Heap leaching technology and determination of its feasibility

Human gold mining has a long history, but due to a large number of low-grade gold mines, tailings and gold-bearing waste stones that have not been utilized due to process technology, there is a shortage of gold resources in the face of human beings. It is also a contradiction between backlog and waste of mineral resources.

With the advancement of mineral processing technology, the US Mining Bureau Reno Research Center first piled up the gold-bearing waste rock in 1969 and published a test report. In the early 1970s, the heap leaching process began to be in some gold. The mine was promoted. The application and development of this process technology has provided a successful and profitable method for the treatment of low-grade gold mines, and at the same time promoted the development of geological prospecting.

The heap leaching method has an economically leached ore grade that is one order of magnitude lower than the conventional grinding leaching method. At present, foreign countries can leaching gold from ore containing 0.03 to 0.01 ounces of gold in a grade of 1 ton. For easy leaching ore, the heap leaching method has low capital cost, simple process, convenient operation, short process, less floor space, less consumption of cyanide leaching solution, strong adaptability, large or small scale, and low investment. Etc. However, the method has a slower leaching speed and stricter requirements on the nature of the ore. Therefore, compared with other gold selection methods, the recovery rate of gold is low. Under normal circumstances, the recovery rate of gold can only reach 65-80%. In addition, since the heap leaching process conditions are difficult to change and the production results cannot be seen very quickly, the leaching behavior of the ore must be known in advance and must be piled up and leached in strict accordance with the best conditions provided by the test. Therefore, before heap leaching Adequate metallurgical testing is required to determine the viability of the ore and provide sufficient design basis.

The so-called heap leaching method is to deposit low-grade gold ore on a leak-proof bottom pad paved with materials such as asphalt , concrete or plastic film, and spray it on the heap with a low concentration of alkaline cyanide solution to make gold. The dissolved, gold-rich rich liquid is leached from the heap, flows into the sump on the bottom pad and is transported to the liquid storage tank, and then the gold is recovered from the rich liquid by activated carbon adsorption or metal zinc displacement precipitation method, adsorption or The replaced lean liquid is returned to the leaching operation. At present, it has developed into an important tool for heap leaching gold and silver processing. As far as the United States is concerned, its heaped gold production has reached 30-40% of total gold production.

Compared with conventional smelting plants, heap leaching plants have much lower infrastructure costs for heap leaching. According to the data, in 1986, Nevada had the same production capacity, but different process smelters were put into production and they were all open-pit mining. However, the HogRanch mine used heap leaching to raise gold, and its infrastructure cost totaled 7 million US dollars. The Paradise Peak mine uses conventional agitation leaching equipment to raise gold, and its infrastructure costs are as high as $80 million.

China's gold heap leaching also has a long history, but so far, the actual production capacity is still very few, mostly in the experimental research stage. In 1988, the Third Geological Team of Shaanxi Bureau of Geology and Minerals, the Institute of Minerals Comprehensive Utilization of the Ministry of Geology and Mineral Resources conducted the first 10,000-ton industrial heap leaching test at the Shuangwang Gold Mine in Shaanxi Province. The amount of ore invested was 10030 tons, and the original ore grade was 1.92. The leaching rate of gold/ton, gold is 66.15%, and the recovery rate of gold is 64.64%. In 1989, China's first 100,000-ton low-grade gold ore heap leaching plant was built in Altay, Xinjiang. In that year, 24,000 tons of ore was piled up and 2,300 gold was produced. The original ore grade was mostly 1.5-3.0 g/t. The pile of 110,000 tons of gold was 1,200, and the per capita profit was 156,700 yuan. This fully shows that although China's gold smelting started later, it has made great breakthroughs in heap leaching technology.

At present, heap leaching is mainly for the treatment of low grade ore and waste rock containing 0.5-3 g/ton of gold. The ore suitable for heap leaching should first be porous and can penetrate cyanide. Relatively no elimination of cyanide and fine gold particles, that is, the particle size of gold must be very small. The ore does not contain gold cyano complex. The carbonaceous material; at the same time, in order to maintain sufficient permeability of the immersion liquid and good contact of the mineral surface with the immersion liquid, the clay content in the ore should be sufficiently low. In addition, the ore does not contain a large amount of a compound which reacts with cyanide, and does not contain an excessive amount of an acidic component which reacts with lime.

For gold leaching, several types of solutions are now available, but currently the effective leaching agent is still an alkaline cyanide solution with a heap leaching concentration ranging from 0.15 to 2.5 g/kg.

However, we also know that the ore is often broken before leaching, but the resulting fine particles and the clay formed by natural air drying and leaching of the leachate will hinder the contact of the ore with the solution. However, small particles leached faster and more completely than small particles. Therefore, if the content of a 10 mm ore in the raw material is high or contains a lot of clay, in the granulator before the heap leaching, the fine ore is mixed with lime and cement, and the water is sprayed to stick the fine particles together, and then solidified. Perform leaching. As for the method used, the pellets should be treated according to the ore particle size composition. It can be seen that it is necessary to determine the most economical range of crushing size and carry out laboratory pellet testing.

For a heap leaching plant, the heap leaching scale can be large or small, with a large annual processing capacity of up to one million tons and a small annual processing capacity of several thousand tons to several tens of thousands of tons. In the United States, the daily processing capacity of a heap leaching plant ranges from a few hundred tons to 18,000 tons. There are two types of heap leaching methods for industrial scale. One is leaching in the form of ore. This method has a long leaching period and is usually called long-term heap leaching. Another method is based on the leaching of broken ore, which has a short leaching period and is commonly referred to as short-term heap leaching.

Cloth is also a very important issue when heaping ore. The most common liquid dispensing system is a pulse shower or a oscillating shower. For some typical heap leaching mines in the United States, the spray rate of the leachate is usually in the range of 0.8-1.2 cm/hr, and the maximum effective application speed is 4.0 cm/hr. The higher the speed, the movement in the oxidized ore is limited. The concentration of the dilute leachate increases the cost of the pump.

In addition, in order to increase the leaching rate of cyanide, it is necessary to control the pH and adjust it to 10 - 1, to prevent a large amount of hydrogen cyanide from being released from the heap. Usually lime is added to the ore to achieve this purpose, and caustic soda can also be used to adjust the pH value, and the recovery rate is higher by this method, but the reagent cost is higher, which is uneconomical for low grade ore. .

In terms of gold recovery, it is mainly recovered by zinc powder replacement or carbon adsorption. For each system, each method has its advantages and disadvantages. Which method is selected depends on the specific conditions and existing equipment in the leaching operation. .

The zinc powder replacement method has a fast reaction kinetics and low cost, does not require desorption equipment, has low investment and operation costs, and has high efficiency, but has poor selectivity and is easy to pollute the environment.

The carbon adsorption method is now more and more popular, and its advantages are: it can adsorb metal from a solution containing a suspended solid solution and a very low metal concentration, and does not require a filtering device, and does not require a deoxidizing tank and a pump, thereby avoiding zinc Environmental pollution caused by salt. However, its carbon transportation and regeneration benefits are poor, and the investment cost is high.

In addition, in addition to carbon adsorption and zinc powder replacement, there are ion exchange adsorption, solvent extraction and direct electrolysis. However, in general, the heap leaching solution may contain suspended solids of 10 to 300 mg/liter. The electrolytic method and the extraction method are used to recover gold, and the suspended solids in the immersion liquid must be removed in advance. Therefore, the zinc powder replacement method and the carbon adsorption method are commonly used at present.

Feasibility determination of heap leaching

For a certain type of ore, to see if it is suitable for the heap leaching method, the first is to determine the feasibility of its heap leaching. According to the type of ore, grade and scale of the deposit, sufficient tests are carried out to provide sufficient design basis data to determine the main technical indicators and process parameters of the heap leaching.

The feasibility test of heap leaching mainly includes the following aspects: one is the roller leaching test; the other is the small column immersion test and the large column immersion test; the third is the field immersion test. They will be introduced separately below.

Rolling bottle leaching test

Take 300-500 grams of ore sample with a particle size of 10 mm and place it in a 4-5 liter leaching bottle. Usually, the coarse ore is adjusted to 40-50% by weight with solid slurry, and then lime is added to adjust the pH value. 10.5, then add 2.0 g / ton of cyanide, open and immersed on the roller mill for several days, take a rich liquid sample at 2, 6, 12, 24, 48 hours, etc. to determine the leaching rate, the leaching The rate can be considered as the maximum leaching rate. This is because the roller bottle test produces friction between the ore particles, and the actual heap leaching is carried out under "static" conditions.

After the end of the test, the content in the solution and the residual slag was analyzed. Through the roller bottle test, it can be determined whether the ore can be cyanated; the relationship between the leaching rate and the ore particle size; the relationship between the leaching rate and the leaching time; and the consumption of the agent. The disadvantages of this method are that there are few samples, the representativeness is poor, and the process parameters are relatively small.

Column leaching test

Column leaching includes both small and large. The small column immersion test usually breaks the ore to a diameter of 12 mm, the leaching column has a diameter of 100-150 mm, a height of 2 m, and an ore capacity of 20-45 kg. During the test, the ratio of the diameter of the column to the diameter of the ore is greater than 6:1, so that the influence of the column wall is minimized. During the test, the amount of lime is determined according to the roller bottle test. The cyanide solution with a flow rate of 0.003 liter/m2·s is sprayed from the top of the column, and the immersion liquid flows through the mineral material for filtering and immersion into the rich liquid tank. The gold content, the concentration of cyanide, and the pH value is adjusted in time. Through the small column immersion test, the feasibility of heap leaching of ore can be further determined, and the amount of cyanide can be provided; the relationship between the recovery rate of gold and silver and the leaching time, and the permeability of the ore can be predicted, and whether granulation is required according to the penetration condition is determined. . The large column leaching test is to carry out a larger amount of ore leaching in a larger column. The leaching process is the same as that of small column immersion, except that large column leaching can provide more reliable parameters such as feed size, leaching rate and dosage. However, Russell has compared small column leaching tests with large-scale tests. The results show that small laboratory tests of 40-80 kg samples can basically provide the same data as large-scale tests with sample weights of several tons.

3. Field reactor test

The heap height of the field reactor test is generally the same as the actual heap leaching, usually 4 meters high, and the ore volume is more than 1500 tons. Otherwise, the mineral material is at the edge of the heap, and the leaching rate on the edge of the heap is not as good as that of the heap. The leaching rate of the flat portion is high, so that the leaching rate of the field pile is low. Through the heap test, on the one hand, the parameters obtained by column leaching can be verified, and the structural factors of the heap, the way of spraying the solution, the feeding speed and the compaction of the heap can be obtained.

In summary, the use of heap leaching to extract gold has made great progress both in research and in actual production; although there are still some technical problems, the promotion of gold extraction by this method is usually a kind of effective resource. The economic decision to get the most profit. At present, the most important issue is to find a way to effectively monitor the heap leaching process so that the gold recovery rate can be accurately calculated and the design or operating conditions of the heap leaching can be evaluated. In addition, the application and development of heap leaching method has also promoted the upgrading of crushing equipment, and also reduced the average grade of industrial use of mineral deposits, making it possible to use low-grade gold mines, tailings and gold-bearing waste rock in China. To this end, it is of great practical significance to study the heap leaching method in detail and make it continuously perfect and reasonable.

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