Ozone oxidation of cyanides is best carried out in the pH range of 9-10, and the oxidation of cyanide to cyanate is extremely rapid (equation 8.3[15]). Further reaction of cyanates is much slower. The addition of copper (2+) salt catalysts accelerates the reaction. A typical ozone control system is shown in Figure 8.3.2.

Cyanide oxidation can also be carried out electrolytically. The more toxic sodium and potassium cyanides can also

FIG. 8.3.2 Cyanide waste oxidation control systems utilizing ozone as the oxidant. Key: pHRC = pH recording controller; ORPR = ORP recorder; PC = pressure controller; LLC = low-level control; AMP = amplifier

be converted to substantially less toxic ferrocyanide complexes by adding ferrous sulfate. This process is not recommended, however, because ferrocyanide releases cyanide when exposed to sunlight.

Chlorination is the most frequently used and best developed process. The addition of chlorine gas is hazardous and requires storage of large quantities of chlorine. Ozone is a faster, more powerful oxidizing agent, requiring smaller holding and reaction tanks. The relative amounts required for each process are shown in Table 8.3.2.

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