Cyanidation Gold Extraction: A Deep Dive into the Agitation Cyanidation Process

In the realm of gold extraction, cyanidation has held a prominent position for over a century. Since its inception in 1887 for the extraction of gold and silver ores, this method has continuously evolved, remaining one of the most widely used techniques due to its high recovery rate, adaptability to various ore types, and feasibility for local production.

1. Understanding Cyanidation in Gold Extraction

Cyanidation is a chemical process that capitalizes on the ability of cyanide ions to form soluble complexes with gold. In the presence of oxygen and water, cyanide ions react with gold atoms. This reaction results in the creation of a soluble compound where gold is bonded with cyanide ions, allowing the gold to dissolve in the solution. While this process is highly effective for extracting gold, it also brings significant environmental and safety concerns because cyanide is a toxic substance.

2. Types of Cyanidation Methods

Cyanidation methods can be broadly classified into two main categories: agitation cyanidation and percolation cyanidation.

• Agitation Cyanidation: This method is primarily used for treating flotation gold concentrates or in all - slime cyanidation scenarios. It involves vigorously mixing the ore pulp with the cyanide solution. By doing so, it ensures that the gold - bearing particles in the ore come into maximum contact with the cyanide ions, facilitating the extraction of gold.

• Percolation Cyanidation: Suited for low - grade gold ores, percolation cyanidation works by allowing the cyanide solution to trickle through a bed of ore. This method consumes less energy compared to agitation cyanidation. However, its application is limited to ores that have good permeability, enabling the cyanide solution to flow through easily.

3. Agitation Cyanidation Gold Extraction Process

The agitation Cyanidation Gold Extraction process encompasses two main sub - processes: the cyanidation - zinc replacement process and the unfiltered cyanidation carbon slurry process.

3.1 Cyanidation - Zinc Replacement Process (CCD and CCF Methods)

• Leaching Raw Material Preparation: The initial step involves getting the ore ready for the leaching process. This often includes crushing the ore into smaller pieces and then grinding it to a fine consistency. In some cases, pre - treatment is also carried out to make the gold particles within the ore more accessible. The aim is to create a pulp with an optimal particle size, which promotes better interaction between the ore and the cyanide solution.

• Agitation Cyanidation Leaching: The prepared ore pulp is then transferred to agitation tanks, where a cyanide solution is added. These tanks are equipped with agitators that keep the pulp and the cyanide solution well - mixed. Oxygen is introduced into the tanks, either through aeration or by adding oxidizing agents. This oxygen helps drive the chemical reaction that dissolves the gold in the cyanide solution.

• Countercurrent Washing for Solid - Liquid Separation: After the leaching process, the resulting slurry consists of solid residues and a liquid phase known as the pregnant solution, which contains dissolved gold. To separate these two components, a series of thickeners or filters are used in a countercurrent washing setup. Methods like Continuous Counter - Current Decantation (CCD) or Continuous Counter - Current Filtration (CCF) are employed to recover as much of the gold - bearing solution as possible while minimizing the amount of gold lost with the solid residues.

• Purification of Leaching Liquid and Deoxidation: The pregnant solution obtained from the solid - liquid separation step may contain impurities and dissolved oxygen. Purification procedures are implemented to remove suspended solids and other contaminants that could disrupt the subsequent gold recovery process. Deoxidation is equally important because oxygen can cause the re - oxidation of the gold - cyanide compound, reducing the effectiveness of the zinc replacement process that follows.

• Zinc Powder (Silk) Replacement and Pickling: Zinc powder or zinc silk is added to the purified and de - oxidized pregnant solution. Zinc is more reactive than gold, so it displaces gold from the compound formed during the leaching process. This results in the formation of a solid precipitate containing gold and zinc, commonly referred to as gold mud. After the replacement reaction, the gold mud is typically treated with an acid solution to remove any excess zinc and other impurities.

• Smelting Ingots: The final stage of the cyanidation - zinc replacement process is to smelt the gold mud to produce pure gold ingots. The gold mud is melted at high temperatures in a furnace, and through a series of refining steps, the remaining impurities are removed, yielding high - purity gold ingots.

3.2 Unfiltered Cyanidation Carbon Slurry Process (CIP and CIL Methods)

• Leaching Material Preparation: Similar to the cyanidation - zinc replacement process, the first task is to prepare the ore for leaching. This requires reducing the ore to an appropriate particle size through crushing and grinding operations.

• Agitation Leaching and Countercurrent Carbon Adsorption: In the Carbon - in - Pulp (CIP) method, the cyanide leaching process takes place first in a series of agitation tanks. Once the gold has dissolved into the solution, activated carbon is added to the pulp. Activated carbon has a strong affinity for the gold - cyanide compound and adsorbs the dissolved gold onto its surface. In the Carbon - in - Leach (CIL) method, the activated carbon is added to the leaching tank simultaneously with the cyanide solution, so the leaching and adsorption processes occur at the same time. In both CIP and CIL, a countercurrent flow of carbon and pulp is maintained to maximize the amount of gold adsorbed by the carbon.

• Gold - Loaded Carbon Desorption: After the adsorption process, the gold - loaded carbon needs to be separated from the pulp. Then, the gold is removed from the carbon using a hot caustic - cyanide solution. This solution breaks the bond between the gold - cyanide compound and the carbon, releasing the gold back into the solution.

• Electrowinning Electrolysis: The gold - rich solution obtained from the desorption process undergoes electrowinning. During this process, an electric current is passed through the solution. This causes the gold ions in the solution to be reduced and deposited onto a cathode, forming a solid deposit of gold that can be further refined.

• Smelting Ingots: The gold obtained from electrowinning is relatively pure but may still contain some impurities. Smelting is performed to further purify the gold and cast it into ingots of the desired purity.

• Carbon Regeneration: The spent carbon, after the gold has been desorbed, can be regenerated and reused. This involves subjecting the carbon to high - temperature treatment to eliminate any adsorbed impurities and restore its ability to adsorb gold.

4. Comparing CIP and CIL Processes

• Process Duration: Generally, the CIP process takes longer overall compared to CIL. This is because in CIP, the leaching and adsorption are separate operations. In CIL, since leaching and adsorption happen simultaneously, the entire process can be completed in a shorter time. However, the CIL process demands more complex control as both processes occur concurrently.

• Carbon and Slurry Management: In the CIL process, there is a larger volume of carbon in circulation, and the concentration of carbon in the slurry is lower than in CIP. As a result, the volume of slurry that needs to be transported for carbon transfer in CIL is usually several times that of CIP (around four times). This has an impact on the sizing of equipment and energy consumption.

• Metal Backlog and Gold Grade in Solution: In the CIP process, there is a significant amount of metal that remains in the system (metal backlog), and this metal is fairly evenly distributed between the activated carbon and the solution. In CIL, most of the metal is adsorbed onto the activated carbon. Additionally, the concentration of gold in the solution in the CIL process is higher than in CIP. This is because in CIL, as the gold is being leached, it is also being continuously adsorbed, which replenishes the dissolved gold in the solution. In CIP, on the other hand, it is a single - step adsorption process with limited replenishment of dissolved gold.

5. Environmental and Safety Considerations

Despite its efficiency, cyanidation, especially agitation cyanidation, presents significant environmental and safety risks. Cyanide is highly toxic, and any leakage or improper handling can lead to severe environmental pollution and pose a threat to human health. To address these risks, gold mining operations adhere to strict safety protocols. These include proper storage and handling of cyanide, installation of containment systems to prevent leaks, and treatment of cyanide - containing wastewater. Moreover, ongoing research aims to develop alternative, less toxic leaching agents to replace cyanide in gold extraction.

6. Conclusion

Agitation cyanidation plays a vital role in the modern gold mining industry, enabling high - rate extraction of gold from diverse ore types. The two main sub - processes, cyanidation - zinc replacement and unfiltered cyanidation carbon slurry, each have their own merits and are chosen based on factors such as ore properties, scale of operation, and economic viability. However, the industry must continue to tackle the environmental and safety challenges associated with cyanide use to ensure the sustainable future of gold extraction.