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The History of Progress in Dimensionally Stable Anodes

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The History of Progress in Dimensionally Stable Anodes The-Historyof Progress in Dimensionally Stable Anodes I -- I- c Paul Duby i it I This article provides a brief history of In an address delivered in 1980 before and made possible the development of dimensionally stable anodes by reviezuing the ElectrochemicalSociety: Henri Beer membrane cells. Over the decade of the j innovations in the chlor-alkali industry, reviewed his research and the subse- 1970s, DSA-type electrodes revolution- electroplating and electrogalvanizing, and quent industrial development of vari- ized the chlorine industry with resulting electrowinning. These anodes are attractive ous types of titanium anodes. His aim savings of about 20% in the specific en- , for numerous reasons (e.g., long life and was to find an appropriate electrocatalyst ergy requirement. Holden and Kolb re- educed energy consumption), but they must for the production of chlorine to be ap- viewed the state of the art of manufac- I 1 still overcome the hurdleof cost togain wider plied to an electrode capable of carrying turing and applications of the metalan- ' acceptance for applications in the metal- high current densities for long periods odes to the chlor-alkali industry? 1 lurgical process industries. of time without being consumed or The success of the industrial appli- changing its geometricaldimensions. He cations of the ruthenium oxide-coated INTRODUCTION started by working with coatings ob- electrode was based only on a limited A nonconsumable anode is preferred tained by electrodeposition of PGMs on investigation of its overpotential for r many electrolytic processes. Techno- titanium, for use in mercury cells. Like chlorine evolution and its stability for other investigators, he discovered the continuous operation as an anode. These importance of the pretreatment of the were empirically related to the mor- titaniumsubstrate and the improvement phology and the preparation process.6 in adhesion and electrocatalytic activity Research of a more fundamental nature obtained by replacing the electrodepo- followed later. Trasatti and OGrady re- sition process by a thermochemical one viewed the work carried out on the consisting of an application of a solution properties of ruthenium oxide electrodes of salts of the desired metals followed by during the 1970s: Trasatti also edited thermal decomposition. He also ob- reviews of all electrodes of conductive served that the passivation and loss of metal oxides.*A better understanding of slower and limited to the platinum metal were greatly reduced by the physico-chemical properties of the alloying theplatinumwithabout30 wt.% single- and mixed-oxide films, in turn, iridium. generated ideas for further improvement Further attempts to reduce the over- of electrodes for practical applications. potential for chlorineevolution, the wear More recently, for instance, ion implan- of the coating, and its cost led to ex- tation has been used to modify and char- perimentation withPGMoxides. In1967, acterize the properties of mixed ruthe- Beer succeeded in preparing more nium-titaniu'm oxide films.9Tilak et a1.I0 electroactive and stable coatings by presented a more fundamental analysis thermochemicalcoprecipitation of PGM of the factors influencing the kinetics of oxides with oxides of titanium or the oxygen versus chlorine evolution on other valve metals. These anodes were-- . ruthenium oxide-based anodes1 They registered under. the trademark DSA? suppressed oxygen evolution by oper- Since that time, many patents have ap- ating with low surface areas and by peared describing various mixtures of adding dopants. metal oxides as the active coatings on Progress is continuing to improve the valve metal substrates. The dimen- design of the structure and coating of the sionally stable anode in use today for electrode^.*^-^^ For diaphragm electro- ~ chlorine production is made of a tita- lyzers, the coatings consist mainly of niumstructureof rods or expanded metal ruthenium and titanium oxides, and with a coating of doped, mixed oxides of service life is reported to be more than 14 ruthenium and titanium having a high years at 2 kA/m2. For the membrane 1960swhenHenri Beer invented the tita- electrocatalytic activity and electrical process, the conditions are more severe nium electrode csated with precious- conductivity. due to a larger current density, higher metal oxides3 Itis worthwhile tonote that thechange alkalinity, and a need to control the oxy- from graphite anodes to coated metal gen and chlorate. An improved coating ' CHLOR-ALKALI INDUSTRY anodes was not a simple substitution, as consisting of ruthenium, titanium, and The development of dimensionally it required modifications to the cell de- iridium oxides is expected to perform stable anodesfor the chlor-alkali industry sign and large capital expenditures. At for more than ten years at 3 kA/m2.12 is an example of a very successful inno- first, the current and cell output were A large number of variations in com- vation. A brief discussion of its history significantly increased with a modest position of oxide coatings have been as well as the technological experience (about 5%)decrease in power consump- investigated. For instance, pall a d'ium gained in the process is relevant to as- tion per tonne of chlorine gas. Later, the oxide can be used to reduce the iridium sess the prospects for metallurgical-in- new anodes led to the design of a new ~0ntent.l~The influence of coating com- 4 E; dustry applications. generation of improved diaphragm cells position has been investigated in order - '1 ~ 3 1993March JOM 41 - to optimize electrodes for the produc- by dissolution or erosion, passivation of nars.387 tion of chlorate or hypochlorite.I5 Non- the valve metal substrate, and various on cop’ precious metal oxides were also consid- types of damage caused by impurities in ith the ered. Alpha-lead dioxide coated on tita- the electrolyte. Id We] nium was suggested for the elec- ELECTROWINNING fferent trosynthesis of sodium perchlorate.16 gated Burke and McCarthy found that ther- Electrowinningis only one of the steps esh or mally produced cobalt oxide (Co30,), of complex metallurgical flow sheets, ,wer et doped with RuO, to improve conduc- and a significant modification of one particular, when investigating the 70- 1cs ani tivity, is a promising anode material.I7 unit operation often affects all theothers. wt.% Pt-30 wt.% Ir appli tivatec These oxides of the spinel type have The successful application of dimen- decomposition process, Tests c been tested for chlorine evolution. Be- sionally stable electrodes for chlorine ;ried 1 cause of their longer service life in alka- evolution is critical for the development per, : line solutions, they were recommended of chloride hydro-electrometallurgy. A e usir on a nickel substrate for water elec- dimensionally stable anode with good newl- trolysis. service life facilitates the design of per- -nce, ELECTROPLATING AND manent anodic compartments and :ed R ELECTROGALVANIZING chlorine removal systems for safe op- tigatt eration of the tankhouse. Given the Inert anodes made by electrodepo- successful commercial application of gster sition of platinum on titanium have been solvent extraction to the treatment of ‘%de used successfully for some time in vari- chloride solutions of nickel and cobalt, >pose< ous baths for electroplating gold and there is an incentive to develop an effi- 1yte.l other precious metals. Information on cient chloride electrowinning process for anode. ium the electrocatalytic properties and be- these metals. Since iridium oxide h yed P havior under different conditions has In 1975, SumitomoMetals andMining to be the electroactive alkal been reviewed.’ Skomoroski et al.I8 in- Company reported the use of DSA an- oxygen evolution, consid h cur vestigated the performance of platinum- odes on a commercial scale for the-elec- been devoted to form 3 sma clad tantalum anodes and compared trowinning of nickel and cobalt from sity 2 them favorably with electroplated pla- chlorideelectrolytes.” The electrodes had ztrol! tinized titanium electrodes in various been selected after being found much e an( electroplating baths. For these applica- superior to graphite anodes in earlier builc tions, the life of the electrodes and the stages of process development. At the costs are quite acceptable. same time, Falconbridge, in Norway, Until recently, soluble anodes of lead- was phasing out the Hybinette electro- :he e silver alloy electrodes were used satis- refining process for nickel and cobalt, ctroc, factorily in sulfate solutions. They are no replacing it by chlorine leach, solvent An wc longer suitable for modern high-speed extraction, and electrowinning.25 The md tc galvanizing lines that produce a pure tankhouse section was equipped with le ag( metal or a zinc alloy exempt of lead DSA-type anodes made of a titanium ning cell. The initial operating potent ,ditivt contamination. Platinum-clad titanium structure coated with an electroactive was about 300 mV lower than that of a! ‘ttrol! anodes suffer from high wear and, thus, layer of a noble metal oxide. The struc- lead anode under the same conditions;it! v!ty ar high costs. The chemical stability of ru- ture is fitted with a hood to which the increased linearly at a rate of 220 mV per:4 :eds f~ thenium oxide is limited in a sulfuric diaphragm bag is attached. The conver- year, which was correlated to the rate of’ &\dam acid solution and the RuOz-Ti02coating sion to the chlorineleachprocess resulted electrochemical corrosion of the coat4 1 direc used successfully
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