Metal ions recovery from mixtures

The number of publications involved with the recovery of rubidium from seawater is very limited. Most of the work in this field is by Russian scientists, who have proposed several schemes for the combined recovery of rubidium, strontium, and potassium with natural zeolites [15, 19, 250-253, 257]. A number of inorganic sorbents with high selectivity toward rubidium were also synthesized for the recovery of rubidium from natural hydromineral sources, including seawater. Ferrocyanides of the transition-metal ions were shown to exhibit the best properties for this purpose [258, 259]. Mordenite (another natural zeolite) has recently been proposed for selective recovery of rubidium from natural hydromineral sources as well [260]. A review of the properties of inorganic sorbents applicable for the recovery of rubidium from hydromineral sources has been published [261]. Studies of rubidium recovery fix>m seawater [15, 19, 250-253] have shown that the final processing of rubidium concentrates, especially the selective separation of Rb -K mixtures remains the major problem. A report was recently published showing that this problem can be successfully solved by countercurrent ion exchange on phenolic resins [262]. 

Although ELM technique is quite efficient essentially due to the thinness of the membrane, large-scale application of this technique is limited in view of the difficulties encountered in the demulsification step needed for the recovery of the trapped metal ion. On the other hand, promise of the SLM technique has been demonstrated in the lab scale experiments. Large-scale applications of SLM require additional work in the area of stability/reusability of the membranes. Apart from the selective extraction, there is a need to develop the membranes that are compatible with the diluent/solvent mixture with respect to physical properties such as surface tension and viscosity. In addition, chemical/radiation environment of the feed/strip solution to which these membranes are subjected over long duration is an area of particular concern. Additional stability can be obtained by developing chemically grafted membranes. 

Metal recovery is feasil)lc when the rinse water streams are separated. This allows the regenerant effluents obtained from the ion exchange units to be appropriately mixed to reproduce the composition of the original bath. Another approach is to actually separate the heavy metal ions from other cations in the mixture, which necessitates a careful choice of resin. 

The separation of target metal ions from a complex mixture is an extremely important area of research for the purpose of recovery of metal values from wastes and for environmental remediation and restoration. Conventional approaches to metal-ion separation and recovery fall into two broad classes, namely (a) solid-liquid and (b) liquid-liquid separations. 

Several investigations were carried out to remove toxic heavy metal ions from waste water by biosorption. Microbial cells loaded with heavy metals were recovered by flotation, e.g. Streptomyces griseus and S clavuUgerus loaded with Pb [108] and Streptomyces pilosus loaded with Cd [109]. In these flotation processes the microbial cells were dead therefore, they are not considered here. The removal of pyritic sulfur from coal slurries such as coal/water mixtures by Thiobacillus ferrooxidans and recovery of this iron-oxidizing bacterium by flotation is a special technique in the presence of high concentrations of solid particles (see e.g. [110]). The flotation of colloid gas aphrons was used for the recovery of yeast in continuous operation [ 111 ] for the recovery of micro algae, and in the presence of flocculants in batch operation [112]. These special techniques are not discussed here. 

Practically, chelating polymers are very useful for selective absorption of certain metal ions from their mixtures and recovery of soluble metal ions from solutions. As a consequence, the metal ion remediation by the graft copolymer can be depicted as shown in Scheme 3.4. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

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