Improvements in the Electrolytical Production of Zinc from its Ores, and Apparatus therefor.
Year: 1895

page_text: Date of Application, 12th July, 1895-Accepted, 31st Aug., 1895.
[Communicated from abroad by Messrs. SIEMENS & HALSKE, of Berlin, in the
German Empire, Electrical Engineers.]
Improvements in the Electrolytical Production of Zinc from its
Ores, and Apparatus therefor.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate,
Westminster, in the County of Middlesex, Electrical Engineers, 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 :-
With the methods heretofore employed for effecting the electrolysis of zinc
solutions, it has not been possible to realize such a profit as would admit of the
process being considered technically valuable. The cause hereof has been the
peculiar phenomenon that the zinc was separated in such a form that it could not
be employed for technical purposes. For when the solution contains the least
impurity there is deposited on the cathode not solid zinc but instead thereof so-called
zinc sponge, i.e., zinc in a fine non-cohering powder of a dark colour, which is only
fusible with great difficulty, and consequently not practically workable.
Now it is a known fact that zinc sponge is mainly produced when the solution
is alkaline and contains other metals in addition to zinc. It occurs however also
when the solution is decidedly acid and the impurities are nearly all removed.
This invention relates to an improved method by which even in the above named
extreme cases the formation of sponge can be entirely avoided.
It has been found that when a zinc lye of any degree of impurity is subjected to
electrolysis a satisfactory result can always be obtained if the circulation of the
lye be increased beyond a certain degree, which increase depends in each case upon
the composition of the lye and the other working conditions.
It has furthermore been shewn by experiments that the lower limit for the speed;
of circulation in each particular case is very considerably above the speed which
has hitherto been employed in similar methods. This fact points to the conclusion
that it is not merely a question of means for imparting an increased circulation to
the liquid as compared with the speed heretofore employed, but that the action of
the exceedingly high speed of circulation used, depends upon some hitherto unknown
molecular process in the electrolysis of zinc which prevents the formation of zinc
sponge. The said high speed of circulation can be carried out by any suitable
means, such for instance as stirring apparatus; it has however been found most
advantageous to effect the circulation by introducing compressed air into the lye
at the lower end of the vats which by its bubbling up produces the desired motion
of the liquid. Experiments have shewn that a solid zinc deposit is in many cases
more easily obtained if the circulation of the electrolyte is effected by blowing in
air, instead of by other mechanical means. This fact is probably due to a chemical
oxidising action of the air upon the impurities of the electrolyte.
The said invention is preferably carried out by means of apparatus which will
be described with reference to the accompanying drawings.
Fig. 1. shews a diagrammatic view of the whole apparatus.
Figs. 2. 3 and 4 shew respectively a longitudinal section, cross section, and plan

page_text: In Fig. 1. Z Z are the cells in which the electrolysis is to be effected; G is a
source of electrical energy of any suitable kind, such as a dynamo, actuated by a
suitable motor M. such as a steam engine, for producing the current required for
the electrolysis, the current being supplied through conductors C. C. to the
cells Z Z. which are here shewn connected in series.
The cells may however be arranged and connected in any other manner most
suitable for each particular case.
The motor M, in addition to driving the dynamo G, also drives a suitable
machine R, such as a Koots blower, for compressing or propelling air. This air
compressing apparatus R is connected with the cells Z Z. by means of a pipe P
leading along the cells and having a double branch P' between each two cells.
The internal arrangement of the cells Z Z is shewn at Figs. 2 and 3; E E and
EE are the electrodes of which the cathodes E are preferably of zinc, while the
anodes E' are, preferably of carbon, lead, or other suitable bodies that are not
materially attacked by oxygen.
On both sides of the electrodes, Fig. 3, are arranged wooden partitions H H
which do not quite reach up to the level of the liquid, nor quite to the bottom.
Into the compartments K K formed behind these partitions the branches P¹ of the
air pipe P. descend to near the lower edges of the partitions H H and are there
connected with horizontal pipes P2 provided with perforations.
The action of the apparatus is as follows:-
The zinc contained in the natural ores is converted in the known manner by
roasting into zinc oxide; it is then preferably dissolved out by means of sulphuric
acid, and after being freed as much as possible by any known purifying process
from any foreign metals contained in the solution, it is led into the cells Z Z.
The motor M is at the same time made to actuate the current producer G and the
Roots blower R, so that the zinc solution is subject to electrolysis, while air is
forced by the Roots blower through the pipe P and branches P' into the compart-
ments, in the lower part of which it issues through the perforations of the pipe P2.
In consequence of the upward currents thus produced, the liquid will be made to
rise in each compartment and will flow continuously over the upper edge of the
wood partitions H H into the middle space of the cell, in which the electrolytic
action takes place.
By this means a downward current of the electrolyte is produced flowing along
the electrode plates E. E and E E¹ to which current any desired velocity can
be given between practical limits by suitable adjustment of the supply of air under
pressure.
It is difficult to give a rule as to such speed of the circulation that will hold
good for all cases, because the most advantageous speed can vary considerably in
each particular case, according to the nature of the electrolyte.
It is therefore preferable not to state any particular rule but to determine in
each case experimentally the speed which suits best the lye being treated.
Naturally the speed of circulation should be chosen as small as possible, because
an increase in the speed of circulation entails an increase in the power absorbed.
On the other hand the speed of circulation must be increased so far that the zinc
is separated from the electrolyte not as zinc sponge, but as a solid deposit on the
cathode.
By way of example it may be stated that good results will generally be
obtained if with a current density of about 50 ampères to the square metre of the
cathode surface, and with cathodes of about 1 metre length and 0-7 metre width
an air current equal to about 1.3 cubic metre per hour for each cathode is
employed.
In employing with the above proportions a zinc sulphate lye of about 10° Bé.
and an acidulation of about 5 to 10 grammes of free sulphuric acid per litre,
the formation of sponge will be avoided and a fine solid deposit be obtained even
in the presence of the most diverse impurities of which however the greater portion
is assumed to have been previously removed.

page_text: The electrolysis is effected in such manner that zinc is deposited on the cathodes
while oxygen is set free at the anodes. The volume of the lye contained in the
electrolytic apparatus remains constant, with the exception of small losses; it may
be made to circulate continuously between the electrolytic apparatus and the
lixiviation vessels by any known means.
In passing through the electrolytic apparatus the percentage of acid in the
lye is increased and the percentage of zinc is diminished; on passing through the
lixiviation vessels, the percentage of acid is decreased and that of the zinc is
increased, so that the mean composition of the lye is constant.
It has been found further that the way in which the free sulphuric acid is
produced in the depositing vats has an influence on the formation of zinc sponge.
If the acid is added to the lye, zinc sponge is formed under some circumstances
even when the acid is chemically pure; if on the other hand the free acid is
produced by taking in the first instance neutral lye and by depositing zinc from it
until the wished for degree of acidity is reached, without regenerating the lye with
zinc oxide, the formation of zinc sponge is obviated. As in this initial process zinc
sponge may be formed, the cathodes after the standard degree of acidity has been
reached, are cleaned from the zinc sponge; and subsequently the special regular
process commences, in which the acid formed in the depositing vats is fixed by
dissolving zinc oxide.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, we declare that what we
claim is:-
1. An improved process for the electrolytic production of zinc, consisting in
subjecting more or less purified zinc solution to electrolysis while at the same time
the electrolyte is caused by means of compressed air or other suitable means to
circulate at so high a speed as to prevent the formation of zinc sponge, substantially
as described.
2. Improved process for obtaining zinc by electrolysis consisting in first roasting
the zinc ore, then lixiviating the same preferably by means of sulphuric acid, and
after separation therefrom of the greater part of the impurities by known means,
subjecting the same to electrolytic action by means of zinc cathodes and anodes of
carbon lead or other bodies not readily attacked by oxygen, while at the same time
air under pressure is forced in at the lower part of the cells so as to produce
a rapid circulation of the electrolyte substantially as and for the purpose
described.
3. Improved process for obtaining zinc by electrolytic action, consisting in first
freeing a zinc solution as much as possible from other metals by known means, then
bringing the same to a concentration of about 10° Bé. then with a degree of acidity
of about 5 to 10 grammes free sulphuric acid per litre of the solution, subjecting the
same to electrolytic action, while at the same time air under pressure is forced in at
the lower part of the cells in the proportion of about 1-3 cubic metres of air per hour
for electrode plates of about 1 metre length and 0-7 metre width and a current
density of about 50 amperes to the square metre, substantially as and for the
purpose described.
4. The use of effecting the electrolytic production of zinc, of apparatus consisting
of the combination or a suitable source of electric energy, connected with the
electrodes of an electrolytic cell or battery of cells, means for producing air under
pressure, and pipes or channels so arranged as to lead such air under pressure to
near the bottom of the cell or cells where the pipes are provided with apertures
through which the compressed air issues into the electrolyte, substantially as and
for the purposes described.
5. The combination with electrolytic cells for the production of zinc of partitions
such as H of neutral material reaching from near the surface of the liquid in the
cell to near the bottom of the vat, so as to form separate compartments, such as K
in the cell, and pipes P1P* passing down to near the lower edge of the said

page_text: partitions and provided at their horizontal part P2 with perforations for the issue
of air under pressure, substantially as described.
6. In the improved process for the avoidance of zinc sponge in the electrolysis of
zinc, the production of the requisite degree of acidity by electrolysis of the lye,
without simultaneous regeneration by zinc oxide, instead of the direct addition of
free acid substantially as described.
Dated this 19th day of Iul 1895
ABEL & IMRAY,
Agents for the Applicants.