Percolation cyanide heap leaching

Percolation-cyanide heap leaching (PCL) was first used in 1752 to treat oxidized copper ore in Spain. In the late 1950s, it was used to leach low-grade and borderline-grade uranium ore. Since 1967, the U.S. Bureau of Mines has used this method to leach low-grade gold ore. PCL is a simple process, easy to operate, requires little equipment investment, has low production costs, and high economic benefits. Therefore, using this process to treat many small gold mines, low-grade gold and silver mines, and gold-bearing waste rock discarded from early mining can bring significant economic benefits. In the late 1970s, with the surge in gold prices, PCL rapidly developed, and many large-scale heap leaching gold processing plants were established in Nevada, Colorado, and Montana in the United States. In 1982, 20% of total U.S. gold production and 10% of total U.S. silver production were produced using heap leaching. Subsequently, this technology saw rapid development in countries such as Canada, South Africa, Australia, India, Zimbabwe, and the former Soviet Union.

In the early 1960s, my country used heap leaching to leach boundary-grade uranium ores; in the late 1960s, it was used to leach copper oxide ores; and in the early 1980s, it was used to leach low-grade gold ores. In the last decade or so, the percolation heap leaching process for low-grade gold ores has developed rapidly, mainly used for low-grade gold-bearing oxide ores and iron-cap gold-bearing ores.

Currently, two processes are used in production: general percolation cyanide heap leaching and granulation cyanide heap leaching for gold-bearing ores and gold-bearing waste rock.

5.4.1 General Percolation Cyanide Heap Leaching

Low-grade gold-bearing ore or waste ore from early mining operations is directly transported to the heap leaching field and piled into ore heaps, or crushed and then transported to the heap leaching field and piled into ore heaps. A cyanide leaching agent is then sprayed onto the surface of the ore heaps, and the leaching agent percolates evenly from top to bottom through the fixed ore heap, allowing gold and silver to enter the leaching solution. The general flow chart of percolation cyanide heap leaching is shown in Figure 5-9, mainly including ore preparation, construction of the heap leaching field, heap building, percolation leaching, washing, and gold and silver recovery.

Figure 5-9 Schematic diagram of heap leaching with cyanide leaching

5.4.1.1 Ore preparation.

Gold-bearing ores used for heap leaching are usually crushed first. The crushing particle size depends on the ore properties and gold grain embedding characteristics. Generally speaking, the finer the ore particle size and the more porous the ore structure, the higher the gold and silver leaching rate during cyanide heap leaching. However, the finer the ore particle size, the lower the leaching rate during heap leaching, and may even prevent the leaching process from proceeding. Therefore, in general, during cyanide heap leaching, the ore can be crushed to below 10 mm, and when the ore has a low mud content, it can be crushed to below 3 mm.

5.4.1.2 Construction of the heap leaching site. The heap leaching site can be located on a hillside, valley, or flat land, generally requiring a slope of 3% to 5%. After the ground is cleaned and leveled, seepage prevention treatment should be carried out. Impermeable materials can be tailings mixed with clay, asphalt, reinforced concrete, rubber sheets, or plastic films. For example, the ground can be first compacted or tamped, then covered with polyethylene plastic film or high-strength polyethylene sheets (approximately 3 mm thick), or with tar paper or artificial felt. The impermeable layer must be leak-proof and able to withstand the pressure of the ore pile. To protect the impermeable layer, fine-grained waste rock and a 0.5–2.0 meter thick layer of coarse-grained waste rock are often laid on top of the foundation layer. Then, low-grade gold ore is transported to the heap leaching site by truck or mine car to form the heap.

To protect the ore pile, drainage ditches should be constructed around the heap leaching site to prevent flooding.

To collect the precious leaching solution, a collection ditch is provided in the heap leaching site. The collection ditch is generally an open ditch lined with plastic sheets and equipped with a corresponding sedimentation tank to allow the ore slime to settle, ensuring that the precious solution entering the precious solution tank is clear.

The heap leaching site can be used multiple times or only once. For single-use heap leaching, the foundation layer can be a 0.5-meter-thick layer of clay laid on a compacted foundation, compacted, and then sprayed with sodium carbonate solution to enhance its impermeability.

5.4.1.3 Heap Construction Commonly used heap construction machinery includes trucks, bulldozers (tracked), cranes, and belt conveyors. Heap construction methods include multi-heap method, layered method, slope method, and hoisting method.

Multi-heap method: First, belt conveyors are used to pile the ore into many heaps about 6 meters high, and then bulldozers are used to level them (Figure 5-10). During belt conveyor heap construction, particle segregation occurs; coarse particles roll to the edge of the heap, while the surface ore is crushed and compacted by the bulldozer. Therefore, during cyanide leaching, channeling occurs. Simultaneously, as the leaching solution flows, slime accumulates within the ore pile, easily clogging pores. This makes it difficult for the solution to seep from the inside of the ore pile, instead causing it to flow through the coarse-grained areas at the pile’s edge. Sometimes, this can even wash away the ore pile slope, resulting in uneven leaching and reduced gold extraction rate.

Figure 5-10 Multi-layer pile method

Multi-layer method: Use trucks or loaders to pile up ore, piling one layer at a time and then leveling it with a bulldozer. Continue piling up layer by layer until the desired pile height is reached (Figure 5-11). This method can reduce particle size segregation and make the ore particles within the pile more uniform. However, each layer of ore can be crushed and compacted by trucks and bulldozers, resulting in poor permeability of the pile.

Figure 5-11, Multi-layer Pile Method

Slope Method: First, a slope transport road is built using waste rock for heavy trucks to transport ore. The slope is 0.6–0.9 meters higher than the ore pile. Trucks unload the ore to be leached onto both sides of the slope, and then bulldozers are used to level the sides (Figure 5-12). In this method, trucks do not crush or compact the ore, and the pressure of the bulldozer is lower than that of the trucks, thus having less impact on the porosity of the ore pile. After the ore pile is completed, the waste rock slope is leveled, and a ripper is used to loosen the waste rock. This pile method can obtain a ore pile with relatively uniform porosity, but it requires a larger area.

Figure 5-12 Slope Construction Method

Lifting Method: A bridge crane is used to pile the ore, and an electric rake is used to level it. This method eliminates the need for ore transport machinery to compact the ore pile, resulting in good permeability and allowing the leaching solution to pass through the pile more evenly, leading to a higher leaching rate. However, this method requires the installation of crane tracks, resulting in higher infrastructure investment and slower pile construction.

5.4.1.4 – Permeation and Washing

After the ore pile is constructed, it can be washed with saturated lime water. When the pH value of the washing solution approaches 10, a cyanide solution is then introduced for permeation. The cyanide leaching agent is pumped through underground pipelines to branch pipes on the surface of the ore pile, and then sprayed evenly onto the surface of the ore pile via sprayers, allowing it to permeate evenly through the ore pile for gold and silver leaching. Commonly used sprayers include swingers, jet sprayers, and drip sprayers. The sprayer should have a simple structure, be easy to maintain, have a large spray radius, and provide uniform spraying. The spray droplets should be relatively coarse to reduce evaporation and heat loss. The leaching process should aim for a uniform and stable solution supply, with a spraying rate typically between 1.4 and 3.4 ml/m²·s.

After percolation cyanide heap leaching, the solution should be washed several times with fresh water. If time permits, the washing liquid should be completely drained after each wash before the next wash to improve the washing efficiency. The total amount of water used for washing depends on factors such as evaporation loss of the washing water and the water content of the tailings.

5.4.1.5 Gold and Silver Recovery The gold content in the precious metal solution obtained from percolation cyanide heap leaching is relatively low. Gold and silver can generally be recovered using activated carbon adsorption or zinc displacement methods, but activated carbon adsorption is more commonly used to achieve higher gold and silver recovery rates. Typically, 4–5 activated carbon columns are used to enrich gold and silver. The desorbed precious metal solution is then sent for electrowinning, and the electrowinning gold powder is smelted to obtain the finished gold product. After gold and silver removal, the lean leaching solution, after adjusting the cyanide concentration and pH value, is returned to the ore heap for percolation leaching.

The waste rock pile after heap leaching is loaded onto trucks using a front loader and transported to the tailings yard for storage. It can then be re-piled and percolated on the heap leaching site. Waste rock from heap leaching sites intended for single use does not need to be removed and becomes a permanent waste rock pile.

5.4.2 Granulation-Percolation Cyanide Heap Leaching

The finer the particle size of the gold-bearing ore to be leached, the more fully the gold and silver minerals are exposed, and the higher the gold and silver leaching rate. However, finer particle size leads to higher crushing costs and a greater amount of fine ore produced. Fine ore in the ore is extremely detrimental to heap leaching. During heaping, particle size segregation occurs, and during percolation, the fine ore moves with the liquid flow, easily causing channeling, preventing the leaching agent from uniformly percolating the ore heap. When the ore has a high mud content, the cyanide solution cannot percolate the ore heap, making heap leaching impossible.

To overcome the adverse effects of fine ore and clay ore on heap leaching, the U.S. Bureau of Mines developed granulated heap leaching technology for fine ore in 1978. This completely changed the situation where heap leaching of fine ore (including clay ore) was impossible, and promoted the further development of heap leaching technology. Currently, this technology is widely used not only in gold mines in the United States but also in gold mines in other countries around the world.

c: In granulated heap leaching, low-grade gold-bearing ore is pre-crushed to -25 mm or finer to liberate or expose gold and silver minerals. The crushed ore is mixed with Portland cement (ordinary Portland cement) at 2.3–4.5 kg/ton dry ore, and then the mixture is moistened with water or concentrated cyanide solution to achieve a moisture content of 8%–16%. The moistened mixture is mechanically tumbled to form spherical pellets. After solidification for more than 8 hours, it is sent to the heap for percolation cyanide heap leaching. The heap formation and leaching methods are the same as conventional heap leaching. Extensive comparative experiments were conducted using lime or cement as binders. Experiments showed that cement is superior to lime; adding 2.3–4.5 kg/ton of cement as a binder produces relatively stable agglomerates with high porosity and good permeability. During leaching, the mineral powder does not move, no channeling occurs, and no additional protective alkali is required. Because the hydrolysis of cement begins within 5 hours,

it reacts with the clay silicate minerals in the ore to produce a large amount of strong and porous bridge-like calcium silicate hydrates, giving the agglomerates sufficient strength and porosity to allow cyanide solution to penetrate and leach the ore heap. The agglomerates produced by the cement binder, after solidification, will not crack when encountering cyanide solution during the heap leaching process. In addition, experiments have also been conducted using magnesium oxide, sintered dolomite, and roasted calcium chloride as binders. Experiments show that roasted dolomite and calcium chloride are not effective binders. When cyanide solution is added to the ore heap, the agglomerates quickly break apart, resulting in fine particle migration and channeling, which affects the smooth progress of heap leaching operations. Magnesium oxide has a strong binding effect on low-grade clayey gold ores and can eliminate fine particle migration, but the resulting agglomerates are too large and prone to channeling. Therefore, ordinary silicate cement is a better binder for granulation heap leaching, followed by lime.

The role of the binder in granulation is to increase the agglomeration of fine ore and improve the strength of the solidified agglomerates. Therefore, the amount of binder is crucial, and its amount is related to the type of binder, the particle size distribution of the ore, the type of ore, and its acidity or alkalinity, and is generally determined through experiments. If the amount of binder is too small, even if the resulting agglomerates are well solidified, it is difficult to obtain high-strength agglomerates, which will break apart during the leaching process due to further wetting by the leachate, reducing the permeability of the ore heap. If the binder dosage is too high, the resulting agglomerates will be too strong, hard, and dense, resulting in poor permeability and hindering gold and silver leaching.

The ore and cement mixture can be wetted with water or cyanide solution. Experiments show that using cyanide solution as a wetting agent is more advantageous. Cyanide solution not only wets the ore-binder mixture but also pre-leaches the ore, shortening the leaching cycle and increasing the gold leaching rate. The wetting agent dosage is related to the ore particle size distribution, moisture content, and binder type. Too little wetting agent is insufficient to form fine ore agglomerates, while too much can reduce the porosity of the agglomerates. The ideal wetting agent dosage is to achieve a moisture content of 8%–16% in the mixture.

After being wetted with the wetting agent, the ore-binder mixture is sent for agglomeration. Currently, two granulation methods are used industrially: multi-belt conveyor method and drum granulation method. The multi-belt conveyor pelletizing method uses mixing rods at the discharge end of each belt conveyor to uniformly mix concentrated cyanide solution, fine ore, and cement to form the desired granules (Figure 5-13). The drum pelletizing method involves feeding the ore and fine ore mixture into a rotating drum, where a concentrated sodium cyanide solution is sprayed. The rotation of the drum uniformly mixes the fine ore, cement, and cyanide solution to form the desired granules (Figure 5-14).

Figure 5-13 Granulation method for multi-belt conveyor

Figure 5-14 Drum Granulation Method. The resulting granules solidify at room temperature for at least eight hours. During solidification, the granules must maintain a certain moisture content. If the granules are too dry, the hydrolysis reaction within them will cease, and such granules will locally crumble upon contact with water. Therefore, maintaining a certain level of humidity during solidification is essential to obtain strong, non-crushing granules. Solidification can be performed separately after granulation or during heap leaching.

During the granulation heap leaching process, because cement is used as a binder, no additional protective alkali is needed when using cyanide solution for leaching, and the pH value of the leachate can be maintained at around 11.

Experiments and production practice show that using ordinary silicate cement, water, or concentrated cyanide solution for granulation and solidification of fine-grained ore can significantly increase the flow rate of the leaching reagent through the ore heap. Compared with conventional heap leaching, this process has the following significant advantages: (1) It can process fine-grained ores with high clay and fine ore content, as well as low-grade fine-grained gold ores suitable for cyanidation; (2) Fine-grained gold ores are directly granulated and heap leached without classification, which can greatly increase the flow rate of the leaching agent through the heap and shorten the leaching cycle; (3) It eliminates particle size segregation during heap building, which can greatly reduce channeling during leaching, allowing the leaching agent to pass evenly through the entire heap. In addition, the ore particle size is finer than that in conventional heap leaching, thus significantly increasing the gold leaching rate; (4) The porosity of the agglomerates results in good ventilation of the entire heap, which can increase the concentration of dissolved oxygen in the solution and accelerate the dissolution of gold; (5) The agglomerates improve the permeability of the heap, which can appropriately increase the height of the heap, reduce the pretreatment liner cost per unit ore and reduce the floor space occupied; (6) The porosity of the agglomerates allows for more thorough washing away of residual cyanide leaching solution after leaching; (7) Aggregates can fix fine ore, control dust content, and have significant environmental benefits.

Key Factors Affecting Heap Leaching with Cyanide Whether gold-bearing ore is suitable for heap leaching depends on a series of factors. A feasibility study is often required before industrial production to determine the suitability of the ore for heap leaching and the optimal process parameters. The heap leaching suitability of gold-bearing ore is related to factors such as ore type, mineral composition, chemical composition, structure, reserves, and grade. Suitable ore for heap leaching must have sufficient permeability, high porosity, loose and porous structure, low content of impurities harmful to cyanide, clean surface of fine gold particles, and low mud content. To determine suitable process parameters for heap leaching, laboratory and scale-up tests should be conducted to determine the optimal particle size, cyanide leaching agent concentration, leaching agent consumption, required saturated solution volume, type, concentration, and consumption of protective alkali, solution spraying method, flow rate, leaching time, washing liquid volume, number of washes and washing time, and solution oxygen content. The design should also consider the size of the heap, the location of the heap leaching site, the subbase material, the location of pumps and storage tanks, and the location of elevated tanks. Correctly selecting the location of the heap leaching site and utilizing favorable terrain can significantly reduce infrastructure investment.

5.4.4 Examples of Heap Leaching Applications As of 1986, 17 mines in the United States (excluding small heap leaching sites) used heap leaching to process gold-bearing ore, and 10 mines used heap leaching to process waste gangue previously mined underground. Most of the ore processed by heap leaching came from surface oxidized ore mined in open-pit mines, with a small number coming from primary gold-bearing ore mined underground. The Smoky Valley gold mine near Garden Hill, Nevada, began production in 1977 after two years of infrastructure construction, producing 1.152 tons of gold that year. It currently employs 140 people and has a projected annual production of 1.71 tons of gold and 0.98 tons of silver. The ore has an average annual gold content of 1.03 g/t and a silver content of 2.18 g/t. Cyanide heap leaching is used, with a gold recovery rate of 86%. The open-pit ore is crushed to 178 mm using a 1066 mm × 1651 mm cone crusher, then to 50 mm using a φ2100 mm standard cone crusher, and finally to -9.5 mm using a short cone crusher for heap leaching. The entire heap leaching site is a rectangular area of ​​640 m × 86 m.

The foundation layer is 178 mm thick, beneath which is a 50 mm thick waterproof layer composed of rubber sheets and asphalt, on which another layer of asphalt is applied. The transport road, constructed of ore fines, is 660 mm higher than the asphalt surface. The entire site is divided into five heap leaching fields, each sloping to one side and one end, with each field capable of leaching 45,000 tons of ore. During operation, leaching is carried out in four fields, while one ore heap undergoes washing, slag removal, and heap building, which takes approximately five days. The leaching agent contains 0.045% NaCN and 0.04% CaO, and is delivered to each ore heap via a main pipe and four plastic pipes. A Baghdad sprayer is installed every 12 meters on each plastic pipe. The sprayers are made of 228 mm long, 6.35 mm diameter medical tubing, with a spray radius of 9 meters. Each ore heap is equipped with approximately 84 nozzles, with a supply rate of 2.7–3.4 ml/m²·s, a spray area of ​​116 m², and a spray volume of approximately 1500 liters per hour. The leaching cycle is 27 days. During the leaching process, the gold content in the leachate is monitored daily. When the gold content falls below the specified value, the leaching agent is shut off, and the solution is washed with clean water for two days. The leached waste rock is transported to the waste rock dump using two loading machines and five 15-ton trucks. The waste rock dump is adjacent to the heap leaching site, and each heap can be transported within 48 hours. To protect the bitumen cushion layer, a 0.2–0.25 meter thick layer of waste rock should be left at the bottom of the heap during unloading. A deep well is located at the bottom of the heap leaching site to check the impermeability of the cushion layer; samples are taken periodically for testing. The precious liquor obtained from heap leaching is pumped to the adsorption-desorption-activation workshop using a turbine pump. It passes through five activated carbon adsorption tanks connected in series, producing gold-loaded carbon with a gold and silver content of approximately 7.775 kg/ton. The precious liquor and activated carbon are adsorbed countercurrently. One ton of gold-loaded carbon is taken from tank 1 daily for desorption; one ton of new activated carbon is added to tank 5. After adsorption, the lean solution is adjusted for sodium cyanide and lime concentrations and then returned to the heap leaching field for recycling. Gold-loaded activated carbon is desorbed using a solution of 0.1% NaCN and 1% NaOH at 85°C for 72 hours. The desorbed activated carbon is regenerated in an indirectly heated rotary kiln at 600°C. The regenerated carbon is washed with nitric acid to remove calcium carbonate and then returned to the adsorption operation. The precious solution obtained from desorption is sent to three electrolytic cells for electrodeposition, and gold is recovered at the cathode.

The Summitville gold mine in southern Colorado, USA, is a large-scale open-pit and heap leaching gold extraction complex with over 100 years of history. The ore contains 1.2 g/t of gold, and the waste rock contains 0.3 g/t of gold. The ore body is continuous for approximately 8 kilometers, with two-thirds being siliceous magnesia and one-third being clayey silt. The open-pit siliceous lump ore and clayey fine ore are transported to two separate roughing bins. The lump ore is crushed by a large 1260 mm × 2100 mm gyratory crusher with a capacity of 1200 tons/hour. The discharged ore is screened by a vibrating screen (30 mm aperture). The oversize product is sent to a φ2100 mm cone crusher. The crushed ore is combined with the undersize product from the -30 mm screen and sent to the intermediate crushing bin. A certain amount of lime is added before entering the intermediate crushing bin. The clayey fine ore is sent to another bin and screened by a 75 mm vibrating screen. Cement is added to the undersize product, and water is sprayed onto it before it is sent to the pelletizing bin via a pellet conveyor belt. The oversize product is crushed by a jaw crusher and then sent to another φ2100 mm cone crusher parallel to the lump ore system. The crushed ore then enters the ore bin. The intermediate crusher has a capacity of 750 tons/hour, and the two crushers together have a capacity of 1500 tons/hour. After crushing, the ore was transported by large dump trucks to the heap leaching site for heap construction. The heaps were constructed using a valley-filling method, and the area occupied increased from 0.1 square kilometers (25 acres) in 1986 to 0.43 square kilometers (106 acres) in 1987. The seepage prevention method involved first leveling the ground, compacting it with a road roller, laying clay and compacting it again, then laying fine sand, followed by a 3 mm thick high-strength polyethylene sheet, and finally a layer of artificial felt to prevent the ore from puncturing the polyethylene sheet. This seepage prevention layer was structurally sound, leak-proof, and load-bearing. 100-ton dump trucks and bulldozers were used for heap construction. The height of the lump ore heap was unlimited, while the height of the granular heap generally did not exceed 4.57 meters. Diluted sodium cyanide solution was pumped from the preparation room to serpentine pipes laid on top of the ore heaps. The solution was sprayed out from small gaps in the pipes, seeping evenly from the top of the heap to the bottom, and then pumped to the gold extraction plant. The local rainfall is low, allowing for 24-hour spraying. Winters are cold and snowy, limiting production to only six months. The spraying rate for lump ore piles is 2.72 ml/m²·s, and for granular ore piles, it is 1.7 ml/m²·s. Sodium cyanide consumption is 0.032 kg/ton, and the pH is adjusted to 10.5 with lime. The gold extraction plant has five activated carbon adsorption columns, a pressurized heating device for gold-loaded carbon desorption, a zinc powder replacement device, a gold refining furnace, and an activated carbon regeneration kiln. All devices are connected by pipelines and controlled by various metering instruments. The plant is odorless, and there are very few production personnel. The five carbon adsorption columns are connected in series. The lean solution, after adding appropriate amounts of sodium cyanide and lime, is returned to the ore pile for leaching. The gold-loaded carbon is desorbed under heating and pressure with a caustic sodium cyanide solution. The precious solution is sent for zinc powder replacement, filtered to obtain gold mud, and then sent to a gas-fired gold refining furnace for gold refining. The desorbed activated carbon is regenerated and returned for reuse. The gold extraction system employs 120 people, working six months a year, seven days a week, one shift per day, 11 hours per shift. In 1986, it processed 3 million tons of ore, and it was projected to process 16 million tons of ore in 1987, producing 4 tons of gold annually. The heap leaching leaching rate was 80%–85%, and the total recovery rate was 70%–75%.

The technical and economic indicators of heap leaching operations in some US gold and silver mines are listed in Table 5-8.

Continued from Table 5-8

my country has relatively abundant low-grade gold ore resources, with diverse ore types and wide distribution. While some deposits have higher gold grades, their reserves are small, making mechanized mining and beneficiation difficult. Therefore, recovering these gold resources using low-cost heap leaching is a feasible option. Since the early 1980s, my country has conducted leaching feasibility tests on ores from relevant deposits using percolation cyanide heap leaching. To date, a considerable number of conventional heap leaching plants and a number of granulation heap leaching plants have been established, achieving good economic benefits. For example, Henan Province has built more than a dozen heap leaching plants, typically with a heap size of 1200-1500 tons of ore per heap. Each female leaching plant can be heaped 2-3 times per year, with a cycle of heap construction, spraying, washing, and unloading taking about three months. Most use temporary heap leaching fields. After leveling and compacting the site, a foundation with a slope of 0.5%-8% is constructed, covered with two layers of polyethylene plastic film, and then a layer of tar paper, ensuring no leakage. Cement bases are prone to cracking and leakage. Drainage ditches should be constructed around the heap leaching area. A 0.2-0.3 meter thick layer of large, low-grade ore is manually laid at the bottom of the ore heap, then the heap is constructed using trucks, with workers leveling the surface. During construction, fine ore and lump ore are layered, with the fine ore with high mud content placed at the bottom and the coarser lump ore at the top. Compaction of the ore should be avoided during construction. If the ore heap has a high mud content, some larger-sized ore should be piled at the leaching outlet, and a settling pond should be constructed accordingly. After the ore heap is constructed, it is first washed with saturated lime water. When the pH of the effluent approaches 10, the heap is then sprayed with a mixture of sodium cyanide and lime. The leaching agent is transported via pipeline and sprayed using a sprayer; the sprayer evenly sprays the leaching agent onto the surface of the ore heap (including the slopes) to ensure uniform leaching. When the gold content in the discharged leachate reaches 3×10⁻⁴%, it is pumped to a high-level tank and enters the activated carbon adsorption system. The leachate entering the carbon adsorption system should be clarified. The lean solution after carbon adsorption is returned to the spray leaching after adjusting the concentration of sodium cyanide and calcium oxide. The carbon adsorption system consists of four adsorption columns connected in series, using GH16-A type activated carbon from Beijing Guanghua Timber Factory and Zx-15 type activated carbon from Taiyuan Xinhua Chemical Plant. The gold-loaded carbon is desorbed at a constant temperature of 95℃ for 2-6 hours using a 5% NaCN + 2% NaOH mixture, and then eluted with 95℃ hot water. The gold content of the eluent is generally 300ppm. The eluent (precious solution) is cooled and filtered before being sent to an electrolytic cell for electrowinning. Gold is deposited in flakes or flocculent form on a titanium plate cathode (or on a steel wool cathode), scraped off and smelted into ingots. The gold-removed carbon is regenerated and reused. The electrolysis tailings are purified by carbon column or ion exchange resin column and then returned to the desorption system as washing water, reused more than 19 times. Finally, they are treated with bleaching powder before discharge. After heap leaching, the ore heap is washed with lime water solution. The washing water is sent to a carbon adsorption column to recover residual gold or directly discharged into a storage tank for the next spraying. When the sodium cyanide concentration in the washing effluent is below 0.01% and contains no gold, an appropriate amount of bleaching powder can be sprinkled on the top surface of the ore heap, followed by washing with clean water until the discharge standard is met before the waste ore heap can be removed. During unloading, slag samples should be taken from each layer for analysis and testing. The high-grade ore mined at Linghu Gold Mine in my country is extracted using the whole-mud cyanidation carbon-in-pulp method. The low-grade ore and gold-bearing host rock mined have a gold grade of about 3 grams per ton and are extracted using the conventional heap leaching method. There are four fixed heap leaching fields, each capable of holding 1,500 tons of ore. One heap is usually maintained for spray leaching. The mined low-grade ore and gold-bearing surrounding rock are crushed to 50 mm using a 250×400 mm jaw crusher. The crushed ore is then piled into a 3-meter-high square truncated cone shape using trucks. The ore is washed with saturated lime water for 5–10 days to bring the pH of the effluent to 10–11. Then, a leaching agent of 0.03%–0.05% NaCN, pH=10–11, is sprayed onto the ore pile at a rate of 2.78–5.56 ml/m²·s for 50 days. The gold leaching rate is 63.76%. The cost of producing one tael of gold is approximately 300 yuan, with underground mining and ore transportation costs accounting for 60%. The Laowan gold mine has four mining areas, which are far apart and mostly small, poorly shaped mines with severe ore oxidation and high mud content. A permanent 25×16 meter heap leaching field has been constructed, filled with rubble concrete mortar and finished with concrete. The ore contains 40%–50% fines, which are piled at the bottom to a height of 2.27 meters during heap construction. A mixed ore method is used to ensure the gold content of the heap. The average grade of the raw ore entering the heap is 2.24 g/t. The ore is relatively acidic, and the leaching water is weakly acidic (pH=6). The washing water is saturated lime water with a small amount of caustic soda solution, and washing takes 15 days. Mobile and fixed sprayers are used to apply the leaching agent; the spraying rate is 4 liters/hour·ton. After 51 days of spray leaching, the gold leaching rate is 75.44%, and the highest gold content in the precious metal solution reaches 104 ppm. Four carbon adsorption columns are used for adsorption at a flow rate of 600 liters/hour. After spray leaching, the tailings are treated with bleaching powder, which is 16 times the amount of cyanide used. The heap washing time is 7 days. The heap leaching cost for each tael of gold in this ore is 349 yuan.