US3691052A

US3691052A - Value metal base electrode coated with pb2ru2o6 or pb2ir2o6

Info

Publication number: US3691052A
Application number: US104797A
Authority: US
Inventors: Robert C Langley
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: BASF Catalysts LLC; Engelhard Minerals and Chemicals Corp
Priority date: 1970-08-24
Filing date: 1971-01-07
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12
Also published as: FR2103472B1; US3691059A; DE2142011A1; FR2121517A1; FR2103472A1; AU448207B2; FR2121517B1; GB1371971A; CA962229A; GB1357188A; DE2200438A1; AU3257671A; CA951266A

Abstract

ELECTROLYSIS OF BRINES IN ACCOMPLISHED WITH ANODES HAVING AN OPERATIVE SURFACE OF LEAD RUTHENATE OR LEAD IRIDATE. THE ANODES ARE PARTICULARLY EFFECTIVE FOR THE PRODUCTION OF CHLORINE. IT HAS BEEN FOUND THAT LEAD RUTHENATE OR LEAD IRIDATE POWDER MAY BE BONDED TO A VALUE METAL SUBSTRATE BY USING A LOW MELTING GLASS FLUX. ONE GROUP OF GLASSES EXISTS IN THE V2O5PBO SYSTEM, CONSISTING OF V2O5 PLUS ABOUT 30 TO 55 MOL PERCENT OF PBO. OTHER TYPES OF FLUXES ARE LEAD BOROSILICATE GLASS OR LEAD BORATE: BISMUTH SUBNITRATE IN THE PROPORTION OF 2:1 PARTS BY WEIGHT.

Description

United States Patent Oflice 3,691,052 VALUE METAL BASE ELECTRODE COATED PbzRllgO Pb II' OS Robert C. Langley, Millington, N.J., assignor to Engelhard Minerals & Chemicals Corporation No Drawing. Filed Jan. 7, 1971, Ser. No. 104,797 Int. Cl. B011 3/04 US. Cl. 204-290 F 6 Claims ABSTRACT OF THE DISCLOSURE Electrolysis of brines is accomplished with anodes having an operative surface of lead ruthenate or lead iridate. The anodes are particularly effective for the production of chlorine. It has been found that lead ruthenate or lead iridate powder may be bonded to a value metal substrate by using a low melting glass flux. One group of glasses exists in the V O .PbO system, consisting of V plus about 30 to 55 mol percent of PhD. Other types of fluxes are lead borosilicate glass or lead boraterbismuth subnitrate in the proportion of 2:1 parts by weight.

BACKGROUND OF THE INVENTION This invention relates to novel anodes for the electrolysis of brines. More particularly it relates to anodes for use in the electrolytic production of chlorine.

Graphite anodes have been in commercial use in chlorine cells for many years despite the unsatisfactory characteristics common to all such anodes. Generally their wear rates are high and impurities such as CO are introduced in the products. In recent years there has been a trend toward substituting platinum group metals as the anode material. Platinum group metals offer several potential advantages over graphite, for example, lower overvoltage, lower erosion rates, and higher purity of products. Limiting factors are the high cost of the precious metals and their tendency to short in mercury cells. In order to be economically advantageous it is of major importance that the shorting tendency be minimized and the anodes exhibit low overvoltage.

Overvoltage in a chlorine cell can be defined as the voltage in excess of the theoretical needed for chlorine to be produced at the anode at an appreciable rate. Lower chlorine overvoltage is translated into savings in operating costs through lower power consumption. The theoretical voltage can be calculated from standard electrical potentials for the cell. Overvoltage is a property of both the anode material and its physical form, e.g. surface roughness, and this must be taken into consideration when comparing materials.

The anodes which have platinum group metals as the operative material may be composed of platinum group metal in bulk or as a coating on a corrosion resistant substrate. Since it is at the surface of the anode that the electrolysis occurs, the anode surface is referred to herein as the operative surface whether the operative material is used in bulk or as a coating. As a practical matter the metal anodes which have potential commercial value are comprised of a coating of the operative material on a suitable substrate. Anodes of this type are well known. The substrate materials are base metals having properties which render them corrosion resistant to the environment in electrolysis cells. Examples of suitable 3,691,052 Patented Sept. 12, 1972 corrosion resistant value metals are Ti, Ta, Hg, Zr, W, Al and alloys thereof. These substrate materials are referred to herein as valve metals. It is also well known to have the valve metal as a layer on a base metal such as copper which is a good electrical conductor but which corrodes in the environment.

In the past it has been proposed to use oxides of platinum group metals rather than the metals as the operative material of the anode. It has also been proposed to use the platinum group metals and oxides such as RuO PtO PdO, in mixtures with various base metal oxides such as oxides of Ti, Ta, Sn, Bi, Pb, etc. for various purposes such as to increase the adherence of the coating, to reduce the shorting tendency of the platinum metals, to increase the threshold overvoltage, and to reduce the loss of platinum metals. Another proposal has been to use the platinum group metal oxides as additives to various oxides such as TiO to introduce semiconductive properties into the TiO in order to make it more conductive. All these proposed coatings of mixed oxides have in fact been mixtures of the oxides. Another approach has been to coat the valve metal substrate and/or the platinum group metal coating with an oxide for various reasons, for example, toprotect the substrate, make the platinum metal coating more adherent, and to protect the platinum group metal coating from shorting. Lead oxide is among the oxides suggested for mixture with platinum group metals and their oxides and also as a coating over a platinum group metal oxide. US. Pat. No. 3,213,004, for example, uses an outer lead dioxide surface on a flash coating of a platinum group metal on a titanium substrate for peroxidation reactions such as the production of sodium perchlorate. British Pat. No. 1,147,442 notes that the threshold overvoltage value of anodes is increased by the addition of oxides such as lead oxide to a platinum group metal oxide and suggests the use of a titanium core coated with a mixture of platinum oxide and lead oxide as an anode for the preparation of perborates or persulfates.

The proposed anodes have met with varying degrees of success, and there has been a continuing effort to find more suitable anodes for use in electrolysis cells.

OBJECTS OF THE INVENTION It is an object of the present invention to provide metallic anodes with improved physical and electrical characteristics. It is another object to provide an anode for the electrolysis of brines which has chemical and electrical stability, and low overvoltage characteristics of the platinum group metals. A further object is to provide an anode having an operative surface of lead ruthenate or lead iridate. It is still a further object to provide a process for the electrolysis of brines which can be effected with relatively low production costs. These and other objects will become obvious from the following description and illustrative examples.

THE INVENTION In accordance with one aspect of this invention a novel anode is provided for the electrolysis of brines said anode having an operative surface of lead ruthenate or lead iridate. These compounds are particularly useful for the production of chlorine by the electrolysis of an aqueous solution in that they are chemically inert to the environment and exhibit low chlorine overvoltage. A further advantage is their relatively low cost compared to an equivalent amount of platinum group metal. Lead ruthenate, for example, has electrical properties equivalent to ruthenium dioxide, but contains only 37.4% Ru by weight.

The lead ruthenate and iridate are chemical compositions having definite compositions and crystal structures. Lead ruthenate has the chemical formula Pb Ru O the Pb and Ru being present in a molar ratio of 1:1. Lead ruthenate is not readily attacked by acids or bases, and it is not attacked by molten glass. It does not react with PbO when heated in an intimate mixture at 850 C. Lead iridate can be similarly defined as Pb Ir O with the Pb and Ir present in a molar ratio of 1:1. The structure and X-ray diffraction pattern of these compounds have been identified in an article entitled Preparation and Properties of Oxygen Deficient Pyrochlores by Longo, Raccah, and Goodenough in Materials Research Bulletin vol. 4 pp. 191-202 (1969).

Of the two compounds lead ruthenate is preferred as the anode material, principally because of its lower cost, and the discussion below will be confined mainly to lead ruthenate.

The lead ruthenate can be prepared by any known method. For example, it can be prepared by heating the oxide, carbonate, or nitrate of lead with ruthenium powder in air at a temperature in the range of about 600 to 1000 C., preferably at a temperature over about 800 C. The lead ruthenate is stable at a temperature of 1000 C. Alternatively, and preferably, it can be prepared from a mixture of PhD and RuO or by co-deposition from salts or resinates. Both of the alternative methods are shown below. Lead iridate can be similarly prepared.

The lead ruthenate or lead iridate may be used in bulk as the anode material or may be used as a coating on a corrosion resistant valve metal substrate, and it is in accordance with another aspect of this invention to provide coatings of lead ruthenate or iridate on suitable substrate materials.

In accordance with one method of preparing a coated anode of this invention, a mixture of Th0 and Ru0 is heated in air at a temperature in the range of about 600 to 1000 C., preferably over about 800. The resultant lead ruthenate is then deposited on a suitable valve metal substrate. It can be noted that below about 600 C. the lead ruthenate is not formed from powders on a practical time scale. Above about 700 C. the valve metals, e.g., Ti or Ta, generally found useful as substrate materials are adversely affected by reaction with oxygen or nitrogen. By first forming the desired chemical compound and then depositing such compound on the substrate the anode with a suitable coating can be prepared without damaging the substrate material. Coatings of lead iridate can be similarly formed.

It will be noted that it is difficult to bond the lead ruthenate powder to the substrate material and while the lead ruthenate is the essential anode material various additives may be used to improve the adherence and continuity of the coating. It has been found, for example, that lead ruthenate powder can be bonded to a valve metal substrate by using a low melting glass flux.

Glass fluxes are widely used in the ceramic art. It is common to use previous metal containing glass fluxes on porcelain for decorative purposes. Printed circuits of composite previous metal-glass flux films on alumina ceramics are used as conductors and resistors in the electronics field. By a flux is meant a glass powder which has been made by melting together its ingredients. The flux, generally a mixture of metal oxide containing glass powders has a melting or softening point below the melting temperature of the principal or operative component of the film to which it is added. In use the composite admixture is applied to a substrate and heated above the melting range of the flux and on cooling it bonds the composite film to the substrate. The flux may be used as a physical mixture of the individual oxide powders, but preferably the ingredients of the flux are premelted together as a homogeneous mixture of the powders to form a glass, and the glass is reduced to a fine powder. In either case the flux is melted or remelted in place. It is also known to add a nucleating agent to the flux, and in such case the glass will be predominantly crystalline when developed in the composite film.

Most of the fluxes used in the ceramic art are intended for firing at 760 C. to 950 C. and their softening temperature ranges are too high for use with valve metal substrates such as titanium. In choosing a flux for use in bonding lead ruthenate or lead iridate films, several requirements must be met. The flux must be inert to the corrosive environment of the electrolysis cells, it must have a thermal expansion close to that of the substrate, it must not react with the lead ruthenate, and its melting or softening temperature should be below that at which the valve metal substrate would be passivated in air. If, for example, the substrate is titanium, the melting point of the flux should be below 600 0., preferably below 500 C., to permit application of the coating in air without degradation of the titanium.

A number of glass fluxes have been found which have the combination of chemical durability, low softening range and thermal expansion needed for use on valve metal substrates. One group of glasses exists in the system, consisting of V 0 plus about 30 to 55 mole percent of PbO, which melt at about 600 C. or lower. Within this range compositions containing about 40 to 52 mol percent are preferred on the basis of lower melting points. The composition containing 50 mol percent PbO is particularly preferred as it is a eutectic melting at 480 C.

Another type of flux useful for composite lead ruthenate or lead iridate films on titanium, is a lead borosilicate glass. Typical of this type of glass is Pyroceram Brand Cement #89, available from Corning Glass Works. This glass has a softening temperature of about 450 C. and a thermal expansion of 89 10-' in./in./ C. It contains a nucleating agent and when held slightly above its softening temperature for 60 minutes or more, converts to a crystalline material with improvement in chemical durability. Still another type of glass suitable for this purpose is made from lead boratezbismuth subnitrate in the proportion of 2:1 parts by weight.

The amount of flux in composite lead ruthenate or lead iridate films can vary widely, e.g. from about 10% to about 70% glass by weight. The amount used depends on the density of the flux, and the density of the lead ruthenate or lead iridate powder used in the composition. To explain, the glass fluxes are usually made by melting all the starting ingredients, and the glass powder obtained from the melt is at its full density. By contrast, the lead ruthenate or lead iridate are preferably powders made at moderate temperatures by solid state reaction between PbO and RuO and IrO Compounds made in this way are of low density and advantageously have high surface area compared to compounds made above the melting point of PhD (888 C.). The low density of lead ruthenate and lead iridate powders are also more economical, since electrical conductivity in composite films is a function of the relative volumes of lead ruthenate or lead iridate and glass. It is convenient to express the compositions in parts by weight, but parts by volume are actually more important.

Lead ruthenate powder, made at 850 C. as described herein, has an apparent bulk density of 1.2. Glass powder made of the eutecic PbO-V O composition by melting the starting materials at 900 C. has an apparent bulk density of 1.5, but when melted for the deposition of the film, e.g. at about 500 to 600 C., the flux goes to full density of 5.2. The density of the Pb Ru O remains at 1.2. A fired film which contains about 60% flux by weight has only about 25% flux by volume.

Another method of developing a lead ruthenate coating on a substrate is by deposit from a solution of salts or resinates of ruthenium and lead, e.g. ruthenium chloride and lead acetate dissolved in a mixture of glycerin and isopropyl alcohol, or a solution of ruthenium resinate and lead resinate. In order to form the compound Pb Ru O from solution, the coating must be heated in an oxidizing atmosphere at a temperature in the range of about 300 to 1000 C., typically about 450 C. Below about 300 C. the compound is not formed. Thin films of lead ruthenate deposited in this way and fired at 450-550" C. have good adherence to valve metal substrates such as titanium.

Evaluation of bulk material for use as anode coatings on titaniumis complicated since many refractory materials, when used in the pure form, do not adhere well when fired on titanium at practical temperatures. To allow comparison of coating materials, it is convenient to use them as powders suspended in a resin solution. After application, this paint is dried at the minimum temperature necessary to remove the vehicle. A toluene solution of an acrylic resin is a suitable medium for this purpose. It can be dried at 70 C., and at this low temperature, the materials being evaluated obviously do not .undergo solid state reactions with titanium. An example is given below of overvoltage measurements made on coatings of RuO Pb RuO O and Bi Ru O prepared in this way.

It is of interest that bismuth ruthenate, having the chemical formula Bi Ru O and having many properties similar to Pb Ru O was found unsuitable as an anode material for the electrolysis of brine, due to high chlorine overvoltage.

EXAMPLE 1 Equimolecular amounts of PhD and Ru powders were mixed together and heated in air at 850 C. for 50 hours. A black powder of uniform color and appearance was formed. The X-ray diffraction of the resultant material was determined and found to match the pattern known for Pb Ru O The X-ray diifraction pattern of the material prepared and the known pattern for Pb Ru O are given in Table I. The known values of Pb Ru O are taken from the above-mentioned article by Longo et al.

In Table I I I, represents a ratio of all intensities to the highest intensity and hkl data are Miller indices. A comparison of these values shows that the pattern of the prepared sample matched closely with the values given in the literature.

As a further check on the prepared sample, the X-ray diffraction pattern of the prepared sample of Pb Ru O was matched agamst the ASTM values for RuO and PhD.

Companson 1s given in Table II. In this Table dA. represents the Bragg spacmgs.

TABLE I X-ray Diffraction Patterns of PbzRuzOo Compared Sample prepared Literature M1 hkl I/I1 hkl 0 0.1 0 553,731 1.5 553,731 0.8 800 733 1.2 733 822,000 0.2 822,000 751,222 20.9 555,751,002 840 18.7 840 753,911 911,753 004 0.1 004' 931 0.0 931 844 10.9 s44,755,933,771 itt??? 1971 "35175913575" 00 1,022,050 0.3 053 1,111,775 0.3 1,111,775 880 0.0 930 1,131,971,955 1.1 1,131,971,955 0 973,1,133,1,002 28.2 973.1,133,1,002 4O 4, 1,200, 18.9 1,200,834

TABLE 11 PbzRmOe (111. I l, 11111 dA A comparison of the values in Table II shows that the prepared lead ruthenate was not a mixture of R110 and PbO melts at 880 C. A sample of the prepared powder tal structure.

In further tests on the prepared powder it was heated to 1000 C. in air and there was no loss in weight and no melting, further evidence of compound formation since 'PbO melts at 80 C. A sample of the prepared powder was boiled for one hour in hydrochloric acid and no Pb dissolved. The latter test is an indication of the inert nature of the compound.

EXAMPLE 2 A sample of the lead ruthenate powder, prepared as described in Example 1, was pressed at 9000 kg./sq. cm. into a wafer 2 mm. thick. The wafer had a circular area of 1 sq. cm. The wafer was then sintered in air overnight at 1000 C. The resultant material was used as an anode in a laboratory scale cell for the electrolysis of sodium chloride and found to have a low chlorine overvoltage, equivalent to that for bulk RuO As noted above, the Pb Ru O is less expensive than RuO since it contains only 37.4% of the expensive RuO by weight.

EXAMPLE 3 A solution containing Ru and Pb in molar ratio of 1:1 was made and used to form thin films of Pb Ru O on titanium. This solution contained the following parts by weight:

Ruthenium resinate dissolved in a mixture of oil or rosemary, nitrobenzene and chloroform (4% The mixture of essential oils was chosen to solubilize the two resinates in a solution having a viscosity suited to application by brushing. A solution of this concentration gives a fired film about 1000 angstroms in thickness for each application.

The above solution was used to prepare thin films of Pb Ru O on titanium sheets and on glass microscope slides, by firing in air to 450 C. with a minute soak at peak temperature. A comparative set of thin RuO films was prepared on these substrates on the same firing cycle by brush application of a solution of RuCl in isopropyl alcohol. The presence of Pb Ru O was determined by measuring electrical resistivity of the film on glass, at room temperature and at liquid nitrogen temperature. The temperature coefiicient of resistance was positive and of the same order as that reported for bulk Pb Ru O in the Longo, Raccah and Goodenough reference cited above. In contrast, the thin film of RuO- on glass had the same electrical resistivity at the two temperatures, i.e. its TCR was zero.

The films on titanium were tested as anodes in a brine cell (22% NaCl) at 20 C. at a pH of 3.7. Overvoltage was measured in the usual way using a Luggin probe connected to a normal calomel electrode. Values for the two coatings were:

POTENTIAL VOLTS It can be seen that the superiority of Pb Ru O ismost pronounced at the higher current densities, the range of practical commercial importance in chlorine production.

EXAMPLE 4 To prepare bismuth ruthenate, powdered Bi O and R1102, in ratio of 1 mole:2 moles, were mixed thoroughly, the mixture was placed in a porcelain dish and heated in air at 750 C. for one hour.

Upon cooling, the product was a black very conductive powder. Microscopic examination showed that the intense orange color of the starting Bi O had completely disappeared.

The bismuth ruthenate powder was sifted and the fraction which passed a 325 mesh sieve was made into a paint using a toluene solution of an acrylic resin. Proportions were chosen to give a film on titanium containing parts by weight BigRuzoq and 5 parts resin after removal of the toluene by baking at 70 C. Paints containing RuO and Pb Ru O prepared in the same way were applied to titanium and baked at 70 C.

The three coating materials were tested at 20 C. in a cell containing a solution containing 22% NaCl at a pH of 3.7. At a current density of amps/square foot, bismuth ruthenate samples had an overvoltage potential of more than 3 volts with reference to a normal calomel electrode. In contrast, RuO samples and Pb Ru O had potentials of 1.2 volts at this current density.

This comparison shows that Bi Ru O is not suitable for the production of chlorine by the electrolysis of brine.

EXAMPLE 5 A composite film, about one mil thick, was prepared on titanium from a mixture of lead ruthenate and a flux. The flux was chosen from the PhD-V 0 system and the composition was equimolar, the eutectic with a melting point of 480 C. It was made from the oxides at 900 C. On cooling to room temperature it did not crystallize. It was ground to a powder to pass through a 325 mesh sieve, applied to titanium and heated to 500 C. On cooling, it was observed that the powder had melted and was firmly bonded to titanium, indicating that the thermal expansion is close to that of titanium. The glass coating was an electrical insulator. It did not generate chlorine when tested as anode in a brine cell. The glass showed no apparent chemical attack from the brine.

The composite film was formed by mixing lead ruthenate powder prepared as in Example 1 and the PhD-V 0 fiux (325 mesh), in proportion of 1:2 by weight. The mixture was suspended in a mixture of essential oils and applied to titanium by brushing. The coated titanium was fired in air to 500 C. with a 20 minute soak. On cooling an adherent, highly conductive composite film about one mil thick, was obtained. This coating was tested under the conditions described in Example 3 and had these values:

Potential volts Amps/sq. ft.: Pb Ru o zflux 2 100 1.18

a 1 2, by weight.

The overvoltage of this composite film is not as low as that of the thin, pure Pb Ru O film described in Example 3. In some applications, the overvoltage of the composite film is acceptable because of the durability and increased operating time obtainable with this relatively thick, very adherent film.

EXAMPLE 6 A solution containing Ir and Pb in a molar ratio of 1:1 was made and used to form a thin film of Pb Ir O on titanium. This solution contained the following parts by weight:

Iridium resinate dissolved in a mixture of oil of rosemary, nitrobenzene and chloroform (6% Ir) 6.23 Lead resinate dissolved in a mixture of oil of rosemary, nitrobenzene and chloroform (27.8%

PB) 1.45 Oil of lavender 0.44 Oil of petitgrain 0.44 Oil of camphor .44 Rosin dissolved in oil of spike (50% rosin 1.00

POTENTIAL VOLTS Amps/sq. ft 100 200 400 600 800 l, 000

This demonstrates the low chlorine overvoltage of the lead iridate coated samples.

EXAMPLE 7 Lead iridate was made by mixing equimolar amounts of IrO and PhD powders, minus 325 mesh. The mixture, green yellow in color, was placed in a porcelain crucible and heated in air to 800 C. It was held at 800 C. for 16 hours and cooled gradually. The product was a flufiy, uniformly black powder which was highly conductive. This was identified by X-ray diffraction as Pb Ir O with a structure similar to lead ruthenate, described above.

Lead iridate powder, -325 mesh, was mixed with eutectic PhD-V 0 glass powder, also 325 mesh, in the ratio of Pb Ir O :glass, 2:3 by weight. After thorough dry mixing in a mechanical shaker, the mixed powders were suspended in a vehicle made by dissolving rosin in a mixture of essential oils. The vehicle contained 16.6%

POTENTIAL VOLIS Amps/sq. ft 100 200 400 600 800 1, 000

PbzIrzOa plus flux 1. 24 1. 26 1. 29 1. 36 1. 45 1. Do. 1. 22 1. 25 1. 29 1. 31 1. 37 1. 48

1 Sample 1. 2 Sample 2.

What is claimed is:

1. An anode for the electrolysis of brines comprising a substrate of a corrosion-resistant valve metal selected from the group consisting of titanium, tantalum, hafnium, zirconium, tungsten, aluminum and alloys thereof, said substrate having an electrically conductive coating consisting essentially of Pb Ru O or Pb lr o 2. An anode of claim 1 wherein the coating consists essentially of Pb 'Ru O 3. An anode of claim 1 wherein the coating consists essentially of Pb Ir O 4. An anode of claim 1 wherein the electrically conductive coating contains from 10% to 70% by weight of an inert, nonconducting glass flux to bond the coating to the substrate.

5. An anode of claim 4 wherein the flux is composed of V 0 and 30 to mol percent PbO.

6. An anode of claim 5 wherein the flux is a eutectic mixture of PbO and V 0 References Cited UNITED STATES PATENTS 3,528,857 9/ 1970 Lieb et a1 204-290 R FOREIGN PATENTS 1,195,871 6/ 1970 Great Britain.

6,606,302 11/1966 Netherlands.

JOHN H. MACK, Primary Examiner R. I. FAY, Assistant Examiner U.S. Cl. X.R. 117-230; 20429l