Weak base resins

Weak base resins are characterized by primary, secondary, or tertiary ami ne functional groups, or mixtures of these. In its basic form this resin can not work and must be protonated in order to recover the gold from a cyanide solution. The protonation can be represented as follow:

] - ⁺NR₂ + HX = ] - ⁺NR₂HX^-

A pKa value can be used to define the conditions at which 50% of the functional groups are protonated. In acidic solutions the equilibrium is shifted to the right and the resin behaves like a strong base anion exchanger. The adsorption of aurocyanide complex is given by;

] - ⁺NR₂HX^- + Au(CN)₂^- = ] - ⁺NR₂HAu(CN)₂^- + X^-

The loading capacity of weak base resins is approximately half than strong base resins under similar conditions. This capacity depends on the number of protonated functional groups per unit volume of resin and their degree of protonation, as well as the concentrations of gold and other competing species in solution. Consequently the capacity is strongly dependent on the pKa of the resin and solution pH as is shown in Figure 4. The higher resin pKa, the higher the pH at which optimum gold loading is achieved. For resin with pKa values between 8 and 9 maximum loadings are achieved in the range pH 6 to 8.

The selectivity of weak resins for gold and silver over metals is shown in Figure 5. Selectivity improves with increasing pH, however, both the capacity and adsorption rate are reduced markedly as the pH is increased above 8. weak base resins which have relatively low pKa values (range 6-8) are generally unsuitable for using in alkaline cyanide solutions, since the ideal pH conditions for cyanide leaching (pH 10-11) trend to strip metals values off the resin. Development work during the late 1980s has produced weak base resins with higher pKa values (8-12) which are more suitable for use in cyanidation systems.