Factors Affecting the Efficiency of Gold Ore Leaching with Sodium Cyanide

In the gold mining industry, the Cyanidation process using Sodium cyanide is widely employed to extract gold from ores. However, the efficiency of this process can be influenced by numerous factors. Understanding these factors is crucial for optimizing the gold extraction process, improving recovery rates, and reducing operational costs. This article delves into the key factors that affect the efficiency of Gold ore leaching with Sodium Cyanide.

Ore Characteristics

Mineral Composition

The mineral composition of gold ores plays a significant role in the cyanidation process. Some minerals can have a detrimental effect on gold leaching. For example, copper, arsenic, antimony, and bismuth present in the ore can increase the consumption of cyanide or deplete the oxygen in the slurry, thereby reducing the leaching rate of gold. When copper minerals are present, copper can react with cyanide to form copper-cyanide complexes, consuming a large amount of cyanide. In the case of arsenic-bearing minerals, they can oxidize in the cyanide solution, consuming oxygen and forming arsenic compounds that may coat the surface of gold particles, hindering the contact between gold and cyanide. Additionally, if the ore contains a high carbon content, carbon can adsorb the dissolved gold, leading to gold losses in the tailings. To mitigate these issues, pre-treatment methods such as roasting or flotation can be used to remove or reduce the impact of these harmful impurities.

Gold Particle Size

The size of gold particles directly affects the leaching time and efficiency. Coarse gold particles (larger than 74μm) have a slower dissolution rate due to their smaller surface area available for reaction with cyanide. In the cyanidation process, it is essential to ensure that the gold particles are sufficiently liberated from the gangue minerals. Grinding the ore to an appropriate fineness is crucial to achieve this. By reducing the particle size, more gold surfaces are exposed, facilitating the reaction with cyanide. However, over-grinding should be avoided as it can lead to increased costs, such as higher energy consumption and wear of grinding equipment. Moreover, over-grinding may cause the release of fine gangue minerals that can interfere with the leaching process or increase the difficulty of solid-liquid separation. For ores with fine-grained gold, achieving a suitable grinding fineness, typically with a high percentage of particles below a certain size (e.g., -38μm), can significantly improve the leaching effect.

Ore Structure and Texture

The internal structure and texture of the ore can also impact the cyanidation process. Ores with complex structures, such as those with fine inclusions or encapsulated gold, may require more intensive grinding or additional pre-treatment steps to expose the gold for leaching. Porous ores can allow the cyanide solution to penetrate more easily, enhancing the leaching efficiency. On the other hand, dense or compact ores may limit the diffusion of cyanide and oxygen, resulting in slower leaching rates. Understanding the ore structure through techniques like microscopy can help in designing more effective leaching strategies.

Leaching Conditions

Cyanide Concentration

The concentration of sodium cyanide in the leaching solution is a critical factor. The dissolution rate of gold initially increases linearly with the increase in Cyanide concentration until it reaches a peak value. Beyond a certain concentration, further increases in cyanide may not significantly improve the gold dissolution rate and may even lead to a decrease. Typically, in gold cyanidation, the cyanide content in the solution is maintained in the range of 0.03% - 0.08%. When the cyanide concentration is too low, the gold leaching effect is poor, and the leaching speed is slow, resulting in longer leaching times and increased costs. Conversely, an excessive amount of cyanide not only causes waste but also increases the environmental risk associated with cyanide handling and disposal. Therefore, determining the optimal cyanide concentration based on the specific ore properties is essential for efficient gold extraction.

Oxygen Concentration

Oxygen is necessary for the oxidation of gold in the cyanidation process. The dissolution rate of gold increases with the increase in oxygen concentration. In most cyanidation plants, air is commonly used as the source of oxygen. By enriching the oxygen in the solution or using high-pressure aeration cyanidation, the dissolution of gold can be enhanced. However, as the temperature rises, the solubility of oxygen in the solution decreases significantly. At 100°C, the oxygen solubility drops to zero, which halts the leaching process. Therefore, maintaining an appropriate oxygen concentration in the leaching slurry, considering factors such as temperature and agitation, is crucial for ensuring efficient gold leaching.

pH Value

Maintaining the correct pH value in the leaching pulp is vital for the cyanidation process. In industrial production, the pH value of the pulp is usually kept between 10.0 - 11.0. Lime is often added to the cyanide solution to act as a protective alkali. It helps to reduce the hydrolysis of cyanide, minimizing the loss of cyanide as hydrogen cyanide gas. Additionally, lime can neutralize acidic substances in the ore and precipitate harmful ions in the slurry, creating ideal conditions for gold dissolution. If the alkalinity is too high (pH > 12) or too low (pH < 9), the leaching rate of gold will decrease. High alkalinity can inhibit the reaction between gold and cyanide, while low alkalinity may accelerate the hydrolysis of cyanide and increase its consumption.

Temperature

The temperature of the leaching process has a complex effect on gold cyanidation. As the temperature rises, the activity of ions increases, which initially accelerates the leaching rate of gold. However, higher temperatures also lead to a significant decrease in the solubility of oxygen in the solution. At the same time, the hydrolysis of cyanide itself increases, and the reaction of base metal cyanides speeds up, resulting in increased cyanide consumption. Moreover, the solubility of calcium hydroxide (from added lime) decreases at higher temperatures, which may cause the pH value of the pulp to drop. Therefore, for most gold cyanidation processes, although a moderate increase in temperature can improve the leaching rate to a certain extent, excessive temperature is not beneficial. In general, cyanidation is often carried out at ambient or slightly elevated temperatures, and the optimal temperature needs to be determined based on the specific ore characteristics and process conditions.

Leaching Time

The leaching time required depends on various factors such as the nature of the ore, the cyanidation method, and the leaching conditions. For stirred cyanidation, the leaching time is usually more than 24 hours and can sometimes be as long as 40 hours or more. In the case of leaching telluride gold ores, it may take up to 72 hours. For percolation cyanidation, the leaching time is even longer, often requiring more than five days. If the leaching time is too short, the gold particles may not be fully dissolved, resulting in low recovery rates. Conversely, if the leaching time is too long, it not only increases production costs but also may cause the dissolution of more impurities in the ore, which can interfere with the subsequent gold recovery process. Therefore, determining the appropriate leaching time through experimental research and process optimization is necessary to achieve efficient gold extraction.

Slurry Concentration

The concentration of the leaching slurry directly affects the diffusion rate of components in the cyanidation process. A high slurry concentration increases the viscosity of the slurry, which is not conducive to the diffusion of cyanide and oxygen to the gold particles, thereby reducing the leaching efficiency. Conversely, if the slurry concentration is too low, although it may improve the diffusion conditions, it will increase the consumption of cyanide and other reagents and also require larger equipment volumes, leading to increased costs. The appropriate slurry concentration needs to be determined through beneficiation tests according to the characteristics of the ore. For ores with less mud and fewer impurities, a higher slurry concentration (usually 40% - 50%) can be used for leaching. For ores with complex mineral compositions and high mud content, a lower slurry concentration (around 25%) is often required.

Other Factors

Presence of Impurities in the Slurry

In addition to the harmful minerals in the ore itself, other impurities in the leaching slurry can also affect the cyanidation process. For example, fine gangue particles, especially those with high clay content, can increase the viscosity of the slurry, impeding the movement of cyanide and oxygen. These fine particles may also adsorb cyanide, reducing its effective concentration for gold leaching. Furthermore, if there are certain heavy metal ions in the slurry, they may react with cyanide to form complexes, consuming cyanide and interfering with the gold leaching reaction. Regular monitoring and appropriate pre-treatment of the slurry to remove or reduce these impurities can help improve the cyanidation efficiency.

Agitation and Mixing

Proper agitation and mixing of the leaching slurry are essential for ensuring uniform distribution of cyanide, oxygen, and ore particles. Agitation helps to bring the reactants into contact more effectively, enhancing the reaction rate. Inadequate agitation may result in local concentration gradients, where some areas of the slurry have insufficient cyanide or oxygen, leading to incomplete gold leaching. However, overly intense agitation can cause excessive wear of equipment and may also lead to the formation of foam in the slurry, which can affect the leaching process. Therefore, optimizing the agitation speed and intensity according to the specific process requirements is important for efficient gold cyanidation.

In conclusion, the efficiency of gold ore leaching with sodium cyanide is influenced by a multitude of factors, including ore characteristics, leaching conditions, and other operational parameters. By carefully considering and optimizing these factors, mining companies can improve the gold recovery rate, reduce costs, and minimize the environmental impact of the cyanidation process.