US3654121A

US3654121A - Electrolytic anode

Info

Publication number: US3654121A
Authority: US
Inventors: Carl D Keith; Alfred J Haley Jr; Robert M Kero
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: BASF Catalysts LLC; Engelhard Minerals and Chemicals Corp
Priority date: 1968-12-23
Filing date: 1968-12-23
Publication date: 1972-04-04
Anticipated expiration: 1989-04-04
Also published as: GB1292130A; FR2105652A5; US3677815A; US3657102A; NL6919307A; DE1964294A1

Abstract

AN IMPROVED ANODE FOR THE ELECTROLYSS OF BRINES IS COMPRISED OF A CORROSION RESISTANT VALVE METAL SUBSTRATE, A THIN POROUS ADHERENT EXTERIOR COATING OF SILICA, AND BETWEEN THE SUBSTRATE AND EXTERIOR COATING A THIN LAYER OF RUTHENIUM OXIDE.

Description

United States Patent 3,654,121 ELECTROLYTIC ANODE Carl D. Keith, Summit, Alfred J. Haley, Jr., Florham Park, and Robert M. Kero, Cranford, N.J., assignors to Engelhard Minerals & Chemicals Corporation, Newark, NJ.

No Drawing. Filed Dec. 23, 1968, Ser. No. 786,438 The portion of the term of the patent subsequent to Dec. 28, 1988, has been disclaimed Int. Cl. B01k 3/04 U.S. 204-290 F 3 Claims ABSTRACT OF THE DISCLOSURE An improved anode for the electrolysis of brines is comprised of a corrosion resistant valve metal substrate, a thin porous adherent exterior coating of silica, and between the substrate and exterior coating a thin layer of ruthenium oxide.

This invention relates to novel anodes for cells used for the electrolysis of brines, and more particularly to improved anodes comprised of platinum group metal coated electrolytic valve metals and a method for obtaining such anodes.

The anodes of the present invention are particularly useful in cells used for the production of chlorine and caustic soda by the electrolysis of an aqueous solution of sodium chloride. In such cells graphite anodes are usually used commercially. Although the graphite anodes are not entirely satisfactory because their wear rates are high and impurities such as CO are introduced in the products, no satisfactory substitutes have yet been found.

Platinum group metal coated electrolytic valve metals have been proposed as substitutes for graphite anodes. These metallic anodes offer several potential advantages over the conventional graphite anodes, for example, lower overvoltage, lower erosion rates, and higher purity products. The economic advantages gained from such anodes, however, must be sufliciently high to overcome the high cost of these metallic anodes. Anodes proposed heretorfore have not satisfied this condition. Therefore commercialization of the platinum group metal anodes has been limited.

One problem is the life of the metallic anodes. A factor which contributes to shortening the anode life is the soealled undercutting elfect. For economic reasons the precious metal coatings are very thin films so that exposure of the substrate is imminent. This is particularly true in the use of low overvoltage coatings which are inherently porous. Although corrosion resistant, the valve metals are attacked through the pores of these coatings thereby shortening the life of the anodes.

Another problem is the loss of precious metal during operation of the cell. Although the loss is gradual, it is costly because the precious metals are expensive and because the erosion of the thin coating shortens the anode life. The loss of precious metal may be from mechanical wear. At the high current densities desirable in commercial installations, the increased rate of flow and the excessive gassing is conducive to such mechanical wear. In mercury cells a contributing factor is amalgamation of the precious metals.

A further problem in mercury cells is shorting of the cell on contact of the precious metal 'with the mercury with consequent elfects, such as amalgamation, change in the surface of the anodes with resultant harmful change in electrolytic properties, and cell stoppage.

A still further consideration which is of major importance in the highly competitive manufacturing processes involving the electrolysis of brines is the power consump- 3,654,121 Patented Apr. 4, 1972 tion associated with the anodes. Power costs represent a substantial percentage of the total production costs and even a small reduction in power consumption produces a material economic advantage.

It was an object of this invention to provide metallic anodes with improved physical and electrical characteristics. It was a further object to provide a process for the electrolysis of brines which can be effected with materially lower production costs.

In accordance with this invention the electrolysis of brines can be effected with a materially lower power consumption. This is achieved by the use of an improved anode. The anode not only reduces the power consumption in the cell, but also it has been found to have long life and low metal losses due to mechanical wear and amalgamation. The resistance to amalgamation makes the anode particularly useful in mercury cells.

The anode of the present invention is comprised of a corrosion resistant metal substrate, a ruthenium oxide coating, and a thin porous adherent coating of silica over the ruthenium oxide. The silica coating has a high surface area, typically of at least about 30 square meters per gram (m. /g.).

The corrosion resistant metal substrates, the so-called valve metals, used for electrolytic anodes are well known in the field. They are much less expensive than platinum group metals and they have properties which render them corrosion resistant to the anodic environments in electrolysis cells. Examples of suitable corrosion resistant valve metals are Ti, Ta, Nb, Hf, Zr, W, Al, and alloys thereof. It is also well known to have the valve metal as a layer on a base metal such as copper which is a good conductor but corrosive to the environment, and such modifications are within the scope of this invention.

The silica coating not only minimizes the contact of the precious metal layer with the electrolyte, but also minimizes penetration of the electrolyte to the valve metal and thus limits the extent of undercutting effects. An other advantage is that it minimizes shorting and the concomitant problems. Surprisingly however, despite the dielectric characteristics of the exterior coating, these advantages are gained without sacrificing the desirable electrical properties of the precious metal anodes. Indeed, the exterior porous high surface area silica coating improves the electrolytic properties of the thin precious metal coatings. A still further advantage of the anodes of this invention is that the high surface area porous exterior coating is conducive to gas evolution.

The anodes of this invention are prepared by first forming a ruthenium oxide layer on the base metal substrate and then depositing a silica coating on the ruthenium oxide.

Many methods are known for forming adherent ruthenium oxide films on a metal substrate. For example, after etching and cleaning the surface of the base metal, ruthenium metal or a ruthenium salt is deposited on the substrate and then the coated substrate is subjected to elevated temperature in an oxidizing atmosphere. The ruthenium metal or salt is deposited in a variety of well known ways, e.g. the ruthenium metal may be deposited as a finely divided dispersion in an organic vehicle or by plating, sputtering, vacuum deposition, the ruthenium salt may be deposited by applying such salt dispersed or dissolved in an organic or aqueous medium. The conversion to the oxide is then effected by firing the coating in an oxygen-containing atmosphere, e.g. air, preferably in the temperature range of about 400 to 800 C. The firing time depends on the temperature, oxidizing atmosphere uesd, and the thickness of the ruthenium metal coating applied. Typically a suitable ruthenium oxide is formed by firing the metal film in air at 500 C. for about five minutes.

The exterior high surface area porous silica coating is deposited from a dispersion or solution containing hydrophilic silica or a silica compound precursor in very fine particle size, and the silica coating is fired at temperatures greater than about 400 C. to promote bonding. When fired at temperatures lower than about 400 C. the coatings are not sufiiciently adherent. A preferred method is to deposit the silica from an aqueous colloidal silica solution. Preferred temperatures for forming an adherent porous coating are 400 to 800 C. Coatings formed in this manner are adherent and porous and have a high surface area. More than one coating of silica may be applied. Generally, the silica coatings are effective at a thickness of up to about 200 microinches. Thicker coatings are often not sutficiently porous. Alternatively, multiple thin coatings may be formed by depositing alternate layers of ruthenium oxide and silica, thereby forming a hard durable multilayer coating on the substrate. Although the multilayer coatings are effective at thickness of over 200 microinches, there is no advantage in forming thicker coatings because of their durability even when exceedingly thin.

The following examples are given by way of illustration and not as a limitation of the invention. It will be appreciated that modifications within the scope and spirit of the invention will occur to those skilled in the art.

Examples 1, 2 and 3 show comparative tests in diaphragm and mercury electrolysis cells using various anodes. For each anode a sheet of commercially pure titanium, /2" x 3 x 0.063", is prepared for coating by etching in concentrated hydrochloric acid for a period of 18 hours at room temprature and cleaning in fiuoboric acid.

RuO coatings are prepared as follows:

An aqueous solution of RuCl (containing 10.35% by Weight of Ru) is applied to one side of a titanium sheet using a brush. Successive coats are applied, each being fired at 500 C. in air for five minutes until a coating of the desired thickness is obtained. Alternatively a ruthenium resinate solution (containing 4% by weight Ru) is applied. In still another alternative method an alcohol based paint is used. This paint is composed of l g. of RuCl;,, 1 ml. of linalool and 30 ml. of 2-propanol. X-ray dilfraction analysis of samples similarly prepared, by firing the deposited coating in air at the indicated conditions, showed that a major portion of the ruthenium was converted to ruthenium oxide.

Porous adherent silica coatings are prepared as follows:

After forming the RuO layer, it is overcoated with S by applying a formulation containing hydrophilic colloidal silica. Ludox HS, an aqueous collodial silica solution, is used in the formulation. The formulations contain about 10% colloidal silica and 90% water. Film forming additives such as sodium titanate, silicate or borate may be incorporated in minor amounts in the colloidal silica solution. For example, suitbale coatings are made from a formulation composed of 10% colloidal silica, 0.5% sodium titanate and 85.5% water. Successive coats of silica are applied and fired in air at 500 C. for 5 minutes until a coating of the desired thickness is obtained.

The thickness of the coatings is determined gravimetrically.

EXAMPLE 1 Two samples are prepared having a RuO coating equivalent to 17 microinches of Ru metal on a titanium substrate.

Sample B is used as prepared.

Sample A is overcoated with 100 microinches of SiO using the method described above. The silica has a surface area of about 70 mF/g.

Sample A and Sample B are used as anodes in a laboratory scale diaphragm cell for the electrolysis of NaCl solution. The tests are run at a temperature of 4 C. and a current density of 1000 amperes per square foot (ASF). The chlorine overvoltage is determined with a conventional Luggin capillary probe, and the results are set forth in Table I.

Cell potential after 210 hours at 1,000 a.s.f. (volts) 3.

l E1102 and SiO: overcoating.

2 After 210 hours, Sample B would not draw the specified current den sity at its initial cell potential. Upon raising the cell potential rapid disintegration of both the coating and the substrate resulted.

This example demonstrates the superior electrical and wear properties of the anode having the SiO exterior coating of this invention over an anode having a RuO layer and no overcoating of silica.

EXAMPLE 2 Samples similar to those described in Example 1 are prepared. Sample C is a titanium substrate with a RuO coating having a thickness equivalent to 17 microinches of Ru. Sample D is a titanium substrate with a RuO layer equivalent to 17 microinches of Ru and microinches overcoating of silica. Each of the samples is masked with pressure tape so that an area of 0.049 in. of coating remains exposed. Samples C and D are then used as anodes in a small cell using a mercury pool as the cathode and a 25% NaCl solution as the electrolyte. The anodes are subjected to a mercury shorting test as follows:

The exposure area of the test coating is allowed to generate chlorine at 1000 ASF in the brine and then it is submerged in the mercury pool and the change in current density is measured. The tests show that Sample D, having the Ru0 layer and the SiO exterior coating is very much less susceptible to shorting than the same coating without the protective exterior coating of SiO It will be appreciated that since the resistance to shorting is higher the anodes of this invention may be positioned in closer spacial relationship with a mercury cathode without danger of shorting and with concomitant lower power requirements.

EXAMPLE 3 Samples similar to those described in Example 1 are prepared, except that the RuO layer is thinner. Two samples are prepared each having a Ru0 coating equivalent to 2 microinches and Ru on a titanium substrate.

Sample E is used as prepared.

Sample F is overcoated with microinches of SiO using the method described above.

Samples E and F are used as anodes in a laboratory scale diaphragm cell and tested for chlorine overvoltage using the procedure described in Example 1. The cell using Sample F, the anode in accordance with this invention, has an initial chlorine overvoltage of 220 millivolts and a cell potential of 4.30 volts. The cell using Sample E as the anode shows erratic behavior. The coating of Sample E is poorly adherent and the erratic results are believed to be attributable to this poor adherence of the coating and also to the insufficient protection provided by the RuO coating of this degree of thinness.

This test demonstrates the improved physical and electrical properties of anodes of this invention. Such improvements not only permit operation of a cell with lower power requirements but also demonstrates the improved life of the anodes since they are operable with thinner coatings of precious metal than anodes without such coatings.

EXAMPLE 4 Two sheets of commercially pure titanium /2 x 3" x 0.063", are prepared for coating by sandblasting the surfaces with aluminum oxide grit followed by cleaning with an abrasive cleanser. Both sheets are then coated on both sides with a formulation composed of (by weight) 11.5% ruthenium chloride, 42.3% 2-propanol, and 46.2% linalool. The coated substrates are heated to 300 to 400 C. for l to 2 minutes and then fired at 500 C. for 5 minutes in an open air furnace to form a RuO coating.

Sample G is prepared by repeating the application of the ruthenium formulation and heat treatment twice, so that a total of three coats of ruthenium oxide are applied.

Sample H is prepared by overcoating the first ruthenium oxide coating with a porous silica coating. The porous silica coating is formed by applying an aqueous colloidal silica solution composed of (by weight) 31.6% Ludox HS (containing 30% SiO 0.5% sodium titanate powder, and 67.9% water. The silica-coated substrate is heated to 500 C. for 5 minutes. Thereafter the procedure of applying and firing alternate coatings of ruthenium oxide and silica is repeated twice.

The composition of the samples is as follows:

Coating Sample G Sample H R1102, percent. 100 43. 3 Si02, percent 0 56. 7

scale diaphragm cell and tested for chlorine overvoltage using the procedure described in Example 1. Sample G having 3 coatings of ruthenium oxide has an initial chlorine overvoltage of 155 millivolts and a cell potential of 4.20 volts. Sample H, a multilayer RuO SiO coating prepared in accordance with the present invention, has an initial chlorine overvoltage of 10 millivolts and a cell potential of 4.30 volts. In addition the multilayer R SiO coating, applied in alternate layers, is more adherent than the R110 coating of Sample G.

This example not only illustrates a method of preparing the RuO- and SiO coating by depositing alternate layers of Ru0 and SiO but also further demonstrates the improved physical and electrical properties of anodes of this invention.

We claim:

1. An electrolytic anode comprising a corrosion resistant valve metal substrate, a thin adherent porous exterior coating of silica, and between the substrate and exterior coating a thin layer of ruthenium oxide.

2. An electrolytic anode of claim 1 wherein the exterior silica coating has a thickness of up to about 200 microinches.

3. An electrolytic anode of claim 1 wherein the exterior silica coating has a surface area of at least 30 m. /g.

References Cited FOREIGN PATENTS 6,606,302 11/ 1966 Netherlands 204290 DANIEL E. WYMAN, Primary Examiner I. VAUGHN, Assistant Examiner