Although the heap leaching method has the advantages of simple process, unsimplified process, less capital investment, easy operation, low cost and quick effect, there are many factors affecting the production index and economic benefit of the process. Possible. Ten issues concerning the impact of gold heap leaching process is analyzed as follows:
1. Ore structure and its physical and chemical properties
Generally speaking, if the ore structure is loose and the fracture is developed, the permeability is good, which is favorable for the penetration and diffusion of the cyanide solution into the ore, so that the gold is fully dissolved, and therefore, the leaching speed is faster. For example, the Gaojiatun gold deposit in Xinshao County, Hunan Province is a argillaceous sandstone type with good permeability. When the mine was used for production and debugging, it was tested. When the spray was leached, the sample was sampled the next day. The concentration is above 4g/m3, which exceeds the adsorption requirement (the concentration of the noble solution required for adsorption is > 1.0g/m3). On the other hand, if the ore structure is dense or contains a high clay , it is not suitable for leaching, and the leaching rate is slow, and the leaching rate is also low. The Fan Lingbao fork quartz vein gold is gold mine have been seven 000 t 1988 Industrial Test heap leaching, the ore dense end result, poor permeability, the leaching rate (40%) failed. It can be seen that the ore structure has a great influence on heap leaching.
If the ore contains carbonaceous materials, it will prevent the solution from entering the ore particles wrapped by it, thereby seriously hindering the dissolution of gold, and the activated carbon can adsorb the dissolved gold, causing the gold to be lost with the tailings.
If the ore contains a non-noble metal compound such as copper , lead , zinc , arsenic or iron , it also affects the dissolution of gold. If the noble solution is replaced by zinc, the effect is poor and the consumption of cyanide is increased.
Further, if the sulfur concentration of the cyanide ions in the solution reaches 5 × 10-7, it will reduce the rate of dissolution of gold, xanthate and dithio phosphate flotation agents and the like will reduce the rate of dissolution of gold.
The size of the gold particles is related to the dissolution rate. The dissolution time of the gold particles of different particle sizes in the 0.12% sodium cyanide solution is shown in Table 1.
2, ore particle size
The size of the ore particle size has a great influence on the leaching rate. Generally, the smaller the particle size, the shorter the leaching time required and the higher the leaching rate.
However, in the production practice, if the fine-grain content is excessive (refer to the content of -0.074 mm exceeding 35%), the leaching rate (generally reduced by about 3 to 5%) is affected. This is because too much fine-grained grade will cause the surface of the heap to form a channel and affect the penetration of the solution.
It can be seen from the curve of 8mm grain size in Figure 2 that the Linghu quartz veined mound-type gold ore contains gold leaching rate of 41.1% and Fanjing quartz vein type of 52.7%. Henan Maotang porphyry gold deposit is 62.8%, Hubei Daye iron cap type gold mine is 73%, and the upper tube altered rock gold mine surface oxide ore is 91.4%. It can be seen that in addition to the ore particle size, the ore type is also one of the important factors affecting the gold leaching rate.
3. Clay content in raw ore
The amount of clay in the ore has a significant effect on leaching. Studies have shown that when the clay is wetted by the cyanidation solution, the volume expands. Its volume increase rate is 25 to 30%. this will
This leads to a decrease in the porosity of the heap and the penetration rate of the solution. As shown in Fig. 3, when the clay content was increased from 15% to 60%, the permeation rate of the solution was lowered from 25 cm/d to 0.4 cm/d, and the leaching time was increased from 15d to 125d.
For ore with a high clay content, the solution penetration rate varies with the leaching time, as shown in Figure 4. When leaching for the 10th day, the solution penetration rate reached a maximum of 6 L/t or d. Thereafter, due to the expansion of the clay mineral, and the porosity of the heap is reduced, the solution permeation rate is reduced to 3.5 to 4 L/t or d.
4 pH
During the leaching process, the cyanide solution must maintain a certain degree of alkalinity to prevent decomposition of cyanide. Therefore, the pH should be controlled between 9.5 and 11. If it is too high, the dissolution rate of gold will be correspondingly reduced. For example, in Gaojiayu Gold Mine in Hunan Province, during the production period in 1993, the control of lime dosage was not strict during the construction. The pH value is above 12, which causes a film to form on the surface of the ore, which affects the dissolution rate of gold and prolongs the leaching time.
The effect of pH on the dissolution rate of gold is shown in Figure 5. It can be seen from the figure that the use of lime as a protective base has a significant inhibitory effect on the dissolution of gold when the pH is >11.5. This is due to the formation of a film of calcium peroxide on the surface of the ore at high pH to prevent it from reacting with cyanide.
5, cyanide concentration
Relationship between dissolution rate of gold and silver cyanide concentration as shown in Figure 6.
It can be seen from the figure that when the cyanide concentration is below 0.05%, the dissolution rate of gold and silver rises linearly with the increase of cyanide concentration. If the concentration is continuously increased, the solubility of gold and silver only rises slowly. Until the cyanide concentration increases to 0.1%. When the concentration exceeds 0.1% or more, the dissolution rate of gold and silver gradually decreases. Therefore, it must be noted in actual production that the higher the cyanide concentration, the faster the dissolution rate of gold and silver. The reason why the dissolution rate of gold and silver in the low concentration cyanide solution is faster is that the dissolution rate of oxygen and the diffusion rate in the dilute solution are large. The solubility of oxygen in low-concentration cyanide solution is almost constant. Therefore, when leaching ore with low-concentration cyanide solution, the dissolution rate of gold and silver is very high, but the solubility of various non-metallic minerals is very small. In this way, the consumption of cyanide can be reduced to a minimum. When the Gaojiayu gold mine heap leaching plant in Hunan was used for production debugging, it was sprayed and leached with a low concentration cyanide solution. The results show that in the low concentration solution ((0.03%~0.08%) gold dissolves faster, the gold concentration of the noble liquid reaches the adsorption requirement on the second day after spraying. Not only the speed is fast, but also the dosage of cyanide is greatly reduced. According to calculations, the consumption of one ton of raw ore is only 135g/t of sodium cyanide (about 100g/t in Guangxi Longbao Gold Mine), and the dosage is 1/3~1/4 of the same type of heap leaching mine in China. Advanced level in foreign countries.
Regarding the relationship between the cyanide concentration and the dissolution rate of gold, the author believes that when the cyanide concentration is low, the dissolution rate of gold depends only on the concentration of the cyanide solution. Conversely, when the cyanide concentration is high, the gold dissolution rate is independent of the cyanide concentration and depends on the oxygen concentration. At different cyanide concentrations, the leaching rate of gold varies with time, ie if the same leaching rate is required, the time required for low concentrations may be longer, as shown in Figure 7. When the cyanide concentration is 0.025%, the leaching rate of 80% is required to be 25d, and when the cyanide concentration is 0.1%, it is only 5d to achieve the above leaching rate. Therefore, in actual production, it is necessary to comprehensively weigh the relevant factors such as cyanide consumption, leaching time and cost.
According to the experience of heap leaching production, when processing gold ore, the cyanide concentration should be controlled within the range of 0.03 to 0.08%, and the silver content should be 0.1% to 0.15%.
6, the height of the heap
The height of the heap depends on the nature of the ore. Generally, the ore with good permeability can be higher, otherwise it is the opposite. At present, the average height of heap leaching mines in China is 2 to 4 m, and that of foreign countries is 4 to 8 m. However, with the renovation and renewal of pile-up equipment, the pile height has reached 10 to 18 m. The United States has tested a 61m high heap, which suggests that some ore is suitable for high heap leaching. It is appropriate to determine how high the heap is, and it can be determined through experiments. Production practices have shown that too high a heap will affect the oxygen supply and permeability of the lower ore, which will reduce the leaching rate.
The height of the pile directly affects the cost, but with the increase of the pile, the cost of the bottom pad and the management fee are correspondingly reduced. Laboratory column immersion test data indicates that an increase in column height will result in a decrease in leaching rate. The stack height will increase from 6.1 m to 18.3 m, and the heap leaching cost can be reduced by 3 to 3.5 yuan/t, but the leaching rate will also be reduced by about 5%. Both are well-prepared and the mine can be accounted for.
In order to reduce production costs, the author suggests that stepped heap leaching is more reasonable. China's Wujialin gold mine has been used successfully in production. The method is to make full use of the natural slope of the terrain to build the yard, as shown in Figure 8. That is to say, after the first pile of spray leaching, it is not necessary to unload the pile, and the pile can be continuously piled on the basis of the original pile. As long as the bottom pad is not broken, 3 to 5 layers can be continuously piled up, and the pile height can reach 15 to 25 m.
7, spray strength
Practice has shown that: properly increasing the spray intensity can shorten the leaching time and increase the leaching rate, as shown in Figure 9. At the same time, the relative movement between the solution and the ore is enhanced to enhance the diffusion.
The spray intensity of the heap leaching mine in China is 8~12L/m2·h, and that of foreign countries is 10~20L/m2·h. The spray strength is large, although it has certain advantages, but the chance of contact with the cyanide solution increases with air, and the oxidation and loss of cyanide also increase. Therefore, too high spray strength is unfavorable for production.
8, leaching time
The leaching rate increases as the leaching time increases. The length of the leaching cycle is related to the embedding properties of gold, ore particle size and permeability. The on-site heap leaching time is usually 3 to 6 times the laboratory column immersion test time.
Proof of production, such as the gold grade in the ore is 2.0 ~ 3.5g / t, the scale is 5000 ~ 7000t / heap, the stack height is 3.5 ~ 4. 5m, the leaching time can be completed 30 ~ 35d (excluding piles, washing and Unloading).
9, oxygen in the solution
Balsky et al. used nitrogen, oxygen and their mixed gases in 0.1% sodium cyanide solution to determine the dissolution rate of gold at different oxygen concentrations as shown in Table 3. As can be seen from the table, the dissolution rate of gold increases with increasing oxygen concentration.
The solubility of oxygen in water varies with temperature and oxygen partial pressure on the liquid surface, depending on the local atmospheric conditions at the altitude elevation. The oxygen concentration in the water is in the range of 5 to 10 mg/L.
When gold ore is subjected to heap leaching cyanidation, other minerals and organic matter also consume dissolved oxygen in the solution. Therefore, in the cyanidation process, the total consumption of oxygen usually exceeds several hundred times, or even thousands, of the theoretically required oxygen amount at the time of the reaction. Therefore, it is advantageous to properly replenish oxygen during the heap leaching process.
The test results of the heap gas leaching test are shown in Table 4. As can be seen from the table, the air is blown into the heap to increase the dissolution rate of gold. The solution does not have to be saturated, and the leaching time is significantly shortened. The leaching time after blowing was reduced from 45d to 32d.
10, temperature
The rate of dissolution of gold in the cyanide solution increases with increasing temperature and is greatest at 85 °C. When the temperature is below 10 ° C, the gold dissolution rate will be greatly slowed down.
Increasing the temperature accelerates the cyanidation reaction and increases the leaching rate of gold. The heap leaching is carried out at a temperature in a natural environment, and it is advantageous to appropriately increase the temperature of the leaching solution in order to continue the leaching in the winter. It is said that heating the solution with a heater is technically and economically feasible. Therefore, the heap leaching mine with the solution heating system can spray leaching even at a temperature of -10 ° C, so the warm leaching is suitable for heap leaching in northeast and northwest China.
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