Cyanide gold extraction is one of the main methods for extracting gold from gold ore. The explanation of the mechanism of cyanide on the dissolution of gold is still inconsistent. Most people think that gold can form a gold complex and dissolve in the presence of oxygen in the cyanide solution. The basic reaction formula is:
4Au+8KCN+2H2O+O2→4KAu(CN)2+4KOH
It is generally believed that gold is cyanide dissolved in a two-step reaction:
2Au+4KCN+O2+2H2O→2Au(CN)2+H2O+2KOH
2Au+4KCN+H2O2→2KAu(CN)2+2KOH
The surface of the gold is gradually dissolved from the surface and the inside in the cyanide solution. The concentration of oxygen in the solution is related to the rate of dissolution of gold.
The factors affecting the leaching rate of gold during cyanidation are: the concentration of cyanide and oxygen, the pH value of the pulp, the raw material properties of the gold mineral, the leaching temperature, the slime content, the pulp concentration and the leaching time.
The concentration of cyanide during leaching is generally 0.03% to 0.08%, and the dissolution rate of gold rises linearly to the maximum with increasing cyanide concentration. Then slowly rise, when the cyanide concentration reaches 0.15%, the dissolution rate of gold is independent of cyanide concentration, and even decreases (due to cyanide hydrolysis).
The dissolution rate of gold increases with increasing oxygen concentration, and the dissolution of gold can be enhanced by an oxygen-enriched solution or high-pressure aeration cyanidation.
Ability to dissolve gold and silver cyanide as reagent: ammonium cyanide> calcium cyanide> sodium> potassium cyanide. The price of potassium cyanide is the most expensive. Most of the current use of sodium cyanide, the consumption of cyanide depends on the nature of the material and the operating factors, often 20 to 200 times the theoretical amount.
The nature of the material affects the leaching rate of gold. Although cyanidation is the main method of extracting gold, gold-bearing ore, but some material not directly employed cyanidation process, if the copper in the ore, arsenic, antimony, tin, sulfur, phosphorus, a magnetic iron ore, iron ore and other white High levels of components will greatly increase cyanide consumption or consume oxygen in the slurry. When the leaching rate of gold is lowered, when carbon is high in the ore, carbon will adsorb dissolved gold and lose with tailings. Pre-oxidation roasting or flotation methods remove the effects of harmful impurities.
The hydrolysis reaction of cyanide is: KCN+H2Oâ†â†’KOH+HCN↑, so toxic HCN is volatilized; lime is added to weaken the cyanide hydrolysis, and the above reaction proceeds to the left to reduce the chemical loss of cyanide. Lime also neutralizes the action of acid substances and can precipitate harmful ions in the slurry, so that the best conditions for the dissolution of gold are dried. Lime is often used as a protective base. The amount of lime added makes the pH value of the slurry reach 11~12, and the pH value of the pulp is too high, which is unfavorable for the gold.
The size of the gold particles mainly affects the cyanide time, and the dissolution rate of the coarse gold (>74 microns) becomes slower. Therefore, it is reasonable to pre-recover coarse gold by amalgamation , re-election or flotation before cyanidation. Dissociation of fine gold particles from the monomer during the grinding process is still a more important factor in increasing the leaching rate of gold.
The clay content and slurry concentration during cyanidation directly affect the diffusion rate of the components. The pulp concentration should be less than 30~33%. The concentration of slurry in the mud should be less than 22~25%, but the concentration should be too low, otherwise the consumption of cyanide will increase.
The cyanide time varies depending on the nature of the material, the cyanidation mode and the cyanide conditions. Generally, the stirring cyanide leaching is often more than 24 hours, sometimes as long as 40 hours or more, 72 hours for cyanidation gold, and 5 days for diafiltration cyanide leaching.
List of 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.
The basic principle of zinc replacement of precipitated gold is that 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+2H20 â†â†’ 2Au↓+2K2Zn(CN)4+K2Zn02+H2↑
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:
Zn+1/2O2+H2O â†â†’ Zn(OH)2↓
Also, K2Zn(CN)4 in the solution will decompose into an insoluble zinc cyanide precipitate:
K2Zn(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.
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