Ten common problems in heap leaching process
While heap leaching offers advantages such as simple process, uncomplicated procedures, minimal capital investment, ease of operation, low cost, and rapid results, numerous factors can influence its production performance and economic benefits, and improper handling can result in losses. The following is an analysis of the ten major issues affecting gold heap leaching:
Ore structure and its physical and chemical properties.
Generally speaking, ore with a loose structure and well-developed fissures exhibits good permeability, facilitating the penetration and diffusion of the cyanide solution into the ore, allowing for sufficient gold dissolution and, consequently, faster leaching rates. For example, the Gaojia'ao gold mine in Xinshao County, Hunan Province, is a argillaceous sandstone ore with good permeability. During commissioning, testing was conducted at the mine. Samples taken the day after spray leaching revealed a gold concentration in the precious solution exceeding 4g/m³, exceeding the adsorption requirement (adsorption requires a precious solution concentration > 1.0g/m³). Conversely, ore with a dense structure or high clay content is less conducive to leaching, resulting in slower leaching rates and lower leaching rates. For example, the Fancha gold mine in Lingbao, Henan Province, is a gold-bearing quartz vein. In 1988, the mine conducted a 70,000-ton heap leaching industrial trial, but ultimately failed due to the dense ore, poor permeability, and low leaching efficiency (approximately 40%). This demonstrates that ore structure significantly influences heap leaching. Carbonaceous materials in the ore can prevent the solution from entering the encapsulated ore particles, severely hindering gold dissolution. Furthermore, activated carbon can adsorb dissolved gold, causing it to be lost in the tailings. Non-precious metal compounds such as copper, lead, zinc, arsenic, and iron in the ore also affect gold dissolution. Replacing precious solution with zinc, for example, is less effective and increases cyanide consumption. Furthermore, if the sulfide ion concentration in the cyanide solution reaches 5×10⁻⁷, the gold dissolution rate will decrease. Flotation agents such as xanthate and dithiophosphates also reduce gold dissolution. Gold particle size is related to its dissolution rate.
2. Ore Particle Size: Ore particle size significantly influences leaching efficiency.
Generally speaking, the smaller the particle size, the shorter the leaching time and the higher the leaching rate. However, in practice, an excessive fines content (defined as a -0.074mm content exceeding 35%) can negatively impact leaching efficiency (typically by approximately 3-5%). This is because excessive fines can cause the ore surface to harden, leading to channeling and impeding solution penetration. The gold leaching rate for Linghu quartz vein-moltenite-type gold-bearing ore is 41.1%, for Fancha quartz vein-type ore it is 52.7%, for Maotang porphyry-type gold deposit in Henan it is 62.8%, for Daye iron cap-type gold deposit in Hubei it is 73%, and for surface oxide ore in Shangguan altered rock gold deposits it is 91.4%. Therefore, in addition to ore particle size, ore type is also a significant factor influencing gold leaching efficiency.
3. Clay Content in the Raw Ore:
The clay content of the raw ore has a significant impact on leaching. Research has shown that when clay is wetted by the cyanide solution, it expands in volume, with the volume increase reaching 25-30%. This leads to a decrease in the porosity of the ore pile and the solution penetration rate. When the clay content increases from 15% to 60%, the solution penetration rate decreases from 25 cm/day to 0.4 cm/day, while the leaching time increases from 15 days to 125 days. For ores with high clay content, the solution penetration rate varies with leaching time, reaching a maximum of 6 L/ton ore/day on the 10th day of leaching. Thereafter, due to the expansion of the clay minerals and the decrease in ore pile porosity, the solution penetration rate decreases to 3.5-4 L/ton ore/day.
4. pH Value:
During the leaching process, the cyanide solution must maintain a certain alkalinity to prevent cyanide decomposition. Therefore, the pH value should be controlled between 9.5 and 11. If it is too high, the dissolution rate of gold will be correspondingly reduced. For example, during production at the Gaojiaao Gold Mine in Hunan Province in 1993, due to lax control of lime dosage during heap construction, the pH value reached over 12, resulting in the formation of a film on the ore surface, which affected the dissolution rate of gold and prolonged the leaching time. The effect of pH on the dissolution rate of gold. Using lime as a protective base, when the pH is > 11.5, it has a significant inhibitory effect on gold dissolution. This is because at high pH values, a calcium peroxide film forms on the ore surface, preventing it from reacting with cyanide.
5. Cyanide Concentration:
When the cyanide concentration is below 0.05%, the dissolution rate of gold and silver increases linearly with increasing cyanide concentration. Further increases in concentration only lead to a slow increase in their solubility until the cyanide concentration reaches 0.1%. When the concentration exceeds 0.1%, the dissolution rate of gold and silver gradually decreases. Therefore, in actual production, it is important to note that higher cyanide concentrations do not necessarily lead to faster dissolution of gold and silver. The faster dissolution rate of gold and silver in low-concentration cyanide solutions is due to the higher dissolution rate of oxygen and its diffusion rate in dilute solutions. The solubility of oxygen in low-concentration cyanide solutions is almost constant. Therefore, when leaching ore with low-concentration cyanide solutions, the dissolution rate of gold and silver is very high, while the solubility of various non-metallic minerals is very low. This minimizes cyanide consumption. During production commissioning, a gold heap leach plant in Hunan Province employed spray leaching with a low-concentration cyanide solution. The results show that the gold dissolution rate is faster in low concentration solution (0.03% to 0.08%), and the gold concentration in the precious solution reaches the adsorption requirement the next day after spraying. Not only is the speed fast, but the amount of cyanide used is also greatly reduced. According to calculations, the sodium cyanide consumption for processing one ton of raw ore is only 135g/t (about 100g/t in a gold mine in Guangxi), which is 1/3 to 1/4 of the amount used in similar heap leaching mines in China, and is lower than the advanced level abroad. Regarding the relationship between cyanide concentration and gold dissolution rate, the author believes that: when the cyanide concentration is low, the gold dissolution rate only depends on the concentration of the cyanide solution. On the contrary, when the cyanide concentration is high, the gold dissolution rate is closely related to the cyanide concentration. The leaching rate of gold varies with time at different cyanide concentrations. For the same leaching rate, lower concentrations may require longer leaching times, as shown in Figure 7. At a cyanide concentration of 0.025%, it takes 25 days to reach an 80% leaching rate, while at a cyanide concentration of 0.1%, this is achieved in just 5 days. Therefore, in actual production, various factors, including cyanide consumption, leaching time, and cost, must be comprehensively weighed. Based on heap leaching experience, the cyanide concentration for gold ore should be controlled within the range of 0.03% to 0.08%, and for silver, 0.1% to 0.15%.
6. Pile Height:
Pile height depends on the properties of the ore. Generally, higher piles are acceptable for ores with good permeability, and vice versa. Currently, the average height of heap leaching mines in my country is 2-4 meters, while overseas, it's 4-8 meters. However, with the renovation and upgrading of heap leaching equipment, heap heights have reached 10-18 meters. A 61-meter-high heap has been tested in the United States, demonstrating that certain ores are suitable for high-heap leaching. The optimal heap height can be determined through experimentation. Production practice shows that excessively high heaps can affect the oxygen supply and permeability of the underlying ore, thereby reducing leaching rates. Heap height directly impacts costs, but as heap height increases, bedding costs and management fees also decrease. Laboratory column leaching test data indicate that increasing column height reduces leaching rates. Increasing heap height from 6.1 meters to 18.3 meters can reduce heap leaching costs by 3-3.5 yuan per ton of ore, but the leaching rate will also decrease by approximately 5%. Mines can consider the optimal balance between the two. To reduce production costs, I recommend the use of a stepped heap leaching method. This method has been successfully used in production at a gold mine in my country. This method utilizes the natural slope of the terrain to construct a stockpile, as shown in Figure 8. This means that after the first heap is finished with spray leaching, the heap does not need to be unloaded. Ore can be piled on top of the original heap. As long as the base pad is intact, 3 to 5 layers can be piled, reaching a height of 15 to 25 meters.
7. Spray Intensity:
Practice has shown that appropriately increasing the spray intensity can shorten leaching time and improve leaching efficiency, as shown in Figure 9. It also enhances the relative movement between the solution and the ore, enhancing diffusion. The spray intensity used in heap leaching mines in my country is 8 to 12 L/m²·h, while in other countries it is 10 to 20 L/m²·h. While high spray intensity has certain advantages, it increases the exposure of the cyanide solution to air, leading to increased oxidation and loss of cyanide. Therefore, excessively high spray intensity is detrimental to production.
8. Leaching Time:
The leaching efficiency increases with leaching time. The length of the leaching cycle is related to the gold embedding characteristics, ore particle size, and permeability. On-site heap leaching typically lasts 3 to 6 times longer than laboratory column leaching tests. Production has proven that, for ore with a gold grade of 2.0 to 3.5 g/t, a production capacity of 5,000 to 7,000 tons per heap, and a heap height of 3.5 to 4.5 meters, leaching can be completed in 30 to 35 days (excluding heap construction, washing, and unloading).
9. Oxygen in Solution.
Barsky et al. measured the dissolution rate of gold in a 0.1% sodium cyanide solution using nitrogen, oxygen, and their mixtures at varying oxygen concentrations (see Table 3). As can be seen from the table, the gold dissolution rate increases with increasing oxygen concentration. The solubility of oxygen in water varies with temperature and the oxygen partial pressure above the liquid level, primarily depending on local atmospheric conditions at altitude. The maximum oxygen concentration in water is between 5 and 10 mg/L. During heap leaching and cyanidation of gold-bearing ore, other minerals and organic matter also consume dissolved oxygen in the solution. Therefore, the total oxygen consumption during the cyanidation process often exceeds the theoretical oxygen requirement by hundreds or even thousands of times. Therefore, appropriate oxygen supplementation is beneficial during heap leaching. Bubbling air into the ore heap can increase the gold dissolution rate. This eliminates the need for saturation in the solution and significantly shortens the leaching time. After aeration, the leaching time was reduced from 45 days to 32 days.
10. Temperature:
The dissolution rate of gold in a cyanide solution increases with increasing temperature, reaching its maximum at 85°C. When the temperature drops below 10°C, the gold dissolution rate slows significantly. Raising the temperature accelerates the cyanidation reaction and improves the gold leaching rate. Heap leaching is conducted in natural ambient air temperatures. To continue leaching in winter, it is beneficial to appropriately raise the leachate temperature. Using a heater to heat the solution is technically and economically feasible. Therefore, heap leaching mines equipped with a solution heating system can continue spray leaching even at temperatures as low as -10°C, making heated leaching highly suitable for heap leaching in Northeast and Northwest my country.
