Cyanidation Process in Gold Ore Processing

Introduction

The cyanidation process in gold ore processing holds a crucial and almost irreplaceable role in the global gold extraction industry. Gold, with its long - standing value as a precious metal, has been sought after by humanity for thousands of years. From being a symbol of wealth and power in ancient civilizations to its modern - day applications in jewelry, electronics, and investment, the demand for gold remains consistently high.

The cyanidation process has been the cornerstone of gold extraction for over a century. Its significance lies in its ability to efficiently extract gold from a wide variety of ore types. Before the development of the cyanidation process, gold extraction methods were often labor - intensive, less efficient, and more environmentally damaging. For example, amalgamation, an earlier method of gold extraction, involved the use of mercury to bind with gold particles. However, this method had significant drawbacks, including the high toxicity of mercury and relatively low recovery rates for some ore types.

In contrast, the cyanidation process revolutionized the gold mining industry. By using cyanide solutions, it can dissolve gold particles, even those that are finely disseminated within the ore, with a relatively high degree of efficiency. This allows mining companies to extract gold from ores that were previously considered uneconomical to process. In fact, a large proportion of the world's gold production today, estimated to be over 80%, relies on the cyanidation process in some form. Whether it is large - scale open - pit mines in South Africa, the United States, or underground mines in Australia and China, the cyanidation process is the go - to method for gold extraction. Its wide - spread use is a testament to its effectiveness and economic viability in the complex and competitive world of gold mining.

What is the Cyanidation Process

The cyanidation process, at its core, is a chemical extraction method that capitalizes on the unique chemical properties of cyanide ions. In the context of gold ore processing, its fundamental prinCIPle is centered around the complexation reaction between cyanide ions (CN^- ) and free gold.

Gold in nature often exists in a free state, even when it is encapsulated within other minerals. Once the encapsulating minerals are broken open, the gold is revealed as elemental gold. The cyanide ions have a strong affinity for gold. When a gold - bearing ore is exposed to a cyanide - containing solution, the cyanide ions form a stable complex with the gold atoms. The chemical reaction can be represented by the following equation:

4Au + 8NaCN+O_2 + 2H_2O = 4Na[Au(CN)_2]+4NaOH. In this reaction, under the action of oxygen, the gold atoms combine with the cyanide ions to form a soluble gold - cyanide complex, sodium dicyanoaurate (Na[Au(CN)_2] ). This transformation allows the gold, which was originally in the solid ore, to dissolve into the solution, separating it from the other non - gold components of the ore.

Strictly speaking, the cyanidation process does not fall within the traditional scope of mineral processing but is classified as hydrometallurgy. Mineral processing typically involves physical separation methods such as crushing, grinding, flotation, and gravity separation to separate valuable minerals from gangue minerals. In contrast, hydrometallurgy uses chemical reactions to extract metals from their ores in an aqueous solution. The cyanidation process, with its reliance on chemical reactions to dissolve gold in a cyanide - containing solution, clearly belongs to the realm of hydrometallurgy. This classification is important as it differentiates the cyanidation process from other more physically - based ore - processing techniques and highlights its chemical - reaction - driven nature in the extraction of gold.

Types of Cyanidation Processes: CIP and CIL

Cyanidation Process in Gold Ore Processing Sodium cyanide gold ore processing cyanidation process CIP CIL No. 1picture

Within the realm of cyanidation processes for gold extraction, two main methods stand out: the Carbon - in - Pulp (CIP) process and the Carbon - in - Leach (CIL) process.

The CIP process is characterized by a sequential operation. First, the gold - bearing ore pulp undergoes an extraction stage. In this stage, the ore is mixed with a cyanide - containing solution. Under the right conditions of oxygen availability, pH, and temperature, the gold in the ore forms a soluble complex with the cyanide ions, as described in the basic cyanidation reaction. After the leaching process is completed, activated carbon is introduced into the pulp. The activated carbon then adsorbs the gold - cyanide complex from the solution. This separation of the leaching and adsorption steps allows for a more controlled and optimized process in some cases. For example, in mines where the ore has a relatively stable composition and the leaching conditions can be precisely maintained, the CIP process can achieve high gold recovery rates.

On the other hand, the CIL process represents an integrated approach. In the CIL process, the leaching of gold from the ore and the adsorption of the gold - cyanide complex by activated carbon occur simultaneously. This is achieved by adding activated carbon directly into the leaching tanks. The advantage of the CIL process lies in its more efficient use of equipment and time. Since the leaching and adsorption are combined, there is no need for additional equipment or time to transfer the pulp between leaching and adsorption stages. This reduces the overall footprint of the processing plant and can lead to cost savings in terms of both capital investment and operational expenses. For instance, in large - scale mining operations where throughput is a crucial factor, the CIL process can handle a larger volume of ore in a shorter time, maximizing production efficiency.

In recent years, the CIL process has been increasingly adopted by cyanidation plants around the world. Its ability to more effectively utilize production equipment gives it an edge over the CIP process in many situations. The continuous nature of the CIL process also leads to a more stable operation, with less variability in the quality of the final product. Additionally, the reduced number of process steps in CIL means there are fewer opportunities for errors or losses during the transfer of materials between different stages of the process. However, the choice between CIP and CIL is not always straightforward. It depends on various factors such as the nature of the ore, the scale of the mining operation, the available capital for investment, and the local environmental and regulatory requirements. Some mines may still prefer the CIP process due to its better - understood and more segmented nature, which can be easier to manage in certain circumstances.

Key Requirements in Cyanidation Process

Grinding Fineness

Grinding fineness plays a pivotal role in the cyanidation operation. Since the effectiveness of cyanidation hinges on the ability to expose the encapsulated gold, meticulous grinding is essential. In typical carbon - in - pulp (CIP) plants, the grinding fineness requirements for the ore to enter the cyanidation operation are quite stringent. Generally, the proportion of particles with a size of -0.074mm should reach 80 - 95%. For some mines where the gold is disseminated in an 浸染 - like pattern, the grinding fineness is even more demanding, with the proportion of -0.037mm particles required to be above 95%.

To achieve such fine grinding, a single - stage grinding operation is often insufficient. In most cases, two - stage or even three - stage grinding is necessary. For example, in a large - scale gold mine in Western Australia, the ore undergoes a two - stage grinding process. The first stage uses a large - capacity ball mill to reduce the particle size to a certain extent, and then the product is further ground in a second - stage stirred mill. This multi - stage grinding process can gradually reduce the particle size of the ore, ensuring that the gold particles are fully exposed and can effectively react with the cyanide solution during the cyanidation process. If the grinding fineness is not met, the gold particles may not be fully exposed, resulting in incomplete dissolution during cyanidation and a significant reduction in the gold recovery rate.

Preventing Cyanide Hydrolysis

The cyanide compounds commonly used in the cyanidation process, such as potassium cyanide (KCN), Sodium cyanide (NaCN ), and calcium cyanide (Ca(CN)_2 ), are all salts of strong bases and weak acids. In an aqueous solution, they are prone to hydrolysis reactions. The hydrolysis reaction of sodium cyanide can be represented by the equation:

NaCN + H_2O\rightleftharpoons HCN+NaOH. As hydrogen cyanide (HCN ) is volatile, this hydrolysis process leads to a decrease in the concentration of cyanide ions (CN^- ) in the pulp, which is detrimental to the cyanidation reaction.

To address this issue, the most effective approach is to increase the concentration of hydroxide ions ( OH^-), which is equivalent to increasing the pH value of the solution. In industrial applications, lime (CaO ) is the most commonly used and cost - effective pH adjuster. When lime is added to the solution, it reacts with water to form calcium hydroxide (Ca(OH)_2 ), which dissociates to release hydroxide ions, thereby increasing the pH value. The reaction of lime with water is: , CaO + H_2O=Ca(OH)_2 & Ca(OH)_2\rightleftharpoons Ca^{2 + }+2OH^- .

However, when using lime to adjust the pH value, it is important to note that lime also has a flocculation effect. To ensure that the lime is evenly dispersed and can play its role effectively, it is usually added during the grinding operation. In a gold mine in South Africa, lime is added to the ball mill during the grinding process. This not only allows the lime to be fully mixed with the ore slurry but also takes advantage of the strong mechanical agitation in the ball mill to ensure that the lime is evenly distributed in the slurry, effectively preventing the hydrolysis of cyanide and maintaining a stable concentration of cyanide ions in the subsequent cyanidation process. Generally, for carbon - in - pulp operations, a pH value in the range of 10 - 11 is found to yield the best results.

Controlling Pulp Concentration

The concentration of the pulp has a profound impact on the contact between gold and cyanide as well as between the gold - cyanide complex and activated carbon. If the pulp concentration is too high, the particles are more likely to precipitate on the surface of the activated carbon, hindering the effective adsorption of the gold - cyanide complex by the activated carbon. On the other hand, if the pulp concentration is too low, the particles tend to settle easily, and to maintain the appropriate pH value and cyanide concentration, a large amount of reagents needs to be added, which increases production costs.

Through years of production practice, it has been determined that for the carbon - in - pulp gold extraction process, a pulp concentration of 40 - 45% and a cyanide concentration of 300 - 500 ppm are more suitable. For instance, in a gold - processing plant in Nevada, USA, maintaining the pulp concentration within this range has consistently achieved high gold recovery rates. However, considering that the final product concentration of the two - to - three - stage grinding operation is generally below 20%, before entering the leaching operation, the pulp needs to undergo a thickening process.

The thickening operation is usually carried out in a thickener. The principle of the thickener is to use the sedimentation effect to separate the solid particles from the liquid in the pulp, thereby increasing the concentration of the pulp. In a modern gold - processing plant, high - efficiency thickeners are often used. These thickeners are equipped with advanced flocculation and sedimentation control systems, which can quickly and effectively increase the pulp concentration to the required level for the subsequent cyanidation leaching operation, ensuring the smooth progress of the cyanidation process and the high - efficiency extraction of gold.

Cyanidation Leaching Mechanism

Aeration and Oxidant

The cyanidation process is an aerobic process, and this can be clearly demonstrated through the chemical reaction equation. The main reaction for the dissolution of gold in the cyanidation process is 4Au + 8NaCN+O_2 + 2H_2O = 4Na[Au(CN)_2]+4NaOH . From this equation, it is evident that oxygen (O_2 ) plays a crucial role in the reaction. During the production process, introducing oxygen can significantly accelerate the leaching rate. This is because oxygen participates in the redox reaction, faCILitating the oxidation of gold and its subsequent complexation with cyanide ions. For example, in many gold - processing plants, compressed air is commonly introduced into the cyanide - containing solution. The oxygen in the air provides the necessary oxidizing environment for the reaction to proceed smoothly.

In addition to aeration, the appropriate addition of oxidizing agents can also enhance the leaching process. Hydrogen peroxide (H_2O_2) is a commonly used oxidizing agent in the cyanidation process. When hydrogen peroxide is added, it can provide additional active oxygen species, which can further promote the oxidation of gold and the dissolution of gold - bearing minerals. The reaction of hydrogen peroxide with gold in the presence of cyanide can be represented by the equation: 2Au+4NaCN+H_2O_2 = 2Na[Au(CN)_2]+2NaOH . This reaction shows that hydrogen peroxide can substitute for some of the oxygen's role in the cyanidation reaction, and under certain conditions, it can lead to a faster leaching rate.

However, it is important to note that an excessive amount of oxidizing agents can have adverse effects. When the amount of oxidizing agent is too high, it can cause the oxidation of cyanide ions. For instance, hydrogen peroxide can react with cyanide ions to form cyanate ions (CNO^-). The reaction is as follows: CN^-+H_2O_2 = CNO^-+H_2O . The formation of cyanate ions reduces the concentration of cyanide ions in the solution, which is essential for the complexation with gold. As a result, the leaching efficiency of gold may be decreased, and the overall production process may be negatively affected. Therefore, the dosage of oxidizing agents needs to be carefully controlled to ensure the optimal performance of the cyanidation process.

Reagent Dosage

Theoretically, the complexation reaction between gold and cyanide has a specific stoichiometric relationship. From the chemical equation 4Au + 8NaCN+O_2 + 2H_2O = 4Na[Au(CN)_2]+4NaOH, we can calculate that 1 mole of gold (Au) requires 2 moles of cyanide ions (CN^-) for complexation. In terms of mass, approximately 1 gram of gold requires about 0.5 grams of cyanide as the leaching reagent. This calculation provides a basic reference for the amount of reagents needed in the cyanidation process.

Nevertheless, in actual production, the situation is much more complex due to the presence of other minerals in the gold - bearing ore. Minerals such as silver (Ag), copper (Cu ), lead ( Pb), and zinc (Zn ) can also react with cyanide ions. For example, copper can form various copper - cyanide complexes. The reaction of copper with cyanide can be expressed as Cu^{2 + }+4CN^-=[Cu(CN)_4]^{2 - } . These competing reactions consume a significant amount of cyanide, increasing the actual dosage required.

Therefore, in practical operation, the determination of reagent dosage cannot be solely based on theoretical calculations. Instead, it should be adjusted according to the final leaching rate. When the ore properties change, continuous tracking and adjustment of the reagent dosage are necessary. In general, it is considered reasonable for the actual cyanide dosage to be 200 - 500 times higher than the calculated value. This wide range of deviation accounts for the variability in ore composition and the complex interactions between different minerals. By closely monitoring the leaching rate and adjusting the reagent dosage accordingly, the gold - extraction process can achieve better efficiency and economic benefits.

Multi - stage Leaching and Leaching Time

To ensure the stability of continuous operation and maintain a relatively stable concentration of cyanide ions in the solution, multi - stage leaching is often employed. In a multi - stage leaching system, the ore pulp sequentially passes through multiple leaching tanks. Each tank contributes to the continuous dissolution of gold and the maintenance of the cyanide - ion concentration. As the pulp moves from one tank to the next, the gold - cyanide complex is gradually formed and the concentration of free cyanide ions is adjusted to ensure that the reaction continues smoothly. This staged approach helps to buffer any fluctuations in the reaction conditions and provides a more stable environment for the cyanidation process. For example, in a large - scale gold - mining operation in Western Australia, a five - stage leaching system is used. The first stage initiates the leaching process, and subsequent stages further extract gold and maintain the cyanide - ion balance, resulting in a high and stable gold - leaching efficiency.

The leaching time is a crucial factor in determining the volume of the leaching tank. However, there is no simple and universal formula for calculating the leaching time. Each carbon - in - pulp (CIP) or carbon - in - leach (CIL) plant must rely on experimental data to determine the appropriate leaching time. This is because the leaching time is affected by multiple factors, including the type and composition of the ore, the concentration of reagents, the temperature, and the agitation intensity. For instance, in a gold - processing plant in South Africa, extensive laboratory - scale and pilot - scale tests were conducted before the plant's construction. These tests involved varying the leaching time and monitoring the gold - leaching rate under different conditions. Based on the experimental results, the optimal leaching time was determined to be 24 hours for the specific ore type processed at that plant.

If a plant blindly relies on experience without conducting proper tests, it is highly likely to encounter production failures. For example, a small - scale gold - mining operation in a certain region attempted to use the leaching time of a neighboring mine as a reference without considering the differences in their ore properties. As a result, the gold - leaching rate was much lower than expected, and the production cost increased significantly due to inefficient leaching and the need for additional reagent consumption. Therefore, accurate determination of the leaching time through experimental data is essential for the successful operation of a cyanidation - based gold - extraction plant.

Post - cyanidation Operations

Once the gold - bearing activated carbon, known as loaded carbon, reaches a gold - adsorption level of over 3000g/t, it is considered that the entire carbon - in - pulp adsorption process is complete. However, the presence of high - content impurities such as copper and silver in the ore can significantly affect the adsorption capacity of activated carbon. These impurities can compete with gold for adsorption sites on the activated carbon, resulting in the failure of the loaded - carbon grade to reach the expected target. When the activated carbon can no longer adsorb gold effectively, it is deemed saturated.

For saturated activated carbon, several methods can be employed to obtain gold. One common approach is desorption and electrolysis. In the desorption process, a chemical solution is used to strip the gold - cyanide complex from the saturated activated carbon. For example, in the high - temperature and high - pressure desorption method, the saturated activated carbon is placed in a desorption system with specific conditions. By adding anions that are more easily adsorbed by the activated carbon, the Au(CN)_2^- complex is displaced from the carbon surface. The reaction mechanism involves the exchange of the gold - cyanide complex with the added anions, causing the gold to be released into the solution. After desorption, the resulting solution, known as the pregnant solution, contains a relatively high concentration of gold ions.

The pregnant solution then undergoes electrolysis. In the electrolysis cell, an electric current is applied. The gold ions in the solution are attracted to the cathode, where they gain electrons and are reduced to metallic gold. The process can be represented by the equation: Au^+ + e^-\rightarrow Au  . The gold accumulates on the cathode in the form of gold mud, which can be further processed to obtain high - purity gold.

In regions where gold production is concentrated, an alternative option is to sell the loaded carbon. This can be a profitable choice as some specialized companies are equipped to handle the further processing of loaded carbon. They have the expertise and facilities to extract gold from the loaded carbon, and the gold - mining companies can obtain revenue by selling the loaded carbon to these entities.

Another relatively simple method is combustion. When the loaded carbon is burned, the organic components of the activated carbon are oxidized and burned off, while the gold remains in the residue in the form of a gold alloy, known as dore gold. Dore gold typically contains a high proportion of gold along with some impurities. After combustion, the dore gold can be further refined through processes such as smelting and purification to obtain high - purity gold products that meet the standards for commercial use in the jewelry, electronics, and investment industries.

Advantages and Disadvantages of Cyanidation Process

Advantages

  1. High Recovery Rate: One of the most significant advantages of the cyanidation process is its high recovery rate. For typical oxidized gold - bearing quartz - vein ores, when using the carbon - in - pulp (CIP) or carbon - in - leach (CIL) process, the total recovery rate can reach over 93%. In some well - optimized operations, the recovery rate can even be higher. This high recovery rate means that mining companies can extract a large proportion of the gold present in the ore, maximizing the economic return from the mining operation. For example, in a large - scale gold mine in the United States, by strictly controlling the process parameters such as grinding fineness, pulp concentration, and reagent dosage, the gold recovery rate of the cyanidation process has been maintained at around 95% for a long - time, which is much higher than many other gold - extraction methods.

  2. Wide Applicability: The cyanidation process is suitable for a wide variety of gold - bearing ores. It can effectively handle not only oxidized gold ores but also some sulfide - bearing gold ores. Whether the gold is in a free - state or encapsulated within other minerals, the cyanidation process can often dissolve the gold with the help of appropriate pre - treatment and process control. For instance, in some mines in South America where the ores contain a mixture of sulfide and oxidized gold minerals, the cyanidation process has been successfully applied. After proper oxidation pre - treatment of the sulfide minerals, the cyanidation process can achieve satisfactory gold - extraction results, demonstrating its strong adaptability to different ore types.

  3. Mature Technology: With a history of over a century, the cyanidation process has become a highly mature technology in the gold - mining industry. The equipment and operation procedures are well - established, and there is a large amount of accumulated experience and data. This maturity means that the process is relatively easy to operate and control. Mining companies can rely on existing technical standards and guidelines to design, build, and operate cyanidation plants. For example, the design of cyanidation leaching tanks, the selection of activated carbon for adsorption, and the control of reagent dosage all have standard procedures and methods. Newly - built cyanidation plants can quickly start up and reach stable production conditions, reducing the risks associated with new technology adoption.

Disadvantages

  1. Toxicity of Cyanide: The most prominent drawback of the cyanidation process is the toxicity of cyanide. Cyanide compounds, such as sodium cyanide and potassium cyanide, are highly toxic substances. Even a small amount of cyanide can be extremely harmful to human health and the environment. If cyanide - containing solutions leak during the mining process, they can contaminate soil, water sources, and air. For example, in some historical mining accidents, the leakage of cyanide - containing wastewater led to the death of a large number of aquatic organisms in nearby rivers and lakes, and also posed a threat to the health of local residents. Inhalation, ingestion, or skin contact with cyanide can cause serious poisoning symptoms in humans, including dizziness, nausea, vomiting, and in severe cases, can be fatal. Therefore, strict safety and environmental protection measures are required in the use of cyanide, which increases the complexity and cost of the mining operation.

  2. Complex and Costly Post - treatment: The post - treatment operations after the cyanidation process are relatively complex and require a large amount of investment. After the gold - bearing activated carbon reaches saturation, processes such as desorption, electrolysis, or combustion are needed to obtain pure gold. The desorption and electrolysis processes require specialized equipment and chemical reagents. For example, in the desorption process, high - temperature and high - pressure equipment may be required, and the use of chemical solutions for desorption also needs to be carefully controlled to ensure the recovery of gold and the recycling of reagents. In addition, the treatment of waste residues and wastewater generated during the post - treatment process is also a challenge. The waste residues may still contain trace amounts of cyanide and other harmful substances, and the wastewater needs to be treated to meet strict environmental discharge standards, which all contribute to the high cost of the entire cyanidation process.

  3. Sensitivity to Ore Impurities: The cyanidation process is highly sensitive to impurities in the ore. Minerals such as copper, silver, lead, and zinc can react with cyanide, consuming a large amount of cyanide reagents. This not only increases the cost of reagents but also reduces the efficiency of gold extraction. For example, when the copper content in the ore is high, copper can form stable copper - cyanide complexes, competing with gold for cyanide ions. As a result, the amount of cyanide available for gold complexation is reduced, and the leaching rate of gold may be significantly affected. In some cases, additional pre - treatment steps may be required to remove or reduce the impact of these impurities, which further increases the complexity and cost of the mining process.

Conclusion

Cyanidation Process in Gold Ore Processing Sodium cyanide gold ore processing cyanidation process CIP CIL No. 2picture

In conclusion, the cyanidation process is an indispensable technology in the gold - mining industry. Its high recovery rate, wide applicability, and mature technology have made it the dominant method for gold extraction globally. It has enabled the extraction of gold from a diverse range of ores, contributing significantly to the global gold supply.

However, the cyanidation process is not without its challenges. The toxicity of cyanide poses a serious threat to human health and the environment. Stringent safety and environmental protection measures must be implemented to prevent cyanide leakage and ensure proper treatment of cyanide - containing wastewater and waste residues. Additionally, the complex and costly post - treatment operations, as well as the process's sensitivity to ore impurities, add to the difficulties and costs of gold production.

Looking ahead, the future of the cyanidation process in gold ore processing is likely to be shaped by technological advancements. The development of more environmentally friendly and efficient cyanidation methods, such as the use of low - toxicity cyanide substitutes, is a promising direction. Automation and intelligent control technologies will also play an increasingly important role. These technologies can improve production efficiency, reduce human - error - related risks, and optimize the use of resources. For example, automated systems can precisely control reagent dosages, pulp concentrations, and other key parameters, ensuring a more stable and efficient production process.

Furthermore, the exploration of new cyanidation - related technologies, such as bio - cyanidation or the integration of cyanidation with other emerging extraction methods, may offer new solutions to the existing problems. With continuous innovation and improvement, the cyanidation process has the potential to maintain its position as a leading technology in gold ore processing while becoming more sustainable and environmentally friendly. As the demand for gold remains strong in various industries, the development and optimization of the cyanidation process will be crucial for the long - term development of the gold - mining industry.

You may also like

Online message consultation

Add comment:

+8617392705576WhatsApp QR CodeScan QR code
Leave a message for consultation
Thanks for your message, we will contact you soon !
Submit
Online Customer Service

Please fill in the product information

Thanks for your message, we will contact you soon!