For weakly alkaline gold- loaded resins, the best method is to use a sodium hydroxide solution with a pH of 13 at normal temperature and pressure for good desorption; for strong alkaline gold-loaded resins, there are more desorbents and sodium hypochlorite. The method, the acid thiourea method, the zinc cyanide complex method and the thiocyanate method are relatively simple and inexpensive, and the zinc cyanide complex method.
The zinc cyanide complex method utilizes zinc cyanide ion Zn(CN) 4 2- to displace the adsorbed gold and silver cyanide complex ions from the resin. The method is as follows: a saturated solution of sodium zinc cyanide is passed through a desorption column containing a gold-loaded resin to desorb the gold from the resin into the solution, and then the gold cyanide solution is passed through the zinc powder bed, so that the gold is replaced by zinc. The gold mud, which is formed on the zinc cyanide, can be returned to the desorption tower.
The regeneration of the resin is also relatively simple, and the zinc cyanide ion can be easily decomposed by the weakly acidic solution, so that the zinc ion and the cyanide are removed from the resin. Further, it is treated with a dilute NaOH solution to remove insoluble matter such as silicate on the resin, and after the regeneration treatment, the resin can be returned to the adsorption operation.
The disadvantage of using the zinc cyanide ion method is that the solubility of zinc cyanide is low. This problem can be solved by dissolving zinc sulfate heptahydrate in sodium cyanide solution. The molar ratio of cyanide to zinc in the solution can be 4.5:1 and the zinc concentration can be as high as 50 g/l. In order to avoid Na 2 [Zn(CN) 4 ] crystallization, for various solutions having a zinc concentration of more than 0.5 mole, the temperature is required to be controlled at about 50 °C.
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