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The basic principle of zinc replacement of precipitated gold is that the zinc in the noble solution will dissolve in the solution to precipitate the gold. The reaction formula of the ion Au(CN) 2- and Zn in the noble solution is usually written as:
2KAu(CN) 2 +3Zn+4KCN+2H 2 0 â†â†’ 2Au↓+2K 2 Zn(CN) 4 +K 2 Zn0 2 +H 2 ↑
There must be enough cyanide and alkali in the solution when the zinc is replaced. Otherwise, the dissolved oxygen in the gold-containing solution will dissolve the precipitated gold powder and oxidize the zinc to Zn(OH) 2 precipitation:
1
Zn+——O 2 +H 2 O â†â†’ Zn(OH) 2 ↓
2
Also, K 2 Zn(CN) 4 in the solution will decompose into an insoluble zinc cyanide precipitate:
K 2 Zn(CN) 4 +Zn(OH) 4 â†â†’2Zn(CN) 2 ↓+2KOH
These zinc hydroxides and zinc cyanide are white precipitates that form a film on the surface of the metal zinc, which prevents the displacement of zinc from the precious liquid. Therefore, the addition of a small amount of lead acetate and lead nitrate to the sedimentation tank contributes to the dissolution of zinc and better replacement of precipitated gold. The inclusion of soluble sulfides, mercury , copper and other impurities in the noble liquid hinder the replacement precipitation of gold.
What are the methods for extracting gold from cyanide leachate?
There are many methods for extracting gold from cyanide leaching solution. If the carbon slurry method (CIP), carbon immersion method (CIC), magnetic carbon method (MCIP) or resin exchange method can be used, the solid-liquid separation operation can be performed. Generally, the cyanide slurry is separated by solid-liquid to obtain a noble liquid (gold-containing solution). From the noble liquid, the method of extracting gold is a zinc displacement precipitation method, an activated carbon adsorption method, an ion exchange resin adsorption method or an electrolytic deposition method. It is a common method to replace gold with a metal zinc wire or zinc powder from a precious liquid. The precious liquid is clarified to remove the slime and suspended matter before entering the displacement precipitation operation, since these impurities are harmful to the next replacement precipitation operation.