Metallic impurities

L. calx, lime) Though lime was prepared by the Romans in the first century under the name calx, the metal was not discovered until 1808. After learning that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, Davy was able to isolate the impure metal. 

Gr. Tantalos, mythological character, father of Niobe) Discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. The first relatively pure ductile tantalum was produced by von Bolton in 1903. Tantalum occurs principally in the mineral columbite-tantalite. 

The fused product is cooled, cmshed, and leached with acidified hot water. The resulting hot solution of potassium hexafiuoto2irconate—hafnate is filtered to remove siUca, then cooled to allow crystaUi2ation of the potassium hexafiuoto2irconate—hafnate. Many of the impurity metals remain in solution. 

The niter and fresh caustic soda, required to maintain the fluidity of the salt bath in the reactor chamber, are added gradually. When the color of the saturated salts turns from a dark gray to white, the impurity metals are at their highest state of oxidation, and the lead content of the spent salts is very low. In a modification, the arsenic and tin are selectively removed as sodium arsenate and sodium stannate, followed by the removal of antimony as sodium antimonate. 

Refining Processes. AH the reduction processes yield an impure metal containing some of the minor elements present in the concentrate, eg, cadmium in 2inc, or some elements introduced during the smelting process, eg, carbon in pig iron. These impurities must be removed from the cmde metal in order to meet specifications for use. Refining operations may be classified according to the kind of phases involved in the process, ie, separation of a vapor from a Hquid or soHd, separation of a soHd from a Hquid, or transfer between two Hquid phases. In addition, they may be characterized by whether or not they involve oxidation—reduction reactions. 

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. 

Arkel refining a sample of tire impure metal, for example zirconium, is heated to a temperature around 550 K in contact with low pressure iodine gas in a sealed system which has a heated mngsten filament in the centre. The filament temperature is normally about 1700K. At the source the iodides of zirconium and some of the impurities are formed and drese diffuse across the intervening space, where tire total pressure is maintained at 10 auiios, and are decomposed on the filament. The iodine then remrns to form fresh iodide at the source, and the transport continues. 

The Kroll process for tire reduction of tire halides of refractory metals by magnesium is exemplified by the reduction of zirconium tetrachloride to produce an impure metal which is subsequently refined with the van Arkel process to produce metal of nuclear reactor grade. After the chlorination of the impure oxide in the presence of carbon... 

Most metals in commercial use contain quite large quantities of impurity (e.g. as alloying elements, or in contaminated scrap). Solid-state transformations in impure metals are usually limited by the diffusion of these impurities through the bulk of the material. 

Depth profiling of halogens and impurities Metallization of surfaces... 

An alloy of nickel was known in China over 2000 years ago, and Saxon miners were familiar with the reddish-coloured ore, NiAs, which superficially resembles CU2O. These miners attributed their inability to extract copper from this source to the work of the devil and named the ore Kupfemickel (Old Nick s copper). In 1751 A. F. Cronstedt isolated an impure metal from some Swedish ores and, identifying it with the metallic component of Kupfemickel, named the new metal nickel . In 1804 J. B. Richter produced a much purer sample and so was able to determine its physical properties more accurately. 

Corrosion of Impure Metals and Single-phase Alloys... 

With increasing purity of aluminium, greater resistance to corrosion is developed. On high-purity materials, however, any pits which develop are likely to be deeper though fewer in number than those formed in more impure metal. In some special applications, notably in contact with ammonia solutions or pure water at elevated temperatures and pressures, the iron and silicon present in commercial-purity metal are beneficial and retard corrosion. Up to about 5% magnesium improves the corrosion resistance to sea-water. 

Impure metals and alloys exhibit all the structural features and crystal defects of the pure meteils already discussed. In addition, however, impure metals and alloys exhibit many structures which are not observed in pure metals, and which, in many instances, have an extremely important effect on the properties, particularly the corrosion resistance. However, before dealing with the structure of impure metals and alloys, it is necessary to consider the concept of metallurgical components, phases, constituents and equilibrium phase diagrams. 

The fundamental difference between pure metals and impure metals and alloys arises from the fact that there is only one atomic species present in the former, while there are two or more present in the latter thus a pure metal is a single-component system, a pure binary alloy is a two-component system, while impure metals and alloys, strictly speaking, are multi-component... 

The principle of the electrorefining process is basically simple plutonium is oxidized at a liquid metal anode containing impure metal feed and the resulting Pu+3 ions are transported through molten salt to a cathode where pure metal is produced. 

Americium Extraction (more commonly referred to as Molten Salt Ex-or MSE). This process is specifically designed to reduce the americium content of the plutonium metal. (Am241 spontaneously grows into plutonium as a result of Pu241 decay.) When the impure metal contains more than 1000 ppm of americium, it is run through the MSE process. Otherwise, it bypasses the MSE step and proceeds directly to electrorefining. 

Figure 6. Impure metal ingot feed for electrorefining.

Plutonium Electrorefining. Plutonium electrorefining principles are summarized in Refs. 1,3,9. Briefly, the process consists of oxidizing plutonium from an impure metal feed at the anode and reducing it to pure metal at the cathode. 

Plutonium pyrochemical processes are now the principal tools at Los Alamos for producing large amounts of high purity plutonium metal from impure metal and oxide scrap. Pyrochemical processing was selected because of its cost effectiveness. The processes are highly compact and require little floor space and manpower to operate. The processes are also operationally efficient in that one or two steps can be used to supplant multi-step operations found in the classical aqueous chemistry flow streams. The... 

Chemical reduction of an ore usually gives metal that is not pure enough for its intended use. Further refining of the metal removes undesirable impurities. Several important metals, including Cu, Ni, Zn, and Cr, are refined by electrolysis, either from an aqueous solution of the metal salt or from anodes prepared from the impure metal. To give one example, ions, obtained by dissolving ZnS or ZnO in acidic solution, can be reduced while water... 

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