JPH0790409A - Method for removing hydrogen in molten aluminum - Google Patents

Abstract

PURPOSE:To float up and remove dissolve hydrogen in molten aluminum together with gaseous nitrogen onto the upper part of the molten metal by blowing the high purity gaseous nitrogen regulated in the purity into the lower part of the molten aluminum. CONSTITUTION:The gaseous nitrogen having 99.95-99.995% purity obtd. by a pressure swing absorption type gaseous nitrogen generator is blown to the lower part of the molten aluminum and dispersed in the whole molten metal as the fine bubbles with an impeller fitted to the lower part of the molten metal, and the gaseous hydrogen dissolved into the molten metal is floated up and removed onto the upper part of the molten metal together with the gaseous nitrogen. At this time, the flow rate of the gaseous nitrogen blown into the molten metal is made to be about 5-3Nl/min/500kg of the molten metal and the dew point is made to be <= about -10 deg.C. By this method, the removal of the hydrogen in the molten aluminum can simply be executed in a high reliability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining molten aluminum, and more particularly to a method for dehydrogenating molten aluminum using nitrogen gas having a purity of 99.95 to 99.995% obtained by a pressure swing adsorption type nitrogen gas generator.

In this specification, "aluminum" includes not only pure aluminum but also aluminum alloys made of aluminum and alloy elements such as copper, silicon, magnesium, nickel and titanium.

[0003]

2. Description of the Related Art In recent years, it has been strongly desired to improve the quality of aluminum and aluminum alloys used for electronic parts, automobile parts, building materials, aviation parts and the like. One of the most important factors for quality control of these aluminum products is the state of the molten metal before casting, and in order to obtain high quality aluminum products, hydrogen gas dissolved in the molten metal, oxides of aluminum and magnesium, etc. It is necessary to remove the non-metallic inclusions, and for that purpose various molten metal refining methods have been developed and put into practical use.

As a general method for dehydrogenating an aluminum melt, there is a method in which a flux of a halogen compound is blown into the melt through an inert gas such as nitrogen or argon. Since the minute bubbles of the inert gas blown in and the bubbles of the gas generated or derived from the flux have a lower partial pressure than the hydrogen gas in the molten metal, the hydrogen gas dissolved in the molten metal follows these rules according to Sieverts' law. It diffuses into the bubbles and floats, and is removed from the surface of the molten metal.

Conventionally, nitrogen gas, which is relatively inexpensive as compared with rare gases such as argon, or high-purity nitrogen gas obtained by gasifying liquid nitrogen, is used as the inert gas blown to remove hydrogen gas in the molten metal. Has been.
However, these nitrogen gases are usually 99.999% or more of high-purity gas and high in price, resulting in an increase in casting cost of aluminum products, which has been a factor limiting the use of aluminum products in a wide range of fields.

Furthermore, when a nitrogen gas cylinder is used, frequent replacement of the cylinder is required, and a lot of labor is required for maintenance and inspection, and out-of-gas, inadequate replacement work, and the like cause an increase in the defective rate of aluminum products. There are problems such as.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have completed the present invention as a result of earnest research to solve the above problems, and the purpose thereof is to provide a simple and highly reliable molten aluminum. To provide a dehydrogenation method of

[0008]

[Means for Solving the Problems] The above object is to continuously supply product air to two or more adsorption towers filled with molecular sieving carbon, and successively repeat pressure adsorption and decompression regeneration to produce product nitrogen gas. Nitrogen gas with a purity of 99.95 to 99.995% obtained with a pressure swing adsorption type nitrogen gas generator is blown into the lower part of the aluminum melt, and this nitrogen gas is blown into the entire melt as fine bubbles by an impeller attached to the bottom of the melt. It is achieved by a method for dehydrogenating an aluminum melt, characterized in that the dissolved hydrogen gas is floated above the melt together with nitrogen gas to be removed.

The pressure swing adsorption type nitrogen gas generator as referred to in the present invention continuously supplies the feed air to two or more adsorption columns filled with molecular sieving carbon and successively repeats pressure adsorption and pressure reduction regeneration. It is a device that can take out product nitrogen gas.

The pressure swing adsorption type nitrogen gas generator used in the present invention is not particularly limited, but is disclosed, for example, in Japanese Patent Laid-Open Nos. 63-104629 and 63-218230. Is preferably used.

The adsorption tower of the pressure swing adsorption type nitrogen gas generator is filled with an adsorption separation material called molecular sieving carbon capable of separating oxygen and nitrogen. The kind of the molecular sieving carbon is not particularly limited, but for example, JP-A-63
-Molecular sieving carbon mainly made of phenol resin disclosed in Japanese Patent Publication No. 201008 is preferably used.

Nitrogen gas blown into the molten metal is conventionally 99.999%
Only high-purity gas of a certain degree was used, and no consideration was made as to whether or not the dehydrogenation treatment should be performed at a gas purity lower than that. It is feared that when the nitrogen gas purity is low, an oxide is generated by the reaction between the molten aluminum and oxygen, which is an impurity contained in the nitrogen gas, to become an inclusion in the metal, resulting in deterioration of the quality of the product aluminum. Because it was.

However, the present inventors have examined the nitrogen gas purity suitable for the dehydrogenation treatment of molten aluminum by using a pressure swing adsorption type nitrogen gas generator,
It was found that a sufficiently good quality aluminum melt can be obtained by using 99.95 to 99.995% nitrogen gas.

When the nitrogen purity of the gas obtained from the pressure swing adsorption type nitrogen gas generator is 99.95% or less, an oxide is formed due to the reaction between the molten aluminum and oxygen contained in the nitrogen gas to deteriorate the quality of the product aluminum. Is not preferable. Also, if the nitrogen purity is 99.995% or more, the efficiency of obtaining nitrogen gas from the air in the pressure swing adsorption device becomes poor and the nitrogen gas cost rises. It causes an increase and is not preferable.

Therefore, the purity of the nitrogen gas obtained by the pressure swing adsorption type nitrogen gas generator used in the present invention is usually 9
9.95-99.995%, preferably 99.97-99.992
%, Most preferably 99.98-99.99%.

The purity of the nitrogen gas obtained by the pressure swing adsorption type nitrogen gas generator of the present invention is the sum of the volume% of nitrogen gas and argon gas, and most of the remaining impurity gas is oxygen gas. .

The flow rate of nitrogen gas blown into the molten aluminum is usually 5 to 30 Nl / m per 500 kg of the molten aluminum.
in, preferably 10 to 25 Nl / min, most preferably 15
~ 20 Nl / min.

When the flow rate of nitrogen gas is less than 5 Nl / min per 500 kg of molten aluminum, the dehydrogenation treatment is insufficient and the dissolved hydrogen gas cannot be sufficiently removed.
It is not preferable because internal defects and surface defects occur in the product after solidification and mechanical properties are deteriorated. Also,
Nitrogen gas flow rate is 30 Nl / mi per 500 kg of molten aluminum
In the case of n or more, when the bubbles of nitrogen gas float on the surface of the molten aluminum and burst, a large amount of the molten metal is scattered and aluminum oxide is generated near the surface of the molten metal, resulting in loss of aluminum and dehydrogenation treatment cost. Is high, which is not preferable.

The time for blowing nitrogen gas into the molten metal is not particularly limited, but per 500 kg of molten aluminum
When blowing about 20 Nl / min, it is usually 3 to 25 minutes, preferably 5 to 20 minutes, and most preferably 10 to 15 minutes.

If the blowing time of nitrogen gas is 3 minutes or less, the dehydrogenation treatment of the molten metal is insufficient, which is not preferable. In addition, dehydrogenation can be sufficiently performed for about 25 minutes, and even if it is longer than that, there is no merit but productivity is reduced.

The smaller the bubbles of nitrogen gas blown into the molten metal, the larger the contact area between the molten aluminum and the bubbles, and the slower the floating speed, so that the degassing effect becomes greater.

If the floating speed of bubbles in the molten metal complies with the Stokes equation, a sedation time is required after the blowing of nitrogen gas until the floating of the bubbles of the gas is completed. The smaller the bubble diameter and the density of the molten aluminum, the deeper the molten metal, and the larger the viscosity of the molten metal, the longer the length. However, due to the presence of convection in the melt, the actual sedation time required may be much shorter than the calculated value, and the sedation time in a commonly used crucible furnace of 300 to 500 kg is 5 to 30 minutes. The degree is enough.

The dew point of nitrogen gas blown into the molten metal is -10 ° C.
If the above is the case, the partial pressure of water vapor increases and the efficiency of dehydrogenation decreases, so the dew point of nitrogen gas blown into the molten metal is usually -10 ° C or lower, preferably -30 ° C or lower, and most preferably -50 ° C or lower.

The molten aluminum referred to in the present invention is, for example, Al-Cu alloy or Al-C in addition to pure aluminum.
u-Si alloy, Al-Si alloy, Al-Si-Cu-Mg alloy, Al-Cu-Ni alloy, Al-Mg alloy, Al-Si-Ni-Mg-Cu alloy, Al-Cu-Si-Mg- Ni
Alloys and molten alloys of various alloys such as Al-Zn-Mg-Cr-Ti alloys are also included.

Next, the quality inspection method used in the present invention will be described. (1) Method for measuring the amount of dissolved hydrogen gas Approximately 100 g of molten metal is sampled in a crucible, placed in a bell jar, and cooled while keeping it under a reduced pressure of 700 mmHg for 5 minutes by a vacuum pump. (Depressurized coagulation method) After that, the central part of the sample taken out from the bell jar was cut, observed with an optical microscope, and the amount of residual hydrogen gas in the molten metal was calculated from the amount of voids in the cut surface, and the amount of residual hydrogen gas in the molten metal was calculated. Is 0.20cc / 100gAl
The following cases were accepted.

(2) Method for measuring the amount of inclusions About 100 g of molten metal was cast into a plate shape (240 × 36 × 6 mm), and after cooling, it fractured and the inclusions on the fracture surface were observed with the naked eye. 20 fracture surfaces were observed, and only when no inclusions were confirmed on any of the fracture surfaces, it was judged as passing.

Hereinafter, the present invention will be described more specifically according to examples, but the present invention is not limited to these examples.

[0028]

Embodiment 1 FIG. 1 shows one embodiment of an apparatus for carrying out the method of the present invention. In FIG. 1, A is a pressure swing adsorption type nitrogen gas generator (hereinafter abbreviated as PSA device), B is a crucible furnace for molten aluminum processing, and C is a nitrogen cylinder. In this device, a PSA device or a nitrogen cylinder can be selected as a supply source of nitrogen gas to the crucible furnace for molten metal treatment by opening / closing valves 17 and 18.

Crucible furnace for processing molten aluminum (internal volume
Put JIS-AC4C aluminum alloy melt in 0.1 m ³ ).
It was kept heated at 750 ° C. As a result of measuring the dissolved hydrogen gas amount of this molten metal, 0.41 cc / 100 g Al of hydrogen was contained.

Purity 99.8% (oxygen concentration 2000
ppm), purity 99.96% (oxygen concentration 400 ppm) and purity 99.988
% (Oxygen concentration 120ppm) nitrogen gas 1.5kgf / cm
^A dehydrogenation treatment was carried out by stirring the impeller at a rotation speed of 300 rpm for 15 minutes while flowing 20 Nl / min at a supply pressure of ² .

After dehydrogenation treatment, sedation time was 5 minutes, 10 minutes, 15
Table 1 shows the measurement results of the amount of residual hydrogen gas and the amount of inclusions in the molten aluminum after 20 minutes and 20 minutes.

In this example, an aluminum alloy having no inclusions was used as the molten metal before the treatment.

30% after dehydrogenation treatment when the nitrogen purity is 99.8%
Even after a lapse of minutes, the amount of residual hydrogen gas was 0.27cc / 100gAl and was unacceptable. With a nitrogen purity of 99.96%, the sample after 15 minutes,
When the nitrogen purity was 99.988%, the amount of residual hydrogen gas was 0.20cc / 100gAl or less in the sample after 5 minutes, and the sample was a pass sample in which inclusions were not confirmed.

[0034]

Example 2 Using the same apparatus as in Example 1, a JIS-AC2B aluminum alloy melt was placed in the crucible of an aluminum melt treatment furnace and heated to 725 ° C. This molten metal contained 0.37cc / 100g Al hydrogen. Then, in the same manner as in Example 1, the purity of 99.8%, 99.96%, 9
20 Nl / mi with 9.994% nitrogen gas at a supply pressure of 1.5 kgf / cm ^2.
While flowing n, the mixture was stirred for 15 minutes at an impeller rotation speed of 300 rpm for dehydrogenation treatment.

After dehydrogenation treatment, sedation time was 5 minutes, 10 minutes, 15
Table 2 shows the measurement results of the amount of residual hydrogen gas and the amount of inclusions in the molten aluminum after 20 minutes and 20 minutes.

If the nitrogen purity is 99.7%, after dehydrogenation treatment, 20
The amount of residual hydrogen gas was 0.25cc / 100gAl even after a lapse of minutes, but it was unacceptable, but when the nitrogen purity was 99.98% and 99.994%, the residual hydrogen gas amount was 0.
It was a passing sample with less than 20cc / 100gAl and no inclusions were confirmed.

[0037]

[Example 3] The same JIS-AC4C aluminum alloy melt as in Example 1 was heated and held at 750 ° C, and the purity was 99.9995%.
The dehydrogenation treatment was carried out by stirring the nitrogen cylinder gas (oxygen concentration 5 ppm) at a supply pressure of 1.5 kgf / cm ² for 20 Nl / min at a supply speed of 1.5 kgf / cm ² for 15 minutes while rotating the impeller at a rotation speed of 300 ppm without changing the cylinder gas in the middle.

After dehydrogenation treatment, sedation time was 5 minutes, 10 minutes, 15
Table 3 shows the measurement results of the amount of residual hydrogen gas and the amount of inclusions in the molten aluminum after 20 minutes and 20 minutes.

Table 4 shows the results of the same dehydrogenation treatment when the cylinder was exchanged for about 1 minute during the dehydrogenation treatment and air was mixed in during that time.

When the cylinder was not exchanged during the dehydrogenation treatment, as shown in Table 3, the amount of residual hydrogen gas in the sample 10 minutes after the dehydrogenation treatment was 0.20cc / 100gAl or less, and inclusions were not confirmed. It became a sample.

However, when the cylinder was exchanged during the dehydrogenation treatment, as shown in Table 4, the residual hydrogen gas amount was 0.26 cc / 100 gAl even in the sample 20 minutes after the dehydrogenation treatment, and inclusions were also confirmed. It became a reject sample.

[0042]

As described above, according to the present invention,
Purity obtained by pressure swing adsorption type nitrogen gas generator 9
By blowing nitrogen gas of 9.95 to 99.995% into the molten aluminum, dehydrogenation in the molten metal can be carried out well. Using the high-quality molten aluminum thus obtained, gravity casting, low-pressure casting, medium-pressure casting, die casting, etc. to produce aluminum castings, high-quality automobile parts, Aluminum products such as machine parts can be manufactured.

[0043]

[Brief description of drawings]

1 is a crucible furnace (B) for treating molten aluminum used in Examples 1 and 2, a pressure swing adsorption type nitrogen gas generator (A), and a nitrogen cylinder (C).

[Explanation of symbols]

1 Air Compressor 2 Air-Dryer 3, 3a Adsorption Tower 4, 4a, 7, 7a, 10, 10a, 13, 13a, 1
5,17,18 Open / close valve 5,5a Air inflow passage pipe 8 Suction passage pipe 9,9a, 11 Product nitrogen gas extraction passage pipe 12 Pressure equalizing pipe 14 Surge tank 16 Product nitrogen gas transportation passage pipe 19 Flux supply hopper-20 Vertical rotary shaft 21 Impeller 22 Crucible 23 Molten metal

[Table 1]

[Table 2]

[Table 3]

[Table 4]

Claims (1)

[Claims] 1. A pressure swing adsorption type nitrogen gas generation method in which raw material air is supplied to two or more adsorption columns filled with molecular sieving carbon, and product nitrogen gas is continuously taken out by sequentially repeating pressure adsorption and pressure reduction regeneration. Purity obtained with the device 99.95
Approximately 99.995% of nitrogen gas is blown into the lower part of the molten aluminum, and this nitrogen gas is dispersed into the entire molten metal as fine bubbles by an impeller attached to the lower part of the molten metal. A method for dehydrogenating molten aluminum, characterized by floating and removing.