[1] In the _Annals of Philosophy_ for November 1821 is a long article
entitled "Electromagnetism" by Oersted, in which he gives a detailed
account of his discovery. He had his thoughts turned to it as far
back as 1813, but not until the 20th of July 1820 had he actually
made his discovery. He seems to have been arranging a compass needle
to observe any deflections during a storm, and placed near it a
platinum wire through which a galvanic current was passed.
[2] See _Trans. Soc. Arts_, 1825, 43, p. 38, in which a figure of
Sturgeon's electromagnet is given as well as of other pieces of
apparatus for which the Society granted him a premium and a silver
medal.
[3] See S.P. Thompson, _The Electromagnet_ (London, 1891); J.A.
Fleming, _A Handbook for the Electrical Laboratory and Testing Room_,
vol. 2 (London, 1903); J.A. Ewing, _Magnetic Induction in Iron and
other Metals_ (London, 1903, 3rd ed.).
ELECTROMETALLURGY. The present article, as explained under
ELECTROCHEMISTRY, treats only of those processes in which electricity is
applied to the production of chemical reactions or molecular changes at
furnace temperatures. In many of these the application of heat is
necessary to bring the substances used into the liquid state for the
purpose of electrolysis, aqueous solutions being unsuitable. Among the
earliest experiments in this branch of the subject were those of Sir H.
Davy, who in 1807 (_Phil. Trans._, 1808, p. 1), produced the alkali
metals by passing an intense current of electricity from a platinum wire
to a platinum dish, through a mass of fused caustic alkali. The action
was started in the cold, the alkali being slightly moistened to render
it a conductor; then, as the current passed, heat was produced and the
alkali fused, the metal being deposited in the liquid condition. Later,
A. Matthiessen (_Quarterly Journ. Chem. Soc._ viii. 30) obtained
potassium by the electrolysis of a mixture of potassium and calcium
chlorides fused over a lamp. There are here foreshadowed two types of
electrolytic furnace-operations: (a) those in which external heating
maintains the electrolyte in the fused condition, and (b) those in which
a current-density is applied sufficiently high to develop the heat
necessary to effect this object unaided. Much of the earlier
electro-metallurgical work was done with furnaces of the (a) type, while
nearly all the later developments have been with those of class (b).
There is a third class of operations, exemplified by the manufacture of
calcium carbide, in which electricity is employed solely as a heating
agent; these are termed _electrothermal_, as distinguished from
_electrolytic_. In certain electrothermal processes (e.g. calcium
carbide production) the heat from the current is employed in raising
mixtures of substances to the temperature at which a desired chemical
reaction will take place between them, while in others (e.g. the
production of graphite from coke or gas-carbon) the heat is applied
solely to the production of molecular or physical changes. In ordinary
electrolytic work only the continuous current may of course be used, but
in electrothermal work an alternating current is equally available.
_Electric Furnaces._--Independently of the question of the application
of external heating, the furnaces used in electrometallurgy may be
broadly classified into (i.) arc furnaces, in which the intense heat of
the electric arc is utilized, and (ii.) resistance and incandescence
furnaces, in which the heat is generated by an electric current
overcoming the resistance of an inferior conductor.
Arc furnaces.
Excepting such experimental arrangements as that of C.M. Despretz
(_C.R._, 1849, 29) for use on a small scale in the laboratory, Pichou in
France and J.H. Johnson in England appear, in 1853, to have introduced
the earliest practical form of furnace. In these arrangements, which
were similar if not identical, the furnace charge was crushed to a fine
powder and passed through two or more electric arcs in succession. When
used for ore smelting, the reduced metal and the accompanying slag were
to be caught, after leaving the arc and while still liquid, in a hearth
fired with ordinary fuel. Although this primitive furnace could be made
to act, its efficiency was low, and the use of a separate fire was
disadvantageous. In 1878 Sir William Siemens patented a form of
furnace[1] which is the type of a very large number of those designed by
later inventors.
In the best-known form a plumbago crucible was used with a hole cut in
the bottom to receive a carbon rod, which was ground in so as to make
a tight joint. This rod was connected with the positive pole of the
dynamo or electric generator. The crucible was fitted with a cover in
which were two holes; one at the side to serve at once as sight-hole
and charging door, the other in the centre to allow a second carbon
rod to pass freely (without touching) into the interior. This rod was
connected with the negative pole of the generator, and was suspended
from one arm of a balance-beam, while from the other end of the beam
was suspended a vertical hollow iron cylinder, which could be moved
into or out of a wire coil or solenoid joined as a shunt across the
two carbon rods of the furnace. The solenoid was above the iron
cylinder, the supporting rod of which passed through it as a core.
When the furnace with this well-known regulating device was to be
used, say, for the melting of metals or other conductors of
electricity, the fragments of metal were placed in the crucible and
the positive electrode was brought near them. Immediately the current
passed through the solenoid it caused the iron cylinder to rise, and,
by means of its supporting rod, forced the end of the balance beam
upwards, so depressing the other end that the negative carbon rod was
forced downwards into contact with the metal in the crucible. This
action completed the furnace-circuit, and current passed freely from
the positive carbon through the fragments of metal to the negative
carbon, thereby reducing the current through the shunt. At once the
attractive force of the solenoid on the iron cylinder was
automatically reduced, and the falling of the latter caused the
negative carbon to rise, starting an arc between it and the metal in
the crucible. A counterpoise was placed on the solenoid end of the
balance beam to act against the attraction of the solenoid, the
position of the counterpoise determining the length of the arc in the
crucible. Any change in the resistance of the arc, either by
lengthening, due to the sinking of the charge in the crucible, or by
the burning of the carbon, affected the proportion of current flowing
in the two shunt circuits, and so altered the position of the iron
cylinder in the solenoid that the length of arc was, within limits,
automatically regulated. Were it not for the use of some such device
the arc would be liable to constant fluctuation and to frequent
extinction. The crucible was surrounded with a bad conductor of heat
to minimize loss by radiation. The positive carbon was in some cases
replaced by a water-cooled metal tube, or ferrule, closed, of course,
at the end inserted in the crucible. Several modifications were
proposed, in one of which, intended for the heating of non-conducting
substances, the electrodes were passed horizontally through
perforations in the upper part of the crucible walls, and the charge
in the lower part of the crucible was heated by radiation.
The furnace used by Henri Moissan in his experiments on reactions at
high temperatures, on the fusion and volatilization of refractory
materials, and on the formation of carbides, silicides and borides of
various metals, consisted, in its simplest form, of two superposed
blocks of lime or of limestone with a central cavity cut in the lower
block, and with a corresponding but much shallower inverted cavity in
the upper block, which thus formed the lid of the furnace. Horizontal
channels were cut on opposite walls, through which the carbon poles or
electrodes were passed into the upper part of the cavity. Such a
furnace, to take a current of 4 H.P. (say, of 60 amperes and 50 volts),
measured externally about 6 by 6 by 7 in., and the electrodes were about
0.4 in. in diameter, while for a current of 100 H.P. (say, of 746
amperes and 100 volts) it measured about 14 by 12 by 14 in., and the
electrodes were about 1.5 in. in diameter. In the latter case the
crucible, which was placed in the cavity immediately beneath the arc,
was about 3 in. in diameter (internally), and about 3-1/2 in. in height.
The fact that energy is being used at so high a rate as 100 H.P. on so
small a charge of material sufficiently indicates that the furnace is
only used for experimental work, or for the fusion of metals which, like
tungsten or chromium, can only be melted at temperatures attainable by
electrical means. Moissan succeeded in fusing about 3/4 lb. of either of
these metals in 5 or 6 minutes in a furnace similar to that last
described. He also arranged an experimental tube-furnace by passing a
carbon tube horizontally beneath the arc in the cavity of the lime
blocks. When prolonged heating is required at very high temperatures it
is found necessary to line the furnace-cavity with alternate layers of
magnesia and carbon, taking care that the lamina next to the lime is of
magnesia; if this were not done the lime in contact with the carbon
crucible would form calcium carbide and would slag down, but magnesia
does not yield a carbide in this way. Chaplet has patented a muffle or
tube furnace, similar in principle, for use on a larger scale, with a
number of electrodes placed above and below the muffle-tube. The arc
furnaces now widely used in the manufacture of calcium carbide on a
large scale are chiefly developments of the Siemens furnace. But
whereas, from its construction, the Siemens furnace was intermittent in
operation, necessitating stoppage of the current while the contents of
the crucible were poured out, many of the newer forms are specially
designed either to minimize the time required in effecting the
withdrawal of one charge and the introduction of the next, or to ensure
absolute continuity of action, raw material being constantly charged in
at the top and the finished substance and by-products (slag, &c.)
withdrawn either continuously or at intervals, as sufficient quantity
shall have accumulated. In the King furnace, for example, the crucible,
or lowest part of the furnace, is made detachable, so that when full it
may be removed and an empty crucible substituted. In the United States a
revolving furnace is used which is quite continuous in action.
Incandescence furnaces.
The class of furnaces heated by electrically incandescent materials has
been divided by Borchers into two groups: (1) those in which the
substance is heated by contact with a substance offering a high
resistance to the current passing through it, and (2) those in which the
substance to be heated itself affords the resistance to the passage of
the current whereby electric energy is converted into heat. Practically
the first of these furnaces was that of Despretz, in which the mixture
to be heated was placed in a carbon tube rendered incandescent by the
passage of a current through its substance from end to end. In 1880 W.
Borchers introduced his resistance-furnace, which, in one sense, is the
converse of the Despretz apparatus. A thin carbon pencil, forming a
bridge between two stout carbon rods, is set in the midst of the mixture
to be heated. On passing a current through the carbon the small rod is
heated to incandescence, and imparts heat to the surrounding mass. On a
larger scale several pencils are used to make the connexions between
carbon blocks which form the end walls of the furnace, while the side
walls are of fire-brick laid upon one another without mortar. Many of
the furnaces now in constant use depend mainly on this principle, a core
of granular carbon fragments stamped together in the direct line between
the electrodes, as in Acheson's carborundum furnace, being substituted
for the carbon pencils. In other cases carbon fragments are mixed
throughout the charge, as in E.H. and A.H. Cowles's zinc-smelting
retort. In practice, in these furnaces, it is possible for small local
arcs to be temporarily set up by the shifting of the charge, and these
would contribute to the heating of the mass. In the remaining class of
furnace, in which the electrical resistance of the charge itself is
utilized, are the continuous-current furnaces, such as are used for the
smelting of aluminium, and those alternating-current furnaces, (e.g. for
the production of calcium carbide) in which a portion of the charge is
first actually fused, and then maintained in the molten condition by the
current passing through it, while the reaction between further portions
of the charge is proceeding.
Uses and advantages.
For ordinary metallurgical work the electric furnace, requiring as it
does (excepting where waterfalls or other cheap sources of power are
available) the intervention of the boiler and steam-engine, or of the
gas or oil engine, with a consequent loss of energy, has not usually
proved so economical as an ordinary direct fired furnace. But in some
cases in which the current is used for electrolysis and for the
production of extremely high temperatures, for which the calorific
intensity of ordinary fuel is insufficient, the electric furnace is
employed with advantage. The temperature of the electric furnace,
whether of the arc or incandescence type, is practically limited to
that at which the least easily vaporized material available for
electrodes is converted into vapour. This material is carbon, and as its
vaporizing point is (estimated at) over 3500 deg. C., and less than 4000
deg. C., the temperature of the electric furnace cannot rise much above
3500 deg. C. (6330 deg. F.); but H. Moissan showed that at this
temperature the most stable of mineral combinations are dissociated, and
the most refractory elements are converted into vapour, only certain
borides, silicides and metallic carbides having been found to resist the
action of the heat. It is not necessary that all electric furnaces shall
be run at these high temperatures; obviously, those of the incandescence
or resistance type may be worked at any convenient temperature below the
maximum. The electric furnace has several advantages as compared with
some of the ordinary types of furnace, arising from the fact that the
heat is generated from within the mass of material operated upon, and
(unlike the blast-furnace, which presents the same advantage) without a
large volume of gaseous products of combustion and atmospheric nitrogen
being passed through it. In ordinary reverberatory and other heating
furnaces the burning fuel is without the mass, so that the vessel
containing the charge, and other parts of the plant, are raised to a
higher temperature than would otherwise be necessary, in order to
compensate for losses by radiation, convection and conduction. This
advantage is especially observed in some cases in which the charge of
the furnace is liable to attack the containing vessel at high
temperatures, as it is often possible to maintain the outer walls of the
electric furnace relatively cool, and even to keep them lined with a
protecting crust of unfused charge. Again, the construction of electric
furnaces may often be exceedingly crude and simple; in the carborundum
furnace, for example, the outer walls are of loosely piled bricks, and
in one type of furnace the charge is simply heaped on the ground around
the carbon resistance used for heating, without containing-walls of any
kind. There is, however, one (not insuperable) drawback in the use of
the electric furnace for the smelting of pure metals. Ordinarily carbon
is used as the electrode material, but when carbon comes in contact at
high temperatures with any metal that is capable of forming a carbide a
certain amount of combination between them is inevitable, and the carbon
thus introduced impairs the mechanical properties of the ultimate
metallic product. Aluminium, iron, platinum and many other metals may
thus take up so much carbon as to become brittle and unforgeable. It is
for this reason that Siemens, Borchers and others substituted a hollow
water-cooled metal block for the carbon cathode upon which the melted
metal rests while in the furnace. Liquid metal coming in contact with
such a surface forms a crust of solidified metal over it, and this crust
thickens up to a certain point, namely, until the heat from within the
furnace just overbalances that lost by conduction through the solidified
crust and the cathode material to the flowing water. In such an
arrangement, after the first instant, the melted metal in the furnace
does not come in contact with the cathode material.
Aluminium alloys.
_Electrothermal Processes._--In these processes the electric current is
used solely to generate heat, either to induce chemical reactions
between admixed substances, or to produce a physical (allotropic)
modification of a given substance. Borchers predicted that, at the high
temperatures available with the electric furnace, every oxide would
prove to be reducible by the action of carbon, and this prediction has
in most instances been justified. Alumina and lime, for example, which
cannot be reduced at ordinary furnace temperatures, readily give up
their oxygen to carbon in the electric furnace, and then combine with an
excess of carbon to form metallic carbides. In 1885 the brothers Cowles
patented a process for the electrothermal reduction of oxidized ores by
exposure to an intense current of electricity when admixed with carbon
in a retort. Later in that year they patented a process for the
reduction of aluminium by carbon, and in 1886 an electric furnace with
sliding carbon rods passed through the end walls to the centre of a
rectangular furnace. The impossibility of working with just sufficient
carbon to reduce the alumina, without using any excess which would be
free to form at least so much carbide as would suffice, when diffused
through the metal, to render it brittle, practically restricts the use
of such processes to the production of aluminium alloys. Aluminium
bronze (aluminium and copper) and ferro-aluminium (aluminium and iron)
have been made in this way; the latter is the more satisfactory product,
because a certain proportion of carbon is expected in an alloy of this
character, as in ferromanganese and cast iron, and its presence is not
objectionable. The furnace is built of fire-brick, and may measure
(internally) 5 ft. in length by 1 ft. 8 in. in width, and 3 ft. in
height. Into each end wall is built a short iron tube sloping downwards
towards the centre, and through this is passed a bundle of five 3-in.
carbon rods, bound together at the outer end by being cast into a head
of cast iron for use with iron alloys, or of cast copper for aluminium
bronze. This head slides freely in the cast iron tubes, and is connected
by a copper rod with one of the terminals of the dynamo supplying the
current. The carbons can thus, by the application of suitable mechanism,
be withdrawn from or plunged into the furnace at will. In starting the
furnace, the bottom is prepared by ramming it with charcoal-powder that
has been soaked in milk of lime and dried, so that each particle is
coated with a film of lime, which serves to reduce the loss of current
by conduction through the lining when the furnace becomes hot. A sheet
iron case is then placed within the furnace, and the space between it
and the walls rammed with limed charcoal; the interior is filled with
fragments of the iron or copper to be alloyed, mixed with alumina and
coarse charcoal, broken pieces of carbon being placed in position to
connect the electrodes. The iron case is then removed, the whole is
covered with charcoal, and a cast iron cover with a central flue is
placed above all. The current, either continuous or alternating, is then
started, and continued for about 1 to 1-1/2 hours, until the operation
is complete, the carbon rods being gradually withdrawn as the action
proceeds. In such a furnace a continuous current, for example, of 3000
amperes, at 50 to 60 volts, may be used at first, increasing to 5000
amperes in about half an hour. The reduction is not due to electrolysis,
but to the action of carbon on alumina, a part of the carbon in the
charge being consumed and evolved as carbon monoxide gas, which burns at
the orifice in the cover so long as reduction is taking place. The
reduced aluminium alloys itself immediately with the fused globules of
metal in its midst, and as the charge becomes reduced the globules of
alloy unite until, in the end, they are run out of the tap-hole after
the current has been diverted to another furnace. It was found in
practice (in 1889) that the expenditure of energy per pound of reduced
aluminium was about 23 H.P.-hours, a number considerably in excess of
that required at the present time for the production of pure aluminium
by the electrolytic process described in the article ALUMINIUM. Calcium
carbide, graphite (q.v.), phosphorus (q.v.) and carborundum (q.v.) are
now extensively manufactured by the operations outlined above.
_Electrolytic Processes._--The isolation of the metals sodium and
potassium by Sir Humphry Davy in 1807 by the electrolysis of the fused
hydroxides was one of the earliest applications of the electric current
to the extraction of metals. This pioneering work showed little
development until about the middle of the 19th century. In 1852
magnesium was isolated electrolytically by R. Bunsen, and this process
subsequently received much attention at the hands of Moissan and
Borchers. Two years later Bunsen and H.E. Sainte Claire Deville working
independently obtained aluminium (q.v.) by the electrolysis of the fused
double sodium aluminium chloride. Since that date other processes have
been devised and the electrolytic processes have entirely replaced the
older methods of reduction with sodium. Methods have also been
discovered for the electrolytic manufacture of calcium (q.v.), which
have had the effect of converting a laboratory curiosity into a product
of commercial importance. Barium and strontium have also been produced
by electro-metallurgical methods, but the processes have only a
laboratory interest at present. Lead, zinc and other metals have also
been reduced in this manner.
For further information the following books, in addition to those
mentioned at the end of the article ELECTROCHEMISTRY, may be
consulted: Borchers, _Handbuch der Elektrochemie_; _Electric Furnaces_
(Eng. trans. by H.G. Solomon, 1908); Moissan, _The Electric Furnace_
(1904); J. Escard, _Fours electriques_ (1905); _Les Industries
electrochimiques_ (1907). (W. G. M.)