Comparison of Discharge Silver Concentrations from Electrolytic Plating and Metallic Replacement Silver Recovery Units

Abstract

Silver-based photographic X-ray film is made of solid crystals of silver chloride or silver bromide suspended in a gelatin and then coated on a film. During the X-ray developing process, the image is processed and the nonimage areas containing solid silver chloride or silver bromide crystals are removed in a solution called the fixer. There may be local environmental regulations that regulate the amount of silver discharged from a facility. To meet these regulations, many facilities have added silver recovery units to their processes. Two different types of recovery processes are in use in a large hospital and three clinics under study. All of the units were claimed by their respective manufacturers to be able to recover silver down to concentrations of 5 mg/L. This concentration would ensure that the building that houses each unit would meet the local county limit of 0.5 mg/L silver for total building silver discharge. The hypothesis for this research is that one system, newer and more expensive, consisting of so-called electrolytic plating units (EPUs) (which are followed by so-called metallic replacement units [MRUs] as a backup), will have better silver recovery than MRUs alone.A total of six units were sampled, three EPUs (in combination with MRUs) and three MRUs. The units were sampled once or twice a day for 10 days for a total of 17 samples from each. The samples then were analyzed by inductively coupled plasma spectroscopy, and an analysis of variance was performed on the results. The range for the electrolytic plating unit/metallic replacement unit combinations was 0.20 -99.9 mg/L (mean of 35.15 mg/L; median of 33.8 mg/L). The range for the MRUs alone was 7.2-1112 mg/L (mean of 565.5 mg/L; median of 720 mg/ L). Many individual results exceeded 5 mg/L, such that extensive dilution would be required to ensure the building effluent did not exceed 0.5 mg/L. It is suggested that the metallic replacement units be changed to EPUs (with metallic replacement backup units) because they had better silver recovery. Also, the EPU combinations need to be sampled regularly to ensure that their silver concentrations are at acceptable levels. INTRODUCTIONSilver is a valuable metal that is used in many applications, including batteries, dental material, jewelry, electrical components, and photographic film. Photographic materials accounted for 45% of the U.S. consumption of silver in 1986. 1 Silver-based photographic materials such as X-ray film are made of solid crystals of silver chloride or silver bromide that are suspended in gelatin and coated on a film for support. 2 For X-ray film, after the image is processed, the remaining image on the film is metallic silver, while the nonimage areas containing solid silver chloride or silver bromide crystals are removed as a soluble silver-thiosulfate complex in a solution called the fixer. It is assumed that approximately 60 -66% of the silver on the film is washed away into the photographic fixer, which can then be processed to recover the silver.For many businesse...

Optimizing Silver Extraction Potential from Waste X-Ray Films Using Acid and Alkaline Leaching Agents

“…Moreover, at this acid concentration, at least 66% of the silver was already recovered (9,830 mg kg -1 out of the possible15,000 mg kg -1 reported to be present in new x-ray films). It should be noted that the concentration of silver in waste x-ray should be lower (not up to 15,000 mg kg -1 ) than that in the new xray estimated to be 1.5 -2 % (w/w), as some silver would have been stripped or converted during the process of developing the image (Harper and Siege, 2003). It is also thought that at this concentration, most of the available silver metal on the polyester film would have been stripped.…”