In the first stage of treatment, hypochlorite oxidizes cyanide to cyanate. This reaction is accomplished most completely and rapidly under alkaline conditions at pH 10 or higher (preferred range is 11.0 to 11.5) (ref. 38). An oxidation period of 10 to 15 minutes is usually adequate (ref. 38); however retention times up to 60 minutes are routinely used (ref. 39, 348). An ORP set-point of approximately +325 millivolts is adequate for most operations. A higher set-point may be needed, depending on the composition of the wastewater. A set-point of +400 millivolts is considered a maximum point (ref. 38). Potassium iodide-starch test paper, which indicates residual chlorine, is sometimes used to determine if the reaction is complete (ref. 243). To avoid producing solid cyanide precipitates, which may resist chlorination, the wastewater should be continuously and vigorously mixed during treatment (ref. 38, 39). The resulting cyanate is further oxidized to carbon dioxide and nitrogen in the second stage. In this stage, the pH is lowered to approximately 8.5 and additional hypochlorite is added. An ORP reading of +600 to +800 generally signals a complete reaction. The retention time of the second stage is typically 30 to 60 minutes; however, times of 120 minutes are sometimes specified (ref. Delta Pollution Control File).
Although a two-tank system is preferred, complete cyanide oxidation to carbon dioxide and nitrogen can be accomplished in a single-stage unit, provided close pH control is maintained (ref. 39).
When sodium hypochlorite is used, the reaction in the first stage is:
NaCN + NaOCl - -> NaCNO + NaCl
and in the second stage,
2NaCNO + 3NaOCl + H2O - -> 3NaCl + N2 + 2NaHCO3
Sodium hypochlorite consumption is usually estimated to be 25 to 100 percent greater than the stoichiometric requirement (approximately 7 lbs of Cl2 or 7.5 lbs of NaOCl per lb of CN); where the excess is consumed by oxidation of organics and raising the valences of metals in the wastewater (ref. 39). The results of the Users Survey indicate that most shops are using substantially higher dosages (up to five hundred percent or more). Higher dosages by respondents may be the result of the formation of metal complexes (e.g., due to inadvertently combining cyanide and nickel or iron bearing wastewaters, use of unlined steel tanks, use of steel anode baskets and not retrieving fallen parts from tank bottoms) and/or poor pH control during treatment. The latter reason is especially apparent for several shops that operate single stage cyanide destruction processes and do not add acid to lower the pH. As a result, the second part of the oxidation reaction is slow and operators probably add higher dosages of NaOCl to compensate for the speed of the reaction. Shops operating under these conditions include: PS 058, PS 135, PS 204 and PS 210. It should be noted that acid additions must be made under the correct conditions or a severe safety problem could arise. Dilute acid is sometimes used in place of concentrated acid to reduce the danger of operating this process (ref. 243). Also, some equipment vendors provide a caustic feed capability that is controlled by an independent set-point, usually at a pH of 7.5 (ref. 38). The use of acid in the alkaline chlorination process is discussed by Roy (ref. 38).
Alkaline chlorination systems have generally proven reliable if well maintained. Use of a well-designed ORP control system is highly recommended. Most problems with the system focus on failures of this element. Exhibit 6-7 shows the response of various electrodes to the cyanide-to-cyanate reaction end point. The graph shows that the gold-plated electrode, although more expensive, gives much better reagent addition control (ref. 39).
