Improvements in the Art of Producing Metallic Zinc.
Year: 1894

page_text: Date of Application, 24th Apr., 1894--Accepted, 2nd June, 1894
COMPLETE SPECIFICATION.
Improvements in the Art of Producing Metallic Zinc.
1, PARKER COGSWELL CHOATE of No. 38 Wall Street New York, in the
County and State of New York, United States of America, Metallurgist, do
hereby declare the nature of my invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:-
"
"
My invention relates to the production of metallic zinc from its ores, and the
object of my improvements is to provide an economical method of producing com-
mercially pure metallic zinc. While my method is applicable to the production of
zinc as a metal from any of its ores it is more especially valuable as applied to that
large class of ores known as "blendons complex," which carry varying per-
centages of zinc, iron, sulphur, lead, copper, gold and silver with accompanying
gangue matter, and also such constituents as cadmium, arsenic, bismuth, antimony,
&c., and which on account of the difficulty of working them have heretofore been
discarded at the mines in great quantities as waste products.
or
In working such ores for the extraction of metals other than zinc as heretofore
practised, there has been a total loss of the zinc, except when utilized as a pigment,
and at the same time its presence causes heavy losses of the other metals such as
gold, silver, &c.
Although in many cases these ores are very rich in zinc, which is usually the
most valuable constituent, all attempts to treat them for the extraction of that
metal commercially has failed even at the sacrifice of the other constituents of
value.
Thus the ordinary method of producing metallic zinc from the ore by distillation is
entirely inapplicable to these complex ores, since so many other metals would be
distilled with the zinc and enter as impurities into the metallic product that it
would be entirely unfit for the commercial use to which metallic zinc is applied.
Even as produced from its purer ores by distillation, zinc of commerce is more
or less impure, carrying various metals, such as lead, cadmium, iron, copper, tin,
bismuth, &c. or some of them, in small quantities.
The production of substantially pure metallic copper from its ores by electrolysis
is well known, but though it is a matter of common knowledge that zinc can be
electro-deposited, I believe that no method has been devised, prior to my improve-
ments by means of which it can be commercially produced by electrolysis in a
sufficiently pure state to be fit for use in the arts.
This is due to the fact that zinc, unlike copper in this respect, is electro-positive
to nearly all the metals which are present with it in the ore and will deposit or
precipitate upon itself most of the metals which are electro-negative to it from an acid
solution in which they are present. Such metals are antimony, arsenic, bismuth,
cadmium, cobalt, copper, gold, lead, nickel, silver, tin &c. and if in the electro-
deposition of zinc any of these be allowed to enter into solution with the zinc, as
has heretofore always been the case, they will be deposited with the zinc and
appear as impurities in the product. It is evident then that in depositing from a
soluble zinc anode if any of these metals are present in the anode and an electrolyte
is used in which they are soluble they will enter into the solution and be deposited
with the zinc, consequently a zinc anode made either direct from an impure ore or
composed of ordinary zinc distilled from an impure ore cannot be used to produce
a pure electro-deposited zinc.
In order therefore to attain the result which I have reached of a commercially
pure electro-deposited zinc produced from a soluble zinc anode it is necessary to
adopt processes and observe precautions in the preparation of the anode which shall

page_text: obviate the presence in the anode of all such of these substances as may be soluble
in the electrolyte employed.
A pure electro-deposited zinc possesses marked advantages over the ordinary
zinc of commerce produced by distillation, being softer and more crystalline in its
structure and by reason of these qualities more available for many of the uses to which
zinc is applied, entirely apart from the advantages incident to its purity. To the
end therefore of producing such a zinc, my invention consists broadly in depositing
zinc by electrolysis from a soluble zinc anode so prepared as to be free from all
metals which are both soluble in the electrolyte used and capable of being deposited
or precipitated by zinc upon itself therefrom.
In order to prepare such an anode from an impure "complex" or " blendons
ore of zinc, I separate the zinc from the less readily volatilizable elements of the
ore by heating or burning the ore mixed with carbonaceous fuel, so as to volatilize
the zinc and leave the non-volatilizable elements of the ore such as copper, gold,
iron, silver, &c. behind in the furnace and condense and collect the volatilized
product in the shape of fume.
Then whenever necessary I subject the collected fume to a moderate roast, so as
to expel all its soluble constituents, which are more readily volatilizable than the zinc,
such as cadmium, arsenic, &c. and finally convert this purified fume into metallic
form suitable for my purpose by distilling it and then collecting and casting the
distilled product into slabs of suitable size and shape for use as anodes. The zinc
contained in the anodes thus prepared is afterward electro-deposited in a pure state
by subjecting the anodes to the action of an electric current in an acid electrolyte,
which is not a solvent of such constituents of the anodes as may be depositable
by zinc.
Any usual or suitable method may be adopted for obtaining the fume to be used
in my process. As an example of one well known method, the ore, after being
crushed may be mixed with carbonaceous fuel, such as coal screenings, and then be
roasted in a furnace, into which air is admitted beneath its grate, at a temperature
high enough to volatilize the zinc until the zinc contained in the ore is driven off.
The volatilized zinc product, and other volatilized matter may then be condensed
and caught, and collected in a bag room in the ordinary manner. The resulting
product will be in the shape of a fine powder known as "fume" and chemically
may be in the form of any of the volatilized compounds of zinc, as produced by the
process employed. There will also be contained in the fume much of the lead
which was present in the ore, and was volatilized with the zinc. There will also
frequently occur with the zinc and lead in the fume the condensed products of
other more volatile metals which may have been present in their compounds in the
ore such as those of arsenic, antimony, cadmium, &c. These substances are among
those which are depositable by zinc upon itself and which are soluble in an acid
electrolyte. They are all however volatilizable at a much lower temperature than
zinc and lead. Hence when they occur is appreciable quantities, I dispose of them
by subjecting the fume mixed with a small percentage of carbon to a moderate
roast at a temperature of from about 500 to 800 C. in any well known form of
muffle furnace, care being taken not to apply sufficient heat to revolatilize any of
the zinc contents. By means of this roast the fume is purified of its lighter,
soluble volatile constituents, which would otherwise subsequently enter into the
anodes, and thence into the solution, and be deposited with and contaminate the
zinc.
The other metals, excepting silver, which may have been contained in the ore,
such as iron, gold, copper, nickel &c. are not driven off with the zinc and lead, but
remain in the furnace in the form of a cinder or slag, and may be afterwards
smelted out and recovered in the usual manner. The greater part of the silver
present will also remain in the furnace, but a portion of it may be volatilized and
appear in the fume.
I do not limit myself to the above described method of producing and purifying
the zinc fume, it being only necessary for my further operations that I should have a

page_text: zinc fume from which all impurities of the class indicated, that is, those which.
would be soluble in the electrolyte to be afterward employed and which would be
deposited by zinc upon itself therefrom, have been eliminated.
The next step is the reduction of the purified fume to a metallic form, which
may be done in the ordinary well known manner of zinc distillation by heating it,.
in company with carbon, in any well known form of closed retort or muffle, and
collecting the reduced and distilled product.
While iron is not included among those metals which are sufficiently electro-
negative to zinc to be depositable upon it from an acid solution and may therefore
be present in the electrolyte in small quantities without injury to the product, I
find that it is extremely difficult to prevent traces of it from being carried over
when it appears as a constituent part of a soluble anode. It will not however
be present in the fume which I employ and can only appear as an accident of
manufacture in the course of distillation. Care should therefore be taken in
distilling the fume not to allow iron to enter the distilled product by reason of
contact with iron tocls or from coal ash.
Where lead is present in considerable quantities a portion of it may be separated
from the zinc by allowing it to settle while in the molten state. The remainder
of the product of distillation is then run into slabs of suitable shape and size for
use as anodes in an electrolytic bath.
Besides zinc the anodes will contain of all the original metallic constituents of
the ore capable of being deposited by zinc upon itself, only lead, where lead is.
present in the ore treated, and sometimes also a small percentage of silver.
The electro-deposition of lead and silver with the zinc may however be prevented
by the use of a proper electrolyte and for the purpose of separating the zinc from.
these metals and producing a pure, soft, crystalline zinc, I now subject the anodes.
to the action of an electric current in an electrolyte, which is a solvent of the zinc,
but in which the lead and silver, or that one of them which may be present, are
insoluble. For this purpose I may use a zinc sulphate bath, which may be prepared
by dissolving pure sulphate of zinc in water or pure oxide of zinc or metallic zinc
in sulphuric acid. If the materials used in preparing the bath are impure,.
i.e. contain any of those metals hereinbefore shown to be inadmissible in the bath,.
they should be deposited out before the deposition of pure zinc is begun. In this.
electrolyte the lead is insoluble. In order to render the. silver insoluble when.
present in the anode, I may keep in the bath a trace of chlorine. A small quantity
of common salt added to the solution will supply the necessary chlorine of which
there needs to be only enough to provide a chemical equivalent for the silver
present, which will be thrown down as insoluble chloride of silver.
Having prepared a suitable bath the slabs or plates of zinc prepared as above
described, are suspended therein as anodes and the zinc is deposited upon the-
cathodes by the application of a current of electricity from any suitable source.
in the manner usual in electrolysis. The cathodes may be plates of zinc which may
be afterwards melted with the deposited zinc, or they may be of copper or other
suitable metal from which the deposited zinc may be stripped after a suitable-
thickness has been attained. The zinc will be deposited in a pure state free
from any lead or silver present, which being insoluble in the bath will settle therein.
and may from time to time be removed.
In order to prevent stratification of the bath, it should be kept in motion, .which
may be done in any well known manner as by pumping the overflow from the bath.
and returning it thereto, so as to maintain a more or less constant circulation.
The bath while practically neutral, will always, while working, carry at the
anode a small amount of free acid set free by the current on the deposition of the-
zinc.
I have specified herein the use of an acid solution for the bath, meaning thereby
5 a solution in which the zinc is carried in solution by an acid solvent as distinguished
from permanent alkaline solutions. These latter I exclude as commercially and
chemically impracticable. Of acid solutions I prefer to use a sulphate solution as-

page_text: above set forth. I do not, however, limit my invention thereto, but may use any
acid in which the metals remaining with the zinc and depositable by zinc upon
itself from a solution in which they are present, are insoluble.
The advantages of my invention will be readily apparent since it affords a simple
and economical method of producing metallic zinc of a purity and quality superior
to any heretofore known from inferior ores, and by its use vast quantities of ore
beretofore waste and of no value can be reduced to their component metals and
made valuable.
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. The hereinbefore described process of producing zinc anodes from an impure
ore of zinc, which consists in first separating the zinc from the less volatile con-
stituents of the ore by heating the ore in the presence of a reducing agent in a
furnace to which air is admitted so as to volatilize the zinc and those constituents
of the ore more volatile than zinc and collecting the volatilized product in the
shape of fume, second, reheating the fume mixed with carbon at a moderate
temperature to revolatilize and eliminate the metallic constituents more volatile
than zinc, and finally subjecting the fume to reduction and distillation and casting
the resulting product into anodes of suitable form, substantially as set forth.
2. The hereinbefore described process of producing commercially pure metallic
zinc which consists in first separating the zinc from the less volatile constituents of
the ore by heating the ore in the presence of a reducing agent in a furnace to
which air is admitted so as to volatilize the zinc and those constituents of the ore
more volatile than zinc and collecting the volatilized product in the shape of fume;
second, reheating the fume mixed with carbon at a moderate temperature so as to
revolatilize and eliminate the volatile constituents which are more volatile than
zinc; third, subjecting the fume to reduction and distillation and casting the
resulting product into anodes and finally depositing the zinc from the anodes by
means of electrolysis, substantially as set forth.
3. The hereinbefore described process of producing commercially pure metallic
zinc which consists in reducing and distilling zinc fume freed from its lighter
soluble constituents and casting the resulting product into anodes and separating
the zinc from such anodes by means of electrolysis in the presence of an acid
electrolyte.
4. The hereinbefore described process of producing commercially pure metallic
zinc which consists in immersing in a zinc sulphate electrolyte containing chlorine,
a zinc anode so prepared as to contain no metal which is both soluble in such
electrolyte and depositable by zinc upon itself therefrom, and depositing the zinc
out of such anode, by means of electrolysis, substantially as set forth.
5. The hereinbefore described process of producing commercially pure metallic
zinc which consists in separating from the zinc as found in the ore those metals,
such as antimony, arsenic, copper, gold &c. which are depositable by zinc upon
itself from an acid solution thereof, forming the zinc with the remaining accom-
panying metals into anodes and depositing the zinc from the anodes by means of
electrolysis in an acid electrolyte substantially as set forth.
6. The hereinbefore described process of producing commercially pure electro-
deposited zinc which consists in immersing in an acid electrolyte a zinc anode so
prepared as to contain no metal, which is both soluble in such electrolyte and
depositable by zinc upon itself therefrom and depositing the zinc from such anode
by means of electrolysis, substantially as set forth.
Dated this 22rd day of Ancil 1004
H. D. FITZPATRICK,
70, Wellington Street, Glasgow, Agent.