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A covered mixing tank with mixer is required for the reaction. <br /> The auxiliary equipment required consists of a mixing tank for <br /> the sodium hypochlorite and the sodium hydroxide. <br /> The individual shops will have a pretreatment system and the <br /> Facility will require an oxidation reactor with auxiliary <br /> equipment. <br /> Metal Precipitation and Sludge Thickening <br /> Soluble metal salts occur in plating bath dumps, etching bath <br /> dumps, metal finishing rinse waters and the ion exchange <br /> regenerant solutions. In order to be removed from the aqueous <br /> solution, the metal salts must be converted to insoluble <br /> hydroxides or sulfides. Sulfide precipitation removes more <br /> metal from the wastewater than hydroxide precipitation. This <br /> is why a two step process is used to remove the bulk of a metal <br /> from solution as a hydroxide and then polishing the solution <br /> with a sulfide precipitation step. Hydroxide precipitation, as <br /> a process, has a greater product flexibility and simpler <br /> discharge pretreatment requirements than sulfide precipitation. <br /> For these reasons hydroxide was chosen as the precipitator for <br /> the Facility. Metal hydroxide sludges can be redissolved in <br /> acids and converted to more soluble forms for reuse. This <br /> cannot be done with metal sulfide sludges because of the danger <br /> of toxic hydrogen sulfide gas being generated. Also, the <br /> • required treatment of wastewaters from hydroxide precipitation <br /> is a simple neutralization with an acid. The removal of excess <br /> sulfide from the waste water may require air stripping which is <br /> much more complicated and expensive. <br /> The choice of base to add to the metal solution is an important <br /> factor affecting both the technical performance and the cost of <br /> the system. Calcium oxide, calcium hydroxide and sodium <br /> hydroxide are all commonly used. <br /> The precipitation of these metal hydroxides is not <br /> instantaneous so proper equipment is required to ensure an <br /> adequate retention time for the settling to occur. The <br /> thickening of the sludge formed from the metal hydroxide <br /> precipitation must also be facilitated by the equipment. Two <br /> standard circular clarifiers with rotating scrapers will be <br /> utilized by the Facility. <br /> A flocculant will most likely be added to the clarifier to aid <br /> in a more complete precipitation of the metal hydroxides. The <br /> most common flocculants are ferric chloride, alum and a variety <br /> of water soluble polymers. <br /> A-7 <br />