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26

MICHAEL COX

di-2-ethylhexylphosphoric acid. The presence of a third phase causes con-siderable Processing difficufties and so, to minimize any incipient formation, Chemicals are added as modifiers tą the organie phase. These organie com-pounds tend to consist of oxygen-containing molecules such as alcohois, phenols and ethers. They should be chosen to have a high organie phase solubility, Iow aqueous solubility and Iow cost; those most commonly used includenonyl phenol, isodecanol, 2-ethylhexanoI, and tri-n-butylphosphate. In addition to inereasing the solubility of the metal-extractant complex modifiers have also been shown to affect other aspects of reagent performance, for exampie reagent strength, selectivity, kinetics, degradation, entrainment, phase separation, and crud control.

One of the most intensive studies of the effect of modifiers on reagent performance has centred around their use with the copper hydroxypxime extractants, in particular PSO. The incorporation of various modifiers has lead to the formulation of the P5000 series (Tumilty et al. 1977a,b) and the M-series of reagents (Dalton and Seward 1986). Nonyl phenol was originally added to the 5-nonylsalicylaldoxime (P50) as a means of modifing its strip-ping performance so that the drcuit could interface with normal spent copper tankhouse electrolyte (see Copper). It had little effect on the extrac-tion circuit so the modifier gave better overall transfer efficiency than the unmodified aldoxime. The incorporation of a different modifier, tridecanol, indicated that the choice of modifier can effect siibtle chańges to other pro-perties, some of which are not yet understood. Thus copper/iron(111) selec-tivity was shown to be better with alcohois tharf' nonyl phenol and than certain esters were even better (Table 1.6).

The PSO reagent is known to exhibit fast kinetics and these are little changed by addition of modifiers at room temperaturę. However, at Iow temperaturę, (0°Q, PS 100 has a much poorer kinetics than PT5050, the

Table 1.6 Comparison of modifier performance for copper/iron(III) seleclivity of P50 (Dalton and Seward 1986)

Cu/Fe Ratio in extract.

Modifier	Reagant	feed A	feed B
Nonyl phenol	P5100	1859	386
alcohol (unspec.)	M5615	6024	976
ester (unspec.)	M5640	7435	1210

feed A = 3 g/dm³ Cu(II); 3 g/dm⁵ Fe(III) pH 2.0 feed B = 3 g/dm³ Cu(II); 30 g/dm³ Fe(III) pH 2.0