Several methods are utilized for recovery and disposal of spent phenolic caustics. Table 8.4.1 briefly describes and compares the major methods.

The best approach to phenolic caustic disposal is to recover the valuable cresylic acids (phenol, cresols, xylenols) and thiophenols for use as base materials in making plastics, wire insulation, lubricating oil additives, rubber reclaiming agents, and other products. Recovery can be achieved by neutralizing (springing) spent caustic with flue gas or mineral acids, forming an aqueous salt solution from which the cresylic acids and thiophenols can be decanted. A typical flue gas neutralization system is shown in Figure 8.4.1.

Both neutralization methods present obstacles. The sodium salt solution is too concentrated for release to a freshwater stream; in addition, it normally contains 5,000

Cf = total chloride concentration in the feedwater

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