On March 1, 1990, AGC Company Sansu Mine invested in production and began to carry out heap leaching operations on clay- rich gold ore. The first batch of gold bars was produced on March 20. By November 1990, the average output of the gold month was about 145 kg.
Sansu Mine is located in the Obuisi area of ​​Ghana, near the old mine area of ​​Ashanti Jintian, about 40 latitude north, with a hot climate, 200km from the coast of Ghana. Most of the rainfall in the area occurred within 7 months from mid-April to mid-November.
The process of processing ore includes: starting to crush ore to -25mm, granulating with cement and lean liquid, pile conveyor, pile of high density polyethylene (HDPE), spraying with sodium cyanide solution, recycling with carbon The gold in the precious liquid is then desorbed by alcohol and deposited by electrolysis. The gold extraction process is shown in the figure below.
The project has a design capacity of 9×10 4 t/month and is produced for 12 months per year. From March 1990 to September, the actual monthly average production capacity was 75,100 t / month, of which the average monthly processing capacity of 7 to 9 months was 84,000 t. In December 1990, the monthly production capacity in weeks was over 11 × 10 4 t. Heap leaching can usually be done quickly, especially in remote areas. It is also rapidly put into production because the system is designed for ore properties and climatic conditions. [next]
The ore of the main resources has been treated by AGC, and gold is either natural gold or closely associated with arsenic and iron sulfide. The ore near the surface is highly oxidized, and the surface is a red earth-rich material containing a small amount of quartz in the fine veins. Below the surface red soil are some fully oxidized soft schist zones that retain their original structure. After the sulfide is completely oxidized, the gold monomer contained is separated, and the surface oxidation and partial decomposition are extended to less than 50 m.
The resources for heap leaching are surface red earth and oxidized schist. Due to the tropical climate, the ore is quite humid. The average grade of gold in the ore is 3g/t, containing a small amount of silver and copper .
The heap leaching test of the Sansu Mine was carried out by KCA in its Nevada laboratory. The test material was a very viscous, highly metamorphic schist, which is representative of the Sansu Mine.
A large number of small column leaching tests are to determine the granulation method of the wet schist, and then carry out a series of tests with a leaching column with a height of not more than 2 m and a diameter of not more than 0.3 m. It is considered that it is not necessary to carry out the column immersion test or the field test of the large column. .
In the laboratory column immersion test, the material was broken to -25 mm and granulated with 1.2% cement content. The leaching rate of gold leached within 60 days was 87%. Based on laboratory data and past experience with similar ores, it is expected that the leaching rate of this ore on-site heap leaching will be close to 83% (90d decomposition into two 45d equilibrium cycles).
At the beginning of production, the metallurgical effect evaluation has always adopted a simple method, because the geological evaluation believes that the deposit is fairly uniform and the total ore grade is high, so it is not necessary to accurately determine the gold leaching rate, and it is believed that it has obvious economic benefits.
In January 1989, the leaching site was cleaned up. In order to minimize the initial capital cost, it was decided to lay only the cushions needed to meet the first year of production. Preparation for laying the mat includes removing the top topsoil, leveling the slope as needed, and tamping the exposed red clay surface. The soil was taken from the vicinity of the immersion pile, and the internal road was built with the soil from the truck. The total slope of the dip layer is 5%.
The lining was made of high density polyethylene (HDPE) with a thickness of 1.0 mm. Then, a layer of geotextile (non-woven fabric) was laid on all the liners, and a series of agricultural drain pipes with a center distance of 6 m and a small hole of 76 mm were installed on the surface of the cushion.
The pile is made of a layer of 350mm crushed sieve powder with a particle size of -25~-6mm on the geotextile and piping system to form the permeable layer foundation, and can prevent the liner from being damaged when the belt conveyor is piled up. At the lower edge of the immersion slope, the header of each unit is connected to a header, and the leaching solution is introduced into the corresponding leaching tank through a large diameter plastic tube located in the sump. The sump is lined with 1.5mm thick HDPE and geotextile.
Gold is recovered by carbon adsorption, and then desorbed from the carbon by ethanol and electrowinned. The gold plant uses a special process from KCA. The company has installed 16 such systems with six top-sealed suspended carbon beds, each containing approximately 2 tons of charcoal, and adsorption, desorption and pickling are carried out sequentially in the same carbon bed.
The two carbon columns (4t charcoal) were simultaneously desorbed, the desorption flow rate was 11m 3 /h, and the desorption solution contained 1% sodium hydroxide and sodium carbonate solution and 20% alcohol. The desorbent is heated in the storage tank with an electric immersion heater. The desorption pressure is the usual atmospheric pressure and the temperature is 83 °C. The solution is electrowinned at a conventional desorption temperature. The actual gold recovery capacity of the desorption section is more than twice the design capacity, and the flexibility of production is large. Usually, the desorption needs to be carried out for 12 hours.
Charcoal is generally not removed from the carbon column, but periodically regenerated. Two carbon regenerations have been carried out within 8 months after commissioning. [next]
Since most of the ore is relatively soft and clay-like, the crushing system consists of a plate feeder and three open-circuit MMD crushers. The usual product size is less than 25mm.
The granulator is a rubber cylinder with a length of 10m and a diameter of 3m. The rotation speed is 10r/min, and the inclination angle varies with the ore property between 80-150. The installed lining has a certain degree of looseness, which prevents the material from accumulating on the wall of the cylinder when it is "burst".
Cement is used as a binder. When it is added to the granulator, it is weighed on the feeding belt. The cement is transported by heavy trucks and stored in two silos. The material is transported to a conveyor by a given speed screw feeder. Weighing on the variable speed feeder. The amount of cement added is controlled by feedback from the belt gauge.
The lean liquid is sprayed into the cylinder through a spray head mounted on a support rod in the granule. A flow meter is installed on the lean liquid pipeline to assist the operation to control the amount of lean liquid added and the metallurgical balance.
At the beginning of the production phase, the amount of cement used in granulation was 1.5%, which was slightly higher than that used in laboratory tests. Considering that the granulated ore in the lower part of the immersion heap is not stable, the amount of cement will increase to 2%.
After granulation, the granulated ore was transferred to the immersion pile using 10 sets of 60 m long moving belt conveyors. At the end of this set of belts, there is a 10m long transverse belt that transports the ore particles to a 60m wheeled self-adjustable conveyor (Grasshopper), which is then unloaded into a 30m long rotating building. Stacker. The rotary piler has a 5m "guide" belt conveyor that can be extended and retracted under the remote control of the heap immersion operation. Typically, the operator operates the device on top of the dip stack so that the surface of the dip is flat and does not require walking on the top of the stack.
This pile-up machine can build a dip pile up to 10m. Once the piler moves, it can build a curved pile with a thickness of 4.7m and a mine volume of about 2600t. The pile-up machine has its own power, which can run radially and up and down the slope. There is a flexible running bogie at the end of the mine. It takes about 30 minutes for the piler to move once.
Because the ore is brittle and fragile, the upper part of the heap should be covered before the leaching line is installed. The covered material can protect the ore particles, weaken the spray intensity and avoid direct contact with the surface of the ore. The Sen-niger 8# vibrating nozzle was installed in a 6m×6m grid. The first leaching cycle of the first 90d was carried out continuously for 24 hours per day, followed by a second leaching cycle of 90 days.
The leachate discharged from the bottom of the first cycle of the heap leaching flows into the pool of precious liquid and then enters the extraction plant. The leachate flowing out of the second cycle flows into the intermediate tank, and then the first cycle of the spray liquid.
The tubing and sprinkler system remain on the immersion heap for several months after the leaching is completed. In the rainy season, old dips that are not completely scrapped should continue to spray. The scrapping of the immersion and the dismantling of the pipeline should be carried out two years after the pile-up.
After the 180d leaching cycle, samples were taken from the immersion pile with a soil drill (spiral drill). A total of 20 samples were taken. The fire slag grade was 0.44 g/t, and the ore geological grade was 2.93 g/t. The total leaching rate was 85%, exceeding the design specifications.
By October 1, 1990, the heap leaching site of Sansu Mine had processed 52.5×10 4 t ore, and the total amount of recovered gold reached 706 kg (22,700 ounces). The total cost of on-site production was about US$174 per ounce. All investment can be recovered in one year.
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