白金族金属分離のための新規抽出剤開発

Abstract

This review describes the knowledge required for the development of new extractants for the platinum group metals (PGMs), along with the properties of the conventional and new PGM extractants. Understanding the extraction mechanism of the PGMs is absolutely essential for accelerating the development of new extractants. The structural information of the PGM complexes both in an aqueous chloride solution and organic solvent is useful to clarify the extraction mechanism, where recognizing the difference between the inner and outer coordination spheres of the PGM ion is needed. During the PGM extraction from acidic chloride media, the extraction reaction can be divided into two broad categories: coordinative and ion-pair. The coordinative and ion-pair extractions mainly occur in the inner and outer coordination spheres, respectively. Moreover, there are various important factors governing the extraction behavior: the HSAB rule, chelate or size recognition effect, synergistic effect, interfacial activity, hydrophobicity of the extractants, etc. As for the conventional extractants, the extraction properties of palladium, platinum and rhodium with commercially available extractants are described. Among the recent studies of the PGM extraction, the size recognition effect in the outer coordination sphere is featured. Our recent work regarding the new extractants for palladium and rhodium is outlined. The thiodiglycolamide compounds can selectively extract Pd(II) much faster and have a higher oxidation resistance than the conventional extractant, di-n-hexyl sulfide; the tertiary amine compounds containing N,N-disubstituted amide groups can extract about 80% of rhodium at 1 to 2 mol/L hydrochloric acid. This value is the highest ever obtained in rhodium extraction from relatively highly concentrated hydrochloric acid.