Improvements in the Electrolytic Production of Zino and Alkalies.
Year: 1896

page_text: Date of Application, 16th Nov., 1396-Accepted, 13th Feb., 1897
COMPLETE SPECIFICATION.
Improvements in the Electrolytic Production of Zino and Alkalies.
I, Dr. OTTO DIEFFENBACH, of Hugel Strasse 8 Darmstadt, Germany, Professor,
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:-
In separating zinc by electrolytic process from alkaline solutions, there may be
employed with great advantage simultaneously and in the same baths, sulphates of
the alkalies for decomposition by the electric current, for the purpose of producing a
solution of potash or soda. In this way, in particular, sulphate of soda which is
produced in considerable quantities as a bye-product of many chemical processes and
which is almost valueless, can be very advantageously utilised.
But the practical execution of this process is successful only with the use of special
electrolytic apparatus.
On attempting to electrolyse simultaneously an alkaline solution of zinc and
sulphate of soda by placing the former in the cathode space, and the latter in the
anode space of a decomposing vessel divided by a diaphragm into two compartments
or spaces (as shewn diagrammatically in Fig. 1 of the accompanying drawing
where A is the anode, C the cathode, and D the diaphragm) it will be soon found that
no increase of the soda solution in the cathode space takes place, and that almost no
free acid at all is produced in the anode space, that is to say, that the sulphate of soda
remains apparently undecomposed.
The reason of this occurrence is the following:-
As is generally known, in the electrolysis of salts of the alkaline metals in baths
with diaphragms, the percentage of free alkali in the cathode space-where the said
cathode space has been charged from the commencement with a dilute alkaline
solution-can only be carried up to a certain limit, namely, at most to from 10 to
With a constant current strength, the increase becomes gradually
smaller, until it ceases altogether.
12
per cen'.
On the other hand a small quantity of free alkali also appears from the commence-
ment in the anode space, and continues to increase until, when the percentage of
alkali in the cathode space has become constant, further alkali is formed only in the
anode space. This last alkali of course does not remain therein as such but enters
into chemical combination with the products of the decomposition which are being
given off at the anode. Therefore if an alkaline solution of zinc be electrolysed
together with sulphate of soda under the above mentioned conditions, and since in
order to obtain any considerable amount of zinc in solution, a solution of soda must
be employed of at least 18 to 20 per cent. the percentage of soda contained in the
said solution in the cathode space will not increase; and fresh soda will be formed
only in the anode space.
This combines with the sulphuric acid separated at the anode to form sulphate of
soda, so that the solution in the cathode space is only very slightly varied.
The conditions are however different if by the employment of two diaphragms
between the anode and cathode spaces a third, middle space is formed, which is
supplied with sulphate of soda like the anode space, as shewn at Fig. 2 of the drawing,
where DD are the two diaphragms between the anode A and the cathode C.
In this case there will be separated from the alkaline zinc solution in the cathode
space, metallic zinc, while its percentage of soda will remain nearly constant; this

page_text: solution is enriched with zine by heating it together with zinc oxide zinc hydroxide,
or even metallic zinc in the same proportion as it loses zine this being effected if
necessary, under pressure for increasing the action. Now whereas with a
decomposing vessel having only one diaphragm free alkali would be formed in the
anode space as above explained, in the apparatus with three spaces this formation will
take place in the middle space.
The
The soda lye here formed is separated from the anode liquid by a diaphragm,
and can therefore not combine with the free sulphuric acid there formed.
latter is partly neutralised with lime &c., the anode lye being made to circulate
regularly.
A transference of zine from the cathode space into the middle space only occurs to
a very slight degree, as the compound ZnONa₂O-in which form the zinc is contained
in the alkaline solution-being a bad conductor of electricity only assists to an
exceedingly slight degree in the conduction of the current, this being mainly effected
by the excess of soda lye.
The soda lye formed in the middle compartment is regularly removed therefrom,
and can be separated from the sulphate of soda by evaporation, the latter separating
as a solid salt from the easily soluble soda.
The percentage up to which the soda lye in the middle space can be advantageously
allowed to increase is not more than from 4 to 5 p.c., as with a higher percentage the
yield becomes worse. But there exists a simple means for raising the lyes to a higher
percentage of soda, namely the following. If, to start with, a nearly saturatel
solution of sulphate of soda at a temperature of from 35° to 40° C. be introduced in
the middle space, it will later on, when allowed to cool outside the vessel, separate-out-
so much sulphate of soda with 10 molecules water that the percentage of the
remaining liquid will be increased about two fold. It is therefore in this way easy
to produce lyes with 8 p.c., and more, of soda, which can then be concentrated by
evaporation in the usual manner.
_ The process can also be modified in such manner, that, on starting a dilute soda lye.
is introduced into the middle space; in this case it will become richer in soda,
up to a certain degree, and thus a lye free from sulphate of soda will be directly
obtained, which is regularly removed from the vessel and concentrated by evaporation
in the manner above described; but the yield is in this case smaller than when
proceeding in the previously described manner.
Lastly, this process, or the described apparatus can also be employed with the
simultaneous electrolysis of halogen alkalies together with alkaline zinc solution, as
has been before proposed.
As there described, this process cannot be carried out, because as previously stated,
the alkaline zinc solution in the cathode space cannot increase as such, already at a
percentage of 10 to 12 p.c. NaOH; it has however as a matter of fact a much higher
percentage as otherwise it cannot dissolve sufficient quantities of zinc.
A new production of soda, or potash, can also in this case take place in a third space
formed by two diaphragms between the anode and cathode spaces, which is filled with
alkaline chlorides, or dilute alkaline lye, and the freshly formed alkalies can only
be gained by removing the lye from the said space at regular intervals (as soon
as it has become sufficiently rich in alkalies) and concentrating the same by
evaporation, or separating the same from the alkaline chlorides that dissolve with
difficulty.
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
claim is :-
1. A process for the electrolytic production of zinc together with soda or potash
from the corresponding sulphates, by means of an electrolytic decomposing apparatu
that is divided by two diaphragms into three compartments or spaces, in the cathode
space of which metallic zinc is separated from an alkaline zinc solution, such process
consisting in the simultaneous production of free sulphuric acid from the sulphate

page_text: solution contained in the anode space and free alkali from the sulphate solution in the
middle space, the alkali being obtained in a pure condition by the regular renewal of
the lye, enriched up to a certain degree, and by the separation of the sulphate salts by
evaporation substantially as described.
2. In the process referred to in the first claim, the introduction of a dilute alkaline
lye in place of the corresponding sulphate in the middle compartment, and the
evaporation of the solution enriched with alkali to a certain extent by evaporation,
substantially as described.
3. In the electrolytical decomposition of halogen alkalies together with alkaline
zinc solution, under employment of the same process and apparatus referred to in the
first and second claims, the regular removal of the lye from the middle compartment,
with the object of obtaining the alkali there formed by concentration and eventual
separation of the chlorides contained in the solution, substantially as described.
Dated this 16th day of November 1896
ABEL & IMRAY,
Agents for the Applicant.