Metallic Replacement

Metallic replacement has been an important means of recovering silver from fixers, bleach-fixes, and washwaters for many years. It can be used as a primary method of recovery or a secondary method following the electrolytic treatment of fixers or bleach-fixes.

Metallic replacement involves passing a silver-bearing solution through a vessel containing a more active metal in elemental form, usually iron. The reaction of dissolved silver with metallic iron is

Fe0+2Ag(S2O3)23-^Fe2++2Ag0+4S2O32-

If conditions were ideal and no other reactions were involved, 1 g of steel wool would recover 3.86 g of silver. In actual practice, usually less than 1 g of silver is recovered per gram of steel wool. Most fixers, such as those used in x-ray and graphic arts processing, are acidic; therefore, a competing reaction that consumes much of the iron is the acidic dissolution of the steel wool. Spontaneous oxidation of iron with air (i.e., rusting) also occurs, particularly upon longterm standing under moist conditions. Insoluble iron hydroxide compounds are also formed when the solutions are at higher pH values, above 7.

A common size of steel wool cartridge (also originally known as a "chemical recovery cartridge" or "CRC," made by Kodak), contains about 8 lb of steel wool. However, many other manufacturers in the United States and overseas now produce such devices in various shapes, configurations, and sizes (categorically known as "metallic recovery cartridges" or "MRCs"), with various types and grades of steel wire filling.

Although a number of active metals (e.g., aluminum, manganese, zinc) can replace silver, steel wool has been shown to be the best choice from several technical, economic, safety, and environmental points of view. Certain metals higher than iron in the electromotive series, particularly aluminum, can react so violently with an acidic fixer that copious amounts of sulfurous gases, which are odorous and noxious, would be given off. Iron reacts more slowly and thus does not form such gases.

There are a number of advantages associated with metallic replacement, including low initial cost, simple nonelectrical installation, small size and low weight, little maintenance, and high efficiency of silver recovery if properly monitored. One disadvantage is the high shipping and refining costs after exhaustion, compared with those for silver flake; this may offset the lower initial cost of MRCs. Another disadvantage is the high iron concentration in the cartridge effluent, which has been measured to be as high as 3000 mg/L. This precludes the reuse of fixer after silver recovery with MRCs, produces a colored effluent, and could cause problems in meeting local sewer codes for color or iron content.

High silver-recovery efficiency can best be achieved if two CRCs are used in series and if they are well maintained. When the first cartridge shows exhaustion, it is removed. The cartridge in the second position is moved to the first position and a fresh cartridge placed in the second position.

Studies were conducted by Cooley [90] to determine how to optimize the use of CRCs. These showed that the optimum pH in actual laboratory practice is between 4.5 and 5.5. At lower pH values, the acidity will consume too much of the steel wool, whereas at higher values the reaction becomes slower as considerable amounts of iron hydroxide are formed that may obscure the surface of the steel wool. Silver deposited on the steel wool may also restrict the mass transfer of silver to the iron surface at higher pHs, while below pH 5.5 the iron may be continuously acid-etched to provide a fresh reaction surface.

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