The Main Process of Agitation Cyanidation Leaching in Gold Mines

Introduction

Cyanidation leaching, which has been used in industrial production since 1887. is a mineral Leaching Process that uses Cyanide solutions as leaching agents to extract gold and silver from gold and silver-containing mineral raw materials. Agitation cyanidation leaching is one of the important methods in cyanidation leaching, and is widely used in the gold mining industry.

Principle of Agitation Cyanidation Leaching

Cyanide, such as Sodium Cyanide (NaCN), is a key reagent in the cyanidation process. Sodium cyanide is a colorless and transparent crystal, often grayish-yellow due to impurities. It is highly soluble in water, with a solubility in water greater than 20%. When the pH of its aqueous solution is acidified to pH = 7. cyanide almost completely decomposes into volatile hydrogen cyanide gas, which is a colorless and highly toxic gas. In the solution, hydrogen cyanide is a weak acid, difficult to ionize, and has no leaching effect on gold and silver. When the pH value is 12. the cyanide in the solution almost completely dissociates into cyanide ions. Therefore, the cyanidation operation must be carried out in an alkaline medium.

Generally, in the presence of oxygen, the cyanidation leaching of gold is an electrochemical corrosion process. Cyanide reacts with gold in an alkaline environment with the participation of oxygen, forming water-soluble gold cyanide complexes, thus enabling the dissolution of gold from the ore.

Process of Agitation Cyanidation Leaching

1. Preparation of Leaching Raw Materials

• Crushing: The mined raw gold ore is first fed into a crusher for crushing. The crushing process is divided into coarse crushing, medium crushing, and fine crushing. Jaw crushers and cone crushers are often used for coarse and medium crushing, mainly to break large pieces of ore into smaller particles, generally with a particle size controlled at a few centimeters. Hammer crushers are mainly used for fine crushing.

• Grinding: The crushed ore enters a ball mill for further grinding to an appropriate particle size. Usually, it is required that the ore particle size reaches -200 mesh accounting for 60% - 90%, so that the gold minerals are fully dissociated. Generally, lattice-type ball mills are used for coarse grinding, and overflow-type ball mills are used for fine grinding.

• Slurry Preparation: The ground ore pulp enters a stirring tank. An appropriate amount of water is added to adjust the ore pulp concentration, generally controlled at 30% - 50%. At the same time, adjusters such as lime are added to adjust the pH value of the ore pulp to 10 - 11. creating an alkaline environment conducive to cyanidation leaching and inhibiting the dissolution of other impurities.

2. Agitation Cyanidation Leaching

• Leaching Reagent Addition: Cyanide agents such as sodium cyanide (NaCN) or potassium cyanide (KCN) are added to the adjusted ore pulp. Under full stirring conditions, cyanide reacts chemically with gold to form water-soluble gold cyanide complexes.

• Leaching Equipment: The leaching process is usually carried out in multiple series-connected agitation tanks. The agitation tanks can be divided into three types according to different mixing methods: compressed air agitation tanks, mechanical agitation tanks, or mixed agitation tanks.

• Leaching Conditions: The concentration of Sodium cyanide in the ore pulp is usually 0.02% - 0.1%. Lime is added during operation to make the ore pulp pH = 9 - 12. Air is filled to maintain the best ratio between the dissolved oxygen concentration and the sodium cyanide concentration in the ore pulp. The leaching time is generally 24 - 48 hours to ensure that gold is fully dissolved.

3. Solid-Liquid Separation and Washing

• Solid-Liquid Separation: After leaching, the ore pulp is subjected to solid-liquid separation through equipment such as thickeners and filters to obtain gold-bearing pregnant solution and leaching residue. Thickeners use the principle of gravitational sedimentation to make solid particles in the ore pulp settle to the bottom, and the supernatant is the gold-bearing pregnant solution. Filters further filter the underflow of the thickener to improve the effect of solid-liquid separation.

• Washing: In order to obtain sufficient separation between the cyanide leachate and the leach residue, a washing process of 3 - 5 stages of thickening, filtration, or a combination of the two is generally used. This is a key operation for cyanidation leaching. The commonly used method is the continuous countercurrent decantation method (CCD method). The thickeners used in this method can be divided into single-layer and multi-layer types. Many cyanidation concentrators in China use 2 - 3 - layer thickeners for solid-liquid separation and continuous countercurrent washing.

4. Gold Recovery

• Zinc Powder Displacement Method: Zinc powder is added to the gold-bearing pregnant solution. Zinc reacts with the gold cyanide complex in a displacement reaction, reducing gold to metallic gold and precipitating it. After filtration, gold mud is obtained, and the gold mud is smelted through processes such as melting to obtain crude gold.

• Activated Carbon Adsorption Method: Activated carbon is added to the gold-bearing pregnant solution, and the gold cyanide complex is adsorbed by the activated carbon. Then, gold is recovered from the activated carbon through processes such as desorption and electrowinning. This method can be further divided into the carbon-in-pulp method (CIP) and the carbon-in-leach method (CIL).

• CIP Process: First, cyanidation leaching is carried out, and then activated carbon is added to the ore pulp to adsorb gold. In the CIP process, leaching and adsorption are two independent operations. In the adsorption operation, the leaching process has basically been completed, and the size, quantity, and operating conditions of the adsorption tanks are determined by adsorption parameters.

• CIL Process: Activated carbon is added to the leaching tank, and leaching and adsorption are carried out simultaneously, that is, leaching and adsorption occur at the same time. In the CIL process, the leaching and adsorption operations are carried out simultaneously. Generally, the leaching operation requires a longer time than the adsorption operation. Therefore, the size, aeration, and dosing of the tank are determined by leaching parameters. Since the adsorption rate is a function of the concentration of dissolved gold in the solution, in order to increase the concentration of dissolved gold in the front - part adsorption tanks and at the same time increase the leaching time, 1 - 2 stages of pre - leaching are usually added before leaching and adsorption.

5. Tailings Treatment

The tailings after Gold Recovery usually contain certain amounts of residual cyanide and other impurities. For environmental protection requirements, the tailings need to be properly treated. Common treatment methods include chemical oxidation methods (such as the sulfur dioxide - air method), natural decomposition, or biodegradation methods to reduce the concentration of residual cyanide to below the national standard and prevent environmental pollution. The treated tailings can be disposed of by stacking or other appropriate means.

Conclusion

Agitation cyanidation leaching is an important process in the extraction of gold from gold ores. Through a series of processes such as raw material preparation, agitation leaching, solid - liquid separation, gold recovery, and tailings treatment, gold can be effectively extracted from gold ores. However, due to the toxicity of cyanide, in the process of using Agitation Cyanidation Leaching, strict attention must be paid to safety production and environmental protection to ensure the sustainable development of the gold mining industry.