JPH1192899A - Device and method for dross removal in hot dip galvanizing equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device and method for dross removal in a hot dip galvanizing equipment, having durability as well as a simplified structure, improved in dross removing efficiency, and practically causing no trouble in operation. SOLUTION: The dross removal device has a dross sedimentation tank 3, provided adjacently to a plating bath 1 and having a larger volume than the plating bath 1, and a transfer means for transferring molten zinc between the plating bath 1 and the dross sedimentation tank 3 and also circulating the molten zinc. The transfer means has transfer pipings 4, 5 and pumps 6, 7 for circulating the molten zinc. The pipings 4, 5 have each an internal pipe composed of castable ceramics, a reinforcing pipe on the outer side of the internal pipe, and an adhesive between them. Further, a straight section of the piping has a falling gradient of <=1/10. Moreover, a bend section has a radius of curvature >=2 times the diameter of the pipe and also has a falling gradient equal to or steeper than that of the straight section of the piping and is regulated to <=2 m per pipe, and further, a joining part between the straight section and the bend section has a packing made of ceramics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dross removing apparatus and method in a hot dip galvanizing facility.

[0002]

2. Description of the Related Art Surface defects of hot-dip galvanized steel sheets caused by dross are the most serious problems among hot-dip galvanized steel sheets. The dross is an intermetallic compound (FeZn ₇ or the like) generated by a reaction between iron and zinc eluted from the steel sheet to be plated, and has a diameter of 5 to 300 μm in terms of sphere diameter. This dross will accumulate at the bottom of the plating tank if it is stationary without the flow of molten zinc.

However, since the molten zinc is agitated by the running of the steel sheet, the rotation of the roll in the bath in the plating tank, or the natural convection of the molten zinc generated by the zinc ingot, the dross having a small difference in specific gravity from the molten zinc is formed at the bottom. It cannot be deposited on the steel sheet, or even if it is deposited, it is rolled up and adheres to the coated steel sheet, and this becomes a surface defect of the hot-dip galvanized steel sheet by dross.

Heretofore, numerous proposals have been made to remove dross. These proposals include a method of pumping molten zinc out of a plating tank to precipitate dross, a method of filtering, and a method of adding aluminum to floatation and separating.

However, despite many proposals, none of the conventional proposals have been put to practical use. The reason is that, although these proposed technologies can be realized on a desk, there are many problems in the complexity, durability, and operability of the mechanism in actual equipment, and it is impossible to actually adopt them. is there.

A typical precipitation method is described in JP-A-53-88.
No. 633, JP-A-56-170260, JP-A-3-2
67357 and JP-A-4-154948.

[0007] JP-A-53-88633 discloses that molten zinc is cooled in a sedimentation tank to form dross in the sedimentation tank.
It is disclosed to remove the precipitate. Further, the embodiment shows that the size of the sedimentation tank is reduced to about 1/4 of the size of the plating tank. However, dross cannot be sufficiently removed by this method.

Japanese Utility Model Application Publication No. Sho 56-170260 discloses that molten zinc and dross are pumped from the bottom of a plating tank, introduced into a sedimentation tank, and after the dross is settled, the molten zinc is returned to the plating tank. I have. However, this method does not describe a method for precipitating dross.

Japanese Patent Application Laid-Open No. 3-267357 discloses providing a baffle plate in a settling tank. But,
The effect of promoting dross sedimentation described in this publication is small. That is, this gazette does not propose any fundamental measures for sedimentation removal of dross.

Japanese Patent Application Laid-Open No. 4-154948 discloses that in order to prevent dross from settling in the plating tank, the plating tank is made to have a specific size and shape, and dross is deposited and deposited on the bottom of the settling tank. It is disclosed that the sedimentation tank has sufficient plane area and depth to facilitate.

By the way, regarding the sedimentation tank, unlike the field of water treatment, the dross treatment does not accumulate and systematize the technology at all. And, as shown below,
Because the growth mechanism is completely different from foreign substances in water, the techniques described in these prior art documents do not show a specific means for facilitating sedimentation of dross.

Dross is an intermetallic compound (FeZn ₁₃ , FN) formed by the chemical reaction of iron as a solute with zinc as a solvent.
eZn _7, an Fe etc. ₅ Zn _21). The iron solubility of the molten zinc is greatly affected by the temperature, for example, 460
0.04% at ℃ and 0.01% at 430 ° C. The change in the temperature of the solubility and the change in the concentration of the solute greatly affect dross generation and growth. In addition, dross is thought to undergo a phase transformation as it grows. Different phases have different physical properties such as density. That is, the sedimentation behavior changes.
The above does not occur in water treatment for exogenous foreign substances having no reactivity with water.

On the other hand, the above-mentioned prior art documents do not sufficiently describe the transfer of molten zinc, and it is considered from these prior art documents that the molten zinc can be easily transferred by installing a pump and piping.

However, according to the experiments conducted by the present inventors, the molten zinc in the plating bath is heated to a temperature required by the sensible heat of the running strip or the installation of a heater (usually 30 ° C. to 50 ° C. from the melting temperature). (Overheated to about ℃), but the heat loss is very large in the transfer pipe with a large specific surface area, and the specific heat of molten zinc is only about 25% of water. The energy up to the required temperature is surprisingly small, so that with a simple transfer method the molten zinc solidifies due to heat loss. Therefore, although it is necessary to select an appropriate means for transferring the molten zinc, there is no example examined so far.

The most basic means for preventing solidification is heating. However, when used for a long time, it promotes erosion which will be described later, generates a large amount of fume, and has a high running cost of heating equipment. Although implementation is possible for reasons such as the device itself becoming complicated, it does not exist as practical equipment.

Further, molten zinc is very liable to react with other metals, and many metals form a eutectic alloy, which lowers the melting point and melts the piping metal. Gold and uranium are metals that do not react with zinc, but are not practical. Therefore, in the past, it was often the case that the piping was designed as a consumable, but this would require labor and cost for replacing the piping.

[0017] Therefore, there is a demand for a means for transferring the molten zinc which minimizes manufacturing or running costs and minimizes damage to the piping due to melting and erosion of the molten zinc. In the field of aluminum smelting technology, the use of special steel has reached a level that satisfies this requirement and has been put to practical use. However, the material is very expensive and is not practical in a hot-dip galvanizing apparatus.

[0018]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a simple structure, is durable, has high dross removal efficiency, and causes troubles in operation. It is an object of the present invention to provide a dross removing apparatus and method for a galvanizing facility that is difficult to perform.

[0019]

In order to solve the above problems, the present invention has been completed as a result of repeated experiments on dross generation / growth behavior, dross sedimentation behavior, and structures and materials suitable for practical equipment. It was done.

That is, the present invention relates to a dross removing apparatus for removing dross generated in a plating tank of a hot dip galvanizing facility, wherein the dross removing apparatus is provided adjacent to the plating tank and has a larger capacity than the plating tank. A dross sedimentation tank for sedimenting, and a transfer means for transferring and circulating molten zinc between the plating tank and the dross sedimentation tank, the transfer means having a straight pipe portion and a curved pipe portion. A pipe for circulating the molten zinc, and a pump for continuously circulating the molten zinc across a predetermined stop period, the pipe is an inner pipe made of castable ceramics,
A reinforcing pipe provided on the outside thereof, an adhesive between the inner pipe and the reinforcing pipe, the straight pipe portion has a descending gradient of 1/10 or less, and the curved pipe portion has a pipe diameter. A molten zinc having a curvature of at least twice as large as that of the straight pipe portion, a slope equal to or greater than that of the straight pipe portion, and a pipe within 2 m, and a ceramic packing at a joint portion thereof. An object of the present invention is to provide a dross removing device in a plating facility.

Further, according to the present invention, in the above dross removing apparatus, there is further provided a gas supply means for supplying any of an inert gas, a combustion exhaust gas, and an oxygen-free gas into the pipe, so as to transfer the molten zinc. Before starting, a dross removing apparatus in a hot-dip galvanizing facility is provided, wherein the gas is supplied from the gas supply means into the pipe to replace the gas in the pipe.

Further, according to the present invention, in any one of the above-described dross removing apparatuses, the transfer means may include an average dross dross, where Q is a depth of the dross settling tank, and V is a sedimentation velocity of the dross to be settled and separated. Residence time becomes Q / V or more,
In addition, the present invention provides a dross removing apparatus for a hot dip galvanizing facility, wherein a circulation flow rate is determined within a range where a suction speed in a plating tank is 1 cm / sec or more.

Still further, the present invention provides a dross sedimentation tank provided adjacent to a plating tank of a hot-dip galvanizing facility and having a larger capacity than the plating tank, for dross sedimentation.
Transfer means for transferring and circulating the molten zinc between the plating tank and the dross settling tank, wherein the transfer means comprises a pipe for circulating the molten zinc having a straight pipe portion and a curved pipe portion. The pipe has an inner pipe made of castable ceramics, a reinforcing pipe provided outside thereof, and an adhesive between the inner pipe and the reinforcing pipe,
The straight pipe portion has a descending gradient of 1/10 or less, the curved pipe portion has a curvature of twice or more the pipe diameter, and has a descending gradient equal to or more than the straight pipe portion. A dross removing method using a dross removing device in a hot-dip galvanizing facility having a ceramic packing at a joint portion thereof, wherein the molten zinc is continuously disposed with a predetermined stop period interposed therebetween. The present invention provides a method for removing dross in a hot-dip galvanizing facility, which is circulated through the pipe without causing local stagnation.

Still further, according to the present invention, in the above dross removing method, before the transfer of the molten zinc is started by the transfer means, the inside of the pipe is replaced with any of an inert gas, a combustion exhaust gas and an oxygen-free gas. Another object of the present invention is to provide a dross removing method in a hot-dip galvanizing facility, which controls an atmosphere in a pipe during circulation of a molten metal to be an inert atmosphere.

Furthermore, the present invention provides the dross removing method according to any one of the above, wherein the dross sedimentation tank has a depth of Q and the sedimentation velocity of the dross to be settled and separated is V, and the average dross residence time is Q / V or more, and a method for removing dross in a hot-dip galvanizing facility, characterized in that molten zinc is circulated at a flow rate in a range where the suction speed in the plating tank is 1 cm / sec or more.

The operation of the present invention configured as described above is as follows. In the present invention, a dross sedimentation tank is provided adjacent to the plating tank in the hot-dip galvanizing equipment, and it is important that the dross sedimentation tank be larger than the capacity of the plating tank. That is, a dross that does not settle in the plating tank needs to be larger than the capacity of the plating tank in order to settle and separate in the sedimentation tank. Conversely, if the plating tank is large, the dross will settle and separate in the plating tank before reaching the sedimentation tank, and the object cannot be achieved.

Before the dross generated in the plating tank grows and reaches a harmful size, the dross is sent to the precipitation tank, and it is important that the molten zinc flow without local stagnation.
The durability of the piping. Therefore, in the present invention, the pipe for transporting the molten zinc has an inner pipe made of castable ceramics, a reinforcing pipe provided outside thereof, and an adhesive between the inner pipe and the reinforcing pipe. The straight pipe portion has a taper with a descending slope of 1/10 or less, the curved pipe section has a curvature of twice or more the pipe diameter, and has the same descending slope as the straight pipe section, Each pipe had a length of 2 m or less, and a ceramic packing was provided at the joint. Further, the molten zinc was continuously circulated through a predetermined stop period using a pump.

Here, the reason why the molten zinc is continuously supplied with a predetermined stop period interposed therebetween is that when the operation is continuously performed for a long time, the operation is stopped and the dross in the settling tank is removed from the tank. Because it is necessary to remove it. Even in that case, dross is steadily generated because the plating tank is operating, and if the operation is resumed to remove the dross in the sedimentation tank, the molten zinc is circulated more than at the beginning and the plating tank is circulated. It is necessary to remove the dross in the as soon as possible.

Before starting the transfer of the molten zinc by the transfer means, the inside of the pipe is gas-replaced with any of an inert gas such as a nitrogen gas, a combustion exhaust gas, or an oxygen-free gas, and the molten metal is circulated. It is preferable to control the atmosphere in the pipe to be an inert atmosphere, thereby preventing the zinc oxide from being inadvertently oxidized to generate an oxidized substance.

Further, assuming that the depth of the dross sedimentation tank is Q and the sedimentation velocity of the dross to be settled and separated is V, the average dross residence time is Q / V or more, and the suction speed in the plating tank is By circulating the molten zinc at a flow rate in the range of 1 cm / sec or more, optimal dross recovery becomes possible.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific example of the present invention will be described with reference to FIGS. FIG. 1 is a sectional view showing a hot-dip galvanizing facility provided with a dross removing apparatus according to an embodiment of the present invention, and FIG. 2 is a plan view thereof. The hot-dip galvanizing equipment has a plating tank 1 in which hot-dip zinc L is stored, and a dross removing device. This dross removing device is composed of a plating tank 1
, A first transfer pipe 4 for transferring molten zinc from the plating tank 1 to the precipitation tank 3,
A second transfer pipe 5 for transferring molten zinc from the precipitation tank 3 to the plating tank 1, a pump 6 provided on the first transfer pipe 4, and a pump 7 provided on the second transfer pipe 5 Have.

At the entrances and exits of the pipes 4 and 5, any one of an inert gas such as nitrogen gas, a combustion exhaust gas, and an oxygen-free gas is supplied into the pipes to replace the gas, and the atmosphere in the pipes is changed to an inert atmosphere. A gas supply device 9 is provided. In the vicinity of the second transfer pipe 5 outside the precipitation tank 3, an induction heating device 10 for controlling the temperature of the returned molten zinc is provided.

The pump 6 used for pumping the first transfer pipe 4 is preferably a ceramic pump. When a ceramic pump is driven, an air pump using compressed air rather than an electric motor is desirable. This is because when the shaft of the pump solidifies due to a trouble, the electric motor is forcibly rotated so that there is a high risk of damaging the ceramics. Since the air pump has a small driving force, the force for forcibly rotating does not work. Plating tank 1
The inlet 6 ′ of the pump for sucking the molten zinc is preferably closer to the bottom of the plating tank 1. This is because the dross concentration is higher at the bottom than at the top, so the dross collection efficiency and transfer efficiency are higher. As shown in the figure, the inlet 6 'of the pump 6 is wide in order to increase the suction efficiency.

As shown in FIG. 3, which is a cross-sectional view taken along the line AA 'of FIG. 2, the plating tank 1 has a sink roll 2 provided so as to be immersed in the molten zinc L therein. 1
After the steel sheet S to be plated passed through 1 is immersed in the molten zinc L stored in the plating tank 1, the steel sheet S is turned by the sink roll 2 and passed out of the plating tank 1.

The first and second transfer pipes 4 and 5
Has the structure shown in FIG. That is, these transfer pipes 4 and 5 have an inner pipe 14 made of castable ceramics such as alumina and a reinforcing pipe 12 provided on the outermost side made of steel or the like. The bonding portion has an adhesive 13 having a buffering property for absorbing thermal expansion. The inside of the inner tube 14 is a molten zinc channel 15. By using a transfer pipe having such a structure, thermal expansion can be minimized, erosion can be prevented, and sufficient strength can be obtained.

The transfer pipes 4 and 5 are set within 2 m per pipe in consideration of the thermal expansion of the pipes themselves. And
To prevent solidification of the molten zinc,
Using ceramic packing and a ceramic filter, the straight pipe part of the pipe has a downward slope of 1/10 or less, the curved pipe part has a radius of curvature of twice or more the pipe diameter,
A downward gradient equal to or greater than the straight pipe portion is formed. This allows the molten zinc to flow without local stagnation.

Assuming that the depth of the dross sedimentation tank is Q and the sedimentation speed of the dross to be settled and separated is V, the average dross residence time of the dross is as follows. Q / V or more, and it is preferable that the suction speed by the motor 6 in the plating tank 1 be in a range of 1 cm / sec or more. This enables optimal dross recovery.

In the hot-dip galvanizing equipment thus configured, the steel sheet S to be plated out of the heating furnace is passed through the snout 11 to the plating tank 1, and
Immersed in the molten zinc L stored in the tank. And FIG.
As shown in (1), after the steel sheet S is turned by the sink roll 2 provided in the plating tank 1, the steel sheet S is passed in the direction shown by the arrow.

During this time, iron is eluted from the steel sheet S to be plated. It is considered that the form of iron elution at this time is the case where the powdery substance adhering to the steel sheet and the fine iron-zinc alloy layer formed on the steel sheet surface are separated and dissolved, or the case where it is directly eluted from the steel sheet. Can be

The iron thus dissolved is reprecipitated and grows into dross. The amount of dross generated is proportional to the throughput of the steel sheet (total surface area of the passing steel sheet). The amount generated when a 1 m steel plate is passed at a speed of 120 m per minute is approximately 50 kg per hour. The dross immediately after generation is extremely small with a diameter of 5 to 10 microns. At this size, the sedimentation velocity is slow. The molten zinc temperature during operation in the plating tank 1 is in the range of 455 to 465 ° C., and this temperature depends on the temperature and heat capacity of the steel sheet to be passed, and the amount of heat released from the plating tank and the amount of heat to the plating tank. I can decide.

The molten zinc L is sent from the plating tank 1 to the precipitation tank 3 by the first transfer pipe 4 as a transfer means.
At this time, the transfer amount of the molten zinc L must be an amount capable of sufficiently removing dross from the plating tank 1. In this case, the transfer amount, that is, the circulation flow rate of the molten zinc is, as described above, the average dross residence time is Q / V or more, and
It is desirable to set the suction speed in the plating tank to be 1 cm / sec or more. For example, assuming that the depth of the sedimentary layer 3 is 2 m and the diameter of the lowest dross in question is approximately 100 μm when approximated by a sphere, the dross sedimentation speed is a gentle end speed of approximately 1 mm per second. The residence time in this case is 2000 seconds, approximately 30 minutes. Therefore, it is desirable to set the circulation flow rate such that the dross residence time is equal to or longer than this time. Although the height of the suction port depends on its shape, usually, a suction port having a height of 5 cm and a width of about 2 m is provided in the plating tank. × 2 × 0.01 × 3600 = 3.6 m ³ / hr circulation flow rate is required. Assuming that the capacity of the plating tank is 20 m ³ , the required circulation flow rate determined from the sedimentation velocity is 40 m ^3.
/ H or less, the circulation flow rate required from the suction port is 3.6
m ³ / h or more, and circulation in this flow rate range enables optimal dross recovery.

Next, an operation example in which the above hot-dip galvanizing equipment is actually operated will be described. Here, castable ceramic made of alumina having an inner diameter of 50 mm is used as the inner pipe 14 as the transfer pipes 4 and 5, and a 80 A steel pipe material is disposed outside as the reinforcing material 12 via the adhesive 13, and the gradient 1 / 200 was installed. The curved pipe was bent so as to have a curvature of twice or more the diameter of the pipe, and the bent pipe was provided with a descending gradient higher than that of the straight pipe, and the pipes were constructed so that there were no protrusions and steps. The step tends to be the solidification starting point when the solidification of the molten zinc occurs, and by minimizing the step,
It was found that clogging in the piping could be completely prevented. In this example, the steps are less than 1 mm at all the connecting parts of the pump and the transfer pipe, the transfer pipe and the transfer pipe, and the outlet from the transfer pipe. The length of one transfer pipe was set to 2 m, and a ceramic fibrous filter having poor wettability of molten zinc was used for a connection portion. This prevented solidification of the molten zinc in the flange portion during disassembly.

In this example, by circulating the molten zinc at 10 m3 / h using a mechanical pump, it was confirmed that the inside of the pipe for transferring the molten zinc was a filling flow. When the flow is filled, the flow resistance itself increases, but the pipe can be heated by the sensible heat of the molten zinc, so that it is possible to achieve operational stability and the pipe temperature is the same on the upper and lower surfaces. Therefore, it was confirmed that no thermal stress problem occurred.

Before supplying the molten zinc to the transfer pipe, the inside of the transfer pipe was replaced with nitrogen gas to prevent the inflow of oxygen even during circulation. Thereby, the oxidation phenomenon of the molten zinc in the pipe was suppressed, and the generation of the oxide of the molten zinc could be eliminated. When pumping molten zinc,
An air pump made of ceramics was used. Since the driving force of the air pump is small and the force for forcibly rotating does not work, there is little risk of breakage even if solidification occurs in the pump.

Thus, after one month of continuous use, the dross in the plating tank was 10% or less of that of the prior art, and it was confirmed that there was no dross that would cause a problem in quality. On the other hand, it was confirmed that 90% of the amount of dross settled in the conventional plating tank was in the sedimentation tank. During the operation, the hot-dip galvanized steel sheet had no significant surface defects.

[0046]

As described above, according to the present invention,
A dross removing device for hot-dip galvanizing equipment with a simple structure, high durability, high dross removal efficiency, and easy discharge of dross out of the facility. A dross removing method in a galvanizing facility can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a dross removing device according to an embodiment of the present invention.

FIG. 2 is a plan view showing a dross removing device according to one embodiment of the present invention.

FIG. 3 is a sectional view taken along line A-A ′ of FIG. 2;

FIG. 4 is a sectional view showing a molten zinc transfer pipe used in the dross removing apparatus according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Plating tank 2 ... Sink roll 3 ... Settling tank 4 ... First transfer pipe 5 ... Second transfer pipe 6,7 ... Pump 6 '... Inlet 8 ... Dross 9 ... Gas supply device 10 Induction heating device 11 Snout 12 Reinforcement tube 13 Adhesive 14 Inner tube 15 Zinc flow path

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Keishi Yamashita Nihon Kokan Co., Ltd. 1-2-1, Marunouchi, Chiyoda-ku, Tokyo

Claims (6)

[Claims] 1. A dross removing apparatus for removing dross generated in a plating tank of a hot-dip galvanizing facility, wherein the dross is provided adjacent to the plating tank and has a larger capacity than the plating tank. A settling tank, and a transfer means for transferring and circulating the molten zinc between the plating tank and the dross settling tank, wherein the transferring means circulates the molten zinc having a straight pipe portion and a curved pipe portion. And a pump for continuously circulating the molten zinc with a predetermined stop period interposed therebetween.The pipe includes an inner pipe made of castable ceramics, a reinforcing pipe provided outside thereof, An adhesive between the inner pipe and the reinforcing pipe, wherein the straight pipe section has a descending slope of 1/10 or less, the curved pipe section has a curvature of at least twice the pipe diameter, and Same as the straight pipe part More has a descending gradient, further within the pipe 1 per present 2m, dross removal apparatus in galvanizing, characterized in that it comprises a ceramic packing on the bonding portion. 2. A gas supply means for supplying any one of an inert gas, a combustion exhaust gas, and an oxygen-free gas in the pipe, and the gas supply means supplies the molten zinc before starting the transfer of the molten zinc. The dross removing apparatus in the hot-dip galvanizing equipment according to claim 1, wherein the gas is supplied into the pipe to replace the gas in the pipe. 3. The transfer means according to claim 1, wherein when the depth of the dross sedimentation tank is Q and the sedimentation velocity of the dross to be sedimented and separated is V, the average dross residence time is Q / V or more, and The dross removing apparatus for a hot-dip galvanizing facility according to claim 1 or 2, wherein the circulation flow rate is determined within a range in which the suction speed at 1 m / sec is 1 cm / sec or more. 4. A dross sedimentation tank provided adjacent to a plating tank of a galvanizing equipment and having a larger capacity than the plating tank, for dross sedimentation, and melting between the plating tank and the dross sedimentation tank. Transfer means for transferring and circulating zinc, the transfer means having a pipe for circulating molten zinc having a straight pipe portion and a curved pipe portion, wherein the pipe is made of castable ceramics. A pipe, a reinforcing pipe provided on the outside thereof, and an adhesive between the inner pipe and the reinforcing pipe, wherein the straight pipe portion is 1/1
0, and the curved pipe portion has a pipe diameter of 2
A dross removing device in a hot-dip galvanizing facility having a curvature of twice or more, and having a descending gradient equal to or greater than that of the straight pipe portion, and having a pipe packing of 2 m or less and having a ceramic packing at a joint thereof. The dross removing method used, wherein in the transferring means, the molten zinc is continuously or intermittently circulated through the pipe without causing local stagnation. Removal method. 5. Before starting the transfer of molten zinc by the transfer means, the inside of the pipe is purged with any of an inert gas, a combustion exhaust gas, and an oxygen-free gas, and the atmosphere in the pipe during the circulation of the molten metal. The method for removing dross in a hot-dip galvanizing apparatus according to claim 4, wherein the dross is controlled to be an inert atmosphere. 6. When the depth of the dross sedimentation tank is Q and the sedimentation velocity of the dross to be sedimented and separated is V, the average dross residence time is Q / V or more, and the suction speed in the plating tank is The dross removing method for a hot dip galvanizing facility according to claim 4 or 5, wherein the molten zinc is circulated at a flow rate in a range of 1 cm / sec or more.