Improvements in the Art of Producing Metallic Zinc.
Year: 1893

page_text: COMPLETE SPECIFICATION.
Improvements in the Art of Producing Metallic Zinc.
I, PARKER COGSWELL CHOATE, Metallurgist, of No. 38 Wall Street, New
York City, State of New York, United States of America, 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 :—
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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 a
commercial brand of spelter substantially free from impurities. While my process
may be used in producing metallic zinc from any of its ores, it is more particularly
intended to be used in connection with an ore carrying both lead and zinc and is
especially valuable as applied to that large class of ores known as "complex." or
blendous galena," which carry zinc, iron, sulphur, lead, gold, silver copper and
gangue matter and also usually varying percentages of the metals such as antimony,
cadmium, selenium, arsenic, bismuth &c. and which on account of the difficulty of
working them economically have heretofore been discarded at the mines in great
quantities as waste products. In working such ores by the processes heretofore
employed, there has been a total loss of zinc, except where utilized as a pigment,
and all attempts to produce a commercial brand of zinc therefrom, even at the
expense of sacrificing the other constituents of value, have resulted in failure. As
is well known to the art zinc of commerce has heretofore been produced only by
distilling it directly from the ore, in retorts or muffles, the ore when necessary being
desulphurized prior to distillation. In all the processes of distillation heretofore
employed very considerable percentages of the other metals associated with the
zinc in the ore appear in the zinc product. Thus such metals as iron, copper &c.
although not volatile, will, when charged into the retort with the zinc, always be
found in the distilled zinc. The more volatile constituents of zinc ores such as
arsenic, antimony, cadmium &c. may be to a certain extent, driven off by the
desulphurizing roast above referred to. They cannot be, however, wholly removed
by any roasting treatment of the body of the ore, but when present in the ore will
always appear in the product of direct distillation.
For these reasons distillation as a method of producing metallic zinc has not
heretofore been applicable to the impure blendous ores of zinc, but has only been
employed to treat ores rich in zinc, and comparatively free from impurities. Even
as produced from such ores by distillation zinc of commerce is universally impure,
the nature and amount of the impurities being dependent upon the character of the
ore. Another serious trouble heretofore met with in treating the body of an ore
directly in retorts or muffles is that the metallic oxides present in zinc ores form
fusible slags with the body of the retort or muffle at the high degree of heat
required for distillation and thus soon destroy the vessel involving heavy loss, and
expense. An attempt has been made to remedy this by volatilizing the zinc before
distillation leaving behind in the furnace the gangue or earthy matter contained in
the ore and charging the muffle or retort with the volatilized and condensed product
or "fume."
On account, however, of the difficulty of distilling the soft, flocculent product
and because of the large quantities of "blue powder" or oxidized zinc, which it was
found resulted from the use of the volatilized zinc product in the retorts, this method
has never been practically employed.
I have found, however, that by heating the zinc fume in the presence of lead,
either in the presence of lead in the shape of lead fume or finally pulverized
metallic lead, the mass becomes condensed and granulated so that it more nearly
resembles in its physical characteristics a crushed ore whereby its rapid and even

page_text: distillation is rendered entirely practicable and the formation of the "blue powder
is to a great extent prevented.
To the end therefore, of producing a commercial brand of zinc from the most
complex zinc ores in an economical manner my invention involves the separation of
the zinc from the less readily volatile constituents of the ore by heating the ore in
the presence of a reducing agent to decompose it and reduce the zinc compounds,
volatilizing the zinc and other readily volatilizable constituents of the ore and
leaving the other constituents behind in the furnace and condensing and collecting
the volatilized products in the shape of "fume;" then heating the zinc fume, in the
presence of lead or lead fume and finally reducing and distilling it and separating
the resulting molten lead from the zinc.
The accompanying drawing shows a sectional elevation of a form of muffle or
retort which may be used with advantage in the distilling part of the process.
By preference, I apply my process to ores carrying together with zinc such an
amount of lead that it will enter the volatilized zinc fume to the extent of at least
five per cent. thereof. In carrying my invention into effect as applied to such ores,
the ore should be first crushed and then when it contains an excess of sulphur, it
may be subjected to the ordinary desulphurizing roast. The ore is then mixed
with carbonaceous fuel such as coal screenings or other suitable reducing agent,
and roasted in any well known type of furnace in which air is admitted underneath
the grate at a temperature sufficiently high to volatilize the zinc, until the zinc
contents of the ore is driven off. With the zinc will also pass off the lead and the
more volatile constituents such as arsenic, cadmium &c. The volatilized zinc and
other volatilized hodies are then caught and collected in a bag room in the ordinary
manner. in the shape of "fume."
The non-volatile matter will be left behind in the furnace and will carry all the
copper and gold and most of the silver contained in the ore, and the whole mass
will be left in an oxidized and semi-fused condition highly suitable for smelting,
whereby the above mentioned metals may be saved.
The object of this first step is threefold, since it separates the zinc from the
metallic oxides which would destroy the retorts, enables the copper, silver and gold
to be saved and yields a mixed zinc and lead fume from which the metallic com-
pounds more volatile than zinc such as cadmium, arsenic, antimony &c. can be
separated leaving the zinc and lead fumes together.
The collected fume which will be in a very soft, flocculent form may then be
reheated in any suitable form of muffle furnace at a temperature of from 300°
to 800° F. or sufficient to drive off the compounds more volatile than zinc, care
being taken not to volatilize any of the zinc or lead contents. The mass should
preferably be agitated while heating, in order that it may all receive the required
degree of heat. As a result of this roast the mass will be freed from the metals
more volatile than zinc such as arsenic, antimony, cadmium &c. only the mixed
zinc and lead fume being left.
In case this roast is employed the desired change in the physical character of
the zinc fume will be partially effected therein, being afterward completed in the
distilling roast. If, however, it is not deemed necessary to eliminate from the fume
the more volatile constituents above mentioned this roast may be omitted and the
effect of granulating and condensing the mass of zinc fume by the melting of the
lead present will be accomplished in the distilling roast which follows, before
distillation begins.
The next step in the process is the reduction of the zinc fume to metallic form
by distillation. For this purpose the mixed zinc and lead fume preferably slightly
nioistened is mixed with carbon in the form of crushed coal and charged into a
retort or muffle, which may be of the form shown in the drawing at A. It is
placed upright in the furnace and is provided with an inner perforated tube B,
having an opening D at the bottom. The retort having been compactly filled with
fume and the cover C luted on, heat is applied and when a temperature sufficient to
volatilize the zinc.fume is reacted, a current of gas and zinc vapor is established

page_text: passing through the perforatious in the tube B and down the interior of the tube.
The .gas will pass off as such while the zinc vapor condenses into molten zinc at
the mouth of the tube and is caught in a suitable vessel underneath. The process
of reduction is facilitated by placing a stick of carbon or of wood, which will
carbonize in the central flue B, which will tend to clear the reduced zinc and the
lead of any oxidized film.
The presence of the lead in the retort not only operates to facilitate the distilla-
tion of the zine fume by granulating and condensing it, but also assists in preventing
the formation of an undue proportion of "blue powder.". It seems to act as a
nucleus and to liquefy and amalgamate with itself such powder of zinc as may be
formed, which would otherwise become condensed in a pulverulent non-liquid
form and which as is well known resists all attempts to melt and reduce it to
liquid form.
The mixed zinc and lead may be allowed to stand in a suitable vessel, being
kept in a molten state, until the lead settles out by gravity, when the lead may be
drawn off by means of a cock at the bottom of the vessel, and the zinc may be
dipped or run off from the top. The zinc cannot be entirely freed from lead by
this process since it will dissolve and carry a percentage of the lead varying in
amount from one and one fourth to one and three fourths per cent. of the zinc. If
the process be carefully carried out and the tools and vessels used are not allowed
to contaminate the molten product, this percentage of lead will be the only
impurity contained in the zinc.
While I prefer the form of retort shown herein both because it is more durable
on account of its position in the furnace, and because its use assists in preventing
the formation of "blue powder" by bringing the molten lead into more intimate
contact with the zinc fume and zinc vapor, I do not limit myself to the use of any
particular form of apparatus, but might use for example, a retort inclined in the
furnace so that the molten lead can be drawn off at the lower end of the retort
while the zinc vapor passes out of a nozzle at the upper end, thereby effecting
the separation of the zinc from the lead in the retort before the zinc is condensed.
While I prefer to apply my invention to those ores of zinc of which lead is a
constituent, it will be evident that it may, if desired, be applied to zinc ores not
containing lead, by mixing lead fume or pulverized metallic lead with the zinc
fume before distilling it, the proportion of lead or lead fume being not less than
five per cent. of the zinc fume.
Instead of using its volatilized product collected in the bag room for subsequent
purification, and distillation, I may also use the so-called "flue-dust" incident to
the manufacture of a zinc lead fume, which may carry besides the zinc and lead
and other more volatile impurities from three to five per cent. of iron oxide or silica,
mechanically carried over from the furnace. This is non-volatile and in the
process of distilling will remain with the coal ash in the retort and not appreciably
affect the purity of the distilled zinc.
The advantages of my invention will be readily apparent to those skilled in the
art, since it prevents the formation of fusible slags with the body of the retort in
the distillation of zinc, and provides an economical method of producing from
impure ores of zinc a commercial metallic zinc carrying lead as its only impurity.
While such a brand of zinc is applicable to numerous purposes its most advan-
tageous use will be in the desilverizing of lead bullion as universally practised.
It may also be refined to great purity by means of the electric current, which is
hot the case with brands of zinc carrying cadmium, copper, tin &c.
I am aware that metallic zinc has heretofore been produced from ores of zinc
containing lead by distillation directly from the ore, and also that attempts have
heretofore been made to distil zinc from a zinc fume free from lead and I do not
broadly claim such processes. I am not aware however, that the use of lead or
ead fume to render practicable the distillation of zinc fume has ever been known
or practised prior to my invention.

page_text: 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 metallic zinc, which consists
in heating an ore of zinc carrying lead in the presence of a reducing agent in a
furnace to which air is admitted so as to volatilize the zinc and lead and those
constituents of the ore more volatile than zinc, and condensing and collecting the
product thereby obtaining a mixed zinc and lead fume, free from the less volatile
constituents of the ore, heating the mixed fume to drive off the constituents more
volatile than zinc and granulate and condense the mass, heating the product mixed
with carbon in a retort to distil the zinc and finally condensing the zinc vapor and
drawing off the molten lead and metallic zinc substantially as set forth.
2. The hereinbefore described process of producing metallic zine which consists
in heating zinc fume mixed with lead fume or pulverized metallic lead to granulate
and condense its fume, heating the product mixed with carbon in a retort to distil
the zinc, and finally condensing the zinc vapor and separating the metallic zinc and
molten lead substantially as set forth.
Dated this 10th day of January 1893
P. C. CHOATE,
By L. W., per F. B.