JP7575670B2 - Fine bubble cleaning method and fine bubble cleaning device - Google Patents

Description

The present invention relates to a fine bubble cleaning method and a fine bubble cleaning device.

During the manufacturing process, oils such as processing oils and lubricants often adhere to the surfaces of various industrial parts. For this reason, there is a need to remove this oil from the surfaces of industrial parts, and various cleaning devices have been proposed in the past.

For example, the following Patent Document 1 proposes a cleaning device that uses ultrasonic waves in combination with a cleaning liquid containing fine bubbles to clean oil adhering to the surface of an object to be cleaned.

International Publication No. 2016/147995

During the steel pipe manufacturing process, metal processing oils such as mineral oils (e.g., rolling processing oils) adhere to the outer and inner surfaces of the manufactured steel pipes. For this reason, the steel pipe manufacturing process generally includes a cleaning process (also called a degreasing process) to clean off such metal processing oils.

In this steel pipe cleaning process (degreasing process), the oil is hydrolyzed by the saponification action of the alkaline cleaning solution, and the temperature of the alkaline cleaning solution is kept at 80°C or higher to promote the hydrolysis reaction, solubilizing the oil.

However, to obtain a high-temperature liquid of 80°C or higher, a heat source is required to heat the cleaning liquid, which is costly. In addition, various problems arise, such as an increase in the amount of chemical solution to be processed due to chemical reactions in the cleaning liquid, and a deterioration of the working environment due to the steam that is generated. Therefore, in addition to further lowering the temperature of the cleaning liquid, there is a demand for further improvements in convenience for workers, such as reducing the amount of chemical solution to be processed and costs, and improving the working environment.

In the cleaning process (degreasing process) of steel pipe manufacturing, long pipes, for example, at least 2 to 30 m long, are cleaned. However, when such long pipes are used as the objects to be cleaned, the degreasing performance of the steel pipe inner surface decreases. Therefore, it is important to further reduce the temperature of the cleaning liquid used for cleaning and further improve convenience for workers without degrading the cleaning performance.

The present invention has been made in consideration of the above problems, and the object of the present invention is to provide a fine bubble cleaning method and fine bubble cleaning device that can further reduce the temperature of the cleaning liquid and further improve convenience for the operator without compromising cleaning performance.

After extensive research into the deterioration of degreasing performance in long pipes as described above, the inventors discovered that the main cause was not the degree of scale adhesion during heat treatment in the manufacture of steel pipes, or the inconsistency in adhesion of metalworking oil during rolling, but rather the change in the flow of cleaning liquid that entered the hollow portion due to the long length.

In other words, it was discovered that the main cause is that when the cleaning liquid enters the hollow part of the long tube, the flow of the cleaning liquid stops, or even if there is a flow, it becomes laminar and liquid exchange is lost, and as a result, the oil solubilization performance decreases the further downstream the flow goes.

As a result of further investigations based on this knowledge, the inventors came up with the idea of supplying air bubbles called fine bubbles to the cleaning liquid and maintaining liquid agitation in the cleaning liquid using such air bubbles, thereby completing the present invention described below.
The gist of the present invention, which was completed based on this concept, is as follows.

[1] A fine bubble washing method for washing a steel pipe, part or all of which is elongated in a longitudinal direction, as an object to be washed, comprising: a treatment tank for holding a washing liquid for washing the object to be washed and in which one or more objects to be washed are immersed; a washing liquid circulating mechanism for circulating the washing liquid in the treatment tank and discharging the washing liquid onto the object to be washed; and a fine bubble supplying mechanism for supplying fine bubbles to the washing liquid in the washing liquid circulating mechanism, wherein the washing liquid circulating mechanism is arranged so that the angle θ between the flow direction of the washing liquid and the longitudinal direction of the steel pipe is within a range of more than 0° and not more than 10.0°. The method uses a fine bubble cleaning device that discharges the cleaning liquid, and controls the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid so as to satisfy the relationship expressed by the following formula (1): where Dpi (unit: m), L (unit: m), average bubble diameter Db (unit: m), and flow velocity V of the cleaning liquid at the inlet side of the hollow part of the steel pipe are respectively 1 to 100 μm for the average bubble diameter Db and 0.2 to 5.0 m/s for the flow velocity V.
When L/Dpi<200, 3.2×10 ⁻¹⁰ ≦Db ² ×V≦5.0×10 ⁻⁸
When 200≦L/Dpi, 6.0×10 ⁻¹³ ≦Db ² ×V≦3.2×10 ⁻⁹
...Equation (1)
[2] The fine bubble cleaning method described in [1], wherein the cleaning liquid circulating mechanism discharges the cleaning liquid so that the angle θ is in the range of more than 0° and not more than 10.0°, and the cleaning liquid flows through the hollow part of the steel pipe from the bottom side of the treatment tank toward the liquid surface side.
[ 3 ] The fine bubble cleaning method according to any one of [1] and [ 2] , wherein the cleaning solution contains a nonionic surfactant having ethylene oxide added to a hydrophilic group, and the HLB value of the surfactant is within a range of 15.0 to 19.0.
[ 4 ] The fine bubble cleaning method according to [ 3 ], wherein the concentration of the surfactant in the cleaning agent is 0.01 times or more and less than 2.00 times the critical micelle concentration of the surfactant.
[ 5 ] The fine bubble cleaning method according to [ 3 ] or [ 4 ], wherein the surfactant has a surface tension of 40 mN/m or more at 25°C.
[ 6 ] The fine bubble cleaning method according to any one of [ 3 ] to [ 5 ], wherein the surfactant is an ether-type nonionic surfactant.
[ 7 ] The fine bubble cleaning method according to any one of [ 3 ] to [ 6 ], wherein the hydrophobic group of the surfactant is a linear carbon chain having a carbon number in the range of 10 to 20.
[ 8 ] The fine bubble cleaning method according to any one of [1] to [ 7 ], wherein the alkali concentration of the cleaning solution is 1.0 mol/L or less.
[ 9 ] The fine bubble washing method according to any one of [1] to [ 8 ], wherein the fine bubble number density is within a range of 1 x 10 ³ to 1 x 10 ⁶ bubbles/mL.
[ 10 ] The fine bubble cleaning method according to any one of [1] to [ 9 ], wherein the cleaning liquid has a liquid temperature within a range of 20 to 60°C.
[ 11 ] The fine bubble cleaning method according to any one of [1] to [ 10 ], wherein the object to be cleaned is a steel pipe having metal processing oil attached thereto.
[ 12 ] A fine bubble washing device for washing a steel pipe, part or all of which is extended in a longitudinal direction, as an object to be washed, comprising: a treatment tank for holding a washing liquid for washing the object to be washed and in which one or more of the objects to be washed are immersed; a washing liquid circulating mechanism for circulating the washing liquid in the treatment tank and discharging the washing liquid onto the object to be washed; and a fine bubble supplying mechanism for supplying fine bubbles to the washing liquid in the washing liquid circulating mechanism, wherein the washing liquid circulating mechanism discharges the washing liquid so that the angle θ between the flow direction of the washing liquid and the longitudinal direction of the steel pipe is within a range of more than 0° and not more than 10.0°.
[13] The fine bubble cleaning device according to [12], wherein the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid are controlled within a range of 1 to 100 μm for the average bubble diameter Db and within a range of 0.2 to 5.0 m/s for the flow velocity V, so as to satisfy the relationship expressed by the following formula (1): Dpi (unit: m), L (unit: m) of the inner diameter of the object to be cleaned, Db (unit: m) of the pipe length, and V (unit: m/s) of the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid.

When L/Dpi<200, 3.2×10 ⁻¹⁰ ≦Db ² ×V≦5.0×10 ⁻⁸
When 200≦L/Dpi, 6.0×10 ⁻¹³ ≦Db ² ×V≦3.2×10 ⁻⁹
...Equation (1)

As described above, the present invention makes it possible to further reduce the temperature of the cleaning liquid and further improve the convenience of the operator without compromising cleaning performance.

FIG. 1 is a schematic diagram for explaining a fine bubble cleaning device according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining the fine bubble cleaning device in the same embodiment. FIG. 2 is a schematic diagram for explaining the fine bubble cleaning device in the same embodiment. FIG. 2 is a schematic diagram for explaining the fine bubble cleaning method according to the embodiment. FIG. 1 is a schematic diagram for explaining a fine bubble cleaning device used in an experimental example. FIG. 1 is a schematic diagram for explaining a fine bubble cleaning device used in an experimental example.

The preferred embodiment of the present invention will be described in detail below with reference to the attached drawings. Note that in this specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals to avoid redundant description.

(Regarding the investigations conducted by the present inventors)
Before describing the fine bubble washing method and fine bubble washing device according to the embodiment of the present invention, we will briefly explain the investigations that the inventors conducted to solve the above problems and the findings obtained through such investigations.

After extensive research into the deterioration of degreasing performance in long pipes as described above, the inventors discovered that, as mentioned above, the main cause was not the deposition of scale during heat treatment in the manufacture of steel pipes, or the inconsistency in the deposition of metalworking oil during rolling, but rather the change in the flow of cleaning fluid that entered the hollow portion due to the long length.

In other words, it was discovered that the main cause is that when the cleaning liquid enters the hollow part of the long tube, the flow of the cleaning liquid stops, or even if there is a flow, it becomes laminar and liquid exchange is lost, and as a result, the oil solubilization performance decreases the further downstream the flow goes.

As a result of further investigation based on this knowledge, the inventors came up with the idea of supplying bubbles called fine bubbles (hereinafter sometimes abbreviated as "FB") to the cleaning solution and using these bubbles to maintain liquid agitation in the cleaning solution. By maintaining liquid agitation in the cleaning solution with fine bubbles, it becomes possible to more efficiently remove oil by utilizing the oil adsorption and oil peeling effects of the fine bubbles in addition to the solubilization of oil by the cleaning solution.

More specifically, since the oil adsorption/peeling effect of fine bubbles is utilized without relying solely on the saponification reaction of oil, it is possible to suppress the temperature rise of the cleaning solution to sufficiently react with the saponification reaction while maintaining the cleaning performance, and the temperature of the cleaning solution can be reduced. In addition, since the temperature rise of the cleaning solution can be suppressed, the deterioration of the working environment due to steam generated by heating can be suppressed. Furthermore, since the oil adsorption/peeling effect of fine bubbles is utilized, the cleaning performance can be maintained by supplying fine bubbles to the cleaning solution until the properties of the cleaning solution are reduced to a level that cannot be ignored, so the amount of chemical solution (cleaning solution) used can be reduced. In this way, by using fine bubbles, it is possible to further reduce the temperature of the cleaning solution and further improve the convenience of the worker without deteriorating the cleaning performance.

Here, the fine bubbles that are the focus of this embodiment of the present invention have a small but buoyant force that causes them to float in the cleaning liquid. Here, if the flow rate of the cleaning liquid held in the treatment tank is low, the fine bubbles will aggregate inside (the hollow part) of the steel pipe immersed in the treatment tank, and not only will the fine bubbles not be able to absorb and remove oil, but it is expected that uneven cleaning will occur in the hollow part of the steel pipe.

Based on the above findings, the inventors first decided to use only fine bubbles without ultrasound, rather than using both fine bubbles and ultrasound as described in the above cited reference 1. This is because ultrasound acts in a direction that causes the fine bubbles to aggregate, and therefore ultrasound is likely to have an undesirable effect in the embodiment of the present invention in which it is desired to suppress the aggregation of fine bubbles.

The inventors also intended to allow some degree of floating of fine bubbles in the cleaning solution, which is the key feature of the present invention, but to prevent aggregation of the fine bubbles. To achieve this, it became necessary to examine the buoyancy acting on the fine bubbles.

Here, the inventors focused on the Stokes' equation shown in the following equation (a). The Stokes' equation is used to calculate the floating speed of particles, but it has been proven to be applicable when focusing on the floating speed of bubbles such as fine bubbles.

Here, in the above formula (a),
v: Sedimentation velocity of particles in water (cm/s)
d: particle diameter (cm)
ρ _s : particle density (g/cm ³ )
ρ _w : density of water (g/cm ³ )
η: Viscosity of water (g/cm s)
g _n :Gravity acceleration (cm/s ² )
It is.

In the case of a long steel pipe of 2 to 30 m or so at the shortest length, as is the focus of the present invention, even if the inner diameter of the steel pipe is small and the pipe length is short, the bubbles will escape from the lower end of the steel pipe (the end relatively closer to the bottom of the treatment tank) even if the flow rate of the cleaning liquid is slow. Therefore, it is presumed that a sufficient cleaning effect can be obtained. On the other hand, if the inner diameter of the steel pipe is small and the pipe length is long, it is presumed that the fine bubbles will aggregate at the top of the hollow part of the steel pipe and will not be able to escape to the outside from the lower end. From this perspective, the inventors have found that the relationship between the inner diameter and length of the steel pipe, the average bubble diameter of the fine bubbles, and the flow rate of the cleaning liquid is an important factor, and that it is important to reduce the average bubble diameter of the fine bubbles and increase the flow rate of the cleaning liquid.

In particular, since the average bubble diameter of fine bubbles has a square effect on the bubble floating speed as shown in the above formula (a), it was expected that the average bubble diameter of fine bubbles would be a factor with a large influence.

Based on the above findings, the inventors came up with the parameter "(average bubble diameter of fine bubbles) ² × flow velocity of cleaning liquid" as a factor indicating the strength of the cleaning action of fine bubbles, and conducted experiments to determine the flow velocity at which bubbles do not aggregate while changing the inner diameter and length of the long steel pipe and the average bubble diameter of the fine bubbles. After summarizing the findings obtained from the experiments based on the above parameters, the inventors came to the conclusion that an excellent cleaning effect can be obtained when the parameter "(average bubble diameter of fine bubbles) ² × flow velocity of cleaning liquid" falls within a certain range.

Below, we will explain in detail the embodiment of the present invention that was created based on this knowledge.

(About steel pipes)
First, a steel pipe that is the focus of the embodiment of the present invention will be briefly described.
As mentioned above, in the embodiment of the present invention, attention is focused on a long steel pipe having a length of at least about 2 to 30 m. In addition, it is assumed that a metal processing oil containing mineral oil as a main component is attached to the surface (outer surface and inner surface) of such a long pipe. The chemical composition and use of the steel constituting the steel pipe are not particularly limited, and various steel pipes can be used as the object to be cleaned.

In addition, it is not necessary for the entire long steel pipe to be elongated in the longitudinal direction, and any part of it may be subjected to various known metal processing treatments.

Metalworking oils, which are primarily composed of mineral oils, come in a wide variety of viscosities and contain impurities. The surfaces of fine bubbles are negatively charged, making it easy to remove impurities contained in metalworking oils and allowing the impurities to float and separate. Even with a wide range of viscosities, cleaning utilizes the contact adsorption and peeling effects of fine bubbles, making it possible to repeatedly generate contact with fine bubbles and enabling more efficient cleaning, compared to general cleaning, which involves lifting up the steel pipe.

(About the fine bubble cleaning device)
Next, a fine bubble washing device used in a fine bubble washing method according to an embodiment of the present invention will be briefly described with reference to Figures 1A to 2. Figures 1A to 2 are schematic diagrams for explaining a fine bubble washing device according to an embodiment of the present invention.

As shown in FIG. 1A and FIG. 1B, the fine bubble cleaning device 1 used in the fine bubble cleaning method according to this embodiment mainly comprises a treatment tank 10, a cleaning liquid circulation mechanism 20, and a fine bubble supply mechanism 30.

The treatment tank 10 holds the cleaning liquid 3 for cleaning the object to be cleaned (i.e., a long pipe with oil on it) S, and one or more objects to be cleaned are immersed in the cleaning liquid 3 so that their hollow parts are filled with the cleaning liquid 3. Here, the objects to be cleaned S are bundled together, for example, with a wire and hung by a hanging device such as a crane (not shown), so that they are immersed in the cleaning liquid 3 held in the treatment tank 10.

The material of the treatment tank 10 is not particularly limited, and may be a treatment tank made of various metal materials such as iron, steel, stainless steel plate, etc., a treatment tank made of various plastic resins such as fiber reinforced plastic (FRP) and polypropylene (PP), etc., or a treatment tank made of various bricks. In other words, an existing treatment tank can be used as the treatment tank 10 constituting the fine bubble cleaning device 1 according to this embodiment.

The size of the treatment tank 10 can also be set appropriately depending on the size and number of objects to be cleaned, and even large treatment tanks of various shapes can be used as the treatment tank 10 of the fine bubble cleaning device 1 according to this embodiment.

The treatment tank 10 is provided with a cleaning liquid circulation mechanism 20 that circulates the cleaning liquid 3 within the treatment tank 10 and discharges the cleaning liquid 3 toward the object to be cleaned S. The cleaning liquid circulation mechanism 20 has a circulation pipe 21 for circulating the cleaning liquid 3, and a circulation pump 23 for circulating the cleaning liquid 3. The circulation pump 23 draws the cleaning liquid 3 from within the treatment tank 10 into the circulation pipe 21, and then discharges the cleaning liquid 3 from the end of the circulation pipe 21 opposite to the drawing side toward the object to be cleaned S in the treatment tank 10.

Here, the circulation pipes 21 and the circulation pumps 23 are not particularly limited, and various types of pipes and circulation pumps can be used to realize the cleaning liquid circulation mechanism 20. In addition, the number of circulation pipes 21 and circulation pumps 23 installed for one treatment tank 10 is not particularly limited, and may be set appropriately depending on the size of the treatment tank 10, etc.

The average flow velocity of the cleaning liquid 3 inside the treatment tank 10, which is achieved by the cleaning liquid circulation mechanism 20, is preferably within the range of 0.2 m/s to 5.0 m/s. By achieving an average flow velocity within this range, it is possible to more reliably enjoy the cleaning effect of fine bubbles, as described in detail below.

A fine bubble supply mechanism 30 is provided for the circulation pipes 21, supplying fine bubbles to the cleaning liquid 3 in the cleaning liquid circulation mechanism 20. In FIG. 1A, the fine bubble supply mechanisms 30 are illustrated as being provided in series with respect to each circulation pipe 21, but the fine bubble supply mechanisms 30 may also be provided in parallel with the circulation pipes 21. There is no particular limit to the number of fine bubble supply mechanisms 30 that can be installed, and the number may be set appropriately depending on the size of the treatment tank 10, etc.

Here, there are various basic methods for generating fine bubbles, such as shearing bubbles, passing bubbles through micropores, pressurized dissolution of gas, high-speed swirling of liquid, ultrasonic waves, electrolysis, chemical reactions, etc. As the fine bubble supply mechanism 30 according to this embodiment, it is possible to use a device having a gas intake mechanism that utilizes the various generation methods described above. However, it is more preferable to use a fine bubble generation mechanism that uses a method of taking in gas into the discharge nozzle that generates fine bubbles, as this makes it possible to more reliably control the average bubble diameter of the fine bubbles.

In the fine bubble supply mechanism 30 according to this embodiment, the average bubble diameter of the fine bubbles generated is preferably within the range of 1 to 100 μm. If the average bubble diameter of the fine bubbles is less than 1 μm, the number of times the fine bubbles come into contact with oil will be reduced, resulting in a reduced amount of oil peeling off and a reduced cleaning effect. By setting the average bubble diameter of the fine bubbles to 1 μm or more, it is possible to more reliably achieve the desired cleaning effect. The average bubble diameter of the fine bubbles is more preferably 5 μm or more, and even more preferably 10 μm or more. On the other hand, if the average bubble diameter of the fine bubbles exceeds 100 μm, the oil adsorption effect may not be achieved. By setting the average bubble diameter of the fine bubbles to 100 μm or less, it is possible to more reliably achieve the oil adsorption effect of the fine bubbles. The average bubble diameter of the fine bubbles is more preferably 80 μm or less, and even more preferably 60 μm or less.

In addition, in the fine bubble supply mechanism 30 according to the present embodiment, the density of the fine bubbles generated is preferably within the range of 1×10 ³ to 1×10 ⁶ bubbles/mL. If the density of the fine bubbles is less than 1×10 ³ bubbles/mL, the number of contacts with oil will be reduced, resulting in a reduced amount of oil peeling, which may result in a smaller cleaning effect. By setting the density of the fine bubbles to 1×10 ³ bubbles/mL or more, it is possible to more reliably achieve the desired cleaning effect. The density of the fine bubbles is more preferably 3×10 ³ bubbles/mL or more, and even more preferably 5×10 ³ bubbles/mL or more. On the other hand, the upper limit of the density of the fine bubbles is not particularly specified, and the higher the better, but if the density of the fine bubbles exceeds 1×10 ⁶ bubbles/mL, the consumption of the surfactant added to the cleaning agent will increase, which may increase costs. By setting the density of the fine bubbles to 1×10 ⁶ bubbles/mL, it is possible to achieve the desired cleaning performance without increasing costs.

The average bubble diameter and concentration (density) of fine bubbles can be measured using known equipment such as a liquid particle counter or a bubble size distribution measuring device.

In order to more reliably control the average flow velocity of the cleaning liquid 3 in the treatment tank 10, a second circulation pipe (not shown) for circulating the treatment liquid 3 may be installed separately from the circulation pipe 21 in which the fine bubble supply mechanism 30 is installed. In this case, a separate circulation pump (not shown) is provided in the second circulation pipe. One end of the second circulation pipe is preferably connected to the treatment tank 10. The other end of the second circulation pipe may be connected to the treatment tank 10 or may be connected to the circulation pipe 21 downstream of the fine bubble supply mechanism 30.

As described above, in the fine bubble cleaning device 1 according to this embodiment, the cleaning liquid circulation mechanism 20 discharges the cleaning liquid 3 so that the angle θ between the flow direction of the cleaning liquid 3 and the longitudinal direction of the steel pipe is in the range of more than 0° and not more than 10.0°, as shown in FIG. 2. The hollow part of the object S to be cleaned is immersed so as to be filled with the cleaning liquid 3, and by satisfying these conditions, the liquid becomes more dirty as it is discharged toward the lower end of the object S to be cleaned, so that when the object S to be cleaned is removed from the cleaning liquid 3, the dirty liquid does not flow back and reattach to the inner surface of the steel pipe.

Moreover, it is more preferable that the cleaning liquid circulation mechanism 20 ejects the cleaning liquid 3 so that the angle θ between the flow direction of the cleaning liquid 3 and the longitudinal direction of the steel pipe is in the range of more than 0° and not more than 10.0°, and so that the cleaning liquid 3 flows through the hollow part of the steel pipe from the bottom side of the treatment tank 10 toward the liquid surface side. This makes it possible to more reliably prevent the dirty liquid from flowing back and reattaching to the inner surface of the steel pipe.

The angle θ between the steel pipe and the longitudinal direction may be adjusted by adjusting the direction of the nozzle axis of the nozzle for discharging the cleaning liquid 3 in the cleaning liquid circulation mechanism 20, or by adjusting the hanging position of the long pipe, which is the object S to be immersed, so that one end of the pipe is inclined vertically downward, or by changing the height of the buffer material provided in the treatment tank 10, so that the above angle θ is realized, or by adjusting both the direction of the nozzle axis of the discharge nozzle and the hanging position of the long pipe or the height of the buffer material. However, it is more convenient to adjust the hanging position of the long pipe or the height of the buffer material before adjusting the nozzle axis of the discharge nozzle, and adjust the nozzle axis of the discharge nozzle when the above conditions are not satisfied by adjusting the hanging position or the height of the buffer material alone. In addition, when adjusting the hanging position or the height of the buffer material, the situation in which the hollow part of the object S to be washed is filled with the cleaning liquid 3 can be realized more easily.

If the angle θ with respect to the longitudinal direction of the steel pipe as shown in Figure 2 exceeds 10.0°, the resistance of the cleaning liquid 3 increases, and the fine bubbles aggregate upward in the hollow portion, making it impossible to obtain a sufficient cleaning effect from the fine bubbles. The angle θ shown in Figure 2 is preferably 0.1° or more, and more preferably 0.2° or more. In addition, the angle θ shown in Figure 2 is preferably 6.0° or less, and more preferably 5.0° or less.

The angle θ' between the flow direction and the horizontal plane is preferably 0 to 10.0°. If the angle θ' with the horizontal plane exceeds 10.0°, the fine bubbles may aggregate or rise to the water surface in the treatment tank 10, resulting in a decrease in the number density of fine bubbles downstream in the treatment tank 10. By keeping the angle θ' with the horizontal plane within the above range, it is possible to more reliably suppress the decrease in the number density of fine bubbles.

The fine bubble cleaning device 1 according to this embodiment has been described above with reference to Figures 1A to 2.

(Fine bubble cleaning method)
A fine bubble washing method using the fine bubble washing device 1 as described above will be described in detail below with reference to Fig. 3. Fig. 3 is a schematic diagram for explaining the fine bubble washing method according to this embodiment.

As explained above, the inventors discovered that excellent cleaning effects can be obtained when the parameter expressed by "(average bubble diameter of fine bubbles) ² × flow rate of cleaning solution" is within a certain range, and came up with the fine bubble cleaning method described in detail below.

Based on the above findings, in the fine bubble cleaning method according to this embodiment, as shown in FIG. 3, when the inner diameter of the object to be cleaned is Dpi (unit: m), the tube length is L (unit: m), the average bubble diameter of the fine bubbles is Db (unit: m), and the flow velocity of the cleaning liquid at the inlet side of the hollow part of the object to be cleaned S is V (unit: m/s), the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid are controlled so as to satisfy the relationship expressed by the following formula (1).

When L/Dpi<200, 3.2×10 ⁻¹⁰ ≦Db ² ×V≦5.0×10 ⁻⁸
When 200≦L/Dpi, 6.0×10 ⁻¹³ ≦Db ² ×V≦3.2×10 ⁻⁹
...Equation (1)

By satisfying the condition expressed by the above formula (1), when the inner diameter Dpi of the pipe of the object to be cleaned is small and the pipe length L is short, even if the average bubble diameter Db of the fine bubbles is large and the flow velocity V is slow, the bubbles will escape from the bottom end of the object to be cleaned. This makes it possible to enjoy the cleaning effect of fine bubbles.

On the other hand, when the pipe inner diameter Dpi of the object to be cleaned is small and the pipe length L is long, the fine bubbles aggregate at the top of the hollow part of the object to be cleaned and cannot escape from the bottom end. However, by reducing the average bubble diameter Db of the fine bubbles and increasing the flow velocity V so that the condition expressed by the above formula (1) is satisfied, it is possible to ensure the circulation of the cleaning liquid that exceeds the floating speed of the fine bubbles, and it is possible to enjoy the cleaning effect of the fine bubbles. As suggested by the above formula (a), the average bubble diameter of the fine bubbles is squared in the floating speed according to the Stokes' law, so reducing the average bubble diameter Db has a large effect.

Here, the average bubble diameter Db of the fine bubbles can be adjusted to a desired value by controlling the pressure of the gas (air) supplied to the fine bubble supply mechanism 30, the pressure of the cleaning liquid supplied to the fine bubble supply mechanism 30, the flow rate ratio of air to cleaning liquid, etc. In addition, the flow velocity V of the cleaning liquid can be adjusted to a desired value by adjusting the output of the circulation pump 23.

In this embodiment, the pipe length L is assumed to be 2 m to 30 m, and the inner diameter Dpi is assumed to be 10 mm to 300 mm, but is not limited to these ranges as long as the above formula (1) is satisfied.

In the fine bubble cleaning method according to this embodiment, the cleaning solution contains a nonionic surfactant with ethylene oxide added to a hydrophilic group, and the HLB value of such a surfactant is preferably within the range of 15 to 19.

In conventional oil cleaning, the surface tension of the cleaning solution is lowered to allow a surfactant to penetrate between the oil and the object being cleaned, and the surfactant then covers the oil with a lift-up effect, forming micelles and causing the oil to float to the surface. In order to achieve this lift-up effect efficiently, due to the specific gravity of oil, a greater amount of surfactant is required than the critical micelle concentration, which is the critical concentration for surfactant micellization.

On the other hand, in the cleaning with fine bubbles that is the focus of this embodiment, the adsorption effect of the fine bubbles is used to peel off oil from the object to be cleaned, and the oil is then adsorbed around the fine bubbles, thereby cleaning the object. In this way, the mechanism of cleaning with fine bubbles that is the focus of this embodiment is significantly different from conventional oil cleaning.

Here, by adding a nonionic surfactant with ethylene oxide added as a hydrophilic group to the cleaning solution, the dispersibility and emulsifiability of the surfactant can be improved, and the conditions for retaining the surfactant around the fine bubbles can be more reliably and easily achieved.

"Nonionic surfactant" is a general term for surfactants that do not have groups that dissociate into ions. The reason why it is preferable to use nonionic surfactants rather than ionic surfactants such as anionic surfactants or cationic surfactants is that the oil we are focusing on (metal processing oil derived from mineral oil) is not removed as an ionized substance, so it does not need to be an anionic or cationic surfactant with strong ionic properties. In addition, using a surfactant that is not affected by ionic properties will not have an unintended effect on the properties of fine bubbles.

In addition, an ionic surfactant requires a certain amount to reach the critical micelle concentration cmc, which is the critical concentration required for cleaning, and this also increases the cost of the chemical solution. However, the nonionic surfactant that is the focus of this embodiment reaches the critical micelle concentration cmc, which has a surfactant effect, in a small amount, and furthermore, when combined with fine bubbles, it can provide a cleaning effect without reaching the critical micelle concentration cmc required without fine bubbles. This allows for further reduction in chemical solution costs.

Here, the concentration of the nonionic surfactant in the cleaning agent is preferably 0.01 to less than 2.00 times the critical micelle concentration cmc of the surfactant. In the fine bubble cleaning method according to this embodiment, the nonionic surfactant as described above is used to stabilize the fine bubbles rather than to cause the surfactant itself to become micellized. Therefore, in this embodiment, the concentration of the nonionic surfactant may be 0.01 to less than 2.00 times the critical micelle concentration cmc. This allows the surfactant components to surround the fine bubbles, more reliably achieving oil adsorption and retention of fine bubbles with the desired average bubble diameter.

If the concentration of the nonionic surfactant is less than 0.01 times the critical micelle concentration cmc, the fine bubble retention effect described above may not be fully achieved. The concentration of the nonionic surfactant is more preferably 0.05 times or more the critical micelle concentration cmc, and even more preferably 0.20 times or more the critical micelle concentration cmc.

On the other hand, if the concentration of the nonionic surfactant is 2.00 times or more the critical micelle concentration cmc, the effect on the cleaning effect of the fine bubbles will be saturated simply by using a large amount of surfactant. The concentration of the nonionic surfactant is more preferably 1.50 times or less the critical micelle concentration cmc, even more preferably 1.00 times or less the critical micelle concentration cmc, and even more preferably less than 1.00 times the critical micelle concentration cmc.

The critical micelle concentration cmc of the nonionic surfactant of interest is the value at normal pressure and 25°C. The critical micelle concentration cmc can be measured by a surface tension method.

In particular, to perform cleaning with fine bubbles more efficiently, it is important that a surfactant is present in the liquid. Therefore, in the fine bubble cleaning method according to this embodiment, it is preferable that the HLB value of the nonionic surfactant is within the range of 15.0 to 19.0.

HLB (Hydrophile Lipophile Balance) value is a numerical value that indicates the degree of balance between hydrophilicity and lipophilicity. The HLB value is divided equally between 0 and 20, with substances that do not have hydrophilic groups (e.g., paraffin, etc.) having an HLB value of 0 and substances that have only hydrophilic groups and no lipophilic groups (e.g., polyethylene glycol, etc.) having an HLB value of 20. For nonionic surfactants, the Griffin method formula (HLB value = 20 x sum of formula weights of hydrophilic moieties / molecular weight) is generally applied. It is possible to identify the HLB value of a nonionic surfactant of interest by measuring the formula weight and molecular weight of the hydrophilic moiety using liquid chromatography/mass spectrometry.

In conventional oil cleaning, surfactants with an HLB value in the range of 13 to 15 have been used to lower the surface tension and maintain a certain level of foaming power and foam film. However, the inventors have come to the conclusion that it is preferable to use a nonionic surfactant with an HLB value in the range of 15.0 to 19.0 for cleaning with fine bubbles. An HLB value in this range means that the surfactant in question exhibits characteristics approaching the hydrophilic side.

When a nonionic surfactant has an HLB value within this range, it is possible to increase the amount of surfactant present in the liquid, and have the surfactant adsorbed around the fine bubbles, forming micelles. This makes it possible to further stabilize the fine bubbles, and also to more reliably maintain adsorption to oil. In other words, it has been found that surfactants that contribute to the stabilization of fine bubbles also have an effect on cleaning properties.

If the HLB value is less than 15.0, the amount of surfactant moving toward the gas-liquid interface (liquid surface) increases rather than surrounding the fine bubbles, and the foaming power of the surfactant increases. As a result, the fine bubbles may aggregate together, weakening the adsorption effect of the fine bubbles on the oil. From this perspective, it is preferable that the HLB value is 15.0 or more. The HLB value of the nonionic surfactant is more preferably 16.0 or more, and even more preferably 16.5 or more.

On the other hand, if the HLB value exceeds 19.0, there will be too few hydrophobic groups and the surfactant will not be able to be present around the fine bubbles, which will likely result in the surfactant not contributing to the stabilization of the fine bubbles. From this perspective, it is preferable that the HLB value be 19.0 or less. The HLB value of the nonionic surfactant is more preferably 18.5 or less, and even more preferably 18.0 or less.

Known nonionic surfactants include ether type (e.g., RO-(CH ₂ CH ₂ O) _n -H), ester type (e.g., RCOO-(CH ₂ CH ₂ O) _n -H), ether ester type, nitrogen-containing type (RCON-(CH ₂ CH ₂ OH) ₂ ), etc. Here, in the fine bubble cleaning method according to this embodiment, it is preferable to use an ether type nonionic surfactant (AE: polyoxyethylene alkyl ether, alcohol ethoxylate).

Ether-type nonionic surfactants have many advantages, such as: 1) they are not easily affected by water hardness or electrolytes and can be used in combination with all other surfactants; 2) they have cleaning power in neutral, acidic, and alkaline conditions; 3) they have superior cleaning power for synthetic fibers compared to other surfactants and can be used in cold water as well; 4) they have high cleaning power even at low concentrations; 5) they are easily biodegradable, have low ecotoxicity, and are low in skin irritation; and 6) they are hygroscopic, so cleaning agents with AE as the main ingredient can be easily liquefied.

Furthermore, according to the inventors' verification, it was found that a complex structure is not necessary for a nonionic surfactant to be adsorbed around fine bubbles, and that what is important is that the surfactant exists as a substance for stabilizing the fine bubbles. Therefore, it is preferable that the ether-type nonionic surfactant used in the fine bubble cleaning method according to this embodiment has a linear carbon chain with a carbon number in the range of 10 to 20 as the hydrophobic group. By having a linear carbon chain with a carbon number in the range of 10 to 20 as the hydrophobic group, it is possible to more reliably achieve the retention of fine bubbles with the desired average bubble diameter.

If a complex structure other than a straight-chain structure is present as a hydrophobic group, the fine bubbles may aggregate together, reducing the number of fine bubbles and making it difficult to manage the fine bubbles.

In addition, if the carbon number exceeds 20, it becomes necessary to increase the carbon number of the hydrophilic group to achieve the desired HLB value, which results in a large molecular weight and a strong expression of solid properties, which may reduce convenience. In addition, if the carbon number exceeds 20, fine bubbles tend to aggregate with each other, which may prevent oil from being removed and reduce cleaning properties. On the other hand, if the carbon number is less than 10, it becomes difficult to synthesize and produce it as a fatty acid derivative, which may increase production costs.

The number of carbon atoms in the hydrophobic group is more preferably within the range of 12 to 18.

In the above explanation, attention is focused on ether-type nonionic surfactants with linear hydrophobic groups, but in the fine bubble cleaning method according to this embodiment, the balance between hydrophilic and hydrophobic groups is the point that should be focused on first, and other structures can also be used as surfactants as long as they are in a range where such a balance is achieved.

In addition, in the fine bubble cleaning method according to this embodiment, the surface tension of the surfactant at 25°C is preferably 40 mN/m or more. In conventional general cleaning, the aim has been to improve cleaning performance by lowering the surface tension, but in cleaning using fine bubbles, which is the focus of this embodiment, there is little need to lower the surface tension. Furthermore, nonionic surfactants, such as those focused on in this embodiment, require a surface state to surround fine bubbles. From this perspective, it is preferable to use a surfactant that does not cause much reduction in surface tension, so that there is more surfactant present in the liquid than at the gas-liquid interface (liquid surface).

If the surface tension at 25°C is less than 40 mN/m, the surfactant may gather on the liquid surface and may not gather around the fine bubbles. Therefore, it is preferable that the surface tension at 25°C is 40 mN/m or more. If the surface tension is 40 mN/m or more, it is possible to retain enough surfactant in the liquid to surround the fine bubbles, making it possible to further stabilize the fine bubbles. It is more preferable that the surface tension at 25°C is 45 mN/m or more. There is no particular upper limit for the surface tension, but it is preferable that the surface tension be 65 mN/m or less as the range in which the surfactant acts. It is possible to measure such surface tension using the ring method.

In the fine bubble cleaning method according to this embodiment, it is preferable to use an alkaline liquid as the cleaning liquid, and the alkaline concentration is preferably 1.0 mol/L or less. By using an alkaline liquid, it is possible to effectively solubilize oil. From the viewpoint of solubilizing oil, a chemical reaction can be promoted by raising the temperature of the cleaning liquid, so that the effect can be more pronounced. However, in the cleaning using fine bubbles that is the focus of this embodiment, by making the liquid alkaline, it is possible to more reliably charge the surface of the fine bubbles with a negative charge, and it is possible to further improve the adsorption of oil. In addition, by using an alkaline liquid, the size of the oil gradually decreases and becomes fine particles, which makes it easier to adsorb to the surface of the fine bubbles.

In the fine bubble cleaning method according to this embodiment, the alkali concentration is preferably 1 mol/L or less. If the alkali concentration exceeds 1.0 mol/L, not only will the working environment deteriorate, but management of the chemical solution may become difficult, and in addition, convenience will decrease due to increased costs for the chemical solution. The alkali concentration of the cleaning solution is more preferably 0.9 mol/L or less, and even more preferably 0.8 mol/L or less.

In the cleaning solution described above, the temperature of the cleaning solution is a factor that affects the solubility of the nonionic surfactant. If the temperature of the cleaning solution is too low, the nonionic surfactant will solidify, and the undissolved surfactant may have a negative effect on the subsequent manufacturing process, and may no longer function as a surfactant. On the other hand, if the temperature of the cleaning solution is too high, not only will the working environment not be improved, but the solubility of the nonionic surfactant will decrease and the degree of micellization on the surface of the fine bubbles may decrease. From this perspective, in the fine bubble cleaning method according to this embodiment, it is preferable that the temperature of the cleaning solution is within the range of 20 to 60°C. The temperature of the cleaning solution is more preferably within the range of 20 to 55°C, and even more preferably within the range of 30 to 50°C.

It is important that the cleaning liquid used in the fine bubble cleaning method according to this embodiment is capable of removing stubborn oil stains, oil stains baked on by heat treatment, and the like. Therefore, it is preferable that the cleaning liquid is one that is highly effective against organic stains, and it is preferable that the cleaning liquid uses an aqueous cleaning agent or an organic solvent to lift and remove the stains, or that the cleaning liquid dissolves and decomposes the stains to remove them. From this perspective, it is preferable to use a cleaning liquid with a viscosity close to that of water or a low viscosity. This makes it possible to fully exert the above-mentioned removal effect. More specifically, it is preferable that the cleaning liquid has a viscosity of 0.1 mPa·s or more and less than 10 mPa·s at the above-mentioned liquid temperature.

As for solid dirt such as inorganic dirt, metal powder, and rust other than the above-mentioned cleaning targets, the fine bubbles can adsorb and remove the dirt more physically than the cleaning solution, providing an additional cleaning effect.

In the fine bubble cleaning method according to this embodiment, the cleaning solution may contain, in addition to the surfactant, a chelating agent, a rust inhibitor, an antifoaming agent, etc. as cleaning solution components.

The above provides a detailed explanation of the fine bubble cleaning method according to this embodiment.

The following provides a detailed explanation of the fine bubble cleaning method and fine bubble cleaning device according to the present invention, showing examples and comparative examples. Note that the examples shown below are merely examples of the fine bubble cleaning method and fine bubble cleaning device according to the present invention, and the fine bubble cleaning method and fine bubble cleaning device according to the present invention are not limited to the examples below.

4A and 4B are schematic diagrams for explaining the fine bubble cleaning device used in this experimental example.
In this experimental example, verification was performed using a fine bubble washing device 1 having the configuration shown in Figures 4A and 4B. Two pumps with a suction volume of 600 ^m3 /h were used as the circulation pumps 23. Two swirl-type fine bubble generators (MBelif, manufactured by Kansai Automated Instruments Co., Ltd.) manufactured by Kansai Automated Instruments Co., Ltd. were used as the fine bubble supply mechanism 30 and were installed midway through the discharge piping of the circulation pump 23. The discharge nozzle of the cleaning liquid 3 was installed so as to be parallel to the horizontal plane (i.e., the above angle θ' = 0°).

A 4% by weight caustic soda solution was used as cleaning solution 3, and the following six types of nonionic surfactants were added to it for verification. The temperature of the cleaning solution was changed between 20 and 80°C. Within this temperature range, the viscosity of the cleaning solution was in the range of 0.1 mPa·s or more and less than 10 mPa·s.

Various steel pipes that had undergone a typical steel pipe manufacturing process and were coated with mineral oil-derived metal processing oil were used as the objects to be cleaned and were immersed for 30 minutes in the treatment tank 10 containing the above-mentioned cleaning solution. The installation angle θ of the steel pipes was adjusted by changing the height of the buffer material (not shown) installed in the treatment tank 10.

In Table 1 below, the case where the steel pipe is immersed so that the end of the steel pipe located downstream of the flow direction of the cleaning liquid 3 faces upward as shown in Figure 4B is indicated by an angle θ having a positive value, and the case where the steel pipe is immersed so that the end of the steel pipe located downstream of the flow direction faces downward is indicated by an angle θ having a negative value.

<Nonionic Surfactant>
a-1: (Ether type) polyoxyethylene monododecyl ether (C ₁₂ H ₂₅ (C ₂ H ₄ O) ₂₅ OH)
a-2: (Ether type) polyoxyethylene monocetyl ether (C ₁₆ H ₃₃ (C ₂ H ₄ O) ₂₃ OH)
a-3: (Ether type) polyoxyethylene monooleyl ether (C ₁₈ H ₃₅ (C ₂ H ₄ O) ₂₀ OH)
a-4: (Ether type) polyoxyethylene monononylphenyl ether (C ₆ H ₄ C ₉ H ₁₉ (C ₂ H ₄ O) ₂₀ OH)
b: (Ester type) polyoxyethylene sorbitan monostearate c: (Nitrogen-containing type) polyoxyethylene alkylamine

Tests were conducted with the steel pipe installation angle θ, pipe inner diameter Dpi, and flow rate conditions shown in Table 1 below. In addition, the flow velocity V was calculated using the CFD module (turbulent flow) of COMSOL Maltiphysics to calculate the flow velocity distribution at the inlet side of multiple steel pipes, and the average value obtained was shown.

The average bubble diameter and number concentration of fine bubbles were measured using image analysis (Microtrac Part An SI, manufactured by Microtrac Bell) by inserting a measuring tube into the outlet of the circulation piping. The surface tension shown is the result of a prior measurement using the ring method for only the surfactant at 25°C.

In this experimental example, the amount of carbon remaining on the inner surface of the pipe as an oil component was measured to evaluate the cleanability of the treated steel pipe. The amount of carbon was measured using a commercially available measuring device (RC612 carbon/hydrogen/moisture analyzer, manufactured by LECO Japan LLC). More specifically, the amount of carbon remaining on the inner surface after degreasing was divided by the amount before cleaning to calculate the percentage of carbon remaining, and the percentage of the amount of oil component removed was used as the cleaning rate.

The evaluation criteria for the degreasing performance are as follows:
Removal rate of carbon (oil components) adhering to the inner surface of the pipe A: 100% or less - 95% or more B: Less than 95% - 90% or more C: Less than 90% - 80% or more D: Less than 80% - 60% or more E: Less than 60% - 40% or more F: Less than 40%

In other words, a rating of "A" or "B" means that the degreasing performance was very good, a rating of "C" means that the degreasing performance was good, a rating of "D" means that the degreasing performance was somewhat poor, and a rating of "E" or "F" means that the degreasing performance was poor.

The results are summarized in Table 1 below.

First, looking at the comparative examples, we can see that when the steel pipe was not installed at the appropriate angle and when the appropriate flow rate was not met, the degreasing performance was poor.

On the other hand, when the inner diameter and flow rate of the pipe were changed under the condition of installation at an appropriate angle, and the relational expression (1) between the given flow rate and the average bubble diameter of the fine bubbles was satisfied, it was confirmed that the degreasing performance was good. It was also found that even better degreasing performance was confirmed when the surfactant had an HLB value of more than 15, or when a linear hydrophobic group or an ether-type component was used.

The above describes in detail preferred embodiments of the present invention with reference to the attached drawings, but the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

REFERENCE SIGNS LIST 1 Fine bubble cleaning device 3 Cleaning liquid 10 Treatment tank 20 Cleaning liquid circulation mechanism 21 Circulation piping 23 Circulation pump 30 Fine bubble supply mechanism

Claims (13)

A fine bubble cleaning method for cleaning a steel pipe, the entire or a part of which is elongated in the longitudinal direction, as an object to be cleaned, comprising:
a treatment tank for holding a cleaning liquid for cleaning the object to be cleaned and in which one or more objects to be cleaned are immersed;
a cleaning liquid circulation mechanism that circulates the cleaning liquid in the treatment tank and discharges the cleaning liquid onto the object to be cleaned;
a fine bubble supply mechanism for supplying fine bubbles to the cleaning liquid in the cleaning liquid circulation mechanism;
having
the cleaning liquid circulation mechanism uses a fine bubble cleaning device that discharges the cleaning liquid so that the angle θ between the flow direction of the cleaning liquid and the longitudinal direction of the steel pipe is within a range of more than 0° and not more than 10.0°;
The fine bubble cleaning method comprises controlling the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid within a range of 1 to 100 μm for the average bubble diameter Db and within a range of 0.2 to 5.0 m/s for the flow velocity V, so as to satisfy the relationship expressed by the following formula (1): Dpi (unit: m), L (unit: m) of the inner diameter of the object to be cleaned, Db (unit: m) of the pipe length, and V (unit: m/s) of the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid.

When L/Dpi<200, 3.2×10 ⁻¹⁰ ≦Db ² ×V≦5.0×10 ⁻⁸
When 200≦L/Dpi, 6.0×10 ⁻¹³ ≦Db ² ×V≦3.2×10 ⁻⁹
...Equation (1)
The fine bubble cleaning method according to claim 1, wherein the cleaning liquid circulation mechanism discharges the cleaning liquid so that the angle θ is in the range of more than 0° and not more than 10.0°, and the cleaning liquid flows through the hollow part of the steel pipe from the bottom side of the treatment tank toward the liquid surface side. The cleaning solution contains a nonionic surfactant having ethylene oxide added to a hydrophilic group,
The fine bubble cleaning method according to claim 1 or 2 , wherein the HLB value of the surfactant is within a range of 15.0 to 19.0. 4. The fine bubble cleaning method according to claim 3 , wherein a concentration of the surfactant in the cleaning agent is 0.01 times or more and less than 2.00 times a critical micelle concentration of the surfactant. 5. The fine bubble cleaning method according to claim 3 , wherein the surfactant has a surface tension of 40 mN/m or more at 25°C. The fine bubble cleaning method according to any one of claims 3 to 5 , wherein the surfactant is an ether-type nonionic surfactant. The fine bubble cleaning method according to any one of claims 3 to 6 , wherein the hydrophobic group of the surfactant is a linear carbon chain having a carbon number in the range of 10 to 20. The fine bubble cleaning method according to any one of claims 1 to 7 , wherein the alkaline concentration of the cleaning solution is 1.0 mol/L or less. The fine bubble washing method according to any one of claims 1 to 8 , wherein the fine bubble number density is within a range of 1 x 10 ³ to 1 x 10 ⁶ bubbles/mL. The fine bubble cleaning method according to any one of claims 1 to 9 , wherein the cleaning liquid has a liquid temperature within a range of 20 to 60°C. The fine bubble cleaning method according to any one of claims 1 to 10 , wherein the object to be cleaned is a steel pipe having metal processing oil attached thereto. A fine bubble cleaning device that cleans a steel pipe, the entire or a part of which is elongated in the longitudinal direction, as an object to be cleaned,
a treatment tank for holding a cleaning liquid for cleaning the object to be cleaned and in which one or more objects to be cleaned are immersed;
a cleaning liquid circulation mechanism that circulates the cleaning liquid in the treatment tank and discharges the cleaning liquid onto the object to be cleaned;
a fine bubble supply mechanism for supplying fine bubbles to the cleaning liquid in the cleaning liquid circulation mechanism;
having
This fine bubble cleaning device has a cleaning liquid circulation mechanism that discharges the cleaning liquid so that the angle θ between the flow direction of the cleaning liquid and the longitudinal direction of the steel pipe is in the range of more than 0° and not more than 10.0°. The fine bubble cleaning device according to claim 12, wherein the average bubble diameter Db of the fine bubbles and the flow velocity V of the cleaning liquid are controlled within a range of 1 to 100 μm for the average bubble diameter Db and within a range of 0.2 to 5.0 m/s for the flow velocity V, so as to satisfy the relationship expressed by the following formula (1): Dpi (unit: m), L (unit: m), average bubble diameter Db of the fine bubbles, and flow velocity V of the cleaning liquid at the inlet side of the hollow part of the steel pipe are controlled within a range of 1 to 100 μm for the average bubble diameter Db, and within a range of 0.2 to 5.0 m/s for the flow velocity V.

When L/Dpi<200, 3.2×10 ^-10 ≦Db ² ×V≦5.0×10 ^-8
When L/Dpi is 200 or less, 6.0×10 ^-13 ≦Db ² ×V≦3.2×10 ^-9
...Formula (1)

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