scan0023 3

10 MICHAEL COX

have an appreciable aqueous solubility, which can, under operating condi-tions, become too high for economic and environmental considerations. The organophosphorus acids however are used commercially in a number of pro-cesses. Their use is generally confined to the monobasic ester derivatives as. aithough some dibasic reagents are available, the formation of polymeric metal complexes with the latter causes solubility problems. The most widel> used reagent is di-2-ethylhexylphosphoric acid, and its Chemical stabilny. Iow aąueous solubility, and good loading and stripping characteristics make it one of the most versatile extractants. It has been shown to exist largely as dimers in organie diluents leading to the formation of polymeric meta species at Iow metal loadings (eqn(l.ll))

(1.11)

Mⁿ⁺ + n{RH)₂ M(R-HR)„ + wH\

However, as metal loading inereases these polymers dissociate,giving species such as MR„, aithough these too may be polymeric with bridging oxyger. atoms (Kolarik and Grimm 1976). Recently interest has been revived in the commercial use of organophosphonic and organophosphinic extractan:s (Table 1.1), especially for the separation of cobalt and nickel (see laten. These reagents also exhibit interesting variations in the selectivity series for zinc and the alkaline earth elements (Table 1.2), which would allow zinc tc be extracted at a lower pH than normally possible with DEHPA and with a greater selectivity over calcium. Also for Cyanex 272 cobalt can be ex-tracted at a lower pH than calcium which has advantages in eliminating any possibility of precipitating gypsum during acid stripping of a cobalt-loadeć organie phase (Rickelton et al. 1984.) The irregular behaviour of calciurr. when compared with the other elements may be attributed to specific inter-actions between the metal and extractant.

Recently commercial reagents based on thiophosphinic acids have becorm available. This type of extractant has been studied for the extraction o second and third row elements for several years (Sekine and Hasagawa 19"" Cote and Bauer 1986) but have only just excited commercial interes: American Cyanamid have now produced two such reagents, Cyanex 30: a dialkyl dithiophosphinic acid, and Cyanex 302, the dialkyl monothi analogue. Cyanex 301 ha$ been developed as an extractant for zinc in th presence of calcium (Brown et al. 1987) and this type of compound can ais

Table 1.2 Selectivity series for organophosphorus acid extractams (Co

and Flett 1987)

Phosphoric acid (DEHPA) Phosphonic acid (PC88A) Phosphinic acid (Cyanex 272)

Zn > Ca > Cu > Mg > Co > > Zn > Cu > Ca > Co > Mg > ^ Zn > Cu > Co > Mg > Ca > l