Note: Descriptions are shown in the official language in which they were submitted.
1317560
The invention relates to a method for ~he
precipitation by electrolysis of metallic mercury from an
electrolyte wherein the electrolyte containing mercury-
(I)-chloridP (Hg2C12~ in suspension reacts with chlorine
which oxidizes the mercury (I) chloride to soluble mercury-
(II)-chloride (HgC12), which latter is cathodically reduced
by electrolysis under the influence of chlorine to liquid
mercury.
The method according to the invention is
advantageously employed whenever mercury-(I)-chloride
available in suspension is to be precipitated and to be
recovered as metallic mercury. Mercury-(I)-chloride is for
instance frequently recovered in processes for the
purification of gases, which are obtained in a reactor with
the aid of a solution of mercury-(II)-chloride.
A method of this kind has been described in
European Patent Publication No. 179,040. The chlorine gas
developed in electrolysis is removed from the electrolysis
vessel and is conducted to a reactor wherein oxidation
takes place. While this has the advantage that the
chlorine gas required in the process is produced by the
method itself, a particular chlor:ine gas duct between the
electrolysis cell and the oxidation reactor is required.
In this connection, particular care must be taken that the
poisonous chlorine gas is not blown into the environment.
By suitable measures, the chlorine gas subsequently has to
be introduced, finely dispersed, into the suspension in the
reactor in order to be able to carry out the oxidation
reaction.
The prior publication does not described the
exact construction of the electrolysis cell employedO It
can be assumed however that an electrolysis cell is used
which is divided by a diaphragm since the prior art
according to German Auslegeschrift No. 2,011,610 advises in
general to keep chlorine away from the cathode in the
electrolysis process since chlorine is reduced at the
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cathode thereby diminishing the current yield of the metal
precipitation taking place (compare in this connection
particularly column 8, line 63 and following of German
~uslegeschrift No. 2,011,610).
5The present invention avoids this disadvantage.
An object of the invention is to provide a method for the
precipitation of metallic mercury by electrolysis from an
electrolyte including the features mentioned above, which
excels by a sensibly simplified procedure in combination
with a sensibly increased yield of precipitated metallic
mercury.
Accordingly, the invention provides a method for
the precipitation by electrolysis of metallic mercury from
an electrolyte containing mercury-~I)-chloride, said method
comprising: (a) providing an electrolysis cell containing
an anode, a cathode and an electrolyte containing mercury-
(I)-chloride, and in which chlorine developed at the anode
during the course of oxidation can migrate without
impediment to the cathode area of the cell; and (b)
su~jecting said electrolyte to electrolysis, whereby: (1)
chlorine is formed at the anode and reacts in situ with
mercury-(I)-chloride to form mercu:ry~ chloride, and (2)
mercury-(II)-chloride is reduced at the cathode to metallic
mercury.
25Thus, the invention is characterized in that when
employing an electrolysis cell wherein the chlorine
developed at the anode during the course of the oxidation
can migrate without any impediment into the cathode area of
the cell, the chlorine developed during electrolysis in the
electrolysis cell is employed in situ for the oxidation of
the mercury(I)-chloride to soluble mercury-(II)-chloride.
By so proceeding, the particular flow the
chlorine gas developed during the electrolysis to the
oxidation reactor is dispensed with. Oxidation and
electrolysis are instead carried out in one and the same
electrolysis cell so that the chlorine gas developing
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during the course of electrolysis can, directly and without
any impediment, also migrate into the cathode area of the
cell. The gaseous chlorine developing during the course of
electrolysis is, in general, dissolved in the electrolyte
and is transported together with the electrolyte, supported
if necessary by a pump or similar device. The whole volume
of the electrolyte is therefore available for the oxidation
of the mercury~ chloride to mercury-(II)-chloride by
means of the gaseous or dissolved chlorine.
To carry out the method as described, a fixed
bed electrolysis cell is advantageously employed, which
does not include a separate anolyte circuit but a diaphragm
preventing direct contact of the counter electrode but does
not impede the material exchange. It has become known in
a basic embodiment through Applicant's German Patent
Specification No~ 2,622,497 and, in an improved embodiment,
through German Patent Specification No. 2,904,539. A
further improved embodiment has been described in
Applicant's German Patent Specificcltion No. 3,532,537. Any
of these embodiments basically may be employed for tha
invention, preferably using the features mentioned above.
The method according to the invention is carried
out such that the electrolyte, at least in the beginning,
is employed in such a concentration of mercury~ chloride
and/or that the electrolysis voltage and thereby the
electrolysis current, at least in the beginning, is
employed at such a level that metallic mercury is
precipitated at the cathode and drips off therefrom~ It
can be withdrawn from the bottom of the electrolysis cell.
As the material for the cathode, iron, silver, nickel,
copper, cadmium, aluminum, zinc, tin or an alloy of these
metals is suitable.
Deviating from the suggestion of German
Auslegeschrift No. ~,011,610 already mentioned, anodically
developed chlorine is employed for oxidation of the
difficultly soluble mercury-(I)-chloxide which dissolves
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this salt so that the starting suspension of the
electrolysis is made usable.
Experiments have shown that it might be
sufficient to apply the electrolysis voltage, and hence the
electrolysis current, only for a relatively short period of
time at the beginning at a voltage peak, that is until a
first mercury film has formed on the surface of the amalgam
cathode. Subsequently, the voltage may be reduced to
normal values and the mercury continues to precipitate in
metallic form on the surface of the amalgam cathode. The
concentration of Hg2Cl2 in the electrolyte is corresponding.
The method according to the invention ~s based on
the theory that, at the anode of the electrolysis device,
Cl- converts into Cl2. The Cl2 reacts with the Hg2Cl2
available in the form of a suspension and results in 2HgCl2
which is dissolved in the electrolyte.
At the cathode, HgCl2 is reduced to Hg+2 Cl .
This Cl is then available for the oxidation of the cathode.
As electrochemical equations, the following can
be written:
Starting reaction (as an example only~ as to how Hg2Cl2 may
be developed as a suspension in an electrolyte:
(A) Hg (gaseous) + HgCl2 (soluble)
= Hg2C12 (difficultly soluble)
Oxidation:
(B) Hg2C12 (difficultly soluble) + Cl2 (gaseous)
= 2HgCl2 (soluble)
Electrolysis.
(C) HgCl2 ~soluble) + electrical energy
= Hg (liquid) + Cl2 (gaseous)
Result: From Hg (gaseous) + Energy,
Hg (~iquid) is obtained.
The C12 (gaseous) developed in step C is used as
such, and in situ, in step B.
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These electrochemical reactions constitute at
least an approximate picture of the processes actually
occurring during electrolysis. In reality, these processes
may be more complicated, for instance via corresponding
complexes or multi reactions.
In one embodiment, where copper was used as the
cathode material, the electrolyte employed had the
following composition:
45 g/l HgzCl2 as suspension
30 g/l sulfuric acid
lO g/l chloride.
The anodic current dansity was 300 A/m2. Since
an expanded metal was employed, this value is based on the
total area inclusive of the gaps. The cathodic current
density also amounted to 300 A/m2.
The initial content of water-insoluble Hg2Cl2 was
equal to the concentration already mentioned of the
suspension of 45 g/l. The initial current density amounted
to 600 A/cm2.
