the basicity α expressed by the mass ratio of CaO and SiO ₂ (hereinafter, the mass of CaO and SiO _{2 in} the slag The basicity expressed by the ratio CaO / SiO ₂ is referred to as “basicity α”) is 1.5 or more and 3.0 or less, and P ₂ O ₅ is 8 mass% or more (−4α ² + 23α−4). Melt slag of 1200 to 1450 ° C. containing iron oxide in an amount of 5% by mass or more and 25% by mass or less in terms of Fe,
(ii) A value obtained by dividing the amount of temperature drop until reaching 600 ° C. by the time until reaching 600 ° C., and cooling by controlling the temperature to be 10 ° C./min or more,
(iii) The total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, the basicity α is 1.5 or more and 3.0 or less, and P ₂ O ₅ is 8% by mass or more ( -4α ² + 23α-4) mass% or less, iron oxide in an amount of 5 mass% to 25 mass% in terms of Fe, and a Ca ₃ (PO ₄ ) ₂ -Ca ₂ SiO ₄ solid solution phase, 5CaO.SiO ₂ .P _A phosphate fertilizer raw material in which the total concentration of one or more of the ₂ O ₅ phase and the 7CaO.2SiO ₂ .P ₂ O ₅ phase is 28% by mass or more can be produced.

  First, a method for producing dephosphorized slag suitable as a raw material of a phosphate fertilizer for growing a plant will be described. FIG. 1 shows a step of producing dephosphorized slag containing phosphoric acid (first step and second step) and a step of dephosphorizing the hot metal produced at that time (third step).

  Usually, a blast furnace hot metal (C: 4.3 to 4.8% by mass) containing 0.08 to 0.15% by mass of P is transferred to a converter, and flux is charged to form slag on the hot metal. Then, an oxygen source is blown, and the hot metal is dephosphorized S01 by the reaction between the hot metal and the slag.

  The steelmaking slag 41 generated by the dephosphorization treatment S01 is discharged from the converter, and thereafter, a flux is charged into the converter to form slag again on the hot metal, and an oxygen source is blown into the decarbonization treatment S02. I do. After subjecting the molten steel obtained in the decarburization treatment S02 to secondary refining S03, a steel slab is produced by continuous casting S04.

  After the dephosphorization treatment S01, the steelmaking slag 41 discharged from the converter contains a large amount of iron together with phosphoric acid in which P in the hot metal is oxidized. In order to recover valuable elements, the steelmaking slag 41 is subjected to a reduction / reforming treatment S11.

In the reducing-reforming process S11, the steelmaking slag 41 melts, as a reducing agent and a modifying agent, pulverized coal, Al ₂ O ₃ source, with the addition of SiO ₂ source, etc., 0.5 to 4 and P. A high P hot metal 42 containing 0% by mass is manufactured.

Then, as a second step, after subjecting the high-phosphorus hot metal 42 containing P to 0.5 to 4% by mass to a Cr removal treatment or a Mn removal treatment S12 as required, for example, the basicity (CaO / SiO ₂ mass Ratio) of 1.5 to 3.0 and a flux containing iron oxide in an amount of 10% by mass or more in terms of Fe (hereinafter sometimes referred to as “iron oxide (Fe equivalent)”). The phosphorus removal treatment S13 (first phosphorus removal treatment) is performed at a processing temperature of ℃ (temperature at the end of the processing).

  The flux used in this treatment includes natural mineral-derived auxiliary materials such as quicklime (CaO: 90% by mass or more) and lightly burnt dolomite (CaO: 60% by mass or more, MgO: 30% by mass or more), and ordinary steelmaking. Converter slag generated in the step, slag generated in a third step described later, or the like can be used. Further, as an oxygen source necessary for the dephosphorization reaction, iron oxide such as iron ore and scale can be used in addition to gaseous oxygen.

By the phosphorus removal treatment S13, for example, the total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, the basicity α is 1.5 or more and 3.0 or less, and P ₂ O ₅ 8 mass% or more (-4α ² + 23α-4) mass% or less, to produce a dephosphorization slag containing 25 mass% or less 5 wt% or more of iron oxide in terms of Fe. If the P concentration of the hot metal after the first dephosphorization is high, this dephosphorization treatment S13 may be repeated a plurality of times.

  When the P concentration of the hot metal is 0.5% by mass or more as a result of performing the phosphorus removal treatment S13 as the second step, the phosphorus removal treatment S13 is performed again. Here, the value of the P concentration in the hot metal is obtained by sampling the hot metal after the dephosphorization treatment and performing a rapid analysis. Then, as a result of the quick analysis, when the P concentration in the hot metal is 0.5% by mass or more, the phosphorus removal treatment S13 is performed again.

  Further, the hot metal 51 dephosphorized to a P concentration of less than 0.5% by mass by the dephosphorization treatment S13 is not suitable for being mixed with hot metal discharged from a blast furnace and supplied to a converter as it is. .

Therefore, as a third step, the hot metal is dephosphorized to a P concentration of 0.1 to 0.15 mass% by a dephosphorization treatment S13 '(second dephosphorization treatment). This dephosphorization treatment S13 'is a treatment for reducing the P concentration in the hot metal to 0.1 to 0.15% by mass, and may be performed under the same conditions as the dephosphorization treatment S01 in the ordinary steelmaking process. It may be performed under the same conditions as in the phosphorus removal treatment S13. For example, similarly to the dephosphorization treatment S13, a dephosphorization slag having a basicity α of 1.5 or more and 3.0 or less and containing iron oxide in an amount of 5% by mass or more and 25% by mass or less in terms of Fe is generated. The temperature after the treatment may be set to 1200 to 1450 ° C. similarly to the phosphorus treatment S13. In this case, P concentration in molten iron to be processed is so low compared to the dephosphorization process S13, P ₂ O ₅ concentration in the dephosphorization slag generated also decreases. The dephosphorization slag generated by the dephosphorization treatment S13 'has no conditions that restrict the composition as a phosphate fertilizer raw material unlike the dephosphorization slag generated by the dephosphorization treatment S13. And the iron oxide concentration can be changed as appropriate.

  The hot metal 52 thus treated is mixed with the hot metal discharged from the blast furnace and supplied to the converter, so that there is no loss in producing hot metal wastefully. Further, the phosphorus removal treatment S13 'may be repeated a plurality of times until the target P concentration is reached. Further, the dephosphorization slag generated by the dephosphorization treatment S13 ′ is suitable for being used as a flux added in the dephosphorization treatment S13 of the second step. Therefore, the manufacturing method of the present invention is a reasonable manufacturing process.

  Next, the reason for defining the manufacturing conditions according to the manufacturing method of the present invention will be described. Specifically, the P concentration of high-P hot metal, the composition of dephosphorized slag (basicity, phosphoric acid concentration, MgO concentration, and iron oxide (Fe equivalent) concentration), and the P concentration of the finally obtained hot metal are specified. The reason for doing so will be described.

P concentration in high P hot metal: 0.5 to 4.0 mass%
If the P concentration of the high P hot metal is less than 0.5% by mass, the phosphoric acid concentration of the slag generated by the dephosphorization treatment in the second step cannot be 8% by mass or more. Therefore, the lower limit of the P concentration in the high-P hot metal is set to 0.5% by mass. Preferably it is 1.0% by mass. On the other hand, if the P concentration of the high-P hot metal exceeds 4.0% by mass, the phosphoric acid concentration of the slag obtained in the second step becomes too high, and the fertilizer effect as a phosphate fertilizer raw material decreases. Therefore, the upper limit of the P concentration in the high-P hot metal is set to 4.0% by mass. The specified range of the phosphoric acid concentration in the slag will be described later.

Phosphoric acid concentration of slag generated by the dephosphorization treatment of the second step: 8% by mass or more When the phosphoric acid concentration of slag generated by the dephosphorization treatment of the second step is less than 8% by mass, phosphoric acid The fertilizer effect is reduced as a fertilizer raw material. Therefore, the phosphoric acid concentration of the slag generated by the dephosphorization treatment in the second step is set to 8% by mass or more.

Slag composition at the end of the dephosphorization treatment in the second step (the total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, and the basicity α represented by the mass ratio of CaO to SiO ₂ : 1.5 or more and 3.0 or less, phosphoric acid concentration: 8 mass% or more (-4α ² + 23α-4) mass% or less, iron oxide (Fe equivalent) concentration: 5 mass% or more and 25 mass% or less)
By controlling the cooling rate to an appropriate value within this composition range, a phosphate fertilizer raw material having a high fertilizer effect can be produced, and thus such conditions are preferable. The reason will be described later.

Slag composition (CaO, SiO ₂ , P ₂ O ₅ , MgO, and iron oxide in total of 70% by mass or more at the end of the second step dephosphorization treatment, basicity expressed by mass ratio of CaO and SiO ₂ α: 1.5 to 3.0, phosphoric acid concentration: 8% by mass (-4α ² + 23α-4)% by mass, iron oxide (Fe equivalent) concentration: 5% by mass to 25% by mass), MgO From 8% by mass to 23% by mass)
By controlling the cooling rate to an appropriate value by setting the composition range containing MgO in a range of 8% by mass to 23% by mass, a phosphate fertilizer material having a higher fertilizer effect can be produced. Is more preferred. The reason for this will be described later.

P concentration of hot metal generated in the third step of dephosphorization: 0.15 mass% or less As described above, mixing with hot metal discharged from a blast furnace does not affect the operation of the steelmaking process. Therefore, the upper limit of the P concentration of the hot metal generated in the dephosphorization treatment in the third step is set to 0.15% by mass.

As described above, the dephosphorization treatment of the second step is operated with a low basicity, and further, in order to utilize the effect of lowering the liquidus temperature (melting point) by P ₂ O ₅ , the Thomas Steelmaking Method (Non-Patent Document 1 The dephosphorization treatment is performed at a temperature lower than the treatment temperature (1550 ° C. or higher) (generally 1200 to 1450 ° C.), and a dephosphorized slag with good slagging properties can be obtained.

  Further, in the production method of the present invention, in the second step, a dephosphorization treatment is performed under conditions that prioritize production of slag suitable as a phosphate fertilizer raw material, and a high-P hot metal is suitable for fertilizer having a high phosphoric acid concentration. Slag is manufactured. Then, in the third step, the dephosphorization treatment is performed under conditions that prioritize the reduction of the P concentration in the hot metal, and a hot metal having a P concentration that does not hinder mixing even in a normal steelmaking process is produced. Therefore, the manufacturing method of the present invention can be said to be a reasonable process with no waste in processing contents.

  Furthermore, in the production method of the present invention, since hot metal is produced from high P molten iron instead of molten steel, decarburization reaction hardly occurs. As a result, it is possible to operate with a simple dephosphorization equipment such as a pot without the need for a converter and exhaust gas equipment of a required scale. Therefore, the production method of the present invention can produce dephosphorized slag as a raw material for phosphate fertilizer at low cost.

Next, a method for producing a phosphate fertilizer raw material from dephosphorized slag obtained by the production method of the present invention will be described. The dephosphorized slag produced by the present invention has a total of CaO, SiO ₂ , P ₂ O ₅ and iron oxide of 60% by mass or more, a basicity α of 1.5 to 3.0 and a P ₂ O ₅ Is 8% by mass or more (-4α ² + 23α-4)% by mass or less, and iron oxide contains 5% by mass or more and 25% by mass or less in terms of Fe. Further, CaO, SiO ₂ , P ₂ O ₅ , the total of MgO and iron oxide is 70% by mass or more, basicity α: 1.5 or more and 3.0 or less, phosphoric acid concentration: 8% by mass or more (-4α ² + 23α-4)% by mass or less, oxidation It is preferable that iron (Fe equivalent) concentration: 5% by mass to 25% by mass) and MgO be contained by 8% by mass to 23% by mass, but even higher by appropriately cooling the slag. Fertilizer effect can be obtained.

The present inventors set the molten slag (dephosphorized slag) produced by the production method of the present invention at a temperature of 10 ° C / min or more from the temperature (1200 to 1450 ° C) during the dephosphorization treatment to 600 ° C. When cooled at a cooling rate, a Ca ₃ (PO ₄ ) ₂ —Ca ₂ SiO ₄ solid solution phase, a 5CaO.SiO ₂ .P ₂ O ₅ phase, and a 7CaO.2SiO ₂ .P ₂ O ₅ phase (hereinafter, three phases) It has been found that one or more of the mineral phases are collectively referred to as a “solid solution phase.” Precisely controlling the precipitation of one or more of them can produce a phosphate fertilizer raw material having an extremely high fertilizer effect.

  The cooling rate control temperature for cooling the molten slag having a temperature of 1200 to 1450 ° C. at the end of the dephosphorization treatment is preferably set to 600 ° C. The present inventors experimentally confirmed that the precipitation of the solid solution phase was completed by cooling to 600 ° C.

  The reason for controlling the cooling of the dephosphorized slag from the temperature after the dephosphorization treatment to 600 ° C. at a required cooling rate is that the phosphorus-containing mineral phase in the dephosphorized slag is determined in a temperature range up to 600 ° C. This is because there is no change in the phosphorus-containing mineral phase in a temperature range below ℃.

  Further, if the cooling rate to 600 ° C. is less than 10 ° C./min, the cooling rate is too slow and the precipitation of the solid solution phase cannot be accurately controlled, so that the cooling rate is preferably 10 ° C./min or more. It is more preferably at least 30 ° C./min. The cooling rate is appropriately set based on the component composition of the derinsed slag. The upper limit is not particularly limited.

According to the findings of the present inventors, (a) the C ₃ P phase, the glass phase, and the soluble phosphoric acid rate of the solid solution phase which precipitate when the derinsed slag is cooled have an order, and (b) C ₃ P The soluble phosphoric acid ratio increases in the order of P phase << glass phase <solid solution phase, (c) the solid solution phase is the phase having the highest soluble phosphoric acid rate, and the C ₃ P phase is the lowest in the soluble phosphoric acid rate. Phase. Therefore, in the phosphate fertilizer raw material, it is preferable to secure the maximum solid solution phase having the highest soluble phosphoric acid rate.

As described above, by cooling the dephosphorized slag at the above cooling rate, the Ca ₃ (PO ₄ ) ₂ -Ca ₂ SiO ₄ solid solution phase, 5CaO · SiO ₂ · P ₂ O ₅ in the phosphate fertilizer raw material to be manufactured is obtained. phase, _{and, 7CaO · 2SiO 2 · P 2} O 5 phase (hereinafter, collectively three mineral phases may be referred to as "solid solution phase".) by controlling one or more deposition, fertilizer effect Can increase the soluble phosphoric acid to the maximum.

The present inventors have studied the conditions under which the solid solution phase is most stably crystallized. FIG. 2 shows a dephosphorization treatment of molten slag in which iron oxide is 10 to 15% by mass in terms of Fe, MnO is 5 to 10% by mass, MgO is 8 to 12% by mass, and Al ₂ O ₃ is 1 to 3% by mass. Phosphoric acid concentration, basicity and phosphoric acid when the cooling rate from 1200 to 1450 ° C., which is the temperature after S13 (see FIG. 1) to 600 ° C., is controlled to be 10 ° C./min or more. The relationship with the contained mineral phase is shown.

That is, FIG. 2 shows the basicity dependence and the P ₂ O ₅ concentration dependence of the phosphorus-containing mineral phase. ○ indicates the case where the solid solution phase was crystallized, and x indicates the case where the C ₃ P phase was crystallized. When the basicity α (= CaO / SiO ₂ ) of the dephosphorization slag is 1.5 or more and 3.0 or less, a solid solution phase is crystallized. Therefore, in order to enhance the fertilizer effect, it is preferable to set the basicity α of the dephosphorized slag to 1.5 or more and 3.0 or less. Further, as described later, when the iron oxide in the dephosphorized slag is 5% by mass or more and 25% by mass or less in terms of Fe and MgO is 23% or less, the basicity α at which the solid solution phase is crystallized and the phosphorus The range of acid concentrations does not change.

When the basicity α of the dephosphorization slag is smaller than 1.5 or larger than 3.0, the C ₃ P phase is crystallized and the fertilizer effect is reduced. Therefore, at the time of the dephosphorization treatment, it is preferable to adjust the amount of the flux such as quicklime or SiO _{2 to be} added, and to adjust the basicity α of the dephosphorization slag to 1.5 or more and 3.0 or less. When the abundance ratio of the solid solution phase to the total slag mass in the slag having a basicity α of 1.5 or more and 3.0 or less was measured by SEM, it was found that phosphoric acid was at least 8 mass% and the solid solution phase was at least 28 mass%. It could be confirmed.

On the other hand, the reason why the upper limit of the phosphoric acid concentration is limited by the quadratic expression of the basicity α will be described. As shown in FIG. 2, even when the basicity α is in the range of 1.5 to 3.0, when the phosphoric acid concentration increases to a certain degree or more, the C ₃ P phase starts to crystallize. Since it was experimentally confirmed that the phosphoric acid concentration at which the C ₃ P phase starts to crystallize increases in a quadratic curve as the basicity α increases, the upper limit of the phosphoric acid concentration increasing in a quadratic curve is set to (−−). 4α ² + 23α-4).

That is, when the basicity is kept constant and the phosphoric acid is increased from 0% by mass, a solid solution phase is crystallized from the condition of 8% by mass, and the phosphoric acid becomes (-4α ² + 23α-4) mass. % Or less, the phosphorus-containing mineral phase is a solid solution phase, but if it exceeds (-4α ² + 23α-4) mass%, the phosphorus-containing mineral phase becomes a C ₃ P phase.

Therefore, if the phosphorus-containing mineral phase of the phosphate fertilizer raw material is a C ₃ P phase, the fertilizer effect is reduced. Therefore, the phosphoric acid content is preferably set to 8% by mass or more and (−4α ² + 23α−4)% by mass or less. Therefore, at the time of the dephosphorization treatment, the amount of the flux added to the slag is preferably adjusted to make the phosphoric acid 8 mass% or more (-4α ² + 23α-4) mass% or less.

If the phosphoric acid content is less than 8% by mass, the phosphoric acid concentration is low and the phase is not a solid solution phase but a C ₃ P phase. As a result, the amount of the phosphate fertilizer used increases, and the commercial value of the fertilizer decreases.

  Further, iron oxide (in terms of Fe) is preferably 5% by mass or more and 25% by mass.

  If the iron oxide (Fe equivalent) is less than 5.0% by mass, a glass phase may be present in the slag, and the fertilizer effect may be reduced. Iron oxide is considered to be present as FeO, is a basic oxide, and is a component that promotes slag crystallization. Therefore, in order to crystallize a solid solution phase which is a crystal phase, it is preferable to contain 5% by mass or more of iron oxide.

On the other hand, if the iron oxide (Fe equivalent) exceeds 25% by mass, the C ₃ P phase is crystallized, so that the fertilizer effect may be reduced. From these results, in order to crystallize the solid solution phase, it is preferable to adjust the amount of oxygen blown during the dephosphorization treatment so that the iron oxide content is 5% by mass or more and 25% by mass or less in terms of Fe. More preferably, the iron oxide content is 10% by mass or less in terms of Fe.

Further, the dephosphorization slag contains CaO, SiO ₂ , P ₂ O ₅ , and iron oxide, and the total of each component is preferably set to 60% by mass or more. If the total of each component is less than 60% by mass, components other than the above components and phosphoric acid form a compound, and it becomes impossible to control the generation of a phosphate-containing mineral phase. It is preferably at least 70% by mass.

  As described above, in order to secure the maximum amount of soluble phosphoric acid that enhances the fertilizer effect, it is preferable to secure the solid solution phase by cooling to 28% by mass or more. The upper limit is not limited because the larger the amount of solid solution phase precipitated, the better.

That is, the total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, the basicity α is 1.5 or more and 3.0 or less, and the phosphoric acid concentration is 8% by mass or more (−4α ² + 23α). -4) mass% or less, iron oxide (Fe equivalent) concentration is 5 mass% to 25 mass%, and Ca ₃ (PO ₄ ) ₂ -Ca ₂ SiO ₄ solid solution phase, 5CaO.SiO ₂ .P ₂ It is possible to produce a phosphate fertilizer raw material in which the total concentration of one or more of the O ₅ phase and the 7CaO.2SiO ₂ .P ₂ O ₅ phase is 28% by mass or more.

During the dephosphorization treatment, the amount of dolomite added as a flux may be adjusted, and a molten slag containing MgO in an amount of 8% by mass to 23% by mass may be generated. Also in this case, by cooling at the above-described cooling rate, the basicity α is 1.5 or more and 3.0 or less, the phosphoric acid concentration is 8 mass% or more (-4α ² + 23α-4) mass% or less, and the iron oxide ( The concentration is 5% by mass or more and 25% by mass or less, the MgO content is 8% by mass or more and 23% by mass or less, and a Ca ₃ (PO ₄ ) ₂ —Ca ₂ SiO ₄ solid solution phase, 5CaO.SiO ₂ .P ₂ O ₅ phase, and can be manufactured one or more presence concentrations total 28 mass% or more phosphate fertilizer material of _{7CaO · 2SiO 2 · P 2 O} 5 phase. When MgO is contained in an amount of 8% by mass or more and 23% by mass or less, the total amount of CaO, SiO ₂ , P ₂ O ₅ , MgO, and iron oxide in the derinsed slag is preferably 70% by mass or more.
As described above, by actively controlling the MgO concentration in the dephosphorized slag, the fertilizer effect when the raw material is a phosphate fertilizer raw material can be further enhanced.

  Next, an example of the present invention will be described. The conditions in the example are one condition example adopted for confirming the operability and effects of the present invention. It is not limited. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
First, as a first step, the steelmaking slag discharged from the converter in the steelmaking step is allowed to flow into the slag supply container in a molten state (solid phase ratio of 25% or less), temporarily stored, and then the slag supply container is By tilting once every 10 minutes, about 8 tons of hot steelmaking slag was flown into the electric furnace once. Then, while supplying electric power to the electric furnace, coke powder was supplied from the auxiliary raw material supply pipe into the electric furnace. Further, fly ash: 378 kg / t-slag and bauxite powder: 47 kg / t-slag as the slag modifier were continuously supplied to the molten slag layer from the auxiliary raw material supply pipe. Then, the reduction treatment was performed by controlling the temperature in the electric furnace.

  By the above-described reduction treatment, hot metal A, B, C, and D having a P concentration of 0.5 to 4.0% by mass and a C concentration of 3.0 to 5.0% as shown in Table 1 below ( Hereinafter, when referring to the hot metal after the first step, it is described as A1, B1, etc.), respectively, and about 30 tons were obtained.

Next, as a second step, using a hot metal ladle, the hot metal (A1) having a P concentration of 0.5 to 4.0% by mass and a C concentration of 3.0 to 5.0% by mass produced in the first step. ~ D1) To about 30 tons, 50 kg of flux having a basicity of 1.5 to 2.5 per 1 t of hot metal is added, dephosphorization treatment is performed by blowing oxygen, and molten slag is scraped out using a drakker. Thereby, de-rinsing slag and hot metal (A2 to D2) were obtained. As a result, in the final dephosphorization slag, the total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, and the basicity (CaO / SiO ₂ mass ratio) α is 1.5 or more. 0 or less, the phosphoric acid concentration is 8 to (−4α ² + 23α−4)% by mass, and t. The Fe concentration (iron oxide concentration in terms of Fe) was 5 to 25% by mass.

  In addition, the molten iron (E1, F1) having a P concentration of less than 0.5% by mass produced by dissolving FeP ore in molten pig iron from a blast furnace and performing a desiliconization treatment is the same as described above. The results of the phosphorus treatment are also shown in Table 2 as a comparative example.

As shown in Table 2, with respect to the hot metal having a P concentration of 0.5 to 4% by mass, such as hot metal A1 to D1, the dephosphorization treatment is performed on the hot metal, such as A2 to D2, at least 8% by mass. Was obtained.
On the other hand, when the dephosphorization treatment is performed on the hot metal (E1, F1) having a P concentration of less than 0.5 mass, the dephosphorized slag generated together with the hot metal E2, F2 has a phosphoric acid concentration of less than 8 mass%. And could not be used as a raw material for phosphate fertilizer.

  Next, since the P concentration in the hot metal of A2 and B2 exceeded 0.5% by mass, the dephosphorization treatment was further repeated. The results are shown in Table 3 as A22 and B22.

As shown in Table 3, by performing the dephosphorization treatment again, a dephosphorized slag having a phosphoric acid concentration of 8% by mass or more could be obtained. In these dephosphorized slags, the total of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide is 60% by mass or more, the basicity α is 1.5 or more and 3.0 or less, and the phosphoric acid concentration is 8 to ( -4α ² + 23α-4)% by mass, and t. The Fe concentration was 5 to 25% by mass.
Further, since the P concentration in the hot metal of A22 and B22 also exceeded 0.5% by mass, when the hot metal was further subjected to a dephosphorization treatment, the dephosphorization slag having a phosphoric acid concentration of 8% by mass or more was obtained. Could be obtained.

  Next, as a third step, the basicity of the hot metal of C2 and D2 in which the P concentration in the hot metal after the second step is less than 0.5% by mass is not less than 1.0 using a hot metal pot. And a dephosphorization treatment was performed to obtain hot metal and dephosphorized slag (C3, D3). The results are shown in Table 4 below.

  As shown in Table 4, the P concentration in the hot metal after the dephosphorization treatment in the third step falls within the range of 0.1 to 0.15% by mass, and becomes hot metal that can be mixed with hot metal discharged from a blast furnace. Was. In addition, the concentration of phosphoric acid in the dephosphorized slag generated at this time is less than 8% by mass, and cannot be used as a raw material for phosphate fertilizer as it is, but the dephosphorized slag of the present composition can be reused as a flux in the second step without any problem. Was confirmed.

(Example 2)
Next, the dephosphorized slag produced by performing the dephosphorization treatment in the same procedure as in Example 1 was cooled to a cooling rate of 10 ° C / min from 1200 to 1450 ° C to 600 ° C after the treatment. The slag was cooled under controlled conditions, and the slag solid solution phase abundance ratio, soluble phosphoric acid ratio, and crystallinity were investigated. The results are shown in Table 5 by comparing the invention examples (examples) implemented with satisfying the requirements according to the present invention with comparative examples partially implemented without partially satisfying the requirements according to the present invention. .

  In Table 5, a soluble phosphoric acid ratio of 0.6 or more was evaluated as ◎, and less than 0.5 was evaluated as x.

In the slag samples of Invention Examples (Examples) 1 to 9 in which the cooling rate was 10 ° C./min, the basicity was 1.5 or more and 3.0 or less, and the phosphoric acid concentration was 8 mass% or more (−4α ² + 23α−4). ) Mass% or less, t. The Fe concentration was 5% by mass or more and 25% by mass or less, and the concentration of the solid solution phase present in the total slag mass was 28% by mass or more.

In the slag samples of Comparative Examples 1 to 8, since the basicity α was as low as less than 1.5 and the phosphoric acid concentration exceeded (-4α ² + 23α-4)% by mass, no solid solution phase was precipitated. In the slag sample of Comparative Example 9, the basicity α was as low as less than 1.5, and t. Since the Fe concentration was as high as more than 25% by mass and in the slag sample of Comparative Example 10, the basicity was higher than 3.0, no solid solution phase was precipitated.

In the slag sample of Comparative Example 11, the phosphoric acid concentration was lower than 8% by mass, and in the slag sample of Comparative Example 12, the phosphoric acid concentration was higher than (-4α ² + 23α-4)% by mass, so that no solid solution phase was precipitated. Was. In the slag sample of Comparative Example 13, t. Since the Fe concentration was lower than 5% by mass, a glass phase was precipitated, and in the slag samples of Comparative Examples 14 and 15, t. Since the Fe concentration was higher than 25% by mass, no solid solution phase was precipitated.

  In the slag sample of Comparative Example 13, the solid solution did not precipitate because the cooling rate was less than 10 ° C./min.

  When Examples 6 and 10 having the same component composition and different cooling rates are compared with Comparative Example 16, when the cooling rate up to 600 ° C. is 10 ° C./min or more, the solid solution phase becomes 28% by mass or more. It can be seen that increasing the cooling rate to 600 ° C. to 30 ° C./min increases the concentration of the solid solution phase.

In the examples in which the requirements according to the present invention were implemented including the control of the cooling rate, the Ca ₃ (PO ₄ ) ₂ -Ca ₂ SiO ₄ solid solution phase, which was found to have a high fertilizer effect as a phosphate fertilizer, had a 5CaO. Phosphate fertilizer raw material suitable for phosphate fertilizer having a total concentration of one or more of SiO ₂ · P ₂ O ₅ phase or 7CaO · 2SiO ₂ · P ₂ O ₅ phase of 28% by mass or more Was manufactured.

  ADVANTAGE OF THE INVENTION According to this invention, the dephosphorization slag used as a phosphate fertilizer raw material with a reasonably high fertilizer effect can be manufactured, without increasing the scale of equipment. Further, hot metal having a low P concentration that can be mixed with hot metal discharged from a blast furnace can also be produced. Therefore, the present invention has high applicability in the steel industry and the plant growing industry.

S11 Reduction / reforming process (first step)
S13 Dephosphorization treatment (second step)
S13 'Dephosphorization treatment (third step)

Claims (6)

A first step of subjecting a steelmaking slag containing P generated in the steelmaking smelting process to reduction treatment to produce molten iron having a P concentration of 0.5 to 4% by mass;
With the first performs the dephosphorization process, dephosphorylated been hot metal and the concentration of phosphoric acid to produce an 8% by weight or more of dephosphorization slag using a flux and an oxygen source with respect to molten iron prepared in the first step A second step of separately separating the dephosphorized hot metal and the dephosphorized slag ,
Performing a second dephosphorization treatment on the hot metal dephosphorized and separated in the second step using a flux and an oxygen source to produce hot metal having a P concentration of 0.15% by mass or less; ,
Have a,
The hot metal treated in the third step is mixed with hot metal discharged from a blast furnace, and the slag generated in the third step is reused as a flux used in the second step. Manufacturing method of rinse slag. 2. The production of the dephosphorized slag according to claim 1, wherein in the second step, the dephosphorized hot metal and the dephosphorized slag are separately separated by scraping out molten slag using a drakker. 3. Method. In a case where the P concentration of the hot metal dephosphorized in the second step is 0.5% by mass or more, in the second step, the first hot metal dephosphorized using the flux and the oxygen source with respect to the hot metal dephosphorized is used. The method for producing a dephosphorized slag according to claim 1 or 2, wherein the dephosphorization treatment is performed again. In the second step, the hot metal produced in the first step is subjected to the first dephosphorization treatment using a flux and an oxygen source for the hot metal subjected to the Cr removal treatment or the Mn removal treatment. The method for producing a derinsed slag according to any one of claims 1 to 3 , characterized in that: The dephosphorization slag contains a total of 60% by mass or more of CaO, SiO ₂ , P ₂ O ₅ , and iron oxide (in terms of Fe), and has a basicity α expressed as a mass ratio of CaO and SiO ₂ of 1.5. Not more than 3.0 and not more than 8% by mass of P ₂ O ₅ (−4α ² + 23α−4)% by mass and not more than 5% by mass and not more than 25% by mass of iron oxide in terms of Fe. The method for producing a de-rinsed slag according to any one of claims 1 to 4 . The dephosphorization slag contains CaO, SiO ₂ , P ₂ O ₅ , MgO, and iron oxide (in terms of Fe) in a total of 70% by mass or more, and has a basicity α expressed by a mass ratio of CaO and SiO ₂ of 1. 5 to 3.0, P ₂ O ₅ is 8% by mass or more (−4α ² + 23α-4)% by mass, iron oxide is 5% by mass to 25% by mass in terms of Fe, and MgO is 8% by mass. The method for producing a dephosphorized slag according to any one of claims 1 to 4 , wherein the content is at least 23 mass%.