Silver Removal from Polluted Effluents Using a Rotating Cylinder Electrode Reactor (RCE) Potential Application for Valuable Metal Removal from Plating Industry Waste Waters

Plating industry generates great amounts of aqueous effluents (rinses and spent baths), which contain in some cases valuable heavy metals in solution originating from the rinses of coated items (i.e. silver). Because its toxicity and complexity, silver effluents generally are confined and in a few cases is removed. The removal of silver, Ag (I), is traditionally carried out by cementation method where metal is precipitated by chemical reduction with zinc powder. Despite this treatment, effluents obtained with high amounts of zinc (II) are prohibited by international environmental legislation.

In this work the application of a rotating cylinder electrode (RCE) in the removal of Ag(I) content from an effluent that contains amounts of Ag (I) similar to wastes generated by plating industry, on the laboratory scale are showed; particularly, it deals with rinse water from the electrolytic silver  process.  The solutions employed was laboratory-made solutions simulated silver rinse (1500 ppm of Ag(I)) and spent bath (23000 ppm). This process was designed to convert silver ions in solution to metallic silver and influence of organic additives was not taken into account. The generation of metallic silver in the RCE was achieved at Reynolds numbers (Re) ranged between 22682-83183 and limiting current densities (J_L) in the range of 7-25 mA cm^-2. The removal of Ag (I) is strongly dependent of Ag (I) initial concentration because the simulated rinse solution (1500 ppm Ag(I)) reached  95 % of silver removal in 20 minutes of electrolysis for Re = 52925 and J = 15 mA cm^-2; meanwhile for spent bath solution (23000 ppm Ag(I)) at similar operational conditions 95% is reached in 180 min. This behavior is associated with elemental thermodynamic predictions described by faraday law and mainly with different mass transfer mechanisms at such studied concentrations during electrolysis. This second statement was evidenced in previous cyclic voltammometric studies.Plating industry generates great amounts of aqueous effluents (rinses and spent baths), which contain in some cases valuable heavy metals in solution originating from the rinses of coated items (i.e. silver). Because its toxicity and complexity, silver effluents generally are confined and in a few cases is removed. The removal of silver, Ag (I), is traditionally carried out by cementation method where metal is precipitated by chemical reduction with zinc powder. Despite this treatment, effluents obtained with high amounts of zinc (II) are prohibited by international environmental legislation.

In this work the application of a rotating cylinder electrode (RCE) in the removal of Ag(I) content from an effluent that contains amounts of Ag (I) similar to wastes generated by plating industry, on the laboratory scale are showed; particularly, it deals with rinse water from the electrolytic silver  process.  The solutions employed was laboratory-made solutions simulated silver rinse (1500 ppm of Ag(I)) and spent bath (23000 ppm). This process was designed to convert silver ions in solution to metallic silver and influence of organic additives was not taken in account. The generation of metallic silver in the RCE was achieved at Reynolds numbers within 22682 < Re < 83183 interval and limiting current densities (J_L) in the range of 7 to 25 mA cm^-2. The removal of Ag (I) it strongly dependent of Ag (I) initial concentration because the simulated rinse solution (1500 ppm Ag(I)) reached  95 % of silver removal in 20 minutes of electrolysis for Re = 52925 and J = 15 mA cm^-2; meanwhile for spent bath solution (23000 ppm Ag(I)) at similar operational conditions 95% is reached in 180 min. This behavior is associated with elemental thermodynamic predictions described by faraday law and mainly with different mass transfer mechanisms at studied concentrations during electrolysis. Second statement was evidenced in previous voltammometric studies.