JPH08246077A - Method for removing impurity of active metal - Google Patents

Abstract

PURPOSE: To remove the contained impurity element in a short time by deposition, by heating a molten Mg-Al alloy at a high temp. below the temp. at which the solidification of the alloy is started. CONSTITUTION: Impurity iron (by about 0.02wt.%) is removed from an Mg alloy (about 600 deg.C solidification starting temp.) contg., for example, 9wt.% Al, 0.7wt.% zinc and 0.3wt.% Mn. In this case, the alloy is heated to about 750 deg.C in the (Ar+0.2% SF6 ) gas atmosphere while preventing the combustion of Mg and completely melted. The molten alloy is kept at 590 deg.C which is lower than the temp. where the solidification of the molten alloy is started, and the iron concn. is reduced to about 0.001wt.% in about 30min. Consequently, the impurity is removed in a short time without using a special molten metal holder, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting and refining high-purity magnesium or magnesium-aluminum alloys, which includes impurities such as iron,
The present invention relates to a method for removing aluminum and manganese.

[0002]

2. Description of the Related Art When producing pure magnesium, it is unavoidable that iron, silicon, etc. are mixed in as impurities, and the presence of these impurities significantly impairs the corrosion resistance of pure magnesium.

Particularly, when pure magnesium is recycled from scrap, the corrosion resistance is further deteriorated because the concentration of impurities such as iron is high. Therefore, establishment of a method for removing iron as an impurity has been conventionally desired.

As a crucible for refining a magnesium alloy, it is difficult to use a refractory crucible because magnesium is an active metal. For example, in a crucible made of magnesium oxide or calcium oxide, since silicon oxide is used as a binder, it becomes a cause of contamination. Therefore, it is common to use iron crucibles and jigs. However, when the melting temperature is raised to around 900 ° C., iron contamination from the crucible and the like occurs even in an iron crucible.

It is generally not easy to remove impurities such as iron from magnesium, which is an active metal, but conventionally, a method of using a flux, a method of introducing some compound, a method of using a filtration device and the like have been used. Are known.

As a method of using the flux, there is known a method of forming a flux layer at the bottom of the processing furnace, charging scrap into the processing furnace, and passing the flux layer (Japanese Patent Publication No. 56-18063). However,
The treatment by this method has drawbacks that the productivity is poor because the target metal is taken away by the flux and the contamination of the flux cannot be avoided.

As a method of introducing any compound, titanium tetrachloride (Japanese Patent Publication No.
504) or a halogenated boron derivative (JP-A-58).
No.-96830), the reaction products of them and impurities are precipitated and removed.

A method using a filtering device is disclosed in Japanese Patent Application Laid-Open No. 61-243133. This method
Filtration is performed by keeping the temperature of the raw material alloy near the crystallization temperature of impurities determined based on the liquidus temperature at a temperature lower than the temperature of the molten metal when it is supplied as a molten metal. Magnesium die casting materials (Mg
When removing impurities mainly composed of oxides from 90 wt%, Al 9 wt%, Zn 0.5 to 0.9 wt%, and the balance), generally, the molten metal temperature is maintained at around 650 ° C. and filtration is performed. Alumina and mullite materials are used for the filtration filter.

[0009]

However, in the above conventional impurity removing method, the size of the inclusions derived from the impurity element is very small, and the sedimentation speed is slow, so that the sedimentation is sufficiently performed. Therefore, it takes a very long time, or in the case of filtration, it takes a long time.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for eliminating the need for preparing a large molten metal holding device and shortening the holding time for removing impurities. To aim.

[0011]

According to the method for removing impurities of an active metal according to the present invention, a raw material metal once melted is kept at a high temperature below a temperature at which solidification of a target pure magnesium or magnesium-aluminum alloy starts. The impurity element is removed by precipitation. The holding temperature at this time is
Needless to say, the temperature must be at least the temperature at which the solidification of the metal to be treated is completely completed, but it is preferably at least 480 ° C.

[0012]

When the compound derived from the impurity element is formed and precipitated from the molten metal of the raw material metal, it is generally considered that the higher the temperature, the better in view of the viscosity of the liquid. This is because the lower the temperature, the worse the fluidity of the liquid, and the more difficult it is for solids to move (precipitate) in the liquid. Therefore, when carrying out the precipitation removal method, it is not generally conducted in a low temperature state where the solid has poor fluidity. However, the inventors of the present invention deliberately studied the case where the temperature was kept low, and completed the present invention.

Next, the mechanism of the present invention will be described. As a case of forming a eutectic in the phase diagram,
For example, a case where aluminum is mixed in magnesium will be described as an example. As in the present invention, when the molten metal of the raw material metal is held at a high temperature equal to or lower than the temperature at which the solidification of the target metal is started to start solidification, the content of impurities in the solid in the solid that occurs first, here Since the content of aluminum is smaller than the aluminum concentration of the molten metal, the composition of the molten alloy liquid temporarily changes with the solidification. For example, the Mg-Al system and the Mg-Al-Z shown in FIG.
In the n-type phase diagram, at 550 ° C., a semi-solidified molten metal composed of a solid having a composition of point c and a liquid having a composition of point d is formed.

In order to generate a compound derived from an impurity element in the molten metal, it is necessary to increase the activity of the elements constituting the compound in terms of reaction energy. For example, FeAl ₃ which is an intermetallic compound of iron and aluminum cannot be formed unless the activity (actually, concentration) of iron or aluminum is high. Therefore, according to the conventional idea, iron or aluminum will be charged into the molten metal to form the above compound. In this case, since the composition of the molten metal itself changes, it is necessary to perform an operation of introducing the alloying element to restore the molten metal composition to the original level in order to meet the product specifications.

On the other hand, in the case of the present invention, a part of Mg is used as described above.
Since the solidification of (3) starts, and the aluminum content in the formed solidified product is low, the aluminum concentration in the liquid melt gradually increases, and it becomes easy to form a compound with the impurity iron without adding aluminum. In this way, an impurity compound is formed.

One of the advantages of the present invention is that it does not purposely add an element that acts to form an intermetallic compound as in the conventional case. Therefore, the present invention can be applied in the same manner to a system in which there is a combination such that an element considered as an impurity and an alloy element react with each other to form an intermetallic compound in a molten metal.

Next, the sedimentation rate of the produced solid will be described.
Generally, the main factors that determine the sedimentation rate are the large difference between the density of the liquid and the solid compound, the large particles of the solid compound, and the change of the state of the interface between the liquid and the compound ( There is slipperiness). Of these, increasing the size of the solid compound particles, that is, increasing the particle size, is considered to be most effective in increasing the sedimentation rate. In the case of the present invention, the factor is not clear, but an increase in sedimentation rate was observed by experiments.

After elapse of the temperature holding time for the precipitation applied to the alloy to be treated, the molten metal is cooled and completely solidified. Cooling after treatment is usually performed from the side of the crucible. At this time, first, the alloy with high Mg purity is solidified from the side surface, and gradually the liquid molten metal containing a large amount of impurities remains at the bottom where Fe compounds are accumulated and finally solidifies. As described above, the present invention does not require a special cooling method and can be carried out by a conventional method. Then, the lower part where impurities are precipitated is cut, and the upper part is melted again and used as a raw material for the alloy. It is also possible to remove the impurity precipitate after the lapse of the precipitation holding time, reheat it as it is, and cast it as a high-purity alloy. As an example of impurities that can be removed by the mechanism of the present invention, pure magnesium can remove impurities such as iron, aluminum, and manganese.

The composition of the main products of Mg-Al alloy is Al
It is thought that the content is up to 10%, but in this range,
In the present invention, the temperature at which solidification starts corresponds to the liquidus temperature on the phase diagram, and the temperature at which solidification ends corresponds to the solidus temperature.

When scrap is used as the raw material metal,
The molten metal contains a large amount of iron, but liquid magnesium has 1
% Iron cannot be dissolved, so iron remains as it is. Therefore, the iron content is 1% or less, and from the Mg-Fe alloy phase diagram, the present invention enables separation and removal even when the Fe content is large (1%). In the case of the conventional method, pretreatment was required, but as described above, the present invention does not require pretreatment, which simplifies the process.

[0021]

Example A graphite crucible was placed in a carbon resistance heating furnace. As the alloy to be treated, a magnesium alloy containing 9% by weight of aluminum, 0.7% by weight of zinc and 0.3% by weight of manganese was used. The solidification starting temperature of this alloy is about 600 ° C., and iron as an impurity is contained at an initial concentration of 0.02% by weight.

This alloy was placed in the crucible. The above alloy was melted while maintaining the atmosphere of argon and 0.2% SF ₆ gas in the furnace to prevent magnesium from burning. The molten metal was heated to 750 ° C. and completely melted, and then held at 650 ° C. (comparative example) and 590 ° C. (example). FIG. 1 shows a comparison of changes over time in iron concentration in this case. Under the conditions of this experiment, the depth of the molten metal was 10 cm, and the sample for component analysis was taken from a position 4 cm from the bottom of the molten metal.

Line a (comparative example) shown in FIG. 1 shows the case where the alloy is held at 650 ° C., which is higher than the temperature at which solidification of the target alloy starts, and the iron concentration is 0.003 even after holding for 1 hour.
It only drops to weight%. Line b (Example) is the change in iron concentration when the temperature is set to 590 ° C., which is lower than the temperature at which solidification of the target alloy starts. In this case, 0.001% by weight in 30 minutes
Has reached the following:

The aluminum content was 8.8% by weight when kept at 650 ° C and 8.7% by weight when kept at 590 ° C. The manganese content was 0.28% by weight when kept at 650 ° C and 0.22% by weight when kept at 590 ° C.

From the above results, it can be seen that when the temperature is kept below the temperature at which the solidification of the target alloy starts as in this example, the precipitation removal can be completed quickly and the impurities can be removed in hardness. Although the crucible having a graphite property is used in the above embodiment, the crucible is not limited to this, and an iron crucible or the like may be used.

[0026]

Since the method for removing impurities of the active metal of the present invention is configured as described above, it is more advantageous than the conventional method of precipitating impurities at a temperature at which the Mg alloy is completely dissolved.
It is possible to shorten the processing time for impurity refining. In addition, no special equipment such as large molten metal holding device or filtration device for processing is required. Further, since an input material for removing impurities is not required, there is an effect that the input material does not adversely affect the alloy and has almost no effect on the alloy composition. Further, impurities other than oxides, which could not be completely removed by the conventional filtration method, can be removed, and refining accuracy can be further improved.

[Brief description of drawings]

FIG. 1 is a graph showing the iron concentration in a molten metal when the method of one example of the present invention and the method of a comparative example were performed.

FIG. 2 is a cutting state diagram of Mg-Al system and Mg-Al-Zn system.

Claims (1)

[Claims] 1. A method for refining a high-purity magnesium or magnesium-aluminum-based alloy by removing impurity elements from magnesium or a magnesium-aluminum-based alloy, wherein the molten metal of the raw material metal is the desired high-purity magnesium. Alternatively, a method for removing impurities of an active metal, characterized in that the impurity element is deposited and removed by holding at a high temperature equal to or lower than a temperature at which solidification of a magnesium-aluminum alloy is started.