Improvements in the Electro-metallurgic Extraction of Zinc.
Year: 1892

page_text: Date of Application, 15th Feb., 1892-Accepted, 19th Mar., 1892
COMPLETE SPECIFICATION.
Improvements in the Electro-metallurgic Extraction of Zinc.
I, GEORG NAHNSEN of Christoph Strasse, Cologne, in the Province of the
Rhine Kingdom of Prussia, German Empire, Engineer 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 :-
My invention relates to improvements in the electro-metallurgic extraction
of zinc and consists in the employment of an electrolyte presenting a great
conductivity and in reducing the polarization by applying a temperature of
about 130° F.; the object of my improvements is to effect a considerable economy
of energy in the extraction of zinc.
Similar improvements have been attempted in order to secure the possibility of
employing currents of a reduced density and thereby to effect a saving of electric
energy; and it was clear that this could not be attained except by two means, the
one consisting in the use of zinc salts sustaining a high temperature and permitting
the employment of currents of low density, and the other in the refrigeration of the
electrolyte containing zinc. I have tried the first means, but have finally found by
experiments a method according to the second means which yields very satisfactory
results.
The basis from which I started was the employment of double salts of sulphate of
alkali and of zinc, as proposed by Herrmann. It is known that these salts allow
the density of the current to be reduced. The conductivity of the same was
however said to be smaller than that of the simple salts, in consequence of their
very small solubility. Moreover these double salts have a very noxious and
disturbing quality preventing their technical use, and I suppose that for this reason
the said salts have not yet been employed in practice. The zinc is in these double
salts an element of the anion and therefore makes its way to the anode, together
with two molecules of SO, whilst the zinc precipitated on the cathode is reduced
by secondary action. This particularity of the double salt has in practice very
disagreeable consequences, which render almost impossible a regular and undisturbed
progress of the electrolysis in the baths.
When a soluble anode is employed for the purpose of separating or refining
metals, a solid salt is formed at the anode even at a very low density of current,
and finally the passage of the current is interrupted. When using a solution of
simple sulphate of zinc at a concentration of 30° Beaumé and a current having at
the anode a density of 418 amperes per square yard, no crystallization takes place,
but when employing a concentrated solution of sulphate of zinc and ammonia the
formation of crystals is already observed with a current of 63 amperes per square
yard. The reason of this chemical process is, that in the last-mentioned salt two
equivalents of sulphate of zinc go to the anode.
When employing an insoluble anode the conditions are still more complicated.
When electrolyzing sulphate of zinc alone, it is decomposed into Zn and SO, the
latter forming sulphuric acid by combining with the water and giving off one atom
of oxygen; this sulphuric acid being specifically lighter than sulphate of zinc, rises
to the surface. A double alkaline salt, for instance sulphate of potash and of zinc is
under ordinary circumstances decomposed by electrolysis into K and Zn SO, SO
so that even when employing an insoluble anode an agglomeration of sulphate of
zinc is formed at the same, whereby the nascent free acid is drawn to the bottom.
The slightest changes in the concentration of the electrolyte, on which changes
largely depends the quality of the zinc of belonging to the anion or to the kathion,
influence this process, and the practical result of the latter is, that part of the
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page_text: nascent acid rises to the surface, but by far the greater portion of the same sinks to
the bottom. Even the best regulated and best conducted circulation is at length
insufficient to prevent the stated inconvenience. A remedy may be found in
employing very diluted solutions, but there the conductivity is so much decreased
that no sensible advantage is secured.
Now I have ascertained by experiments that the decomposition of the double
salts of sulphate of alkali and zinc is influenced by the temperature in the same
measure as by a dilution of the electrolyte, a fact the probability of which was
also indicated by the theory. It is possible to ascertain experimentally the changes
in the decomposition from degree to degree. Between 104 and 122° F. the point is
reached at which the decomposition of the double salt into the simple_salts is
completed. In practice I prefer to employ a temperature of 122 to 140° F. and I
obtain thereby with perfect security a decomposition taking place in the same
manner as when simple salts are used; and all the nascent acid, as in the latter
case, rises to the surface.
The simple conditions under which the circulation is effected in the electrolysis
of sulphate of zinc alone, a dilution taking place at the cathode-by the zinc being
precipitated out of the solution-and at the anode -by the formation of free
sulphuric acid without any agglomeration of sulphate of zinc-these same simple
conditions are by my improvements established for double salts of sulphate of alkali
and zinc. At the high temperature which I employ the salts are no more double
salts from the electro-chemical point of view. Notwithstanding the character of a
double salt being completely wanting in the electro-chemical sense, the relation
between the density of the current at the cathode and the deposit of zinc has
remained the same, and it is thereby clearly shown, that it is not the quality of the
electrolyte of being a solution of a double salt which is the first condition for the
employment of currents of low density, but only the partial or complete secondary
reduction of the zinc by the primary alkali.
Having ascertained these facts by experiments I still improve the electrolyte by
reducing the proportion of zinc or of sulphate of zinc contained in the same, as far
as the intended density of the current permits. Thus I employ a very diluted
solution of sulphate of zinc, containing but 1 to 24 oz. of crystallized sulphate of
zinc per liter according to the density of the current, and enrich this solution with
sulphate of alkali-I may also, but with less favourable result, employ sulphate of
magnesia, of which I take as much as will be dissolved at the temperature at
which it is to be used, or, for practical reasons, at a temperature about 18° F. below
the first. The electrolyte which I employ contains 13 to 24 oz. of crystallized
sulphate of zinc and 10 to 5 oz. of sulphate of alkali per liter according to the
density of the current. It will be seen that even in the chemical sense there is no
double salt.
The advantages I realize by the employment of this electrolyte are the
following:
1. A very simple circulation.
2. An extremely high conductivity of the electrolyte, being 300 to 400% higher
than that of the sulphate of zinc at the degree of concentration yielding the best
conductivity.
3. The possibility of employing any density of current of practical value, that is
to say from 8 to 125 amperes per square yard.
4. The possibility of employing soluble anodes for this electrolyte.
5. The diminution of the polarization in consequence of the higher temperature.
6. The small amount of energy needed.
When employing my improved electrolyte and steam engines fitted with
condensers it is possible to extract a ton of zinc from the ore with a consumption
of but 3 to 3,5 tons of coal.
.
In order to show how very little the temperature influences the zinc deposit in
my improved electrolyte I will mention that I performed an experiment with a
temperature of 198° F. and a current having a density of 84 amperes per square

page_text: yard, and that the deposit of zinc did not become spongious during this experiment,
though steam bubbles were formed at the kathode.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that
what I claim is :-
In the electro-metallurgic extraction of zinc: The employment of an electrolyte
artificially heated and kept heated during the extraction, said electrolyte con-
taining zinc and an alkali or magnesium, for the purpose as described.
Dated Berlin, this 12th day of February 1892.
By Robert Deissler,
GEORG NAHNSEN,
38, Alexander Strasse, Berlin, C., Germany, Agent for the Applicant.