Anode for electrolytic processes

#The present invention relates to improved electrodes particularly adapted for use as anodes in electrochemical processes involving the electrolysis of brines.

#A variety of materials have been tested and used as anodes in electrolytic cells. In the past, the material most commonly used for this purpose has been graphite. However, the problems associated with the use of graphite anodes are several. The chlorine overvoltage of graphite is relatively high, in comparison for example with the noble metals. Furthermore, in the corrosive media of an electrochemical cell graphite wears readily, resulting in substantial loss of graphite and the ultimate expense of replacement as well as continued maintenance problems resulting from the need for frequent adjustment of spacing between the anode and cathode as the graphite wears away. The use of noble metals and noble metal oxides as anode materials provides substantial advantages over the use of graphite. The electrical conductivity of the noble metals is substantially higher and the chlorine overvoltage substantially lower than that of graphite. In addition, the dimensional stability of the noble metals and noble metal oxides represents a substantial improvement over graphite. However, the use of noble metals as a major material of construction in anodes results in an economic disadvantage due to the excessively high cost of such materials.

#In attempts to avoid the use of the expensive noble metals various other anode materials have been proposed for use as coatings over valve metal substrates. In U.S. Pat. No. 3,627,669, it is disclosed that mixtures of tin dioxide and oxides of antimony can be formed as adherent coatings on a valve metal substrate to form an anode useful in electrochemical processes. In the electrolytic production of chlorine, anodes of this type provide the advantage of economy in the elimination of the use of expensive noble metals or noble metal oxides. In addition the tin oxide coating provides an effective protection for the substrate. However, the tin oxide compositions, although useful as an anode material, exhibit a chlorine overvoltage that is substantially higher than that of the noble metals or noble metal oxides. Thus, despite the elimination of expensive noble metals, the cost of chlorine production, in processes using such anodes, is relatively high.

#A strip of titanium plate is prepared by immersion in hot oxalic acid for several hours to etch the surface, then washed and dried. A solution of 2.2 parts of SnCl.sub.4.sup.. 5H.sub.2 O and 0.2 parts of (NH.sub.4).sub.2 MoO.sub.4 in 5 parts of water is brushed on to the cleaned titanium surface. The coating is dried by heating at 100.degree.C for about 2 minutes than heated in an oven with a forced flow of air at 400.degree.C for about 5 minutes to convert the coating substantially to the oxide form. The process is repeated three times with a final heating in air at 400.degree.C for 30 minutes.

#The conductive tin oxide coated titanium is further coated in the following manner:

#A solution of 1 gram of ruthenium trichloride in 0.4 cubic centimeters of 36 percent hydrochloric acid and 6.2 cubic centimeters of butyl alcohol is brushed several times on to the tin oxide surface and then allowed to dry in air at a room temperature. After drying, the samples are heated in air at 560.degree.C for 25 minutes to decompose the RuCl.sub.3 and form RuO.sub.2. An additional coating of RuCl.sub.3 is similarily applied, dried and thermally treated, to yield a final coating of RuO.sub.2 having a coating weight of about 5.0 to 7.0 grams of ruthenium per square meter.