METHOD FOR DESULFURIZING MOLTEN METAL

Abstract

Proposed is a method for efficiently desulfurizing molten metal in a short time without passing an excessive current when applying a potential difference between slag and metal. Using a direct-current power source, this method for desulfurizing molten metal applies a potential difference between molten slag and molten metal through electrodes, of which one electrode contacting the molten metal serves as a negative electrode and the other electrode contacting only the molten slag serves as a positive electrode. An applied current density J.sub.a is determined according to an equilibrated S concentration [S].sub.e0 before application of a potential difference such that an equilibrated S concentration [S].sub.ea when a potential difference is applied becomes equal to or lower than a target S concentration [S].sub.ft.

Claims

1. A method for desulfurizing molten metal that, using a direct-current power source, applies a potential difference between molten slag and molten metal through electrodes, of which one electrode contacting the molten metal serves as a negative electrode and the other electrode contacting only the molten slag serves as a positive electrode, characterized in that an applied current density J.sub.a is determined according to an equilibrated S concentration [S].sub.e0 before application of a potential difference such that an equilibrated S concentration [S].sub.ea when a potential difference is applied becomes equal to or lower than a target S concentration [S].sub.ft.

2. The method for desulfurizing molten metal according to claim 1, wherein the molten metal is molten iron, and wherein determining the applied current density J.sub.a(A/m.sup.2) involves: step 1 of, using Formulae (1) and (2) below, determining a value of A and a value of B based on coefficients listed in Table 1 that are obtained from an equilibrated S concentration [S].sub.e0 (mass ppm) before application of a potential difference; and step 2 of, using Formula (3) below, calculating an equilibrated S concentration [S].sub.ea (mass ppm) when a potential difference is applied based on the value of A and the value of B determined in step 1, and determining the applied current density J.sub.a such that the calculated equilibrated S concentration [S].sub.ea becomes equal to or lower than a target S concentration [S].sub.ft (mass ppm):
A=A0+A1.Math.J.sub.aA2.Math.J.sub.a.sup.2+A3.Math.J.sub.a.sup.3(1)
B=B0+B1.Math.J.sub.aB2.Math.J.sub.a.sup.2+B3.Math.J.sub.a.sup.3(2)
[S].sub.ea=ep(B/A)(3) TABLE-US-00003 TABLE 1 [S].sub.e0 A0 A1 A2 A3 B0 B1 B2 B3 mass ppm No. 10.sup.2 10.sup.4 10.sup.7 10.sup.10 10.sup.1 10.sup.4 10.sup.7 10.sup.11 1 Exceeding 10 6.4408 3.6497 3.8200 1.8218 1.1981 2.3088 1.4831 5.2189 2 Exceeding 6 but 7.4338 3.1714 2.9497 1.3469 1.2493 2.1868 1.3061 4.3146 not exceeding 10 3 Exceeding 5 but 8.1965 2.8943 2.4923 1.1083 1.2833 2.2380 1.5197 5.8263 not exceeding 6 4 Exceeding 4 but 10.012 2.4337 1.8788 8.3450 1.4139 2.0718 1.4623 6.2218 not exceeding 5 5 Not exceeding 4 14.236 1.6594 0.91710 4.0404 1.7774 1.4833 0.79567 3.4848

3. The method for desulfurizing molten metal according to claim 2, wherein the applied current density J.sub.a is set to 1000 A/m.sup.2 or lower.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 (a) is a schematic view showing one example of devices suitable for a method of the present invention, and FIG. 1 (b) is a schematic view showing another example of devices suitable for the method of the present invention.

[0023] FIG. 2 is a graph showing the influence of an equilibrated S concentration [S].sub.e0 before application of a potential difference on a relationship between an applied current density J.sub.a and an equilibrated S concentration [S].sub.ea.

[0024] FIG. 3 is a graph showing a relationship between an overall mass transfer coefficient k of S between slag and molten iron and the applied current density J.sub.a.

DESCRIPTION OF EMBODIMENT

[0025] An embodiment of the present invention will be specifically described below. The drawings are schematic and may differ from the reality. The following embodiment exemplifies a device and a method for embodying the technical idea of the present invention, and is not intended to limit the configuration to the following one. That is, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0026] FIG. 1 (a) is a schematic view showing one example of devices suitably used for a method for desulfurizing molten metal according to one embodiment of the present invention. Similarly, FIG. 1 (b) is a schematic view showing another example of devices suitably used for the method for desulfurizing molten metal. Examples of molten metal include molten iron and copper alloy. In particular, molten iron is preferably used as molten metal. Here, the term molten iron refers to melted metal composed mainly of Fe and covers molten pig iron, melted cold iron source, and molten steel. Hereinafter, an embodiment of the present invention will be described using molten iron as an example.

[0027] In the device of FIG. 1 (a), molten iron 3 is charged into a vessel 1, such as a ladle, that is lined with an insulating refractory 2, and slag 4 is added onto the molten iron 3. Electrically conductive substances 5 are disposed on the side of the slag 4 and the side of the molten iron 3 (in the example of FIG. 1 (a), on the furnace bottom), and are connected to a stabilized direct-current power source 7 through lead wires 6. Here, the lead wire 6 connected to the electrically conductive substance 5 disposed on the side of the slag 4 is connected to the plus side (positive electrode) of the stabilized direct-current power source 7. The lead wire 6 connected to the electrically conductive substance 5 disposed on the side of the molten iron 3 is connected to the minus side (negative electrode) of the stabilized direct-current power source 7. While the electrically conductive substance 5 on the side of the molten iron 3 is disposed on the furnace bottom in the example of FIG. 1 (a), this electrically conductive substance 5 may instead be disposed on a furnace wall.

[0028] As shown in FIG. 1 (b), the electrically conductive substance 5 disposed on the side of the molten iron 3 may be partially in contact with the slag 4. In this case, it is necessary to dispose this electrically conductive substance 5 such that a shortest distance L 1 from a leading end of the electrically conductive substance 5 immersed on the side of the slag 4 to a slag-molten iron interface becomes smaller than a shortest distance L.sub.2 between surfaces of the electrically conductive substances 5 in the slag 4.

[0029] In the device configuration like that of FIG. 1 (a) or (b), applying a potential difference from the external power source 7 can pass a current between the slag and the molten iron.

[0030] In steelmaking, Al.sub.2O.sub.3-based bricks or refractories of indefinite shape are often used as the insulating refractory 2. It may be substituted by another refractory that does not have electrical conductivity.

[0031] As the electrically conductive substance 5, a graphite shaft or an MgOC-based refractory can be used. Any other substances that have electrical conductivity and do not melt in a range of molten iron temperature (1300 to 1700 C.) can be used as a substitute.

[0032] An injection lance or a bottom-blowing porous plug may be added to the device, and a gas may be blown into the molten iron 3 through it to apply agitation to the molten metal.

[0033] Next, a method of determining an applied current density according to this embodiment will be described.

[0034] First, an equilibrated S concentration [S].sub.e0 (mass ppm) before application of a potential difference in a desulfurization reaction between the slag 4 and the molten iron 3 is obtained. This concentration can be determined based on a sulfide capacity of the slag 4 used as described above, oxygen activity in the molten iron, the temperature, and also the amount of slag 4 used. The calculation method will not be described in detail here, as it is already commonly known from papers etc. and any person skilled in the art can easily perform the calculation. For example, the concentration can be calculated by using the theoretical optical basicity of oxides presented in Non-Patent Literature 3 for the theoretical optical basicity of slag in the formula of a sulfide capacity proposed in Non-Patent Literature 2.

[0035] Other than the aforementioned calculation method using the sulfide capacity, a calculation in past operation under similar conditions or an actually measured S distribution ratio Ls=(mass % S)/[mass % S] may be referred to, and the equilibrated S concentration [S].sub.e0 (mass ppm) before application of a potential difference may be obtained from this and the amount of slag. Here, (mass % S) and [mass % S] represent a S concentration in the slag and a S concentration in the molten iron, respectively. Similar conditions can be selected in terms of the slag composition, the element composition of the molten iron, the processing temperature, etc.

[0036] First, as step 1 in determining an applied current density J.sub.a (A/m.sup.2), with reference to the value of the equilibrated S concentration [S].sub.e0 (mass ppm) before application of a potential difference obtained by the above technique, the coefficients of the following General Formulae (1) and (2) are specified according to the classification of cases below. Then, a formula for determining the value of A and the value of B is determined. Symbol J.sub.a in the formulae denotes the applied current density (A/m.sup.2), with J.sub.a>0 holding true.

A=A0+A1.Math.J.sub.aA2.Math.J.sub.a.sup.2+A3.Math.J.sub.a.sup.3(1)

B=B0+B1.Math.J.sub.aB2.Math.J.sub.a.sup.2+B3.Math.J.sub.a.sup.3(2)

(1) Case where [S].sub.e0>10 mass ppm

A=6.440810.sup.2+3.649710.sup.4.Math.J.sub.a3.820010.sup.7.Math.J.sub.a.sup.2+1.821810.sup.10J.sub.a.sup.3

B=1.198110.sup.1+2.308810.sup.4J.sub.a1.483110.sup.7J.sub.a.sup.2+5.218910.sup.11.Math.J.sub.a.sup.3

[0037] (2) Case where 10 mass ppm [S].sub.e0>6 mass ppm

A=7.433810.sup.2+3.171410.sup.4.Math.J.sub.a2.949710.sup.7.Math.J.sub.a.sup.2+1.346910.sup.10.Math.J.sub.a.sup.3

B=1.249310.sup.1+2.186810.sup.4.Math.J.sub.a1.306110.sup.7.Math.J.sub.a.sup.2+4.314610.sup.11.Math.J.sub.a.sup.3

[0038] (3) Case where 6 mass ppm [S].sub.e0>5 mass ppm

A=8.196510.sup.2+2.894310.sup.4.Math.J.sub.a2.492310.sup.7.Math.J.sub.a.sup.2+1.108310.sup.10.Math.J.sub.a.sup.3

B=1.283310.sup.1+2.238010.sup.4.Math.J.sub.a1.519710.sup.7.Math.J.sub.a.sup.2+5.826310.sup.11.Math.J.sub.a.sup.3

[0039] (4) Case where 5 mass ppm [S].sub.e0>4 mass ppm

A=1.001210.sup.1+2.433710.sup.4.Math.J.sub.a1.878810.sup.7.Math.J.sub.a.sup.2+8.345010.sup.10.Math.J.sub.a.sup.3

B=1.413910.sup.1+2.071810.sup.4.Math.J.sub.a1.462310.sup.7.Math.J.sub.a.sup.2+6.221810.sup.11.Math.J.sub.a.sup.3

[0040] (5) Case where 4 mass ppm[S].sub.e0

A=1.423610.sup.1+1.659410.sup.4.Math.J.sub.a9.171010.sup.8.Math.J.sub.a.sup.2+4.040410.sup.10.Math.J.sub.a.sup.3

B=1.777410.sup.1+1.483310.sup.4.Math.J.sub.a7.956710.sup.8.Math.J.sub.a.sup.2+3.484810.sup.11.Math.J.sub.a.sup.3

[0041] Next as step 2, using the value of A and the value of B determined in step 1, an equilibrated S concentration [S].sub.ea (mass ppm) when a potential difference has been applied is calculated by Formula (3) below. FIG. 2 shows the applied current density J.sub.a (A/m.sup.2) on the axis of abscissas and the equilibrated S concentration [S].sub.ea (mass ppm) on the axis of ordinates, as plotted according to the above-described classification of cases. From FIG. 2, the applied current density J.sub.a and the corresponding equilibrated S concentration [S].sub.ea can be obtained.

[S].sub.ea=ep(B/A)(3)

[0042] Using FIG. 2, one can obtain, for the equilibrated S concentration [S].sub.e0 before application of a potential difference, information on a minimum applied current density J.sub.a required for making the equilibrated S concentration [S].sub.ea when a potential difference is applied equal to or lower than a target S concentration [S].sub.ft upon completion of the desulfurization process. It is preferable that the equilibrated S concentration [S].sub.ea when a potential difference is applied be made lower than the target S concentration [S].sub.ft upon completion of the desulfurization process.

[0043] For example, in the case where the equilibrated S concentration [S].sub.e0 before application of a potential difference is 8 mass ppm and the target S concentration [S].sub.ft upon completion of the process is desired to be 5 mass ppm, from the result of the calculation using the determination formula (2), the applied current density J.sub.a can be estimated to be about 20 A/m.sup.2 or higher.

[0044] Since the desulfurization speed is defined by d[S]/dt=k.Math.(A/V) ([S][S].sub.e) as described above, other than lowering the equilibrated S concentration [S].sub.e, increasing the overall mass transfer coefficient k can also increase the desulfurization speed.

[0045] When an overall mass transfer coefficient k.sub.a(m/s) when a potential difference has been applied and an overall mass transfer coefficient k.sub.0(m/s) before application of a potential difference were compared, it was found that the ratio k.sub.a/k.sub.0() between these overall mass transfer coefficients had dependency on the applied current density J.sub.a as shown in FIG. 3.

[0046] From FIG. 3, the inclination of the increasing rate of the overall mass transfer coefficient is small at an applied current density J.sub.a exceeding 1000 A/m.sup.2. Thus, when 1000 A/m.sup.2 is exceeded, increasing the applied current density J.sub.a cannot be expected to have an increasing effect on the overall mass transfer coefficient. It is therefore preferable that the applied current density J.sub.a be set to 1000 A/m.sup.2 or lower.

[0047] When calculating an amount of current I a (A)=J.sub.aA to be passed to the circuit from the applied current density J.sub.a (A/m.sup.2) obtained as described above, it is preferable that an area of contact (m.sup.2) between slag and molten iron charged in a vessel be used as the area A. In reality, however, the accurate area of contact is often unknown due to agitation applied to the molten iron etc. Therefore, it is also acceptable to use for the calculation an inner cross-sectional area (m.sup.2) of the device at the position of the slag-molten iron interface that takes the insulating refractory 2 lining the vessel 1 into account. This is because the slag 4 itself behaves as a positive electrode (anode) and the molten iron 3 itself behaves as a negative electrode (cathode) through the electrically conductive substances. The present inventor has confirmed that changing the ratio between the cross-sectional areas of leading end portions of the respective electrically conductive substances 5 on the side of the slag 4 and on the side of the molten iron 3 did not affect the S concentration reached or the desulfurization speed.

EXAMPLES

[0048] In the following, examples of the present invention will be described in detail.

[0049] Using a device having the configuration of FIG. 1 (a), CaOAl.sub.2O.sub.3 binary slag, CaOAl.sub.2O.sub.3MgO ternary slag, or CaOAl.sub.2O.sub.3SiO.sub.2MgO quaternary slag was added onto molten iron of 1400 C. to 1650 C. In the slag, a lead wire was connected to a plus electrode (positive electrode) of an external power source through a graphite shaft, and in the molten iron, a lead wire was connected to a minus electrode (negative electrode) of the external power source through an MgOC brick, and a current was passed to the circuit by a constant-current application method using a stabilized direct-current power source.

[0050] The slag composition, the oxygen activity in the molten iron, the temperature, and the slag feed amount were adjusted such that the equilibrated S concentration [S].sub.e0 before application of a potential difference assumed the following five levels: 11.0, 8.0, 5.5, 4.5, and 3.8 mass ppm.

[0051] For the applied current density J.sub.a (A/m.sup.2), the value of A and the value of B were obtained from the above-described determination formulae (1) to (5), and an applied current density J.sub.min (A/m.sup.2) at which the equilibrated S concentration [S].sub.ea when a potential difference is applied meets the target S concentration [S] ft upon completion of the process was obtained by the above-described steps 1 and 2. A current was passed from the stabilized direct-current power source with the applied current density J.sub.a set according to the following three patterns: (a) lower than the obtained value J.sub.min; (b) not lower than the obtained value J.sub.min and not higher than 1000 A/m.sup.2; and (c) higher than 1000 A/m.sup.2.

[0052] For the current to be passed, a value obtained as the product of the cross-sectional area A (m.sup.2) of the device at an upper end of the part where the molten iron was charged and the applied current density J.sub.a (A/m.sup.2) was used as a set current value I.sub.a (A) of the stabilized direct-current power source, and this current was maintained for the duration of a desulfurization process time ta (min).

[0053] Tests were conducted within a range of 0.018 to 18 m.sup.2 of the cross-sectional area A of the device at the upper end of the part where the molten iron was charged, and a range of 6.8 kg to 280 t of the amount of molten iron. Table 2 shows test conditions and results. In the column Judgment, circles are indicated for those tests in which the S concentration [S].sub.fa reached was not higher than the target S concentration [S].sub.ft, and crosses are indicated for those tests in which the S concentration [S].sub.fa reached was higher than the target S concentration [S].sub.ft. Table 2 also lists the equilibrated S concentration [S].sub.ea when a potential difference is applied, the ratio k.sub.a/k.sub.0 () between the overall mass transfer coefficients in desulfurization, the process time ta, and the desulfurization speed d[S]/dt (mass ppm/min). Here, for the process time ta, the equilibrated S concentration [S].sub.ea when a potential difference is applied and the overall mass transfer coefficient k a were substituted for the values in the linear formula (shown above) of temporal changes in S concentration [S] relating to the desulfurization reaction to thereby predict changes in S concentration [S] over time. A time with which the S concentration [S].sub.fa reached became lower than a value obtained by adding 0.5 mass ppm to the equilibrated S concentration [S].sub.ea was predicted, and that time was used as the process time ta.

TABLE-US-00002 TABLE 2 [S].sub.e0 [S].sub.ft [S].sub.fa [S].sub.ea -d[S]/dt mass mass J.sub.min J.sub.a mass mass k.sub.a/k.sub.0 ta mass No. ppm ppm A/m.sup.2 A/m.sup.2 ppm ppm Judgement min ppm/min Remarks 1 11.0 6.4 10 0 11.4 11.0 1 80 0.61 Comparative Example 2 11.0 6.4 10 5 6.6 6.2 1.05 73 0.71 Comparative Example 3 11.0 6.4 10 10 6.3 6.0 1.10 66 0.81 Invention Example 4 11.0 6.4 10 60 5.2 4.8 1.45 49 1.12 Invention Example 5 11.0 6.4 10 100 4.4 4.3 1.57 44 1.27 Invention Example 6 11.0 6.4 10 600 3.9 3.2 1.85 36 1.54 Invention Example 7 11.0 6.4 10 1000 3.4 3.0 1.87 35 1.60 Invention Example 8 11.0 6.4 10 1200 3.4 2.8 1.87 35 1.61 Invention Example 9 8.0 4.4 100 0 8.4 8.0 1 80 0.65 Comparative Example 10 8.0 4.4 100 90 4.6 4.2 1.57 48 1.14 Comparative Example 11 8.0 4.4 100 100 4.4 4.1 1.57 46 1.21 Invention Example 12 8.0 4.4 100 300 3.9 3.4 1.76 40 1.40 Invention Example 13 8.0 4.4 100 1000 3.4 3.0 1.87 37 1.53 Invention Example 14 8.0 4.4 100 1100 3.4 2.9 1.87 37 1.53 Invention Example 15 5.5 4.4 90 0 5.9 5.5 1 80 0.68 Comparative Example 16 5.5 4.4 90 80 4.6 4.1 1.53 51 1.08 Comparative Example 17 5.5 4.4 90 100 4.4 4.0 1.59 48 1.16 Invention Example 18 5.5 4.4 90 500 3.7 3.2 1.83 41 1.39 Invention Example 19 4.5 4.4 20 0 4.9 4.5 1 80 0.69 Comparative Example 20 4.5 4.4 20 10 4.6 4.1 1.10 72 0.77 Comparative Example 21 4.5 4.4 20 20 4.4 4.0 1.20 66 0.85 Invention Example 22 4.5 4.4 20 400 3.7 3.3 1.81 42 1.33 Invention Example 23 3.8 3.4 500 0 4.2 3.8 1 80 0.70 Comparative Example 24 3.8 3.4 500 400 3.6 3.2 1.80 43 1.30 Comparative Example 25 3.8 3.4 500 500 3.4 3.1 1.82 42 1.33 Invention Example 26 3.8 3.4 500 700 3.4 3.1 1.85 42 1.35 Invention Example 27 3.8 3.4 500 900 3.4 3.0 1.87 41 1.37 Invention Example

[0054] At each of the levels, in tests in which the minimum required applied current density J.sub.min was not met (No. 2, 10, 16, 20, and 24), the S concentration [S].sub.fa reached with the same process time was lower than that in tests in which no current was passed (No. 1, 9, 15, 19, and 23), but failed to achieve the target S concentration [S].sub.ft. On the other hand, in tests in which a current with a density not lower than the minimum required applied current density L.sub.min (No. 3 to 8, 11 to 14, 17, 18, 20, 21, and 25 to 27) was passed, the S concentration [S].sub.fa reached was reduced to the target S concentration [S] ft or lower.

[0055] In tests in which a current was passed at an applied current density higher than 1000 A/m.sup.2 (No. 8 and 14), both the S concentration [S].sub.fa reached and the process time to were equivalent to those in tests in which a current was passed at an applied current density of 1000 A/m.sup.2 (No. 7 and 13). This demonstrates that passing a current at a current density exceeding 1000 A/m.sup.2 does not significantly affect at least the S concentration reached and the desulfurization reaction speed.

INDUSTRIAL APPLICABILITY

[0056] The present invention can reduce the S concentration reached per fixed process time while keeping power consumption down, and can achieve a reduction in the production cost. The present invention is suitably applied not only to desulfurization of molten metal but also to refining in which an electrochemical reaction at an interface is dominant.

REFERENCE SIGNS LIST

[0057] 1 Vessel [0058] 2 (Insulating) refractory [0059] 3 Molten iron [0060] 4 Slag [0061] 5 Electrically conductive substance [0062] 6 Lead wire [0063] 7 Stabilized direct-current power source