Organophosphorus acids rare earths

The use of organophosphorus acids, such as di(2-ethylhexyl)phosphoric acid (D2EHPA di(2-ethylhexyl) monohydrogen phosphate 2 R = C4H9CH(Et)CH2), is now well established in the recovery of base metals. This reagent has found commercial application in the separation of cobalt from nickel,67 68 the separation of zinc from impurities such as copper and cadmium,69 the recovery of uranium,68 beryllium70 and vanadium,71 and in separations involving yttrium and the rare-earth metals.72 73... 

Other commercial operations known to have used organophosphorus acids for the recovery of rare earths are those of Thorium Ltd. in England,39the Megon Company in Norway,73-95 and Denison Mines in Canada.96,97... 

The lanthanides form many compounds with organic ligands. Some of these compounds are water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating AGENTS). The complex formation is pH-dependent. Oil-soluble compounds are used extensively in the industrial separation of rare earths by liquid—liquid extraction. The preferred extractants are carboxylic acids, organophosphorus acids and esters, and tetraalkylammonium salts. 

In Table 1, typical extracting reagents used for separation and enrichment of inorganic elements are summarized. Organophosphorus extractants are often used because of their solubility properties. Di(2-ethylhexyl) phosphoric acid is commonly applied to industrial separations because of its high extractability and high separation factors between many inorganic elements, especially for rare earth elements. Other metal ions are extracted as well as the trivalent metal ions. 

The purpose of the present work consists in the investigation of polydentate neutral organophosphorus compounds as extractants for isolation and concentration of TPE from acid solutions and quaternary ammonium bases and alkylpyrocatechols for isolation and separation of TPE and rare earth elements from alkaline solutions. 

Preconcentration by an acidic organophosphorus extractant is also effective, though the lanthanides are extracted from dilute acid solutions (10 -10 M) and stripped into acidic (>1M) solutions when HDEHP is employed as the extractant. A representative application of liquid-liquid extraction to rare-earth preconcentration is the 100 to 200-fold concentration of the rare earths in seawater prior to ICP/MS determination using a mixture of HDEHP and mono-(2-ethylhexyl)phosphoric acids (Shabani et al. 1990, Shabani and Masuda 1991). When the same extractant system was adsorbed on a Cig cartridge and used in an extraction chromatographic mode, the rare earths were concentrated 200-1000-fold (Shabani et al. 1992). 

Preston, J. S. and du Preez, A. C. 1988. The solvent extraction of cobalt, nickel, zinc, copper, calcium, magnesium, and the rare earth metals by organophosphorus acids. Mintek Report M378. 

---