Year: 1889

page_text: I, CHARLES MARTIN HALL, of Oberlin, in the County of Lorain, State of Ohio, and
United States of America, Chemist, do hereby declare the nature of my said invention
and in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
The invention described therein relates to certain improvements in the reduction of
aluminium from alumina and also the simultaneous production of alloys of aluminium
with other metals. The method of reducing aluminium generally stated consists in
fusing together the fluorides of aluminium and potassium in a suitable crucible,
dissolving alumina in the bath or solvent and then separating the material held in
solution into its constituent elements aluminium and oxygen by passing an electric
current through the solution. When pure or practically pure aluminium is desired
the electric current is introduced into the bath by means of a positive electrode formed
of copper or other suitable material and a carbon negative electrode.
When an alloy
is desired the negative electrode is formed of the metal with which it is desired to
alloy the aluminium, the positive electrode being formed of copper, platinum or other
suitable material. The bath or solvent may be rendered more fusible and its capacity
for dissolving alumina increased by the addition thereto of suitable proportions of the
fluoride of lithium. In the production of alloys the fluoride of sodium, or of other
metals more electro-positive than aluminium may be substituted for the fluoride of
potassium.
In the accompanying drawings forming a part of this specification, Fig. 1 is a view
partly in section and partly in elevation of a form of apparatus applicable in the
practice of my invention, and; Fig. 2 is a view in perspective, of a modified form of
the apparatus, a portion being broken away to show the manner of connecting one of
the conductors to the carbon lining when using the latter as the negative electrode.
In the practice of my invention, 1 form a fused bath or solvent for the alumina
by melting in a crucible a combination of the salts known as the fluoride of aluminium
and the fluoride of potassium. This combination, which may also be termed the
double fluoride of aluminium and potassium is preferably formed by mixing together

page_text: 169 parts of aluminium fluoride and 116 parts of potassium fluoride, such proportions
of the ingredients corresponding to the formula K,AF. A variation in these pro-
portions within certain limits, produces only immaterial changes in operativeness of my
process, as for example (as I now believe the fact to be) a larger proportion of
potassium fluoride increases the capacity of the bath for dissolving alumina at the
same time lessens its fusibility, whereas a larger proportion of aluminium fluoride
renders the bath more fusible, but decreases its capacity for dissolving alumina.
The combination of the fluorides of aluminium and potassium may be rendered more
fusible and its capacity for dissolving alumina may be increased by the addition of
lithium fluoride or a partial substitution thereof for potassium fluoride. As for
example, the combinations of the fluorides of aluminium, potassium and lithium
represented by the formulas K Li Al, F, or K, Li, Al F₁s are effective as regards their
capability for dissolving alumina and are quite fusible. These compounds or com-
binations of the fluorides of aluminium, potassium and lithium can be conveniently
made by saturating and neutralizing with hydrofluoric acid suitable proportions of
hydrate of alumina and carbonate of potassium or lithium.
18
On account of the affinity which aluminium has for other metals, and also the
corrosive action of fluorides on earthy materials, the crucible 2 is formed of metal as
iron, steel, copper, etc., and protected from the action of the aluminium by a carbon
lining 7. In lining the crucible any mixture of carbon and a carbonaceous binding
material, as coal tar, sugar or caking bituminous coal, used in the arts in manufac-
turing carbons for electrical purposes, may be employed. The bottom of the
crucible is first covered with a stiff paste formed of gas-carbon or other suitable
form of carbon and coal tar, a wooden mold or core is then placed in the crucible,
the diameter of the-core being, smaller than, the inside diameter of the crucible, so
as to leave an open space between the core and crucible equal to the thickness of lining
desired. The carbonaceous paste is then packed tightly into the space between the core
and crucible which is then placed in a suitable furnace or oven and-baked at a red heat,
thereby bardening the paste and so charring or partially burning the surface of the
core, as to permit of its easy removal. In order to render the lining more compact.
and durable I prefer to soak the lining with coal tar and bake it at a red heat
repeatedly. Care should be taken to carefully exclude all earthy materials from the
carbonaceous material forming the lining for the reason that the solution will dissolve
any earthy material with which it comes in contact, and would therefore, in case of
the presence of such material in the lining, form holes therethrough and thus permit
the aluminium to come into contact with the metal.
The crucible 2 containing the fluorides of aluminium and potassium, with or
without the fluoride of lithium, as hereinbefore described, is placed in a suitable
furnace B and heated sufficiently to melt the bath, i.e. to or approximately to a low
red heat. The electrodes 3 and 4 having suitable connections by conductors a and b
with a dynamo electric machine A or other suitable source of electric energy, are
then inserted into the bath and a sufficient amount of alumina placed therein. The
alumina is dissolved by the bath, and by the action of the electric current alumina is
reduced at the negative electrode 4 and being melted sinks down to the bottom of the
crucible. In making pure aluminium the negative electrode 4 should be formed of
carbon or metal coated with carbon so as to protect the metal from the aluminium,
which has a great affinity for other metals. The oxygen goes to the positive
electrode 3 which may be formed of copper, platinum or other suitable material.
When the positive electrode is formed of copper it soon becomes coated with an
oxide of copper, which, while not materially affecting its conducting qualities under
these conditions, forms a protecting covering over the surface of the copper as against
the action of the oxygen, such oxygen thereafter escaping in a free state at the
positive electrode. The aluminium may be removed from the crucible as it accu-
mulates by any suitable means or the contents of the crucible may be poured out
and cooled, and the aluminium then separated from the solvent.
In the manufacture of aluminium alloys, the negative electrode 4 is formed of the
inetal with which it is desired to alloy the aluminium, which is reduced at the

page_text: negative electrode and being fused by the heat of the bath, takes up a portion of the
electrode and forming an alloy, sinks to the bottom of the crucible.
A large number of the double fluorides of aluminium and metals more electro-
positive than aluminium, may be substituted in the manufacture of alloys for the
fluoride of potassium, as for example combinations of the fluoride of aluminium with
the fluoride of sodium, potassium, lithium, barium, strontium or calcium. Good
results are obtained in the manufacture of aluminium bronze by employing a salt
composed of the fluorides of sodium and aluminium in the proportions represented
by the formula Na, Al, F, or corresponding salts of the fluorides of aluminium and
potassium or lithium, strontium, barium or calcium, the negative electrode being
formed of copper.
The double fluoride of aluminium and barium, calcium, or lithium may be added
to the combination of the fluoride of aluminium and the fluoride of sodium repre-
sented by the formula Na, Al, Fs, thus the lithium salt Li, Al, F, if added in amounts
up to about one third of the total mass lowers the melting point and so does also the
potassium salt K, Al, F, if added in amounts up to a little more than half the mass.
When an electric current is passed through the bath, alumina being dissolved
therein, oxygen goes to the positive electrode, and if such electrode is formed of
copper, forins a thin protecting coating of copper oxide on its surface. After the
positive electrode is thus coated, it acts like a platinum electrode evolving free oxygen
gas as electrolysis continues. Aluminium is reduced by the current at the negative
copper electrode and having a strong affinity for the copper forms with it an alloy
which drops to the bottom of the crucible in a fused state. This alloy is of a white
crystalline character and being melted with more copper forms the well known
aluminium bronze, which consists of copper with about ten per cent. more or less of
aluminium.
.
In the above described method of forming an alloy of aluminium and copper it is
not essential that the positive electrode should be formed of copper. Although
copper is preferable, a carbon positive electrode may be used, which, however, is
consumed and must be renewed from time to time. It is advantageous to use a copper
positive electrode as it is not consumed and a purer aluminium copper alloy pro-
duced, than is possible when using carbon which gives off an ash as it is consumed.
And further the bath works clearer and better when a positive copper oxide-coated-
electrode is employed.
By substituting iron for copper in forming the negative electrode, a valuable alloy
of aluminium and iron is formed, the process being the same in other respects as
described in relation to the production of the aluminium-copper alloy. If desired
the positive electrode may be forined of iron, which will become coated with a thin
protecting coating of iron oxide and act in the same manner as a copper electrode
under like circumstances. The alloy thus produced consisting of about 60 to 90%
of aluminium, the remainder being principally iron, is valuable for use in the manu-
facture of steel and iron.
In the manufacture of the aluminium and iron alloy I prefer to form the bath or
solvent of the fluorides of aluminium, sodium and calcium in the proportions repre-
sented by the formula Na, Al, F. + Ca Al, F, and to form the positive electrode
of carbon. The impurities which come from the carbon anode do not injure the alloy
as is the case when manufacturing the aluminous copper alloy. Beauxite may be
advantageously employed in the manufacture of the alloy of aluminium and iron, as
the silicon and iron contained therein do not injure the resulting alloy for the purposes
of iron or steel manufacture.
In the manufacture of alloys the metal cathode is not simply a substitute for or an
equivalent of the carbon cathode used in the production of pure aluminium, for the reason
that, while transmitting the current as does also the carbon cathode, the metal cathode
effects a further and additional function or office i.e. that of presenting a metal with
which the aluminium can combine as fast as it is reduced, by this it is secured that a
much larger amount of aluminium is reduced by the same current or at least remains
reduced, when it goes directly into an alloy than when it is reduced unalloyed. This

page_text: is on account of the affinity which aluminium has for other metals, such affinity
rendering it possible to reduce aluminium into. its alloys under conditions where it
cannot be reduced pure as in the electric furnace.
The method of manufacturing aluminium alloys herein described is not limited to
baths or solvents specifically mentioned but can be carried out with any bath formed
of the fluorides of aluminium and any metal more electro-positive than aluminium,
which dissolves alumina.
Having now particularly described and ascertained the nature of my inven
tion and in what manner the same is to be performed, I declare that what I
claim is:
1. As an improvement in the art of manufacturing aluminium, the herein described
method, which consists in dissolving alumina in a fused bath composed of the fluorides
of aluminium and potassium, and then passing an electric current through the fused
mass, substantially as set forth.
2. As an improvement in the art of manufacturing alloys of aluminium with other
metals, the method herein described, consisting in dissolving alumina in a fused bath
of the double fluorides of aluminium and a metal more electro-positive than aluminium
and passing an electric current through the fused mass by means of electrodes sub-
merged therein, the negative electrode. being formed of the metal with which the
aluminium is to be alloyed, substantially as set forth.
3. As an improvement in the art of manufacturing alloys of aluminium with copper,
the method berein described, which consists in dissolving alumina in a fused bath of
the double fluoride of aluminium and a metal more electro-positive than aluminium,
and then passing an electric current through the fused mass by means of copper
electrodes, substantially as set forth.
4. In an apparatus for the manufacture of aluminium, the combination of a crucible
adapted to contain a suitable solvent and electrodes arranged in the crucible and con-
nected to an electric generator, the positive electrode being formed of copper and
coated with an oxide thereof, substantially as set forth.
5. In an apparatus for the manufacture of aluminium, the combination of a crucible
formed of a metal casing and a carbon lining, a solvent formed of the fluorides of
aluminium and a metal more electro-positive than aluminium contained in said
crucible, and electrodes arranged in the solvent and connected to an electric generator,
substantially as set forth.
Disund at Dittsburgh Pa ILSA this Twelfth day of February A.D. 1889.
Witnesses
DARWIN S. WOLCOTT,
J. SNOWDEN BELL
CHARLES MARTIN HALL.