DD216966A1 - METHOD AND ARRANGEMENT FOR CORROSION PROTECTION OF DIMENSION STABILIZED ANODES - Google Patents

Abstract

The invention relates to a method and arrangement for corrosion protection of dimensionally stable anodes, which are used in electrolysis processes, for example in the chloralkali electrolysis, when using bipolar electrolyzers with ion exchange membranes, in metal refining or even in organic electrolysis. The object of the invention is to reduce the corrosion of noble metal and / or noble metal oxide coated dimensionally stable anodes (DSA), in a significant reduction of the loss of precious metal and a substantial service life of these anodes. The corrosion of the DSA when decommissioning individual cells from a series connection is significantly reduced by replacing the usual short circuit with a switch with suitably dimensioned resistor. The voltage drop across the resistor must be smaller than the equilibrium cell voltage of the electrolysis process and at the same time the anode potential must be above the potential of the reductive corrosion.

Description

Method and arrangement for corrosion protection of dimensionally stable 'anodes'

Field of application of the invention

The invention relates to a method and arrangement for corrosion protection of dimensionally stable anodes, which are used in electrolysis processes, for example in the chloralkali electrolysis, when using bipolar Slektrolyseure with ion exchange membranes, in metal refining or even in organic electrolysis,

Characteristic of the well-known "technical solutions case of inorganic and organic electrolysis process ¹ are used in the electrolysis cells often dimensionally stable anodes (DSA), which are bfiiso-hichtet on its surface with a noble metal or noble metals and / or their oxides. These electrodes are well known, both in short-circuit , for example _for;... purpose of the shutdown operation of a cell, as well as when restarting a considerable corrosion this results in losses of noble metal, .the service life of the electrodes is substantially reduced to eliminate this corrosion are previously no suggestions published · respectively, have been disclosed are known only a few patents relating to the construction of switching elements for large DC currents or the arrangement of the same in series-connected electrolysis cells, for example, DE 20 55 16.1 discloses a device

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protected, which consists essentially of a chassis / which can be moved between the cells and on which two flexible contacts for connection to the cells and the switching elements are constructed. In the patent DE OS 24 48 194 a short-circuiter is described, which is arranged mobile under the cells. The advantage of these devices is the easy and quick handling of the short circuiters. In all these constructions, although a low resistance der'Energieverlust is kept low, but the corrosion of the electrode surface is not avoided, _χ

Object of the invention

The object of the invention is to reduce the corrosion of dimensionally stable anodes coated with noble metal and / or noble metal oxide, with a decisive reduction in the loss of noble metal and a substantial extension of the service life of these anodes.

Presentation of the essence, the invention

The invention has for its object to develop a method and arrangement for the corrosion protection of DSA, which makes it possible to put an electrolytic cell from a battery of cells without damaging the DSA by corrosion out of service and to put into operation again. According to the invention, in a series connection of electrolysis cells parallel to a cell which is to be taken out of operation / maintained a voltage at the electrolysis cell, which is smaller than that. Equilibrium cell voltage of the electrolysis process and at the same time keep the anode potential above the reductive corrosion potential of the anode to carry out this method can be arranged parallel to the electrolytic cell, a switching element whose resistance realized the above conditions to avoid large energy losses in technical

Cells, the resistance after disassembly of the Ström- 'supply or the electrodes may be replaced by a conventional short-circuiting »/. The inventive method will be explained with reference to the schematic representation (Figure 1), Spll example, the cell 2 are taken out of the circuit, .so is a combined switch, consisting of Part I and II, as shown in Figure 1, to cell 2 and 3 connected. . , Part I represents a switch with resistance whose magnitude depends on the load on the cells and which, when shorted, gives the cell a residual voltage which depends on the process parameters, e.g. for the chloralkali electrolysis after the diaphragm, η 'is about 1 volt. Part II is a common short circuiter. , , '. ,

By closing the switch I, the cell 2 is de-energized 'made. After interruption of the power supply A between cell. 2 and 3, the short-circuiter II is closed and cell 2 can be disassembled while cells 1 and 3 continue to operate normally. When restarting the cell 2, after the assembly and connection of the power supply A is still open before pulling the cell 2 with electrolyte of the short-circuiter II. 'Thereafter, the cell 2 is made kqmplett ready for operation and connect your circuit again by subsequent opening of the switch I.

It is known that according to the general behavior of electrochemical cells for a currentless state, no reduction of the voltage to zero is necessary, but only to a voltage which is less than ^7, the equilibrium cell voltage in question. Thus, based on the anodic reaction, only one pier displacement of the potential to values below the equilibrium potential is required to produce a

To achieve powerlessness. Unexpectedly it was found that a strong corrosion of the DSA only in connection with a reduction of surface oxides or valence changes of surface compounds > (reductive corrosion) at potentials below the. Anodic main reaction begins (see Embodiment 1) and therefore a potential area exists in which the electrolessness of the cell can be achieved without causing corrosion of the DSA surface. ,

The inventive method prevents corrosion of the noble metal / Edelmatalloxi'd layer of the DSA when necessary dismantling a cell, thereby the precious metal loss is significantly reduced, and significantly extended the life of the DSA.

_ Embodiment SExample 1

.An with a Pt / Ir coated DSA chlorine was in a laboratory cell develops (Conditions: 2 m WACL, T m HClO _4, E = .1,2 V / SCE, T = 25 ⁰ C) (range I). Without interruption the potential then became linear.

varies in the cathodic direction (area II) * equal

192

early was on the basis of the isotope Ir. the corrosion; the DSA radioche.inisch. tracked.

The result of these measurements is shown in Fig. 2. »It can be seen that the corrosion first decreases and only rises sharply in the potential range 0.7 - 0.3 V / SCE. This area corresponds to the area of the reduction of the surface oxides "Surprisingly occurs in this area a strong corrosion, which is the Ursaohe" of the corrosion with the short ο The potential area below the equilibrium potential of the anode reaction and above the reductive corrosion is to be kept thus during a decommissioning of the cells ,

Embodiment 2 . ';

In.Labordiaphragmazellen with Pt / Ir-coated

¹ 2

Anodes (current density 1.8 KA'm ", cell voltage 3.6 ^V,: anolyte concentration 180 g / l NaCl) were short-circuit tests are carried out" by a der.Zellen was directly short-circuited, so that a residual voltage of 0.3 V abutted, and another cell was short-circuited via a parallel-connected carbon resistor (see Fig. 1, switch I) so that a residual voltage of 1.1 Y persisted in the cells.All other cells continued to operate under normal conditions The corrosion rates in the short-circuit tests without parallel resistance were 20 times higher than in the experiments with parallel resistances.

Claims (4)

Erf in.dungsan.apr uch 1. A method for corrosion protection of dimensionally stable anodes with precious metal and / or precious metal oxide coating on commissioning and recommissioning an electrolytic cell from a battery of such cells in series, characterized in that a voltage is maintained at the electrolytic cell which is smaller than the equilibrium cell voltage of the electrolysis process and at the same time keeps the anode potential above the reductive corrosion potential of the anode. Arrangement for carrying out the method according to _x point 1, characterized in that parallel to the electrolysis cell when switching off a switching element is arranged whose resistance is so dimensioned that its voltage drop is smaller than the Gleichgewichtsaellspannung the electrolysis process and that it is the anode potential over. ; the reductive corrosion potential of the anode holds. 3 · Arrangement according to point 2, characterized in that after decommissioning optionally another conventional switching element is arranged parallel to the switched-off electrolysis cell. Arrangement according to items 2 and 3, characterized in that a switching element is arranged for the chloralkali electrolysis according to the diaphragm method with iron cathodes, at whose resistance a voltage drop of at least 0.9 V occurs. ; .jt 2a'JAH.1983 * O65lG4