Photographic silver recovery

Reaction (13) is particularly problematic in photographic silver recovery as a rapid chemical reaction results in the formation of silver sulphide ... 

In the majority of cases, metal is removed from the cathode at intervals, usually by manual peeling or scrapping the deposit. A rotating-cyfinder cell for photographic silver recovery is shown in Fig, 7,6, Such cells are available with rotating cathodes (small-scale) or rotating anodes (on a larger scale). 

Another practical approach is to promote electrolyte turbulence via the use of an expanded-mesh electrode in a solution containing fluidized inert particles (typically 1 mm glass ballotini). This is the principle of the Chemelec cell (Fig. 7.4) which has been widely applied to metal finishing, photographic silver-recovery and precious metal reclamation processes. 

Fig. 7.6 A 300 A rotating-anode reactor for photographic silver recovery. A view of the top of the cell. The 6.5 cathode is surrounded by 72 carbon rod anodes mounted on rotating rings inside a glass-reinforced plastic tank. (Courtesy Wilson Process Systems Ltd.) The anode mounting rings which lie inside and outside the cathode rotate at 30 rev min" Cell liquor capacity is 750 dm the tank has an approximate diameter of 1.4 m and is 1.2 m high. The electrolyte is pumped around the cell.

A simple and economical method for recovering silver residues by dissolution in used photographic fixer (thiosulfate) solution, then precipitation by addition of zinc powder, is detailed [1]. After the acid digestion phase of silver recovery operations, addition of ammonia followed immediately by addition of ascorbic acid as reducant gives a near-quantitative recovery of silver metal, and avoids the possibility of formation of silver nitride [2],... 

The most popular method of silver recovery is electrolytic deposition. In an electrolytic recovery unit, a low voltage direct current is created between a carbon anode and stainless steel cathode. Metallic silver plates onto the cathode. Once the silver is removed, the fixing bath may be able to be reused in the photographic development process by mixing the desilvered solution with fresh solution. Recovered silver is worth about 80% of its commodity price. Used silver films also constitute a significant quantity of waste. The film can be sold for silver recovery to many small recyclers. 

Fig. 5. A flow-through reactor designed for silver recovery from photographic process waste streams CE, counter electrode RE, reference electrode. (Adapted from [24]).

Application of Polymer-Bound Reducing Agents for Silver Recovery from Photographic Fixer. Several 10-mL aliquots of a spent photographic fixer solution containing 0.55% silver were diluted to 100 mL and adjusted to 5 different pH levels 4.5, 5.5, 6.5, 7.5, and 8.5. One gram of borohydride-form A-26 anion-exchange resin was added to each sample. The samples were stirred and allowed to stand at 20 °C for 30 min. 

Several processes have been developed [41-43] to overcome the technological drawbacks of plastics incineration cited above. These include continuous rotary-kiln processes a process for glass-reinforced PET a combined system for wood fiber and PET to provide steam to power equipment and a fluidized system for pyrolysis, in combination with silver recovery from photographic film. Incineration of photographic film raises the additional problem of the formation of toxic halogenated compounds due to the presence of silver halides. 

Recycled PET from used x-ray film must be carefully analyzed for contamination if it is to be used for the production of new photographic base film. This is because caustic residues from the silver recovery process can cause fogging of the photographic emulsion. ... 

Since 1951, silver consumption has exceeded its extraction from ore. Secondary silver production involves the recovery of silver from new and old scrap, resulting from silver-containing wastes generated by industry and the consumer. Recycled silver accounted for 40% of U.S. refinery production in 1971 and had increased to 67% by 1974 (Smith and Carson 1977). It was estimated to be 61% and 56% in 1988 and 1989, respectively (The Silver Institute 1990). The estimated world-wide recovery of silver from the photographic industry is about 67% of the total used (The Silver Institute 1988). It has been estimated that 80%, 68%, and 75% of today s annual consumption by the electrical, industrial-alloy, and art industries, respectively, is recycled silver, but these estimates may be high. 

Recovery of silver from spent photographic process streams using a graphite felt cathode in a flow-through reactor (Fig. 5) was studied by Tricoli et al. [24]. Initially, the current increased with time because of the higher exchange current density of silver deposition on silver than on graphite. On the other hand, currents obtained from... 

Exchange resins are also employed for the concentration of ions present in very dilute solutions instances are the recovery of silver from photographic residues, chromate from the waste liquor of chromium plating and magnesium from sea water. They have also been used for the separation of rare earths (p. 426), and of uranium, plutonium and radio-active fission products (p. 437), and for plutonium and uranium-233 purification. A striking application was the historic separation of single atoms of mendelevium on a sulphonated polystyrene resin and their elution therefrom, at 87 , with a-hydroxyisobutyrate (Seaborg, 1955). 

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