The principle of gold deposition by zinc replacement
2020-11-11 12:01Introduction：The principle of gold deposition by zinc replacement Metal replacement from pregnantsolutioncontaining gold cyanide gold precipitation can use metal zinc or metal aluminum as replacement agent.Zinc replacement deposition is often used in p
The principle of gold deposition by zinc replacement
Metal replacement from pregnant solution containing gold cyanide gold precipitation can use metal zinc or metal aluminum as replacement agent.Zinc replacement deposition is often used in production, while aluminum replacement deposition is rarely used.Zinc silk or zinc powder can be used as replacement agent when zinc replacement deposition.
According to the theory of electrode reaction kinetics, the electrochemical reactions of conjugate cathode and anode are performed on any metal surface in contact with the electrolyte solution.When the more electronegative metal and the more electronegative metal ion solution contact, the ion exchange between the metal surface and the solution will be generated immediately.The electron will flow from the replacement metal (anode area) to the replaced metal (cathode area). The anode area is where the replacement metal is oxidized, making the replacement metal ionized into the solution.As the potential of metallic zinc is -0.763 volts, which is more negative than the potential of cyanocyanide anion (-0.60 volts), metallic zinc can replace gold from the cyanide pregnant solution.
The anode zone Zn-2e→Zn2+
The cathode zone Au(CN)2-+e→Au+2CN-
The total reaction can be expressed as:
2Au(CN)2-+Zn→2Au↓+Zn(CN)42-
It can be seen from above that the impetus of metal zinc to exchange gold from gold containing cyanide liquor is great, and the replacement deposition rate is quite high.The main factors influencing the gold precipitation of metal zinc replacement include oxygen content, pH value, gold content, temperature, cyanide content, mercury, copper, lead, soluble sulfide content, etc.
Zinc hydroxide precipitates in cyanide solution.When the cyanide concentration and alkali concentration in the cyanogen solution are small, the oxygen in the solution can also make the gold deposition dissolved, and the zinc cyanogen ions generated when the zinc is dissolved in the cyanation solution are also dissolved and precipitated in the form of zinc cyanide precipitation.The reaction can be expressed as:
Zn+1/2O2+H2O→Zn(OH)2↓
4Au+O2+8CN-+2H2O→4Au(CN)2-+4OH-
Na2Zn(CN)4+Zn(OH)2→2Zn(CN)2↓+2NaOH
The reaction leads to the formation of zinc hydroxide and zinc cyanide films on the surface of metallic zinc.Therefore, the dissolved oxygen in the cyanide solution not only increases the consumption of metal zinc, but also reduces the gold deposition efficiency and the gold content of gold mud.In production, before replacement, pregnant solution is deoxygenated by deoxygenation tower to remove or reduce harmful effects of dissolved oxygen.
The pH (alkalinity) of the cyanide solution should generally be kept around 10.When the alkalinity of cyanide solution is low, it is easy to produce zinc hydroxide and zinc cyanide film on the surface of metallic zinc, which hinders gold and silver precipitation.Generally, the alkalinity of cyanide solution should be controlled from 0.03% to 0.05% CaO.
When metal zinc displaces gold, the recovery of gold is closely related to the content of gold in cyanide solution.As the gold content in the gold removal solution (barren solution) after zinc replacement is almost constant, the recovery rate of gold increases with the increase of gold content in the cyanidation solution (pregnant solution).When the gold content in the cyanide solution is too low, the zinc replacement method should not be used directly to precipitate gold, and the activated carbon or ion-exchange resin should be used in advance after adsorption and enrichment, followed by zinc replacement method or electrodeposition method to recover the gold in the pregnant solution
