1 Overview
Using the heap leaching process to extract the metal from the ore already has a long history, it was first used copper ore, and later for leaching uranium mine, to the late 1960's, with the technical and economic development The heap leaching method is widely used for gold extraction in low-grade oxidized ore. Later, with the improvement of activated carbon adsorption technology, the heap leaching technology is more perfect and widely developed worldwide, especially in the United States. Widely, its gold production accounts for one-third of the total output, and its production scale is huge. Some of the annual processing capacity reaches several million tons. In recent years, in order to deal with ore containing clay with high clay content, granulation heap leaching technology has been developed, and the superiority of heap leaching gold extraction technology has been increasingly shown. In the late 1970s, China began the experimental research on heap leaching and gold extraction, and then applied it to industrial production in the early 1980s. By the turn of the century, Zijin Group vigorously used heap leaching technology, currently in the national gold industry. Occupy an important position.
Through experimental research and production practice, the ore suitable for heap leaching gold must meet the following conditions:
First, the gold particles in the stone are small and the surface is clean;
Second, the stone itself has good porosity and permeability;
Third, substances that are substantially free of harmful cyanide (such as copper, arsenic , antimony , carbon, etc.);
Fourth, the ore has low acid content;
Fifth, the ore does not contain excessive amounts of fine mud and clay;
The heap leaching gold extraction process basically comprises the following two processes: one is to use cadmium leaching solution to leach gold from the heap to produce gold-containing precious liquid; the other is to use various methods to extract gold from the leaching solution.
2, heap leaching gold extraction process
There are three common processes: (1) heap leaching - zinc replacement - gold mud baking; (2) heap leaching - carbon adsorption - desorption of electricity; (3) heap leaching - carbon adsorption - incineration of gold Charcoal - ash smelting.
The above three processes are most suitable for the process of "heap leaching - carbon adsorption - desorption of gold-loaded carbon".
3. Factors affecting heap leaching
3.1, the structure, physical and chemical properties of ore and the occurrence of gold particles
The ore to be immersed must have loose structure, large porosity, surface pores and capillary pores, which will facilitate the diffusion of leaching liquid and oxygen into the interior of the ore and accelerate the leaching rate of gold. Gold-bearing oxidized ore has the characteristics of loose porosity, so it is more suitable for heap leaching than dense ore sulfide ore.
The finer the gold particles in the immersed ore, the faster the leaching rate. When the fine gold is stored in the cross section and crack of the ore, it is easy to leaching. However, when fine-grained gold is wrapped by other minerals, it is difficult or even impossible to be leached.
The shape of the gold particles also has an effect on the leaching, such as the flaky gold compared to the coarse spherical gold.
3.2, other components in the ore
The ore mainly contains: mineral composition such as As, Sb, C, Cu, etc. Some minerals will react with cyanide and oxygen to consume cyanide and oxygen, affecting the leaching of gold, and some will passivate the gold surface. The film hinders the leaching of gold, so care must be taken in the incoming condition to take measures to reduce its effect on leaching.
3.3, the size of the infiltrated ore and the permeability of the heap
The particle size of the infiltrated ore has a great influence on the leaching of gold. The smaller the particle size of the ore is broken, the larger the surface area exposed by the gold particles, the faster the leaching speed, but the smaller the particle size, the greater the influence of the leaching solution. Forming a dead angle, in addition, too much fine ore powder will cause the amount of residual liquid in the slag to increase, resulting in metal loss.
Factors affecting permeability, in addition to particle size, are also related to the amount of ore contained in the ore, the clay content, and the method of pile-up and the height of the pile.
3.4. Composition and pH of leaching solution
The cyanide heap leaching process is mainly controlled by the diffusion step of each component in the leaching ore solution. When the concentration of cyanide in the leaching solution is low, the effect is more obvious. Practice has proved that when other conditions are the same, the same is achieved. The leaching rate, the leaching time of the 0.1% NaCN solution is about one quarter of the leaching time of the concentration of 0.25% leaching solution. Usually during the heap leaching process, the cyanide concentration should be controlled between 0.025 and 0.1% depending on the ore composition and the different stages of leaching.
Ensuring that the leaching solution has sufficient alkalinity is a necessary condition for heap leaching, so it is necessary to constantly control the alkalinity of the leaching solution, and often keep the pH of the leaching solution at 10-11.
3.5, spray method and spray intensity
The spray method affects the uniform distribution of the leaching solution and the atomization loss. In the production, the leaching solution should be distributed as evenly as possible, and there is no spray dead angle to reduce the atomization loss. Spraying intensity refers to the amount of leaching ore discharged to a certain amount of ore per unit time. Increasing the amount of spray can accelerate the circulation of the leaching solution and increase the leaching speed of gold. However, excessive spray intensity will increase the impurities in the ore. Leaching, correspondingly reduce the grade of gold in the leachate, affecting the recovery of gold-containing precious liquid.
3.6. Temperature and meteorological effects
Increasing the temperature increases the leaching speed, but for heap leaching, manual temperature adjustment is generally not appropriate. In rainy seasons and regions, the impact on leaching should be considered in production. In high winds, high temperature and dry seasons, the area should consider the loss of spray.
4. Industrial production of heap leaching
4.1. Selection and construction of the site of the heap leaching plant
The heap leaching site must be selected in an area where the soil texture is uniform and has sufficient strength to carry thousands of tons of ore and equipment, to ensure that there is no local subsidence to damage the dip pad, the gold-containing leachate is lost, and the site is required to have a certain slope. So that the leachate collects the outflow, but it should not be too large to prevent the bottom pad from sliding.
4.2, heap immersion construction
The construction of heap leaching directly affects the porosity of the inner part of the heap and the uniformity of solution penetration. Therefore, in the pile building, the occurrence of coarse and fine particle segregation should be avoided as much as possible to avoid compaction of the heap. The height of the heap is determined by the test to be generally 2 to 9 meters.
4.3. Technical conditions for heap leaching
(1) The porosity of the loose porous infiltrate may be generally -50 or -30 mm, and the dense ore may be controlled at -10 mm.
(2) The concentration of cyanide in the leaching solution is generally controlled between 0.025 and 0.1%. In practice, it is better to control the stage according to the third stage of the junior high school. The initial stage is higher and the later stage is lower.
(3) The pH value of the leaching solution is generally controlled between 9.5 and 11. The usual operation is to wash the metal impurities with saturated lime water or caustic soda after washing and removing metal impurities after immersion. The immersion gold can be sprayed until the pH of the bottom of the stack reaches 9.5 or higher.
(4) Spray intensity and leaching time
4.4, adsorption recovery of gold-containing solution
Carbon recovery is usually carried out by carbon adsorption. The specific adsorption devices are various, the same in nature, and the efficiency is different. The small and medium-sized storage yards are mostly adsorbed by tandem carbon columns.
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