Selective leach

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Fluxes. Fluxes, composed mostly of salts or oxides of metals, serve to protect underlying metal from the air. This prevents the formation of surface oxides that impede fusion and the formation of a strong solder joint. Fluxes may also act to selectively leach elements from the surface of the underlying metal. The result is a surface free of obstacles to fusion, and of a composition readily wetted by the solder. 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

Dezincification is explained by two theories. The first is that the alloy dissolves, with a preferential redeposition of copper. The other proposes a selective leaching of the zinc, leaving the copper behind. There is evidence that both mechanisms may operate, depending on the specific environment. 

If the metallic compound in the ore can be selectively leached by acid or base without dissolving much of the remaining ore, then the energy requirement is only about 10 J/kg. Examples are leaching of oxide ores of copper, zinc, or uranium with sulfuric acid. 

The various types of localised corrosion have been enumerated in Table 1.2 in Section 1.1, and many of them are dealt with in some detail in other sections of this volume. For this reason this section will be confined to a consideration of the factors that give rise to crevice corrosion, filiform corrosion, pitting, selective leaching and erosion-corrosion and of the mechanisms of these forms of localised attack. 

Two theories have been proposed to explain dezincification, but since both have considerable support the precise mechanism remains unresolved. One theory proposes that the zinc is selectively leached from the alloy leaving a porous residue of metallic copper in situ (c/. parting of Ag-Au alloys), whilst the other proposes that the whole of brass dissolves and that the copper immediately redeposits at sites close to where the brass was dissolved. 

Dezincification As either plug dezincification (localized corrosion) or layer dezincification (general corrosion) and refers to zinc (Zn) being selectively leached out of brass. It can be prevented either by reducing the Zn content to below 15% or by the addition of trace amounts of inhibiting elements such as arsenic (As)... 

NOTE Do not confuse graphitization with graphitic corrosion, which is different. Graphitic corrosion causes the iron in cast iron to selectively leach out, leaving behind a porous graphite structure. 

At the stage of Kuroko mineralization, evolved reacted seawater enriched in Eu, Ca, and Sr formed at low seawater/rock ratio (ca. 1 by mass) and at relatively reduced condition (Eu +/Eu + greater than 1). Selective leaching of Eu, Ca and Sr occurred from the dacitic rocks underlying the Kuroko ores. The hydrothermal solution enriched... 

D and 8 0 data on fluid inclusions and minerals at main stage of epithermal Au-Ag mineralization clearly indicate that the dominant source of ore fluids is meteoric water. Meteoric water penetrates downwards and is heated by the country rocks and/or intrusive rocks. The heated water interacts with country rocks and/or intrusive rocks and extracts sulfur, Au, Ag and other soft cations (e.g., Hg, Tl) from these rocks. If hydrothermal solution boils, it becomes neutral or slightly alkaline, leading to the selective leaching of soft cations such as Au, Ag, Hg and Tl from country rocks. However, a contribution of sulfur gas and other components from magma cannot be ruled out. 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

Chitwood (2) found that copper compounds exhibited only a short period of maximum catalytic activity for the dehydrogenation of ethanolamine to glycine salt. In this study, the catalytic activity of a skeletal copper catalyst was tested in repeated use. The catalyst used was prepared by selectively leaching CuAl2 particles in a 6.1 M NaOH solution at 293 K for 24 hours. Figure 1 shows the profiles of hydrogen evolved versus reaction time. 

McClenaghan, M.B., Hamilton, S.M., Hall, G.E.M., Burt, A.K., Kjarsgaard, B.A. 2006. Selective Leach Geochemistry of Soils Overlying the 95-2, B30, and A4 Kimberlites, Northeastern Ontario. Geological Survey of Canada, Open File 5069. 

Hall, G.E.M., Vaive, J.E., Beer, R., Hoashi, M. 1996. Phase selective leaches for use in exploration geochemistry. In EXTECH I a multidisciplinary approach to massive sulphide research in the Rusty Lake-Snow Lake greenstone belts, Manitoba. Geological Survey of Canada Bulletin, 169-200. 

Hamilton, S.M. 1998. Electrochemical mass transport in overburen a new model to account for the formation of selective leach geochemical anoamalies in Glacial Terren. Journal of Geochemical Exploration, 63, 155-172. 

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