TOP-BLOWING LANCE FOR CONVERTER, METHOD FOR ADDING AUXILIARY RAW MATERIAL, AND METHOD FOR REFINING OF MOLTEN IRON

Abstract

A method that, regarding a process of refining molten iron, can increase the thermal margin and the amount of cold iron source to be used. A burner having jetting holes for jetting a fuel and a combustion supporting gas is provided at a leading end part of one lance that top-blows an oxidizing gas to molten iron contained in a converter-type vessel, or at a leading end part of another separate lance. A powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is blown into the molten iron from the one lance or the other lance passes through a flame formed by the burner. This top-blowing lance for a converter is configured to secure a predetermined heating time and powder-fuel ratio. Also, a method for adding an auxiliary raw material and a method for refining of molten iron that use this top-blowing lance.

Claims

1. A top-blowing lance for a converter, wherein: a burner having jetting holes for jetting a fuel and a combustion supporting gas is provided at a leading end part of one lance that top-blows an oxidizing gas to molten iron contained in a converter-type vessel, or at a leading end part of another lance that is installed separately from the one lance; a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is blown into the molten iron from the one lance or the other lance passes through a flame formed by the burner; and the top-blowing lance is configured to secure a predetermined heating time as well as a predetermined powder-fuel ratio.

2. The top-blowing lance for a converter according to claim 1, wherein a distance l.sub.h (m) from a leading end of the lance having the burner to a molten metal surface and a discharge speed u.sub.p (m/s) of powder constituting the powdery auxiliary raw material or the auxiliary raw material processed into a powder form are determined so as to meet Expression 1 below, and a supply flow rate Q.sub.fuel (Nm.sup.3/min) of the fuel and a supply amount V.sub.p (kg/min) of the auxiliary raw material per unit time are determined so as to meet the relationship of Expression 2 below: $\begin{array}{cc}\left[\mathrm{Expression}1\right]& \\ \frac{l_h}{u_p}{t}_0& \left(1\right)\end{array}$ $\begin{array}{cc}\left[\mathrm{Expression}2\right]& \\ \frac{V_p}{Q_{\mathrm{fuel}}.Math.H_{\mathrm{combustion}}}{C}_0& \left(2\right)\end{array}$ where t.sub.0 represents a required heating time (s) obtained from a particle diameter of the powdery auxiliary raw material or the auxiliary raw material processed into a powder form, H.sub.combustion represents an amount of heat (MJ/Nm.sup.3) generated by fuel combustion, and C.sub.0 represents a constant (kg/MJ).

3. The top-blowing lance for a converter according to claim 2, wherein the required heating time to of the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is determined from a particle diameter d.sub.p of the powdery auxiliary raw material or the auxiliary raw material processed into a powder form, an adiabatic flame temperature of the fuel, a flow velocity of a combustion gas of the fuel, and the discharge speed u.sub.p of the powder.

4. The top-blowing lance for a converter according to claim 2, wherein the constant C.sub.0 in Expression 2 is determined by a type of fuel gas to be used.

5. A method for adding an auxiliary raw material when performing a refining process on molten iron contained in a converter-type vessel by supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 1, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

6. A method for performing a refining process on molten iron contained in a converter-type vessel by adding an auxiliary raw material and supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 1, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

7. The top-blowing lance for a converter according to claim 3, wherein the constant C0 in Expression 2 is determined by a type of fuel gas to be used.

8. A method for adding an auxiliary raw material when performing a refining process on molten iron contained in a converter-type vessel by supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 2, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

9. A method for adding an auxiliary raw material when performing a refining process on molten iron contained in a converter-type vessel by supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 3, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

10. A method for adding an auxiliary raw material when performing a refining process on molten iron contained in a converter-type vessel by supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 4, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

11. A method for adding an auxiliary raw material when performing a refining process on molten iron contained in a converter-type vessel by supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter according to claim 7, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

12. A method for performing a refining process on molten iron contained in a converter-type vessel by adding an auxiliary raw material and supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter claim 2, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

13. A method for performing a refining process on molten iron contained in a converter-type vessel by adding an auxiliary raw material and supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter claim 3, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

14. A method for performing a refining process on molten iron contained in a converter-type vessel by adding an auxiliary raw material and supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter claim 4, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

15. A method for performing a refining process on molten iron contained in a converter-type vessel by adding an auxiliary raw material and supplying an oxidizing gas to the molten iron, wherein, using the top-blowing lance for a converter claim 7, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form that is part of the auxiliary raw material is blown into the molten iron so as to pass through a flame formed by the burner, and the powdery auxiliary raw material or the auxiliary raw material processed into a powder form is heated for a predetermined heating time or longer and jetted at a predetermined powder-fuel ratio.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 is a schematic vertical sectional view showing an overview of a converter used in embodiments of the present invention.

[0030] FIG. 2 is schematic views of a burner according to one embodiment of the present invention, with FIG. (a) showing a vertical sectional view of a leading end of a lance, and FIG. (b) showing a bottom view of jetting holes as seen from below.

[0031] FIG. 3 is a graph showing a relationship between a powder-fuel ratio V/QH and heat transfer efficiency in the case where powder was supplied after being heated using the burner of the embodiment.

[0032] FIG. 4 is a graph showing an influence of a distance l.sub.h from a lance leading end to a molten metal surface on a relationship between a powder particle diameter d.sub.p and heat transfer efficiency in the case where powder was supplied after being heated using the burner of the embodiment.

[0033] FIG. 5 is a graph showing changes over time in particle temperature and combustion gas temperature for each powder particle diameter d.sub.p in the case where powder was supplied after being heated using the burner of the embodiment.

[0034] FIG. 6 is a graph showing suitable ranges of the present invention in a relationship between the powder-fuel ratio V/QH and an in-flame retention time l.sub.h/u.sub.p of powder.

DESCRIPTION OF EMBODIMENTS

[0035] Embodiments of the present invention will be specifically described below. The drawings are schematic and may differ from the reality. The following embodiments exemplify a device and a method for embodying the technical idea of the present invention, and are not intended to limit the configuration to the one described below. Thus, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0036] FIG. 1 is a schematic vertical sectional view of a converter-type vessel 1 having a top and bottom blowing function and used in a method for refining of molten iron of one embodiment of the present invention. FIG. 2 is schematic views of a leading end of a lance showing the structure of a burner having a powder supply function, with FIG. 2 (a) representing a vertical sectional view and FIG. 2 (b) being a view of section A-A.

[0037] For example, first, iron scrap as a cold iron source is charged into the converter-type vessel 1 through a scrap chute (not shown). Then, molten pig iron is charged into the converter-type vessel 1 using a charging ladle (not shown).

[0038] After the molten pig iron is charged, an oxygen gas is top-blown toward molten iron 3 from one lance 2 that is configured to top-blow an oxidizing gas. An inert gas, such as argon or N.sub.2, is supplied as a stirring gas from a tuyere 4 installed at the furnace bottom to thereby stir the molten iron 3. Then, auxiliary raw materials, such as a heating agent and a slag forming material, are added, and a dephosphorization process is performed on the molten iron 3 inside the converter-type vessel 1. Meanwhile, a powdery auxiliary raw material or an auxiliary raw material processed into a powder form (hereinafter, both may be collectively referred to as a powdery auxiliary raw material), such as lime powder, is supplied, using a carrier gas, through a powder supply pipe provided in the one lance 2 that top-blows an oxidizing gas or a powder supply pipe provided in another lance 5 that is installed separately from the one lance. Here, a burner having jetting holes for jetting a fuel and a combustion supporting gas is further provided at a leading end part of the one lance 2 or a leading end part of the other lance 5 installed separately from the one lance 2. During at least a part of the period of the dephosphorization process, the powdery auxiliary raw material supplied through the powder supply pipe is blown in so as to pass through a flame formed by this burner. FIG. 2 shows a schematic view of the leading end part of the lance 5 in the case where the lance 5 is installed separately from the one lance 2 and the burner is provided at the leading end of the lance 5. A powder supply pipe 11 having a jetting hole is disposed at the center, and a fuel supply pipe 12 and a combustion supporting gas supply pipe 13 both having a jetting hole are disposed in this order around the powder supply pipe 11. On the outer side, an outer shell having a cooling water passage 14 is provided. A fuel gas 16 and a combustion supporting gas 17 are supplied through the jetting holes provided in an outer periphery of the powder supply pipe 11 to form a burner flame. Then, the powdery auxiliary raw material (powder 15) is heated inside this burner flame. This turns the powdery auxiliary raw material into a heat transfer medium, so that the efficiency of heat transferred into the molten iron can be increased. As a result, the amount of heating agent to be used, such as a carbon source and a silicon source, can be reduced, and an increase in the dephosphorization process time can be avoided. To efficiently transfer heat to the powder, it is important to secure a time for which the powder 15 is retained inside the burner flame. As the oxidizing gas, other than pure oxygen, a mixture gas of oxygen with CO.sub.2 or an inert gas can be used. As the combustion supporting gas, air, oxygen-enriched air, or an oxidizing gas can be used. As the fuel to be supplied, a fuel gas, such as a liquefied natural gas (LNG) or a liquefied petroleum gas (LPG), a liquid fuel, such as heavy oil, or a solid fuel, such as coke powder, can be used. From the viewpoint of reducing the amount of CO.sub.2 generation, a fuel with less carbon source is preferable.

[0039] Using a converter-type vessel, the present inventors conducted a test of heating lime powder by a burner, with the flow rate of the carrier gas and the height of the lance changed to various values. As a result, we found that setting the retention time of powder in the burner flame to approximately 0.05 s to 0.1 s could achieve high heat transfer efficiency. For securing the in-flame retention time, reducing the flow velocity of the powder is effective. However, transporting the powder through a pipe requires supplying the carrier gas at a certain flow rate. Under realistic operation conditions, the flow velocity of the powder is within a range of 30 m/s to 60 m/s. Therefore, to secure the in-flame retention time, it is desirable to set the powder discharge hole (the leading end of the burner lance) to the position of a height (a lance height) of about 2 to 4 m from the molten iron surface. Details will be described below.

[0040] In the device configuration of FIG. 1, CaO powder having a mean particle diameter of 50 m was supplied as a powdery auxiliary raw material to a 330-ton-scale converter-type vessel 1 from the burner lance 5 at 500 kg/min. FIG. 3 shows an influence on the heat transfer efficiency of changing a powder-fuel ratio (V/QH) by changing the flow rate of the fuel gas 16 in this case. Here, the powder-fuel ratio (V/QH) is, as shown in Formula (2) of Expression 3 below, obtained by dividing the amount of powdery auxiliary raw material supplied per unit time by the product of the supply flow rate of the fuel and the amount of heat generated by fuel combustion. The heat transfer efficiency (%) is expressed as a percentage of an amount of heat transfer (MJ) as calculated from a change in the molten iron temperature relative to an amount of heat input (MJ) due to combustion of the fuel gas. The same applies hereinafter. Increasing the powder-fuel ratio resulted in increased heat transfer efficiency. This demonstrates that the heat transfer efficiency increases when heat generated by burner combustion is input into the powder and the heated powder is brought into the molten iron. It has been shown that obtaining such an increasing effect on the heat transfer efficiency requires maintaining appropriate amounts of gas and powder inside the burner flame. It has been shown that when the powder is too little relative to the flame gas, the heat transfer efficiency decreases as the ratio of heat discharged to the outside of the furnace as gas sensible heat increases. Next, as for the influence of the gas type, as has been clarified by FIG. 3, when an LPG is used, the heat transfer efficiency becomes constant at a powder-fuel ratio of 0.3 kg/MJ or higher. When an LNG is used, the heat transfer efficiency becomes constant at a powder-fuel ratio of 0.45 kg/MJ or higher. Therefore, it is necessary to control the powder-fuel ratio according to the type of fuel gas to be used. That is, Formula (2) below needs to be met. In Formula (2), V/QH represents a powder-fuel ratio (kg/MJ); V.sub.p represents an amount of powdery auxiliary raw material supplied per unit time (kg/min); Q.sub.fuel represents a supply flow rate (Nm.sup.3/min) of the fuel; H.sub.combustion represents an amount of heat (MJ/Nm.sup.3) generated by fuel combustion; and C.sub.0 represents a constant (kg/MJ) determined by the type of fuel gas to be used. The upper limit of the powder-fuel ratio is determined by a condition under which the temperature of the heated powder becomes equal to or lower than the molten iron temperature.

[00002]$\begin{array}{cc}\left[\mathrm{Expression}3\right]& \\ V/\mathrm{QH}=\frac{V_p}{Q_{\mathrm{fuel}}.Math.H_{\mathrm{combustion}}}{C}_0& \left(2\right)\end{array}$

[0041] In the device configuration of FIG. 1, CaO was supplied as a powdery auxiliary raw material to a 330-ton-scale converter-type vessel 1 from the burner lance 5 at 700 kg/min. FIG. 4 shows an influence exerted on the heat transfer efficiency by the mean particle diameter d.sub.p (m) of the powder and the distance (l.sub.h) from the leading end of the lance to the molten metal surface in this case. An LPG was used as the fuel gas, and the powder-fuel ratio (V/QH) was set to 0.5 kg/MJ. The heat transfer efficiency was found to decrease as the mean particle diameter of the CaO powder increased, and at the same particle diameter, the heat transfer efficiency was higher when the lance height was greater. The discharge flow velocity of the powder was within a range of 30 to 60 m/s.

[0042] A possible explanation is that how much the powder was heated while the powder was passing through the burner flame had an influence. Therefore, temperature transition of the powder passing through the flame was estimated by the following method with reference to Non Patent Literatures 1 to 3. A specific heat capacity C.sub.p, P of the powder was 1004 J/(kg.Math.K); a particle density was 3340 kg/m.sup.3; a particle radiation factor .sub.p was 0.9; and heat conductivity of the gas was 0.03 W/(m.Math.K). The fuel gas was an LPG, and the powder supply speed/fuel flow rate (V/Q) was set to 100 kg/Nm.sup.3. The combustion reaction is based on Chemical Reactions (a) to (e) shown in Chemical Formulae 1 to 5 below. The equilibrium constant K.sub.i of each reaction can be obtained by a partial pressure P.sub.G (G is a chemical formula of the gas type) of a gas involved in the reaction (i). Here, the suffix i represents Chemical Reaction Formulae (a) to (e) shown in Chemical Formulae 1 to 5 below. A total pressure P inside the combustion flame is, as the sum of the partial pressures of the respective gas types, 1 atm in total as in Formula (3) shown in Expression 4 below.

[00003]$\begin{array}{cc}\left[\mathrm{Formula}1\right]& \\ \begin{array}{cc}{\mathrm{CO}}_2.Math.\mathrm{CO}+\frac{1}{2}{O}_2& \left(K_a=\frac{P_{CO}.Math.P_{O_2}^{1/2}}{P_{C{O}_2}}\right)\end{array}& \left(a\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Formula}2\right]& \\ \begin{array}{cc}{H}_{2}O.Math.H_2+\frac{1}{2}{O}_2& \left(K_b=\frac{P_{H_2}.Math.P_{O_2}^{1/2}}{P_{H_{2}O}}\right)\end{array}& \left(b\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Formula}3\right]& \\ \begin{array}{cc}{H}_{2}O.Math.\frac{1}{2}{H}_2+\mathrm{OH}& \left(K_c=\frac{P_{OH}.Math.P_{H_2}^{1/2}}{P_{H_{2}O}}\right)\end{array}& \left(c\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Formula}4\right]& \\ \begin{array}{cc}\frac{1}{2}{H}_2.Math.H& \left(K_d=\frac{P_H}{P_{H_2}^{1/2}}\right)\end{array}& \left(d\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Formula}5\right]& \\ \begin{array}{cc}\frac{1}{2}{O}_2.Math.O& \left(K_e=\frac{P_O}{P_{O_2}^{1/2}}\right)\end{array}& \left(e\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Expression}4\right]& \\ P{P}_{\mathrm{CO}}+P_{{\mathrm{CO}}_2}+P_{O_2}+P_{H_2}+P_{H_{2}O}+P_{OH}+P_H+P_O=1\mathrm{atm}& \left(3\right)\end{array}$

[0043] Formula (4) is a formula for calculating an equilibrium flame temperature. The equilibrium flame temperature was estimated by a trial-and-error method such that the difference between an enthalpy change of the particles (H.sup.0H.sup.0.sub.298).sub.p from a base temperature to the equilibrium flame temperature and an enthalpy change of the gas (H.sup.0H.sup.0.sub.298).sub.g from the base temperature to the equilibrium flame temperature became equal to an enthalpy change (H.sup.0.sub.298) due to the gas reactions (a) to (e) that meets Formula (3).

[0044] Formula (5) is a formula that estimates a change in temperature of the particles as the sum of a heat input due to heat transfer and a heat input due to radiation.

[0045] Formula (6) is a formula for obtaining a heat flux of heat transfer.

[0046] Formula (7) is a formula for obtaining a heat flux of radiation.

[0047] Formula (8) is a formula that expresses a dimensionless relationship relating to forced convection with the flame as a heat fluid. Symbols Nu, Re.sub.p, and Pr represent a Nusselt number, a Reynolds number, and a Prandtl number, respectively.

[0048] Symbol m is the mass (kg) of the powder; C.sub.p, P is the specific heat capacity (J/(kg.Math.K)) of the powder; A.sub.S, P is the surface area (m.sup.2) of the particles; T.sub.g and T.sub.p are respectively the gas temperature and the particle temperature (K); q.sub.p and q.sub.R are respectively a convection heat transfer term and a radiation heat transfer term; is the heat conductivity (W/(m.Math.K)) of the gas; d is the particle diameter as a representative length; .sub.p is the radiation factor () of the particles; and is a Stefan-Bolzmann coefficient. The powder temperature T.sub.p was calculated by the fourth-order Runge-Kutta method.

[00004]$\begin{array}{cc}\left[\mathrm{Expression}5\right]& \\ {\left(H^0-H_{298}^0\right)}_P={\left(H^0-H_{298}^0\right)}_g-H_{298}^0& \left(4\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Expression}6\right]& \\ {\mathrm{mC}}_{p,P}\frac{d\left(T_P\right)}{dt}=A_{S,P}.Math.q_P+A_{S,P}.Math.q_R& \left(5\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Expression}7\right]& \\ q_P=\frac{Nu}{d}\left(T_g-T_P\right)& \left(6\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Expression}8\right]& \\ q_R={}_P.Math.\left(T_g^4-T_P^4\right)& \left(7\right)\end{array}$$\begin{array}{cc}\left[\mathrm{Expression}9\right]& \\ \mathrm{Nu}=2+0.6.Math.R{e}_P^{1/2}{\mathrm{Pr}}^{1/3}& \left(8\right)\end{array}$

[0049] FIG. 5 shows the influence of the particle diameter d.sub.p, as estimated by the above relational expressions, on the relationship between the change in the combustion gas temperature T.sub.g and the change in the particle temperature T.sub.p in the case where the powder passes through the flame. As can be seen from FIG. 5, the time taken for the temperature T.sub.p of the powder inside the flame to become equal to the gas temperature T.sub.g on the flame side varies greatly with the particle diameter d.sub.p. A required heating time t.sub.0 of the powdery auxiliary raw material can be set, for example, to such a time that the difference between the gas temperature T.sub.g and the particle temperature T.sub.p becomes 10 C. or smaller. Specifically, it is important that the powder discharge speed u.sub.p and the lance height l.sub.h meet the relationship of the following Formula (1), to control the heat transfer efficiency.

[00005]$\begin{array}{cc}\left[\mathrm{Expression}10\right]& \\ \frac{l_h}{u_p}{t}_0& \left(1\right)\end{array}$

[0050] To sufficiently heat the powdery auxiliary raw material by the flame of the burner, the burner lance 5 constituting the top-blowing lance for the converter of this embodiment is configured such that, for example, the lance height l.sub.h can be adjusted so as to set the in-flame retention time (l.sub.h/u.sub.p) of the powder to be equal to or longer than the required heating time t.sub.0. The required heating time t.sub.0 can be calculated, using the above estimation formula, from the particle diameter d.sub.p of the powdery auxiliary raw material, the adiabatic flame temperature of the fuel, the flow velocity of the combustion gas of the fuel, and the powder discharge speed u.sub.p. The lance height l.sub.h is subject to a facility restriction that prohibits the leading end of the lance from sticking out beyond the throat. An appropriate range of the powder discharge speed u.sub.p is obtained from the viewpoint of stably delivering the powder by a carrier gas. For example, the nozzle diameter of the burner lance 5 is designed such that the powder-fuel ratio (V/QH) can meet the above Formula (2).

[0051] FIG. 6 shows suitable ranges based on Formula (1) and Formula (2). In FIG. 6, the axis of abscissas represents the powder-fuel ratio V/QH (kg/MJ) and the axis of ordinates represents the in-flame retention time l.sub.h/u.sub.p (s) of the powder. The hatched areas indicate suitable ranges in the case where the powder particle diameter d.sub.p is 50 m and the fuel gas type is an LPG and the case where the powder particle diameter d.sub.p is 150 m and the fuel gas type is an LNG.

EXAMPLES

[0052] Using a 300-ton-capacity top and bottom blowing converter (with an oxygen gas top-blown and an argon gas bottom-blown) of the same form as the converter-type vessel 1 shown in FIG. 1, decarburization refining of molten iron was performed. As the top-blowing lance 2 for blowing oxygen, a lance having five Laval nozzle-type jetting nozzles at the leading end part was used. The top-blowing lance 2 used had the nozzles disposed at regular intervals on the same circumference relative to the axial center of the lance, with the jetting angle of the nozzles set to 15. The jetting nozzles had a throat diameter dt of 73.6 mm and an outlet diameter de of 78.0 mm.

[0053] First, iron scrap was charged into the converter. Then, 300 tons of molten pig iron that had been subjected to a desulfurization process and a dephosphorization process in advance was charged into the converter. Table 1 shows the chemical components of the molten pig iron and the temperature of the molten pig iron.

TABLE-US-00001 TABLE 1 Molten pig iron Chemical components of molten pig iron (mass %) temperature C Si Mn P S Cr Fe ( C.) 3.4 to 0.01 to 0.16 to 0.015 to 0.008 to tr bal. 1310 to 1360 3.6 0.02 0.27 0.036 0.016

[0054] Next, while an argon gas was blown into the molten iron 3 as a stirring gas from the bottom blowing tuyere 4, an oxygen gas was blown onto the bath surface of the molten iron 3 as an oxidizing gas from the top-blowing lance 2 to start decarburization refining of the molten iron 3. The amount of iron scrap to be charged was adjusted such that molten steel upon completion of decarburization refining had a temperature of 1650 C.

[0055] Then, quicklime was fed as a CaO-based flux during decarburization refining from the burner lance 5 for feeding auxiliary raw materials, and decarburization refining was performed until the concentration of carbon in the molten iron became 0.05% by mass. The amount of quicklime to be fed was adjusted such that the basicity ((mass % CaO)/(mass % SiO.sub.2)) of slag generated inside the furnace became 2.5. An LNG was used as the fuel gas, and the flow rate of the oxygen gas for combusting the fuel was controlled so as to achieve an air-fuel ratio of 1.2. The powder supply speed u.sub.p, the fuel gas flow rate Q.sub.fuel, and the lance height l.sub.h of the burner lance 5 for feeding auxiliary raw materials were controlled as shown in Table 2.

TABLE-US-00002 TABLE 2 Amount Amount Heat of heat of heat transfer d.sub.p u.sub.p l.sub.n l.sub.n/u.sub.p t.sub.o V.sub.p Q V/QH input transfer efficiency No. m m/s m s s kg/min N m.sup.3/min kg/MJ MJ/t MJ/t % Remarks 1 50 30 2.5 0.08 0.02 700 35 0.48 36.0 29.2 81 Invention Example 2 50 60 4.0 0.07 0.02 700 35 0.48 36.0 29.9 83 Invention Example 3 50 60 4.0 0.07 0.02 700 25 0.67 25.7 21.1 82 Invention Example 4 50 30 3.5 0.12 0.02 700 35 0.48 36.0 31.7 88 Invention Example 5 100 30 3.5 0.12 0.06 700 25 0.67 25.7 22.3 87 Invention Example 6 100 60 3.5 0.06 0.06 500 25 0.48 25.7 20.8 81 Invention Example 7 150 30 3.5 0.12 0.11 700 25 0.67 25.7 22.3 87 Invention Example 8 50 30 2.5 0.08 0.02 350 35 0.24 36.0 14.0 39 Comparative Example 9 50 30 2.5 0.08 0.02 500 35 0.34 36.0 15.5 43 Comparative Example 10 100 50 2.0 0.04 0.06 700 25 0.67 25.7 13.4 52 Comparative Example 11 100 60 2.5 0.04 0.06 350 35 0.24 36.0 12.2 34 Comparative Example 12 150 30 3.0 0.10 0.11 350 35 0.24 36.0 22.0 61 Comparative Example 13 150 60 3.0 0.05 0.11 550 35 0.38 36.0 20.9 58 Comparative Example

[0056] As is clear from Table 2, the heat transfer efficiency in the examples of the present invention was dramatically increased relative to that in the comparative examples. Further, the status of slag formation in the sequence of operation was evaluated. The components of slag were analyzed and the concentrations of non-slagged CaO (% fCaO) were compared. In processing conditions No. 1 to 7, (% fCaO) was 0 to 0.05% by mass, whereas in processing conditions No. 10 to 13, (% fCaO) was 0.4 to 2.6% by mass. Thus, the present invention was found to be also effective in promoting melting of CaO.

INDUSTRIAL APPLICABILITY

[0057] The top-blowing lance for a converter, the method for adding an auxiliary raw material, and a method for refining of molten iron of the present invention increase the heat transfer efficiency, making it possible to shorten the processing time and reduce the slag generation amount. Moreover, the time taken to melt slag is shortened and metallurgical efficiency increases. These advantages make the present invention useful for industrial purposes. The present invention is suitably applied to processes not only in a converter type but also in electric furnaces etc. that require a heat source.

REFERENCE SIGNS LIST

[0058] 1 Converter-type vessel [0059] 2 Top-blowing lance for oxidizing gas [0060] 3 Molten iron [0061] 4 Bottom blowing tuyere [0062] 5 Burner lance [0063] 10 Leading end part of burner lance [0064] 11 Powder supply pipe [0065] 12 Fuel supply pipe [0066] 13 Combustion supporting gas supply pipe [0067] 14 Cooling water passage [0068] 15 Powder [0069] 16 Fuel [0070] 17 Combustion supporting gas [0071] 18 Cooling water