Silver recovery

Up to the Middle Ages silver rich ores were smelted with charcoal in hearih or shaft furnaces as previously described, usually with the addition of lead and the use of a lead pool in the forehearth to collect a lead-silver alloy. Silver was then separated from the lead by cupellation. 

The separation of silver by the Pattinson process used fractional crystallisation, in which molten lead was cooled and partly solidified in a pan while being briskly stirred. The solid lead crystals were relatively pure, leaving silver in the remaining liquid. A row of about nine pans were used, each heated by a fire from below. Crude bullion from the smelter was fed to the middle pan, from which solid crystals were transferred to the first pan on one side and the remaining liquid to the first pan on the other side. This process was repeated from one pan to the next up and down the line to give a purified lead with low silver content at one end and a residual liquid of around 9 kg of silver per tonne at the other end. The silver rich lead was subjected to cupellation to recover a silver bullion. Today, precious metals are separated from lead bullion using the Parkes Process, following the removal of copper, arsenic and antimony. In this process, zinc is added and the lead bulUon is cooled to precipitate a zinc-silver alloy, which is removed and separately treated. 

Copper smelting in the Balkans Lead metal produced in Egypt Silver recovered by cupellation 

Iron blast fiimace Stamp mills for ore crushing Hearth smelters for lead 

1713 Use of coke for iron smelting 1720 Reverberatory hearth smelter for lead 

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The solutions we offer are based on two main technologies electrolytic silver recovery from fixer solutions and cascade fixing. In what follows we will give more teclmical details about these teclmologies. We will clarify the key-factors to obtain reliable and more ecological solutions for the silver in the rinsing water. 

Electrolytic silver recovery is a common technique to desilver fixing solutions. It has been known for decades, although it never really reached a point where it was massively introduced into the industrial radiology market. In the past, the main reasons to implement silver recovery were twofold. 

The desilvering speed of an electrolytic silver recovery unit can be characterized by two parameters. 

Silver readily forms alloys with lead. Lead is often used as a base metal solvent for silver recovery processes. The lead—silver system is a simple eutectic having the eutectic point at 2.5 wt % silver and 304°C. The soHd solubihty of silver in lead is 0.10 wt % at 304°C, dropping to less than 0.02 wt % at 20°C. 

Secondary Silver Recovery. The consumption of sdvei normally exceeds its mine production therefore recovery from scrapped products. ... 

CHOICES—Choosing the Right Silver-Recovery MethodforYour Needs, Kodak Pubhcation J-21, Rochester, New York, 1989. 

Chemical precipitation for removal of dissolved metals Settling or filtration to remove SS Ion exchange for silver recovery Chemical reduction of chromium Settling... 

Anon., Environment, Safety Health Bull., 1993, 93(2), 1 As part of an analytical procedure prior to silver recovery, 10 ml commercial hypochlorite solution (15% available chlorine ) was added to highly alkaline waste developer solution. The redox reaction was violent enough to spray the worker, causing alkali bums. 

Paiva, A. P. Solvent extraction and related studies on silver recovery from aqueous solutions. Sep. Sci. Technol. 1993, 28, 947-1008. 

Silver oxide lead ores have much different flotation processing characteristics. Although this ore responds to sulphidization-xanthate system, silver recovery in the lead concentrate was usually poor and amounted to about 30 40%. Floatability of lead minerals also was not satisfactory. 

Figure 20.6 Effect of CuS04 additions on oxide lead silver recovery.

Figure 20.7 Effect of conditioning time with Na2S on lead and silver recoveries.

The use of CuS04 was extremely beneficial for improvement in silver recovery contained in the fine fractions. 

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],... 

Investigation into the 12-kW SILVER II plant membrane failures Silver recovery from silver chloride Evaporator trials for the full-scale SILVER II EDP plant CST/CATOX testing Desktop studies (literature surveys)... 

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

Demonstrate silver recovery from (contaminated) silver chloride. 

Caustic washing and thermal treatment of silver chloride precipitate are performed to achieve a 5X decontamination level for shipment to an offsite silver recovery operation. 

AEA. 2001n. Silver Recovery from Silver Chloride, Report Number 20/ 3034/13/003 (final Rl), December 15. Pittsburgh, Pa. AEA Technology Engineering Services, Inc. 

The generator may want to reclaim silver from the x-ray waste itself. If its silver recovery equipment is hard-piped and connected to the photoprocessor, this generator is currently exempt from recycling permits. 

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. 

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. 

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

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. 

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