Hypochlorite construction materials

Chemical methods, where compatible, can be used on a wider variety of construction materials. These methods typically employ oxidizing agents such as hypochlorite, hydrogen peroxide, ozone, or per-acetic acid. Hypochlorites are effective sanitizers but are difficult to flush from the system and tend to leave biofilms intact. 

Typically, the entire contents of a calcium hypochlorite container are emptied into a mixing tank where they are readily and completely dissolved in water. The resulting corrosive solution is stored in and fed from a stock solution vessel constructed of corrosion-resistant materials such as plastic, ceramic, glass, or rubber-lined steel. Solutions of 1% or 2% available chlorine can be delivered by a diaphragm-type, chemical feed pump or metering pump. 

Two possible approaches to lower the concentrations of these contaminants are precipitation as oxides or hydroxides and ion exchange. Oxides can be removed by adding NaOCl to the brine at a pH of 10-12, followed by filtration. Nickel levels as low as 5 ppb can be realized by this method. However, the practicality of instituting this scheme in an operating pleuit should be carefully assessed because of the possibility of corrosion and contamination of the brine by reaction of materials of construction with hypochlorite. 

Materials of Construction. Preventing contamination of process materials is one criterion in the selection of materials of construction. With hypochlorite solutions, it is especially important. The presence of contaminants in a solution is itself objectionable. Section 15.3.2.2 discussed the fact that certain contaminants also catalyze the decomposition of hypochlorite, reducing the useful life of the product and even creating process hazards and mechanical difficulties. 

Given the cost of titanium, hypochlorite handling systems usuaOy are constructed from nonmetallic materials. The most widely used materials are halogenated polymers, polyethylene, and fiber-reinforced plastics (FRP). They often appear as linings on metal substrates, and PVC wrapped with FRP is also quite common. Polymeric systems in outdoor use should have ultraviolet (UV) light barriers. 

The economic design of high-quality tanks for storing hypochlorite solutions has been a long-standing problem [75]. Iitanium would be an excellent material of construction, but only at a yery high cost. Rubber-lined carbon steel has been widely used in both horizontal and vertical tanks. Fiber-reinforced polymers are attractive candidates, but results often have been unsatisfactory. Vinylester resins (e.g., Derakane 411) are preferred, and polyester resins also may be satisfactory. Those with high chemical resistance are best, and bisphenol A resins, for example, are preferred to those based on isophthalic acid. 

Safety. Because of the presence of active chlorine and the possibility of evolution of elemental chlorine, hypochlorite solutions are highly corrosive and toxic. No one should be allowed to handle these substances without thorough training. Plant systems should be carefully designed, using the proper materials of construction (Section 15.3.2.3), and in particular should rigidly exclude the possibility of accidental contact with excess acid. 

J.K. Nelson, Materials of Construction for Alkalies and Hypochlorites. In B.J. Moniz and W.I. Pollock (eds). Process Industries Corrosion, National Associatioii of Corrosion Engineers, Houston, TX (1986), p. 297. 

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