Phosphazenes as Collective Extractants of Rare-Earth Elements
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ISTRY AND TECHNOLOGY OF RARE, TRACE, AND RADIOACTIVE ELEMENTS
Phosphazenes as Collective Extractants of Rare-Earth Elements M. D. Ryabochenkoa, K. V. Shumikhina, O. A. Sinegribovaa, *, and A. M. Chekmareva aMendeleev
Russian University of Chemical Technology, Moscow, 125047 Russia *e-mail: [email protected]
Received June 3, 2018; revised June 18, 2018; accepted June 24, 2018
Abstract—The extraction ability of some linear phosphazenes with respect to rare-earth elements (REEs) in a nitrate medium is studied. Some possible mechanisms of interaction between phosphazenes of different types and REEs at various acidities are discussed. It has been established that imido- and amidophosphazenes are very strong extractants of REEs within a broad range of nitric acid concentrations in the aqueous phase. It has been shown that phosphazenes can be efficient collective extractants of REEs. Keywords: phosphazenes, imide-amide rearrangement, rare-earth elements, extraction, complexation DOI: 10.1134/S004057951905021X
INTRODUCTION Phosphazenes represent a class of heterogeneous compounds with a cyclic or linear structure in which the main chain is composed of alternating nitrogen and phosphorus atoms, and the phosphorus atom is covalently bonded to the nitrogen atom by means of a double bond and other atoms or radicals by means of ordinary bonds. The phosphazenes containing hydroxyl substituents at the phosphorus atom are characterized by so-called phosphazene–phosphazane (imide–amide) rearrangement (PPR) HO P N
O P NH .
Organosubstituted derivatives of phosphazenes are obtained via the preliminary synthesis of chlorophosphazenes with the further nucleophilic substitution of halide atoms by organic elements [1]. Alkoxy phosphazenes containing linear alkoxy substituents also sustain PPR RO P N
O P NR .
In the synthesis of phosphazenes, the substitution of chlorine atoms by bulkier alkoxy substituents is complicated due to the steric factor. The hydrolysis of residual chlorine atoms in alkoxy phosphazene with the resulting formation of hydroxy (alkoxy) phosphazene groups also leads to PPR in the long run. The presence of bulky substituents at the phosphorus atom inhibits PPR. Being synthesized with 2-ethylhexyloxy radicals, phosphazenes get a certain number of OH groups due to their interaction with water in the course of synthesis, so the obtained product may have –P=O and –OH
groups at different ratios. In this work, the extraction properties were studied first and foremost for imidophosphazenes, which also have OH groups able to exchange a proton for an extracted metal ion in addition to Р=О groups. The most important characteristic of OH groups is the acid number (AN), which characterizes the number of gram equivalents of protons exchangeable for a metal ion in 1 L of an extractant and is determined by titration with an alkali solution. Despite a great number of studies, compounds of this class have a considerable untapped potential of application in different fields of science and engineering, particularly as hydrophobic
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