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4 MICHAEL COX

with extensive recycling, some impurities do build up in the process streams, so it is usual to incorporate purification procedures into bleed streams off the main process stream.

Although it is not intended to cover exhaustively the theory of liąuid-liquid extraction eąuilibria and kinetics in this chapter, to appreciate the criteria involved in the design and selection of extraction reagents for hydro-metallurgical processes, it is necessary to outline the underlying principles.

As stated earlier, elements can exist in aqueous solution in a number of different forms, which may be charged, for example as aqueous cations or chloroanions, or uncharged, as in some aquated complex species such as U0₂(N0₃)_2aq. Ali these species have little or no tendency to transfer to a non-polar organie phase as used in liquid-liquid extraction. Thus, to achieve the desired extraction, the aqueous species have to be modified in some way to make them compatible with the organie phase. This modification can be carried out by neutralization of any ionic charge and/or replacement of water of hydration by other coordinating species. These requirements deter-mine the naturę and modę of operation of the organie extractants used in hydrometallurgy, which can be conveniently divided into the following three classes.

1.    Acidic extractants which possess ionizable hydrogen atoms that can be replaced by metal ions to give an overall neutral species (eqn(l.l))¹;

M_aq + nRH ^ MR„ + nH_a⁺q.    (1.1)

2.    Basic, or anion exchangers, which are normally the protonated form of high molecular weight amines or quaternary compounds. These form ion-pairs with anionie species, thus reducing the effective ionic charge of the lat ter (eqn(1.2»;

(R₃NH⁺)_p_„ (MXjf“ⁿ⁾ ~) + H₂0.    (1.2)

This equation also represents an ion association reaction, hence the alter-native names of ion-association or ion-pair extractants.

3.    Solvating extractants, which compete with water for the first solvation shell of the metal atom. The replacement of water molecules by these reagents, which often contain donor oxygen atoms, facilitates the transfer of a metal-containing complex into a nón-polar organie phase (eqn(1.3))

MX_p,_aq + gś ^ MX_PS, + H₂0.    (1.3)

Ali the above reactions are equilibria so the essential requirement that the extraction process is easily reversible is obscrved. Thus, with acidic extract-ants, change of acidity will cause the equilibrium to be shifted, whereas strip-ping for the other two extractants may be carried out with water. Before

1

In this and all subseąuent extraction equations the barred species occur in the organie phase.