Cathodes, aluminum silver

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

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,... 

Zinc electrowinning takes place in an electrolytic cell and involves running an electric current from a lead-silver alloy anode through the aqueous zinc solution. This process charges the suspended zinc and forces it to deposit onto an aluminum cathode (a plate with an opposite charge) that is immersed in the solution. Every 24 to 48 h, each cell is shut down, the zinc-coated cathodes removed and rinsed, and the zinc mechanically stripped from the aluminum plates. The zinc concentrate is then melted and cast into ingots, and is often as high as 99.995% pure. 

Zinc electrowinning Zinc in a sulfuric acid/ aqueous solution, lead-silver alloy anodes, aluminum cathodes, barium carbonate, or strontium, colloidal additives... 

A term used to describe how easily a metal is oxidized is active. A more active metal is one that is more easily oxidized. A listing of metals in order of activity is known as an activity series. The activity series is used to determine which substances will be oxidized and reduced in an electrochemical cell the element higher on the list will be oxidized. For example, in a cell with aluminum and silver electrodes in their appropriate solutions, aluminum is oxidized and silver is reduced. Therefore, aluminum is the anode and silver is the cathode. If you have ever bitten a piece of aluminum foil and experienced discomfort, you had this electrochemical process occur in your mouth. Silver (or mercury) fillings and the aluminum serve as electrodes and your saliva serves as an electrolyte between the two. The resulting current stimulates the nerves in your mouth resulting in the discomfort. 

Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions as well as the metal object to be plated. The metal object acts as a cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous metal objects are typically electroplated with aluminum, brass, bronze, cadmium, chromium, copper, iron, lead, nickel, tin, and zinc, as well as precious metals such as gold, platinum, and silver. Common electroplating bath solutions are listed in Table 7-1. 

Zinc is extracted from the purified solution in cells using lead/silver alloy anodes and aluminum cathodes at a current density of 38-60 amperes/square foot (400-650 amperes/square meter) The product is normally SHG zinc, particularly if strontium carbonate is added and/or lead/silver anodes of greater than 0.596 silver content are used. After deposition of 24 to 72 horns, the cathodes are removed from the cells and the zinc is stripped by automatic machines in modem plants, melted, and cast for market. The move to automated handling of large cathodes was a major factor in lowering the overall labor requirement in producing zinc. 

Accidentally chewing on a stray fragment of aluminum foil can cause a sharp tooth pain if the aluminum comes in contact with an amalgam filling. The filling, an alloy of silver, tin, and mercury, acts as the cathode of a tiny galvanic cell, the aluminum behaves... 

Milton and Hutton [21] evaluated aluminum, copper, silver, indium, lead, and tantalum as possible secondary cathode materials. The first three candidates (Al, Cu, and Ag) sputtered at rates too high to allow production of ions characteristic of the glass sample (i.e., tended to produce too thick a metallic layer). Indium and lead are soft materials that lead to the overcompression of the insulator-cathode-sample sandwich, consistently resulting in electrical short-circuiting between the anode and cathode. Finally, tantalum does indeed exhibit the desirable characteristics for application as secondary cathode materials. Although not explicitly required, the fact that Ta is a getter element is likely to provide some added benefits as well. 

Probably the most common solid electrode is platinum, although it dissolves anodically in some melts, for example in halides. The choice of gold and silver [86] is also frequently made. Graphite is very often used because it is cheap and can be obtained in a wide range of sizes and qualities. These electrodes can be used over long periods of time, and they have a wide electrochemical stability, both anodic and cathodic. Vanadium and molybdenum are also used in appropriate systems. Studies for the use of some inert anodes made of semiconducting ceramics have been made, especially for aluminum electrolysis [87],... 

Electrolysis is used in a wide variety of ways. Three examples follow (1) Electrolysis cells are used to produce very active elements in their elemental form. The aluminum industry is based on the electrolytic reduction of aluminum oxide, for example. (2) Electrolysis may be used to electroplate objects. A thin layer of metal, such as silver, can be deposited on other metals, such as steel, by electrodeposition (Eig. 14-2). (3) Electrolysis is also used to purify metals, such as copper. Copper is thus made suitable to conduct electricity. The anode is made out of the impure material the cathode is made from a thin piece of pure copper. Under carefully controlled conditions, copper goes into solution at the anode, but less active metals, notably silver and gold, fall to the bottom of the container. The copper ion deposits on the cathode, but more active metals stay in solution. Thus very pure copper is produced. The pure copper turns out to be less expensive than the impure copper, which is not too surprising when you think about it. (Which would you expect to be more expensive, pure copper or a copper-silver-gold mixture )... 

The second type of cell is a mercury pool type. A mercury cathode is particularly useful for separating easily reduced elements as a preliminary step in an analysis. l or example, copper, nickel, cobalt, silver, and cadmium are readily separated from ions such as aluminum, titanium, the alkali metals, and phosphates. The precipitated elements dissolve in the mercury little hydrogen evolution occurs even at high applied potentials because of large overvoltage effects. A coulomet-ric cell such as that shown in Figure 24-5b is also useful for coulometric determination of metal ions and certain types of organic compounds as well. 

The other category of impurities includes elements that are detrimental to the process or equipment. This category includes iron, lead, chloride and fluoride. Iron is required in the process as a purification agent. However, it also generates residue that cannot be treated on site. Iron exits the Big River Zinc plant in the lead-silver concentrate. If the iron content is too high, the value of the concentrate is reduced, resulting in a cash flow reduction. Lead, as stated earlier, will cause problems in the roasters if its concoitration is too high. In the case of chloride and fluoride, their major impact is equipment corrosion. As with sodium and potassium, the leach and purification circuit does not eliminate chloride and fluoride. When the chloride or fluoride contents increase, we observe increased corrosion rates in the equipment, especially in the cellroom aluminum cathodes. 

The intensity of emission from multiple-element hollow cathodes is generally lower than single-element tubes, although several successful combinations are possible if proper choice of elements is made, taking into account volatilization rates and noninterfering emission spectra. For example, a multiple-element combination of magnesium, calcium, and aluminum is available, as is a combination of silver, lead, and zinc. 

The electrons for the radical anions are injected by the cathode, consisting of a metal with a low work function. Usually, calcium, a coevaporated magnesium-silver alloy with a Mg Ag ratio of 10 1, or aluminum can be used. The corresponding work functions are 2.9 eV, 3.7 eV, and 4.3 eV, respectively. The injection of the electrons can be facilitated by an additional layer of lithium fluoride [40]. Several mechanisms have been proposed to explain the electron injection improvement [41,42]. The most plausible mechanism is the dissociation of LiF toward metalHc hthium, which acts as a redox dopant for the electron-transport layer. From the cathode the electrons are then transported through the electron- transport layer on the LUMO level via hopping transport, which is in principle a mutual solid-state redox reaction. 

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