Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
34
Applicant: Vacmetal GmbH, Dortmund
Inventor: Ir G.W.M. van Wijk of Delfzijl
Title: Method of Reducing the Sodium Content in
Aluminum and Aluminum Alloys
The invention relates to a method of reducing the Na
content in a bath of liquid aluminum, either alloyed or not,
in particular a magnesium-alloyed aluminum, by means of a melt
treatment, which is known as such, in a vacuum furnace.
The treatment of molten metals in a vacuum furnace
in order to remove undesired elements from the melt, is known.
Two methods are known for the vacuum treatment of aluminum
alloys. The static treatment in which a quantity of metal is
exposed to the vacuum in a furnace, and the dynamic vacuum
treatment in which the metal is sucked as a jet or a spray
into the furnace which has previously been evacuated.
For instance it is known that in a static treatment,
when applying a pressure reduction which is usual for this
purpose, the Na content in a bath of molten aluminum may be
~ reduced from 25 p.p.m. to ~ 8 p.p.m., although this requires
considerable time. In a similar way it also is possible to
remove other residual alloying elements from the molten
aluminum. ~t is also known for instance that the sodium con-
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tent is drastically reduced if aluminum which does not contain
~ magnesium is subjected to a dynamic treatment.
Usually aluminum is used in the form of an aluminum-
magnesium alloy. In this case the alloy should have an extre-
mely low Na content in order to be suitable for rolling and
pres~ing. ~
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It has now been found particularly that the d~amic
treatment in a vacuum furnace has in practice almost no, or very
little~ effect i~ the treatment is carrîed out on an aluminum-
~agnesium alloy instead of on a magnesiu~-free aluminum. In
practice it has been found that, partly because of physical-
chemical causes which are not yet understood, in the dynamic
treatment of Mg-alloyed Al, it is difficult to get the residual
content of sodium below 15 p.p.m. For alloys to be rolled and
pressed a sodium content of 15 p.p.m. is too high. It is to be
noted that in a static treatment the sodi~m can be removed, ~ut
at the cost of much time and the cost of burnt-of~ magnesium.
The in~ention aims to provide a method by which
the Na content may be further reduced drastically in the
treatment of aluminum and/or its alloys i~ a vacuum furnace.
- More particularly the inve~tion aims to provide
a method ln which an aluminum-magnesium a~loy, with up to 5%
magnesium, may be produced in a melting furnace, whereafter
i . . . . . .
this alloy is sucked into a vacuum ~oundry furnace and is
simultaneously treated ln such a way that as well as the hydrogen
- content the content of sodium is also drastically reduced.
... . .
It is remarked that vacuum furnaces and ~acuum
foundry furnaces and the operation thereof, are sufficiently
generally known that it is not necessary to describe these
installations and methods at length here.
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The invention consists in th~t, in the known
meth~ds, at least during part of the duration of the vacuum
treatment, there is brought to reaction in the vacuum furnace
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i ~ a material which has a greater affinity to sodium than aluminum
andlor its alloying elements. This causes the material supplied
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to form a compound with the sodiu~ either via an intermediate
compound with the liquid aluminum which is present in excess
and/or with magnesium invapour form or not via such an inter-
mediate compound.
Although the invention is not restricted to this,
particularly good results have been obtained when the reac-
ting material is selected from the group comprising halogen
and halogen compounds which have a smaller heat of formation
than their reaction products with sodium. It is also of
importance that the halogen compounds used have a sufficiently
high vapour pressure under the conditions of the process, in
order that they may react with the sodium which escapes from
the stream of metal, and that as a consequence thereof the
vapour pressure of the sodium inside the vacuum furnace i~
kept sufficiently low in order that the sodium may separate
from the melt.
As magnesium is an often used alloy element for
aluminum, the invention in particular relates to AlMg-alloys.
Good results were obtained by using gaseous chlorine.
Particularly good results were also obtained when
employing magnesium-chloride as reaction material, whereby
magnesium in the chloride is replaced by sodium during the re-
action. Instead of MgCl, one or more chlorides of the metals
Si; Fe; Mn and/or Zn may be used successfully. Very good
results however were also obtained when using a chlorinated or
fluorinated hydrocarbon as the reaction material. For instance
;~ carbon tetrachlorid0; hexachloroethane and chlorofluoro car-
bons such as ~hose sold under the trade mark FREON may be used
for this purpose.
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11~3~34
It is remarked that methods are known for producing
aluminum-magnesium with a low Na content, but for ...........
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that purpose aluminum which was ~ree of magnesium had first to
be treated in order to reduce the Na content therein, whereafter
this product was then alloyed with magnesium. Apart from the
fact that this treatment is more complicated than that presently
proposed, it also has the disadvantage that the vacuum furnace
should be absolutely free of magnesium, and also that it is not
possible to feed magnesium-containing scrap into the melting
furnace.
The invention may be applied in the dynamic vacuum
treatment of the melt.
Good results are obtained if the reaction material
is supplied into a vacuum furnace in solid or liquid form, and
that therea~ter the molten metal is sucked in under vacuum. It
has also been found to be feasible according to the invention
to supply the reaction material to the jet of molten metal while
this jet enters the evacuated vacuum furnace. In this so-called
dynamic treatment of the metal, powdery, liquid and gaseous
material may be supplied for the reaction. Apart from supplying
the reaction material to the ~et stream in a dynamic vacuum
treatment, it is also possible to dose the reacting material
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into the vacuum furnace directly durin~ the vacuum treatment.
For this purpose the furnace should be pro~ided with a dosing
apparatus.
` In all these cases a ~urther improvement is obtained
; by atomizing the metal as it enters the va~uum furnace, which
creates a greater reaction sur~ace. The invention has been
explained above ~or the purpose of reducing the Na content in
the liquid alloy, However, it ~s found that the method is also
very suitable for simultaneously removing minor quantities
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~3~34
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of elements such as lithium; calcium; potassium and/or barium.
It is remarked that the invention also comprises
application to an alloyed aluminum. The most remarkable results
have, however, been obtained by application to AlMg alloys,
as in that case the dynamic vacuum treatment alone gives a less
favorable result. Also it is remarked that chlorination of
molten aluminum without vacuum is known per se. The mechanism
which occurs in this is quite a different one, however, and in
combination with a vacuum treatment a clear synergistic effect
is obtained.
Apart from the above described method the
invention also relates to a magnesium-containing aluminum alloy
when treated according to the method described.
.
The invention will be described below with
reference to two examples.
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The initial material was an aluminum alloy which
contained 4% magnesium. This alloy contained 45 p.p.m. sodium
be~ore the vacuum treatment.
A quantity of MgCl sufficient to combine with the
sodium contained in the alloy was supplied first to a vacuum
furnace. Thereafter the alloy was sucked into the vacuum furnace
at a rate of 16 tons per hour. A pressure of less than ~0 torr
was maintained in the vacuum furnace whereby sodium was
eliminat:ed from the jet by means of the MgCl vapour during the
sucking in. Directly after the treatment, a sodium content of
about 4 p.p.m. was found. The product obtained was very
suitable for rolling and pressing purposes.
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ample 2
In this example, the initial material was an
aluminum alloy containing 2% magnesium. During the dynamic
vacuum treatment in the vacuum furnace, powdery MgCl was
injected in a quantity of 500 gr/ton of treated aluminum.
After the vacuum treatment the Na content of the metal was found
to have been reduced *rom 30 p.p.m. down to 6 p.p.m.
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