Zinc removal method and zinc removal device

Abstract

The present invention provides a method for removing zinc, the method being capable of removing zinc by determining the heating temperature of a galvanized steel sheet, while taking the oxidation of lead into consideration.　The present invention provides a method for removing zinc from a galvanized steel sheet that is contained in a container, wherein the temperature and the oxygen partial pressure within the container satisfy relational expressions (1) to (3).　(1): T ≥ 17.6log(PO2) + 1031 (2): 700 ≤ T ≤ 1200 (3): PO2 ≤ 10.13 In relational expressions (1) to (3), T represents the temperature (°C) within the container; and PO2 represents the oxygen partial pressure (kPa) within the container.

Description

[Technical Field]

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

[0002] Galvanized steel sheets have excellent corrosion resistance and are used in a variety of applications, including automobile bodies and building materials. In recent years, in order to make effective use of steel materials, many steel materials available in the market, including galvanized steel sheets, are collected as scrap and reused as raw materials in electric furnace steelmaking or converter steelmaking processes. Furthermore, since the electric furnace steelmaking process, which uses scrap as raw material, is more effective in reducing CO2 emissions in the steelmaking process than the integrated blast furnace / converter steelmaking process, which uses iron ore as raw material, it is expected that the use of scrap raw materials, including galvanized steel sheets, will continue to increase in the future.

[0003] However, because the temperature of molten iron in these steelmaking processes reaches approximately 1,700°C, the zinc contained in galvanized steel sheets volatilizes and becomes mixed into the steelmaking dust or migrates into the molten iron.Zinc present in molten iron also volatilizes easily because its saturated vapor pressure is higher than that of molten iron, so much of it ultimately accumulates in the steelmaking dust.

[0004] In recent years, technological developments have been underway to reuse by-products such as steelmaking dust as iron sources. For example, steelmaking dust is reused in the iron ore sintering process, but when zinc-containing steelmaking dust is reused in the sintering process, zinc accumulates in the sintered ore. When zinc-containing sintered ore is used in a blast furnace as a raw material for ironmaking, metallic zinc vapor is generated inside the furnace. The metallic zinc then cools and precipitates, adhering to the furnace interior and growing, which can worsen the air permeability inside the furnace or cause any remaining zinc to be picked up in the molten iron, making it difficult to apply this method to the recycling of galvanized steel sheets.

[0005] When the zinc concentration in steelmaking dust is 50% by mass or more, it can be sold to zinc refineries as a raw material for producing crude zinc. However, the zinc concentration in blast furnace dust and converter dust is generally at most 1% by mass, and even in electric furnace dust, it is at most 10 to 40% by mass, making it difficult to use as a raw material in the crude zinc production process. Therefore, these zinc-containing dusts require industrial waste disposal, which is one of the factors that increase the production costs of steel materials.

[0006] In light of this, technologies for removing zinc from galvanized steel sheets in advance have been studied. For example, Patent Document 1 discloses a method for removing zinc from scrap by supplying superheated steam into a treatment chamber containing scrap raw materials. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-145499 Summary of the Invention [Problem to be solved by the invention]

[0008] When zinc is heated, it is oxidized by oxygen in the air, forming zinc oxide on the surface of the zinc plating, which inhibits the volatilization and removal of zinc. Therefore, to efficiently remove zinc, it is necessary to determine the heating temperature of zinc taking into account the effects of zinc oxidation, but Patent Document 1 does not take into account the effects of zinc oxidation at all. Therefore, the method disclosed in Patent Document 1 has the problem that zinc may not be efficiently removed due to zinc oxidation.

[0009] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a zinc removal method and zinc removal device that can remove zinc by determining the heating temperature of a zinc-plated steel sheet while taking into consideration the oxidation of zinc. [Means for solving the problem]

[0010] The means for solving the above problems are as follows. [1] A method for removing zinc from a zinc-plated steel sheet stored in a container, wherein the temperature and oxygen partial pressure in the container satisfy the relationship of the following equations (1) to (3). T ≥ 17.6 log(P O2 )+1031···(1) 700≦T≦1200 (2) P O2 ≦10.13 (3) In the above formulas (1) to (3), T is the temperature inside the container (°C), and P O2 is the oxygen partial pressure (kPa) in the container. [2] The zinc removal method described in [1], wherein the zinc-plated steel sheet is placed in the container for 5 minutes or more. [3] A method for removing zinc according to [1] or [2], wherein the inside of the container is heated by the sensible heat of exhaust gas generated from the melting furnace. [4] The zinc removal method according to [3], wherein the inside of the container is heated by the sensible heat of the molten iron contained in the melting furnace. [5] The method for removing zinc according to [3], wherein exhaust gas satisfying the relationships of the following formulas (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 equations (4) to (6), T g is the temperature of the exhaust gas (°C), and P O2,g is the oxygen partial pressure (kPa) of the exhaust gas. [6] A zinc removal device for removing zinc from a zinc-plated steel sheet, comprising: a container for accommodating the zinc-plated steel sheet; a thermometer for measuring the temperature inside the container; a component concentration meter for measuring the concentration of gas components inside the container; and a control device for controlling the temperature and the concentration of gas components inside the container, wherein the control device controls the temperature and oxygen concentration inside the container so as to satisfy the relationships of the following equations (1) to (3). T ≥ 17.6 log(P O2 )+1031···(1) 700≦T≦1200 (2) P O2 ≦10.13 (3) In the above formulas (1) to (3), T is the temperature inside the container (°C), and P O2 is the oxygen partial pressure (kPa) in the container. [7] The zinc removal device described in [6], wherein the container is connected to an exhaust gas flow path of a melting furnace containing molten iron, and the control device controls the temperature and oxygen concentration within the container by controlling the flow rate of exhaust gas flowing into the container. [Effects of the Invention]

[0011] According to the present invention, the heating temperature and oxygen partial pressure in the container that houses 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 explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional schematic diagram of a zinc removal device that can carry out the zinc removal method according to this embodiment. [Figure 2] FIG. 2 is a cross-sectional schematic view of another zinc removal device that can be used to carry out the zinc removal method according to this embodiment. [Figure 3] FIG. 3 is a graph showing the range of the temperature and oxygen partial pressure inside the container that satisfies the relationships of formulas (1) to (3). [Figure 4] FIG. 4 is a graph showing the range of the exhaust gas temperature and oxygen partial pressure that satisfies the relationships of formulas (4) to (6). DETAILED DESCRIPTION OF THE INVENTION

[0013] First, the results of experiments that led to the conception of the present invention will be described. The inventors carried out heat treatment of galvanized steel sheets using a laboratory-scale electric resistance furnace and a full-scale electric furnace. In this treatment, the temperature, atmospheric conditions, heat treatment time, etc. during the heat treatment were variously changed, and the removal rate of zinc contained in the galvanized steel sheets after the heat treatment was investigated. As a result, it was found that heat treatment of the galvanized steel sheets under predetermined temperature and oxygen partial pressure conditions is effective for removing zinc from the galvanized steel sheets.

[0014] First, the temperature inside the container in which the galvanized steel sheet is heat-treated and the oxygen partial pressure of the atmosphere inside the container will be described. The temperature inside the container in which the galvanized steel sheet is heat-treated and the oxygen partial pressure of the atmosphere inside the container must satisfy the relationships of the following formulas (1) to (3).

[0015] T ≥ 17.6 log(P O2 )+1031···(1) 700≦T≦1200 (2) P O2 ≦10.13 (3) In the above formulas (1) to (3), T is the temperature inside the container (°C), and P O2 is the oxygen partial pressure in the container (kPa).

[0016] Maintaining the temperature inside the vessel above the boiling point of zinc (907°C) is considered effective for volatilizing and removing zinc. However, when a galvanized steel sheet is heat-treated in the atmosphere at 1000°C, the zinc is oxidized by oxygen in the atmosphere, forming zinc oxide on the surface of the galvanized steel sheet. This zinc oxide inhibits the volatilization and removal of zinc. Furthermore, the base steel sheet is also oxidized, which may lead to a deterioration in iron yield when the heat-treated galvanized steel sheet is used as a raw material to produce molten steel. In response to this, the inventors have found that a lower oxygen partial pressure inside the vessel suppresses the above-mentioned adverse effects of oxygen and reduces the temperature required to efficiently remove zinc from the galvanized steel sheet. Therefore, the temperature and oxygen partial pressure inside the vessel must satisfy the relationship shown in Equation (1) above.

[0017] However, if the heating temperature is below 700°C, the temperature difference with the boiling point of zinc (907°C) is large, slowing the rate at which zinc volatilizes, and sufficient zinc removal cannot be achieved. On the other hand, if the heating temperature exceeds 1200°C, the zinc in the coating layer is more likely to diffuse into the base steel sheet and form an Fe-Zn alloy phase, which could result in the zinc not volatilizing but being mixed into the base material. For this reason, the temperature inside the container must satisfy the relationship in equation (2) above.

[0018] Furthermore, if the oxygen partial pressure exceeds 10.13 kPa, even if the heating temperature and oxygen partial pressure satisfy the relationship in formula (1), the oxidation of zinc by oxygen in the atmosphere cannot be suppressed, and the volatilization and removal of zinc is hindered. Therefore, the oxygen partial pressure inside the container must satisfy the relationship in formula (3) above.

[0019] Thus, the inventors have found that by maintaining the temperature and oxygen partial pressure inside a container that houses a galvanized steel sheet at a level that satisfies the relationships of the above formulas (1) to (3), it is possible to efficiently volatilize and remove zinc while suppressing oxidation of the zinc and even the steel sheet itself, and have completed the present invention. The present invention will be described below through embodiments of the invention.

[0020] Fig. 1 is a cross-sectional schematic diagram of a zinc removal apparatus 1 in which the zinc removal method according to this embodiment can be implemented. As shown in Fig. 1, the zinc removal apparatus 1 includes a container 3 that contains a galvanized steel sheet 2 and a control device 4 that controls the temperature and gas component concentration within the container 3. The container 3 is not limited to a specific shape or structure, and may be a holding container for containing scrap raw materials such as galvanized steel sheets, a bucket for transporting and charging scrap raw materials, or the like.

[0021] Since the container 3 is heated to 700°C or higher, it is preferably made of a heat-resistant material such as firebricks, steel plate, etc. The container 3 has a thermometer 5 that measures the temperature inside the container 3, a component concentration meter 6 that measures the concentration of gas components inside the container 3, a heating device 7 that heats the inside of the container 3, and an inert gas supply device 8 that supplies various gases into the container 3.

[0022] The thermometer 5 is a device, such as a thermocouple, that measures the temperature inside the container 3. The component concentration meter 6 is a device, such as a mass spectrometer or laser analyzer, that measures the component concentrations of gases such as oxygen, CO, CO2, and hydrogen inside the container 3. Note that the measurement limit of oxygen concentration in the atmosphere with a typical component concentration meter is about 0.01% by volume, and the oxygen partial pressure that can be directly measured under atmospheric pressure is about 1.01 kPa.

[0023] Therefore, when measuring oxygen partial pressure in the range of less than 1.01 kPa, the equilibrium oxygen partial pressure when the reaction between CO gas and CO2 gas is in equilibrium, or the equilibrium oxygen partial pressure when the reaction between hydrogen gas and water vapor is in equilibrium, may be used, as shown in the following equations (7) to (10): In other words, when the oxygen concentration in the atmosphere is lower than the lower measurement limit of the concentration meter and the oxygen concentration cannot be measured directly, the oxygen concentration may be calculated from the concentrations of CO gas, CO2 gas or hydrogen gas, and water vapor measured simultaneously, using the following equations (7) to (10).

[0024] [ka]

[0025]

number

[0026] [ka]

[0027]

number

[0028] 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)), and P CO , P CO2 , P H2 and P H2O are the CO partial pressure (kPa), CO2 partial pressure (kPa), hydrogen partial pressure (kPa), and water vapor partial pressure (kPa), respectively.

[0029] The heating device 7 is a device that heats the inside of the container 3. The heating device 7 may heat the inside of the container 3 by heating the galvanized steel sheet 2 housed in the container 3. The heating device 7 is, for example, an electromagnetic induction heating device or an oxygen combustion burner.

[0030] The inert gas supply device 8 is a device that supplies an inert gas such as N2 or Ar into the container 3 and adjusts the oxygen concentration inside the container 3. Note that instead of the inert gas supply device 8, a reducing gas supply device that supplies a reducing gas such as CO gas or hydrogen gas may be used, or a pressure reducing device that evacuates the inside of the container 3 to create a reduced pressure atmosphere may be used.

[0031] The control device 4 controls the temperature and oxygen gas component concentration 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 acquires temperature data indicating the temperature in the container 3 from the thermometer 5 and determines, for example, whether the temperature data is less than the intermediate value (950°C) of the above equation (2). If the control device 4 determines that the temperature data is less than 950°C, it outputs a heating signal to the heating device 7, causing the heating device 7 to heat the container 3 until the temperature in the container 3 reaches 950°C or higher. If the control device 4 determines that the temperature data exceeds 1200°C, it outputs a heating stop signal to the heating device 7, causing the heating device 7 to stop heating until the temperature in the container 3 falls below 950°C. In this manner, the control device 4 may control the temperature in the container 3.

[0032] The control device 4 also acquires concentration data indicating the concentration of the oxygen gas component in the container 3 from the component concentration meter 6, and determines whether the oxygen partial pressure calculated from the concentration data and the temperature data in the container 3 acquired from the thermometer 5 satisfy the relationship in equation (1) above. Similarly, the control device 4 acquires 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 calculated from the concentration data satisfies equation (3) above. If the control device 4 determines that the oxygen partial pressure in the container 3 has increased and no longer satisfies one or both of the relationships in equations (1) and (3), it outputs an inert gas supply signal to the inert gas supply device 8, causing the inert gas supply device 8 to supply inert gas into the container 3, thereby reducing the oxygen concentration until the temperature and oxygen partial pressure satisfy the relationships in equations (1) and (3). In this manner, the control device 4 may control the oxygen partial pressure in the container 3.

[0033] In such a zinc removal apparatus 1, the zinc-plated steel sheet 2 is accommodated in the container 3, whereby the zinc from the zinc-plated steel sheet 2 is volatilized and removed. The longer the time the zinc-plated steel sheet 2 is accommodated in the container 3, the longer the heating time of the zinc-plated steel sheet 2, and therefore the greater the amount of zinc that is volatilized and removed. For this reason, it is preferable to accommodate the zinc-plated steel sheet 2 in the container 3 for 5 minutes or more, and more preferably for 10 minutes or more. This makes it possible to increase the amount of zinc removed from the zinc-plated steel sheet 2, and to improve the rate of zinc removal from the zinc-plated steel sheet 2.

[0034] 2 is a cross-sectional view of another zinc removal device 20 that can implement the zinc removal method according to this embodiment. In the zinc removal device 20, elements common to the zinc removal device 1 are designated by the same reference numerals, and redundant explanations will be omitted.

[0035] As shown in Fig. 2, a container 21 of a zinc removal device 20 is connected to an electric furnace 11 having graphite electrodes 12 via an adjustment valve 9 and an exhaust gas flow path 10. By connecting the container 21 and the electric furnace 11 through the exhaust gas flow path 10 in this way and introducing exhaust gas generated from the electric furnace 11 into the container 21, the inside of the container 21 can be heated by the sensible heat of the exhaust gas. This makes it possible to remove zinc from the galvanized steel sheet 2 without using the heating device 7 or while reducing the amount of heating by the heating device 7.

[0036] In such a zinc removal device 20, the control device 4 controls the opening of the regulating valve 9 and the flow rate of the exhaust gas flowing into the container 21, thereby controlling the temperature and oxygen partial pressure inside the container 21 so as to satisfy the relationships of the above equations (1) to (3).

[0037] The vessel 21 also has an extruder 14 that extrudes the steel plate after zinc removal toward the electric furnace 11. Providing the extruder 14 in the vessel 21 allows the steel plate after zinc removal to be supplied to the electric furnace 11 while still at a high temperature. The extruder 14 allows the heated steel plate to be supplied to the electric furnace 11 as scrap raw material, thereby reducing the thermal energy required to melt the scrap raw material in the electric furnace 11. The electric furnace 11 is an example of a melting furnace that accommodates molten iron, and a converter or an induction heating furnace may be used instead of the electric furnace 11.

[0038] Alternatively, the vessel 21 and the molten iron 13 contained in the electric furnace 11 may be placed close to each other. This allows the inside of the vessel 21 to be heated also by the sensible heat of the molten iron 13. Furthermore, when refining molten iron in the electric furnace 11, the exhaust gas from the refining can be used to adjust the oxygen concentration inside the vessel 21. This is because an oxidizing gas supplied into the molten iron for the purpose of decarburization or the like reacts with the carbon in the molten iron to generate CO gas and CO2 gas, and such a process generates exhaust gas with a low oxygen concentration.

[0039] When adjusting the temperature and oxygen partial pressure inside the container 21 using the sensible heat of the molten iron contained in the electric furnace 11 and the sensible heat of the exhaust gas generated from the melting furnace, instead of measuring the temperature and the concentration of the oxygen gas component inside the container 21, it is also possible to continuously measure the temperature and the concentration of the oxygen gas component of the exhaust gas introduced into the container 21, and supply the exhaust gas that satisfies the relationships of the following equations (4) to (6) into the container 21.

[0040] T g ≧17.6log(P O2 , g )+1031···(4) 700≦T g ≦1200 (5) P O2 , g ≦10.13 (6) In the above equations (4) to (6), T g is the temperature of the exhaust gas (℃), and P O2,g is the oxygen partial pressure of the exhaust gas (kPa).

[0041] In this way, by supplying exhaust gas that satisfies the relationships of the above formulas (4) to (6) to container 21, the temperature and oxygen partial pressure inside container 21 are controlled to satisfy the relationships of the above formulas (1) to (3). This eliminates the need for temperature control and atmosphere control inside container 21, thereby reducing the cost of materials and utilities required for volatilizing and removing zinc. Note that the means for continuously measuring the temperature of the exhaust gas introduced into container 21 and the concentration of the oxygen component in the exhaust gas can be achieved by providing measuring devices similar to thermometer 5 and component concentration meter 6 in exhaust gas flow path 10, respectively, and continuously measuring the exhaust gas temperature and the concentration of the exhaust gas components using these measuring devices.

[0042] Next, the procedure of the zinc removal method according to this embodiment will be described. First, the zinc-plated steel sheet is placed in a container in which the zinc-plated steel sheet is heat-treated. Next, the oxygen partial pressure in the container is controlled to be within a predetermined range. Methods for controlling the oxygen partial pressure in the container include supplying an inert gas such as Ar gas or nitrogen gas, supplying a reducing gas such as CO gas or hydrogen gas, or evacuating the container 3 to create a reduced pressure atmosphere.

[0043] Furthermore, the temperature inside the container is controlled within a predetermined range by heating the galvanized steel sheet inside the container. The means for heating the galvanized steel sheet inside the container is not limited to a specific method, and methods that use electromagnetic induction heating, an oxygen-fuel burner, or the sensible heat of the molten iron contained in the melting furnace or the sensible heat of the exhaust gas generated from the melting furnace, as shown in Figure 2, can be applied.

[0044] The temperature and oxygen partial pressure inside the container are controlled by these methods, and the zinc contained in the zinc-plated steel sheet is placed in a container in which the temperature and oxygen partial pressure inside the container satisfy the relationships of the above formulas (1) to (3), and the zinc contained in the zinc-plated steel sheet is removed by volatilization. Since the steel sheet after zinc removal may oxidize in the atmosphere, it is preferable to keep the container in a state where an inert gas or a reducing gas is supplied into the container, or in a state where the container is kept in a reduced-pressure atmosphere. [Example]

[0045] First, Example 1 will be described, in which zinc removal from a galvanized steel sheet was carried out using a 10 kg-scale electric resistance furnace. After adjusting the output of the electric resistance furnace to raise the temperature inside the furnace to a predetermined level, a gas supply lance was inserted into the electric resistance furnace, and a mixed gas of CO gas and CO2 gas was supplied from the lance. After the atmosphere inside the electric resistance furnace was replaced with a CO-CO2 mixed gas, a galvanized steel sheet having a zinc concentration in the coating of 99 mass% or more was placed in the electric resistance furnace, and the galvanized steel sheet was heat-treated. Examples and comparative examples were carried out in which the temperature, oxygen partial pressure, and heat treatment time inside the electric resistance furnace were variously changed during this heat treatment. The oxygen partial pressure inside the electric resistance furnace was the equilibrium oxygen partial pressure when the reaction between CO gas and CO2 gas reached equilibrium, as shown in the following equations (7) and (8).

[0046] [ka]

[0047]

number

[0048] After a predetermined time had elapsed, the galvanized steel sheet was removed from the electric resistance furnace and cooled to room temperature under a CO2-CO2 mixed gas atmosphere with the same composition as the heat treatment. The heat-treated galvanized steel sheet was immersed in hydrochloric acid to dissolve only the galvanized portion. The hydrochloric acid that dissolved the galvanized portion was measured using an ICP atomic emission spectrometer to quantify the zinc concentration (mg / L) in the hydrochloric acid. The zinc removal rate from the galvanized steel sheet was calculated from the zinc concentration in the hydrochloric acid. The zinc removal rate is the difference between the zinc concentration in the galvanized steel sheet before and after heat treatment, expressed as a percentage of the zinc concentration in the galvanized steel sheet before heat treatment. The input energy was calculated from the product of the output of the electric resistance furnace and the heat treatment time during treatment. The heating conditions, zinc removal rate, and input energy for the galvanized steel sheet are shown in Table 1 below.

[0049] [Table 1]

[0050] In Examples 1 to 8, the galvanized steel sheets were heat-treated in an electric resistance furnace that satisfied the relationships of the above formulas (1) to (3). On the other hand, in Comparative Examples 1 to 7, the galvanized steel sheets were heat-treated in an electric resistance furnace that did not satisfy at least one of the above formulas (1) to (3).

[0051] Figure 3 is a graph showing the range of temperature and oxygen partial pressure inside the vessel that satisfies the relationships of formulas (1) to (3). In Figure 3, the horizontal axis represents the common logarithm (-) of the oxygen partial pressure, and the vertical axis represents the furnace temperature T (°C). Note that (-) means that the value is dimensionless. In Figure 3, black circles plotted represent examples of the invention, and white circles plotted represent comparative examples.

[0052] 3, it was confirmed that in Examples 1 to 8, in which the galvanized steel sheet was heat-treated under heating conditions that satisfied the relationships of the above formulas (1) to (3), the zinc removal rate from the galvanized steel sheet was 80% or more. On the other hand, in Comparative Examples 1 to 7, in which the galvanized steel sheet was heat-treated under conditions that did not satisfy at least one of the above formulas (1) to (3), it was confirmed that the zinc removal rate from the galvanized steel sheet was less than 25%. Furthermore, as shown in Table 1, the results obtained in Examples 1 to 3 confirmed that by setting the heat treatment time to 5 minutes or more, the zinc removal rate from the galvanized steel sheet was 90% or more. These results confirmed that zinc can be efficiently removed from the galvanized steel sheet by using the zinc removal method according to this embodiment. [Example]

[0053] Next, Example 2 will be described, in which zinc was removed from a galvanized steel sheet using a 100-ton electric furnace facility. Scrap raw material not containing galvanized steel sheet was charged into the electric furnace, and then the scrap raw material was melted by arc heating to produce molten steel. Here, a carbon-containing substance was added to the electric furnace as needed, and an oxidizing gas was supplied from a top-blowing lance so that the temperature and oxygen partial pressure of the exhaust gas generated from the electric furnace were set to predetermined values.

[0054] The temperature of the exhaust gas was continuously measured using a thermocouple installed in the exhaust gas flow path. The oxygen partial pressure of the exhaust gas was calculated using the above formulas (7) and (8) based on the CO concentration and CO2 concentration in the exhaust gas, which were continuously measured using a mass spectrometer installed in the exhaust gas flow path, and the exhaust gas temperature, as in Example 1.

[0055] Exhaust gas generated during electric furnace operation was introduced into a scrap holding vessel directly connected to the electric furnace body, and the temperature inside the scrap holding vessel was raised to a predetermined temperature. After the atmosphere inside the scrap holding vessel was sufficiently replaced with electric furnace exhaust gas, a galvanized steel sheet having a zinc concentration in the coating of 99 mass% or more was charged into the scrap holding vessel and subjected to heat treatment. The temperature inside the scrap holding vessel, the oxygen partial pressure, and the heat treatment time during this heat treatment were variously changed to carry out inventive examples and comparative examples. The zinc removal rate of the galvanized steel sheet after heat treatment was calculated using the same procedure as in Example 1. The exhaust gas temperature, oxygen partial pressure, zinc removal rate, and input energy are shown in Table 2 below.

[0056] [Table 2]

[0057] In Example 2, the temperature and oxygen partial pressure inside the scrap holding vessel are controlled by introducing exhaust gas, so the temperature and oxygen partial pressure inside the scrap holding vessel are the same as those of the exhaust gas. That is, in Examples 9 to 11, exhaust gas satisfying the relationships of the above formulas (4) to (6) is introduced into the scrap holding vessel, so the inside of the scrap holding vessel is controlled to satisfy the relationships of the above formulas (1) to (3). On the other hand, in Comparative Examples 8 to 14, exhaust gas that does not satisfy at least one of the above formulas (4) to (6) is introduced into the scrap holding vessel, so the inside of the scrap holding vessel is in a condition that does not satisfy at least one of the above formulas (1) to (3).

[0058] Fig. 4 is a graph showing the range of exhaust gas temperature and oxygen partial pressure that satisfies the relationships of formulas (4) to (6). In Fig. 4, the horizontal axis represents the common logarithm (-) of the oxygen partial pressure of the exhaust gas, and the vertical axis represents the exhaust gas temperature T g(° C.) In FIG. 4, the black circles plotted represent examples of the present invention, and the white circles plotted represent comparative examples.

[0059] Table 2 and Figure 4 confirm that in Examples 9 to 11, in which the galvanized steel sheet was heat-treated under conditions satisfying the relationships of the above formulas (1) to (3) by introducing exhaust gas satisfying the relationships of the above formulas (4) to (6), a zinc removal rate of 80% or more from the galvanized steel sheet was achieved. On the other hand, in Comparative Examples 8 to 14, in which the galvanized steel sheet was heat-treated under conditions satisfying the relationships of the above formulas (1) to (3), by introducing exhaust gas that did not satisfy at least one of the above formulas (4) to (6), a zinc removal rate of less than 25% from the galvanized steel sheet was achieved. These results confirm that introducing exhaust gas that satisfies the relationships of the above formulas (4) to (6) into the container enables the galvanized steel sheet to be heat-treated under heating conditions satisfying the relationships of the above formulas (1) to (3), thereby enabling efficient zinc removal from the galvanized steel sheet. Furthermore, in Example 2, the scrap holding container was heated using exhaust gas from an electric furnace, so the input energy was zero. In this way, it was confirmed that it is preferable to heat the inside of the container with the sensible heat of the exhaust gas generated from the melting furnace, as this reduces the amount of energy input. [Explanation of symbols]

[0060] 1. Zinc removal device 2. Galvanized steel sheet 3 containers 4. Control device 5 Thermometer 6 component concentration meter 7 Heating device 8. Inert gas supply equipment 9. Regulating valve 10 Exhaust gas flow path 11 Electric furnace 12 Graphite electrode 13 Molten Iron 14 Extruder 20 Zinc removal device 21 Container

Claims

1. A method for removing zinc from a zinc-plated steel sheet contained in a container, comprising: the temperature and oxygen partial pressure in the container satisfy the relationships of the following formulas (1) to (3), A method for removing zinc, in which the inside of the vessel is heated by the sensible heat of exhaust gas generated from the melting furnace. 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 inside the container (°C), and P Ｏ２ is the oxygen partial pressure in the container (kPa).

2. 2. The method for removing zinc according to claim 1, wherein the inside of the vessel is heated by sensible heat of molten iron contained in the melting furnace.

3. 2. The zinc removal method according to claim 1, wherein exhaust gas satisfying the relationships of the following formulas (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), and P Ｏ２，ｇ is the oxygen partial pressure (kPa) of the exhaust gas.

4. A zinc removal device for removing zinc from a galvanized steel sheet, a container for containing a galvanized steel sheet; a thermometer for measuring the temperature inside the container; a component concentration meter for measuring the concentration of the gas component in the container; a control device for controlling the temperature and the concentration of the gas components in the container; and the control device controls the temperature and oxygen concentration in the container so as to satisfy the relationships of the following formulas (1) to (3), the vessel is connected to an exhaust gas flow path of a melting furnace that contains molten iron, The control device controls the temperature and oxygen concentration in the container by controlling the flow rate of exhaust gas flowing into the container. 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 inside the container (°C), and P Ｏ２ is the oxygen partial pressure in the container (kPa).

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