Metal Recovery from Leached Plating Sludge

he 23 US. Army installations engaged in plating employ a variety of techniques to treat wastewater. While some sites have implemented recovery and T reuse, a majority still find it advantageous to precipitate the metals and discharge the wastewater. Sludge is collected in clarifiers, treated by gravity thickening, then dewatered in pressure or vacuum filters, lagoons or drying beds. The dewatered sludge is incinerated, used in landfills or hauled away by contractors. Because in most cases the sludge IS classified as hazardous, options for landfilling are limited. The sludge contains metals with potential economic and in some cases strategic value. A study was therefore conducted to review the state of the art in metal recovery from plating sludge' and an extensive database was assembled. Earlier studies of waste treatment in military finishing facilities established a typical composition for the sludge and a representative synthetic material containing metal hydroxides.''' A detoxification and recovery process should be sufficiently flexible to treat a mixture of elements at various stages of aging. The secondary sludge remaining from a metal recovery operation must also pass EPA's extraction procedure (EP leach test)' for hazardous constituents such as chromium and m. tes at Army depots originate primarily from the treatment of rinsewaters, a synthetic sludge was produced by liming dilute solutions. Rinse baths generally contain sulfates, so liming causes a significant amount of gypsum to coprecipitate with the metal hydroxides. Iron is frequently found in untreated sludge.' After the recovery of valuable constituents, the secondary sludge consists primarily of gypsum and iron hydroxide compounds. The treatment of dissolved metal