Note: Descriptions are shown in the official language in which they were submitted.
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A Process for Electrolytically Detoxifying or
- Regenerating an Aqueous Solution Containing Cyanide
and an Apparatus for Carrying out the Process
The invention relates to a process for electrolytically detoxifying or regenerating
5 an aqueous solution containing cyanide, wherein cyanide is converted by
means of anodic oxidation into carbon dioxide and/or nitrogen and/or
ammonia, also an apparatus for carrying out the process.
German Patent Application DE-OS 26 00 084 describes the electrochemical
treatment of dilute metal cyanide solutions such as occur, for example, in the
10 forrn of washing solutions in electroplating installations; in the described
process, cyanide is anodically oxidized while rnetal is simultaneously
cathodically deposited. The anode and cathoda are designed as reticulate
structures or wire-mesh baskets, and both electrodss may be made from
platinum-coated titanium; however, it is also possible to uss a copper cathode.
15 In accordance with one embodiment of the invention, the solution to be treated
contains, in addition to copper cyanide, also sodium cyanide and sodium
carbonate as well as Rochelle salt.
When carrying out electrohJtic oxidation in low concentration ranges, the
unsatisfactory current efficiency proves to be a problem so that, in view of the20 customarily strictwaste discharge conditions (environmental restrictions~
imposed by the authorities, it is essential to use an additional process for
detoxifying low cyanide concentrations. Furthermore, high rates of srosion (=
short seMce iives) occur at the platinum coated titanium electrodes, so that in
practice the process for the electrochemical oxidation of cyanide is not very
25 economical because of the high resulting costs (anode costs~.
Furthermore, German Patent DE PS 28 36 720 describes a procsss for the
continuous electrolytic regeneration of a washing solution containing silver
cyanide, such as occurs during electroplating processes, wherein the
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concentration of cyanide is greatly reduced by anodic oxidation to ammonia,
and at the same time the silver is deposited in spongy form on the cathode,
from which it is periodically stripped and collected in a funnel arranged in thebottom of the regeneration cell and then filtered off. In accordance with one
5 embodiment of the invention, the anode possesses a coating of titanium and
platlnum while the cathode is plated with purs silver, so that here again the
problem of high erosion rates of platinum-coated titanium electrodes is
encountsr~d, thereby resulting in high attendant costs in practice.
It is an object of the invention to propose a means by which the large volumes
10 of cyanidic waste water accumulating, in particular, in electroplating operations
may be elactrochemically treated in a cost-effective manner, with high current
efficiencies beirlg achieved even at low concentrations, thus rendering further
detoxification processs superfluous; furthermore, the purpose is also to ~.
propose an apparatus for the electrolytic detoxification or regeneration of an
15 aqueous solution containing cyanide; and an additional goal is to increase the
Iifetime of the anodes.
In general terms, ths invention provides a process for electrolytically detoxifying
or regenerating an aqueous solution containing cyanide, wherein cyanide is
converted by means of anodic oxidation into carbon dioxide and/or nitrogen
20 and/or ammonia, characterized in that in order to improve the efficiency of the
cyanid~ conversion process a metal and/or metal ions having cathodic
potential are introduced into the solution, and further characterized in that the
oxidation takes place at a dimensionally stable anode.
An important advantage of the process is that, due to the high degree of
25 efficiency, a rapid reaction sequence is achieved, and thereby not only the
powe~ consumption costs but also the electrode costs (coating costs) may be
held relatively low.
A further advantage is obtained by recycling tne cathodically deposited metallic`
component of the cyanide compound.
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According to another general aspect of the present invention, apparatus is
provided for electrolytically detoxifying or regenerating an aqueous solution
containing cyanide by means of anodic oxidation, wherein at least two
eiectrodes are immersed in the solution, and at least one of these electrodes
5 consists substantially of valve metal, and the surface of the anode contains at
least a noble metal and/or noble metal oxide, and at least the surface of the
cathode contains silver or copper, characterized in that the anode is a
dimensionally stable electrode having an electrocatalytic coating consisting of
oxides of the platinum group oF metals and/or of valve metal, and consisting
10 also of platinum metals, and the spacing between the anode and the cathode is in the range of 5 - 50 mm.
The term "expanded metal grid" used in the present disclosure and claims also
relat~s to an expanded metal foil.
One important advantage of th0 equipment is that the anode has a long service
15 life because of its dimensionally stable structure and the high degree of
resistance of its coating. Furthermore, because of the high degree of
efficiency, the size and thus also the costs of the equipment may be kept :
relatively low.
In the following, the subject of the invention is explained on the basis of various
20 embodiments.
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An electrochemical cell having a solution container with a superjacent cell
housing containing a paGkage of elongated, plate-shaped electrodes arranged
parallel to and at a distance from each other, with the anodes being arranged
25 in the centre of the housing and the cathodes at the ends of the housing, is
used to electrochemically detoxiFy a pure cyanidic solution containing 5 - 100
g/L cyanide. The cathode used consists of an expanded metal grid of copper-
plated titanium (copper surface), while the anode takes the form of a
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dimensionally stable titanium-based anode having an electrocatalytic coating
consisting oF oxides of the platinum group of metals and/or of titanium, and
also corisisting of metals from the platinum group. The fundamental structure
of such anodes is known, for example, from German Patent DE-PS 20 41 250.
5 The volume o~ the solution container is in the range of 50 - 1000 L; the anodic
current density is in the range of 10û - 2500 A/m2. The distance between the
electrodes is in the range of 0.5 - 5 cm. The reac~ion takes place at a
temperature in the range of 20 - 50C. The electrolytic detoxification is carried
out until a final cyanide concentration of 0.2 mg/L and a final copper
10 concentration of 0.1 mg/L is achieved.
EXAMPLE 2
When treating a solution of the kind mentioned in Example 1 with the addition
of copper in the range of 10 - 500 mg/L copper ions, it is also possible to carry
out the first phase of the treatment (first run-through) using a cathode made of15 expanded titanium metal without any copper-plated surface or copper coating.
The copper is added either in the form of copper sulphate (solid or dissolved)
or in the form of a waste water solution containing copper ions. The remaining
process parameters mentioned in Example 1, as well as the structure of the
dimensionally stable anode and the electrode spacing mentioned in that
20 example, are retained.
EXAMPLE 3
A Gyanidic silver solution containing 13.5 g/~ silver ions and 1.8 g/L free
cyanide ions is demetallized using an electrochemical apparatus having a
dimensionally stable anods in accordance with Examples 1 and 2 and squipped
25 with a cathode made of expanded copper metal foil. The electrochemical
treatment is carried out until a silver ion concentration of less than 0.1 mg/L is
obtained; the residual cyanide concentration following demetallizing is 8.~ g/L
free cyanide ions. The remaining process parameters, such as the anodic and
cathodic current density, the electrode spacing and the temperature
30 correspond to those quoted in Example 1.
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However, it is also possible, instead of an expanded copper metal foil, to use
an expanded silver metal foil.
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The demetallized solution obtained in this manner may be detoxified accordingto the process described in Example 1 or according to the process described
5 in the following Example 4.
EXAMPLE 4
A silver cyanide solution from which the silver has been removed (silver ion
concentration less than 0.1 mg/L) and containing 8.6 g/L free cyanide is
supplied to a suitably constructed electrochemical cell whose cathode consists
10 of copper-plated expanded titaniurn metal. The current density and electrode
spacing, as well as the operating temperature, are maintained at the sama
values as those quoted in the preceding examples. The electrolytic
detoxification (anodic oxidation) i5 carried out until a cyanide content of 0.1
mg/L is measured.
15 In an apparatus for carrying out the processs of Examples 1 to 4, solution issupplied by means of a recirculating device to ~he bottom area of the cell
housing1 and once it has passed through the package of electrodes it flows via
an overflow over the side walis of the cell housing into a discharge container,
arran3ed in cascade fashion with its own overflow, for the purpose of
20 degassing the solution; once the solution has passed a second overFlow stage
it has become substantiaily degassed and it is returned to the solution
container and recirculated, and it continues to undergo electrochemical
treatment until the desired or legally prescribed residual concentration is
attained.
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