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Chromium Reduction A covered mixing tank with mixer is required for the reaction. The auxiliary equipment required consists of a mixing tank for Chromium exists in the hexavalent form in etching, plating and the sodium hypochlorite and the sodium hydroxide. chromating baths and rinse waters. Hexavalent chromium is much more reactive and toxic than the other forms of chromium. The individual shops will have a pretreatment system and the Hexavalent chromium must be reduced to the trivalent form in Facility will require an oxidation reactor with auxiliary order to be less toxic upon discharge of small amounts to the equipment. environment and to avoid destruction of the ion exchange resins. Metal Precipitation and Sludge Thickening To accomplish this reduction, an acidic solution of sulfur Soluble metal salts occur in plating bath dumps, etching bath dioxide or sodium bisulfate is reacted with the chrome dumps, metal finishing rinse waters and the ion exchange solution. Sulfur dioxide is often used to cure resins but it regenerant solutions. In order to be removed from the aqueous is more difficult to use than sodium bisulfite. Although solution, the metal salts must be converted to insoluble sodium bisulfate is slightly more expensive, it was chosen as hydroxides or sulfides. Sulfide precipitation removes more the more preferable reagent for chromium reduction. metal from the wastewater than hydroxide precipitation. This is why a two step process is used to remove the bulk of a metal The reduction reaction may take place in a simple mixing tank from solution as a hydroxide and then polishing the solution with mixer. An auxiliary mixing tank is needed for the sodium with a sulfide precipitation step. Hydroxide precipitation, as bisulfite and acid. a process, has a greater product flexibility and simpler discharge pretreatment requirements than sulfide precipitation. One chromium reduction system will be installed in each shop to For these reasons hydroxide was chosen as the precipitator for protect the ion exchange reins. There will also be one in the the Facility. Metal hydroxide sludges can be redissolved in Facility. acids and converted to more soluble forms for reuse. This cannot be done with metal sulfide sludges because of the danger Cyanide Oxidation of toxic hydrogen sulfide gas being generated. Also, the required treatment of wastewaters from hydroxide precipitation Cyanide destruction or removal is also required for the is a simple neutralization with an acid. The removal of excess wastewater to meet the MWCC pretreatment regulations. In order sulfide from the waste water may require air stripping which is to remove the cyanide for reuse it must be complexed and much more complicated and expensive. precipitated from the solution. This process, however, is very complicated and the cyanide may not be in a saleable form when The choice of base to add to the metal solution is an important recovered. Destroying the cyanide through oxidation to carbon factor affecting both the technical performance and the cost of dioxide and nitrogen is much more simple. the system. Calcium oxide, calcium hydroxide and sodium 3 hydroxide are all commonly used. Cyanide is used in plating solutions and ends up in the wastewater from electroplating and printed circuit board plants The precipitation of these metal hydroxides is not and is trapped by the ion exchange resins. The cyanide will instantaneous so proper equipment is required to ensure an subsequently occur in the ion exchange regenerant solution at adequate retention time for the settling to occur. The the Facility. The required oxidation reaction by which cyanide thickening of the sludge formed from the metal hydroxide is destroyed is performed by reacting sodium hypochlorite, j precipitation must also be facilitated by the equipment. Two chlorine or chlorine dioxide with the regenerant solution. standard circular clarifiers with rotating scrapers will be Chlorine is toxic, corrosive and difficult to control. It is utilized by the Facility. the least expensive of the three; however, the Facility and the individual shops are probably too small to take advantage of A flocculant will most likely be added to the clarifier to aid this lower cost. Chlorine dioxide is also dangerous to use in a more complete precipitation of the metal hydroxides. The because of its tendency to explode at 10°C, its boiling point. most common flocculants are ferric chloride, alum and a variety Sodium hypochlorite is only available in an aqueous solution, of water soluble polymers. 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-7 A-6 1 e