Improvements in the Art of Producing Metallic Zino.
Year: 1892

page_text: Date of Application, 19th Apr., 1892-Accented, 21st May, 1892
COMPLETE SPECIFICATION.
Improvements in the Art of Producing Metallic Zino.
I, PARKER COGSWELL CHOATE, Metallurgist, of No. 38 Wall Street, New
York, County and State of New York, United States of America, do hereby declare
the nature of my invention for IMPROVEMENTS IN THE ART OF PRODUCING
METALLIC ZINC, 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 by means of
electrolysis and the object of my improvements is to provide an economical method
While
of producing commercially pure metallic zinc.
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 class of ores known as "blendous" or "complex," which contains
besides zinc numerous other metals such as gold, silver, iron, lead, arsenic, antimony,
bismuth, cadmium, etc. in varying proportions 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 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 etc. By my process, however, I am able to save the zinc as a metal
and also to prevent the loss of the other metals. Heretofore the production of pure
metallic zinc by electrolytic deposition has not been possible on a profitable com-
mercial scale. The reason of this is, that to insure purity of the product the zinc
"electrolyte," or solution in the electrolytic baths upon which the electric current
is to act to affect the deposition of the metal, must itself be pure,-that is to say,
must be free from the other metals which were present in the ore and which if
carried with the zinc into the product would contaminate the latter; and although
the desirabity of this purity of solution may have been recognized by some
metallurgists no adequate mode of attaining it has hitherto been proposed. The
serious obstacle which has been encountered was that as almost all these other
metals and bodies are more readily acted upon by the electric current than zinc
they would enter into the deposited product in the bath with the zinc, and no
satisfactory practical means bas heretofore been found to eliminate them from the
solution and thereby prevent this result. The only method proposed for the
purpose has been to clear the solution by the use of successive precipitants, but
these have failed to remove all the impurities and, moreover, their employment is
so inconvenient and costly as to render impracticable any process which depends
upon them.
Another difficulty heretofore encountered has been the inability-unless at
prohibitory expense-of neutralizing the large volume of free acid which quickly
appears in the bath through the action of the current upon the solution, and which,
unless neutralized or annulled, soon brings the operation to a stand still by
redissolving the zinc from the cathode as fast as it is deposited thereon.
Hence, even though a pure zinc solution be originally formed and the operation
of depositing a pure metallic zinc be begun, it can only be maintained and carried
on by neutralizing or annulling the free acid, and in such a way that soluble,
depositable impurities shall not enter into the bath where they will be a source of
prohibitory trouble and expense.
By my improvement I overcome these difficulties, for it is a result of my process
that the condition in which I take the zinc to subject it to the action of its solvent
in order to produce the electrolytic solution enables me to obtain this solution in
great purity and also provides an efficient and economical means of neutralizing the

page_text: acid set free in the bath and maintaining the solution without impairing its purity
although I dispense altogether with the use of precipitants.
The defects of prior processes have largely come from the fact that their electro-
lytic solutions were made and maintained direct from the ore so that all the soluble
foreign matters present necessarily went into the solutions and then required to be
removed. In my process on the contrary, I at the outset take the zinc in a state
in which all the soluble impurities can easily be eliminated before the solution is
formed, while such insoluble impurities as go into the electrolytic baths will readily
I
separate themselves out during the working of the charges, so that in this way
insure and constantly maintain a practically pure solution for the action of the
electric current.
From the point of view of my process, all metals and other bodies which if
they were to enter into the zinc product would contaminate it are treated as
impurities. To the end, therefore, of securing and maintaining the purity of
solution referred to, my invention consists in producing commercially pure metallic
zinc by first converting the zinc in the ore into zinc oxide by decomposing and
volatilizing it in the presence of free oxygen in order that all impurities in the
ore which are volatilizable more quickly than, or as quickly as, the zinc may
.be driven off before, or with the latter to be eliminated from the volatilized
product afterwards, leaving the non-volatilizable bodies behind in the furnace ;
then whenever necessary submitting the condensed volatilized product to a
moderate roast so as to expel all its impure and soluble constituents which are
more readily volatilizable than the zinc, such as the oxides of antimony, arsenic,
cadmium etc.; then treating the purified product thns obtained with an acid
solvent of the zinc oxide in which lead is insoluble, such as dilute sulphuric
acid; and finally treating the solution of zinc thus obtained by electrolysis to
produce the pure metallic deposit; the continuity of such production being made
possible by using the purified oxide of zinc to neutralize or annul the acid set
free in the bath by the current. The accompanying drawing shows a sectional
elevation of an apparatus containing the vessel in which the solution of zinc is
formed, and the baths for the electrolytic treatment of it, together with feed
and overflow tanks, and also a pump and connecting pipes for keeping the solution
in circulation. Any usual or suitable mode may be adopted for obtaining the
zinc oxide to be used in my process. As an example of one well known method,
the ore may be first crushed and then, when it contains an excess of sulphur,
it may be roasted at a moderate temperature in order to drive off the excess of
sulphur. The partially desulphurized ore may then be mixed with carbonaceous
fuel, such as coal screenings, in a furnace in which air is admitted beneath its
grate, and may be roasted at a temperature high enough to cause decomposition
and volatilization of zinc until the zinc contained in the ore is driven off.
volatilized zinc and other volatilized bodies may then be caught and collected
in a bag room in the ordinary manner. The resulting sublimed product will be
substantially pure oxide of zinc in the shape of a fine powder known as "fume
mixed with such lead as was contained in the ore and which is volatilized with
the zinc. There may also frequently occur with the zinc and lead fume, as
condensed products over volatilizable oxides which may have been present in
the ore in their metallic form, such as the oxides of antimony, arsenic, selenium,
cadmium etc. these substances are all volatilizable at a lower temperature than
zinc and lead. Hence 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 them off,
care being taken not to apply sufficient heat to cause revolatilization of the zinc
contents. By means of this roast the fume is purified from its more volatile
constituents thereby easily eliminating all those impurities which would be soluble
in the clectrolyte employed and which would therefore otherwise enter into
solution with the zinc and contaminate the deposited product.
The
"
Of the metals originally present in the ore the volatilized product will now

page_text: contain only the zinc and lead; other metals which may have been contained in the
ore, such as iron, copper, gold, etc. are not driven off with the volatilized product
but remain in the furnace in the form of cinder or slag aud may afterwards be
smelted out and recovered in the usual manner. I do not confine myself to the
above described mode of producing or purifying the zinc oxide, as this may be done
in any suitable way, it being only necessary to my invention that I should have for
my subsequent operations an oxide of zinc which has been properly purified from
its more volatile soluble constituents.
The next step in the process is the treatment of the purified zinc oxide electro-
lytically in order to separate the metallic zinc. For this purpose the oxide is
dissolved in an acid solvent in which lead is insoluble, such as dilute sulphuric acid,
in a treating tank shown in the drawing at A, in such proportions as to form a well
concentrated solution. The zinc oxide will be dissolved, while other constituents,
lead and silver sublimates which are the only impurities now remaining, being
insoluble are not acted upon and consequently separate out and settle to the bottom
of the tank, leaving a clear solution which contains out of all the elements originally
present in the ore only the zinc, from which the others will now have been separated
by the preceding steps of the process.
It will thus be seen that I am able to obtain an electrolytic solution of a high
degree of purity without the use of any precipitant and in a rapid and economical
manner, thereby avoiding the great cost of handling bulky solutions as well as of
the precipitants themselves.
The clear solution is next drawn off through a pipe, seen in the drawing at 1, into
a feed tank B, whence it is conveyed to the electrolytic baths cc c, by pipes 2, 2, 2,
opening into the bottom of the baths. The baths are supplied with anodes of
suitable material, such as lead or carbon, insoluble, in the solution, and catholes
which may be plates of zinc or copper. The anodes and cathodes are connected
with mains L L leading from a suitable source of electricity D, such as a dynamo
of proper capacity. If desired, the solution may be heated in order to lower the
resistance of the baths. On the application of the current the zinc in the solution
will be deposited upon the cathode, while the freed acid will rise to the surface whence
it is drawn off through overflow pipes, 3, 3, 3, and carried to the tank E.
treating and feed tanks and the baths and overflow tanks, should be set upon
different levels so as to produce a natural flow from the treating tank and through
the baths thence to the overflow tank.
The
By means of a pump P, the acidulated solution in the tank E, is raised to the
treating tank A, through the pipe 4. It is there neutralized by the addition to it of a
fresh supply of the purified zinc fume, the zinc being soluble in the dilute free acid.
After any insoluble constituents of the fume have settled out the solution is led to the
feed tank B and thence, as required, to the baths to maintain the solution therein.
Since the overflow from the baths will contain a considerable proportion of zinc a
portion of it may, if desired, be returned directly to the feed tank B, through the
pipe 5 to be passed again through the baths instead of being regenerated in the first
instance. In this manner I easily and effectually annul the free acid, and maintain
the solution in its original state, since I always have a sufficient supply of purified
oxide of zinc therefor, and thus enable the process to be carried on indefinitely.
At the same time I maintain a continual circulation through the baths and prevent
the stratification of the acid which would take place if the solution were not kept
in motion.
If suitable staple acids be used there will practically be no loss of the solvent as
it can be used again and again in carrying on the process. Water will have to be
added to the solution in small quantities from time to time to supply the slight loss
which will take place. The deposited zinc may be stripped off the plates and
melted, or where zinc plates are used as cathodes the plates may be melted whenever
a sufficiently heavy deposit has been formed upon them. The residual metallic
sulphates which settle to the bottom of the treating tank may be smelted in the
ordinary manner to recover the metals contained therein.

page_text: Any other convenient electrolytic apparatus may be employed, and I wish it to
be understood that I do not limit myself in any portion of my process to any
particular form of apparatus for putting it into practice. The advantages of my
invention will be readily appreciated since it affords an easy and economical method
of producing metallic zinc from its ores, and by its use vast quantities of ore heretofore
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 J
claim is :-
1. The above described process of producing metallic zinc which consists in
separating the zinc and the equally volatile and more volatile constituents from the
less volatile constituents of the ore by decomposition and volatilization in the
presence of free oxygen, thereby obtaining a condensed oxidized zinc fume, subjecting
this fume to a moderate heat in order to expel its soluble constituents more volatile
than zinc, then treating the remaining product with an acid solvent of the zinc in
which lead is insoluble such as dilute sulphuric acid, and finally obtaining
metallic zinc from the solution thus formed by means of electrolysis, substantially
as set forth.
2. The above described process of producing metallic zinc which consists in first
separating the zinc from the less readily volatilizable constituents of an ore by
decomposing and volatilizing the zinc element in the presence of free oxygen,
thereby obtaining a condensed oxidized zinc fume, treating this fume with an acid
solvent of the zinc in which lead is insoluble, such as dilute sulphuric acid, to form
á practically pure electrolytic solution and finally subjecting this solution to the
action of an electric current to obtain the zine as a metallic deposit, substantially as
set forth.
3. The above described process of producing metallic zinc, which consists in
forming an electrolytic solution by dissolving in dilute sulphuric acid, oxide of zinc,
however obtained, in the state of fume, free from the lighter soluble impurities, such
as the oxides of antimony, arsenic cadmium etc. and then treating this solution by
electrolysis, substantially as set forth.
4. The hereinbefore described process of producing metallic zinc which consists
in depositing the zinc by electrolysis from an acidulated solution of a zinc salt
maintained and regenerated by dissolving in the acid set free in the bath during
electrolysis, oxidized zinc, in the state of fume, purified from its more volatile
impurities such as the oxides of arsenic, antimony, bismuth etc.
5. In the process of depositing metallic zinc by electrolysis from an acid zinc
electrolyte, the hereinbefore described method of regenerating and maintaining the
electrolyte which consists in dissolving in the acid set free in the bath during elec-
trolysis oxidized zinc in the state of fume, freed from its more volatile soluble
impurities, and maintaining the electrolyte with the solvent thus formed.
1
11000
P. C. CHOATE,
By L. W., per F. B.