Corrosion chlor-alkali industry

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

In terms of scale of production (around 2x10 ton yr V worldwide) aluminium electrolysis is second in importance only to the chlor-alkali industry. This is because aluminium is both light and strong and therefore suitable for many engineering and construction applications, may readily and cheaply be treated by anodizing (see Chapter 7) to retard corrosion and is the principal alternative to copper as a conductor of electricity. Moreover, the known reserves of aluminium ores are relatively high. 

The corrosion rate of iron under these conditions can be reduced either by cathodic protection during shutdowns or by adjusting the pH of the medium to a value where iron is stable. The latter approach is commonly practiced in the chlor-alkali industry. 

Even though the involvement of graphite in the chlor-alkali industry is now history, this application remains important to this chapter, because it remains the largest and most reliable source of information about the mechanism of graphite corrosion and ways to limit the rate of corrosion [1 -3]. The graphite lost from the anode was found partly as sludge in the base of the cells, but CO and CO2 were also found to contaminate the chlorine off-gas. Evidently, graphite consumption results from anodic oxidation... 

Owing to its catalytic activity, ruthenium dioxide (RuO,) is used extensively in industrial anodes for the chlor-alkali industry and the production of perchlorates. These ruthenium-dioxide-based anodes consist of a thin catalytic layer coated onto a titanium base metal. Iridium-dioxide-based anodes are used for the production of persulfates, in electroplating, and in hydrometallurgy for evolving oxygen. These composites anodes, because of their corrosion resistance in chloride-containing media or concentrated acids and their ability to decrease the overpotential of chlorine and oxygen evolution, are called by the trade name dimensionally stable anodes (acronyms DSA ). Other uses are in fuel cells electrodes electrocatalysts. 

Today, a large number of important technologies are based on or related to electrodes reactions. Besides the chlor-alkali and aluminium industries, energy conversion in batteries and fuel cells, electrodeposition, electrorefining, organic electrosynthesis, industrial and biomedical sensors, corrosion and corrosion protection, etc. are amogst those technologies. In many of them, kinetic, catalytic or specificity aspects of electrode processes are of enormous importance. 

In practice, we remain far from meeting these apparently trivial requirements so-called inert electrodes have a finite lifetime due to corrosion and physical wear while it is common, even normal, to accept an overpotential of several hundred millivolts. Only in the chlor-alkali process and, to a lesser extent, in water electrolysis has significant progress towards improved electrode materials been made. Generalizations concerning electrode materials are probably unwise and the choice of electrodes for particular industrial processes will be discussed in... 

Nickel, which is not as electrocatalytically active as iron toward the HER, exhibits excellent corrosion resistance in hot, concentrated, alkaline solutions. Motivated by the stability of Ni in caustic solutions (Fig. 4.6.3) and the extensive investigations by tire water electrolysis industry to develop Ni-based cathodes, significant efforts have been made to develop catalytic cathodes for application in chlor-alkali cells. 

Industrially, many of the areas of electrochemical technology have developed in isolation, with little interaction or transfer of concepts, experience or hardware, e.g. batteries, electroplating, corrosion monitoring and chlor-alkali processing are often viewed as entirely separate disciplines, each with its own technology, mythology and market. 

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