JP7610114B2 - Method for dephosphorizing molten iron - Google Patents

Description

The present invention relates to a method for dephosphorizing molten pig iron that can be used in a method for dephosphorizing and refining molten pig iron with a high phosphorus concentration and using the resulting slag as a raw material for phosphate fertilizer.

Pig iron produced by the blast furnace process contains about 0.1% phosphorus. Dephosphorization is carried out during the steelmaking process, where molten pig iron is refined. In dephosphorization, slag for dephosphorization is formed on the surface of the molten pig iron, and the phosphorus in the molten pig iron is oxidized by oxidation refining and transferred to the slag, thereby reducing the phosphorus concentration in the molten steel after refining is completed. The slag recovered after refining (hereinafter referred to as "steelmaking slag") contains about 2% of the phosphoric acid that transferred to the slag during dephosphorization. Steelmaking slag also contains iron oxide. Steelmaking slag discharged from the system is used as civil engineering material such as roadbed material, or for ocean reclamation.

It is known that by subjecting such steelmaking slag to a reduction process, the iron oxide and phosphoric acid in the steelmaking slag are reduced, resulting in high-phosphorus molten pig iron containing 0.5% or more phosphorus, and that this high-phosphorus molten pig iron is used as a raw material for a dephosphorization process, and the resulting dephosphorization slag is used as a raw material for phosphate fertilizer.

Patent Document 1 discloses a method for producing phosphate fertilizer in which molten pig iron with a phosphorus concentration of 1% or more is dephosphorized and the slag produced by the dephosphorization is used as a raw material for phosphate fertilizer, in which the molten pig iron is charged into a refining vessel, a refining agent molten by a melting projection type fuel burner is added to the surface of the molten pig iron, and oxygen gas is supplied to the surface of the molten pig iron from a top blowing lance to dephosphorize the molten pig iron. This method for producing phosphate fertilizer does not require the use of heat from the oxidation of carbon in the molten pig iron as a heat source for melting and heating the slag raw material.

Patent Document 2 discloses a powder injection lance with a burner function, and a molten iron refining method and melting reduction method using the powder injection lance, which efficiently imparts (heats) the combustion heat of the burner to the molten iron. This molten iron refining method uses a top-blowing lance having separate powdered refining agent supply passages, fuel gas supply passages, fuel gas combustion oxidizing gas supply passages, and refining oxidizing gas supply passages to form a flame on the front of the nozzle of the top-blowing lance toward the bath surface of the molten iron contained in a reaction vessel, supplies the powdered refining agent from the powdered refining agent supply passage toward the bath surface of the molten iron, and sprays the powdered refining agent toward the bath surface of the molten iron while heating it with the flame. It is said that the thermal efficiency is increased by controlling the ratio of the top-blowing lance height and flame length and adjusting the ratio of the fuel gas and the combustion oxidizing gas.

JP 2017-128747 A International Publication No. WO2013/057927

The present invention relates to a method for dephosphorizing molten iron using a fuel burner capable of projecting a refining agent. The fuel burner projects the refining agent through the flame while injecting fuel and gaseous oxygen toward the surface of the molten iron in a refining vessel to form a burner flame.

It is preferable to transfer as much of the heat generated by the combustion of fuel supplied from a fuel burner as possible to the molten pig iron and the sprayed refining agent, thereby contributing to the increase in sensible heat of the molten pig iron and the refining agent.The present invention defines the heat transfer efficiency as the ratio of the amount of heat transferred to the molten pig iron and the sprayed _refining agent and contributing to the increase in sensible heat of the molten pig iron and the refining agent to the amount of heat generated when the fuel supplied from a fuel burner reacts with gaseous oxygen to combust H in the fuel into H2O and C into _CO2 , and aims to improve the heat transfer efficiency in a molten pig iron dephosphorization method for performing a dephosphorization treatment of the molten pig iron using a fuel burner capable of spraying a refining agent.

That is, the gist of the present invention is as follows.
[1] A method for dephosphorizing molten iron using a fuel burner capable of projecting a refining agent, comprising:
The fuel burner injects fuel and gaseous oxygen toward the surface of the molten pig iron in the refining vessel to form a burner flame, and projects the refining agent through the burner flame,
The distance between the tip of the fuel burner and the molten metal surface is L, and the length of the burner flame is _L0 .
0.3≦L/L ₀ ≦0.8 (1)
and
The gaseous oxygen supply amount required to burn H in the fuel supplied from the fuel burner into _H2O and C into _CO2 is defined as the theoretical gaseous oxygen supply amount, and the value obtained by dividing the actual gaseous oxygen supply amount by the theoretical gaseous oxygen supply amount is defined as the excess oxygen supply ratio (R);
1.1≦R≦1.5 (2)
The method for dephosphorizing molten iron, comprising supplying the gaseous oxygen in a range of
[2] The method for dephosphorizing molten iron according to [1], characterized in that the refining agent contains iron oxide and CaO.
[3] A method for dephosphorizing molten iron according to [1] or [2], in which molten iron having a phosphorus concentration of 1% or more is dephosphorized.

The present invention is a method for dephosphorizing molten pig iron using a fuel burner capable of projecting refining agents. By setting the distance L between the tip of the fuel burner and the molten iron surface and the excess oxygen supply ratio R to the theoretical gaseous oxygen supply amount required for the combustion of the fuel supplied from the fuel burner within appropriate ranges, it is possible to improve the heat transfer efficiency, which is the contribution of heat generated by fuel combustion to the transfer of heat to the molten pig iron and refining agents.

FIG. 2 is a diagram showing an example of the relationship between a refining vessel and a fuel burner used in the molten iron dephosphorization method of the present invention. FIG. 1 is a diagram showing the relationship between the distance L between the tip of a fuel burner and the molten metal surface and the length of the burner flame, _L0 .

The method of dephosphorization of molten pig iron using a fuel burner capable of projecting a refining agent, which is the subject of the present invention, will be described with reference to Fig. 1. Molten pig iron 3 is contained in a refining vessel 1. A fuel burner 2 injects fuel and gaseous oxygen toward the molten iron surface 4 of the molten pig iron 3 in the refining vessel 1 to form a burner flame 7, while projecting a refining agent through the burner flame 7. The distance between the tip of the fuel burner 2 and the molten iron surface 4 is defined as L. With the distance L sufficiently large, and with the tip of the burner flame 7 away from the molten iron surface 4, the length _L0 of the burner flame 7 can be visually measured (see Fig. 2(C)).

The mechanism by which heat generated by the burner flame 7 is transferred to the molten pig iron 3 and the projected refining agent can be considered as follows: first, radiant heat from the burner flame 7 is transferred to the molten pig iron 3. Part of the radiant heat from the burner flame 7 is transferred to the molten pig iron 3, and the other part of the radiant heat is dissipated to the furnace body and furnace cover of the refining vessel 1 and consumed. Second, the radiant heat can be transferred as sensible heat of the projected refining agent through the burner flame 7.

Here, we consider the efficiency of heat transfer from the burner flame 7 to the molten pig iron 3 and the injected refining agent. Assume that CaO and iron oxide are used as the refining agent. Predetermined amounts of fuel, gaseous oxygen, CaO, and iron oxide are injected into the molten pig iron 3 through the fuel burner 2 for a specified time.

The burner combustion heat value is calculated by multiplying the amount of heat generated per fuel injection amount when the fuel reacts with gaseous oxygen to convert H in the fuel into H ₂ O and C into CO ₂ by the fuel injection amount.
The initial temperature of the molten iron is T ₁ , and the temperature of the molten iron after a certain time has passed is T ₂ (ΔT=T ₂ -T ₁ ) (°C). Regarding the change in the components of the molten iron over a certain time, the change in the P content is ΔP, and the change in the C content is ΔC. The reduction heat (endothermic heat) generated when the injected iron oxide is reduced in the molten iron, and the oxidation heat (exothermic heat) generated when the P and C in the molten iron are oxidized to CO are calculated. The amount of heat transferred is calculated by adding the sensible heat increase due to the temperature rise (ΔT) of the molten iron and slag to the reduction heat (endothermic heat) of the iron oxide and the amount of heat dissipated (endothermic heat), and subtracting from this the oxidation heat (exothermic heat) generated when the P and C in the molten iron are oxidized. If the molten iron before treatment contains Si, the oxidation heat (exothermic heat) generated when the Si is oxidized is also taken into consideration.
Based on the above preparations and the calculated heat generation amount and heat transfer amount, the heat transfer efficiency of the burner combustion heat is determined as follows.
Heat transfer efficiency = (increase in sensible heat of molten iron + increment in sensible heat of slag + heat dissipation + heat of reduction of iron oxide - heat of oxidation of P - heat of oxidation of C - heat of oxidation of Si) / heat of combustion by burner (3)
Increase in sensible heat of molten iron: Amount of molten iron × specific heat of molten iron × ΔT
Increase in sensible heat of slag: Amount of slag × Specific heat of slag × ΔT
The amount of heat dissipated per unit time can be calculated, for example, by the following method. The temperature is measured every few minutes without the burner being irradiated, and the amount of temperature drop is measured. The amount of heat dissipated per unit time is calculated using the following formula. The value obtained by multiplying the amount of heat dissipated per unit time by the specified time is the amount of heat dissipated in formula (3).
Amount of heat dissipated per unit time = Amount of temperature drop per unit time × (amount of molten iron × specific heat of molten iron + amount of slag × specific heat of slag) (4)

Below, we will consider a method for dephosphorizing molten iron that can maximize the heat transfer efficiency defined above.

First, regarding the relationship between the distance L between the tip of the fuel burner 2 and the molten iron surface 4 and the length L ₀ of the burner flame 7, the effect of the parameter defined as L/L ₀ on the heat transfer efficiency is considered. FIG. 2(A) illustrates the case where L/L ₀ =0.3, and FIG. 2(B) illustrates the case where L/L ₀ =0.8. If L/L ₀ is too large, for example, L/L ₀ >1, the portion of the radiant heat from the burner flame 7 that contributes to heating the molten iron 3 decreases, and it is assumed that the heat transfer efficiency decreases. On the other hand, if L/L ₀ is too small, it is assumed that the distance that the projected refining agent passes through the burner flame 7 becomes short, and the increase in the sensible heat of the refining agent becomes insufficient. Therefore, it is decided to change L/L ₀ in various ways and determine the optimal range of L/L ₀ for maximizing the heat transfer efficiency.

The gaseous oxygen supply amount required to burn H in the fuel supplied from the fuel burner 2 into _H2O and C into _CO2 is defined as the theoretical gaseous oxygen supply amount. The actual gaseous oxygen supply amount can be made larger or smaller than the theoretical gaseous oxygen supply amount defined above. Therefore, the value obtained by dividing the actual gaseous oxygen supply amount by the theoretical gaseous oxygen supply amount is defined as the excess oxygen supply ratio (R). The excess oxygen supply ratio (R) is then changed in various ways to determine the optimum range of R for maximizing the heat transfer efficiency.

A ladle was used as the refining vessel 1, and molten pig iron 3 with [P] = 1 mass% was contained in the refining vessel 1. The molten pig iron was dephosphorized using a fuel burner 2 (also called a powder projection type fuel burner) capable of projecting a refining agent. Powder (refining agent), fuel gas such as LPG, and gaseous oxygen were supplied from the tip of the fuel burner 2. The powder (refining agent) supplied from the fuel burner 2 was a mixture of iron ore and quicklime. The length _L0 of the burner flame 7 of the fuel burner 2 was 2 m.

The distance L between the tip of the fuel burner 2 and the molten metal surface 4 was changed in a range where L/ _L0 was 0.2 to 0.9, and the amount of oxygen gas supplied was changed in a range where the excess oxygen supply ratio (R) was 1.0 to 1.7.
As a result, in the range satisfying the following formulas (1) and (2), a good result of heat transfer efficiency of 35% or more was obtained. Therefore, in the present invention, the range satisfying formulas (1) and (2) was defined as the range of the present invention.
0.3≦L/L ₀ ≦0.8 (1)
1.1≦R≦1.5 (2)

The reason why the heat transfer efficiency was good when L/ _L0 satisfied the above formula (1) will be considered. As described above, if L/ _L0 is too large, the portion of the radiant heat from the burner flame 7 that contributes to heating the molten pig iron 3 decreases, and it is considered that the heat transfer efficiency decreases, but it was found that if L/ _L0 is 0.8 or less, good heat transfer efficiency can be obtained. On the other hand, if L/ _L0 is too small, the distance that the projected refining agent passes through the burner flame 7 becomes short, and it is considered that the increase in the sensible heat of the refining agent becomes insufficient, but it was found that if L/ _L0 is 0.3 or more, good heat transfer efficiency can be obtained.

The reason why the heat transfer efficiency was good when the excess oxygen supply ratio R satisfied the above formula (2) will be considered.
When the excess oxygen supply ratio R is greater than 1, the actual gaseous oxygen supply amount is excessive compared to the theoretical gaseous oxygen supply amount required to burn H in the fuel supplied from the fuel burner to H ₂ O and C to CO _2. This is called excess gaseous oxygen. In the space above the molten iron surface 4 in the refining vessel 1, C in the molten iron 3 is oxidized to CO, and CO gas separated from the molten iron 3 is contained. It is considered that the excess gaseous oxygen supplied from the fuel burner 2 reacts with CO gas present in the space above the molten iron surface 4 in the refining vessel 1 to generate CO ₂ gas (secondary combustion). And, if the excess oxygen supply ratio R is 1.1 or more, it is estimated that the secondary combustion proceeds to generate secondary combustion heat, and this secondary combustion heat is transferred to the molten iron 3, increasing the heat transfer efficiency.

On the other hand, the excess oxygen gas from the fuel burner 2 also reacts with the carbon in the molten iron 3. When a decarburization reaction occurs, the slag foams, and excessive decarburization reaction can cause slopping, which can lead to operational problems. For this reason, it is necessary to adjust the amount of gaseous oxygen supplied from the fuel burner so that the excess oxygen supply ratio R is 1.5 or less, and so the right-hand side of equation (2) is specified.

As described above, in the present invention, the following effects can be obtained under high heat supply conditions.
When the powder (refining agent) projected from the fuel burner 2 is quicklime or decarburization slag, the quicklime reaches a higher temperature in the burner flame 7 and melts rapidly into the slag, improving the dephosphorization efficiency, compared to when the heat transfer efficiency is not controlled to a high value as in the present invention. When the powder (refining agent) projected from the fuel burner 2 is a mixture of iron oxide-containing material (e.g., iron ore) and quicklime, calcium ferrite with a low melting point is generated in the burner flame 7 and is supplied in a molten state, improving the dephosphorization efficiency.

Molten pig iron with a phosphorus concentration of 1 mass% or more is charged as a raw material into a refining vessel 1, and the molten pig iron dephosphorization method of the present invention is carried out, whereby the slag formed on the surface of the molten pig iron after dephosphorization can be used as a phosphate fertilizer raw material. By carrying out dephosphorization refining using molten pig iron with a phosphorus concentration of 1 mass% or more as a raw material, the phosphorus concentration in the molten pig iron after dephosphorization is reduced to around 0.2 mass%. In this case, _{the P2O5} in the slag on the surface of the molten pig iron after dephosphorization can be made 10 mass% or more, and can be used as a phosphate fertilizer raw material.

A ladle was used as the refining vessel 1, and molten pig iron 3 with [P] = 1 mass % and [C] = 4 mass % was contained in the refining vessel 1. The molten pig iron was dephosphorized using a fuel burner 2 (also called a powder-projecting fuel burner) capable of projecting refining agents. The material injection part at the tip of the fuel burner 2 is a triple tube, and powder (refining agent) is supplied from the inside (center hole) of the innermost tube using air as carrier gas. Fuel gas such as LPG is supplied from the gap between the innermost tube and its outer tube (middle tube). Gaseous oxygen is supplied from the gap between the middle tube and the outermost tube. The powder (refining agent) supplied from the fuel burner 2 was a mixture of iron ore and quicklime. The length L ₀ of the burner flame 7 of the fuel burner 2 was 2 m.

As shown in Table 1, the distance L between the tip of the fuel burner 2 and the molten iron surface 4 was changed so that L/ _L0 was in the range of 0.2 to 0.9. The oxygen gas supply amount was also changed so that the excess oxygen supply ratio (R) was in the range of 1.0 to 1.7. The dephosphorization treatment time, fuel supply amount, iron oxide and quicklime supply amounts were the same under all conditions. As shown in Table 1, the P content in the molten iron before treatment was 1.20% for No. 7 and 1.00% for the others.

The P content after treatment and the amount of decarburization during treatment are shown in Table 1. Using these values, the heat transfer efficiency was calculated based on formula (3) above, and is also shown in Table 1. As is clear from Table 1, as described above, good results were obtained with a heat transfer efficiency of 35% or more in the range that satisfied formulas (1) and (2).

For No. 11 in Table 1, R was 1.7, and slopping occurred. Therefore, it was necessary to adjust the amount of gaseous oxygen supplied from the fuel burner to make the excess oxygen supply ratio R 1.5 or less, as mentioned above, and so the right side of equation (2) was regulated.

As described above, Nos. 1 to 7 shown in Table 1 are examples of the present invention, which achieved high heat transfer efficiency and reduced [P] after treatment. On the other hand, Nos. 8 to 11 are comparative examples, which had low heat transfer efficiency and high [P] after treatment. In addition, in comparative example No. 11, which had too much excess oxygen with an excess oxygen supply ratio R of 1.7, excessive decarburization occurred, slopping occurred, and operation was difficult.

1 Refining vessel 2 Fuel burner 3 Molten iron 4 Molten iron surface 7 Burner flame

Claims (2)

A method for dephosphorizing molten iron having an initial phosphorus concentration of 1.0 to 1.2 mass% by using a fuel burner capable of projecting a refining agent, comprising:
The fuel burner injects fuel and gaseous oxygen toward the surface of the molten pig iron in the refining vessel to form a burner flame, and projects the refining agent through the burner flame,
The distance between the tip of the fuel burner and the molten metal surface is L, and the length of the burner flame is _L0 .
0.3≦L/L ₀ ≦0.8 (1)
and
The gaseous oxygen supply amount required to burn H in the fuel supplied from the fuel burner into _H2O and C into _CO2 is defined as the theoretical gaseous oxygen supply amount, and the value obtained by dividing the actual gaseous oxygen supply amount by the theoretical gaseous oxygen supply amount is defined as the excess oxygen supply ratio (R);
1.1≦R≦1.5 (2)
The method for dephosphorizing molten iron, comprising supplying the gaseous oxygen in a range of The method for dephosphorizing molten iron according to claim 1, characterized in that the refining agent contains iron oxide and CaO.

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