Research project title : Development of Micro Reaction System for On-site Separation of Rare Metals with Host Compounds

Metal recycle from spent small home appliance products is important to reserve metal resources with increased demand for advanced materials. However, since they contain a variety of different elements in trace or slight amounts, individual separation is very difficult. Solvent extraction technique is one of the most effective processes. The role of solvent extraction reagents is predominant to improve extraction and separation efficiencies. Although various kinds of the extraction reagents have been prepared and examined, only a few extraction reagents have been found that satisfies the complete mutual separation of such elements. Nobel reagents, e.g. three-dimensionally designed extraction reagents with high discriminating property for certain target elements have been required for this purpose.

Calixarenes are macrocyclic oligomers that provide three-dimensional coordination sites. The discriminating ability for a certain metal ion is one of the remarkable features. Although calixarene derivatives are suitable to be based on such reagents, they have some serious drawbacks as solvent extraction reagents for commercial applications, such as poor solubility in organic diluents, slow kinetics, and high price.

Microreactor liquid-liquid extraction system was focused on to accomplish the complete mutual separation. It would contribute to many advantages, such as micro system for reduced amounts of extraction reagent and solvent employed, save energy and space to set up, extremely larger interfacial area for fast extraction rate. Most of them can improve the drawbacks observed when the calixarene derivatives employed for the metal extraction.

In this project, eight derivatives of cone conformational p-t-octylcalix[4]arene were successfully prepared. From the results of batch-wise extraction of precious metals with derivatives, it was found that each derivative exhibits its own selectivity for precious metals in nitric acid media as follows;

Methyl ketonic derivative ( $ ^{t}Oct[4]CH_{2}COCH_{3} $ ): Ag > Pd(II)

Phenyl ketonic derivative (tOct[4]CH2COPh) : Pd(II) > Pt(IV) > Ag (Ag > Pt(IV) at high nitric acid conc.)

2-Pyridyl derivative (tOct[4]CH22Py) : Pd(II) > Ag ≈ Pt(IV)

2-Quinolyl derivative (tOct[4]CH22Qu) : Pd(II) > Ag

Diethylamido derivative ( $ ^{t}Oct[4]CH_{2}CONEt_{2} $ ): Ag > Pd(II)

Dibenzylamido derivative ( $ ^{t}Oct[4]CH_{2}CONBz_{2} $ ): Ag > Pd(II).

Based on this result together with that of stripping, stepwise recovery of precious metals from the real waste solution by batch-wise method were investigated. Although a problem on stripping still remained, selected extractants exhibited high selectivity for a certain precious metal.

Phosphonic derivative of cone conformational p-t-octylcalix[4]arene exhibited extremely high extraction ability but just poor selectivity. Dipropyl-diacetic acid derivative exhibited specific stoichiometry for alkali metal ions based on size-discriminating property, although it showed poor selectivity.

From the relationship between the results for extraction rate and interfacial activity of the extractants, the extraction rate is significantly affected by interfacial tension, that is, the adsorption rate of the extractants on the interface. The interfacial tension is related to the structures of the extractants, that is, polar functional groups.

Two types of microreactor systems, that is parallel two phase flow type and slag flow type of microreactor system, have been designed and prepared for precious metal recovery. The silver extraction with methyl ketonic derivative was drastically improved from 72 h by batch-wise method to 15 s and 4 s by two microreactor systems. Such drastic improvement of extraction rate was contributed by the increase of interfacial area using microreactor system. The microreactor system also accelerated the stripping rate of the loaded silver. Finally, stepwise recovery of precious metals from the real waste solution by microreactor system were investigated. Although there are some problems to be optimized, selected extractants exhibited high selectivity for a certain precious metal. It was found that microreactor systems can contribute to the stepwise recovery of precious metals from the real waste solution.

By preparation of adequate extraction reagents and finding the most optimal stripping reagents, the stepwise recovery not only for precious metals but also other valuable and toxic metals will be successfully carried out by using microreactor systems with the reduced amounts of the reagents and diluents, together with the reduced energy and space.