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Chromium Reduction Chromium exists in the hexavalent form in etching, ng and chromating baths and rinse waters. Hexavalent chromiumiis much more reactive and toxic than the other forms of chromium. Hexavalent chromium must be reduced to the trivalent form in order to be less toxic upon discharge of small amounts to the environment and to avoid destruction of the ion exchange resins. To accomplish this reduction, an acidic solution of sulfur dioxide or sodium bisulfite is reacted with the chrome solution. Sulfur dioxide is often used to cure resins but it is more difficult to use than sodium bisulfite. Although sodium bisulfite is slightly more expensive, it was chosen as the more preferable reagent for chromium reduction. The reduction reaction may take palace in a simple mixing tank with mixer. An auxiliary mixing tank is needed for the sodium bisulfite and acid. One chromium reduction system wily. be installed in each shop to protect the ion exchange reins. There will also be one in the Facility. Cyanide Oxidation Cyanide destruction or removal is also required for the wastewater to meet the MWCC pretreatment regulations. In order to remove the cyanide for reuse it must be complexed and precipitated from the solution. This: process, however, is very complicated and the cyanide may not be in a saleable form when recovered. Destroying the cyanide through oxidation to carbon dioxide and nitrogen is much more simple. Cyanide is used in plating solutions and ends up in the wastewater from electroplating and printed circuit board plants and is trapped by the ion exchange resins. The cyanide will aubsequently occur in the ion exchange regenerant solution at the Facility. The required oxidation reaction by which cyanide is destroyed is performed by reacting sodium hypochlorite, chlorine or chlorine dioxide with the regenerant solution. Chlorine is toxic, corrosive and difficult to control. It is the least expensive of the three; however, the Facility and the individual shops are probably too small to take, advantage of this lower cost. Chlorine dioxide is also dangerous to use because of its tendency to explode at 10°C, its boiling point. Sodium hypochlorite is only available in an aqueous solution, however, it is the easiest of the three reagents to control,, For this reason, sodium hypochlorite was chosen as the oxidizing reagent for cyanide destruction. Sodium hydroxide is also required as an additional reagent for the oxidation reaction. A-6