Metal Finishing Guide Book


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to its trivalent (Cr3+) form is necessary to ensure removal by precipitation. Commonly, trivalent chromium replacement processes are being employed for safety considerations and the elimination of the reduction wastewater step. Exercise care in selecting trichromium replacements that may contain ammonia and other chemicals, which can cause complexing of other metals in waste treatment. The reduction of hexavalent chromium is achieved by reaction with sulfur dioxide gas (SO2), or more commonly sodium metabisulfite (MBS). The speed of the reaction is pH dependent. At pH 2.5-3, the reaction is virtually instantaneous. Above pH 4, the reaction slows to a point where it becomes impractical for use in continuous flow systems. The use of pH and oxidation-reduction potential (ORP) controllers is common. Without automatic pH controllers, care must be exercised to ensure complete reaction, particularly in batch reactors where the pH is manually adjusted to pH 2.5 prior to MBS addition. MBS addition raises the pH of the solution, often to ranges where reduction times are lengthy. As batch processes are usually controlled visually by color change, a significant MBS overfeed often results. Although MBS and SO2 are the most common chemical reducers used in hexavalent chromium reduction, any strong reducing agent will suffice. Ferrous iron in many forms, including ferrous sulfate, ferrous chloride, ferrous hydrosulfide, or electrochemical ferrous production from iron electrodes, is used. The primary benefit of ferrous reduction is that Fe2+ will reduce hexavalent chromium at near neutral pH values. For low concentration applications (moderate chromating use processes), ferrous addition can eliminate the complete chromium reduction stage. The ferric ion formed in the process becomes an excellent coagulant in the precipitation stage. The only drawback to ferrous reduction is the additional sludge generated by its use, as three parts Fe2+ is required to reduce one part Cr6+. Chromium Reduction Process Precautions 1. SO2 and MBS form noxious acidic vapors. Avoid excess formation and inhalation of the vapors. 2. pH control is very important. Allowing pH to drift below 2 increases SO2 gassing vapors. Allowing pH drift upward to 4 increases reaction times to impractical levels. 3. Underfeed of SO2/MBS causes chrome carryover. Overfeed of MBS/SO2 causes increased metal solubilities in neutralization, and reverses the particle charge and, consequently, results in poor flocculation. Cyanide Oxidation Treatment of cyanide (CN) in metal finishing wastewaters is most commonly performed by oxidation in an alkaline chlorination process using sodium hypochlorite (NaOCl) or chlorine gas (Cl2). Because of the toxic danger of Cl2 gas, NaOCl processes are considerably more common. The alkaline chlorination process either involves only first-stage CN oxidation, whereby simple cyanides are converted to cyanates (OCN), or the addition of a second-stage reactor to convert cyanates to carbon dioxide (CO2) and nitrogen (N2). First-stage CN oxidation is carried out at a pH of 10.5 or higher. The reaction slows greatly at pH values below 10 and virtually ceases at pH values below 9. The process only oxidizes simple cyanides, such as NaCN, KCN, 630

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