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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 />!fide from the waste water may require air stripping which is <br />-h 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 />