Leaching differential extraction

FSL Bohuions, which can be used for differential leaching, can be obtained from analogous heal transfer solutions, which are used to predict solid temparatures in nonndiabatic immersion calorimeters,1,2 These solutions are similar in form to Eqs. (10.7-3) and (10.13-1), but the dimansionless concentration involved is MX - Y-.nViMXr, lfe) and the qt and C, depend on both a and an additional dimensionless parameter, y - EMa /DJt, where E b the voluma flow rate into and out of the extractor, in which the extract volume E remains constant. The q, and C,- for various solid shapes are defined in Table 10.15-1. 

After comminution, soluble impurities either inherent to the raw materials or introduced during processing can be extracted by washing (e g. with water), followed by filtration [6, 23]. Chemical leaching and magnetic separation are also used to purify raw materials. In a more specialized process, a frothing agent can be used to promote differential adsorption of impurity particles onto gas bubbles to separate out the desired product [23]. 

Differentiation of sedimentary metal phases was performed on grain size fractionated samples from the lower Rhine River by successive chemical leaching (review). Pollution affects the significant increase of nonresidual associations of chromium, cop er, lead, and zinc. Except for manganese the metal contents in most of the extracted phases decrease as the grain size increases. Phase concentration factors (PCF relative enrichment of metal content in major carrier substances) are high for chromium in moderately reducible phases (20-fold increase in clay-sized particles), for manganese and zinc in the easily reducible sediment fraction (30- and 55-fold enrichment), and for copper and zinc in the carbonates (15 or 25 times compared with total sediment). 

At the same time, we want to focus on the role of diffusion in extraction, for that is the subject of this book. As a result, we emphasize the case of a dilute solute being extracted between two immiscible liquids. This defers complicated issues of ternary phase equilibria to more specialized texts and lets us focus on the issues of mass transfer, which can be obscured in those texts. Specifically, we discuss extraction equipment in Section 14.2, we analyze differential extractors as a parallel to gas absorption in Section 14.3, and we describe staged extraction in Section 14.4. Leaching, which can be either staged or differential, is treated in Section 14.5. The result is a brief summary that emphasizes the role of mass transfer. 

Liquid liquid extraction and leaching use different equipment, even though the analysis of this equipment can be similar. Liquid-liquid extraction can be accomplished either in differential contactors or in staged extractors (Godfrey and Slater, 1995). The differential contactors are analyzed in ways that parallel the analysis of gas absorption the staged extractors depend heavily on ideas developed for distillation. In both cases, an enormous variety of equipment is used, with specific apparatus often being optimized for particular separations. 

We now turn from liquid-liquid extraction to solid-liquid extraction. In more casual terms, we turn from extraction to leaching. As expected, we can use the same ideas of differential and staged separations to handle this case as well. We begin with target separations, we list separation equipment, and we then describe differential and staged leaching. The summary leads to the case of unsteady-state leaching, a transitional case best handled by parallels with the analysis of adsorption in the next chapter. 

While the analysis of differential leaching is much like that of differential gas absorption or differential distillation, the definitions of the concentrations vary. The concentration x in the solvent stream is easy enough it is just the amount of solute per amount of solvent. It may be expressed as a mole fraction, a mass fraction, or a mass ratio. The concentration y in the feed is harder. When the solute is being extracted from the solid itself, y will be expressed as amount of solute per amount of solid. When the solute completely dissolves in the solvent to leave none in the solid, then the concentration y will be expressed as solute per retained solvent, and the slope of the equilibrium line will be one. We will return to this point below, in the discussion of staged leaching. 

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