Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 531 of 707

below 9. The process only oxidizes simple cyanides, such as NaCN, KCN, Zn(CN)2 , CdCN, CuCN, etc. Complexed cyanides, commonly found in met- al finishing wastewater as iron complexes, are not destroyed in alkaline chlorination processes. In fact, complexed cyanides are not destroyed effi- ciently by any common cyanide oxidation process, including ozone. The use of high-pressure/high-temperature thermal processes will, however, destroy complexes. Also, lengthy exposure to sunlight will convert complexed cyanides to simple cyanides, to a small extent. As federal and local regulations are generally written for total cyanide moni- toring and limiting, complex cyanides are often the species causing violations. Complexed cyanides are most commonly formed by poor housekeeping, con- trol, and rinsing. Drag-out or drippage of CN from baths or bath rinses into acids and chromates is very common. Steel electrode use in plating baths causes a sig- nificant amount of complexed cyanide input to the bath from constant decom- position. Clean steel parts allowed to fall and accumulate in CN baths are anoth- er major source of complexed CN formation. Although complexed cyanide formation cannot be totally eliminated, reduced formation through housekeeping and improved rinsing can reduce the con- centration to nonproblem levels. Complexed cyanides are generated in both soluble and insoluble forms. The insoluble form is removed via mass settling in the clarifier. Conversion of solu- ble complexes to insoluble complexes can be achieved to some extent by the addi- tion of MBS to the neutralization tank. The efficiency is improved in the presence of copper ion. Permanganate addition also has been reported to accomplish improved precipitation of complexed cyanides. The second-stage CN oxidation process is carried out at a pH of 8.0-8.5. An amount of Cl2 comparable to that required in first-stage oxidation (3.5 lb Cl2:1 lb CN) is necessary to complete the conversion of OCN to CO2 and N2. Most sewer use ordinances do not require cyanate oxidation or limit cyanate in the discharge. Consequently, many treatment systems only employ first-stage processes. A common problem associated with first-stage-only systems is the propensity to gassing in the neutralization tank, with resultant clarifier floating problems. This is caused by an uncontrollable cyanate breakdown, particularly when excess residual Cl2 is present in the first-stage dischare. Although reaction times for most simple cyanides and cyanates are 10-15 minutes, it is advisable to size reaction tanks at 1 hour and longer if afford- able/practical. Certain simple cyanides, including cadmium and copper, only start breaking down after the sodium, potassium, and zinc cyanides are destroyed, thus requiring longer contact periods. Furthermore, the longer the reaction, the more efficient the gas venting becomes, reducing the incidence of clarifier floating. Because precise control of pH and Cl2 is important, pH and ORP controllers are recommended in all continuous control reaction tanks. Summary of Cyanide Process Precautions 1. First-stage oxidation must be controlled at pH 10.5 or higher. (The higher the pH, the faster the reaction.) 2. Control the formation of complexed cyanides, as treatment processes do not destroy them. Add MBS to the neutralization tank if soluble complexes cause effluent violations. 530

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