Ion exchange technology Process

Ion ex change steps.: Ion exchange technology adapted to the treatment of aurocyanide solutions comprises three steps: loading, elution, and recovery.

Loading: Consists in employ an anionic resin for recovering the gold from a pregnant solution obtained in a cyanidation process. The solution can be passed through a resin bed or the extraction can be done directly from cyanided slurry. The latter case receives the name of Resin-in-Pulp (RIP).

Ion exchange resins have higher loading capacities than activated carbon and can be employed in the treatment of diluted solutions previously clarified such as liquors obtained in a heap leaching process.

The equipment to be used in this operation includes fluidized columns or agitated tanks.

Resins have different loading capacity; some capacities are shown in table 2.

Table 2. Adsorption capacities of some ion exchange resins for gold. [6]

Resin Alkalinity Capacity (kg Au/m3) Amberlite IRA 400 Strong 7.0 Amberlite IRA 401 Strong 4.2 Dowex 1 Strong 5.2 Dowex 2 Strong 5.0 AV-17 Strong 7.4 Vlanlt AT-1 Strong 8.8 Myklon PA Strong 6.9 Wofallt L-150 Weak 1.4 AN-18 Weak 5.9 AE-1A Weak 1.7 Zerolit G Weak 3.2

During the cyanidation process, gold is leached and forms a complex ion (Au(CN)₂^-) which is extracted from the solution by ion exchange. The presence of other functional groups affect adsorption efficiency. Also, there is a competition among several cyano complexes of copper, nickel, and zinc, against gold and silver, which affects the resin loading capacity.

Loading capacity can be affected by the presence of cations (i.e. Cu, Zn, Ni, Co) and anions (i.e. carbonate, chloride, sulphate) which trend to displace precious metals.

A strong base resin contains quaternary amine functional groups and extracts aurocyanide according to the equation:

] - ⁺NR₃HX^- + Au(CN)₂^- = ] - ⁺NR₃Au(CN)₂^- + X^-

 A weak base resin can contain tertiary amine functional groups and in the free form the resin is uncharged. Then it is necessary for the resin to be protonated prior to the extraction of aurocyanide.

pH < pKa

] - ⁺NR₂ + HX = ] - ⁺NR₂HX^-

pH > pKa

The protonation of the resin occurs at pH values below the pKa of the tertiary amine functional group and, therefore, before a weak base resin can be used in a RIP process, it is necessary for the ph value of the pulp to be reduced to below the pKa of the resin. It has been shown that the best absorption capacities are at pH 7 and 9. This is sufficiently for no problems to arise with the evolution of hydrogen cyanide, and the favorable economic on elution more than compensate for the acid consumption in reducing pH of the slurry to 9.