The Influence of the Coexistence of Sodium Cyanide and Sodium Chloride on Leaching

1. Introduction

In the field of metallurgy, especially in the extraction of precious metals such as gold and silver, the cyanideleaching process holds a pivotal position. Sodium cyanide (NaCN) is widely utilized because it can selectively dissolve gold and silver from ores. However, in many ore bodies, various substances coexist, and sodium chloride (NaCl) is one of the common accompanying salts. Understanding the impact of the coexistence of (NaCN) and (NaCl) on the leaching process is crucial for optimizing the extraction efficiency, reducing costs, and minimizing environmental impacts. This article aims to comprehensively explore this issue.

2. The Role of Sodium Cyanide in Leaching

2.1 Chemical Reaction Mechanism

During the cyanide leaching process, cyanide ions play a key role in forming soluble complexes with gold and silver atoms. Oxygen is also essential as it acts as an oxidizing agent, facilitating the oxidation of gold and silver and promoting their dissolution in the cyanide solution. This chemical interaction enables the extraction of these precious metals from the ore.

2.2 Factors Affecting the Efficiency of Sodium Cyanide in Leaching

• Concentration of Sodium Cyanide: The concentration of sodium cyanide significantly affects the leaching rate. Theoretically, a specific amount of Sodium Cyanide is required for dissolving a certain quantity of gold based on electrochemical reactions. However, in practice, the actual consumption of Sodium cyanide is often much higher than the theoretical amount. In processes like Carbon - in - Pulp (CIP) and Carbon - in - Leach (CIL), the sodium cyanide concentration is generally maintained within a specific range. For more complex ores or those with high impurity levels, the concentration may need to be increased accordingly.

• pH Level: Sodium cyanide hydrolyzes in solution, producing hydrocyanic acid, a highly toxic gas. The degree of hydrolysis depends on the pH of the solution. To minimize cyanide loss through hydrolysis and ensure the stability of the cyanide solution, the pH is usually kept within a certain alkaline range in gold CIP plants. This pH environment also impacts the optimal sodium cyanide concentration for effective gold leaching.

• Dissolved Oxygen Concentration: Oxygen is indispensable for the dissolution of gold and silver in a cyanide solution. The reaction requires both cyanide ions and oxygen. The maximum solubility of oxygen at room temperature and pressure is limited. If the dissolved oxygen concentration in the slurry is too low, it can restrict the dissolution rate of gold and silver. In such cases, methods like injecting air into the slurry or adding hydrogen peroxide can be used to increase the oxygen concentration. The ratio of oxygen to cyanide is crucial; an imbalance can lead to a decrease in the leaching rate.

3. The Impact of Sodium Chloride on the Leaching Process

3.1 Effects on the Chemical Environment

• Ionic Strength and Activity Coefficient: When sodium chloride is present in the leaching solution, it increases the ionic strength of the solution. According to relevant theories, an increase in ionic strength can affect the activity coefficients of ions in the solution. In the cyanide leaching system, this change in activity coefficients can influence the chemical equilibrium of the reactions related to the dissolution of gold and silver. For example, it may change the effective concentration of cyanide ions available for reacting with gold, thereby affecting the leaching rate.

• Competition for Reactive Sites: Chloride ions can compete with cyanide ions for reactive sites on the surface of the ore particles. In some situations, if the concentration of chloride ions is high enough, they may adsorb on the surface of gold or silver particles, preventing cyanide ions from accessing and thus reducing the leaching efficiency. However, in certain circumstances, the presence of chloride ions can also have a positive effect. For instance, in some ores containing copper minerals, chloride ions can form complexes with copper, reducing the consumption of cyanide by copper and potentially improving the leaching of gold and silver.

3.2 Influence on the Leaching of Associated Metals

• Copper - Containing Ores: In ores with high copper content, copper minerals react strongly with sodium cyanide, consuming a significant amount of cyanide. The presence of sodium chloride can affect this reaction. Chloride ions can form complexes with copper, and these complexes may have different stabilities compared to copper - cyanide complexes. If the formation of copper - chloride complexes is favored, it can reduce the amount of cyanide consumed by copper, leaving more cyanide available for the leaching of gold and silver.

• Other Metals: Sodium chloride can also interact with other metals present in the ore, such as zinc, lead, and iron. For example, chloride ions can enhance the solubility of some zinc and lead compounds, which may in turn affect their behavior during the leaching process and their impact on the cyanide leaching of gold and silver. In the case of iron, the presence of chloride ions can influence the formation and stability of iron - containing precipitates or complexes, which may either promote or inhibit the leaching process depending on the specific conditions.

4. The Combined Effects of Sodium Cyanide and Sodium Chloride on Leaching

4.1 Synergistic or Antagonistic Effects

• Synergistic Effects: In some cases, the coexistence of sodium cyanide and sodium chloride can have a beneficial, or synergistic, effect on the leaching process. For example, in certain refractory gold ores, adding an appropriate amount of sodium chloride can improve the permeability of the ore structure, allowing cyanide ions to penetrate more easily and react with gold particles. This can lead to an increase in the gold leaching rate. Additionally, as mentioned earlier, in ores with copper minerals, the formation of copper - chloride complexes by sodium chloride can reduce the cyanide consumption by copper, which is beneficial for the cyanide leaching of gold and silver, showing a synergistic effect.

• Antagonistic Effects: However, there are also situations where sodium cyanide and sodium chloride have opposing, or antagonistic, effects. High concentrations of chloride ions can compete with cyanide ions for the surface of gold and silver particles, as well as disrupt the chemical equilibrium of the cyanide leaching reactions, resulting in a decrease in the leaching efficiency. Moreover, if the presence of sodium chloride causes the formation of certain precipitates or complexes that coat the surface of the ore particles, it can prevent cyanide ions from coming into contact with the valuable metals, further reducing the leaching rate.

4.2 Optimization of the Leaching Process in the Presence of Both

• Adjustment of Reagent Concentrations: When both sodium cyanide and sodium chloride are present, it is necessary to optimize their concentrations. This requires a detailed analysis of the ore composition. For ores with a high content of metals that can react with cyanide, such as copper, an appropriate increase in the sodium chloride concentration may be considered to reduce cyanide consumption. At the same time, the concentration of sodium cyanide should be adjusted according to the actual leaching effect to ensure the efficient leaching of gold and silver.

• Control of Process Conditions: In addition to reagent concentrations, other process conditions such as pH, temperature, and aeration also need to be carefully controlled. The pH value needs to be maintained within an appropriate range to ensure the stability of the cyanide solution and the effectiveness of the leaching reaction, taking into account the influence of both sodium cyanide and sodium chloride. The temperature of the solution is also important. Although there is a theoretically optimal temperature for gold dissolution in a cyanide solution, in the presence of sodium chloride, the impact of temperature on the leaching process may change, and it is necessary to find the optimal temperature through experimental research. Adequate aeration is crucial to ensure sufficient oxygen supply for the leaching reaction, and the presence of sodium chloride may affect the solubility and distribution of oxygen in the solution, which needs to be considered.

5. Case Studies and Experimental Results

5.1 Case Study 1: Gold - Silver Ore with High Copper Content

In a gold - silver ore deposit with a high copper content, traditional cyanide leaching using only sodium cyanide resulted in a low gold leaching rate due to significant cyanide consumption by copper. When sodium chloride was added to the leaching system at a certain concentration and the sodium cyanide concentration was adjusted, the gold leaching rate increased. The analysis showed that the addition of sodium chloride led to the formation of copper - chloride complexes, reducing the amount of cyanide consumed by copper and thus increasing the availability of cyanide for gold leaching.

5.2 Case Study 2: Refractory Gold Ore

For a refractory gold ore, the initial cyanide leaching without sodium chloride achieved a low gold leaching rate. After adding sodium chloride at a specific concentration and optimizing the cyanide concentration and other process conditions, the gold leaching rate increased. Microscopic observation of the ore particles revealed that the addition of sodium chloride improved the permeability of the ore structure, allowing cyanide ions to reach the gold particles more effectively, thereby enhancing the leaching efficiency.

6. Environmental and Safety Considerations

6.1 Toxicity of Cyanide

Cyanide is a highly toxic substance. Any release of cyanide - containing solutions into the environment can have severe consequences for aquatic life, soil quality, and human health. When sodium chloride coexists with sodium cyanide in the leaching process, it is necessary to ensure that the management and treatment of cyanide - containing waste are still carried out in strict accordance with environmental regulations. The presence of sodium chloride may affect the behavior of cyanide in waste treatment processes, such as in methods used to destroy cyanide like alkaline chlorination or biological treatment. For example, the increased ionic strength caused by sodium chloride may influence the reaction rate and efficiency of these treatment methods.

6.2 Safety in Handling

Both sodium cyanide and sodium chloride need to be handled with care. Sodium cyanide is extremely toxic and requires strict safety measures during storage, transportation, and use. Sodium chloride, although relatively less hazardous, can still pose risks such as corrosion to equipment and potential impacts on the working environment if not properly managed. In a leaching operation where both are used, workers need to be trained to handle these chemicals safely, and appropriate safety equipment and procedures should be in place to prevent accidents and ensure the well - being of the workforce.

7. Conclusion

The coexistence of sodium cyanide and sodium chloride in the leaching process has a complex impact on the extraction of precious metals. Sodium chloride can affect the chemical environment of the leaching solution, interact with associated metals, and have both synergistic and antagonistic effects with sodium cyanide. Understanding these effects is essential for optimizing the leaching process. By adjusting reagent concentrations, controlling process conditions, and considering environmental and safety factors, it is possible to achieve more efficient and sustainable extraction of gold, silver, and other valuable metals. Further research and experimental studies are still needed to fully explore the potential of this co - existing system under different ore types and process conditions, aiming to continuously improve the metallurgical industry's extraction technology.