Silver electrorefining

The anode material for silver electrorefining is mainly derived from anode slimes from copper, lead, nickel and zinc electrorefining processes together with diverse contributions from the secondary recovery and scrap-recovery processes. 

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. 

Other Meta.Is, Although most cobalt is refined by chemical methods, some is electrorefined. Lead and tin are fire refined, but a better removal of impurities is achieved by electrorefining. Very high purity lead is produced by an electrochemical process using a fluosiUcate electrolyte. A sulfate bath is used for purifying tin. Silver is produced mainly by electrorefining in a nitrate electrolyte, and gold is refined by chemical methods or by electrolysis in a chloride bath. 

Fite refining adjusts the sulfur and oxygen levels in the bhster copper and removes impurities as slag or volatile products. The fire-refined copper is sold for fabrication into end products, provided that the chemistry permits product specifications to be met. Some impurities, such as selenium and nickel, are not sufficiently removed by fire refining. If these impurities are detrimental to fabrication or end use, the copper must be electrorefined. Other impurities, such as gold, silver, selenium, and tellurium, are only recovered via electrorefining. Virtually all copper is electrorefined. 

Although some changes occur in the melting furnace, cathode impurities are usually reflected directly in the final quaUty of electrorefined copper. It is commonly accepted that armealabiUty of copper is unfavorably affected by teUurium, selenium, bismuth, antimony, and arsenic, in decreasing order of adverse effect. Silver in cathodes represents a nonrecoverable loss of silver to the refiner. If the copper content of electrolyte is maintained at the normal level of 40—50 g/L, and the appropriate ratio of arsenic to antimony and bismuth (29) is present, these elements do not codeposit on the cathode. 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

A sample of silver is to be purified by electrorefining. This will separate the silver from an impurity of gold. The impure silver is made into an electrode. Which of the following is the best way to set up the electrolytic cell ... 

Insoluble impurities fall to the floor of the cell as anode slime. Despite the derogatory name, this material contains precious metals such as gold, silver, and platinum. Anode slime from the electrorefining of nickel11 at Sudbury, Ontario, is a significant source of platinum and palladium as byproducts ( 0.34 g Pt and 0.36 g Pd per metric ton of ore), whereas deposits in the Bushveld complex (Transvaal, South Africa) are so rich in platinum-group metals (Ru, Os, Rh, Ir, Pd, Pt) that the associated Co, Ni, and Cu recovered are considered to be by-products of the lucrative platinum production (4.78 g Pt and 2.03 g Pd per metric ton of ore).7... 

Closely related to electrorefining is electroplating, the coating of one metal on the surface of another using electrolysis. For example, steel automobile bumpers are plated with chromium to protect them from corrosion, and silver-plating is commonly used to make items of fine table service. The object to be plated is carefully cleaned and then set up as the cathode of an electrolytic cell that contains a solution of ions of the metal to be deposited. 

Electrorefining in aqueous media is extensively applied for the production of copper, nickel, lead, tin, cobalt, silver, and gold, while in molten salt electrolytes it is practically limited to aluminum. 

In electrorefining, the metal to be refined is used as the anode whic dissolves in the electrolyte and is deposited as electrolytic-grade mett at the cathode. The impurities present in the anode remain on it, fall oi to the bottom of the cell as slime, or go into solution but are prevente. from moving toward the cathode by precipitation with some chemics reagent such as another metal added to the electrolyte. The buildup c metallic impurities that are dissolved but not deposited at the cathode i reduced by circulation of fresh electrolyte through the cells. Electrorefinin techniques are used in producing gold, silver, copper, nickel, cobalt, lead tin, antimony, bismuth, indium, and mercury. 

Metals that have been produced by pyrometallurgical methods, such as copper, silver, nickel, and tin, are too impure for many purposes, and electrorefining is used to purify them further. Crude metallic copper is cast into slabs, which are used as anodes in electrolysis cells that contain a solution of CUSO4 in aqueous H2SO4. Thin sheets of pure copper serve as cathodes, and the copper that dissolves at the anodes is deposited in purer form on the cathodes (Fig. 17.18). Impurities that are more easily oxidized than copper, such as nickel, dissolve along with the copper but remain in solution elements that are less easily oxidized, such as silver and gold, do not dissolve but fall away from the anode as a metallic slime. Periodically, the anode slime and the solution are removed and further processed for recovery of the elements they contain. 

Only weakly electropositive metals can, obviously, be purified by electrorefining, since the concerned metal must not react with water and must be easily oxidized (at anode) and reduced (at cathode) relative to hydrogen. Beside copper, some other metals for which electrorefining has been employed are Silver, gold, tin and lead. In the electrolytic purification of these metals the relevant electrolyte is the one listed below against each metal ... 

They explained this phenomenon in terms of ion pair transport. Lee et al. (M) studied copper extraction using two types of liquid membranes. More recently, Gokalp et al. (70) used liquid membranes containing polyethers to electrorefine silver and copper. They explored two geometries for the liquid membrane. They placed the membrane between the anode and cathode and also coated the cathode with the membrane in separate experiments. They were able to demonstrate the technical feasibility of this process. 

Dore is electrorefined in conventional Balbach Thum cells to produce 99.99% silver cathode which is then cast or granulated according to market requirements. Anode slimes are periodically recovered from the cells and are leached to produce a 98% gold product. 

Lead bullion is clectrorcfined to produce refined lead and slimes. The slimes arc treated in the silver refinery where a series of furnaces process the slimes to produce dore metal and bismuth, as well as various slags and baghouse dusts that contain antimony and arsenic. Some of the baghouse dust is directed to the copper products plant and the lead alloys plant to recover antimony and arsenic values, and the remainder of the baghouse dust and the slags are recycled to the smelter. Done metal is electrorefined to produce pure silver and gold bullion. 

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