Processing of Caustic Liquors

The paragraph above describes the main lines of the processes. Other processing common to all types of cell includes dilution to a concentration suitable for general 

Caustic soda and potash react rapidly with aluminum, zinc, tin, and copper. Many failures have been caused by use of these metals in minor components of equipment Some of these reactions present another hazard by evolving hydrogen. The loss of zinc or aluminum can also destroy brasses and bronzes. This effect depends on the severity of the exposure bronze is sometimes used in ambient-temperature service in utility (20-25%) caustic. 

With certain other metals, caustic solutions react to form passivating films of insoluble hydroxides on the metal surfaces. Hotter, more concentrated solutions tend increasingly to react directly with the metals. As the severity of the application increases, therefore, the various metals gradually become unsuitable. KOH is at least as corrosive as NaOH, and its applications are on the whole more sensitive to contaminants. 

Solid caustic itself is not corrosive and it, too, is handled and stored in carbon steel equipment. It is very hygroscopic, however. Its corrosivity and the flow properties of the commercial forms are sensitive to moisture. Packages are lined and carefully sealed to prevent corrosion and product degradation. 

FIGURE 9.59. Materials of constructioo for NaOH. I. Cast iron, steel, stainless steel, copper alloys, nickel, many plastics and elastomers U. SteeL Monel, Hastelloy B and C, Tliconium, nickel and its alloys, neoprene m. Monel, Hastelloy, Inconel, nickel IV. Cast iron, nickel, silver V. Steel, stainless steel, nickel and its alloys. 

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