EP0042984A1 - Electrode free from noble metals and process for its manufacture - Google Patents

Abstract

In the case of an electrode which is suitable for the electrolysis of aqueous sodium or potassium chloride solutions, at least the outer layer contains spinels. The spinels consist of a mixture of the individual spin parts of iron and cobalt, with the proviso that the weight ratio of iron spinels: cobalt spinels is 30:70 to 90:10. The electrodes are produced by applying a powdery mixture of iron spinel and cobalt spinel to a substrate by means of the plasma spiking process, argon being used as the plasma gas and the injection energy being 6 to 30 kW.

Description

In the manufacture of chlorine or chlorine compounds by electrolysis of aqueous sodium or potassium chloride solutions, titanium anodes with active metal-containing active layers or graphite electrodes are generally used today. These so-called dimensionally stable titanium anodes have the advantage over the graphite electrodes that the external dimensions do not change during operation. The disadvantage of these anodes is the relatively high production costs due to the use of noble metal in the active layer.

It is also known that magnetite can be used as an anode material for the separation of chlorine, but this material has a very high overvoltage with regard to chlorine, so that its use has been discontinued for a long time due to the high energy consumption.

However, there has been no shortage of attempts to provide non-precious metal electrodes on the basis of the much cheaper iron oxide, which on the one hand have a technically and economically satisfactory low deposition voltage and on the other hand have sufficient chemical resistance for chlorine.

Thus, in the GDR patent 98 838 an electrode consisting predominantly of trivalent iron oxide with additions of one or more metal oxides is described. To produce this electrode, an oxide mixture is obtained from an iron salt solution via a carrier precipitation, which is then pressed and sintered in an oxygen-containing atmosphere. Titanium dioxide, zirconium dioxide and / or tin dioxide are mentioned as additional oxides. However, this electrode has a deposition potential for Chlorine of 1.65 V GKE (measured against saturated _K alomel electrodes), at a current density of 1 kA / m ² , which corresponds to a chlorine separation voltage of 1.9 V based on the normal hydrogen potential. With increasing current density, the deposition potential increases considerably, so that this electrode achieves an impermissibly high deposition potential at the current densities of 1.5 to 2.0 kA / m ² that are currently used in technical systems.

DE-OS 23 20 883 describes anodes which consist of sintered bodies with a spinel structure of the general formula MxFe3-x04 and are said to be suitable as chlorine anodes. In this formula, M means a metal from the group consisting of manganese, nickel, cobalt, magnesium, copper, zinc and / or cadmium and x stands for 0.05 to 0.4. With these electrodes, particular reference is made to the improved corrosion resistance compared to conventional magnetite electrodes, while the deposition potentials which are essential for the assessment of an electrode are not emphasized. As our own investigations (cf. Comparative Example 1) have shown, these separation potentials are at technically customary current densities of 1.5 kA / m ² , at 1750 mV to 2000 mV (measured against H ₂ normal electrode).

In US Pat. Nos. 3,977,958 and 4,142,005 electrodes are described which consist of an electrically conductive substrate to which a single metal spinel of the formula Co ₃ 0 _{4 is applied} as an electrochemically active substance, and additionally modifying oxides of groups IIIB -VIIB, IIIA-VA, and may contain lanthanides or actinides. However, the deposition potential of these electrodes also does not meet the technical requirements.

The present invention had for its object to provide the electrol, whose electrochemically active layer contains spinels, which are particularly suitable as anodes for the separation of chlorine in electrolysis cells and which, in addition to good corrosion resistance to the electrolyte and the electrolysis products, are associated with a high level Service life, have a low separation voltage for chlorine.

It has been found that this object is achieved by an electrode containing spinel, in which the spinels consist of a mixture of the individual spinels of iron and cobalt, with the proviso that the weight ratio of iron spinel to cobalt spinel is from 30:70 to 90:10.

It is essential that the electrode according to the invention contains the two spinels as individual spinels and that they do not form a mixed spinel. The presence of the two substances next to one another can be proven in a known manner by means of an X-ray fine structure analysis.

The active layer preferably has the two spinels in a weight ratio of Fe ₃ O ₄ : CO ₃ O ₄ of 40:60 to 70:30.

The active layer can be on an electrically conductive support, e.g. a valve metal, graphite, magnetite. However, it is also possible to dispense with this substrate entirely, i.e. that the entire thickness of the electrode consists of the active layer.

The electrodes according to the invention are produced under conditions such that mixed spinel formation cannot take place, special conditions having to be observed since CO ₃ O ₄ tends to easily separate into two finished cobalt oxide and vice versa Fe ₃ O _{4 has} the tendency to easily transition into trivalent iron oxide, with the formation of a cobalt-iron mixing spinel.

A suitable method to achieve this goal is the plasma spraying process. To do this, the two powdered spinels are mixed thoroughly before processing. They should be useful grain sizes of 10 to 200 _/ um, to which preferably of <125 _/. The mixture is then placed in the storage container of a plasma spray gun, taking care to ensure that no segregation occurs both during metering and during transport. A conventional plasma spraying system can be used for the coating, with either argon alone or argon in a mixture with up to 10% by volume of hydrogen being considered as carrier gas. It is also essential that the plasma spraying system is operated in a low energy range, ie that values of 30 kW are not exceeded, with a minimum amount of 6 kW being adhered to for design reasons.

The body to be coated should be degreased beforehand in a known manner and then the surface should be prepared by sandblasting, pickling and the like.

The distance between the plasma flame and the body to be coated should suitably be 7 to 12 cm. The plasma flame is moved back and forth in front of the body to be coated until the spray layer has reached the desired thickness. The active layer is effective even with a relatively small thickness of 20 to 30 µm, although of course much thicker layers are permissible, up to electrodes which consist exclusively of the electrochemically active material.

To increase the application rate of the plasma spraying system, a powder of a valve metal can also be added to the spinel mixture to be sprayed. Of course, other substances can also be added if special properties are desired and if these other substances do not impair the electrochemical activity of the spinel layer.

The electrodes according to the invention, when used as anodes in the electrolysis of aqueous alkali metal chloride solutions, have a chlorine separation potential of 1395 mV at current densities of 0.15 kA / m ² , based on the normal H2 electrode, ie the overvoltage is only approx . 35 mV. But even with the higher current densities of 1.5 kA / m ² to 6 kA / m ² , which are of particular technical interest, the electrodes are characterized by a low overvoltage, with the deposition potential at 1.5 kA / m ² depending on the substrate between approx 1450 and a maximum of about 1600 mV. In contrast, in the GDR patent 98 838 already cited above, at lower current densities of 1.0 k _A / m ^2, deposition potentials of 1650 to 1730 mV, measured against a calomel electrode, are mentioned, which means a potential against the normal H ₂ electrode of approx Corresponds to 1900 to 1980 mV.

In addition, the electrodes according to the invention are notable for good chemical and mechanical resistance, and even if graphite is used as the substrate, practically no erosion can be ascertained even with longer standing times.

example 1

• a) On a titanium expanded metal grid (11 x 6 x 2 x 1.5 mm) with the geometric area of about 20 cm ² , which is provided with a central electrical lead made of titanium, a mixture of Fe is using a plasma torch ₃ O ₄ and CO ₃ O _{4 applied} in a weight ratio of 70:30. Powders with a grain size in the range of <125 µm and argon as carrier gas with an injection energy of 18 kW are used. After 3 spraying cycles per side at a distance of 90 mm, the layer thickness is 30 μm.
• b) Under otherwise identical conditions, a mixture of Fe ₃ O ₄ and CO ₃ O ₄ in a weight ratio of 50: 5 ₀ and
• c) applied in a molecular weight ratio of 30:70.

The anodes produced in this way are subjected to a voltage test under the operating conditions of chlor-alkali electrolysis. The following deposition potentials are measured (against normal H ₂ electrodes):

Example 2

• a) The anode is produced as described in Example 1, a mixture of 90% by volume Ar as the plasma gas, 10 vol .-% H _{2 is used} with an injection energy of 17.2 kW. The weight ratio Fe ₃ O ₄ : CO ₃ O ₄ (grain size <125 _/ um) is 90:10. The current-voltage test shows the following results:
  

• b) When using a plasma gas made of pure argon and an injection energy of 19.2 kW, anodes are obtained which show the following potentials:
  

Example 3

An active layer of Fe ₃ 0 ₄ : C ₀₃ 0 ₄ (weight ratio 70:30) is applied to a base body made of electrographite with the dimensions of the electrode area of 20 x 15 x 10 mm. Argon serves as the carrier gas, the injection energy is 18 kW and the distance of the plasma flame from the electrographite base body is 9 cm.

The determination of the separation potential results in:

Values of the base body measured under the same conditions without activation:

Example 4

A powder mixture of Fe ₃ O ₄ : CO ₃ O ₄ im Weight ratio 66 2/3: 33 1/3, which was added 70 wt .-% titanium powder, sprayed. After a layer thickness of 1.5 mm has been reached, the coating process is stopped, the sprayed-on layer is detached from the aluminum and the negative mold thus produced is measured as an electrode. The following separation potentials are determined:

Comparative Example 1

Compounds of the type M _x Fe _3x O ₄ (A = CO _0.3 Fe _2.7 O ₄ and B = CO _0.4 Fe _2.6 O ₄ , according to DE-OS 23 20 883) are applied analogously to Example 1 with the aid of a plasma spray gun onto a corresponding anode base body Titanium applied and the deposition potentials determined under the same conditions as described in Examples 1 to 4.

The following values are determined:

A comparison of these deposition voltages measured at 1.5 kA / m ² with the Ab measured in Examples 1 to 4 cutting voltages in the electrodes according to the invention shows a difference of more than 250 mV.

Comparative Example 2

As described in Example 1, an electrode is made using pure Co ₃ 0 ₄ (corresponding to US Pat. No. 3,977,958). The voltage test shows the following results:

This electrode also has a deposition potential increased by approximately 200 mV at 1.5 _{kA / m} ² compared to the electrodes according to the invention _.

Comparative Example 3

The anode is produced as described in Example 1, argon being used as the plasma gas at an injection energy of 32 kW. The weight ratio Fe ₃ O ₄ : CO ₃ O ₄ (grain size <125 µm) is 70:30. The deposition potential is determined under the same conditions as in Examples 1 to 4. The following values are determined:

A comparison of these deposition potentials with the deposition potentials of the electrode according to Example 1a, the active layer of which was produced with an injection energy of 18 kW, shows that the latter has a deposition potential which is 90 to 170 mV lower.

Claims (6)

1. Electrode in which at least the outer electrochemically active layer contains spinels, characterized in that the spinels consist of a mixture of the individual spinels of iron and cobalt, with the proviso that the weight ratio of iron spinel: cobalt spinel from 30:70 to 90:10 is. 2. Electrode according to claim 1, characterized in that the weight ratio of iron spinel: cobalt spinel is from 40:60 to 70:30. 3. A process for producing the electrodes according to claims 1 to 2, characterized in that a homogeneous, powdery mixture of iron spinel and cobalt spinel is applied to a substrate by means of the plasma spraying process, argon being used as the plasma gas and the injection energy being 6 to 30 kW. 4. The method according to claim 3, characterized in that the plasma gas contains up to 10 vol .-% hydrogen. 5. The method according to claims 3 to 4, characterized in that the distance plasma flame / substrate is 7 to 12 cm. 6. Process according to Claims 1 to 5, characterized in that the substrate is removed after the spinel layer has been applied.