Mechanism of elution by thiocyanate

Thiocyan ate. The efficiency of thiocyanate for eluting gold and silver cyano complexes from strong base resins has been known for some time. However, due to its high affinity for the resin active groups, thiocyanate is difficult to remove once it has been absorbed. The mechanism of elution by thiocyanate is anion exchange displacement by mass action.

] - ⁺NR₃Au(CN)₂^- + SCN^- = ] - ⁺NR₃SCN^- + Au(CN)₂^-

A salt like thiocyanate is very soluble in water and at concentrations of 2 and 10M, the equilibrium lies to the right side. Provided that the gold cyanide is removed continuously from the recirculating eluate, total displacement of the gold from the resin can be done in few hours. The success of this method is based on the affinity of anion exchange resins for the highly polarized thiocyanate anion, and this has created the major obstacle to the implementation of the thiocyanate elution method. The drawback is the problem encountered in displacing the thiocyanate anion from the resin after elution. This is an important aspect for two reasons. [8] Firstly, the thiocyanate anion is not readily displaced by other anions and therefore, if the resin is recycled, directly to adsorption, its anion exchange capacity is reduced significantly. Secondly, it is important that a large percentage of the thiocyanate absorbed onto the resin during each elution cycle should be recovered and recycled to the next elution cycle. This can be done by using a solution with ferric ions to elute the thiocyanate ions.

] - ⁺NR₃SCN^- + Fe₂(SO₄)₃ = (] - ⁺NR₃)₂SO₄ + 2 (FeSCN)SO₄

Thiocyanate is recovered by the addition of sodium hydroxide or other suitable base. A proposed flowsheet is shown in Figure 7.

FeSCN² + 3OH- = SCN^- + Fe(OH)₃