Solid extractants classification

The solid and liquid extract classification is based on method of preparation. 

FIGURE 7.1 The classification of solid extractants based on fnnctional ligand or donor atoms. (From C. Kantipuly, S. Katragadda, A. Chaw, D. H. Gesser. Tcdanta 37 491—517,1990. With permission.)... 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

The general objective, principle, and scope of application of the pT-method are succinctly described in Section 1 and also reported elsewhere in this book (see Chapter 3 of this volume, Section 5.1), where readers will appreciate that this hazard assessment scheme is adaptable to both liquid and solid media. Briefly recalled here in the context of solid-media samples such as dredged material, the pT-value, which relates to a single bioassay, and the pT-index, derived from the most sensitive organism in a test battery, permit a numerical classification of environmental samples on the basis of ecotoxicological principles. Sediment from any aquatic ecosystem (freshwater, brackish, marine) and from any of its phases (whole sediment, porewaters, elutriates or organic extracts) can be appraised provided that the proper standardized toxicity tests are available. There are whole-sediment test protocols standardized for many agencies (e.g., Environment Canada, ASTM). 

Numerous extraction methods and techniques have been developed and reported, especially if one takes into account the variety of modifications. The most common and simple general classification of these methods is similar to that introduced in chromatography and based on the kind of phase to which the analyte is transferred. One can distinguish the extractions as liquid, solid, gas, and supercritical fluid phase extractions. More precise description specifies the two phases between which the analyte is distributed (e.g., liquid-liquid or solid-liquid [leaching] extractions). The latter methods are all called solvent extraction. 

Arden et al. (A7) used the jigged-bed ion exchange technique for the extraction of uranium from acid-leached pulps. This method treats de-sandcd pulp containing 2(M0% by weight of solids with the -1-300 mesh particles removed by classification in single hydrocycloncs. The volume of the slime pulp is 1.2-2.0 times the volume of the leach solution compared to 2-3 times if conventional filtration is used. 

Solving the problem on the interaction of a solid particle, drop, or bubble with the surrounding continuous phase underlies the design and analysis of many technological processes. The industrial applications of such interaction include classification of suspensions in hydrocyclones, sedimentation of colloids, pneumatic conveyers, fluidization, heterogeneous catalysis in suspension, dissolving solid particles, extraction from drops, absorption, and evaporation into bubbles [69, 107, 111, 122,137,478,505],... 

In the classification scheme in Sec. 1.4.1, the first three entries under liquid-solid separation methods, i.e., ion-exchange, adsoiption, and sorbent extraction, all belong to column separation techniques. While in the batch approach, separations based on these principles may be performed either by static equilibration or by a column technique, online columns are invariably used in FI separations, both for convenience and efficiency. FI column separation systems based on different sorptive mechanisms do not differ strongly in the principles of system design and optimization of operational parameters. Therefore, the principles discussed in the following sections are generally applicable to the different approaches. 

The results using the modified procedure are shown in Table IV. The pressure-filtration values in SCT-87 compare well with the Soxhlet-extraction values from SCT 65 and 69. In other experiments (SCT 91 and 95), part of the reacted slurry was used to perform solvent classification by pressure filtration and the remainder for solvent classification by Soxhlet extraction. Agreement of cyclohexane solubility values within 3-6jt is a considerable improvement over the difference of approximately 25% obtained with the standard pressure-filtration procedure, but further improvements in the technique are being sought. The solubility values for the two methods compare fairly well, considering that these "high solids content" slurries are difficult to extract with cyclohexane. 

Gravitational settling of particles in liquids is an age-old process which can be used for a variety of purposes. For example, it is used for the classification of solids, washing, particle size measurement or mass transfer, and in solvent extraction. The majority of applications of gravity sedimentation, however, are in solid-liquid separation duty. The object here is to remove the solids from the liquid either because the solids and/or the liquid are valuable or because the two phases have to be separated before disposal. 

The viability of continuous extraction depends on the availability of lock systems which will enable solid material to be injected into the high pressure extractor and then removed from it without incurring unacceptable loss of solvent. These systems have been discussed by Reimert [24]. (It should be remembered that some solvent will normally be present in voids within the bed of extracted material.) The author finds it useful to classify lock systems into form preserving systems (required for example in the decaffeination of coffee beans), form-changing systems (which could be used, for example for de-oiling seeds or the extraction of the bitter components from hops) and systems in which a solid to be processed is injected into the extraction zone as a suspension in a suitable fluid. This classification applies irrespective of whether the solid is being conveyed into or out of the pressure chamber and is shown in Table 8.5. 

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