U.S. patent application number 10/298043 was filed with the patent office on 2003-12-11 for method of removing coating from coated magnesium alloy product, method of making recycled magnesium alloy and method of recycling coating.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Kimura, Koichi, Nishii, Kota.
Application Number | 20030226424 10/298043 |
Document ID | / |
Family ID | 29714372 |
Filed Date | 2003-12-11 |
United States Patent
Application |
20030226424 |
Kind Code |
A1 |
Nishii, Kota ; et
al. |
December 11, 2003 |
Method of removing coating from coated magnesium alloy product,
method of making recycled magnesium alloy and method of recycling
coating
Abstract
A method is provided for removing a coating from a coated
magnesium alloy product. The method includes a first treatment step
of immersing the coated magnesium alloy product in a first alkaline
solution, and a second treatment step of immersing the magnesium
alloy product, which has undergone the first treatment step, in a
second alkaline solution or in an acid solution. The second
alkaline solution is different from the first alkaline
solution.
Inventors: |
Nishii, Kota; (Kawasaki-shi,
JP) ; Kimura, Koichi; (Kawasaki-shi, JP) |
Correspondence
Address: |
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
29714372 |
Appl. No.: |
10/298043 |
Filed: |
November 18, 2002 |
Current U.S.
Class: |
75/377 ;
75/600 |
Current CPC
Class: |
C23G 1/22 20130101; Y02P
10/20 20151101; C22B 26/22 20130101; C23G 1/00 20130101; C22B 7/006
20130101 |
Class at
Publication: |
75/377 ;
75/600 |
International
Class: |
C22B 026/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 2002 |
JP |
2002-168957 |
Jun 21, 2002 |
JP |
2002-181823 |
Claims
1. A method of removing a coating from a coated magnesium alloy
product, the method comprising: a first treatment step of immersing
the coated magnesium alloy product in a first alkaline solution;
and a second treatment step of immersing the magnesium alloy
product, which has undergone the first treatment step, in a second
alkaline solution or in an acid solution, the second alkaline
solution being different from the first alkaline solution.
2. The method according to claim 1, wherein the first alkaline
solution and/or the second alkaline solution contains potassium
hydroxide as a main alkaline component.
3. The method according to claim 1, wherein the acid solution
contains an organic acid as a main acid component.
4. The method according to claim 1, wherein the first treatment
and/or the second treatment using the second alkaline solution is
performed at 60-90.degree. C.
5. The method according to claim 1, wherein the first treatment is
performed at 60-90.degree. C., whereas the second treatment using
the acid solution is performed at 20-70.degree. C.
6. The method according to claim 1, further comprising the step of
rinsing the magnesium alloy product with water after the first
treatment step and/or the second treatment step.
7. The method according to claim 1, wherein the coating includes an
undercoat layer and an overcoat layer, the first treatment step
being performed for removing the overcoat layer, the second
treatment step being performed for removing the undercoat
layer.
8. A method of making a recycled magnesium alloy comprising: a
coating removing step which includes a first treatment of immersing
a coated magnesium alloy product in a first alkaline solution, and
a second treatment of immersing the magnesium alloy product in a
second alkaline solution or in an acid solution, the second
alkaline solution being different from the first alkaline solution;
a melting step of melting the magnesium alloy product to provide
molten metal; a component analyzing step for determining a content
of at least one component contained in the molten metal; and a
component controlling step for controlling the content of the
component in the molten metal.
9. The method according to claim 8, wherein the melting step
includes melting a first flux together with the magnesium alloy
product, the first flux functioning to separate impurities
contained in the magnesium alloy product from the molten metal.
10. The method according to claim 8, further comprising the step of
putting a second flux into the molten metal before the component
controlling step, the second flux functioning to generate a gas for
preventing the molten metal from oxidizing.
11. The method according to claim 8, further comprising the step of
rinsing the magnesium alloy product with water after the first
treatment and/or the second treatment.
12. A coating recycling method comprising the steps of: removing a
coating from a coated magnesium alloy product by a first treatment
of immersing the coated magnesium alloy product in a first alkaline
solution, and a second treatment of immersing the magnesium alloy
product, which has undergone the first treatment step, in a second
alkaline solution or in an acid solution, the second alkaline
solution being different from the first alkaline solution;
recovering the coating removed from the magnesium alloy product in
the first treatment and the second treatment; and pulverizing the
recovered coating to provide a recycled coating material.
13. The method according to claim 12, further comprising the steps
of rinsing the magnesium alloy product with water after the first
treatment and/or the second treatment, and recovering the coating
removed from the magnesium alloy product in the rinsing step.
14. The method according to claim 12, wherein the recycled coating
material is added to a resin composition for molding a resin
product.
15. The method according to claim 14, wherein the resin composition
contains a thermoplastic resin selected from the group consisting
of ABS resin, polycarbonate resin, polypropylene resin, polystyrene
resin, polyamide resin and polyphenylene sulfide resin.
16. The method according to claim 12, wherein the recycled coating
material is added to asphalt.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of recycling a
coated magnesium alloy product. More particularly, the present
invention relates to a method of removing a coating from a coated
magnesium alloy product such as a magnesium alloy casing of a
notebook computer or a cellular phone, a method of making a
recycled magnesium alloy, and a method of recycling a coating of a
coated magnesium alloy product.
[0003] 2. Description of the Related Art
[0004] The casings of electronic devices such as notebook
computers, cellular phones, PDAs are required to be strong, to
efficiently diffuse heat generated by the inside components and to
be suitable for efficient recycling. To meet such requirements,
instead of conventional resin casings, metal casings are
increasingly used for such mobile electronic devices.
[0005] As materials for forming such metal casings for electronic
devices, attention is focused on light alloys mainly composed of
light metals such as magnesium (Mg) or aluminum (Al) for making
light electronic devices. Particularly, Mg has the highest specific
strength among simple metals which can be used as a structural
material, has a high heat dissipation capability which is
comparable to Al, and has a low specific gravity which is about 70%
of that of Al. Therefore, Mg alloy mainly composed of Mg can be
suitably used for making the casings of electronic devices.
[0006] For molding articles from Mg alloy, die casting or
thixomolding may be utilized. When a casing of a notebook computer,
for example, is formed from molten Mg alloy by die casting or
thixomolding, about 30-50 volume % of the molten metal injected by
a single time of injection finally becomes portions of the casing.
The remaining about 50-70 volume % of the molten Mg alloy
solidifies in a sprue or a runner of the mold and is later cut
away. In this way, with the die casting or thixomolding, a large
part of the material is wasted in each time of the molding.
Therefore, to attain the cost reduction and good use of resources
of the earth, there is a demand for a technique for recycling the
portions of the Mg alloy product which have conventionally been
wasted in the molding process and the Mg alloy obtained from
collected products to reuse as a material for injection
molding.
[0007] JP-A-2000-47257 discloses a method of making a recycled Mg
alloy from the portions of the Mg alloy product cut away after the
injection molding. With this method, the portions of the Mg alloy
product cut away after the injection molding are remelted together
with flux, and its composition is controlled in the molten state.
Then, the material is cooled for solidification to provide a
recycled Mg alloy having an intended composition.
[0008] Generally, commercially available Mg alloy parts or products
have coated surfaces. Unfavorably, when such Mg alloy product
formed with a coating is melted for recycling, a large amount of
organic gas is generated due to the burning of acrylic resin or
urethane resin contained in the paint. Further, the molten metal
may be unacceptably contaminated due to titanium or the like
contained in the paint. Therefore, for getting a recycled Mg alloy
from wasted products, the coating on the surfaces of the Mg alloy
product need be removed before melting the Mg alloy product.
[0009] Techniques for removing coatings from the surfaces of the Mg
alloy product are disclosed, for example, in JP-A-2000-263443 and
JP-A-2001-20018. Specifically, JP-A-2000-263443 discloses wet
blasting, wherein the coating is physically removed by spraying
water working as fluid with particles of inorganic substance such
as alumina. With the wet blasting, however, the proper removal of
the coating is difficult when the coated surface of the Ng alloy
product is irregular including projections and recesses.
Specifically, the recesses are unlikely to be hit by the inorganic
particles so that the coating at the recesses may not be
sufficiently removed. Conversely, the projections are likely to be
excessively hit by the inorganic particles so that the Mg alloy may
be removed together with the coating.
[0010] JP-A-2001-20018 discloses thermally treating the coated Mg
alloy product at a temperature at which the paint vaporizes in
non-oxidizing atmosphere. However, this method requires a chamber
for performing the thermal treating under particular conditions.
Moreover, since a large amount of organic gas is generated after
the paint is decomposed, this method further requires an apparatus
for properly treating such gas.
[0011] As an alternative known method, the coating may be removed
from the surfaces of the Mg alloy product using alkaline solution.
Specifically, with this method, the coating is removed by immersing
the coated Mg alloy product in alkaline solution for swelling the
coating due to the action of the alkaline solution followed by
rinsing for example. Conventionally, in such a method, the
immersing of the coated Mg alloy product is performed until the
coating entirely swells. Specifically, since such a coating is
generally made up of an undercoat layer and an overcoat layer, the
coated Mg alloy product continues to be immersed in alkaline
solution until the both layers swell, which may take a long time of
about two hours. Further, when the coating swells, metal powder and
the like contained in the coating dissolves or diffuses,
contaminating the alkaline solution. Therefore, the alkaline
solution used until the entirety of the coating swells tends to be
degraded relatively early during its repetitive use for the
immersion.
[0012] JP-A-5-84746 and JP-A-6-114842 also disclose a method for
removing a coating. However, these methods are intended for
removing coating from a molded resin product, not an Mg alloy
product.
[0013] Moreover, conventionally, the coating removed from the Mg
alloy product by the immersion and the following rinsing is treated
as industrial waste. Such treatment of the coating is not desirable
in view of the good use of natural resources and the environmental
preservation.
SUMMARY OF THE INVENTION
[0014] It is, therefore, an object of the present invention to
provide a method of removing a coating from a magnesium alloy
product properly in a short period of time while prolonging the
lifetime of the treatment liquid used for the removal.
[0015] Another object of the present invention is to provide a
method of making a recycled magnesium alloy utilizing the coating
removing method.
[0016] Still another object of the present invention is to provide
a method of recycling a coating utilizing the coating removing
method.
[0017] According to a first aspect of the present invention, there
is provided a method of removing a coating from a coated magnesium
alloy product. The method comprises a first treatment step of
immersing the coated magnesium alloy product in a first alkaline
solution, and a second treatment step of immersing the magnesium
alloy product, which has undergone the first treatment step, in a
second alkaline solution or in an acid solution. The second
alkaline solution is different from the first alkaline solution
[0018] With this method, the coating formed on the surfaces of the
magnesium alloy product can be removed properly in a relatively
short time. Generally, a coating on a magnesium alloy product
includes an overcoat layer for providing a desired color, and an
undercoat layer for providing good adhesion of the overcoat layer
to the surfaces of the base material, i.e. the magnesium alloy. The
overcoat layer contains a resin component for paint, a color
pigment, a dye and the like by a relatively large amount, which
become factors for deteriorating the treatment liquid used for the
removal of the coating. The overcoat layer tends to swell
relatively quickly when immersed in alkaline solution. On the other
hand, the undercoat layer tends to swell relatively slowly when
immersed in an alkaline solution. In the conventional coating
removing method using an alkaline solution, the coated magnesium
alloy product continues to be immersed in the same alkaline
solution until the overcoat layer and then the undercoat layer
swell. In such immersion, the overcoat layer first swells due to
the action of the alkaline solution. At this time, a relatively
large amount of components contained in the overcoat layer dissolve
or diffuse in the alkaline solution, thereby degrading the alkaline
solution to a considerable degree. The alkaline solution thus
degraded takes a long time for swelling the undercoat layer which
is inherently unlikely to swell. For this reason, the conventional
method takes a relatively long time for removing the entire
coating.
[0019] According to the present invention, however, the coating is
immersed in different solutions at least twice. Specifically, the
first immersion treatment using the first alkaline solution ends
when the overcoat layer is swelled for example. Then, after the
overcoat layer is removed, the second immersion treatment is
performed using a second alkaline solution which is different from
the first alkaline solution, or an acid solution. Herein, the
second alkaline solution "which is different from the first
alkaline solution" means an alkaline solution which has not been
used in the first treatment. Therefore, the second alkaline
solution may have the same composition as that of the first
alkaline solution. Alternatively, the second alkaline solution may
differ in composition from the first alkaline solution. When the
second treatment is performed using the second alkaline solution,
the second alkaline solution directly acts on the undercoat layer
because the overcoat layer has already been removed. Further, since
the second alkaline solution has not been used for swelling the
overcoat layer, its ability as treatment liquid is not degraded.
Therefore, the second alkaline solution can reliably swell the
undercoat layer which is inherently unlikely to swell, thereby
causing the layer to be removed from the base alloy. As a result,
the total time required for swelling and removing the coating
entirely is shorter than that in the conventional method. When the
second treatment is performed using an acid solution, the acid
solution directly acts on the undercoat layer because the overcoat
layer has already been removed. The acid solution decomposes or
dissolves the undercoat layer due to the chemical action.
Alternatively, the acid solution swells the undercoat layer for
removal from the base material. Also in this case, the total time
required for swelling and removing the coating entirely is shorter
than that in the conventional method.
[0020] The method according to the first aspect of the present
invention can prolong the lifetime of the treatment liquid used for
the removal of the coating. Specifically, in the prior art method,
the coated magnesium alloy product is continuously immersed in a
same alkaline solution until the overcoat layer and then the
undercoat layer are swelled. Therefore, the components which
deteriorate the solution dissolve or diffuse in the alkaline
solution both from the overcoat layer and from the undercoat layer.
Therefore, the alkaline solution is considerably degraded in terms
of its ability as treatment liquid, so that it cannot be
repetitively used many times. That is, the treatment liquid used in
the conventional method has a relatively short lifetime.
[0021] On the other hand, with the coating removing method
according to the present invention, the coating is immersed in at
least two solutions. Therefore, all of the components coming from
the coating do not dissolve in either of the first alkaline
solution or the second alkaline or acid solution to degrade the one
solution considerably. Therefore, the deterioration of each
solution is suppressed. Thus, each solution used in the method of
the present invention can have a relatively long lifetime.
[0022] In this way, according to the first aspect of the present
invention, the coating formed on the magnesium alloy product can be
removed properly in a short period of time while prolonging the
lifetime of treatment liquid used for the coating removal.
[0023] According to a second aspect of the present invention, there
is provided a method of making a recycled magnesium alloy. This
method comprises a coating removing step which includes a first
treatment of immersing a coated magnesium alloy product in a first
alkaline solution and a second treatment of immersing the magnesium
alloy product in a second alkaline solution (which is different
from the first alkaline solution) or in an acid solution, a melting
step of melting the magnesium alloy product to provide molten
metal, a component analyzing step for determining the content of at
least one component contained in the molten metal, and a component
controlling step for controlling the content of the component in
the molten metal.
[0024] This method of making a recycled magnesium alloy includes
the coating removing process according to the first aspect of the
present invention. With this method, therefore, the coating can be
removed properly from the coated magnesium alloy product in a short
period of time while prolonging the lifetime of treatment liquid
used for the coating removal, which leads to the efficient making
of a recycled magnesium alloy.
[0025] According to a third aspect of the present invention, there
is provided a method of recycling a coating. This method comprises
the steps of removing a coating from a coated magnesium alloy
product by a first treatment of immersing the coated magnesium
alloy product in a first alkaline solution and a second treatment
of immersing the magnesium alloy product in a second alkaline
solution (which is different from the first alkaline solution) or
in an acid solution, recovering the coating removed from the
magnesium alloy product in the first treatment and the second
treatment, and pulverizing the recovered coating to provide a
recycled coating material.
[0026] This coating recycling method includes the coating removing
process according to the first aspect of the present invention.
With this method, therefore, the coating can be removed properly
from the coated magnesium alloy product in a short period of time
while prolonging the lifetime of treatment liquid used for the
removal. Therefore, the coating can be efficiently recovered from
the treatment liquid, which leads to the efficient making of
recycled coating material. The recycled coating material may be
added to a resin composition for molding a resin product.
Preferably, the resin composition may contain a thermoplastic resin
such as ABS resin, polycarbonate resin, polypropylene resin,
polystyrene resin, polyamide resin or polyphenylene sulfide resin,
for example. Alternatively, the recycled coating material may be
added to asphalt as extender.
[0027] Preferably, in the methods according to the first through
the third aspects of the present invention, the first alkaline
solution and/or the second alkaline solution may contain potassium
hydroxide as the main alkaline component. Further, the acid
solution used in the second treatment may contain an organic acid
as the main acid component.
[0028] Preferably, the first treatment and/or the second treatment
using the alkaline solution may be performed at 60-90.degree. C.,
and more preferably, at 70-80.degree. C. The treatment at a
temperature below 60.degree. C. is not preferable, because the
coating does not sufficiently swell. Further, the treatment at a
temperature higher than 90.degree. C. is not preferable, because
the alkaline solution excessively acts on the metal powder
contained in the coating to generate a relatively large amount of
metal hydroxide, which promotes the degradation of the alkaline
solution.
[0029] Preferably, in the case where acid solution is used for the
second treatment, the second treatment may be performed at
20-70.degree. C., and more preferably, at 30-60.degree. C. The
treatment at a temperature below 20.degree. C. is not preferable,
because the coating does not sufficiently swell or melt. Further,
the treatment at a temperature higher than 70.degree. C. is not
preferable, because the acid solution excessively acts on the
coating to dissolve even the magnesium alloy beneath the
coating.
[0030] Preferably, each of the methods according to the first
through the third aspect of the present invention may further
include the step of rinsing the magnesium alloy product with water
after the first treatment and/or the second treatment. After the
first and/or the second treatment, the adhesion of the coating to
the magnesium alloy product is weakened due to the swelling or the
like. Therefore, when the magnesium alloy product in such a state
is rinsed with water at a predetermined water pressure, the coating
is duly removed from the magnesium alloy product. Preferably, the
method according to the third aspect of the present invention
further includes the step of recovering the coating removed from
the magnesium alloy product in the rinsing step.
[0031] Preferably, in the method according to the second aspect of
the present invention, the melting step may include melting a first
flux together with the magnesium alloy product. The first flux
functions to separate impurities contained in the magnesium alloy
product from the molten metal. With this method, impurities can be
properly removed as sludge or precipitate in the molten metal.
[0032] Preferably, a second flux which generates a gas for
preventing the molten metal from oxidizing may be put into the
molten metal before the component controlling step. With this
method, it is possible to prevent the molten metal from oxidizing
from its surfaces before the component controlling step, or during
when the molten metal is left for cleaning for example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a flow chart of a coating removing method
according to a first embodiment of the present invention;
[0034] FIG. 2 illustrates an example of magnesium alloy casing of a
notebook computer;
[0035] FIG. 3 is a flow chart of a method of making a recycled
magnesium alloy according to a second embodiment of the present
invention;
[0036] FIG. 4 is a flow chart of a coating recycling method
according to a third embodiment of the present invention; and
[0037] FIG. 5 illustrates flowerpots provided by the coating
recycling method according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] FIG. 1 is a flow chart illustrating a method of removing
coating according to a first embodiment of the present invention.
For example, this method may be utilized for removing the coating
formed on the casing of a notebook computer formed of a magnesium
alloy, as shown in FIG. 2. Specifically, the method is applicable
to removing of the coating consisting of an undercoat layer mainly
composed of an epoxy resin and an overcoat layer mainly composed of
an acrylic resin, for example.
[0039] Firstly, a first treatment step S11 is performed, in which
the Mg alloy product provided with a coating is immersed in a first
alkaline solution. Specifically, in this step, the Mg alloy product
is immersed in an alkaline solution of a volume 10-20 times that of
the Mg alloy product for 20-60 minutes. During the immersion, the
alkaline solution is kept at 60-90.degree. C. Preferably, the first
alkaline solution contains a hydroxide of alkali metal or alkali
earth metal, particularly potassium hydroxide, as the alkaline
component. Preferably, the concentration of potassium hydroxide in
the first alkaline solution is 2-40 wt %. As a result of the first
treatment step S11, the overcoat layer swells, weakening its
adhesion to the undercoat layer. Thus, the overcoat layer is
partially removed. The undercoat layer also swells at the portion
where the overcoat layer has been removed, thereby weakening its
adhesion to the Mg alloy product.
[0040] Subsequently, in a rinsing step S12, the Mg alloy product is
pulled out from the first alkaline solution and is rinsed with
water. Specifically, in this step, water is sprayed to the Mg alloy
product at a pressure of 0.5-2 kgf/cm.sup.2 using a shower.
Alternatively, the Mg alloy product may be immersed in wash water
stored in an appropriate bath. As a result of the rinsing step S12,
the overcoat layer swelling on the Mg alloy product or pieces of
the overcoat layer adhering to the Mg alloy product are physically
removed from the Mg alloy product. Thus, by the steps S11 and S12,
mainly the overcoat layer is removed from the Mg alloy product.
[0041] Subsequently, in a second treatment step S13, the Mg alloy
product is immersed in a second alkaline solution. Specifically, in
this step, the Mg alloy product is immersed in an alkaline solution
of a volume 10-20 times that of the Mg alloy product. During the
immersion, the second alkaline solution is kept at 60-90.degree. C.
The immersion may be continued for 30-70 minutes as long as the
total immersion time of the immersion in the first treatment step
S11 and the immersion in the second treatment step S13 does not
exceed 90 minutes. The second alkaline solution may have the same
composition as that of the first alkaline solution. Alternatively,
an alkaline solution which differs from the first alkaline solution
in composition may be used in view of the composition of the
undercoat layer. As a result of the second treatment step S13, the
undercoat layer swells, weakening its adhesion to the Mg alloy
product. The undercoat layer is partially removed from the Mg alloy
product.
[0042] In the second treatment step S13, acid solution may be used
instead of the second alkaline solution. Specifically, in this
case, the Mg alloy product is immersed in an acid solution of a
volume 10-20 times that of the Mg alloy product for 5-30 minutes.
During the immersion, the acid solution is kept at 20-70.degree. C.
Preferably, the acid solution contains an organic acid such as
formic acid, acetic acid or benzoic acid as the acid component. In
the case where the acid solution containing formic acid is used, it
is preferable that the concentration of formic acid in the acid
solution is 2-40 wt %. As a result of the second treatment step
S13, the undercoat layer decomposes or dissolves for removal from
the Mg alloy product.
[0043] Subsequently, in a rinsing step S14, the Mg alloy product is
pulled out from the second alkaline solution or acid solution and
is rinsed with water. Specifically, similarly to the rinsing step
S12, water is sprayed to the Mg alloy product at a pressure of
0.5-2 kgf/cm.sup.2 using a shower. Alternatively, the Mg alloy
product may be immersed in wash water stored in an appropriate
bath. As a result, the undercoat layer swelling on the Mg alloy
product or pieces of the undercoat layer adhering to the Mg alloy
product are physically removed from the Mg alloy product.
[0044] Subsequently, a drying step S15 is performed to dry the Mg
alloy product. Specifically, for example, the Mg alloy product is
left in a drying chamber kept at 50-80.degree. C. for 30-60 minutes
for drying.
[0045] Through the above steps, coating is duly removed from the
magnesium alloy product. In the case of a notebook computer casing
as shown in FIG. 2, the coating is removed not only from a flat
portion 1 but also from side walls 2, a retreated portion 3 and an
irregular portion 4. In this method, an alkaline solution or acid
solution which has not been degraded in terms of treatment ability
is used in the second treatment step 13. Therefore, the total
immersion time of the first treatment step 11 and the second
treatment step 13 required for the removal of the coating is
shorter than the time of the single immersion in the conventional
method. Further, it is possible to prevent all of the components
which are contained in the coating and which contaminate the
treatment liquid dissolve in either of the first alkaline solution
or the second alkaline or acid solution. Therefore, each of the
first and the second treatment liquids has a longer lifetime than
the liquid used in the conventional method.
[0046] FIG. 3 is a flow chart illustrating a method of making a
recycled magnesium alloy according to a second embodiment of the
present invention. It is known that, through the recycling process
of an Mg alloy product, the content of Fe in the alloy tends to
increase, whereas the content of Mn in the alloy tends to decrease.
Therefore, in the embodiment described below, the Fe content and
the Mn content in the Mg alloy are controlled to provide a recycled
Mg alloy for forging which is equivalent to JIS MD1D or AZ91d.
[0047] First, as shown in FIG. 3, the process steps from a first
treatment step S11 through a drying step S15 are carried out
similarly to the first embodiment described with reference to FIG.
1. As a result, an Mg alloy product from which a coating has been
removed is obtained.
[0048] Subsequently, in a melting step S21, the Mg alloy product is
melted together with a first flux in a melting furnace.
Specifically, the coating-free Mg alloy as pellets or an ingot is
put into a melting furnace heated to about 680.degree. C. in
advance, together with a first flux in powder form. The temperature
of the furnace is then raised to about 720.degree. C. for melting
the Mg alloy and the flux. At this time, the molten Mg alloy in the
melting furnace is stirred by rotating a mechanical impeller for
homogenization. The first flux is added mainly to reduce oxides
generated during the melting and to settle impurities such as oil
in the molten metal as a sludge in the subsequent process step. For
the first flux, use may be made of halide of alkali metal or alkali
earth metal. Specifically, the first flux may be a powdery mixture
containing 40-60 wt % MgCl.sub.2, 15-35 wt % KCl, 1-10 wt %
CaF.sub.2 and 10-30 wt % BaCl.sub.2. If the surface of the molten
metal ignites, the first flux may further be added as required.
[0049] Subsequently, in a sludge separation step S22, impurities in
the molten metal are separated as a sludge by continuing the
agitation of the molten metal. In this step, when the separation of
the sludge is confirmed, a second flux in powder form is added to
the molten metal to promote the separation of the sludge. For the
second flux, use may be made of a flux contains the same components
as the first flux but differs therefrom with respect to the
proportions of the respective components. Preferably, the
proportion of BaCl.sub.2, which has a relatively large specific
gravity, in the second flux is lower than that in the first flux.
Specifically, the second flux may be a powdery mixture containing
60-75 wt % MgCl.sub.2, 20-35 wt % KCl, 1-5 wt % CaF.sub.2 and 1-10
wt % BaCl.sub.2. If the surface of the molten metal ignites, the
second flux may further be added as required.
[0050] Then, in a cleaning step S23, the molten metal is cleaned.
Specifically, the molten metal is kept still for 10-30 minutes
after the stirring is stopped, thereby allowing the sludge to
settle at the bottom of the melting furnace. At this time, a third
flux is added to the molten metal to form an anti-oxidation gas
layer covering the surface of the molten metal. Specifically, the
third flux thus added decomposes due to the heat of the molten
metal, generating a gas for preventing the oxidation of the molten
metal. As the gas fills the melting furnace, the surface of the
molten metal is covered with the anti-oxidation gas layer. For the
third flux, use may be made of a powdery mixture containing 60-90
wt % sulfur (S) and 10-40 wt % MgF.sub.2.
[0051] Subsequently, in a component analysis step S24, analysis is
performed with respect to the components contained in the cleaned
molten metal. Specifically, part of the molten metal is extracted
from the melting furnace to cast a columnar sample which is 5 cm in
diameter and 5 cm in length, for example. Then, the component
analysis is performed with respect to the sample to determine the
contents of Fe and Mn in the molten metal. The analysis may be
carried out by the arc emission spectrochemical analysis for
example.
[0052] Then, in a component controlling step S25, the content of Fe
and Mn each is controlled to a desired range based on the analysis
results obtained in the former step. For example, to provide an Mg
alloy for forging equivalent to AZ91D, the content of Fe need be
controlled to no more than 40 ppm (0.004 wt %), whereas the content
of Mn need be controlled to the range of 0.17-0.4 wt %.
[0053] As described before, it is known that the content of Fe in
an Mg alloy increases through the recycling process. Therefore, to
properly control the Fe content, an appropriate amount of
Fe-settling agent is added to the molten metal in accordance with
the amount of excess Fe. For the Fe-settling agent, use may be made
of Al--Mn intermetallic compound. On the other hand, it is known
that the content of Mn in an Mg alloy decreases through the
recycling process. Therefore, to properly control the Mn content,
an appropriate amount of Mn-supplying medium is added to the molten
metal in accordance with the shortage amount of Mn. For the
Mn-supplying medium, use may be made of pure Mn or a compound
containing Mn. For example, Al--Mn intermetallic compound may be
used also as the Mn-supplying medium. In the case where the content
of Fe and Mn lies in a proper range from the beginning, neither the
Fe-settling agent nor the Mn-supplying medium is not added.
[0054] The addition of the Fe-setting agent and/or the Mn-supplying
medium to the molten metal is followed by stirring and cleaning of
the molten metal in the component controlling step S25. Thereafter,
the component analysis step S24 is carried out again. In this way,
the component controlling step S25 and the component analysis step
S24 are repeated until the contents of Fe and Mn lies in a proper
range.
[0055] Subsequently, in a casting step S26, the molten metal having
a desired composition is cast into an ingot of a predetermined
size. In this way, a recycled Mg alloy is obtained in the form of
an ingot.
[0056] With the method of making a recycled Mg alloy according to
the present invention, a coating can be efficiently removed from
the coated Mg alloy product by the first treatment step S11 through
the rinsing step S14, so that a recycled Mg alloy can be
efficiently obtained. Further, since the Mg alloy product is melted
after removal of the coating, the generation of organic gases due
to the burning of the resin material contained in the coating can
be avoided. For the same reason, the molten metal is prevented from
being excessively contaminated by metals (e.g. Ti contained in the
paint) which are undesirable for a Mg alloy. Moreover, although the
contents of Fe and Mn in the Mg alloy may vary depending on the
number of recycle times and other factors, the method of the
present invention makes it possible to control these contents
individually and flexibly. Thus, with the method according to the
present invention, it is possible to efficiently and properly make
a recycled Mg alloy having an intended composition, i.e., the
composition equivalent to that of a virgin Mg alloy.
[0057] FIG. 4 is a flow chart illustrating a method of recycling
coating according to a third embodiment of the present invention.
In this method, as shown in the figure, the process steps from a
first treatment step S11 through a rinsing step S14 are carried out
similarly to the first embodiment described with reference to FIG.
1. As a result, coating is removed from a coated Mg alloy
product.
[0058] Subsequently, in a recovering step S31, the pieces of the
coating removed from the magnesium alloy product in the first
treatment step S11 through the rinsing step S14 are recovered.
Specifically, filtration is performed with respect to the first
alkaline solution used in the first treatment step S11, the second
alkaline solution or acid solution used in the second treatment
step S13 and water used in the rinsing steps S12 and S14, thereby
separating the pieces of the coating removed from the Mg alloy
product from these treatment liquids.
[0059] Then, in a rinsing step S32, the recovered pieces of the
coating are rinsed with water to satisfactorily remove the
treatment liquid adhering thereto. Subsequently, a drying step S33
is performed to dry the pieces of the coating. Specifically, for
example, the coating pieces are left in a drying chamber kept at
80.degree. C. for 2 hours for drying. Then, a pulverizing step S34
is performed to pulverize the dried coating pieces to a desired
particle size. Specifically, for example, the coating pieces are
put into a mortar and pulverized to a particle size of no more than
1 mm. Thus, pellets of a recycled coating material are
obtained.
[0060] Subsequently, in a resin molding step S35, a resin product
is molded utilizing the pellets of the recycled coating as an
extender. Specifically, a thermoplastic resin material and the
recycled coating material are mixed for melting at a predetermined
mixing ratio using a twin-screw extruder, for example.
Specifically, the proportion of the recycled coating pellet added
is 10-60 wt %. For the thermoplastic resin, use may be made of ABS
resin, polycarbonate resin, polypropylene resin, polystyrene resin,
polyamide resin and polyphenylene sulfide resin. Then, the resin
composition thus obtained is molded into an article of a desired
configuration using an injection molding machine. As an alternative
usage, the recycled coating pellets may be added to asphalt as
extender.
[0061] The present invention will now be described through examples
and comparative examples.
EXAMPLE 1
[0062] [Preparation of Recycled Mg Alloy Product]
[0063] The casing of a notebook computer as shown in FIG. 2 was
immersed in 2L of a first alkaline solution. The casing was made of
an Mg alloy and had coated surfaces, and the coating on the
surfaces of the casing was made up of an undercoat layer mainly
composed of epoxy resin and an overcoat layer mainly composed of
acrylic resin. The first alkaline solution used contained 3.5 wt %
potassium hydroxide, 2.5 wt % sodium hydroxide, 20 wt % anionic
surface active agent and 40 wt % diethylene glycol monoethyl ether.
During the immersion, the first alkaline solution was kept at
70.degree. C. The immersion was performed for 10 minutes. As a
result, mainly the overcoat layer of the coating swelled. Then, the
Mg alloy casing was pulled out from the first alkaline solution and
rinsed with running water at 1 kgf/cm.sup.2. As a result, mainly
the overcoat layer was removed.
[0064] Subsequently, the Mg alloy casing was immersed in a second
alkaline solution having the same composition as that of the first
alkaline solution. During the immersion, the second alkaline
solution was kept at 70.degree. C. The immersion was performed for
40 minutes. As a result, the undercoat layer of the coating
swelled. Then, the Mg alloy casing was pulled out from the second
alkaline solution and rinsed with running water at 1 kgf/cm.sup.2.
As a result, the undercoat layer was removed. The Mg alloy casing
was then left in a drying chamber for drying. Thus, the coating
formed on the flat portion 1, the side walls 2, the retreated
portion 3 and the irregular portion 4 of the casing shown in FIG. 2
was properly removed by the immersion which took 50 minutes in
total.
[0065] Subsequently, the Mg alloy casing which was 30 kg after the
removal of coating and 1 kg of first flux were put into a melting
furnace (60L) heated to about 680.degree. C. in advance.
Thereafter, the temperature in the melting furnace was raised to
720.degree. C. for melting the materials therein. The first flux
used was powder mixture containing 50 wt % MgCl.sub.2, 25 wt % KCl,
5 wt % CaF.sub.2 and 20 wt % BaCl.sub.2. During the melting, the Mg
alloy was stirred with a mechanical impeller rotating at 100 rpm
for providing homogeneous molten metal.
[0066] While the stirring is continued, 0.2 kg of second flux was
added to the molten metal when sludge started to separate from the
molten metal. The second flux used was powder mixture containing
67.5 wt % MgCl.sub.2, 27.5 wt % KCl, 1 wt % CaF.sub.2 and 4.5 wt %
BaCl.sub.2. The second flux was further added by 0.1 kg each in
accordance with the inflammation state at the surface of the molten
metal. The addition of the second flux promoted the separation of
sludge from the molten metal.
[0067] Subsequently, the stirring was stopped and the molten metal
was kept still for 20 minutes, allowing sludge to settle in the
molten metal for cleaning the molten metal. At this time,
immediately after the stirring was stopped, 0.2 kg of third flux
was added to the molten metal. The third flux used was powder
mixture containing 80 wt % sulfur (S) and 20 wt % MgF.sub.2 The
addition of the third flux caused the generation of SF.sub.6
filling the melting furnace, so that anti-oxidation gas layer of
SF.sub.6 was formed to cover the surfaces of the molten metal.
Thus, sludge was settled to a sufficient degree while preventing
the oxidation of the molten metal.
[0068] After the molten metal was thus cleaned, 0.2 kg of molten
metal was extracted, which was cast into a columnar sample which
was 5 cm in diameter and 5 cm in length. Then, this sample was
subjected to component analysis using an arc emission
spectrophotometer (Tradename:PDA-5500II, Supplier:Shimadzu
Corporation). As a result, the Fe content was found to be 0.0058 wt
% whereas the Mn content was found to be 0.120 wt %.
[0069] Subsequently, to decrease the Fe content while increasing
the Mn content, Al--Mn intermetallic compound serving as both
Fe-settling agent and Mn-supplying medium was put into the melting
furnace. Specifically, to provide an Mg alloy for forging
equivalent to AZ91D (Fe: no more than 0.004 wt %, Mn: 0.17-0.4 wt
%), 4 kg of Al--Mn intermetallic compound was added. After the
component control was performed in this way, the molten metal was
stirred and then kept still for 10 minutes for cleaning.
[0070] Subsequently, 0.2 kg of molten metal was extracted, which
was cast into a columnar sample which was 5 cm in diameter and 5 cm
in length. Then, this sample was subjected to component analysis
using an arc emission spectrophotometer (Tradename: PDA-5500II,
Supplier: Shimadzu Corporation). As a result, it was found that the
Fe content decreased to 0.0015 wt % whereas the Mn content
increased to 0.210 wt %. Thus, the molten metal having the desired
composition was obtained. Then, the molten metal was cast into five
ingots (5 kg). In this way, a recycled Mg alloy was obtained from a
coated Mg alloy casing of a notebook computer.
[0071] [Measurement of Bending Strength]
[0072] From the recycled Mg alloy obtained in the above-described
manner, five samples corresponding to JIS Z 2204 1 (10 mm.times.50
mm.times.3.2 mm) were prepared by die casting for measuring the
bending strength. Specifically, the measurement was performed with
respect to each of the samples by three-point bending test based on
JIS K 7055 using an universal testing machine (Tradename:
INSTORON5581, Supplier: Instron Japan Co Ltd). In the measurement,
with the sample supported at two support points spaced from each
other by 40 mm (span:40 mm), load was applied to the sample at the
generally middle of the span while moving at a speed of 2 mm/min.
The three-point bending test was performed under the same condition
also with respect to five samples corresponding to JIS Z 2204 1
obtained from a virgin material (AZ91D). As a result, the recycled
Mg alloy sample was found to have an average bending strength of
about 400 MPa, which was generally equal to that of the virgin
material. The difference between the maximum value and the minimum
value was about 13%.
[0073] [Corrosion Test]
[0074] From the recycled Mg alloy obtained in the above-described
manner, five samples corresponding to JIS Z 2204 1 (10 mm.times.50
mm.times.3.2 mm) were prepared by die casting. With respect to
these samples, corrosion resistance was measured by the salt spray
test based on JIS Z 2371. The test was performed under the same
condition also with respect to five samples corresponding to JIS Z
2204 1 obtained from a virgin material. As a result, the amount of
corrosion of the recycled Mg alloy was found to be generally equal
to that of the virgin material.
[0075] [Formability Test]
[0076] From the recycled Mg alloy obtained in the above-described
manner, 50 casings (320 mm.times.240 mm.times.1.2 mm) of notebook
computers as shown in FIG. 2 were formed by die casting. Similarly,
50 casings of notebook computers were formed from a virgin
material. As a result, the recycled Mg alloy product was found to
have a formability which was almost equal to that of the virgin
material. For example, the non-defective product rate was about 80%
which was generally equal to that of the virgin material. The
non-defective product rate herein means the rate of products which
are free from cracks or shrink marks in the state of molded-piece
due to short shot of the molten metal and the like and which
therefore have good appearance to meet the requirement as a
product.
EXAMPLE 2
[0077] Similarly to Example 1, the casing of a notebook computer as
shown in FIG. 2 as a coated Mg alloy product was immersed in the
first alkaline solution and then rinsed to remove the overcoat
layer of the coating. Subsequently, the Mg alloy casing was
immersed in 2L of acid solution. The acid solution used contained 5
wt % formic acid as organic acid, 15 wt % benzyl alcohol and 40 wt
% ethylene glycol monobutyl ether. During the immersion, the acid
solution was kept at 40.degree. C. The immersion was performed for
5 minutes. Then, the Mg alloy casing was pulled out from the acid
solution and rinsed with running water at 1 kgf/cm.sup.2. As a
result, the undercoat layer of the coating was removed. The Mg
alloy casing was then left in a drying chamber for drying. Thus,
the coating formed on the flat portion 1, the side walls 2, the
retreated portion 3 and the irregular portion 4 of the casing shown
in FIG. 2 was properly removed by the immersion which took 15
minutes in total.
[0078] Similarly to Example 1, the Mg alloy casing from which the
coating had been removed was then subjected to the process steps
from the melting in the melting furnace through the casting of
ingots of 5 kg. The recycled Mg alloy thus obtained was checked
with respect to the bending strength, corrosion resistance and
formability in the same manner as in Example 1. As a result, the
recycled Mg alloy of this example was found to have characteristics
or physical properties similar to that of Example 1.
COMPARATIVE EXAMPLE 1
[0079] Similarly to Example 1, the casing of a notebook computer as
shown in FIG. 2 as a coated Mg alloy product was immersed in 2L of
first alkaline solution. The first alkaline solution used had the
same composition as that of Example 1. During the immersion, the
first alkaline solution was kept at 70.degree. c. The Mg alloy
casing was then rinsed with running water at 1 kgf/cm.sup.2. For
the sufficient removal of the coating, the immersion in the first
alkaline solution needed be continued for no less than 90 minutes.
If the immersion is stopped after the lapse of about 50 minutes and
the Mg alloy product still carrying the coating is used for making
a recycled Mg alloy, organic toxic gas and a large amount of
precipitates are generated during the melting step and the
resulting alloy has a composition which differs largely from the
desired one. Therefore, the productivity of a recycled Mg alloy is
considerably deteriorated.
EXAMPLE 3
[0080] Two hundred casings of notebook computers as shown in FIG. 2
as coated magnesium alloy products were held on a jig and immersed
in 800L of first alkaline solution stored in a treatment bath (1
m.sup.3) made of stainless steel. The coating formed on the
surfaces of the computer casing was made up of an undercoat layer
mainly composed of epoxy resin and an overcoat layer mainly
composed of urethane resin. The first alkaline solution used
contained 3.5 wt % potassium hydroxide, 2.5 wt % sodium hydroxide,
20 wt % anionic surface active agent and 40 wt % diethylene glycol
monoethyl ether. During the immersion, the first alkaline solution
was kept at 70.degree. C. The immersion was performed for 10
minutes. As a result of the immersion, mainly the overcoat layer of
the coating swelled and partially removed. The casings were then
pulled out from the first alkaline solution and immersed in 800L of
first rinsing water stored in a treatment bath (1 m.sup.3). As a
result, about 90% of the coating was removed from the casings.
[0081] Subsequently, the 200 casings were immersed in 800L of acid
solution stored in a treatment bath (1 m.sup.3). The acid solution
used contained 5 wt % formic acid as organic acid, 15 wt % benzyl
alcohol and 40 wt % ethylene glycol monobutyl ether. During the
immersion, the acid solution was kept at 40.degree. C. The
immersion was performed for 5 minutes. As a result of the
immersion, mainly the undercoat layer of the coating swelled and
was partially removed. The casings were then pulled out from the
acid solution and immersed in 800L of second rinsing water stored
in a treatment bath (1 m.sup.3). As a result, the coating was
entirely removed. Then, the Mg alloy casings were left in a drying
chamber for drying. In this way, the coating formed on the flat
portion 1, the side walls 2, the retreated portion 3 and the
irregular portion 4 of each casing as shown in FIG. 2 was properly
removed by the immersion which took 15 minutes in total. Using the
Mg alloy casing from which the coating had been removed, a recycled
Mg alloy was prepared in the same manner as in Example 1.
[0082] The pieces of coating mainly composed of epoxy resin or
urethane resin were recovered from the first alkaline solution, the
first rinsing water, the acid solution and the second rinsing water
used for removal of the coating. Specifically, filtration was
performed with respect to each of these treatment liquids in which
pieces of coating precipitated or floated, thereby separating the
coating pieces from the liquids. Then, the recovered pieces of
coating were rinsed with water to satisfactorily remove the
treatment liquid adhering thereto. Subsequently, the coating pieces
were left in a drying chamber kept at 80.degree. C. for 2 hours for
drying. Then, the dried pieces of coating were put into a mortar
and pulverized to a particle size of no more than 1 mm. In this
way, about 4 kg of recycled coating material in a pellet state was
obtained from 200 Mg alloy casings.
[0083] Subsequently, the recycled coating pellet and polypropylene
resin were mixed at the mixing ratio of 1:1 for melting using a
twin-screw extruder (Tradename: KZW 15-30MG, Supplier: Technovel
Corporation). The mixing was performed at 220.degree. C. with the
mixing unit rotating at 100 rpm. From the resin compound thus
obtained, flowerpots as shown in FIG. 5 (height:300 mm, opening
diameter:300 mm, wall thickness:2.5 mm) were molded using an
injection molding machine (Tradename: AUTOSHOT, Supplier: Fanuc
Ltd.). The flower pots had good appearance and sufficient strength
for practical use. In this way, the coating of the Mg alloy product
was properly recycled.
* * * * *