The use of electricity in the reduction of metals from their ores is
extending so rapidly, and the methods of its generation and
application have been so greatly improved within a few years, that the
possibility of its becoming the chief agent in the metallurgy of the
future may now be admitted, even in cases where the present cost of
treatment is too high to be commercially advantageous.
The refining of copper and the separation of copper, gold, and silver
by electrolysis have thus far attracted the greatest amount of
attention, but a commercial success has also been achieved in the dry
reduction by electricity of some of the more valuable metals by the
Cowles Electric Smelting and Aluminum Company, of Cleveland, Ohio.
Both this method of manufacture and the qualities of the products are
so interesting and important that it is with pleasure we call
attention to them as steps toward that large and cheap production of
aluminum that the abundance of its ores and the importance of its
physical properties have for several years made the unattained goal of
many skillful metallurgists.
The Messrs. Cowles have succeeded in greatly reducing the market value
of aluminum and its alloys, and thereby vastly extending its uses, and
they are now by far the largest producers in the world of these
important products. As described in their patents, the Cowles process
consists essentially in the use for metallurgical purposes of a body
of granular material of high resistance or low conductivity interposed
within the circuit in such a manner as to form a continuous and
unbroken part of the same, which granular body, by reason of its
resistance, is made incandescent, and generates all the heat required.
The ore or light material to be reduced--as, for example, the hydrated
oxide of aluminum, alum, chloride of sodium, oxide of calcium, or
sulphate of strontium--is usually mixed with the body of granular
resistance material, and is thus brought directly in contact with the
heat at the points of generation, at the same time the heat is
distributed through the mass of granular material, being generated by
the resistance of all the granules, and is not localized at one point
or along a single line. The material best adapted for this purpose is
electric light carbon, as it possesses the necessary amount of
electrical resistance, and is capable of enduring any known degree of
heat when protected from oxygen without disintegrating or fusing; but
crystalline silicon or other equivalent of carbon can be employed for
the same purpose. This is pulverized or granulated, the degree of
granulation depending upon the size of the furnace. Coarse granulated
carbon works better than finely pulverized carbon, and gives more even
results. The electrical energy is more evenly distributed, and the
current can not so readily form a path of highest temperature, and
consequently of least resistance through the mass along which the
entire current or the bulk of the current can pass. The operation must
necessarily be conducted within an air-tight chamber or in a
non-oxidizing atmosphere, as otherwise the carbon will be consumed and
act as fuel. The carbon acts as a deoxidizing agent for the ore or
metalliferous material treated, and to this extent it is consumed, but
otherwise than from this cause, it remains unimpaired.
Fig. I. of the accompanying drawings is a vertical longitudinal
section through a retort designed for the reduction of zinc ore,
according to this process, and Fig. II. is a front elevation of the
same. Fig. III. is a perspective view of a furnace adapted to
withstand a very high temperature, and Figs. IV. and V. are
respectively longitudinal and transverse sections of the same.
[Illustration: THE COWLES ELECTRIC SMELTING PROCESS.]
This retort consists of a cylinder, A, made of silica or other
non-conducting material, suitably embedded in a body, B, of powdered
charcoal, mineral wool, or of some other material which is not a good
conductor of heat. The rear end of the retort-cylinder is closed by
means of a carbon plate, C, which plate forms the positive electrode,
and with this plate the positive wire of the electric circuit is
connected. The outer end of the retort is closed by means of an
inverted graphite crucible, D, to which the negative wire of the
electric circuit is attached. The graphite crucible serves as a plug
for closing the end of the retort. It also forms a condensing chamber
for the zinc fumes, and it also constitutes the negative electrode.
The term "electrode" is used in this case as designating the terminals
of the circuit proper, or that portion of it which acts simply as an
electrical conductor, and not with the intention of indicating the
ends of a line between which there is no circuit connection. The
circuit between the "electrodes," so called, is continuous, being
established by means of and through the body of broken carbon
contained in the retort, A. There is no deposit made on either plate
of the decomposed constituents of the material reduced. The mouth of
the crucible is closed with a luting of clay, or otherwise, and the
opening, _d_, made in the upper side of the crucible, near its
extremity, comes entirely within the retort, and forms a passage for
the zinc fumes from the retort chamber into the condensing chamber.
The pipe, E, serves as a vent for the condensing chamber. The zinc ore
is mixed with pulverized or granular carbon, and the retort charged
nearly full through the front end with the mixture, the plug, D, being
removed for this purpose.
A small space is left at the top, as shown. After the plug has been
inserted and the joint properly luted, the electric circuit is closed
and the current allowed to pass through the retort, traversing its
entire length through the body of mixed ore and carbon. The carbon
constituents of the mass become incandescent, generating a very high
degree of heat, and being in direct contact with the ore, the latter
is rapidly and effectually reduced and distilled. The heat evolved
reduces the ore and distills the zinc, and the zinc fumes are
condensed in the condensing chamber, precisely as in the present
method of zinc making, with this important exception that, aside from
the reaction produced by heating carbon in the presence of zinc oxide,
the electric current, in passing through the zinc oxide, has a
decomposing and disintegrating action upon it, not unlike the effect
produced by an electric current in a solution. This action accelerates
the reduction, and promotes economy in the process.
Another form of furnace is illustrated by Fig. III., which is a
perspective view of a furnace adapted for the reduction of ores and
salts of non-volatile metals and similar chemical compounds. Figs. IV.
and V. are longitudinal and transverse sections, respectively, through
the same, illustrating the manner of packing and charging the furnace.
The walls and floors L L', of the furnace are made of fire bricks, and
do not necessarily have to be very thick or strong, the heat to which
they are subjected not being excessive. The carbon plates are smaller
than the cross section of the box, as shown, and the spaces between
them and the end walls are packed with fine charcoal.
The furnace is covered with a removable slab of fireclay, N, which is
provided with one or more vents, _n_, for the escaping gases.
The space between the carbon plates constitutes the working part of
the furnace. This is lined on the bottom and sides with a packing of
fine charcoal, O, or such other material as is both a poor conductor
of heat and electricity--as, for example, in some cases, silica or
pulverized corundum or well-burned lime--and the charge, P, of ore and
broken, granular, or pulverized carbon occupies the center of the box,
extending between the carbon plates. A layer of granular charcoal, O',
also covers the charge on top. The protection afforded by the charcoal
jacket, as regards the heat, is so complete, that with the
covering-slab removed, the hand can be held within a few inches of the
exposed charcoal jacket; but with the top covering of charcoal also
removed and the core exposed, the hand cannot be held within several
feet. The charcoal packing behind the carbon plates is required to
confine the heat and to protect them from combustion.
With this furnace, aluminum can be reduced directly from its ores; and
chemical compounds from corundum, cryolite, clay, etc., and silicon,
boron, calcium, manganese, magnesium, and other metals are in like
manner obtained from their ores and compounds. The reduction of ores
according to this process can be practiced, if circumstances require
it, without any built furnace.
At present, the Cowles company is engaged mostly in the producing of
aluminum bronze and aluminum silver and silicon bronze. The plant,
were it run to its full capacity, is capable of turning out eighty
pounds of aluminum bronze, containing 10 per cent. of aluminum daily;
or, were it to run upon silicon bronze, could turn out one hundred and
twenty pounds of that per day, or, we believe, more aluminum bronze
daily than can be produced by all other plants in the world combined.
This production, however, is but that of the experimental laboratory,
and arrangements are making to turn out a ton of bronze daily, and the
works have an ultimate capacity of from eight to ten thousand horse
power. The energy consumed by the reduction of the ore is almost
entirely electrical, only enough carbon being used to unite with the
oxygen of the ore to carry it out of the furnace in the form of the
carbon monoxide, the aluminum remaining behind. Consequently, the
plant necessary to produce aluminum on a large scale involves a large
number of the most powerful dynamos. These are to be driven by
water-power or natural gas and marine engines of great capacity.
The retail price of standard 10 per cent. aluminum bronze is $1 per
pound avoirdupois, which means less than $9 per pound for aluminum,
the lowest price at which it has ever been sold, yet the Cowles
company has laid a proposition before the Government to furnish this
same bronze in large quantities at very much lower prices than this.
The Hercules alloy, castings of which will stand over 100,000 pounds
to the square inch tensile strain, sells at 75 c. a pound, and is also
offered the Government or other large consumers at a heavy discount.
The alloys are guaranteed to contain exactly what is advertised; they
are standardized into 10 per cent., 7.5 per cent., 5 per cent. and 2.5
per cent. aluminum bronze before shipment.
The current available at the Cowles company's works was, until
recently, 330 amperes, driven by an electromotive force of 110 volts
and supplied by two Edison dynamos; but the company has now added a
large Brush machine that has a current of 560 amperes and 52 volts
electromotive force. We shall, on another occasion, give some
particulars of experiments in the reduction of refractory ores by the
process.--_Eng. and Mining Jour._
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