Silver recovery methods

These tests were conducted to provide sufficient data to select a silver recovery method (AEA, 200In). They had three objectives ... 

Figure 6 illustrates a combined system involving the use of both the electrolytic cell and the in situ ion-exchange unit. The combined system (Fig. 6) produces an excellent effluent with lower residual silver in comparison with the chemical recovery cartridge method (Fig. 2), electrolytic silver recovery method (Fig. 3), the conventional ion-exchange method (Fig. 4), and... 

Selection of a suitable method for silver removal depends on many factors what processes the company uses, what volume of wastes the company produces, what kind of training and technical knowledge the company s personnel has, whether the company wants to reuse the company s fixer or bleach-fix, how much the company wants to spend for recovery equipment, and what the environmental concerns are, such as how strict the effluent discharge limits are. Just considering these factors makes choosing a silver recovery method very much an individual decision for each company. 

Kodak Company. Choices Choosing the Right Silver Recovery Method for Your Need-Environment, Kodak Company Rochester, NY, 1987. 

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],... 

However, the company s wastewater volume was actually very low, and chemical recovery cartridges, hydroxide precipitation tanks, and sulfide precipitation tanks became reasonable choices for silver recovery. Chemical recovery cartridges and the two types of precipitation tanks were all very simple to install. The costs for purchasing, installing, operating, and monitoring this equipment are very low compared with other methods. 

In comparison with the silver recovery/removal efficiencies of the chemical recovery cartridge (CRC) method, the hydroxide precipitation method, and the sulfide precipitation method shown in Table 6, the two precipitation methods appeared to be a better choice than the CRC method. 

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. 

Another method used for silver recovery is ion exchange. Small ion exchange columns can be employed for approximately the same cost as electrolytic recovery (Hart 1987). Effluent from some ion exchange systems can have silver concentrations as low as 0.1 ppm. High-performance units can process as much as 500 gallons per hour. 

This method of silver recovery can be applied to any silver precipitates from quantitative analysis, such as silver chloride, bromide, or thiocyanate. First wash the residues well with water by decantation and on the Buchner funnel, then spread out on paper to dry. A little nitrobenzene absorbed on the precipitate from the Volhard titration will not do any harm the greater part of the nitrobenzene will be removed by washing, in any case. 

Daskalakis and co-workers recently evaluated several procedures for digesting the tissues of oysters and mussels prior to analyzing the samples for silver. One of the methods used to evaluate the procedure is a spike recovery in which a known amount of silver is added... 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

The classical method for the determination of iodide in seawater was described by Sugawara [5]. Various workers [6,7] have improved the original method. Matthews and Riley [6] modified the method by concentrating iodide by means of coprecipitation with chloride using silver nitrate (0.23 g per 500 ml seawater). Treatment of the precipitate with aqueous bromine and ultrasonic agitation promote recovery of iodide as iodate which [15] when reacted with excess of iodide ions under acid conditions, yields Ij, which are determined either spectrophotometrically or by photometric titration with sodium thiosulfate. Photometric titration gave a recovery of 99.0 0.4% and a coefficient of variation of 0.4% compared with 98.5 0.6% and 0.8%, respectively, for the spectrophotometric procedure. 

In this method, a metal (usually iron) in a chemical recovery cartridge (CRC) reacts with the silver thiosulfate in the spent fixer and goes into solution. The less active metal (silver) settles out as a solid. To bring the silver into contact with the iron, the spent fixer is passed through the CRC container, which is filled with steel wool. The steel wool provides the source of iron to replace the silver. The main advantages of this CRC method are the very low initial cost (cartridges cost about US 60) and the simplicity of installation only a few simple plumbing connections (shown in Fig. 2) are required. 

With the in situ ion-exchange method (Fig. 5), dilute sulfuric acid is used to precipitate the silver in the resin beads as silver sulfide instead of removing it with regenerant. The resin that is inside the ion exchange unit is used for many cycles without a loss in capacity. When the resin eventually loses its capacity to recover silver, or when there is sufficient silver to make recovery worthwhile, it is sent to a silver refiner who incinerates it to remove the silver. This may occur after between six months and a year. 

The ninth book is devoted to arts connected with chemistry, as distillation and sublimation, the arts of coining and the manufacture of jewelry, mirrors of metal, etc. In this book appears a description of methods for extracting all gold or silver from the waste of the mines or sweepings of the mint. It is of especial interest because it describes, apparently for the first time, recovery of silver by the method of amalgamation, a process first apparently utilized on a large scale by the Spaniards in America later in that century. He says ... 

The covalent bond formed between precious metals (PGM, gold and silver) and functional groups with a sulfur donor atom are often so strong that elution without destruction of the resin is generally difficult. In such cases, because of the very high capacity of these resins and because of the value of the metal loaded, incineration of the resin is usually the most economically viable method for recovery of the precious metals. 

Hung et al. (1982) developed a sensitive and selective method for silver analysis by reacting silver (I) with 2(3,5-dibromo-2-pyridylazo)-5-diethyl amino phenol in the presence of an anionic surfactant, sodium lauryl sulfate. The ternary complex formed is red and exhibits an absorption peak at 570 nm. Hung and his co-workers employed EDTA as a chelating agent, thereby reducing the interference of common ions. Recoveries were good, and a detection limit of 0.39 ppm of silver was achieved. 

The limitations of the Gutzeit method for determining arsenic are well-known. The spectrophotometric molybdenum blue or silver diethyldithio-carbamate procedures tend to suffer from poor precision. Sandhu [34] has described a spectrophotometric method for the direct determination of hydrochloric acid-releasable inorganic arsenic in soils and sediments. The method provides reliable data on the quantitative recovery of 2.0 xg of arsenic(V) added to 5.0 g (0.4 mg/kg) of soil, clay, sand and sediment samples. The method is simple, reliable and relatively rapid 24 samples can be analysed in about an hour. It does not require elaborate equipment and can be routinely used for the quantitative determination of arsenic in soil and soil-like material. The detection limit has been established as 0.5 xg of arsenic. The extent of ionic interference when this method is used for arsenic determination in soil was also quantitatively evaluated. 

Tanaka et al. [ 16] have described a spectrophotometric method for the determination of nitrate in vegetable products. This procedure is based on the quantitative reaction of nitrate and 2-sec-butylphenol in sulfuric acid (5 + 7), and the subsequent extraction and measurement of the yellow complex formed in alkaline medium. The column reaction is sensitive and stable and absorbances measured at 418 nm obey Beer s law for concentrations of nitrate-nitrogen between 0.13 and 2.5 xg/ml. In this procedure, the vegetable matter is digested at 80 °C with a sodium hydroxide silver sulfate solution, concentrated sulfuric acid and 2-sec-butylphenol are added, and after 15 minutes of standing time the nitrated phenol is extracted with toluene. Finally, the toluene layer is back-extracted with aqueous sodium hydroxide and evaluated spectrophotometrically at 418 nm. The standard deviation of the whole procedure was 1.4%, and analytical recoveries ranged between 91 and 98%. 

Total recoverable metals are defined as the concentrations of metals in an unfiltered aqueous sample treated with hot nitric and hydrochloric acids according to EPA Method 3005. The method is suitable for the preparation of digestates for the FLAA and ICP-AES analysis. This procedure is known to produce low recoveries of silver due to precipitation and of antimony due to evaporation. 

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