EP1953270A1 - Method for improving the performance of nickel electrodes - Google Patents

Abstract

Method for improving the performance of nickel electrodes used in sodium chloride electrolysis comprises adding a platinum compound, especially hexachloroplatinic acid or alkali platinate, soluble in water or in alkali during the electrolysis.

Description

The invention relates to a method for improving the performance of nickel electrodes in alkali chloride electrolysis.

Sodium chloride electrolytic cathodes where hydrogen is developed in alkaline solution are usually made of iron or nickel. If nickel electrodes are used, they can be made entirely of nickel, or only nickel surfaces are used in which substrates of other metals are surface-plated nickel-plated.

As in the published patent application EP 298 055 A1 mentioned nickel electrodes can be coated with a metal from the VIII subgroup, especially the platinum metals (including Pt, Ru, Rh, Os, Ir, Pd) of the Periodic Table of the Elements or an oxide of such a metal or mixtures thereof. After a calcination process, the corresponding noble metal oxides are usually present on the surface.

The electrode thus prepared may, for. B. in the sodium chloride electrolysis used as a cathode for hydrogen evolution. In this case, many coating variants are known, because the coating of metal oxides can be modified in many different ways, so that different compositions are formed on the surface of the nickel electrode. According to the US-A-5 035 789 is z. For example, a ruthenium oxide based coating on nickel substrates is used as the cathode.

During operation of the nickel-based electrodes, a decreasing quality of the electrode over time is observed, in the form that the cell voltage rises during the electrolysis, so that, if necessary, a new coating of the electrode is required. This is technically complex, since the electrolysis must be switched off and the electrodes must be removed from the electrolysis cells. The object of the invention is therefore to find a simpler form of performance increase or performance restoration.

The company ELTECH has published and offered under the website www.eltechsvstems.com/contact.asp a technique with which a voltage reduction of 200 to 300 mV is achievable compared to untreated nickel electrodes. In this case, a precious metal-containing solution of unnamed composition and ingredients in situ, that is used in the operation of electrolysis, on the cathode side of sodium chloride electrolysis in membrane cells. The solution should be added during operation of the cell and decrease the cell voltage.

According to the teaching of the patent US-A-4 555 317 For example, iron compounds or finely divided iron are added to the catholyte to lower the cell voltage in sodium chloride electrolysis. Contradictory to this, however, is the publication of the company ELTECH, since according to the information from ELTECH the assignment of the cathodes with iron should have a disruptive effect on the electrolysis and increase the cell voltage.

According to the published patent application EP 1 487 747 A1 the addition of a 0.1 to 10 wt .-% platinum-containing compound for sodium chloride electrolysis. Here, the solution of the platinum-containing compound is added to the water which forms the catholyte, wherein per liter of water 0.1 to 2 liters of the aqueous solution of the platinum compound-containing solution is added.

According to the JP 1011988 A For example, in order to restore the activity of a deactivated cathode based on a Raney Nickel structure with low hydrogen overvoltage, a soluble compound of a platinum group metal salt of sodium hydroxide is added to the catholyte during operation of the sodium chloride electrolysis. For example, in a sodium chloride electrolysis cell with 32 wt .-% sodium hydroxide solution, a salt concentration of 200 g / l sodium chloride at 90 ° C and a current density of 2.35 kA / m ² operated. The cathode is electroless nickel plated for pretreatment and then nickel-plated in a nickel bath. Platinum chlorate, for example, was added as the active compound in the catholyte, which led to a lowering of the cell voltage by 100 mV.

According to the US-A-4,105,516 During the electrolysis of alkali metal chlorides, metal compounds are added to the catholyte, which reduce the hydrogen overvoltage and thus reduce the cell voltage. The in the US-A-4,105,516 Examples given in turn describe the dosage and effects that result from the addition of an iron compound added to the catholyte of a sodium chloride diaphragm laboratory cell. The cell has an anode made of titanium expanded metal coated with ruthenium and titanium oxide. The cathode consists of iron in the form of expanded metal. The examples show the use of cobalt or iron solution on the iron cathode. The disadvantages of iron compounds in the treatment of coated nickel electrodes have already been mentioned above.

According to the further known patent US-A-4 555 317 It is known that the sodium chloride electrolysis can be started with a nickel-coated copper cathode. An initial dosage under electrolysis conditions of the cell was carried out with hexachloroplatinic acid in three steps. In the first step, 2 mg platinum per 102 cm ² , ie 0.02 mg / cm ² , added in the second step about 0.03 mg / cm ² and in the third step about 0.2 mg / cm ² . The cell voltage was lowered by a total of about 157 mV.

According to the US-A-4,160,704 For example, metal ions having a low hydrogen overvoltage may be added to catholyte of a membrane electrolytic cell of sodium chloride electrolysis to coat the cathode. The addition takes place during the electrolysis. By way of example, however, only the addition of platinum oxide for improving an iron or copper cathode is mentioned.

The sodium chloride electrolysis according to the membrane process is carried out as follows. A solution containing sodium chloride is supplied to an anode chamber with an anode, a sodium hydroxide solution is fed to a cathode chamber with a cathode. The two chambers are separated by an ion exchange membrane. Several of these anode and cathode compartments are assembled into an electrolyzer. The anode chamber leaves next to the chlorine formed a less concentrated sodium chloride-containing solution than it was supplied. The cathode chamber leaves next to hydrogen, a higher concentration of sodium hydroxide solution as this was supplied. The sodium hydroxide volume flow which is supplied to the cathode chamber depends on the current density and the cell design. At a current density of, for example, 4 kA / m ² and the cell design of the company. UHDE, version BM 3.0, the volume of liquor to the cathode space, for example, between 100 and 300 1 / h, at a concentration of the effluent sodium hydroxide from 30 to 33 wt. -%. The geometrically projected cathode area is 2.71 m ² , which corresponds to the membrane area. The cathode consists of a nickel-expanded metal, which is provided with a special coating (hereinafter called Coating) (manufacturer eg DENORA) to reduce the hydrogen overvoltage.

Usually, the cathode coatings in the sodium chloride electrolysis of platinum metals, platinum metal oxides or mixtures thereof such as a ruthenium - ruthenium oxide mixture. Like in the EP 129 374 Ruthenium, iridium, platinum, palladium and rhodium are among the useful platinum metals. The cathode coating is not long-term stable, especially not under conditions in which no electrolysis takes place or in case of interruptions of the electrolysis, which can lead to Umpolungsprozessen example. Thus, a more or less severe damage of the coating over the operating time of the electrolyzer takes place. Likewise, impurities which, for example, from the brine into the liquor, such as iron ions, can be deposited on the cathode or especially on the active centers of the noble metal-containing coating and thereby deactivate this. The result is that the cell voltage increases, which increases the energy consumption for the production of chlorine, hydrogen and caustic soda and significantly worsens the economics of the process.

Also, only individual elements can have damage to the cathode coating, and it is not always economical to turn off the entire electrolyser and remove the damaged coating element, as this involves significant production losses and costs.

Methods for improving nickel electrodes for the sodium chloride electrolysis, which are coated with elements of platinum metals (VIII. Subgroup of the Periodic Table), hereinafter platinum metals, their oxides or mixtures thereof, are so far not directly known from the prior art.

The object of the invention is therefore to develop a special method for improving nickel electrodes which are coated with platinum metals, platinum metal oxides or mixtures thereof, for use as cathodes in the electrolysis of sodium chloride that can be used in ongoing electrolysis operation and a longer interruption of the electrode operation to restore the cathode activity avoids.

The invention relates to a method for improving the performance of nickel electrodes having a coating based on platinum metals, platinum metal oxides or mixtures of platinum metals and platinum metal oxides, for the sodium chloride electrolysis according to the membrane process, characterized in that in the electrolysis of sodium chloride, a water-soluble or in Alkali-soluble platinum compound, especially hexachloroplatinic acid, or more preferably an alkali platinate, more preferably sodium hexachloroplatinate (Na ₂ PtCl ₆ ) and / or sodium hexahydroxyplatinate (Na ₂ Pt (OH) ₆ ) is added to the catholyte.

In this case, in particular, either the sodium hexachloroplatinate can be metered in as an aqueous solution or in alkaline solution, or the hexachloroplatinic acid is metered directly into the catholyte, in particular the sodium hydroxide solution, in which case a reaction with the lye to give the chloroplatinate takes place.

The addition of the platinum compound takes place here in particular during electrolysis under normal electrolysis conditions at a current density of 0.1 to 10 kA / m ² , more preferably at a current density of 0.5 to 8 kA / m ² .

A further preferred embodiment of the addition of platinum is that after the addition of the platinum compound, the electrolysis voltage, in particular pulse-like in the range of 0 to 5 V is varied to deposit finely divided platinum on the cathode. The voltage here describes the voltage between anode and cathode.

Depending on the rectifier used, it may be sufficient to generate the electrolysis DC voltage in order to lower the cell voltage in order to utilize the residual ripple of the rectifier for this purpose. The residual ripple of the rectifier may result in an alternating voltage in said voltage range with an amplitude of 0.5 to 500 mV. Modern rectifiers hardly have any residual ripple, but there is the possibility of artificially generating a residual ripple. The residual ripple is, for example, between 20 and 100 Hz.

If the amplitude is also regulated, this can be around the quiescent potential for the time of the precious metal metering with +100 or -100mV. The rest potential is the voltage at which no current flows anymore. Normally, this potential is about 2.1 to 2.3 V, depending on the cell technology and membrane used. However, there is also the special possibility of performing the noble metal dosing when the cell voltage is 0 V, then the amplitude must be greater than the quiescent potential.

Higher modulated amplitudes are also conceivable.

Platinum metals, which may be present as metal or metal oxide in the context of the invention as electrode coating on the nickel, are in particular ruthenium, iridium, palladium, platinum, rhodium and osmium.

A further preferred embodiment of the novel process consists in that, in addition to the platinum compound, other further soluble compounds of subgroup 8 of the Periodic Table of the Elements, in particular compounds of palladium, iridium, rhodium, osmium or ruthenium, are additionally added. These are used in particular in the form of water-soluble salts or complex acids.

Preferably, the addition in a first dose after detection of deactivation is such that a platinum compound in the catholyte in the feed to the cathode chamber at a cathode area of 2.71m ² between 0.02 and 11 g Pt per cathode element, corresponding to 0.007 g / m ² to 4 g / m ² , at a current density of 1 to 8 kA / m ² , takes place. The surface used is the geometrically projected cathode surface, which also corresponds to the membrane surface. The metering rate can be carried out in such a way that the platinum-containing solution, based on the platinum content per meter ^{2 of} cathode area, is metered between 0.001 g Pt / (hm ² ) and 1 g Pt / (hm ² ).

The addition can be made at a current density preferably under normal operating conditions, as well as at higher or lower current density. Thus, the addition can be carried out at a current density in particular of 0.1 to 10 kA / m ² .

The temperature at which the dosage preferably takes place of the platinum compound is 70 to 90 ° C. The dosage can also be done at a lower temperature.

If a voltage increase is observed again after the dosage has been completed, it can be compensated directly by re-dosing. At this dosage, a significantly lower amount of noble metal is necessary to restore the original tension. Depending on the quality of the brine, lye or stoppages, a renewed, but then lower platinum addition may be required in a period of 1 to 3 weeks. In this case, the addition of the platinum compound in the catholyte can also be in the feed to the cathodes. The required platinum quantities are to be measured according to the scope of damage. For more severe damage, corresponding to a high voltage increase, more platinum must be dosed, with less damage, correspondingly lower voltage increase, correspondingly less. An overdose of platinum, however, leads to no further improvement in terms of lowering the cell voltage.

With particular preference the proportion based on the platinum of the further soluble compounds in the solution to be added from the 8th subgroup is from 1 to 50% by weight.

The variation of the electrolysis voltage can be effected, for example, in a preferred embodiment by superposing an AC voltage on the electrolysis voltage. The frequency of the superposed AC voltage is in particular from 10 to 100 Hz, the amplitude can then be between 10 and 200 mV.

With the aid of the method according to the invention, it is possible for the first time to realize a voltage reduction of up to 200 mV in the case of damaged nickel electrodes which are coated with ruthenium and / or ruthenium oxides or mixtures thereof.

The preparation of the Alkaliplatinates can be carried out by reaction of hexachloroplatinic acid with alkali. This can be done separately or directly in situ if, for example, hexachloroplatinic acid is metered directly into the sodium hydroxide feed to the elements or to the electrolyzer. Particularly preferably, the hexachloroplatinic acid is metered directly into the feed to the elements.

Examples example 1

A technical electrolyzer with 144 elements, whose nickel cathodes were provided with a coating based on ruthenium / ruthenium oxide from Denora, was operated at an average voltage of 3.12 V. Of these 144 elements, one showed an increased voltage of more than 100 mV from the mean. The following treatment cycle was started: 65.88 g of a hexachloroplatinate solution (1.19 g Pt / l) was added at 10.98 l / h during operation in the sodium hydroxide solution (conc. 31.5%) of a membrane electrolyzer at a current density of 4.18 kA / m ^{2 added} over 6 h. Thus, 78.25 g platinum reached the surface of 144 cathodes (surface of a cathode: 2.71 m ² ). This corresponds to a platinum amount of 0.21 g Pt / m ² . The cell voltage fell on average to 3.08 V, the power consumption increased to 4.57 kA / m ² . Converted to 4 kA / m ² , this corresponds to a lowering of the voltage by 80mV, thus from 3.09 to 3.01. Elements with significantly higher voltage were no longer available. The next day was again dosed with 16.44 l of the same solution, corresponding to 0.05 gPt / m ² . The cell voltage did not improve anymore.

After 9 days, the mean voltage increased to 3.02 V (based on 4 kA / m ² ), so that a further dosage of platinum took place in the form of hexachloroplatinic acid. During this time, 4.12 g of the hexachloroplatinate solution (1.19 g Pt / l) were metered uniformly over the course of 2 hours, so that 4.9 g of platinum reached the surface of 144 cathodes (0.012 g Pt / m ² ). The electrolysis was continued, the average voltage was then 3.01 V.

The cell voltage was 3.09 V at a current density of 4 kA / m ² before dosing, 3.01 V after dosing, which corresponds to a voltage reduction of 80 mV.

Example 2

A laboratory electrolysis cell was operated at a current density of 4 kA / m ² at a cell voltage of 3.05 V using a standard cathode coating from Denora on the nickel cathode as described in Example 1. After decommissioning without application of a protective potential, damage to the cathode coating occurred. Normally, a protection potential is applied during decommissioning in order to protect the coating of the cathode from damage. After restarting, the cell voltage was 3.17 V.

A solution of hexachloroplatinate with a platinum content of 1250 mg / l Pt was added to the running cell in the catholyte. After dosing the solution for 2 hours with a At a dose level of 5 ml / h, the voltage dropped to 3.04 V. A total of 12.5 mg of platinum (12.5 mg / 100 cm ² ) was added.

Example 3

The experiment of Example 2 was repeated, but a solution with a platinum concentration of 250 mg / l was dosed (same dosing and same flow rate). Addition hereby 2.5 mg Pt / 100 cm ² . The voltage dropped from 3.16V to 3.07V, ie around 90mV.

Further replenishment brought no further decrease in voltage.

Example 4 (comparison)

A laboratory electrolysis cell was operated at a current density of 4 kA / m ^{2 of} a cell voltage of 3.08 V with a standard cathode coating of Fa. Denora on nickel electrodes as in Example 1. After decommissioning without application of a protective potential, damage to the cathode coating occurred. Normally, a protection potential is applied during decommissioning in order to protect the coating of the cathode from damage. After restarting, the cell voltage was 3.21 V.

A solution of rhodium III chloride with a rhodium content of 125 mg / l was dosed for 4 hours at 5 ml / h. Thereafter, a further 2 hours with a solution of a concentration of 1250 mg / l and 5 ml / h were dosed, whereby a further 50 mV of voltage reduction were achieved. The voltage reduction was only 60 mV.

Claims (8)

A method for improving the performance of nickel electrodes comprising a coating based on platinum metals, platinum metal oxides or mixtures of platinum metals and platinum metal oxides, for sodium chloride electrolysis according to the membrane process, characterized in that in the electrolysis of sodium chloride, a water-soluble or alkali-soluble platinum Compound, in particular hexachloroplatinic acid or an alkali platinate, more preferably Na ₂ PtCl ₆ and / or Na ₂ Pt (OH) _{6 is} added to the catholyte. 2. The method according to claim 1, characterized in that after the addition of the platinate, the electrolysis voltage in the range of 0 to 5 V, preferably by a difference of 0.5 to 500 mV, is varied. 3. The method according to claim 2, characterized in that the electrolysis voltage is pulsed or by superposition of the electrolysis voltage with an AC voltage in the range of 0 to 5 V, preferably by a difference of 0.5 to 500 mV, is varied. 4. The method according to any one of claims 1 to 3, characterized in that the platinum compound further other water-soluble compounds of the 8th subgroup of the Periodic Table of the Elements, in particular compounds of the platinum group, particularly preferably of palladium, iridium, platinum, rhodium, osmium or ruthenium be added. 6. The method according to claim 4, characterized in that the proportion of further soluble compounds of the 8th subgroup based on platinum of the platinum compound 1 to 50 wt .-%. is. 7. The method according to at least one of claims 1 to 6, characterized in that the metering rate of the platinum-containing solution based on the platinum content per m ² cathode area and hour 0.001 g Pt / (hm ² ) to 1 g Pt / (hm ² ) is. 8. The method according to at least one of claims 1 to 7, characterized in that the temperature at which the metering of the platinum compound takes place, 70 to 90 ° C. 9. The method according to at least one of claims 1 to 8, characterized in that the addition of the platinum compound takes place during electrolysis and under a current density of 0.1 to 10 kA / m ² .