Zinc removal method and zinc removal device

Abstract

Provided is a method for removing zinc, with which it is possible to determine the heating temperature of a galvanized steel sheet and remove zinc while taking into account oxidation of lead. A method for removing zinc, in which zinc is removed from a galvanized steel sheet housed in a container, in which the temperature and the partial pressure of oxygen in the container satisfy the relationships of formulae (1)-(3). T > = 17.6 log (PO4) + 1031 (1) 700 < = T < = 1200 (2) PO2 < = 10.13 (3) In the formulae (1)-(3), T is the temperature (DEG C) in the container, and PO2 is the partial pressure (kPa) of oxygen in the container.

Description

Technical Field

[0001] The present invention relates to a zinc removal method and a zinc removal device for removing zinc from a galvanized steel sheet. Background Art

[0002] Galvanized steel sheets have extremely excellent corrosion resistance and are therefore used in various applications such as automobile bodies and building materials. In recent years, from the perspective of effective utilization of steel materials, a large amount of steel materials that have entered the market, represented by galvanized steel sheets, have been collected as scrap steel and reused as raw materials for electric furnace steelmaking or converter steelmaking processes. In addition, compared with the blast furnace / converter conventional ironmaking process using iron ore as raw material, the electric furnace steelmaking process using scrap steel as raw material is more effective in reducing CO2 emissions in the ironmaking process, so it is expected that the use of scrap steel raw materials including galvanized steel sheets will further increase in the future.

[0003] However, the temperature of the molten iron in these steelmaking processes also reaches about 1700°C, so the zinc contained in the galvanized steel sheet volatilizes and mixes into the ironmaking dust or transfers to the molten iron. As for the zinc in the molten iron, since the saturated vapor pressure of zinc is higher than that of the molten iron, it is easy to volatilize, so most of it is finally accumulated in the ironmaking dust.

[0004] In recent years, technology development has been underway to reuse byproducts such as ironmaking dust as an iron source. For example, ironmaking dust is reused in the sintering process of iron ore, but when ironmaking dust containing zinc is reused in the sintering process, zinc accumulates in the sintered ore. When the sintered ore containing zinc is used as an ironmaking raw material in a blast furnace, metallic zinc vapor is generated in the blast furnace, and then the metallic zinc precipitated by cooling adheres to the furnace and grows. As a result, the air permeability in the blast furnace deteriorates, or the zinc that is not completely removed is picked up into the molten iron, making it difficult to apply it to the recycling of galvanized steel sheets.

[0005] When the zinc concentration in ironmaking dust is 50% by mass or more, it can also be considered to be sold to zinc refiners as a raw material for crude zinc production. However, in general, the zinc concentration in blast furnace dust and converter dust is about 1% by mass, even if it is high, and even electric furnace dust is about 10% to 40% by mass at most, so it is difficult to use it as a raw material in the crude zinc production process. Therefore, these zinc-containing dusts need to be treated as industrial waste, which has become one of the main reasons for the deterioration of the manufacturing cost of steel materials.

[0006] Under such circumstances, technologies for removing zinc contained in galvanized steel sheets in advance have been studied. For example, Patent Document 1 discloses a method of supplying superheated steam into a treatment chamber containing scrap steel raw materials to remove zinc contained in the scrap steel.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2018-145499 Summary of the invention

[0010] Problems to be solved by the invention

[0011] When zinc is heated, zinc is oxidized by oxygen in the atmosphere, and zinc oxide is formed on the surface of the zinc coating, which hinders the volatilization and removal of zinc. Therefore, in order to efficiently remove zinc, it is necessary to consider the influence of zinc oxidation and determine the heating temperature of zinc, but in Patent Document 1, the influence of zinc oxidation is not considered at all. Therefore, the method disclosed in Patent Document 1 has the problem that zinc cannot be efficiently removed by zinc oxidation in some cases.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a zinc removal method and a zinc removal device capable of removing zinc by determining the heating temperature of a galvanized steel sheet while taking zinc oxidation into consideration.

[0013] Methods used to solve problems

[0014] A method for solving the above-mentioned problem is as follows.

[0015] [1] A method for removing zinc from a galvanized steel sheet contained in a container, wherein the temperature and oxygen partial pressure in the container satisfy the relationship of the following equations (1) to (3):

[0016] T≥17.6log(P O2 )+1031…(1)

[0017] 700≤T≤1200…(2)

[0018] P O2 ≤10.13…(3)

[0019] In the above formulas (1) to (3), T is the temperature in the above container (°C), P O2 is the oxygen partial pressure in the above container (kPa).

[0020] [2] The method for removing zinc according to [1], wherein the galvanized steel sheet is contained in the container for 5 minutes or more.

[0021] [3] The method for removing zinc according to [1] or [2], wherein the interior of the container is heated using sensible heat of exhaust gas generated from a smelting furnace.

[0022] [4] The method for removing zinc according to [3], wherein the interior of the container is heated using sensible heat of the molten iron contained in the smelting furnace.

[0023] [5] The method for removing zinc according to [3], wherein an exhaust gas satisfying the following relationships (4) to (6) is supplied into the container.

[0024] T g ≥17.6log(P O2,g )+1031…(4)

[0025] 700≤T g ≤1200…(5)

[0026] P O2,g ≤10.13…(6)

[0027] In the above formulas (4) to (6), T g is the temperature of the above exhaust gas (℃), P O2,g is the oxygen partial pressure of the above exhaust gas (kPa).

[0028] [6] A zinc removal device, which is a zinc removal device for removing zinc from a galvanized steel sheet, comprising a container for accommodating the galvanized steel sheet, a thermometer for measuring the temperature in the container, a component concentration meter for measuring the concentration of a gas component in the container, and a control device for controlling the temperature and the concentration of the gas component in the container, wherein the control device controls the temperature and oxygen concentration in the container in a manner that satisfies the relationship of the following equations (1) to (3).

[0029] T≥17.6log(P O2 )+1031…(1)

[0030] 700≤T≤1200…(2)

[0031] P O2 ≤10.13…(3)

[0032] In the above formulas (1) to (3), T is the temperature in the above container (°C), P O2 is the oxygen partial pressure in the above container (kPa).

[0033] [7] The zinc removal device according to [6], wherein the container is connected to a waste gas flow path of a smelting furnace that contains molten iron, and the control device controls the temperature and oxygen concentration in the container by controlling the flow rate of the waste gas flowing into the container.

[0034] Effects of the Invention

[0035] According to the present invention, the heating temperature and the oxygen partial pressure in the container for accommodating the galvanized steel sheet are determined in consideration of the oxidation of zinc, so that zinc can be efficiently removed from the galvanized steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1It is a schematic cross-sectional view of a zinc removal apparatus capable of carrying out the zinc removal method of the present embodiment.

[0037] Figure 2 It is a schematic cross-sectional view of another zinc removal apparatus capable of carrying out the zinc removal method of the present embodiment.

[0038] Figure 3 This is a diagram showing the range of the temperature and oxygen partial pressure in the container that satisfies the relationship of equations (1) to (3).

[0039] Figure 4 It is a graph showing the range of the exhaust gas temperature and the oxygen partial pressure that satisfies the relationship of equations (4) to (6). DETAILED DESCRIPTION

[0040] First, the experimental results for conceiving the present invention are described. The inventors used a laboratory-scale resistance furnace and a real electric furnace to heat a galvanized steel sheet. In this treatment, various changes were made to the temperature, atmosphere conditions, and heating time during the heating treatment, and the removal rate of zinc contained in the galvanized steel sheet after the heating treatment was investigated. As a result, it was found that in order to remove zinc from the galvanized steel sheet, it is effective to heat the galvanized steel sheet under the conditions of a specified temperature and oxygen partial pressure.

[0041] First, the temperature in the container for heating the galvanized steel sheet and the oxygen partial pressure in the atmosphere in the container are described. The temperature in the container for heating the galvanized steel sheet and the oxygen partial pressure in the atmosphere in the container need to satisfy the following relationships (1) to (3).

[0042] T≥17.6log(P O2 )+1031…(1)

[0043] 700≤T≤1200…(2)

[0044] P O2 ≤10.13…(3)

[0045] In the above formulas (1) to (3), T is the temperature in the container (°C), P O2 is the partial pressure of oxygen in the container (kPa).

[0046] It is believed that maintaining the temperature in the container above the boiling point of zinc (907°C) is effective for volatilizing and removing zinc. However, when the galvanized steel sheet is heated in the atmosphere at 1000°C, the zinc is oxidized by the oxygen in the atmosphere, and zinc oxide is formed on the surface of the zinc coating. The zinc oxide hinders the volatilization and removal of zinc. Moreover, the steel sheet as the base material is also oxidized, so when the galvanized steel sheet after the heat treatment is used as a raw material to produce molten steel, it may lead to a deterioration in the iron yield. In contrast, the inventors have found that the lower the oxygen partial pressure in the container, the more the above-mentioned adverse effects of oxygen can be suppressed, and the temperature required to efficiently remove zinc from the galvanized steel sheet is reduced. Therefore, the temperature and oxygen partial pressure in the container need to satisfy the relationship of the above formula (1).

[0047] However, when the heating temperature is lower than 700°C, the temperature difference with the boiling point of zinc (907°C) is large, and the volatilization rate of zinc slows down, so the effect of removing zinc cannot be fully obtained. On the other hand, when the heating temperature exceeds 1200°C, the zinc in the coating diffuses into the steel plate as the base material, and it is easy to form a Fe-Zn alloy phase, and the zinc may not volatilize but mix into the base material. Therefore, the temperature in the container needs to satisfy the relationship of the above formula (2).

[0048] In addition, when the oxygen partial pressure exceeds 10.13 kPa, even if the heating temperature and the oxygen partial pressure satisfy the relationship of formula (1), the oxidation of zinc caused by oxygen in the atmosphere cannot be suppressed, and the volatilization and removal of zinc will be hindered. Therefore, the oxygen partial pressure in the container needs to satisfy the relationship of formula (3) above.

[0049] It was found that by setting the temperature and oxygen partial pressure in the container containing the galvanized steel sheet to satisfy the relationship of the above formulas (1) to (3), the oxidation of zinc and even the steel sheet itself can be suppressed, and zinc can be volatilized and removed efficiently, thereby completing the present invention. The present invention is described below through the embodiments of the invention.

[0050] Figure 1 2 is a schematic cross-sectional view of a zinc removal device 1 capable of implementing the zinc removal method of the present embodiment. Figure 1 As shown, the zinc removal device 1 has a container 3 for accommodating a galvanized steel sheet 2 and a control device 4 for controlling the temperature and concentration of gas components in the container 3. The container 3 is not limited to a specific shape or structure, and a holding container for accommodating scrap steel materials such as galvanized steel sheets, a bucket for transporting and loading scrap steel materials, etc. can be used.

[0051] The container 3 is heated to 700° C. or higher, and is therefore preferably formed of a heat-resistant material such as a refractory brick or a steel plate. The container 3 includes a thermometer 5 for measuring the temperature inside the container 3, a component concentration meter 6 for measuring the concentration of gas components inside the container 3, a heating device 7 for heating the inside of the container 3, and an inert gas supply device 8 for supplying various gases into the container 3.

[0052] The thermometer 5 is a device for measuring the temperature in the container 3, such as a thermocouple. The component concentration meter 6 is a device for measuring the component concentration of gases such as oxygen, CO, CO2 and hydrogen in the container 3, such as a mass spectrometer or a laser analyzer. It should be noted that the measurement limit of the oxygen concentration in the atmosphere of a general component concentration meter is about 0.01 volume %, and the oxygen partial pressure that can be directly measured under atmospheric pressure is about 1.01 kPa.

[0053] Therefore, when measuring the oxygen partial pressure within a range of less than 1.01 kPa, as shown in the following formulas (7) to (10), the equilibrium oxygen partial pressure value when the reaction of CO gas and CO2 gas is in equilibrium, or the equilibrium oxygen partial pressure value when the reaction of hydrogen gas and water vapor is in equilibrium, can also be used. That is, when the oxygen concentration in the atmosphere is lower than the lower limit of the concentration meter and the oxygen concentration cannot be directly measured, the oxygen concentration can be calculated using the following formulas (7) to (10) from the concentration values ​​of CO gas, CO2 gas or hydrogen gas, and water vapor measured simultaneously.

[0054]

[0055]

[0056]

[0057]

[0058] ΔG7 in the above formula (8) is the Gibbs standard free energy change (J / mol) in the above formula (7), and ΔG9 in the above formula (10) is the Gibbs standard free energy change (J / mol) in the above formula (9). a is the absolute temperature (K), R is the gas constant (J / (mol·K)), P CO , P CO2 , P H2 and P H2O They are CO partial pressure (kPa), CO2 partial pressure (kPa), hydrogen partial pressure (kPa) and water vapor partial pressure (kPa).

[0059] The heating device 7 is a device for heating the inside of the container 3. The heating device 7 can heat the inside of the container 3 by heating the galvanized steel sheet 2 accommodated in the container 3. The heating device 7 is, for example, an electromagnetic induction heating device or an oxygen burner.

[0060] The inert gas supply device 8 is a device that supplies an inert gas such as N2 or Ar into the container 3 to adjust the oxygen concentration in the container 3. It should be noted that a reducing gas supply device that supplies a reducing gas such as CO gas or hydrogen gas may be used instead of the inert gas supply device 8, or a decompression device that exhausts the inside of the container 3 to create a decompression atmosphere may be used.

[0061] The control device 4 is a device for controlling the temperature and the concentration of the oxygen component in the container 3. The control device 4 is, for example, a general-purpose computer such as a workstation or a personal computer. The control device 4 obtains temperature data representing the temperature in the container 3 from the thermometer 5, and determines, for example, whether the temperature data is lower than the intermediate value (950°C) of the above formula (2). When the control device 4 determines that the temperature data is lower than 950°C, the control device 4 outputs a heating signal to the heating device 7 to heat until the temperature in the container 3 reaches 950°C or higher. In addition, when the control device 4 determines that the temperature data exceeds 1200°C, the control device 4 outputs a heating stop signal to the heating device 7 to stop heating until the temperature in the container 3 is lower than 950°C. In this way, the control device 4 can control the temperature in the container 3.

[0062] In addition, the control device 4 obtains concentration data indicating the concentration of the oxygen component in the container 3 from the component concentration meter 6, and determines whether the oxygen partial pressure obtained from the concentration data and the temperature data in the container 3 obtained from the thermometer 5 satisfy the relationship of the above-mentioned formula (1). Similarly, the control device 4 obtains concentration data indicating the concentration of the gas component in the container 3 from the component concentration meter 6, and determines whether the oxygen partial pressure obtained from the concentration data satisfies the above-mentioned formula (3). When the control device 4 determines that the oxygen partial pressure in the container 3 has increased and does not satisfy the relationship of either or both of the above-mentioned formulas (1) and (3), the control device 4 outputs an inert gas supply signal to the inert gas supply device 8, and the inert gas is supplied from the inert gas supply device 8 to the container 3, so that the oxygen concentration is reduced until the temperature and the oxygen partial pressure satisfy the relationship of the above-mentioned formulas (1) and (3). In this way, the control device 4 can control the oxygen partial pressure in the container 3.

[0063] In such a zinc removal device 1, the zinc of the galvanized steel sheet 2 is volatilized and removed by accommodating the galvanized steel sheet 2 in the container 3. The longer the galvanized steel sheet 2 is accommodated in the container 3, the longer the heating time of the galvanized steel sheet 2 is, and thus the amount of zinc removed by volatilization increases. Therefore, it is preferred to accommodate the galvanized steel sheet 2 in the container 3 for more than 5 minutes, and more preferably for more than 10 minutes. Thus, the amount of zinc removed from the galvanized steel sheet 2 can be increased, and the removal rate of zinc from the galvanized steel sheet 2 can be improved.

[0064] Figure 2It is a schematic cross-sectional view of another zinc removal device 20 capable of implementing the zinc removal method of this embodiment. In the zinc removal device 20, the same reference numerals are used for the elements common to the zinc removal device 1, and duplicate descriptions are omitted.

[0065] like Figure 2 As shown, the container 21 of the zinc removal device 20 is connected to the electric furnace 11 having the graphite electrode 12 via the regulating valve 9 and the exhaust gas flow passage 10. In this way, by connecting the container 21 and the electric furnace 11 using the exhaust gas flow passage 10 and introducing the exhaust gas generated from the electric furnace 11 into the container 21, the container 21 can be heated by the sensible heat of the exhaust gas. Thus, zinc can be removed from the galvanized steel sheet 2 without using the heating device 7 or by reducing the heating generated by the heating device 7.

[0066] In such a zinc removal device 20, the control device 4 controls the opening of the regulating valve 9 and controls the flow rate of the exhaust gas flowing into the container 21, thereby controlling the temperature and oxygen partial pressure in the container 21 to satisfy the relationship of the above formulas (1) to (3).

[0067] In addition, the container 21 has an ejector 14 for ejecting the steel sheet after zinc removal toward the electric furnace 11. By having the ejector 14, the container 21 can supply the steel sheet after zinc removal to the electric furnace 11 at a high temperature. By using the ejector 14, the heated steel sheet can be supplied to the electric furnace 11 as a scrap steel raw material, thereby reducing the thermal energy required for melting the scrap steel raw material in the electric furnace 11. It should be noted that the electric furnace 11 is an example of a melting furnace for storing molten iron, and a converter or an induction heating furnace may be used instead of the electric furnace 11.

[0068] In addition, the container 21 can be brought close to the molten iron 13 contained in the electric furnace 11. Thus, the inside of the container 21 can also be heated by utilizing the sensible heat of the molten iron 13. Furthermore, when the molten iron is refined in the electric furnace 11, the exhaust gas from the refining can be used to adjust the oxygen concentration in the container 21. This is because the oxidizing gas supplied to the molten iron for the purpose of decarburization treatment and the like reacts with carbon in the molten iron to generate CO gas and CO2 gas, and therefore, exhaust gas with a low oxygen concentration is generated in such treatment.

[0069] When the temperature and oxygen partial pressure in the container 21 are adjusted by utilizing the sensible heat of the molten iron contained in the electric furnace 11 and the sensible heat of the exhaust gas generated by the smelting furnace, instead of measuring the temperature and the concentration of the oxygen component in the container 21, the temperature and the concentration of the oxygen component in the exhaust gas introduced into the container 21 can be continuously measured, and the exhaust gas that satisfies the relationship of the following equations (4) to (6) can be supplied to the container 21.

[0070] T g ≥17.6log(P O2, g )+1031…(4)

[0071] 700≤T g ≤1200…(5)

[0072] P O2 , g ≤10.13…(6)

[0073] In the above formulas (4) to (6), T g is the exhaust gas temperature (℃), P O2,g is the oxygen partial pressure of the exhaust gas (kPa).

[0074] In this way, by supplying the exhaust gas satisfying the relationship of the above formulas (4) to (6) to the container 21, the temperature and oxygen partial pressure in the container 21 are controlled to satisfy the relationship of the above formulas (1) to (3). As a result, the temperature control and atmosphere control in the container 21 are no longer necessary, and the material cost and service cost required for the volatilization and removal of zinc can be reduced. It should be noted that as a means for continuously measuring the exhaust gas temperature introduced into the container 21 and the concentration of the exhaust gas oxygen component, measuring instruments similar to the thermometer 5 and the component concentration meter 6 can be respectively provided in the exhaust gas flow passage 10, and the exhaust gas temperature and the exhaust gas component concentration can be continuously measured using the measuring instruments.

[0075] Next, the steps of the zinc removal method of the present embodiment are described. First, the galvanized steel sheet is placed in a container for heat treatment of the galvanized steel sheet. Next, the oxygen partial pressure in the container is controlled to be within a specified range. As a method for controlling the oxygen partial pressure in the container, the following methods can be applied: a method of supplying an inert gas such as Ar gas or nitrogen; a method of supplying a reducing gas such as CO gas or hydrogen; a method of exhausting the inside of the container 3 to create a reduced pressure atmosphere, etc.

[0076] Furthermore, by heating the galvanized steel sheet in the container, the temperature in the container is controlled to be within a predetermined range. The method for heating the galvanized steel sheet in the container is not limited to a specific method, and can be applied to: a method using electromagnetic induction heating; a method using an oxygen / fuel burner; and Figure 2 As shown in the figure, there is a method of using the sensible heat of molten iron contained in the melting furnace and the sensible heat of exhaust gas generated from the melting furnace.

[0077] By controlling the temperature and oxygen partial pressure in the container by these methods, the galvanized steel sheet is contained in a container where the temperature and oxygen partial pressure in the container satisfy the relationship of the above formulas (1) to (3), and the zinc contained in the galvanized steel sheet is volatilized and removed. Since the steel sheet after zinc removal may be oxidized in the atmosphere, it is preferably maintained in a state where an inert gas or a reducing gas is supplied to the container, or in a state where the container is made into a reduced pressure atmosphere.

[0078] Example 1

[0079] First, Example 1 of zinc removal from galvanized steel sheet using a 10kg-scale resistance furnace is described. The output of the resistance furnace is adjusted to raise the temperature in the furnace to a specified temperature, and then a gas supply gun is inserted into the resistance furnace to supply a mixed gas of CO gas and CO2 gas from the gun. After the resistance furnace is replaced with the CO-CO2 mixed gas, a galvanized steel sheet having a zinc concentration of 99% by mass or more in the coating is loaded into the resistance furnace and the galvanized steel sheet is subjected to a heat treatment. Inventive examples and comparative examples in which various changes are made to the temperature, oxygen partial pressure, and heating treatment time in the resistance furnace during the heat treatment are implemented. It should be noted that the oxygen partial pressure in the resistance furnace uses the value of the equilibrium oxygen partial pressure when the reaction of CO gas and CO2 gas reaches equilibrium as shown in the following formulas (7) to (8).

[0080]

[0081]

[0082] After a prescribed period of time, the galvanized steel sheet is taken out of the resistance furnace and cooled to room temperature in a CO-CO2 mixed gas atmosphere of the same composition as that of the heat treatment. The galvanized steel sheet after the heat treatment is immersed in hydrochloric acid to dissolve only the galvanized part. The hydrochloric acid that has dissolved the zinc coating is measured using an ICP emission spectrometer, and the zinc concentration (mg / L) in the hydrochloric acid is quantified. The zinc removal rate from the galvanized steel sheet is calculated from the zinc concentration in the hydrochloric acid. The zinc removal rate is expressed as a percentage of the difference between the zinc concentration in the galvanized steel sheet before the heat treatment and the zinc concentration in the galvanized steel sheet after the heat treatment relative to the zinc concentration in the galvanized steel sheet before the heat treatment. In addition, the input energy is calculated by the product of the output power of the resistance furnace during the treatment and the heating treatment time. The heating conditions, zinc removal rate and input energy of the galvanized steel sheet are shown in Table 1 below.

[0083]

[0084] In Inventive Examples 1 to 8, the galvanized steel sheet was heated in a resistance furnace that satisfied the relationships of the above equations (1) to (3). On the other hand, in Comparative Examples 1 to 7, the galvanized steel sheet was heated in a resistance furnace that did not satisfy at least one of the above equations (1) to (3).

[0085] Figure 3 : is a graph showing the range of temperature and oxygen partial pressure in the container that satisfies the relationship of equations (1) to (3). Figure 3 In the graph, the horizontal axis is the common logarithm of the oxygen partial pressure (-), and the vertical axis is the furnace temperature T (°C). It should be noted that (-) means dimensionless. Figure 3 The black circles represent inventive examples, and the white circles represent comparative examples.

[0086] From Table 1 and Figure 3 It was confirmed that in Invention Examples 1 to 8 in which the galvanized steel sheet was heat-treated under the heating conditions satisfying the relationship of the above-mentioned formulas (1) to (3), the zinc removal rate from the galvanized steel sheet was 80% or more. On the other hand, it was confirmed that in Comparative Examples 1 to 7 in which the galvanized steel sheet was heat-treated under the conditions that did not satisfy at least one of the above-mentioned formulas (1) to (3), the zinc removal rate from the galvanized steel sheet was less than 25%. In addition, as shown in Table 1, the results obtained in Invention Examples 1 to 3 confirmed that by setting the heating treatment time to more than 5 minutes, the zinc removal rate from the galvanized steel sheet reached more than 90%. From these results, it was confirmed that by using the zinc removal method of the present embodiment, zinc can be efficiently removed from the galvanized steel sheet.

[0087] Example 2

[0088] Next, Example 2 in which zinc is removed from a galvanized steel sheet using an electric furnace facility of 100 tons is described. Scrap steel raw materials not including galvanized steel sheets are charged into the electric furnace, and then the scrap steel raw materials are melted by arc heating to produce molten steel. Here, carbon-containing materials are added to the electric furnace as needed so that the temperature and oxygen partial pressure of the exhaust gas generated from the electric furnace reach predetermined values, and an oxidizing gas is supplied from a top-blowing lance.

[0089] The temperature of the exhaust gas is continuously measured using a thermocouple set in the exhaust gas flow path. In addition, the oxygen partial pressure of the exhaust gas is calculated using the above formulas (7) to (8) in the same manner as in Example 1 based on the CO concentration and CO2 concentration in the exhaust gas continuously measured using a mass spectrometer set in the exhaust gas flow path and the temperature of the exhaust gas.

[0090] The exhaust gas generated by the electric furnace operation is introduced into a scrap steel holding container directly connected to the furnace body of the electric furnace, and the temperature in the scrap steel holding container is raised to a specified temperature. After the scrap steel holding container is fully replaced by the electric furnace exhaust gas, a galvanized steel sheet with a zinc concentration of 99% by mass or more in the coating is placed in the scrap steel holding container and heat-treated. The invention examples and comparative examples are implemented by making various changes to the temperature, oxygen partial pressure and heating treatment time in the scrap steel holding container during the heating treatment. The zinc removal rate of the galvanized steel sheet after the heating treatment is calculated according to the same procedure as in Example 1. The temperature, oxygen partial pressure, zinc removal rate and input energy of the exhaust gas are shown in Table 2 below.

[0091]

[0092] In Example 2, the temperature and oxygen partial pressure in the scrap steel holding container are controlled by introducing exhaust gas, so the temperature and oxygen partial pressure in the scrap steel holding container are the same as the temperature and oxygen partial pressure of the exhaust gas. That is, in Inventive Examples 9 to 11, exhaust gas satisfying the relationship of the above-mentioned formulas (4) to (6) is introduced into the scrap steel holding container, so the scrap steel holding container is controlled to satisfy the relationship of the above-mentioned formulas (1) to (3). On the other hand, in Comparative Examples 8 to 14, exhaust gas not satisfying at least one of the above-mentioned formulas (4) to (6) is introduced into the scrap steel holding container, so the scrap steel holding container becomes a condition that does not satisfy at least one of the above-mentioned formulas (1) to (3).

[0093] Figure 4 : is a graph showing the range of exhaust gas temperature and oxygen partial pressure that satisfies the relationship of equations (4) to (6). Figure 4 In the figure, the horizontal axis is the common logarithm of the oxygen partial pressure of the exhaust gas (-), and the vertical axis is the exhaust gas temperature T g (℃). Figure 4 The black circles represent inventive examples, and the white circles represent comparative examples.

[0094] From Table 2 and Figure 4 It was confirmed that in Invention Examples 9 to 11, in which the galvanized steel sheet was heated under the conditions satisfying the relationship of the above-mentioned formulas (1) to (3) by introducing exhaust gas satisfying the relationship of the above-mentioned formulas (4) to (6), the zinc removal rate from the galvanized steel sheet reached 80% or more. On the other hand, it was confirmed that in Comparative Examples 8 to 14, in which the galvanized steel sheet was heated under the conditions satisfying at least one of the above-mentioned formulas (1) to (3) by introducing exhaust gas that did not satisfy at least one of the above-mentioned formulas (4) to (6), the zinc removal rate from the galvanized steel sheet was less than 25%. From these results, it was confirmed that by introducing exhaust gas satisfying the relationship of the above-mentioned formulas (4) to (6) into the container, the galvanized steel sheet can be heated under the heating conditions satisfying the relationship of the above-mentioned formulas (1) to (3), thereby efficiently removing zinc from the galvanized steel sheet. In addition, in Example 2, the exhaust gas of the electric furnace was used to heat the scrap steel holding container, so the input energy was 0. It has been confirmed that by heating the inside of the container using the sensible heat of the exhaust gas generated from the melting furnace in this way, it is possible to reduce input energy, and therefore it is preferable to heat the inside of the container using the sensible heat of the exhaust gas generated from the melting furnace.

[0095] Explanation of symbols

[0096] 1. Zinc removal device

[0097] 2Galvanized steel sheet

[0098] 3 Containers

[0099] 4 Control device

[0100] 5. Thermometer

[0101] 6-component concentration meter

[0102] 7. Heating device

[0103] 8Inert gas supply device

[0104] 9. Regulating valve

[0105] 10 Exhaust flow duct

[0106] 11 Electric furnace

[0107] 12 Graphite Electrode

[0108] 13 Molten Iron

[0109] 14 Ejector

[0110] 20 Zinc removal device

[0111] 21 Containers

Claims

1. A method for removing zinc from a galvanized steel sheet contained in a container, wherein: The temperature and oxygen partial pressure in the container satisfy the following relationship (1) to (3): T≥17.6log(P O2 )+1031…(1) 700≤T≤1200…(2) P O2 ≤10.13…(3) In the above formulas (1) to (3), T is the temperature in the container (°C), P O2 is the oxygen partial pressure in the container (kPa).

2. The method for removing zinc according to claim 1, wherein: The galvanized steel sheet is housed in the container for more than 5 minutes.

3. The method for removing zinc according to claim 1 or claim 2, wherein: The interior of the container is heated using sensible heat of exhaust gas generated from the smelting furnace.

4. The method for removing zinc according to claim 3, wherein: The interior of the container is heated by utilizing sensible heat of molten iron contained in the smelting furnace.

5. The method for removing zinc according to claim 3, wherein: The exhaust gas satisfying the following relationships (4) to (6) is supplied into the container. T g ≥17.6log(P O2,g )+1031…(4) 700≤T g ≤1200…(5) P O2,g ≤10.13…(6) In the above formulas (4) to (6), T g is the temperature of the exhaust gas (°C), P O2,g is the oxygen partial pressure of the exhaust gas (kPa).

6. A zinc removal device for removing zinc from a galvanized steel sheet, wherein: A container for accommodating a galvanized steel sheet, a thermometer for measuring the temperature in the container, a component concentration meter for measuring the concentration of a gas component in the container, and a control device for controlling the temperature in the container and the concentration of the gas component, The control device controls the temperature and oxygen concentration in the container in a manner that satisfies the relationship between the following equations (1) to (3). T≥17.6log(P O2 )+1031…(1) 700≤T≤1200…(2) P O2 ≤10.13…(3) In the above formulas (1) to (3), T is the temperature in the container (°C), P O2 is the oxygen partial pressure in the container (kPa).

7. The zinc removal device according to claim 6, wherein: The container is connected to the exhaust gas flow passage of the smelting furnace containing molten iron. The control device controls the temperature and oxygen concentration in the container by controlling the flow rate of the exhaust gas flowing into the container.

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