Preparation and Applications of Graphite Substrate Lead Dioxide (GSLD) Anode

Autor: K. C. Narasimham, H. V. K. Udupa
Rok vydání: 1976
Předmět:
Zdroj: Journal of The Electrochemical Society. 123:1294-1298
ISSN: 1945-7111
0013-4651
DOI: 10.1149/1.2133063
Popis: The GSLD anodes have been developed for obtaining sui table size anodes for use in high amperage cells in the production of chlorates and perchlorates. The performance characteristics of the anodes in the preparat ion of chlorates and perchlorates are described. The use of the GSLD anode in other inorganic preparat ions like bromates iodates, and periodates is also included. In recent years the quest for the development of in destructible anodes either as a substi tute for costlier anodes or to increase the life of anodes in electrochemical processes has intensified. Increasing interest in the scientific development of iner t and insoluble anodes provided a heal thy atmosphere mer i t ing considerable research effort both in the improvement of existing anodes and in the development of new anodes. The complex na ture of the evaluat ion problem stems from the number of variables involved, such as electrode life, operating conditions of the cell, and replacement costs. Graphite and p la t inum are widely we l l -known anodes in electrochemical processes and, less frequently, materials like magnetite, lead, and lead-si lver or l ead-an t imony alloy are employed. But the recent researches on the indestruct ible or iner t anodes are largely centered around the development of (i) plat i n u m or its alloy coated over t i t an ium and (ii) coating of oxide or mixed oxides of certain metals on suitable substrates. The main requirements for an oxide anode are: (i) the possibility of forming ions of different valences to provide for high electrical conductivity, (ii) a high anodic potential at evolution of oxygen, (iii) absence of rectifying contacts at the boundary of oxide-metal current lead, and (iv) chemically inert. The high cost of p la t inum has prompted several attempts to replace this metal by cheaper material. In the last two decades interest in the use of lead dioxide as anode in the place of p la t inum for the preparat ion of inorganic and organic electrochemicals (1) has been very much in evidence as seen by the considerable amount of work carried out to obtain lead dioxide deposits in a form suitable for anodes in the production of chlorates and perchlorates. The successful development of a suitable GSLD anode for commercial scale operations in chlorates and perchlorates has been engaging the at tent ion of the Central Electrochemical Research Institute. This paper reviews the developmental work on this project. Earlier attempts to prepare the lead dioxide electrodes as well * E l e c t r o c h e m i c a l Soc ie ty Ac t ive Member. K e y w o r d s : e l e c t r o d e p o s i t i o n , c h l o r a t e s , perchlerates , insoluble anodes. as the difficulties encountered by previous workers have been reviewed earlier by Naras imham and Udupa (2-4). Recently, Carr and Hampson (5) reviewed the studies on the electrodeposition of lead dioxide laying emphasis on the kinetics of eIectrodeposition. Preparation of GSLD Anode A survey of the l i terature shows a continuous interest in the preparat ion of lead dioxide anodes beginning in 1934. Lead dioxide satisfies the major requirements given above for the oxide anode. Angel and Mellquist (6) reported the deposition of lead dioxide from lead tar t ra te bath. Though electrolysis of almost all soluble salts of lead (5, 7-10) gives lead dioxide deposit under suitable conditions on the anode, the lead ni t ra te bath is preferred since it may be readily controlled over long plat ing periods and also due to the high quali ty of deposit obtained from the bath over a wide range of operating conditions. ~-PbO2, which has an oxygen overvoltage higher than a-PbO2, is obtained from the ni t rate bath. Japanese workers (11-16) used this ni t rate bath extensively for depositing lead dioxide on nickel or mild steel substrate. When lead nitrate alone is used, a dendrit ic form of lead is also deposited on the cathode result ing in the shorting of the electrodes. Even though a diaphragm cell has been suggested (17) to prevent lead deposition on the cathode, the addition of copper salt to the extent of 2-3% to the bath for prevent ing the deposition of lead is by far the most important contr ibut ion (12) in the field of electrodeposition of lead dioxide. The copper, being more electropositive than lead in the electrochemical series, deposits preferent ia l ly on the cathode. Grigger et aI. (18) and Schumacher et aI. (19) reported the formation of lead dioxide from the ni t rate bath on tan ta lum or p la t inum-clad tantalum. Although much work had been done as described above, the preparat ion of lead dioxide electrodes in volved certain disadvantages. In the process developed by the Japanese workers (11-16), massive lead dioxide electrodes were prepared by depositing lead dioxide up to 1 cm thick or more, then removing the same from the substrate, and finally cutt ing the deposit into suitable shapes with either an a lundum or carborundum Downloaded 10 Jan 2012 to 139.80.123.34. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp VoI. 123, No. 9 GSLD ANODE 1295 grinding stone. This operation needs special care as the deposit is bri t t le as well as very hard. The convent ional clamp current-contact produces local heating and hence suitable modifications have to be done. On the other hand, costlier substrates were described for depositing lead dioxide in the methods described by Grigger et al. (18) and Schumacher et al. (19). The successful electrodeposition of lead dioxide from lead ni t ra te-copper ni t ra te bath on graphite substrate carried out s imultaneously by the authors (2, 20, 21) and by Gibson (22, 23) in the U.S. obviates the difficulties experienced by earlier workers. While Gibson (22) used a nonionic surface active agent in the bath, Naras imham and Udupa (2, 20, 21) employed rotat ion for the cylindrical rods and to-and-f ro motion for the plates dur ing deposition to inhibit gas bubbles from sticking to the surface, thereby avoiding pinholes and pi t t ing in the coating. Although hydrodynamic factors, such as the decrease in the thickness of the diffusion layer and also the easy t ranspor t of ions to the in te r phase, do exist dur ing the movement of the electrode, the impor tant aspect in this case happens to be the dislodging of gas bubbles adhering to the surface of the electrode. Similarly, the use of surfactant lowers the interracial tension, thereby enabl ing the easy release of gas bubbles from the anode surface. The electrodes thus developed have the following advantages: (i) a thin coating of lead dioxide on graphite is adequate; (it) the graphite provides mechanical s trength for the deposit and is completely protected against anodic attack; (iii) electrical contact to lead dioxide can convenient ly be made on graphite; and (iv) preparat ion of such anodes for large scale operation does not present undue difficulty. Cell assembly for oxide deposition differed with the size and shape of the anodes required to be coated. The electrolyte contained 325-350 g/ l i ter lead ni t ra te and 25-30 g/ l i ter copper nitrate, with an init ial pH between 4 and 4.5. During electrolysis the pH became acidic due to the production of nitr ic acid and was main ta ined at 1-1.5 by adjust ing the flow rate of the electrolyte. The acid produced was neutral ized outside the cell by the addit ion of lead carbonate or lead monoxide and copper carbonate. Deposition was carried out at current densities of 3-5 A/d in 2 and at a temperature of 58 ~ 65~ It is most impor tant to have precleaning operations for the graphite anode prior to deposition, which consist in electrolyzing a 10% (W/V) sodium hydroxide solution with the graphite as anode for 30 min, dipping the anode in 10% (V/V) nitric acid for 10 rain, and finally washing it thoroughly with distilled water. Different rods and plates of GSLD prepared are given in Table I. These have been used in 200, 800, and 5000A cells for the production of chlorates and perchlorates. The thickness of the deposit depends on the process in which GSLD is used, e.g., 1.5-2.5 mm thick is sufficient for chlorate production while the thickness must be more than 3.5 mm for the production of perchlorate. Even if the lead dioxide peels off without any attack on the graphite surface, the lead dioxide can be deposited again on the same graphite. Some of the GSLD anodes given in Table I include such once or twice redeposited electrodes. Calculations for optimizing the size of the anode with respect to weight of graphite needed for different diameter graphite rods show that either 7.5 or 10 cm diameter GSLD rods are preferable (24). Table I. GSLD rods and plates
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