Method for supplying cover gas and magnesium melting device

Abstract

This method is for supplying a cover gas to a furnace to cover the surface of molten magnesium or a magnesium alloy held in the furnace, and the method comprises: a first step of, while supplying into the furnace the cover gas including a fluoroketone and a dilution gas, adjusting the supply flow rate of the cover gas so that the oxygen concentration in the furnace becomes 7% or less; a second step of determining the space volume that can be filled with the cover gas in the furnace; and a third step of adjusting the fluoroketone concentration in the cover gas on the basis of the supply flow rate of the cover gas and the space volume, and further supplying the cover gas into the furnace.

Description

Cover gas supply method and magnesium melting device

[0001] The present invention relates to a method for supplying a cover gas that prevents oxidation and combustion of molten magnesium, and a magnesium melting apparatus. This application claims priority based on Japanese Patent Application No. 2024-048254, filed on March 25, 2024, the contents of which are incorporated herein by reference.

[0002] When magnesium or a magnesium alloy is melted for casting, a gas (cover gas) is used to cover the molten metal to prevent oxidation and evaporation of the molten metal. Patent Document 1 discloses a method for detecting the presence or absence of combustion of magnesium in a melting furnace by detecting the CO concentration in the melting furnace and issuing an alarm or increasing the flow rate of the cover gas.

[0003] A mixture of sulfur hexafluoride and a carrier gas such as an inert gas is commonly used as a cover gas. However, sulfur hexafluoride has a high global warming potential. For this reason, a cover gas consisting of fluoroketone and carbon dioxide, which has a lower global warming potential than sulfur hexafluoride, has been proposed (Patent Document 2). This document discloses a method for determining the optimal concentration of fluoroketone in the cover gas by calculating the moisture concentration in the melting furnace using the moisture content of the air outside the melting furnace based on the moisture concentration in the atmosphere inside the melting furnace.

[0004] JP 2006-200001 A JP 2008-116108 A

[0005] Because fluoroketone is more expensive than sulfur hexafluoride, there are cases where the fluoroketone concentration in the cover gas is reduced, but this poses a problem in that the combustion conditions of magnesium in the melting furnace change, making it impossible to maintain an appropriate fluoroketone concentration.

[0006] The present invention has been made in view of the above circumstances, and provides a method for supplying a cover gas containing fluoroketone that is safe and has excellent cost performance, in order to prevent combustion of molten magnesium.

[0007] [1] A method for supplying a cover gas into a furnace to cover the surface of molten magnesium or magnesium alloy held in the furnace, the method comprising: a first step of adjusting the supply flow rate of the cover gas containing fluoroketone and a dilution gas while supplying the cover gas into the furnace so that the oxygen concentration in the furnace is 7% or less; a second step of determining a spatial volume in the furnace that can be filled with the cover gas; and a third step of adjusting the fluoroketone concentration in the cover gas based on the supply flow rate of the cover gas and the spatial volume, and then supplying the cover gas into the furnace. [2] The method for supplying a cover gas according to [1], wherein the third step adjusts the fluoroketone concentration (y2) in the cover gas based on the following formula: Formula 1: 66234X - 1980 < Y < 66234X + 150 Formula 2: Y = y1 ・ y2 [where X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.] [3] The cover gas supply method according to [1] or [2], wherein in the third step, the fluoroketone concentration in the cover gas is adjusted based on the supply flow rate of the cover gas, the space volume, and the minimum relative humidity outside the furnace. [4] The cover gas supply method according to any of [1] to [3], wherein in the third step, when the minimum relative humidity outside the furnace is 75% or more, the fluoroketone concentration (y2) in the cover gas is adjusted based on the following formula: Formula 3: 66234X - 930 < Y < 66234X + 920 Formula 4: Y = y1 y2 [wherein X represents the space volume (unit: m ３), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.] [5] The cover gas supply method according to [3] or [4], wherein the fluoroketone concentration in the cover gas is adjusted according to the minimum relative humidity within the past 24 hours in an area where the furnace is installed. [6] The cover gas supply method according to any of [1] to [5], wherein the supply flow rate of the cover gas and the fluoroketone concentration in the cover gas are corrected according to a change in the volume of the molten metal. [7] A magnesium melting apparatus comprising: a furnace for melting magnesium or a magnesium alloy to produce molten metal; a cover gas supply unit that prepares a cover gas by mixing fluoroketone and a dilution gas in a predetermined ratio for the purpose of covering the surface of the molten metal held in the furnace and supplies the cover gas into the furnace; a cover gas flow meter that measures the supply flow rate of the cover gas supplied from the cover gas supply unit into the furnace; a flow control valve that adjusts the supply flow rate of the cover gas supplied from the cover gas supply unit into the furnace; an oxygen concentration meter that measures the oxygen concentration in the furnace; and a melt level sensor that measures the melt level position of the molten metal in the furnace, and further comprising: a control unit that controls at least the fluoroketone concentration in the cover gas prepared by the cover gas supply unit and the supply flow rate of the cover gas adjusted by the flow control valve, based on the cover gas supply method described in any one of [1] to [6].

[0008] According to the present invention, a cover gas containing fluoroketone, which is safe and has excellent cost performance, can be supplied to prevent oxidation and combustion of molten magnesium.

[0009] 1 is a side view showing a schematic configuration of a magnesium melting apparatus capable of carrying out an example of a method for supplying a cover gas according to the present invention.

[0010] An example of an embodiment of the device according to the present invention will be described below with reference to the drawings. For the sake of convenience, the drawings may show characteristic parts enlarged, and the dimensional proportions of the components may not necessarily be the same as those in reality.

[0011] <Magnesium Melting Apparatus> The magnesium melting apparatus 1 illustrated in Figures 1 and 2 is an apparatus that can carry out one example of the cover gas supply method of the present invention, and is equipped with a melting furnace body 2 that accommodates molten magnesium or magnesium alloy (molten metal) M, a cover gas inlet 3 that introduces a cover gas into the furnace of the melting furnace body 2, and a discharge mechanism 4 that discharges the molten metal M to the outside of the apparatus.

[0012] The melting furnace body 2 includes a storage section 11 (furnace) for storing molten metal M, a cover section 13 for covering a portion of an upper opening 12 of the storage section 11, and a lid section 14. The storage section 11 has a substantially rectangular parallelepiped shape. The cover section 13 is formed to cover a portion of the upper opening 12 except for an opening 15 located at one end of the storage section 11.

[0013] As shown in Figure 2, the opening 15 is rectangular and extends from one side wall 11a to the other side wall 11b of the accommodation section 11. The opening 15 can be used for operations such as adding materials to the accommodation section 11. The lid 14 is sized and shaped to fit the opening 15, and is rotatably attached to one end 13a of the cover 13 so as to be able to open and close the opening 15. Since the melting furnace body 2 is not normally airtight, gas can enter and exit through the opening 15 or the top opening 12.

[0014] The cover gas introduction section 3 includes a supply line 21 for supplying cover gas and an introduction nozzle 22 for introducing the cover gas from the supply line 21 into the melting furnace body 2. The introduction nozzle 22 is designed to obtain a spray speed according to the inner diameter of the nozzle, and is attached to the cover 13. As shown in Figure 1, the introduction nozzle 22 is preferably inclined toward the opening 15 (to the right in Figure 1) toward the tip. This inclination makes it easier to prevent external air from entering the melting furnace body 2 through the opening 15.

[0015] The inclination angle A of the introduction nozzle 22 is preferably set so that the cover gas can be sprayed toward the molten metal. For example, as shown in Fig. 1, the inclination angle A is preferably set to an angle equal to or less than the angle at which the cover gas is sprayed toward the liquid surface position B of the molten metal on the end wall 11c at one end of the container 11. Furthermore, as shown by the arrow in Fig. 2, the introduction nozzle 22 is preferably provided so as to face approximately the center of the opening 15 in the horizontal plane.

[0016] The number of introduction nozzles 22 may be one or more. In the illustrated example, three introduction nozzles 22 are provided. If there is a portion (external air inlet portion) other than the opening 15 where external air is likely to flow into the melting furnace body 2, the introduction nozzles 22 may be formed at an angle toward this external air inlet portion. This allows the injection direction of the cover gas to be inclined toward the external air inlet portion, thereby preventing external air from flowing in through this external air inlet portion. An example of an external air inlet portion is a portion where there is a large gap between the cover portion 13 and the storage portion 11.

[0017] The discharge mechanism 4 includes a cylinder 31 provided in the accommodation section 11, a piston 32 inserted into the cylinder 31, and a discharge pipe 33 for discharging the molten metal M in the cylinder 31 to the outside of the furnace.

[0018] The molten metal level sensor 5 is a sensor that detects the liquid level position B of the molten metal M contained in the container section 11 (furnace). In the illustrated example, the rod-shaped detection part of the sensor is inserted into the molten metal to detect the liquid level position B in a contact manner, but the liquid level position B may also be detected in a non-contact manner using a laser, ultrasonic waves, a camera, or the like. A known liquid level sensor can be used as the molten metal level sensor 5.

[0019] The tip of the supply pipe 21 of the cover gas inlet 3 is provided with an introduction nozzle 22, while the base of the supply pipe 21 is provided with a cover gas supply unit 6 that prepares a cover gas by mixing fluoroketone and a diluent gas in a predetermined ratio and supplies this cover gas into the storage unit 11 (furnace). The cover gas supply unit 6 has, for example, a cylinder filled with fluoroketone, a cylinder filled with a diluent gas, a mixer where pipes leading from these cylinders join and which can arbitrarily adjust the mixing ratio of the gases in the cylinders, and an optional valve.

[0020] The supply pipe 21 is provided with a cover gas flow meter 7 that measures the supply flow rate of the cover gas supplied from the cover gas supply unit 6 into the accommodation unit 11 (furnace), and a flow control valve 8 that adjusts the supply flow rate. Also, an oxygen concentration meter 9 is provided at an arbitrary position in the space that can be filled with the cover gas inside the accommodation unit 11. The cover gas flow meter 7, flow control valve 8, and oxygen concentration meter 9 can be publicly known gas flow meters, flow control valves, and oxygen concentration meters, respectively.

[0021] The melt level sensor 5, cover gas supply unit 6, cover gas flow meter 7, flow control valve 8, and oxygen concentration meter 9 are connected to a control unit 10 so as to be able to send and receive electrical signals. The control unit 10 is composed of a known information processing device such as a computer and software that controls the above-mentioned devices connected to it. The control unit 10 may be composed of a single information processing device and software, or may be composed of multiple information processing devices and software.

[0022] The control unit 10 controls the cover gas supply unit 6 to mix fluoroketone and diluent gas at an arbitrary ratio in the cover gas supply unit 6 to prepare a cover gas with a predetermined fluoroketone concentration. The control unit 10 receives from the cover gas flow meter 7 the supply flow rate of the cover gas supplied through the supply line 21 into the accommodation unit 11 (furnace). The control unit 10 controls the aperture of the flow control valve 8 to adjust the supply flow rate of the cover gas supplied through the supply line 21 into the accommodation unit 11 (furnace). The control unit 10 receives from the oxygen concentration meter 9 the oxygen concentration in the space in the accommodation unit 11 (furnace) that can be filled with the cover gas. As described below, the control unit 10 determines the spatial volume in the furnace that can be filled with the cover gas. The control unit 10 controls the fluoroketone concentration in the cover gas prepared by the cover gas supply unit 6 and the supply flow rate of the cover gas adjusted by the flow control valve 8 based on the cover gas supply method described below. The control unit 10 may be operated manually by an operator's input, automatically by a pre-programmed algorithm, or by a combination of manual and automatic operations.

[0023] <<Cover Gas Supply Method>> As an example of the cover gas supply method according to the present invention, an example in which the magnesium melting apparatus 1 is used will be described.

[0024] A magnesium or magnesium alloy raw material (ingot) is placed into the accommodation section 11 through the opening 15 and heated to melt it. The temperature inside the accommodation section 11 (furnace) is set to, for example, 630 to 700° C. The molten metal M spreads inside the furnace to form molten metal.

[0025] When raw materials are introduced into the container 11, air containing moisture is entrained, increasing the oxygen and moisture concentrations in the furnace, which can lead to oxidation and combustion of the molten magnesium. To prevent or reduce this oxidation and combustion, the cover gas supplied from the cover gas supply unit 6 is injected at a sufficient supply flow rate through the introduction nozzle 22 toward the molten metal in the container 11 at a desired speed. The cover gas flows over the surface of the molten metal M, covering the surface of the molten metal M. Fluoroketone in the cover gas reacts with magnesium to form MgF ２ A coating containing the above is formed on the surface of the molten metal. This coating prevents the molten metal from oxidizing or evaporating.

[0026] In the method for supplying a cover gas into the furnace to cover the surface of molten magnesium or magnesium alloy held in the accommodation section 11 (furnace), the following first to third steps are carried out.

[0027] The first step is to supply a cover gas containing a fluoroketone and a diluent gas into the furnace while adjusting the flow rate of the cover gas so that the oxygen concentration in the furnace is 7% or less. By reducing the oxygen concentration in the furnace, oxidation and combustion of the molten metal can be prevented or reduced.

[0028] For example, if the oxygen concentration information received from the oxygen concentration meter 9 exceeds 7%, the control unit 10 controls the flow rate control valve 8 to increase the opening of the flow rate control valve 8 and increase the supply flow rate of the cover gas supplied into the furnace. At this time, the control unit 10 controls the cover gas supply unit 6 to make the oxygen concentration contained in the cover gas 7% or less. As a result, the occupancy rate of the cover gas contained in the gas inside the furnace increases, and the oxygen concentration inside the furnace can be made the target 7% or less.

[0029] Normally, a small amount of air (oxygen concentration of approximately 21%) flows into the furnace, so the decrease in oxygen concentration due to the supply of cover gas and the increase in oxygen concentration due to the inflow of air are balanced, and the oxygen concentration inside the furnace can be kept at 7% or less.

[0030] The fluorinated ketone contained in the cover gas is preferably a perfluoroketone, a hydrogenated fluoroketone, or a mixture thereof.

[0031] The perfluoroketone preferably has 5 to 9 carbon atoms. Specifically, CF ３ CF ２ C(O)CF(CF ３ ) ２ , (CF ３ ) ２ CFC(O)CF(CF ３ ) ２ , C.F. ３ (CF ２ ) ２ C(O)CF(CF ３ ) ２ , C.F. ３ (CF ２ ) ３ C(O)CF(CF ３ ) ２ , C.F. ３ (CF ２ ) ５ C(O)CF ３ , C.F. ３ CF ２ C(O)CF ２ CF ２ CF ３ , C.F. ３ C(O)CF(CF ３ ) ２ and perfluorocyclohexanone.

[0032] The hydrogenated fluoroketone preferably has 4 to 7 carbon atoms. ２ CF ２ C(O)CF(CF ３ ) ２ , C.F. ３ C(O)CH ２ C(O)CF ３ , C ２H ５ C(O)CF(CF ３ ) ２ 、CF ２ CF ２ C(O)CH ３ 、(CF ３ ) ２ CFC(O)CH ３ 、CF ３ CF ２ C(O)CHF ２ 、CF ３ CF ２ C(O)CH ２ F、CF ３ CF ２ C(O)CH ２ CF ３ 、CF ３ CF ２ C(O)CH ２ CH ３ 、CF ３ CF ２ C(O)CH ２ CHF ２ 、CF ３ CF ２ C(O)CH ２ CHF ２ 、CF ３ CF ２ C(O)CH ２ CH ２ F、CF ３ CF ２ C(O)CHFCH ３ 、CF ３ CF ２ C(O)CHFCHF ２ 、CF ３ CF ２ C(O)CHFCH ２ F、CF ３ CF ２ C(O)CF ２ CH ３ 、CF ３ CF ２ C(O)CF ２ CHF ２ 、CF ３ CF ２ C(O)CF ２ CH ２ F、(CF ３ ) ２ CFC(O)CHF ２ 、(CF３ ) ２ CFC(O)CH ２ F, CF ３ CF (CH ２ F) C(O)CHF ２ , C.F. ３ CF (CH ２ F) C(O)CH ２ F, and CF ３ CF (CH ２ F)C(O)CF ３ Preferably, one or more selected from the group consisting of:

[0033] The fluoroketone concentration in the cover gas is preferably 50 ppm or more, more preferably 100 ppm or more. If it is 50 ppm or more, oxidation, combustion, and evaporation of the molten metal M can be reliably prevented. Furthermore, the fluoroketone concentration in the cover gas is preferably 1000 ppm or less, more preferably 500 ppm or less. If it is 1000 ppm or less, harmful substances (COF ２ The occurrence of such substances can be suppressed. Note that 1 ppm is based on the volume (1 μL / 1 L).

[0034] The diluent gas contained in the cover gas is also called a carrier gas. The diluent gas is a gas other than fluoroketone, and is preferably one or more selected from carbon dioxide, nitrogen, and argon. Air may be mixed as part of the diluent gas, but from the viewpoint of sufficiently reducing the oxygen concentration in the first step, it is preferable that the amount of air contained in the diluent gas is as small as possible.

[0035] The second step is a step of determining the spatial volume that can be filled with the cover gas out of the total volume inside the accommodation section 11 (furnace).

[0036] The total volume of the furnace is the total volume of the space surrounded by the bottom surface of the accommodation section 11, the cylindrical side surface rising from the periphery of the bottom surface, and the cover section 13 and the lid section 14. This total volume can be determined in advance before forming the molten metal. For example, the total volume can be determined in advance by measuring and calculating the dimensions of the accommodation section 11 with a ruler or by determining the amount of water required to fill the accommodation section 11.

[0037] The volume of the space that can be filled with the cover gas is the total volume of the furnace minus the volume of the molten metal M, the volume of the components that make up the discharge mechanism 4 housed in the furnace, the volume of the molten metal level sensor 5, the volume of the oxygen concentration meter 9, and the volume of other items housed in the furnace. Of these, all volumes except for the volume of the molten metal M can be determined in advance by a conventional method before the molten metal is formed.

[0038] The volume of the molten metal M can be calculated based on information on the liquid level position B of the molten metal M from the molten metal level sensor 5. For example, a calibration curve showing the relationship between the liquid level position B and the volume of water when a predetermined amount of water is placed in place of the molten metal M can be prepared in advance, and the volume of the molten metal M that has reached the liquid level position B can be calculated based on the calibration curve. The control unit 10 may calculate the volume of the molten metal M based on the calibration curve and the information on the liquid level position B of the molten metal M received from the molten metal level sensor 5.

[0039] The control unit 10 subtracts the volume of the molten metal M, the volume of the components constituting the discharge mechanism 4 housed in the furnace, the volume of the metal level sensor 5, the volume of the oxygen concentration meter 9, and the volume of other items housed in the furnace from the total volume of the furnace, and calculates the volume of the space in the furnace that can be filled with the cover gas.

[0040] The third step is a step of adjusting the fluoroketone concentration in the cover gas prepared in the cover gas supply unit 6 based on the supply flow rate of the cover gas and the spatial volume that can be filled with the cover gas in the furnace, and then supplying the cover gas into the furnace via the supply pipe 21 and the nozzle 22.

[0041] As one embodiment of the present invention, for example, the control unit 10 adjusts the fluoroketone concentration in the cover gas prepared by the cover gas supply unit 6 based on the cover gas supply flow rate measured by the cover gas flow meter 7 and the spatial volume.

[0042] The fluoroketone in the cover gas reacts with the molten magnesium to form MgF ２ This coating prevents oxidation and evaporation of the molten metal M. However, if an excessive amount of fluoroketone is supplied into the furnace, harmful substances (HF, COF ２etc.) will be generated.

[0043] As described above, in order to avoid an excessively low or high concentration of fluoroketone in the furnace and to maintain an appropriate concentration, it is preferable that the control unit 10 adjusts the fluoroketone concentration (y2) in the cover gas based on the following formulas 1 and 2. Here, it is preferable to use the flow rate adjusted in step 1 as the cover gas supply flow rate (y1), but y1 may be adjusted again in this step. Formula 1: 66234X - 1980 < Y < 66234X + 150 Formula 2: Y = y1 y2 [wherein X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.

[0044] The above formula 1 is preferably the following formula 1': Formula 1': 66234X-1081<Y<66234X+681 Within the above range of 1', an effect of reducing the amount of oxides can be obtained.

[0045] In one embodiment of the present invention, for example, the control unit 10 adjusts the fluoroketone concentration in the cover gas prepared by the cover gas supply unit 6 based on the cover gas supply flow rate measured by the cover gas flow meter 7, the space volume, and the minimum relative humidity outside the furnace.

[0046] The minimum relative humidity outside the furnace may be input by an operator to the control unit 10, or may be obtained from a communication network to which the control unit 10 is connected, or may be obtained from a hygrometer (not shown) connected to the control unit 10. The hygrometer may be installed, for example, near the magnesium melting apparatus 1.

[0047] Generally, relative humidity is the ratio of the amount of water vapor to the saturated amount of water vapor at the temperature at that time, expressed as a percentage, and the smallest value observed throughout the day is called the "minimum relative humidity." For example, the minimum relative humidity of the present invention may be that published from time to time by the Japan Meteorological Agency, and is preferably the minimum relative humidity (unit: %) observed within the past 24 hours in the area where the furnace is installed.

[0048] Moisture that enters the housing 11 reacts with magnesium to generate heat and hydrogen, and the generated hydrogen may react with oxygen in the air and burn violently. When the minimum relative humidity of the air outside the furnace is low, such as from February to April in Japan, the amount of moisture that enters the furnace is small, so the fluoroketone concentration in the cover gas can be reduced. On the other hand, when the minimum relative humidity of the air outside the furnace is high, such as from June to September in Japan, it is preferable to increase the fluoroketone concentration in the cover gas to suppress oxidation and combustion of magnesium in the furnace.

[0049] From the viewpoint of reducing the risk associated with a high minimum relative humidity outside the furnace as described above, when the minimum relative humidity outside the furnace is 75% or higher, the control unit 10 preferably adjusts the fluoroketone concentration (y2) in the cover gas based on the following formulas 3 and 4. Here, it is preferable to use the flow rate adjusted in step 1 as the cover gas supply flow rate (y1), but y1 may be adjusted again in this step. Formula 3: 66234X - 930 < Y < 66234X + 920 Formula 4: Y = y1 y2 [wherein X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.

[0050] When the minimum relative humidity outside the furnace is 50% or less, the fluoroketone concentration (y2) in the cover gas may be adjusted to lower the fluoroketone concentration based on the following formulas 5 and 6. The cover gas supply flow rate (y1) may also be adjusted. Formula 5: Y<66234X-4895 Formula 6: Y=y1·y2 [wherein X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.

[0051] The order of steps 1 to 3 described above is preferably to perform steps 1 and 2 first, followed by step 3. The order of steps 1 and 2 is not particularly limited, and either step may be performed first.

[0052] In the discharge mechanism 4, a portion of the molten metal M flows into the cylinder 31. By lowering the piston 32, the molten metal M in the cylinder 31 is discharged to the outside of the furnace through the discharge pipe 33. The molten metal M discharged from the melting furnace body 2 is supplied to, for example, casting in a molding device.

[0053] When raw materials are added to the furnace or when a portion of the molten metal M is discharged from the discharge mechanism 4 to the outside of the furnace, the amount of molten metal (volume of the molten metal) changes, causing a change in the liquid level B of the molten metal M. This change in the amount of molten metal corresponds to a change in the liquid level B. When the control unit 10 detects the change in the liquid level B using the molten metal level sensor 5, it is preferable that the control unit 10 performs at least one of the first to third steps again and re-adjusts (corrects) at least one of the fluoroketone concentration in the cover gas and the cover gas supply flow rate.

[0054] According to the cover gas supply method of the present invention described above, a cover gas containing fluoroketone is supplied to the surface of molten magnesium, thereby preventing oxidation and combustion of the molten magnesium. By taking into consideration the volume of the furnace to which the cover gas is supplied, the oxygen concentration inside the furnace when the cover gas is supplied, and, if necessary, the minimum relative humidity outside the furnace, optimal cover gas conditions can be calculated, and sufficient oxidation and combustion prevention effects can be achieved. As a result, safe operation and reduced running costs can be achieved.

[0055] [Test Example 1] Eleven users (A to K) tested the cover gas supply method using magnesium melting equipment (12 models) equipped with various melting furnaces. The temperature at which magnesium was melted in the furnace of the magnesium melting equipment is shown in Table 1. Based on empirical rules, the initial fluoroketone concentration in the cover gas and the initial cover gas supply flow rate were determined, and the cover gas was supplied into the furnace. Next, the cover gas supply flow rate was adjusted so that the oxygen concentration in the furnace was 7% or less, and the supply flow rate (y2) (unit: L / min) was determined. The oxygen concentration in the furnace at this time is shown in Table 1. Next, the spatial volume in the furnace that could be filled with cover gas was determined from values ​​calculated based on measurements taken in advance and a molten metal level sensor. This spatial volume (X) was defined as the "volume inside the melting furnace (unit: m ３)) in Table 1. Finally, based on the above formulas 1 and 2, the fluoroketone concentration (y1) (unit: ppm) in the cover gas was adjusted so that either the condition "satisfies formula 1" or "does not satisfy formula 1" was met, and this was supplied into the furnace. If there was a combustion prevention effect (combustion prevention effect) of magnesium at this stage, the result was marked as "Good" in Table 1. Furthermore, among the cases where there was a combustion prevention effect, those that showed particularly excellent results were marked as "◎". On the other hand, if there was no combustion prevention effect, the result was marked as "Poor" in Table 1. Furthermore, the gas in the furnace was sampled, and the HF concentration was measured by a conventional method (unit: ppm), and the results are also shown in Table 1. The above results are summarized in Table 1.

[0056]

[0057] How to read Table 1 will be explained using the top case of User A as an example. The volume inside the melting furnace corresponding to the space volume (X) is 0.055 m ３ The melting temperature of magnesium was 630°C. When the oxygen concentration in the furnace was 5.35%, the supply flow rate (y1) of the cover gas was 16 L / min, and the fluoroketone concentration (y2) in the cover gas was 150 ppm. The calculated value Y obtained from the above formula 2 (Y = y1 · y2) was 2400. This Y value satisfies the above formula 1.

[0058] In the test examples shown in Table 1 that had no flame retardant effect, the calculated value Y obtained from the above formula 2 (Y = y1 · y2) did not satisfy the above formula 1. In the test examples shown in Table 1 that were NG because the HF concentration in the furnace was too high, the calculated value Y obtained from the above formula 2 (Y = y1 · y2) did not satisfy the above formula 1.

[0059] [Test Example 2] A user evaluated seasonal cover gas conditions. The average temperature and minimum relative humidity at that time were data published by the Japan Meteorological Agency. Magnesium was melted at 690°C in the furnace of a magnesium melting apparatus. Based on empirical rules, the initial fluoroketone concentration in the cover gas and the initial cover gas supply flow rate were determined, and the cover gas was supplied into the furnace. Next, the cover gas supply flow rate was adjusted so that the oxygen concentration in the furnace was 7% or less, and the supply flow rate (y2) (unit: L / min) was determined. The oxygen concentration in the furnace at this time is shown in Table 2. Next, the spatial volume in the furnace that could be filled with cover gas was determined from calculations based on previous measurements and a melt level sensor. This spatial volume (X) was 0.122 m ３ and "Volume inside the melting furnace (unit: m ３ The results are shown in Table 2. Finally, the following procedures were performed depending on the time of the test.

[0060] During the period from August to September when the minimum relative humidity was 75% or higher, the fluoroketone concentration (y1) (unit: ppm) in the cover gas was adjusted based on the above-mentioned formulas 3 and 4 so that it was within the range that satisfied formula 3, and this was supplied into the furnace. At this time, the combustion prevention effect (combustion prevention effect) of magnesium was sufficient. In addition, when the gas in the furnace was sampled and the HF concentration was measured using a conventional method, it was found that almost no HF was generated. On the other hand, when the cover gas conditions did not satisfy formula 3, problems occurred such as no combustion prevention effect (NG) or the generation of HF at a high concentration (3 ppm). The above results are summarized in Table 2.

[0061] During the period from February to April when the minimum relative humidity was below 50%, the fluoroketone concentration (y1) (unit: ppm) in the cover gas was adjusted to satisfy Equation 5 based on Equations 5 and 6, and this was supplied into the furnace. At this time, the magnesium combustion prevention effect (combustion prevention effect) was sufficient. Furthermore, sampling of the gas inside the furnace and measurement of the HF concentration using conventional methods revealed that no HF was being generated.

[0062]

[0063] REFERENCE SIGNS LIST 1 magnesium melting apparatus 2 melting furnace body 3 cover gas inlet 4 outlet mechanism 5 molten metal level sensor 6 cover gas supply 7 cover gas flow meter 8 flow rate control valve 9 oxygen concentration meter 10 control unit 11 accommodation unit (furnace) 11a side wall 11b side wall 11c end wall 12 upper opening 13 cover 13a one end 14 lid 15 opening 21 supply pipe 22 inlet nozzle 22 nozzle 31 cylinder 32 piston 33 outlet pipe A tilt angle B liquid level position M molten metal

Claims

1. A method for supplying into a furnace a cover gas that covers the surface of molten magnesium or magnesium alloy held in the furnace, comprising: a first step of adjusting the supply flow rate of the cover gas so that the oxygen concentration in the furnace is 7% or less while supplying the cover gas containing fluoroketone and a dilution gas into the furnace; a second step of determining the spatial volume in the furnace that can be filled with the cover gas; and a third step of adjusting the fluoroketone concentration in the cover gas based on the supply flow rate of the cover gas and the spatial volume, and then supplying the cover gas into the furnace.

2. The method for supplying a cover gas according to claim 1, wherein in the third step, the fluoroketone concentration (y2) in the cover gas is adjusted based on the following formula: Formula 1: 66234X - 1980 < Y < 66234X + 150 Formula 2: Y = y1 · y2 [wherein X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.

3. The cover gas supply method according to claim 1, wherein in the third step, the fluoroketone concentration in the cover gas is adjusted based on the supply flow rate of the cover gas, the space volume, and the minimum relative humidity outside the furnace.

4. The cover gas supply method according to claim 3, wherein in the third step, when the minimum relative humidity outside the furnace is 75% or higher, the fluoroketone concentration (y2) in the cover gas is adjusted based on the following formula: Formula 3: 66234X - 930 < Y < 66234X + 920 Formula 4: Y = y1 · y2 [wherein X is the spatial volume (unit: m ３ ), y1 represents the supply flow rate of the cover gas (unit: L / min), y2 represents the fluoroketone concentration in the cover gas (unit: ppm), and Y represents the product of y1 and y2.

5. The cover gas supply method of claim 3, wherein the fluoroketone concentration in the cover gas is adjusted according to the minimum relative humidity in the area where the furnace is installed within the past 24 hours.

6. A cover gas supply method according to any one of claims 1 to 5, wherein the supply flow rate of the cover gas and the fluoroketone concentration in the cover gas are corrected according to the amount of change in the volume of the molten metal.

7. A magnesium melting apparatus comprising: a furnace for melting magnesium or a magnesium alloy to produce molten metal; a cover gas supply unit for preparing a cover gas by mixing fluoroketone and a dilution gas in a predetermined ratio for the purpose of covering the surface of the molten metal held in the furnace and supplying the cover gas into the furnace; a cover gas flow meter for measuring the supply flow rate of the cover gas supplied from the cover gas supply unit into the furnace; a flow control valve for adjusting the supply flow rate of the cover gas supplied from the cover gas supply unit into the furnace; an oxygen concentration meter for measuring the oxygen concentration in the furnace; and a molten metal level sensor for measuring the level of the molten metal in the furnace, the magnesium melting apparatus comprising: a control unit for controlling at least the fluoroketone concentration in the cover gas prepared by the cover gas supply unit and the supply flow rate of the cover gas adjusted by the flow control valve, based on the cover gas supply method described in claim 1.

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