Strong base resins

Th e functional groups of strong base resins are quaternary, and occasionally tertiary, amines which have a permanent positive charge in aqueous solution. The adsorption of aurocyanide complex is represented as follows:

] - ⁺NR₃ X^- + Au(CN)₂^- = ] - ⁺NR₃Au(CN)₂^- + X^-

Where ] – represents the inert part of the resin (matrix) and X- is an anion such as sulphate or bisulphate, depending on the elution and regeneration methods used. The resin gold loading capacity depends on the resin structure, the concentration of functional groups present, the concentration of various ions in solution and their properties. Gold loadings above 100 g/l resin can be achieved when there are little or not competing ions present in solution, however the capacity decreases with the increasing ionic strength and with increasing temperature. The effect of temperature in resin performance is similar, though less significant (particularly loading capacity) than its effect on metal adsorption by activated carbon. See Figures 2 and 3.

Strong base resins also readily adsorb silver, nickel, cobalt, copper, zinc, Fe (+2), and Fe (+3) cyanide complexes and consequently show poor selectivity for gold. Usually copper, nickel and zinc are absorbed more readily than gold and silver, and consequently must be extracted almost completely onto the resin before the precious metal can be effectively recovered. The selectively of adsorption of the different metal cyanides is pH dependant. At low pH, metal cyanide salts are precipitated which reduces competition for resin adsorption sites. Selective precipitation of base metal prior to precious metal extraction by strong base resins is an option.

The rate of gold adsorption is first order respect to gold activity in solution at low resin loadings (approximately 50 g/l) and for solution gold concentrations up to approximately 40 ppm. The adsorption rate decreases as the resins becomes loaded, is largely unaffected by pH between pH 2 and 12, increases with increasing temperature, and increases with increasing agitation up a limiting level.

The activation energy for gold adsorption onto a strong base resin has been estimated at 16.5 kJ/mol, which is within the range of mass transport control. The rate controlling step is either diffusion across the resin solution layer or diffusion along resin pores. Pore diffusion is strongly affected by solution pH and ion strength, and is favoured by high pH and low ionic strength. The layer diffusion significantly affects the rate of gold extraction under most conditions encountered in commercial RIP circuits, and hence the rate maximized by good mixing