Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 532 of 707

3. Allow 1 hour or more reaction time to ensure completion of the reactions, and for problem gas venting. 4. Underfeed of chemical allows CN pass through; overfeeds cause increased gassing and reoxidation of trichrome. Coagulation/Neutralization Process Considerations Effluents from hexavalent chromium reduction and cyanide oxidation stages com- bine with other alkaline and acid wastewater streams in a neutralization tank. The express purpose of the neutralization tank is to create a suitable environment and retention time for soluble pollutants to react and form insoluble precipitates for eventual physical separation. The principal precipitation process employed in con- ventional wastewater treatment systems is that of hydroxide precipitation. Heavy metals, the prime targets of neutralization-precipitation, have varying solubili- ties depending on pH. In common mixed-metal wastewater streams, control of the neutralization tank at pH 9.2-9.5 is generally suitable to lower metal solu- bilities, as hydroxides, to concentration ranges where compliance is achievable. In many cases, it is necessary to add chemical coagulants to the wastewater in order to achieve minimum solubilities and superior flocculation/solids separa- tion in the clarifier. A proper coagulant will effectively tie up anionic surfactants, wetters, and species such as phosphates, which interfere with polymer floccula- tion; and also add bulk density for improved solids separation. Where coagulants are required for good process performance, it is recom- mended that two-stage neutralization reaction tanks be employed, as coagulants perform better when reacted with the wastewater at pH values in the 5.5-6.5 range. Common chemical coagulants include calcium chloride, ferrous salts, ferric salts, and alum. For improved coagulation, certain specialty coagulants are available from chemical suppliers. These chemicals usually contain one of the above base salts, which are sometimes blended with polymers, generally of a cationic nature. Although these specialty products are expensive, with costs ranging from $400 to $1,000 per drum, their use is often necessary to achieve compliance. Neutralization is generally achieved using caustic soda (NaOH) and sometimes potassium hydroxide (KOH). Hydrated lime and magnesium hydroxide also have wide utilization. Although these neutralization chemicals present certain han- dling and feeding problems associated with their solids content, lower metals sol- ubilities are achieved at maintenance of lower neutralization tank pH (8.0-8.5). The introduction of strong chemical complexers used in production process- es commonly impedes the pollutant precipitation process. Common complex- ers/chelators include ethylene diamine tetra acetic acid (EDTA), nitrilotriacetic acid (NTA), quadrol, glucconates, glutamates, ammonia, and various amies. Complexing agents are commonly used in electroless baths, electroplating bath brighteners, alkaline cleaners, parts strippers, and numerous other applications. Eliminating their use, where practicable, is the simplest means of mitigating their adverse wastewater treatment effects. Where critical to the process, special means and practices must be employed, which vary with the type and strength of the complexer, as well as the metal(s) being complexed. Often off-line pretreatment is necessary, as in the case of high volume electroless bath use. In other cases, the use of specialty chemical precipitants, metered into the complexed waste stream or into the neutralization tank, is suitable and effective. Specialty chemical precipitants include dithiocarbamates, dithiocar- 531

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