Note: Descriptions are shown in the official language in which they were submitted.
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Method and Apparatus for Regenerating
an Aqueous Solution Containing
Metal lons and Sulphuric Acid,
and the Use of the Method
5 The invention relates ~o a method for regenerating an aqueous solution
containing metal ions and sulphuric acid, in particular a solution containing zinc
ions, nickel ions, iron ions and/or copper ions, in an elec~rolytic cell, wherein
the metal ions are cleposited on the surface of the cathode, and oxygen and
protons are formed by hydrolysis at the anode; the invention also includes the
10 utilization of the method and an apparatus for carrying it out.
The textbook "Praktische Galvanotechnik" [Practicai Electroplating], published in
1970 by the Leuze Verlag, Saulgau/Wurttemberg describes on pages 537 and
538 how zinc may be cathodically precipitated from sulphate electrolytes. Such
sul,chate electrolytes are formed when zinc chloride solutions are converted into
15 zinc sulphate solutions by means of ion exchange methods; the purpose of thispreliminary procedural step is to avoid electrolytic processing of chloride
electrolytes becaus~, when zinc chloride electrolytes are electrolytically
processed, chlorine would be generated and this cowld pose a considerable
source of danger.
20 Such direct regenera~ion of a zinc chloride solution is described in German
patent application DE-(:3S 25 39 137, according to which the solution containingohioride ions is introduced into a cathode chamber in an electrolytic cell whichis subdivided into thrae chambers, namely an anode chamber, a cathode
chamber and an electrolyte chamber arranged between them; the anode
25 chamber is bordered by a porous diaphragm of low permeability separating the
anolyte, which contains sulphuric acid, from the electrolyte. The anions of the
anolyte possess a sufficiently high oxidation potential to guarantee that
essen~ially only water is decomposed at the anode under operating conditions,
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while the cathode chamber is bordered by a diaphragm of relatively high
permeability. The anolyte contains a substance which is capable of combining
with the chloricle ions which enter into the anode chamber in order thus ~o
prevent oxidation of chloride ions at the anode. The liquid level of the anolyte5 is maintained in such a way, possibly by adding more anolyte, that it remains
above the liquid level of the adjacent electrolyte, so that the desired flow
velocity through the diaphragm is maintained in order to achieve the desired
technical goals. In order to prevent chloride ions which penetrate through the
anode diaphragm from being oxidized into chlorine gas, the anolyte contains an
10 addition of silver sulphate in order to guarantee that the chloride is precipitated
out as silver chloride.
With this arrangement, problems are created by the relatively complicated
subdivision of the electrolyte space into three chambers, as well as by the use
of diaphragms whose permeability can vary greatly in the course of the
15 electrolytic process. Other problems are caused by the addition of silver
sulphate, the formation of silver chlorids, and the removai of the latter from the
cell, and also by the risk of the diaphragms becoming clogged due to
precipitations of silver chloride. I
Furthermore, on page 210 of the book "Angewandte Elektrochemie" [Applied
20 Electrochemistry] by A. Schmidt, published by Verlag Chemie Weinheim 1976,
conditions are mentioned according to which zinc, despite its electronegative
standard potential of -0.763V, can be precipitated from aqueous solutions
because of the hlgh overvoltage of hydrogen on zinc; on this point it is stated
that in order for zinc to precipitate out, a relatively high zinc ion concentration is
25 needed at the cathode otherwise, from a certain poin~ onwards, hydrogen
instead of zinc would be cathodically deposited because of the increasing
sulphuric acid concentration. Various examples of zinc electrolysis methods are
described on page 213 of the same book.
It is an object ot the present invention to provide a method and an apparatus
30 by means of which sulphuric-acid-based pickling or extraction solutions
containing a heavy load of metal ions can be substantially demetallized, and at
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the sam~ time pure, highly concentrated sulphuric is recovered. In the
process, the cathodic deposition of hydrogen, which may occur in particular in
aqueous solutions with relatively low metal ion concentrations, must be reiiablyavoided.
5 The method is intended to be used as an intermediate stage in the
regeneration of pickling or extraction solutions without the production of
chlorine gas.
Furthermore, it is an object of the invention to provide a use for the substances
contained in the solution, so that to a very large extent they can be recycled.
10 The task is solved by the rnethod according to the invention in that the solution
containing the metal ions is introduced as the catholyte into an electrolytic cell
subdivided using an anion exchange membrane which is resistant to sulphuric
acid; because of the voltage applied to the electrodes, sulphate ions from the
catholyte migrate through the anion exchanye membranc into the anolyte,
15 sulphuric acid is generated with the anodically formed protons, and the
concentration of sulphuric acid in the anolyte is constantly increased.
In a preferrad embocliment of the method, the concentrated sulphuric acid is
removed frorn the anolyte.
One major advantage of the method is that the concentrated sulphuric acid can
20 be resupplied to the pickling or extraction process, in a closed circuit, as a
fresh component of the solution, and the cathodically deposited metal can also
be supplied to a recycling process.
The method may be operated either batchwise or continuously. When
batchwise operation is selected, a catholyte solution is supplied whose
25 sulphuric acid concentration matches in each case the initial concentration of
the anolyte; if, on the other hand, the catholyte solution is continuously
supplied, its sulphuric acid concentration must always be below the sulphuric
acid concentration of the anolyte. A catholyte solution having a sulphuric acid
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concentration in the range of 60 to 80 g/L is supplied. Cathodic deposition
occurs at a current density in the range of 250 - 1500 A/m2. Once the layer of
deposited metal has attained a predetermined thickness, the cathode is
removed from the catholyte chamber.
5 The equipment used to solve the task according to the invention consists of anelectrolytic cell which is subdivided into an anolyte chamber and a catholyte
chamber by means of an anion exchange membrane which is resistant to
sulphuric acid; the catholyte chamber possesses at least one opening through
which the solution containing metal ions is supplied, and the anolyte chamber
10 possesses at least one opening through which the sulphuric acid formed by
electrolysis is removed; and the cathoda is electrically and mechanically
detachable for the purpose of removing the deposited metal.
The method according to the invention is advantageously used as a second
procedural step in a pickling or extraction process, following a first procedural
15 step in which a solution containing chloride ions is converted by means of an ion exchange procedure into a solution containing sulphate ions.
One important aclvantage of the invention is that the metal can be precipitated
in a simple and cost-effective manner from a sulphate solution containing metal
ions, and at the same tirne the sulphuric acid concentration in the anolyte is
20 increased as in a closed circuit, and the acid is then reused to continue the regeneration process.
The invention is described in more detail below With reference to the
accompanying drawings. In the drawings:
Figure 1 is a diagrammatic view of a longitudinal section through an electrolytic
cell; and
Fiyure 2 shows in diayrammatic form the sequence of the rnethod in the form
of a closed circuit.
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ln accordance with Figure 1, the electrolytic apparatus consists of a tank 1
whose interior is subdivided by means of an anion exchange membrane 2 into
an anolyte chamber 3 and a catholyte chamber 4. The anode 5 located in the
anolyte chamber 3 consists of a dimensionally stable valve-metal electrode, in
5 particular a ~itanium electrode, which is connected with the positiv6 pole 6 of a
DC voltage source 7. The fundamentai structure of such dimensionally stable
valve-me~al electrodes, in particular titanium electrodes, is known from the
process of chloralkali electrolysis and is described, for example, in German
patent appiication DE-OS 20 41 250.
10 The cathode 8 located in catholyte chamber 4 consists of expanded copper
metal and it is connected via a detachable electric connection 9 to the negativepole 10 of the DC voltage source. Anolyte chamber 3 contains an aqueous
solution of sulphuric acid which is supplied at the start of the method via the
supply line 11 to produce the conduction of ions; water may be added during
15 the electrolysis process, and the additionally formed sulphuric acid can be
removed via outlet 1~ of the anolyte chamber 3 and can be returned to the
regeneration process or the pickling process.
The sulphate solution containing zinc ions is, for example, continuously
supplied to the catholyte chamber 4 via the supply line 13, and as a rule the
20 sulphuric acid concentration of the catholyte does not exceed that of the
anolyte; the sulphuric acid concentration of the anolyte is in the range of 70
g/L. Once the anolyte and catholyte chamber have been filled, the electrolytic
process commences, and by applying power from the voltage source 7, the
charge is transported during electrolysis through the ion exchange membrane 2
25 by means of the sulphate ions, which are symbolically identified by the
reference number 13. The zinc ions are symbolically indicated by reference
number 14 and they are discharged at the cathode 8, with metallic zinc being
deposited.
In anolyte chamber 3 the water component of the anolyte solution is broken
30 down and the oxygen is removed in the form of gas from tank 1, which is open
at the top, and the hydrogen ions are recombined with the sulphate ions to
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form sulphuric acid, which increases in concentration during the electrolytic
process, and the acid is then removed via outlet 12 and supplied to the picklingprocess. - The sulphuric acid concentration of the anolyte is adjusted by means
of devices which measure the pH value, and by a control loop which maintains
5 the predetermined sulphuric acid concentration, or adapts it to the sulphuric
acid concentration of the catholyte, by removing the concentrated sulphurio
acid and adding water by supply line 11. The catholyta added in the form of a
pickling solution possess a zinc ion concentration of approxirnately 170 g/L
and a sulphunc acid concentration in the range of 70 g/L. The cathode 8 is
10 made from expanded copper metal while the anode 5 is a dimensionally stable
titanium anode, to which reference has already been made. A compact layer of
zinc is deposited out on the cathode 8. The current density of the cathode is
in the range from 250 to 1500 A/m2.
In principle, the same electrolytic apparatus may also be used for batchwise
15 operation, in which case the anolyte is removed in batches or continuously
within certain concentration ranges while the catholyte side is intermittently
replenished.
According to Figure 2, the sulphate solution containing zinc ions which flows
out of outlet 21 of a pickling apparatus 20 is supplied via supply line 13 to the
20 catholyte chamber 4 oF an electrolytic cell consisting of a tank 1 containing an
ion exchange membrane 2, and the zinc deposited in the catholyte chamber -
indicated schematically by reference number 22 is removed from the catholyte
charnber 4. The aqueous sslution of sulphuric acid whose concentration builds
up in anolyte chamber 3 is removed via outlet 12 and supply line 23 and
25 supplied via inlet 24 to the pickling apparatus 20 as a fresh component in the
pickling process.
The circuit followed by the sulphuric acid solution according to the method i5
shown in Fiyure 2. The spent pickling solution is supplied to the electrolytic cell
in the form of an aqueous sulphate solution containing metal ions, via outlet 2130 of the pickling apparatus 20 and supply line 13; the practically pure
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concentrated sulphuric acid is returned to the pickling process via supply line
23.
The deposited zinc can also be reused and it is removed from this cyclical
process by taking it out of the cell. The anion exchange mernbrane used is a
5 membrane of the ARA type manufactured by the firm of MS:)RGANE (France).
