Silver electrolytic refining

Silver is also recovered during electrolytic refining of copper. Commercial fine silver contains at least 99.9% silver. Purities of 99.999+% are available commercially. 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Fig. 1. Recovery of copper from sulfide ore. The residue from electrolytic refining is processed to recover gold, silver, and selenium. Courtesy of Kennecott...

Potassium cyanide is primarily used for fine silver plating but is also used for dyes and specialty products (see Electroplating). Electrolytic refining of platinum is carried out in fused potassium cyanide baths, in which a separation from silver is effected. Potassium cyanide is also a component of the electrolyte for the analytical separation of gold, silver, and copper from platinum. It is used with sodium cyanide for nitriding steel and also in mixtures for metal coloring by chemical or electrolytic processes. 

The unsoftened lead obtained after the softening operation contains silver in small but significant quantities. Such unsoftened lead is cast into anode and subjected to electrolytic refining. The anode mud that is formed adhering to these anodes is removed by scraping. It contains bismuth, silver, gold, and other impurity metals. Silver is obtained from this anode mud by methods similar to the extraction of anode mud from the copper refining process discussed earher. 

In the electrolytic refining of copper, blister copper is used as the anode and oxidized. The copper(II) ion that is produced from its oxidation is then reduced at the cathode to give a metal with a much higher purity. The impurities in the blister copper include iron, nickel, silver gold, cobalt, and trace amounts of other metals. The material that is not... 

These are very unreactive metals. Silver exists mainly as silver sulfide, Ag2S (silver glance). The extraction involves treatment of the pulverised ore with sodium cyanide. Zinc is then added to displace the silver from solution. The pure metal is obtained by electrolysis. Silver also exists to a small extent native in the Earth s crust. Gold is nearly always found in its native form (Figure 10.17). It is also obtained in significant amounts during both the electrolytic refining of copper and the extraction of lead. 

Each industrial chemical process has as its objective the economical production of a particular primary product. It is frequently true that, in attaining this objective, one or more by-products may become available. If these by-products can be disposed of at a profit, this serves to decrease the overall cost of operation and to permit the sale of the primary product at a lower, more favorable, competitive price. Thus, the cost of electrolytically refined copper is dependent on the recovery and sale of the by-products—silver, gold, platinum, and palladium. These precious metals are recovered in large quantities from accumulated anode sludges. Fully one-fourth of the total production of silver, about one-eighth of the gold, and lesser quantities of platinum and palladium are obtained as by-products of the electrolytic refining of copper. 

When this kind of diaphragm is used the composition of both anolyte and catholyte is the same. Similar conditions hold good for tho electrolytical refining of certain metals when the slime formed on the anode which contains precious metals is caught in a cloth bag covering the anode (e. g. when refining silver). 

In the electrolytic refining of copper (p. 249) both silver and gold are deposited in the insoluble sludge at the bottom of the vessel, and are subsequently extracted from this sludge, the silver being dissolved by boiling with sulphuric acid, and subsequently precipitated by copper. 

Silver is often removed from lead by the Parkes process, described in Chapter 27. Some pure lead is made by electrolytic refining. 

Electrolytic refining. Lead of very high purity can be produced from the electrolytic process. Most electrolytic refineries utilize the Betts process [17]. In this process, lead bullion is cast into anodes and placed in an electrolytic cell which contains an electrolyte of fluorosilicic acid and lead fluorosilicate. The cathode is a thin sheet of high-purity lead referred to as the starter sheet . Lead is deposited on the cathode while the impurities form an adherent, but porous, slime layer on the anode. The slimes are collected for recovery and refining as they contain valuable impurities such as silver, gold, copper, and bismuth. 

Silver occurs both in a native form and in ores, such as argentite (Ag2S) and horn silver (AgCl) lead, lead-zinc, copper, gold, and copper-nickel ores are principal sources. Mexico, Canada, Peru, and the U.S. are the primary silver producers in the Western Hemisphere. Silver is also recovered during electrolytic refining of copper. Conunercial fine silver contains at least 99.9% silver. Purities of 99.999+% are available commercially. 

Potassium cyanide is used for electrolytic refining of platinum fine silver plating as an electrolyte for the separation of gold, silver, and copper from platinum and for metal coloring. 

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