Leaching extract displacement

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

Sometimes these operationally defined procedures have a sound theoretical basis. For example, it is quite reasonable to suppose that leaching with magnesium nitrate solution will displace zinc from cation exchange sites in soils, or leaching with ammonium acetate will displace exchangeable calcium, magnesium, sodium, and potassium. Flame spectrometry, especially flame AAS, is widely used for the analysis of such extracts. 

Unlike ASE, because the leaching agent used in the dynamic mode is most often water, no desiccant need be added to the extraction cell together with the sample. As in ASE, however, additional materials such as surfactants [47] or sorption discs [39] can be placed in the cell to facilitate displacement of the analytes from the sample to the micellar medium or preconcentrate them in the sorbent, respectively. 

Solids feed preparation usually plays a crucial role in leaching processes. The use of line particles usually win spied up diffusive transfer of solutes from the solid to the extract. However, if too fine a size is used, extract may not be able to flow Ibrongh solid beds as rapidly as desired, flow pressure drop may be excessive and cause undesirable bed compaction, displacement instability will he enhanced, extract cling will increase, solid-liquid separation difficulty may increase, and excessive amocrats of intracellular colloidal material may be released. Therefore, the use of a compromise particle size is almost invariably desirable. Except for flaked oilseeds, where flake thicknesses of the order of 0.2 mat are necassery to obtain adequate oil release, particle diameters or thicknesses in the 2-5 mm range usually represent a good choioe for industrial scale extractions. 

In this study, the practicalities of three methods for extracting and characterizing pore-waters from potential containment materials are discussed. These are (i) mechanical squeezing using a hydraulic press (ii) centrifugation using a heavy liquid displacent and (iii) aqueous leaching of residual salts from crushed core material. 

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