The nature of the leaching solution refers to the concentration of cyanide in the leaching solution, the concentration of oxygen, and the pH of the leaching solution.
1. Cyanide concentration The cyanide leaching reaction is a reaction controlled by cyanide diffusion and then controlled by diffusion of oxygen. Therefore, the size of the cyanide diffusion flow directly affects the rate of the leaching reaction. In particular, the concentration of cyanide ions in the solution is relatively low, and the influence of cyanide on the reaction is more obvious, and the diffusion flow of cyanide ions increases as the concentration thereof increases. Therefore, the leaching speed of gold rises linearly as the cyanide concentration increases.
When other conditions are the same, a similar leaching rate of gold is achieved, and the leaching time required to use a 0.1% sodium cyanide solution is one quarter of the leaching time required to use a 0.025% sodium cyanide solution (see The following figure). In the process of heap leaching and gold extraction, the concentration of sodium cyanide in the leaching solution is generally controlled to be 0.1 to 0.03%.
2. Oxidation concentration From the dissolution mechanism of gold, the reaction rate is controlled by diffusion, and the concentration of oxygen is undoubtedly an important factor. Since the magnitude of the oxygen concentration also determines the size of the oxidized diffusion stream, especially when the cyanide concentration is sufficiently high, the rate of dissolution of gold depends on the concentration of oxygen. However, simply increasing the concentration of cyanide or oxygen does not achieve the desired leaching speed. In order to maximize the leaching speed, it is important to keep the molar ratio of free cyanide to oxygen in the leaching solution equal to 6. In heap leaching, it is difficult to use natural ventilation to ensure this condition, but create conditions to keep the mine well ventilated and the leaching solution is in full contact with the air during spraying to ensure that there is enough oxygen to participate in the reaction. Conducive to the leaching of gold.
3. pH The pH value of the leaching solution has an influence on the thermodynamics of the leaching, that is, the reduction potential of gold increases with the increase of pH value, which is beneficial to the leaching of gold. More importantly, it is beneficial for cyanide to undergo hydrolysis reaction in aqueous solution to produce HCN gas:
CN-+H 2 O→HCN↑+OH -
This pollutes the surrounding environment and consumes a lot of cyanide. It is therefore necessary to keep the solution alkaline. However, if the alkalinity is too high, the dissolution rate of gold will be lowered, which is disadvantageous for leaching. Therefore, the leaching solution should be kept alkaline in the leaching process to obtain a higher leaching speed. Lithium or sodium hydroxide is commonly used in production to control the pH of the leaching solution between 10 and 11.
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