Improvements in the Art of Preparing Solutions Carrying Salts of Zinc.
Year: 1894

page_text: Date of Application, 9th Jan., 1894-Accepted, 10th Feb., 1894
COMPLETE SPECIFICATION.
Improvements in the Art of Preparing Solutions Carrying Salts
of Zinc.
I, 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 this invention and in what manner the same is to
be performed, to be particularly described and ascertained in and by the following
statement:
" blendous
"
My invention relates particularly to the preparation of solution carrying salts of
zinc from those "mixed or
ores which contain besides zinc numerous
other metals such as gold, silver, lead, copper, iron, antimony, arsenic, bismuth,
cadmium &c., and the object of my improvements is to prepare from such ores, or
from any ores in which zinc is associated with other soluble metals, a solution of
zinc which shall be free from the salt of other metals and of such purity as to be
suitable for use as an electrolyte in producing commercially pure metallic zinc by
electrolysis or for any of the purposes for which a pure solution of zinc is necessary
or useful.
Attempts have heretofore been made to purify impure solutions of zinc by
removing the metals other than zinc therefrom by the use of chemical precipitants.
The precipitation of other metals from a solution in which they are present with
zinc so as to obtain a pure solution of zinc is, however, extremely difficult, even as
a laboratory experiment, and has proven utterly impracticable on a commercial
scale.
In carrying out the first part of my improvements I provide a quantity of zinc
oxide fume freed from its more volatile constituents. This may be obtained in any
usual or couvenient manner, as by roasting a zinc ore, mixed with carbonaceous
fuel, in a furnace into which air is admitted, so as to volatilize and oxidize the zinc
contents, and collecting the volatilized product in a bag room. This product will
66 fume," mixed
be substantially pure oxide of zinc in the shape of a fine powder as
with such lead as was contained in the ore, which is volatilized with the zinc.
There may also be present with the mixed zinc and lead fume, other volatilized
products, such as the oxides of antimony, arsenic, bismuth, cadmium &c., when these
were represented in the ore. These substances are all volatilizable at a lower
temperature than zinc and lead, and when they occur in appreciable quantities I
dispose of them by subjecting the fume to a moderate roast at a temperature of
from 500° to 800° F. in any well known form of muffle furnace, thereby driving
The fume
them off, care being taken not to revolatilize any of the zinc contents.
is thus freed from those soluble elements which would otherwise, when it is used to
neutralize the free acid as hereinafter set forth, be dissolved with the zinc, and
keep the solution in an impure state.
any
Having thus provided for a supply of purified zinc oxide fume, I now proceed to
the purification of an impure zinc solution, which may be one obtained in usual
manner as by treating a mixed zinc ore, such as is above referred to, or the impure
"flue product" from a zinc volatilizing furnace with a solvent of its zinc constituent.
Such a solution will contain besides zinc various other metals and to the end of
removing these I place the solution in a tank or vat supplied with suitable anodes
and cathodes and pass through it a current of electricity from a suitable source, as
a dynamo electric machine.
In case the solution is to be afterward used as an electrolyte in depositing pure
metallic zinc the density of the current applied should be not less than and

page_text: preferably somewhat greater than that which is to be afterward used in depositing
zinc.
On the application of the current some of the depositable impurities in the bath
will be deposited upon the cathode while the others, through the action of the
current and of the secondary reactions which will take place, will be thrown down
as precipitates in the bath.
The impurities in the solution cannot however be permanently removed in this
manner, since with the deposition of metal free acid appears in the bath which soon
accumulates to such an extent as to dissolve the metals from the cathode as fast as
they are deposited.
In order to prevent the redissolving of the deposited and precipitated impurities,
without at the same time introducing fresh ones, I neutralize the free acid liberated
in the bath by the action of the current, by means of the oxidized zinc fume above
described.
For this purpose the fume may be introduced directly into the bath in sufficient
quantities to keep the solution neutral, or the freed acid, which will rise to the top
of the bath, may be continuously drawn off into a separate tank and after being
neutralized with the fume be returned to the depositing tank.
In case the zinc solvent used in making up the solution is also a solvent of lead,
which will be the only impurity associated with zinc in the fume, enough sulphuric
acid should be introduced into the bath, when lead is present in the fume to render
it insoluble. With this precaution any desired solvent of zinc may be used in
making up the solution.
The application of the current to the solution should be continued until all the
depositable impurities are deposited or precipitated, when the precipitate may be
allowed to settle and the clear solution drawn off ready for use.
When the impure solution of zinc to be purified is a sulphate solution I may
instead of treating it with an electric current as above set forth, proceed according
to the second part of my invention as follows. The sulphate solution may, as
before, be one formed in any usual or suitable manner as by first crushing the ore
and then if it is a sulphate of zinc submitting it to a sulphate roast to form sulphate
salts of the zinc, and then treating it with water as a solvent. Or if the ore is of
such a nature that the zinc will not be converted into a sulphate by roasting, the
solution may be formed by treating the ore in the first instance with dilute sulphuric
acid as a solvent.
However the solution is formed, the insoluble constituents such as lead, gangue &c.
are allowed to settle out and the solution is then drawn off and heated to evaporate
the water and crystallize out the contained salts which will be recovered mostly in
the form of hydrated sulphates.
This product I then charge into a furnace of the muffled type and of any well
known form and heat it in a sufficiently oxidizing atmosphere at a temperature of
from 500° to 900° F., care being taken not to apply sufficient heat to decompose
the sulphate of zinc. I prefer to stir the mass while heating so that the heat may
be equally distributed throughout. By this roast the whole mass will be dehydrated,
the metallic constituents more volatile than zinc will be volatilized and driven off,
while the sulphate of those metals which are less readily volatilizable than zinc
will be decomposed, the acid passing off as vapour, and converted into forms
insoluble in water, for the most part oxides. The zinc being less readily decom-
posed than any of the other soluble metallic sulphate will remain as an hydrous
zinc sulphate.
After the roast is completed the product is removed from the muffle and when
cool is ready for treatment with a solvent of the zinc constituent. It will contain
besides anhydrous sulphate of zinc, which will be in a state of fine subdivision, very
suitable for entering readily into solution, only oxides or compounds insoluble in
water of other metals such as iron, copper, nickel &c. I next treat the mass with
water as a solvent in any usual or suitable form of tank or treating vessel. The
sulphate of zinc will be dissolved by the water, while the insoluble compound of

page_text: other metals which may be present will be unaffected and will settle in the treating
tank leaving a clear solution containing only salts of zinc, which is then ready for
use and may be drawn off as desired.
The sediment remaining in the tank may be afterward treated by any well known
smelting process for the recovery of the contained metals.
The advantage of my invention will be readily apparent to those skilled in the
art, since by means of it I am able to utilize the impure ores of zinc, heretofore
discarded at the mines in great quantities as waste products and to prepare therefrom
concentrated solutions of the salts of zinc of great purity.
The value of a pure zinc solution is obvious since only from such a solution can
a pure precipitated oxide of zinc be prepared or a pure crystallized sulphate of
zinc be obtained by evaporation.
In obtaining metallic zinc by electro deposition, also such a solution is of the
greatest importance, since it is essential to the production of a pure electro
deposited zinc that the electrolytic solution shall be free from all metals which
may be deposited along with the zinc.
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 purifying an impure solution of zinc
salts, which consists in subjecting the solution to the action of an electric current
so as to precipitate and deposit the depositable impurities and meanwhile neutral-
izing the acid set free in the bath with an oxidized zinc fume freed from its more
volatile soluble constituents, substantially as set forth.
2. The hereinbefore described process of purifying an impure solution of zinc
salts which consists in subjecting the solution to the action of an electric current to
precipitate and deposit the depositable impurities and at the same time preventing
the resolution of such impurities by neutralizing the acid set free in the bath with a
neutralizing agent free from any depositable impurities soluble in the solvent
element of the bath, substantially as set forth.
3. The hereinbefore described process of forming from a zinc ore a solution
carrying salts of zinc which consists in forming a sulphate solution of the soluble
element of the ore and recovering the same therefrom by evaporation and crystal-
lization, heating the crystallized product to drive off the salts of metals more
volatile than zinc and convert those less volatile than zinc into compounds insoluble
in water and finally treating the mass with water to dissolve the zinc element.
4. The herein before described process of forming a solution carrying salts of
zinc which consists in recovering the soluble elements of the ore from a solution
thereof in the form of sulphates, heating the product thus obtained to volatilize
those elements more volatile than zinc and convert those less volatile than zinc
into compounds insoluble in water, leaving the zinc in the form of an anhydrous
sulphate, and finally dissolving the zinc sulphate in water to form the solution.
Dated thin 9th dan of Lonuar 1801
H. D. FITZPATRICK,
70, Wellington Street, Glasgow, Agent.
H. D. FITZPATRICK,
70, Wellington Street, Glasgow, Agent.