Metal separation/extraction

This shift in emphasis by the mining iadustry has led to the development and use of a variety of improved techniques, in particular the commercial avadabihty of several metal specific extractants. These techniques are particularly useful in the separations and recycling of metals from metal sludges and metal salt solutions. 

Hydrochloric acid digestion takes place at elevated temperatures and produces a solution of the mixed chlorides of cesium, aluminum, and other alkah metals separated from the sUiceous residue by filtration. The impure cesium chloride can be purified as cesium chloride double salts such as cesium antimony chloride [14590-08-0] 4CsCl SbCl, cesium iodine chloride [15605 2-2], CS2CI2I, or cesium hexachlorocerate [19153 4-7] Cs2[CeClg] (26). Such salts are recrystaUized and the purified double salts decomposed to cesium chloride by hydrolysis, or precipitated with hydrogen sulfide. Alternatively, solvent extraction of cesium chloride direct from the hydrochloric acid leach Hquor can be used. 

Gold may also be separated from hydrochloric acid solutions of the platinum metals by extraction with diethyl ether or with ethyl acetate (compare Chapter 6) except in special cases these methods do not offer any special advantages over the reduction to the metal. 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

Many hydroxyp5rranones and hydroxypyridinones and their metal complexes have been of importance in analytical chemistry, solvent extraction, and metal separation. Here their excellent chelating properties in conjunction with the possibility of synthesizing strongly lipophilic derivatives make this class of ligands particularly useful. 

Many industrial processes begin with a leaching step, yielding a slurry that must be clarified before solvent extraction. The solid-liquid separation is a costly step. The solvent extraction of unclarified liquids ( solvent-in-pulp ) has been proposed to eliminate solid-liquid separation. The increased revenue and reduced energy cost make this an attractive process, but many problems remain to be solved loss of metals and extractants to the solid phase, optimization of equipment design, effluent disposal, etc. 

Temperature can have a considerable effect on both the extraction and stripping properties of a solvent extraction system relative to equilibrium, kinetics, and metal separations [5] (see Chapters 3-5). Therefore, it is advisable to investigate these effects, especially when the solvent tends to be viscous or high loadings of metal are to be obtained in the solvent (Fig. 7.5). 

Many of these experimental methods can be used in the development of systems more complex than the extraction of a single substance, such as metal separations. Metal separation processes may involve as few as two metals and as many as 15, as in the rare earths (see Chapter 11). 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

We see that the distribution coefficient for metal ion extraction depends on pH and ligand concentration. It is often possible to select a pH where D is large for one metal and small for another. For example. Figure 23-4 shows that Cu2+ could be separated from Pb2+ and Zn2+ by extraction with dithizone at pH 5. Demonstration 23-1 illustrates the pH dependence of an extraction with dithizone. Box 23-1 describes crown ethers that are used to extract polar reagents into nonpolar solvents for chemical reactions. 

Plasticizers can be separated by extraction with diethyl ether. Stabilizers based on pure organic or organo-metallic compounds may only be partially separated. Extraction time depends on particle size and on the amount of plasticizer in the sample. 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

Fig. 4.13 Separation of a standard trace metal sample (extracted with dithizone solution). Divisions on baseline 1 min per division peaks, dithizone complexes of (a) Hg, (b) Cu, (c) Ni,...

Solvent extraction was first applied to metal separation in the nuclear industry in the late 1940s. Nuclear power generation by uranium fission produces spent fuel containing 238U, 235U, 239Pu, 232Th, and many other radioactive elements collectively known as fission products. 

In 1960, Hewitt and Notton estimated the phosphorus in fractions of tomato and cauliflower leaves using 32P coupled with extractions. Bowen et al. (1962) fed tomato plants with a number of radioactive elements, the fresh tissues were then extracted with a series of extractants. The results posed a number of interesting questions, for example, the association of cobalt and iron with proteins, and the authors suggested that a high speed centrifuge should be used to separate intracellular particles of different sizes. The nature of the metal compounds extracted was also questioned. Some inconclusive paper chromatography was also reported. 

The dabblings of homo sapiens into separations are very recent indeed when viewed in the context of these primordial separations. But even here the beginnings are lost in human antiquity. Crude food processing, such as milling, metallurgical processes to obtain metals, the extraction of dyes, flavors, and medicines, and the concentration of many materials by evaporation are certainly very ancient on a human time scale. 

A series of 4-alkylamido-2-hydroxybenzoic acids containing a different number of carbon atoms in the alkyl-amido group has been studied as model ligands for metal ion extraction in aqueous micellar solutions of nonionic surfactants. Their acid-base properties and reactivity towards metal ions in the presence of micelles were investigated. By operating at a proper temperature, the separation of the iron (III) chelate complexes into a micellar rich phase was achieved and the extraction efficiency was correlated with the ligand hydrophobicity. 

Solvent extraction is one of the methods widely used for concentration and separation. Most heavy metals are extracted with chelating reagents into organic solvents [35]. Some chelating agents commonly used in atomic absorption spectrometry are shown in Fig. 3. APDC and DDC are most commonly used in AAS. Solvents such as ketones, esters, ethers, alcohols, and other oxygen-containing hydrocarbons are suitable for the flame atomic absorption technique. Of these solvents, MIBK (methyl isobutyl ketone,... 

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