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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.3 Cyanide Oxidation
Nearly all electroplating shops that generate dilute cyanide bearing wastewaters employ alkaline chlorination treatment. This process, which has been in commercial use for over 35 years, is suitable for destroying free dissolved hydrogen cyanide and for oxidizing all simple and some complex inorganic cyanides in aqueous media (ref. 348). If properly designed, maintained and operated (good pH and oxidation reduction potential control), the process will oxidize cyanides which are amenable to chlorination (i.e., the cyanide that can be oxidized by the alkaline chlorination process), to less than 1.0 mg/l cyanide (ref. 39). Extensive sampling by EPA showed that the average effluent concentration from a well operated cyanide destruction system contains 0.18 mg/l of total CN and 0.06 mg/l of amenable CN.
The cyanide in very stable complexes, such as ferrocyanides or ferricyanides, is basically unaffected by chlorination (ref. 243). Cyanide that is complexed with copper, nickel and precious metals is amenable to chlorination, but reacts more slowly than free cyanide and therefore requires excess chlorine for efficient cyanide destruction. Concentrated cyanide wastes, such as spent plating or stripping solutions, should not be reacted with hypochlorite because the reaction can be violent, with the emission of chlorine gas. These wastes can be batch treated by electrolytic oxidation and thermal destruction (ref. 242).
A schematic of an alkaline chlorination process is shown in Exhibit 6-6. Most often the process is operated in two stages, with separate tanks for each stage. A common exception is batch treatment systems, where one tank is typically employed (ref. 38). Destruction of dilute solutions of cyanide by chlorination can be accomplished by direct addition of sodium hypochlorite (NaOCl), or by addition of chlorine gas plus sodium hydroxide (NaOH) to the wastewater. With direct chlorine gas addition, sodium hydroxide reacts with the chlorine to form sodium hypochlorite. Selection between the two methods is based primarily on economics and safety. The chemical costs for chlorine gas treatment are less than half those of direct hypochlorite addition (see Section 6.3.2), but handling is perceived to be more dangerous and equipment costs are higher (ref. 39). Relative to the safety issue, one expert source argues that modern gas handling equipment is highly reliable and designed to be relatively fail safe (ref. 38). The results of the Users Survey indicate that 95% of the respondents with alkaline chlorination processes use sodium hypochlorite and 5% use chlorine gas plus sodium hydroxide. The shops that use chlorine gas tend to be the same shops that select sulfur dioxide for chromium reduction (e.g., PS 025 and PS 093). Generally, these shops have substantial cyanide wastestream flow rates.
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).
There are several alternatives to the alkaline chlorination process. These are discussed in the following paragraphs.
Ozone oxidation of cyanide has been practiced as a substitute technology for alkaline chlorination. It is effective in destroying cyanide to the levels required by EPA. The advantage of using ozone lies in reduced operating costs. Ozone is generated on-site (typically by the silent electrical discharge method) and is less expensive than chlorine or sodium hypochlorite. The equipment cost is significantly higher, however, owing to the expense of an ozone generator. Another disadvantage of the process is that it does not oxidize cyanide past the cyanate stage, unless excess ozone is used. Ozone oxidation requires 1.8 to 2.0 lbs of ozone per pound of CN to reach the cyanate stage and 4.6 to 5.0 lbs to reach complete oxidation. An advantage of ozone oxidation is the absence of chlorine that can combine with organics present in the wastewater to produce toxic compounds (ref. 39). Another advantage is the ability of ozone to destroy zinc, copper and nickel cyanide complexes (ref. 243). The ozonation process has also been combined with UV radiation for the treatment of halogenated organics (ref. 348, 386). None of the respondents to the Users Survey employ ozone oxidation of cyanides.
Other alternatives to alkaline chlorination include: alternative chemistries; electrochemical oxidation; thermal oxidation; and precipitation.
Alternative chemistries include hydrogen peroxide (PS 020 and PS 273) (ref. 386) and calcium hypochlorite (PS 174).
Electrochemical oxidation is sometimes used to destroy concentrated cyanide wastes (e.g., >50,000 mg/l CN). A description of the required equipment, including a small commercial unit, is presented in AESF literature (PS 243).
Thermal treatment of cyanide wastes is performed by two of the respondents to the Users Survey (PS 142 and PS 245). Their equipment is provided on a rental basis by Cyanide Destruct Systems (Canada). The equipment can be purchased for $40,000 or rented for $600 per week. PS 142 also indicated that they required a facility modification/ancillary equipment purchase costing of $700. The equipment is intended for concentrated cyanide wastes (at both shops dilute CN wastes are treated by alkaline chlorination). The rental unit is essentially a heated pressure chamber (35 gal, 450 to 470°F, 600 psi) with an agitator. PS 142 used the unit to treat cyanide wastes containing 100,000 mg/l CN to approximately 25 mg/l. Disposal costs for these wastes were previously $800 to $1,000 per 55-gal drum. In operation, CN wastes are transferred into the vessel and treated on a batch basis for 10 hours and then discharged to conventional treatment. Ammonia gas generated by the unit is vented to the atmosphere. PS 142 reported operating costs, excluding labor, of $4,000 per year and a labor requirement of 750 man-hours. They also indicated that they experienced temperature and pressure control problems due to a seal leak (N2 and oil seal) at the point of agitator entry into the vessel. PS 142 indicated that the unit had a downtime of 50% and that they have discontinued its use.
Chemical precipitation of cyanide can be achieved using ferrous sulfate. This process precipitates the cyanide as a ferrocyanide, which can be removed in a subsequent sedimentation process (ref. 386). Results of EPA sampling of such a process showed that an average influent concentration of 2.7 mg/l CN was treated to 0.023 mg/l CN.