Percolation cyanide bath immersion

Percolation cyanidation is a relatively simple and economical cyanide gold extraction method, suitable for extracting gold from ores, loose and porous gold-bearing mineral raw materials, roasted sand, and slag. It is commonly used to process gold-bearing materials with a particle size of -10 ± 0.074 mm. Its advantages include low cyanidation reagent consumption, low power consumption, and the ability to directly obtain a clear leaching solution without expensive solid-liquid separation operations; it is widely used in small gold mines both domestically and internationally. However, percolation cyanidation has a long leaching time, requires a large equipment footprint, and results in incomplete washing after leaching, leading to a lower gold leaching rate. Materials with high mud content require pre-classification. Therefore, the application of percolation cyanidation is somewhat limited.

5.3.1 Percolation Leaching Tank (Pool) The structure of the percolation leaching tank is shown in Figure 5-8. The leaching tank can be constructed using carbon steel, wood, brick, stone, or concrete; its cross-sectional shape can be circular, square, or rectangular. It should be able to withstand pressure, be leak-proof, and easy to operate. The bottom should be slightly inclined towards the outlet (slope of approximately 0.3%) and equipped with a false bottom. The volume of the leaching tank depends on the processing capacity and the particle size distribution of the raw material. Small gold mines typically use leaching tanks with a diameter of 5–12 meters and a height of 1.5–2.5 meters, processing 75–150 tons of ore per tank at a time. Gold mines in my country generally use rectangular cement pools with smaller volumes, processing 15–30 tons of ore per tank at a time. Large leaching tanks can have a diameter of over 17 meters and a height of 3 meters, processing over 1000 tons of ore per tank.

Figure 5-8 Percolation Leaching Tank

1—Tank body; 2—Cement lining; 3—Mineral sand layer; 4—False bottom; 5—Outlet pipe

The false bottom of the percolation tank is approximately 100-200 mm from the bottom of the tank. Its construction varies, generally adapting to local conditions and using locally available materials. The false bottom is usually constructed of square timber strips forming a grid, upon which is laid a supporting material such as reed mats or bamboo mats. Then, canvas, burlap sacks, or a layer of mineral sand is laid on top to support the leached material and allow the leachate to pass smoothly. There is an outlet hole between the bottom and the false bottom in the percolation tank wall, through which the leachate flows out of the tank. Some percolation tanks have movable doors on the side walls or bottom for unloading leaching residue, but most percolation tanks do not have movable doors; the leaching residue is directly excavated from the tank. 5.3.2 Immersion Operation in the Percolation Tank

5.3.2.1 Loading

After the false bottom of the percolation tank is laid, the material to be leached can be loaded into the tank. There are two methods for loading the percolation tank: dry and wet. For dry loading, the material can be manually or mechanically fed into the tank and then leveled. Manual dry loading typically involves using a handcart to feed the material into the tank and then leveling it. Its advantage is that it ensures a loose and porous material layer with relatively uniform particle size, but it is labor-intensive. Mechanical dry loading often uses a belt conveyor to deliver the material to a disc spreader located in the center of the tank. The disc has radial ribs on its surface, and the centrifugal force generated by the high-speed rotation of the disc evenly loads the material into the percolation tank. Mechanical dry loading results in more severe particle segregation and is prone to channeling during leaching. Dry loading allows air to fill the gaps in the material layer, which can improve the gold leaching rate. Dry charging is suitable for materials with a moisture content of less than 20%. Wet-milled ore must be pre-dehydrated before dry charging, making the process more complex.

Wet charging is mainly used in large-scale gold mines operating year-round. In this method, the material to be leached is diluted with water to form a slurry, which is then pumped or gravity-fed into a leaching tank with a false bottom. The ore settles naturally in the tank, and excess water and some sludge are discharged through a ring overflow channel. Once the tank is full, feeding is stopped, and the leaching solution outlet is opened to drain all water from the ore layer. With wet charging, there is less air in the ore layer, resulting in a higher moisture content and a slower gold leaching rate.

When using lime as a protective alkali, it is evenly added to the tank along with the material to be leached. When using caustic soda as a protective alkali, it is dissolved in cyanide solution before being added to the tank.

5.3.2.2 Percolation Cyanide Tank Leaching

After the material to be leached is placed in the percolation tank, cyanide leaching agent can be added for percolation cyanide leaching. There are two ways the cyanide leaching agent passes through the material layer: one is that the cyanide leaching agent percolates from top to bottom through a fixed material layer under gravity; the other is that the cyanide leaching agent is placed in a high-level tank, and the leaching agent percolates from bottom to top through a fixed material layer under pressure. The first method is usually used. The disadvantage of this method is that the sludge in the material percolates along with the leaching agent through the sand layer and accumulates on the false bottom filter medium, gradually reducing the percolation rate. The pressure method can overcome this disadvantage, but

its power consumption and operating costs are higher. During percolation in a percolation tank, it is crucial to control the percolation rate, monitor the pH value and gold content of the leachate, and strictly prevent channeling and “collapse” to ensure that the cyanide leaching agent can percolate evenly through the entire layer of material to be leached.

Based on the method of adding the cyanide leaching agent and discharging the leachate, percolation in a percolation tank can be divided into two operation methods: intermittent and continuous. In intermittent operation, the addition of leaching agent and the discharge of leachate are both intermittent. Typically, a more concentrated leaching agent (e.g., 0.1-0.2% NaCN solution) is first added to the tank, with the liquid level above the material layer. The tank is soaked for 6-12 hours, the leachate is drained, and the tank is allowed to stand for 6-12 hours to allow air to fill the pores of the material layer. Then, a medium-concentration leaching agent (e.g., 0.05%-0.08% NaCN solution) is added to the tank, with the liquid level above the material layer. The tank is soaked for 6-12 hours, the second leachate is drained, and the tank is allowed to stand for 6-12 hours. Next, a lower concentration leaching agent (e.g., 0.03%-0.06% NaCN solution) is added and soaked for 6-12 hours. The third leachate is drained, and clean water is added for washing. After draining the washing liquid, the leaching residue can be discharged. In continuous operation, the cyanide leaching agent is continuously added to the tank, and the leachate obtained after percolation through the material layer is also continuously discharged from the tank. During the percolation process, the liquid level in the tank is always slightly higher than the material layer. In intermittent operation, the pores between the material layers are intermittently filled with air, increasing the dissolved oxygen concentration in the leaching agent. Therefore, under the same conditions, the gold leaching rate of intermittent operation is generally higher than that of continuous operation.

Several percolation tanks can be operated simultaneously. Mixing the leachates from these tanks ensures a more stable gold content in the precious metal solution. Circulating leaching or countercurrent leaching methods can also be used to increase the gold leaching rate and reduce cyanide consumption, resulting in a precious metal solution with a higher gold content. After cyanide leaching is terminated, the leaching residue should be washed with clean water to displace as much of the precious metal solution contained between the material layers as possible, achieving a higher gold leaching rate.

5.3.2.3 Unloading Leaching Residue

The unloading of cyanide leaching residue can be done using either dry or wet methods. Dry unloading can be done manually or with a digging bucket. When there is a central access door at the bottom of the leaching tank, a hole can be drilled from above using an iron bar, through which the cyanide tailings can be unloaded into mine cars and transported away. Generally, it is dug from the top manually or with a digging bucket, and then transported to the tailings dam by mine cars. Wet unloading involves flushing the leaching residue into the tailings ditch with high-pressure water (150–300 kPa),

and then diluting it with water before gravity flow or pumping it to the tailings dam for storage.

Key Factors Affecting Cyanide Leaching: During cyanide leaching, the gold leaching rate mainly depends on factors such as gold particle size, grinding fineness, ore structure, content of impurities harmful to cyanide, concentration and dosage of cyanide leaching agent, leaching rate, leaching time, and the degree of washing of the leaching residue. The optimal values ​​for each factor depend on the properties of the material to be leached and are generally determined experimentally. Loose, porous gold-bearing ores with coarser gold particles are more suitable for cyanide leaching. If the gold particles are mainly in the form of fine particles, under the grinding fineness conditions of leaching, the gold particles are basically present as inclusions, with very few exposed gold particles, resulting in a very low gold leaching rate. Therefore, gold particle size and the looseness and porosity of the ore are decisive factors in determining whether leaching can be used. The leaching rate and infiltration rate of gold are related to the grinding fineness. Higher grinding fineness results in greater exposure of gold particles, increasing the leaching rate, but decreasing the infiltration rate. After grinding, it is best to classify the ore to remove fine mud, allowing only the ore sand to be leached, while the fine mud is sent for agitated cyanidation. This approach increases the exposure of gold particles while maintaining a certain infiltration rate.

During infiltration, the cyanide concentration in the cyanide leaching agent is generally higher than that during agitated cyanidation, typically ranging from 0.1% to 0.2%. Cross-flow leaching is generally performed using multiple batches of cyanide leaching agent with gradually decreasing concentrations. The total amount of cyanide leaching agent passing through the material layer is generally 0.8 to 2 times the weight of the material. The amount of reagent consumed depends on the properties of the material being leached; the sodium cyanide consumption is typically 0.25 to 0.75 kg and lime 1 to 2 kg (or caustic soda 0.75 to 1.5 kg) per ton of dry ore.

The leaching rate in a percolation tank is typically expressed as the linear velocity of the liquid level’s rise or fall, generally controlled between 50 and 70 mm/hour. If the leaching rate is less than 20 mm/hour, the material is considered difficult to leach. The leaching rate depends on factors such as the particle size, shape, and particle size distribution of the material to be leached, the uniformity of the charge, the height of the material bed, the sludge content, and the characteristics of the false bottom filter medium. An excessively high leaching rate may be due to particle segregation or channeling caused by uneven charge. An excessively low leaching rate may be due to high sludge content or sludge and calcium carbonate precipitates clogging the filter medium. Therefore, the false bottom filter medium should be periodically sprayed with water or washed with a dilute hydrochloric acid solution to remove calcium carbonate precipitates.

All impurities detrimental to cyanide leaching can reduce the gold leaching rate in a percolation cyanide tank. When the iron sulfide content is high and oxidation is severe, the ore sand can be washed with water, alkali, or acid before leaching to remove free acid and soluble salts. Alkaline washing can neutralize acids. Washing with dilute sulfuric acid solution can remove copper oxides and carbonates. Adding a certain amount of lead salt to the cyanide leaching agent can reduce the harmful effects of sulfides, arsenic, antimony, and other components.

The leaching time depends on factors such as the properties of the ore, the leaching rate, the loading and unloading methods, the cyanide concentration, and the consumption. In production, a leaching cycle in a percolation tank is generally 4–8 days. When the material classification efficiency is low or the ore slime content is high, a leaching cycle can last as long as 10–14 days.

The degree of washing of the cyanide residue is one of the main factors affecting the gold leaching rate. Generally, after percolation cyanide leaching, 1–2 washings should be performed. A cyclic washing method with progressively increasing concentration can be used for 1–3 washes, but finally, 1–2 washes with clean water are necessary to completely remove the precious solution from the interlayer.

Intermittent rest operation can fill the gaps between the ore layers with air. This can be achieved by blowing air into the ore layer, pre-aerating the cyanide leaching agent, or adding an appropriate amount of oxidant to the cyanide leaching agent. All these methods increase the dissolved oxygen concentration in the cyanide leaching agent, which is beneficial for improving the gold leaching rate and extraction rate.

When treating gold-bearing quartz sand in a percolation tank, the gold leaching rate can reach 85%–90%. When the grinding particle size is coarse and classification is insufficient, the gold leaching rate can drop to 60%–70%. When the gold-bearing material contains high levels of impurities harmful to cyanide leaching, such as copper, arsenic, antimony, and carbon, the gold leaching rate is quite low, and the treatment of the resulting precious metal solution is also more complex.

5.3.4 Example of Cyanide Percolation Leaching A gold beneficiation plant in my country processes gold-bearing quartz vein oxide ore. The main metallic minerals are native gold, galena, pyrite, limonite, malachite, etc., and gangue minerals are quartz, mica, calcite, kaolin, etc. The raw ore contains 8-10 g/t of gold and 1% of lead. The gold particles are fine, and the ore has a high content of mud and soluble salts. The plant uses a combined process of amalgamation-gravity separation-cyanide percolation leaching to recover gold. The ore is ground to 60%-0.074 mm. Amalgamation plates are installed at the ball mill discharge end and the classifier overflow to recover coarse gold particles. The gold recovery rate during amalgamation is about 20%. The amalgamation tailings are classified by hydraulic classifier and then fed into shaking tables for particle size separation to obtain a lead concentrate containing 50% lead and 11.6 g/t of gold. The tailings from the shaking table are settled in coarse and fine sand sedimentation tanks, with the overflow being discarded. The coarse sand obtained after sedimentation has a particle size of 98%-0.42 mm; the fine sand has a particle size of 98%-0.125 mm. The coarse and fine sands are excavated separately using an excavator, dried, and then mixed in a coarse sand:fine sand ratio of 3:1. The mixed sand is the raw ore for leaching in the cyanide percolation tank, containing 3.48 g/t of gold, with a water content of 5%–6%, and a particle size of 40%-0.074 mm.

The percolation tank is rectangular, with dimensions of 4 m × 3 m × 1.2 m. Each tank holds 16 tons of ore, with a bottom slope of 0.3%. The false bottom is 100 mm from the tank bottom and consists of a bamboo curtain covered with burlap sacks. There is no working door at the bottom of the tank. The cyanide ore was manually loaded into a leaching tank, and then a 0.5% concentration leaching solution with a pH of 9-10 was added. Intermittent leaching was used, with sodium hydroxide as a protective alkali.

The precious metal leaching solution was then transferred to a zinc wire displacement settling tank for gold precipitation. The settling tank was divided into seven compartments, each measuring 0.2m × 0.2m × 0.3m, with a displacement time of 7 minutes.

The cyanide tailings were excavated by an excavator and transported to a tailings dam using a ramp winch. The lean solution after displacement was returned for leaching use for one and a half months, then treated with bleaching powder before being discharged.

When the cyanide ore contained 3.48 g/t of gold, the cyanide tailings contained 0.64 g/t of gold, resulting in a gold leaching rate of 81.3%. The sodium cyanide consumption was 2 kg/t, and the sodium hydroxide consumption was 0.5 kg/t. The overall gold recovery rate of the plant was 74.4%, of which approximately 20% was recovered from amalgamation, approximately 15% from gravity separation, and approximately 40% from cyanidation.