Cover gas feed method and magnesium melting apparatus

Abstract

To provide a method for feeding a cover gas including fluorinated ketone which is safe and excellent in cost performance for the purpose of preventing the combustion of a magnesium molten metal.SOLUTION: In a method for feeding a cover gas covering a surface of a molten metal of magnesium or a magnesium alloy held in a furnace into the furnace, the cover gas feed method comprises: a first step of, while feeding the cover gas including fluorinated ketone and a dilution gas into the furnace, adjusting the feed flow rate of the cover gas so that an oxygen concentration in the furnace reaches 7% or less; a second step of obtaining a space volume at which the cover gas in the furnace can be filled; and a third step of adjusting a fluorinated ketone concentration in the cover gas based on the feed flow rate of the cover gas and the space volume, and further feeding the cover gas into the furnace.SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[0002] When magnesium or a magnesium alloy is melted for casting, a gas (cover gas) is used to cover the molten metal in order to prevent oxidation and evaporation of the molten metal. A method has been disclosed in which the CO concentration in a melting furnace is detected and an alarm is issued or the flow rate of the cover gas is increased in order to detect whether or not magnesium is burning in the melting furnace (Patent Document 1).

[0003] A mixture of sulfur hexafluoride and a carrier gas such as an inert gas is commonly used as a cover gas, but sulfur hexafluoride is a substance with 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 from the moisture concentration in the atmosphere inside the melting furnace. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-200001 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-116108 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[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, a first step of supplying the cover gas containing fluoroketone and a diluent gas into the furnace while adjusting the supply flow rate of the cover gas so that the oxygen concentration in the furnace is 7% or less; a second step of determining a space volume in the furnace that can be filled with the cover gas; 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; A cover gas supply method comprising: [2] The cover gas supply method according to [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 Equation 2: Y=y1·y2 [Wherein, X is the spatial volume (unit: m 3 ), y1 is the supply flow rate of the cover gas (unit: L / min), y2 is the fluoroketone concentration in the cover gas (unit: ppm), and Y is 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 one 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 Equation 4: Y=y1·y2 [Wherein, X is the spatial volume (unit: m 3 ), y1 is the supply flow rate of the cover gas (unit: L / min), y2 is the fluoroketone concentration in the cover gas (unit: ppm), and Y is 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 the area where the furnace is installed. [6] The cover gas supply method according to any one 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 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 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]. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a side view showing a schematic configuration of a magnesium melting apparatus capable of carrying out an example of a cover gas supply method according to the present invention. FIG. [Figure 2] FIG. 2 is a plan view showing a schematic configuration of the magnesium melting apparatus of FIG. DETAILED DESCRIPTION OF THE 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 equipment> The magnesium melting apparatus 1 illustrated in Figures 1 and 2 is an apparatus capable of carrying out an example of the cover gas supply method of the present invention, and is equipped with a melting furnace body 2 in which molten magnesium or magnesium alloy (molten metal) M is accommodated, a cover gas inlet section 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) that stores molten metal M, a cover section 13 that covers 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, opening 15 is rectangular and extends from one side wall 11a to the other side wall 11b of storage section 11. Opening 15 can be used for tasks such as adding materials to storage section 11. Lid 14 is sized and shaped to fit opening 15, is rotatably attached to one end 13a of cover 13, and is designed to freely open and close opening 15. Since melting furnace body 2 is not normally airtight, gas can enter and exit through opening 15 or upper opening 12.

[0014] The cover gas introduction section 3 includes a supply line 21 for supplying the 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 FIG. 1, the introduction nozzle 22 is preferably tilted toward the opening 15 (to the right in FIG. 1) as it faces the tip. This tilted position makes it easier to prevent external air from flowing into 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 smaller 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 so as to be inclined 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 portion 11, a piston 32 inserted into the cylinder 31, and a discharge pipe line 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 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 supply pipe 21 of cover gas introduction section 3 is equipped with introduction nozzle 22, while the base of supply pipe 21 is equipped with cover gas supply section 6 that prepares cover gas by mixing fluoroketone and diluent gas in a predetermined ratio and supplies this into storage section 11 (furnace). Cover gas supply section 6 has, for example, a cylinder filled with fluoroketone, a cylinder filled with 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] Supply pipe 21 is provided with cover gas flow meter 7, which measures the supply flow rate of cover gas supplied from cover gas supply unit 6 into accommodation unit 11 (furnace), and flow rate adjustment valve 8, which adjusts the supply flow rate. Also, oxygen concentration meter 9 is provided at an arbitrary position in the space that can be filled with cover gas inside accommodation unit 11. Known gas flow meter, flow rate adjustment valve, and oxygen concentration meter can be used for cover gas flow meter 7, flow rate adjustment valve 8, and oxygen concentration meter 9, 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 so that the fluoroketone and the diluent gas are mixed in an arbitrary ratio in the cover gas supply unit 6 to prepare a cover gas having a predetermined fluoroketone concentration. The control unit 10 receives from the cover gas flow meter 7 the supply flow rate of the cover gas that flows through the supply pipeline 21 and is supplied into the container unit 11 (furnace). The control unit 10 controls the opening of the flow rate adjusting valve 8 to adjust the supply flow rate of the cover gas that flows through the supply pipe 21 and is supplied 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 will be described later, the control unit 10 determines the volume of space 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 method is carried out using the magnesium melting apparatus 1 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. 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 and causing oxidation and combustion of the molten magnesium. To prevent or reduce this oxidation and combustion, cover gas supplied from the cover gas supply unit 6 is sprayed at a sufficient 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 it. The fluoroketone in the cover gas reacts with magnesium to form a coating containing MgF2 and other compounds on the surface of the molten metal. This coating prevents oxidation and evaporation of the molten metal.

[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 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 adjustment valve 8 to increase the opening of the flow rate adjustment 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, allowing the oxygen concentration inside the furnace to be kept below 7%.

[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 a carbon number of 5 to 9. Specifically, it is preferably at least one selected from the group consisting of CF3CF2C(O)CF(CF3)2, (CF3)2CFC(O)CF(CF3)2, CF3(CF2)2C(O)CF(CF3)2, CF3(CF2)3C(O)CF(CF3)2, CF3(CF2)5C(O)CF3, CF3CF2C(O)CF2CF2CF3, CF3C(O)CF(CF3)2, and perfluorocyclohexanone.

[0032] The hydrogenated fluoroketone preferably has 4 to 7 carbon atoms. Specifically, HCF2CF2C(O)CF(CF3)2, CF3C(O)CH2C(O)CF3, C2H5C(O)CF(CF3)2, CF2CF2C(O)CH3, (CF3)2CFC(O)CH3, CF3CF2C(O)CHF2, CF3CF2C(O)CH2F, CF3CF2C(O)CH2CF3, CF3CF2C(O)CH2CH3, CF3CF2C(O)CH2CHF2, CF3CF2C(O)CH2CHF2, CF3CF2C(O)CH2CHF2, CF3CF2C(O)CH2CH2F, CF3CF2 One or more selected from the group consisting of C(O)CHFCH3, CF3CF2C(O)CHFCHF2, CF3CF2C(O)CHFCH2F, CF3CF2C(O)CF2CH3, CF3CF2C(O)CF2CHF2, CF3CF2C(O)CF2CH2F, (CF3)2CFC(O)CHF2, (CF3)2CFC(O)CH2F, CF3CF(CH2F)C(O)CHF2, CF3CF(CH2F)C(O)CH2F, and CF3CF(CH2F)C(O)CF3 are preferred.

[0033] The fluoroketone concentration in the cover gas is preferably 50 ppm or more, more preferably 100 ppm or more. At 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. At 1000 ppm or less, the generation of harmful substances (such as COF2) 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 diluent gas contain as little air 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 container 11 (furnace).

[0036] The total volume of the furnace interior 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 melt 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 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 determined based on the calibration curve. The control unit 10 may determine 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 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] 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 and the spatial volume.

[0042] The fluoroketone in the cover gas reacts with the molten magnesium to form a film containing MgF2 and other substances on the surface of the molten metal. This film prevents oxidation and evaporation of the molten metal M. However, if an excessive amount of fluoroketone is supplied to the furnace, harmful substances (HF, COF2, etc.) are generated.

[0043] To avoid an excessively low or high concentration of fluoroketone in the furnace and maintain an appropriate concentration, the control unit 10 preferably adjusts the fluoroketone concentration (y2) in the cover gas based on the following equations 1 and 2. Here, the cover gas supply flow rate (y1) is preferably the flow rate adjusted in step 1, but y1 may also be adjusted again in this step. Formula 1: 66234X—1980 <Y<66234X+150 Equation 2: Y=y1·y2 [Wherein, X is the spatial volume (unit: m 3 ), y1 is the supply flow rate of the cover gas (unit: L / min), y2 is the fluoroketone concentration in the cover gas (unit: ppm), and Y is 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 range of 1', the amount of oxides can be reduced.

[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 spatial 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 near the magnesium melting apparatus 1, for example.

[0047] Generally, relative humidity is the ratio of the amount of water vapor to the amount of saturated 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." The minimum relative humidity in this invention can be, for example, 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 equations 3 and 4. Here, it is preferable to apply the flow rate adjusted in step 1 as the cover gas supply flow rate (y1), but y1 may also be adjusted again in this step. Formula 3: 66234X-930 <Y<66234X+920 Equation 4: Y=y1·y2 [Wherein, X is the spatial volume (unit: m 3 ), y1 is the supply flow rate of the cover gas (unit: L / min), y2 is the fluoroketone concentration in the cover gas (unit: ppm), and Y is 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. At the same time, the cover gas supply flow rate (y1) may be adjusted. Formula 5: Y<66234X-4895 Equation 6: Y=y1·y2 [Wherein, X is the spatial volume (unit: m 3 ), y1 is the supply flow rate of the cover gas (unit: L / min), y2 is the fluoroketone concentration in the cover gas (unit: ppm), and Y is 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. There are no particular restrictions on the order of steps 1 and 2, 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 line 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, and the liquid level B of the molten metal M changes. This change in the amount of molten metal corresponds to the 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. [Example]

[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 apparatus is shown in Table 1. Based on empirical rules, the fluoroketone concentration in the initial cover gas and the initial supply flow rate of the cover gas were determined, and the cover gas was supplied into the furnace. Next, the supply flow rate of the cover gas (y2) (unit: L / min) was adjusted so that the oxygen concentration in the furnace was 7% or less. The oxygen concentration in the furnace at this time is shown in Table 1. Next, the volume of the space inside the furnace that could be filled with the cover gas was calculated from measurements taken in advance and values ​​calculated using the melt level sensor. This volume (X) was defined as the "volume inside the melting furnace (unit: m 3 )" in Table 1. Finally, the fluoroketone concentration (y1) (unit: ppm) in the cover gas was adjusted based on the above equations 1 and 2 so that either "Equation 1 is satisfied" or "Equation 1 is not satisfied," 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. On the other hand, if there was no combustion prevention effect, the result was marked as "Poor" in Table 1. In addition, the gas inside the furnace was sampled, and the HF concentration was measured using the usual method (unit: ppm). The results are also shown in Table 1. The above results are summarized in Table 1.

[0056] [Table 1]

[0057] We will explain how to read Table 1 using the top case, User A, as an example. The volume inside the melting furnace corresponding to the space volume (X) is 0.055m 3The melting temperature of magnesium was 630°C. When the oxygen concentration in the furnace was 5.35%, the supply flow rate of the cover gas (y1) was 16 L / min, and the fluoroketone concentration (y2) in the cover gas was 150 ppm. The calculated value Y obtained from the above equation 2 (Y = y1 · y2) was 2400. This Y value satisfies the above equation 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 where the HF concentration in the furnace was too high and was NG, the calculated value Y obtained from the above formula 2 (Y=y1·y2) did not satisfy the above formula 1.

[0059] [Test Example 2] One user evaluated seasonal cover gas conditions, using data published by the Japan Meteorological Agency for mean temperature and minimum relative humidity. Magnesium was melted in the furnace of a magnesium melting apparatus at 690° C. Based on empirical rules, the fluoroketone concentration in the initial cover gas and the initial supply flow rate of the cover gas were determined, and the cover gas was supplied into the furnace. Next, the supply flow rate of the cover gas (y2) (unit: L / min) was adjusted so that the oxygen concentration in the furnace was 7% or less. The oxygen concentration in the furnace at this time is shown in Table 2. Next, the volume of the space inside the furnace that could be filled with the cover gas was calculated based on measurements taken in advance and the melt level sensor. This volume (X) was 0.122 m 3 and "Volume inside the melting furnace (unit: m 3 )" 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 to satisfy Equation 3 based on Equations 3 and 4, and this was supplied into the furnace. In this case, the magnesium combustion prevention effect (combustion prevention effect) was sufficient. Furthermore, sampling the gas inside the furnace and measuring the HF concentration using conventional methods revealed that almost no HF was generated. On the other hand, when cover gas conditions did not satisfy Equation 3, there were problems such as no combustion prevention effect (NG) or generation of high concentrations of HF (3 ppm). These 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. 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] [Table 2] [Explanation of symbols]

[0063] 1. Magnesium melting equipment 2 Melting furnace body 3 Cover gas inlet 4 Derivation mechanism 5 Water level sensor 6 Cover gas supply unit 7 Cover gas flow meter 8 Flow control valve 9. Oxygen concentration meter 10 Control Unit 11. Storage section (furnace) 11a side wall 11b Side wall 11c End wall 12 Top opening 13 Cover 13a One end 14 Lid 15 Opening 21 Supply pipeline 22 Inlet nozzle 22 nozzles 31 cylinders 32 piston 33 Outlet pipeline A Tilt angle B Liquid level position M Molten metal

Claims

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

2. 2. The cover gas supply method 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. 2. 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. 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. 4. The method for supplying a cover gas according to claim 3, further comprising adjusting the concentration of fluoroketone in the cover gas in response to the minimum relative humidity in the area where the furnace is installed within the past 24 hours.

6. 6. The cover gas supply method according to claim 1, 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 furnace for melting magnesium or a magnesium alloy to produce a 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 for measuring a supply flow rate of the cover gas supplied from the cover gas supply unit into the furnace; a flow rate adjusting 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; a melt level sensor for measuring the melt level position of the molten metal in the furnace; A magnesium melting apparatus 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 rate control valve based on the cover gas supply method of claim 1; A magnesium melting apparatus comprising:

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