Improvements in the Production of Aluminium.
Year: 1889

page_text: Date of Application, 2nd Apr., 1889-Accepted, 1st June, 1889'
Improvements in the Production of Aluminium.
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
The invention described herein relates to certain improvements in the method of
reducing aluminium from alumina, and generally stated, consists in dissolving alumina,
in a fused bath composed of the fluoride of aluminium and the fluoride of an alkaline
earth metal, and then passing an electric current through the fused mass, thereby
separating the alumina into its constituent elements, oxygen and aluminiuin.
alkaline earth metals, which I have found applicable in the practice of my invention,
are calcium, barium, and strontium. The fluoride of either of these metals is mingled in
suitable proportions with the fluoride of aluminium with or without other ingredients,
and the compound thus formed melted in a suitable vessel preferably one formed of a
metal shell lined with carbon. Electrodes connected to a suitable electric generator
are then placed in the bath and alumina added in sufficient quantities to form a saturated
solution. By the action of the electric current the alumina held in solution is
separated into its constituents, the aluminium going to the negative electrode and the
oxygen to the positive electrode. These electrodes if pure aluminium is desired
are formed of carbon, but if an alloy is desired, the negative electrode is formed
of the metal with which the aluminium is to be alloyed, the positive electrode being
formed of carbon. The bath or solvent herein before mentioned may be altered by
mixing therewith the fluoride of sodium, to which may be added the chloride of
calcium, thereby increasing the effectiveness of the bath or solvent.
In the accompanying drawings forming a part of this specification, Fig. 1 is a view
partly in elevation and partly in section of the apparatus employed in carrying out
iny invention; Fig. 2 is a sectional view of a modified form of crucible, and, Fig. 3
is a perspective view of another form of crucible a part of the wall being broken away
to illustrate the negative conductor connected to the carbon lining.

page_text: 8°
In carrying out my invention I employ a double fluoride of aluminium and an
alkaline earth metal, such as calcium, having a composition represented by the
formula Ca Al, Fg. This composition is formed by 169 parts of aluminium fluoride
to 78 parts of calcium fluoride or fluorspar, the aluminium fluoride being artificially
prepared by saturating hydrated alumina with hydrofluoric acid. The flourides of
calcium and aluminium unite in forming a double fluoride more fusible than the
fluoride of calcium, and as I believe more fusible than either constituent alone.
12
While I prefer to use the double fluoride of calcium and aluminium represented by
the formula Ca Al, Fs in carrying out my process, there is a large number of similar
double fluorides of the same metals which may be used and which closely resemble
the one named and answer almost, if not quite as well as a bath, for dissolving the
alumina. Thus the double salt of the formula Ca, Al, F₁ has been successfully
employed in the electrolytic reduction of aluminium. Whether the bath employed
contains proportionately more or less aluminium fluoride within wide limits seeins
to make little difference. In lieu of forming the bath or solvent of the double
fluorides of an alkaline earth metal and aluminium alone as herein before stated, the
In such cases I form the bath or solvent of
fluoride of sodium may be added.
the fluorides of an alkaline earth metal (as calcium) sodium and aluminium, the
fluorides of calcium and sodium being obtained in the form of fluorspar and cryolite
respectively and the fluoride of aluminium being obtained partly from the cryolite
and partly, as hereinbefore stated, by saturating hydrated alumina (Al, (HO)) with
hydrofluoric acid. The production of the materials employed in forming this bath
or solvent are approximately as follows: Huoride of calcium 234 parts, cryolite (the
double fluoride Na, Al, F₁) 421 parts, and fluoride of aluminium 507 parts by
weight. The proportions of this compound, which is represented approximately by
the formula Na Al Fis + Ca, Al, F1, can however, be widely varied without
materially changing the efficiency of the bath or solvent.
In the practice of my invention, either of the baths or solvents hereinbefore
described are placed in a crucible 2 arranged over a suitable furnace B. On account.
of the permeating quality of the bath or solvent and the affinity of aluminium for
other metals, it is necessary to employ a crucible constructed not only to prevent
the escape of the bath, but also the pollution of the aluminium. To these ends
the crucible is formed of a suitable metal capable of withstanding considerable heat
and then lined with pure carbon, as at 7, such lining being impermeable by the
aluminium and not affected by the salts composing the bath or solvent, or by the
aluminium. 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
manufacturing 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 that 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 hardening the paste and so charring or partially
In order to render
burning the surface of the core, as to permit of its easy removal.
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 there-
through and thus permit the aluminium to come into contact with the metal.
The materials composing the bath or solvent. having been fused and thoroughly
mingled, electrodes 3 and 4 formed of carbon when pure aluminium is desired and
connected by conductors a and b to a suitable electric generator as for example a
If an alloy is desired the
dynamo electric machine A, are then arranged in the bathi.

page_text: negative electrode 4 is formed of the metal with which it is desired to alloy the
aluminium. The carbon lining 7 of the crucible may be employed as the negative
electrode if desired as shown in Fig 3 where the conductor bis arranged between the
carbon lining and the metallic shell. Alumina in the form of beauxite, anhydrous
oxide of aluminium or any other suitable form of alumina, preferably the .pure
anhydrous oxide Al, O,, artificially prepared, is then dissolved in the fused bath and
being subjected to the action of the electric current, aluminium is reduced at the
negative electrode 4 and oxygen is liberated at the positive electrode 3, and combining
therewith when formed of carbon, forms carbonic acid gas or carbonic oxide, the
carbon being gradually consumed.
When the bath composed of the fluorides of aluminium and calcium or other
alkaline earth metal is employed, the solution formed by the addition of alumina is
apparently heavier than the metal aluminium, and hence this metal if unalloyed rises
after being reduced and melted and floats on the surface of the bath, where it is liable
to loss by oxidation on contact with the air, or with the positive electrode, where it is
subjected to a strong oxidizing action. Hence when reducing pure aluminium the
crucible is provided with a cover 5 provided with openings for insertion of the electric
conductors and the escape of gas from the crucible and with a partition 6 of carbon
extending down into the bath and from side to side of the crucibles and separating the
two electrodes as shown in Fig. 2. As the aluminium is reduced at the negative
electrode alone, it will rise on the same side of the partition with said electrode, and
hence will be protected by the cover and partition from any oxidizing influences the
carbonic acid gas and carbonic oxide being formed on the opposite side of the
partition. As the operation continues the cover is removed from time to time and
the aluminium removed from the surface of the bath. When employing the bath
composed of the fluorides of aluminium and calcium or other alkaline earth metal, for
the production of alloys of aluminium with iron, copper or other metals, the negative
electrode is formed of iron, copper or other metal, with which the alloy is to be
formed. The aluminium which, as above stated is reduced at the negative electrode,
takes up a portion thereof, and, if the electrode is formed of a heavy metal the alloy
thus formed sinks to the bottom of the crucible, and will be protected by the bath from
oxidation. Hence the partition may be dispensed with in forming alloys with heavy
metals.
It may be stated in this connection that the specific gravity of the bath composed of
the fluorides of aluminium and calcium inay be lowered by the addition to it of other
salts lighter than these double fluorides, so that pure aluminium will sink in the
combination and thus be protected fron oxidation. Thus by the addition to this bath
of about two thirds of its weight of the double fluorides of potassium and
aluminium (K, Al, F), which is comparatively a very light salt, is formed a
composition lighter than aluminium, which will therefore sink when reduced and be
thoroughly protected.
When using the bath composed of the fluorides of aluminium, sodium and calcium,
the aluminium will be reduced, as before stated, at the negative electrode, and being
melted by the heat of the bath will sink down to the bottom of the crucible, the bath
being of less specific gravity than the aluminium.
During the reduction of the aluminium, the positive electrode 3, when formed of
carbon is slowly consumed and must be renewed from time to time, but the bath or
solvent remains unchanged for a long time. A partial clogging of the bath formned of
the fluorides of aluminium, sodium, and an alkaline earth metal, such as calcium,
when mixed in certain proportions will occur in time. This clogging without
rendering the bath wholly inefficient necessitates an increase in the electro-motive
force of the reducing current, the resistance of the bath being increased in proportion
to the degree to which the bath becomes clogged, thereby increasing the cost of
reduction. In order to entirely prevent any clogging of this bath, I add approximately
3 or 4 per cent. more or less of calcium chloride to the bath or solvent, thereby
preventing any clogging or increase of resistance in the bath. Hence a bath
composed of the fluorides of aluminium, sodium and an alkaline earth metal, such

page_text: as calcium and the chloride of calcium, can be used continuously without renewals or
additions, except such as may be needed to replace any loss by evaporation, and
without increasing the electro-motive force of the reducing current. And further the
addition of the calcium chloride enables each atom of carbon of the positive electrode
to take up two atoms of oxygen forming CO, thereby reducing the amount of carbon
consumed in proportion to the amount of aluminium produced. The calcium chloride
being quite volatile, is subjected to loss faster than the rest of the bath and must be
renewed from time to time on this account.
The double fluoride of aluminium and the alkaline earth metals calcium, strontium
and barium, resemble each other very closely and may be employed interchangeably
in this process, as for example the salts represented by the formulas Ca Al F, Sr AlF and
Ba Al F, may be employed in the same way, and the salts of strontium and barium as
well as other double fluorides of strontinm and aluminium, and barium and aluminium
may be used in the manner stated in regard to the double fluoride of calcium and
aluminium. The salts of strontium and barium are heavier than the calcium salt but
more fusible, the salt of barium being the most fusible, and appear especially well
adapted for the reduction of aluminium from alumina dissolved in them, into its alloys
with heavy metals, especially copper, the negative electrode being formed of the metal
with which the aluminium is to be alloyed. In this connection it may be stated that
the fluoride of magnesium, an alkaline earth metal, may be employed in combination
with the fluorides of aluminium and sodium in forming the bath or solvent for the
reduction of aluminium, as herein before described.
In reducing aluminium as above described I prefer to employ an electric current of
about six volts electro-motive force, but the electro-motive force can be varied within
large limits.
As the aluminium is reduced and collects in a molten state at the bottom of the
crucible, it can be removed by means of a dipper or ladle covered with carbon to
prevent any pollution of the aluminium or a suitable outlet may be provided at the
bottom of the crucible for drawing off the aluminium from time to time. The
alumina is charged into the bath from time to time or continuously so as to maintain
the bath or solvent in a saturated condition. The addition of more alumina than can
be dissolved at one time is not detrimental, provided the bath is not chilled, as such
excess will sink to the bottom and be taken out as required. In this manner the
operation may be carried on continuously without change or renewal of the bath,
except such additions as may be required to replace loss due to volatilization and
other causes. As the chloride of calcium volatilizes more rapidly than the other
ingredients, a more frequent renewal thereof is necessary to preserve the required
percentage. The necessity of replenishing the chloride of calcium is indicated by a
falling off of the electric current due to an increased resistance in the bath and also
by the character of the gas given off at the positive electrode, carbonic acid gas being
evolved when a proper percentage of the chloride is present in the bath, while carbonic
oxide indicates a low percentage of the chloride.
Having now particularly described and ascertained the nature of my said 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 method herein
described, which consists in fusing a combination of the fluoride of aluminium
and the fluoride of an alkaline earth metal, adding alumina to the bath so formed
and passing a current of electricity through the fused mass, substantially as set
forth.
2. As an improvement in the art of manufacturing aluminium, the method herein
described, which consists in fusing a combination of the fluoride of alumini um, the
fluoride of sodium and the fluoride of an alkaline earth metal, adding alumina to the
bath so formed and passing a current of electricity through the fused mass, substantially
as set forth.
3. As an improvement in the art of manufacturing aluminium, the method herein

page_text: described, which consists in fusing a combination of the fluoride of aluminium, the
fluoride of sodium, the fluoride of an alkaline earth metal and the chloride of calcium,
adding alumina to the bath so formed and then passing a current of electricity through
the fused mass, substantially as set forth.
4. As an improvement in the art of manufacturing aluminium, the method herein
described, which consists in fusing a combination of the fluoride of aluminium and the
fluoride of calcium, adding alumina to the bath so formed and passing a current of
electricity through the fused mass substantially as set forth.
5. As an improvement in the art of manufacturing aluminium, the method herein
described, which consists in fusing a combination of the fluoride of aluminium, the
Huoride of sodium and the fluoride of calcium, adding alumina to the bath so formed
and passing a current of electricity through the fused mass, substantially as set
forth.
Signed at Pittsburgh Po ILS A this TILL J
Witnesses,
DARWIN S. WOLCOTT,
J. SNOWDEN BELL.
CHARLES MARTIN HALL.