The adsorption and destruction of cyanide by activated carbon have long been discovered. In the practice of recovering gold by carbon slurry method, it is found that activated carbon can not only adsorb precious metals such as gold and heavy metals such as copper, zinc and iron, but also adsorb cyanide from waste water, and the adsorption capacity of thiocyanate is also large. The density of activated carbon in pulp is only about 2%, and the air filled is limited. The existence of activated carbon reduces the cyanide concentration in pulp by 20% ~ 70%, which proves that activated carbon has a strong ability to adsorb and destroy cyanide. Through the in-depth study on the conditions required for the destruction of cyanide by activated carbon adsorption, the process technology of treating cyanide-containing wastewater by activated carbon method was developed.
1968 Canada studied the role of copper salts in the catalytic decomposition of activated carbon, and thought that activated carbon needed to be regenerated. 1987, South Africa began to treat cyanide-containing wastewater from cyanide plants with activated carbon, mainly recovering gold, and achieved good results. There are also many studies on activated carbon method in China. In addition to using copper salt as catalyst, an S catalyst is also used to act with copper salt, which is said to have a good effect. The research on activated carbon method in China's gold industry is close to the stage of industrial application.
7. Characteristics of1activated carbon method
I. Advantages
1) The process equipment is simple and easy to operate and manage.
2) Only a small amount of inorganic acid is consumed (sometimes copper sulfate is needed as a catalyst), so the treatment cost is low.
3) The investment is small, and the investment in supporting equipment of 50t/d all-sliming cyanidation plant is less than 200,000 yuan.
4) At the same time of cyanide removal, it has a high removal rate of heavy metal impurities in wastewater.
5) Trace gold and silver in wastewater can be recovered, which has good economic benefits.
Two. shortcomings
1) can only treat clarified water, not pulp.
2) The equipment must undergo anti-corrosion treatment.
3) When the pH value of wastewater is higher than 9, it is necessary to add acid to adjust the pH value, otherwise the treatment effect will become worse.
4) The coagulation of silicate on activated carbon will deactivate and scrap activated carbon, and the regeneration effect of activated carbon will become worse, which needs to be further solved.
5) Waste gas containing HCN and (CN)2 may be generated. If its content is large, it should be treated by absorption device, otherwise it may cause air pollution in the workplace.
6) When the wastewater contains high concentration of thiocyanate, the regeneration of activated carbon becomes more complicated.
7.2 Characteristics of Activated Carbon
Activated carbon is manufactured artificially, and the raw materials used are wood, stone, coal, oil and crops. It is shaped by an appropriate method and then activated to produce the finished product. The shapes of activated carbon are powder, spherical, columnar and flaky, and its activation methods are steam activation and zinc chloride activation. So the nature of the products produced is very different. For example, the activated carbon used for gold adsorption in the pulp mill is coconut shell carbon and apricot core carbon, which are flaky and have good strength and wear resistance, while the activated carbon used for treating cyanide-containing wastewater is generally coal-based carbon with low price and large specific surface area, but poor strength. Activated carbon manufacturers generally use benzene absorption, iodine value, specific surface area and total porosity to express the adsorption performance of activated carbon. These indexes are in the range of 20 ~ 400 mg/L, 600 ~ 800 mg/L, 300 ~ 1000 m2/g and 0.35 ~ 0.8 1 cm3/g respectively. The greater the porosity, the higher the others.
7.2. 1 adsorption
Adsorption is the main feature of activated carbon, which is considered as a surface phenomenon. When cyanide-containing wastewater passes through activated carbon, the surface of activated carbon faces the corresponding wastewater surface, and the area surrounded by two surface layers is an interface, so adsorption occurs in this interface area. Activated carbon has both physical adsorption and chemical adsorption, and the two adsorption cannot be completely separated. Take the adsorption of gold by activated carbon as an example. First, gold is Au (CN).
The adsorption of cyanide by activated carbon is different from that of gold. Heavy metal cyanide is adsorbed in ionic form, free cyanide is adsorbed in ionic form, and free cyanide is adsorbed in HCN form. Therefore, when the pH value of wastewater decreases, the adsorption rate of activated carbon to cyanide is high. Before the adsorbed cyanide is oxidized on the carbon surface to form CNO, it can be eluted with acid.
The adsorption rate depends on the speed of cyanide diffusion to the carbon surface and the speed of cyanide diffusion from the outer carbon to the unoccupied surface of the inner layer. This is not difficult for HCN gas, but it is difficult for cyanide in water. Therefore, when using new carbon to treat wastewater, we see that the adsorption speed is very fast at first, but after a period of time, the external surface area is occupied, the adsorption speed is controlled by internal diffusion, and the adsorption speed is obviously slow down, which is why we choose small granular activated carbon in the catalytic decomposition method of activated carbon.
7.2.2 Specific surface area and pore structure
The total active surface area of activated carbon is generally 300 ~ 1000m2/g, and cyanide is adsorbed on the surface of activated carbon. It is generally believed that the larger the specific surface area, the more active sites (active centers) there are on the active surface of activated carbon. But whether cyanide can be adsorbed depends on the pore structure of activated carbon. If the pore size is smaller than the diameter of HCN molecules or complex ions, cyanide cannot reach the active surface, so activated carbon cannot adsorb cyanide. It is generally believed that the microcrystalline aggregates of activated carbon contain a network with irregular gaps, and there are pores in this network. Macropores provide channels for adsorbable molecules to enter the interior. Micropores provide surface area for adsorption. It should be pointed out that not all micropores have the same adsorption capacity, but often have specific and selective adsorption capacity in different surface parts, so people put forward the hypothesis of active center, and this property of activated carbon is related to the manufacturing process.
Catalysis of activated carbon
Because of the large specific surface area of activated carbon, cyanide in cyanide-containing wastewater is adsorbed by activated carbon when it contacts with activated carbon, and oxygen in the air is also adsorbed by activated carbon when it contacts with air. In this way, the concentration of cyanide and oxygen on the surface of activated carbon is much higher than that in wastewater, and the activation energy of reaction is also reduced, so it is easier to react with oxygen than cyanide in water. Therefore, the catalytic function of activated carbon is to enrich reactants and reduce the activation energy required by melon tips.
According to the literature, when air is introduced into the activated carbon bed, hydrogen peroxide can be detected at the exhaust port, which shows the great catalytic effect of activated carbon:
activated carbon
H2O+0.5O2———→H2O2
The mechanism of gold adsorption by activated carbon researchers also believe that Au(CN)2- finally reacts with hydrogen peroxide to generate elemental gold and cyanate.
activated carbon
Gold (cn) 2+2h2o 2-→ gold +2CNO-+H2O
Table 7- 1 is the result of cyanide oxidation test at 18℃ by adding-160 mesh activated carbon to cyanide-containing wastewater.
Table 7- Loss of cyanide in the presence of 1 activated carbon *
Solution PH Aeration and Carbonization Cyanide Loss (%) Cyanide Loss Path (24 hours)
(1/h. L wastewater) (G/L) was oxidized and hydrolyzed by HCN after 8 hours and 24 hours.
9.6 4 20 46 80 5 75
10.5 4 0 — 5 2 3
10.5 4 20 59 84 4 80
10.5 4 60 87 >95 & lt 1 & gt； 95
10.5 15 0 — 13 8 5
10.5 15 20 42 88 3 85
10.5 60 0 — 60 55 5
10.5 60 20 57 >95 16 84
* l: l in the table; H: Inflating unit per hour: Inflating liters per liter of water per hour.
Activated carbon used to treat cyanide-containing wastewater by activated carbon method should have the characteristics of large cyanide adsorption capacity, fast adsorption speed and good regeneration effect, which makes wastewater treatment economically and technically feasible. These conditions can be determined through a series of experiments, and the methods can be found in books on activated carbon, which are not introduced here.
7.3 Mechanism of cyanide removal by activated carbon method
Although activated carbon can adsorb cyanide, there are three main ways to remove cyanide by activated carbon: oxidation, hydrolysis and blowing off. According to different conditions, there are mainly one or two ways to remove cyanide. The first two ways are based on the adsorption of cyanide on activated carbon.
Cyanide oxidation on activated carbon
When activated carbon is in contact with wastewater and air at the same time, oxygen in the air will be adsorbed on activated carbon, and its adsorption capacity is as high as 10 ~ 40g/kg. It is thousands of times higher than dissolved oxygen in water, and oxygen chemisorbs on the surface of activated carbon to form peroxide and hydroxyl acid functional groups, which together with other functional groups such as phenolic aldehyde and benzoquinone form active surfaces.
activated carbon
O2+2H2O+2e——→H2O2+2OH-
Metal cyanide complexes are attracted to these active surfaces, and the adsorption process of cyanide complexes is completed. Activated carbon has a large specific surface area, which can generally reach 1000 m2/g, and the porosity is 0.6 ~ 0.9, which can adsorb a large number of metal cyanide complexes. According to the literature, the adsorption sequence of various metal cyanide complexes is as follows:
Au(CN)2->Ag(CN)2- > iron (CN)64- > nickel (CN)42- > zinc (CN)42- > cuprous cyanide
The physical adsorption of HCN by activated carbon is obvious, and cyanide in wastewater can be removed at a high rate.
Because H2O2 is produced in the process of adsorption of oxygen by activated carbon, and the concentration of cyanide on activated carbon is much higher than that in wastewater, it is much easier to oxidize cyanide on the surface of activated carbon with hydrogen peroxide than in wastewater.
The optimum conditions of activated carbon catalytic oxidation method are naturally consistent with hydrogen peroxide oxidation method.
Copper ions in wastewater play an important role in the catalytic oxidation of activated carbon. It is believed that copper hydrolyzes CNO into ammonia and carbon dioxide, and it is also believed that cyanide forms complex ions first, which are more easily adsorbed on activated carbon. After the activated carbon is impregnated with copper salt, the treatment capacity is improved several times. In any case, the role of copper should not be underestimated.
After cyanide complex of heavy metals in wastewater is cyanided, heavy metals and anions such as carbonate form insoluble substances and remain on activated carbon. With the passage of time, the active surface of activated carbon is full of heavy metal impurities. Calcium ions in wastewater can also form calcium carbonate precipitation on activated carbon. Ferricyanide and ferrocyanide finally exist on carbon in the form of hydroxide, which leads to the deactivation of activated carbon.
activated carbon
CN-+0.5O2——→CNO-
Copper ion
Cno-+2h2o-→ bicarbonate -+NH3
HCO3-+OH-——→CO32++H2O
2Cu2++CO32++2OH-→CuCO3 Cu(OH)2
Ca2++CO32-→CaCO3↓
7.3.2 Hydrolysis of Cyanide on Activated Carbon
It has been found that activated carbon soaked in wastewater has the ability to remove cyanide even if different activated carbons are aerated with air. On the one hand, activated carbon does adsorb some cyanide, but it will be saturated for a certain concentration of cyanide-containing wastewater, but in fact, activated carbon is not saturated quickly, but has a certain oxidation removal rate, which shows that the cyanide adsorbed by activated carbon is hydrolyzed to produce ammonium formate under the condition of insufficient oxygen.
HCN+H2O=HCONH2
This reaction is not obvious in aqueous solution at room temperature, but the reaction speed is obviously accelerated under the action of activated carbon, and the generated ammonium formate is heated to decompose CO and NH3.
7.3.3 filler function of activated carbon
If the inherent characteristics of activated carbon are not considered, it is only used as a filler. Because the hydrophilicity of activated carbon is much better than other packings, the packed column composed of cylindrical activated carbon with a diameter of φ 1.0 ~ 3.5 mm and a length of 1.5 ~ 4 is undoubtedly a good HCN stripper, but under the process conditions of catalytic decomposition of activated carbon, the reaction pH value is 6 ~ 9, which is better than acidification recovery.
Table 7-2 Volatilization of Cyanide during Catalytic Oxidation of Activated Carbon
The pH value of the reaction is 5.0 5.7 7.2 8.0.
Volatile cyanide (calculated by CN-)%15.13.211.29.6
The volatilization of HCN in the reaction process is undoubtedly not a good thing, but if the volatilization amount of HCN is very low and the exhaust pipe design is reasonable, it will not cause environmental pollution and workplace pollution, because HCN exists in the air for a short time. If activated carbon is used as filler and the pH value of wastewater is controlled to be 2 ~ 3, then the blowing-off rate of HCN is naturally high, and then this part of cyanide can be absorbed by absorption method, and cyanide can be recovered. Moreover, due to the characteristics of adsorption, oxidation and hydrolysis of cyanide by activated carbon, the cyanide content in the treated wastewater is reduced to a much lower level than that of the waste liquid by acidification recovery method, which can kill two birds with one stone. This is the advantage of activated carbon stripping method. It is worth noting that this method can not replace the acidification recovery method, because the recovery rate of activated carbon stripping is low and the decomposition is high.
7.4 Regeneration method of activated carbon
After using activated carbon for a period of time, the cyanide removal ability of activated carbon is greatly reduced because impurities occupy the active surface and pores. At this time, activated carbon needs to be regenerated. The service life of activated carbon depends on the composition of wastewater. The simpler the composition, the lower the content of iron, zinc and calcium in wastewater and the longer the service life.
As the impurities accumulated on the activated carbon are mainly zinc, iron, calcium and copper, they mainly exist in the form of Zn2Fe (CN) 6, ZnCO3 Zn (OH) 2, Fe(OH)3, Fe(OH)2, CaCO3 and CuCO3 Cu(OH)2) 2, so the activated carbon can be regenerated by soaking with inorganic acids. However, a part of Cu2+ will occupy the active surface and still act as a catalyst, and the excess will be desorbed from the carbon by Ca2+, and Prussian blue will also be leached. Oxidized cyanide and SCN- adsorbed on carbon will be washed away by acid. However, under the condition of non-oxidizing acid elution at room temperature, the elution and decomposition of ammonium formate are not much, and the total elution rate can not reach a high level. The cyanide removal effect of acid regenerated carbon is far less than that of new carbon.
The commonly used regenerant (eluent) is 2% ~ 5% hydrochloric acid, nitric acid or sulfuric acid. Nitric acid has the best regeneration effect, and hydrochloric acid is better than sulfuric acid. In addition, the mixture of 8g/L sodium hypochlorite and 6% ammonium sulfate (volume 1: 1) was used as a regenerant to oxidize the reducing substances on these carbons, and the copper was eluted in the form of cuprammonia complex. The regeneration effect of activated carbon with nitric acid under heating condition is good, but it is costly, corrosive and produces nitrogen oxides. . Soak in sulfuric acid at room temperature, then dry the carbon and regenerate it at above 450℃. The effect is very good. In a word, after several times of recycling, although the activated carbon is regenerated by pickling, its performance is still declining, which is caused by the accumulation of organic matter and silicate in wastewater on the carbon. Thermal regeneration or high temperature regeneration is needed to restore its activity. There is little practice in this field.
The high-temperature regeneration method is effective, and the regeneration cost is not high, and the equipment investment is not large, so the high-temperature regeneration method can be used. High temperature regeneration equipment can be divided into three types: electric heating, coal coke heating and gas heating. Can be selected according to the actual situation. If the regeneration amount is below 1t/d, electric heating regeneration equipment can be used, such as JHR series internal heating regeneration equipment; If the treatment capacity is large, gas, oil or coal-fired regeneration equipment should be used, such as the multi-tube gas regeneration furnace used in Australian gold mines.
In the process of treating cyanide-containing wastewater with activated carbon, trace gold will be adsorbed. When activated carbon fails, if the gold product is as high as 100g/t, it is equivalent to the value of activated carbon. That is to say, taking 0.03mg/L gold-bearing wastewater as an example, if 4000m3 wastewater is treated per ton of carbon, it is more economical to buy new activated carbon with the profit of recovering gold. In addition, when the impurity content in wastewater is high, measures can be taken to remove impurities in advance to ensure the use effect of activated carbon. This is the development direction of activated carbon method.
7.5 Catalytic decomposition process and equipment of activated carbon
Although there are three ways to remove cyanide by activated carbon, one of the three ways may be the main one in a specific device. Therefore, there are three methods: activated carbon catalytic oxidation with cyanogen oxide as the main method, activated carbon catalytic hydrolysis with hydrolysis as the main method and activated carbon bed stripping with stripping as the main method. The process, equipment and process conditions of these methods are quite different, which are introduced below.
7.5. 1 activated carbon catalytic oxidation method
According to the previous hypothesis, the oxidation of cyanide by activated carbon is the oxidation of cyanide by hydrogen peroxide, so the optimum pH value of faith oxidized activated carbon is the same as that of cyanide removal by hydrogen peroxide, that is, in the range of 6 ~ 9, as shown in the figure. In addition, the reaction conditions also include aeration rate, wastewater spraying density and catalyst dosage. Because the wastewater from cyanide plant often contains copper, copper catalyst cannot be added. The ventilation rate is determined according to the oxygen required for the reaction. Because the oxygen adsorption speed of activated carbon is controlled by the liquid film, it is necessary to increase the linear velocity of air flowing through the reaction tower. However, excessively increasing the gas velocity will not only increase the resistance of the carbon bed, but also increase the power consumption of the fan, and also reduce the contact opportunity between cyanide in wastewater and activated carbon, resulting in cyanide production. Therefore, the aeration rate should be determined by experiments. If calculated according to the gas-liquid ratio, it is generally around 100, which is far less than the gas-liquid ratio of acidification recovery method.
Activated carbon catalytic oxidation device is divided into four parts, namely wastewater pretreatment, cyanide oxidation, wastewater secondary treatment and activated carbon regeneration. The process flow chart is shown in Figure 7- 1.
Air waste gas
↓ ↑
Cyanide-containing wastewater → precipitation or filtration → oxidation reaction → precipitation or filtration → discharge.
↓ ↓ ↑ ↓
Waste residue activated carbon regeneration waste residue
Fig. 7- 1 schematic diagram of catalytic oxidation process of activated carbon
I. Pretreatment device
Cyanide-containing wastewater contains Ca2+ and CNO-, which will continuously decompose CO32-. Because the wastewater is alkaline, CO2 in the air will be continuously absorbed into the water, and these CO32- will form CaCO3 precipitation with Ca2+, which is the main component of suspended solids in the water. If these suspended substances enter the reaction tower and come into contact with the filled activated carbon, it will block the pores between the micropores of the activated carbon and the activated carbon particles, inactivate the activated carbon and increase the bed resistance. In addition, the waste water discharged from tailings pond often contains sediment, which must be treated before it can enter the reaction tower.
Generally, wastewater is pretreated by filtration, such as activated carbon filter tower, activated carbon filter tank, fiber ball filter tower or filter cloth filter. Among them, activated carbon filter has the advantages of low investment, adsorption of gold, silver, copper and zinc in wastewater, easy management and desliming (suspended matter and sediment).
When the pH value of wastewater is higher than 9, it must be neutralized with acid until the pH value is in the range of 7 ~ 9. In the process of adjusting pH value, precipitation such as Zn2Fe(CN)6 may occur. At this time, the pretreatment device is complicated, including neutralization, solid-liquid separation and filtration. In addition, containers should be sealed to prevent HCN from escaping, and the equipment used should be strictly preserved. Otherwise, the iron will be treated with Fe (CN) 64.
Two. Cyanide oxidation unit
The oxidation of cyanide is completed by 2 ~ 3 oxidation towers in series, the wastewater is lifted by the water pump between the towers, and the air enters the towers in parallel. Oxidation tower is the core equipment of activated carbon catalytic decomposition device, and its structure is similar to that of adsorption tower.
The oxidation tower shall have the following functions:
1) The oxidation tower is evenly filled with enough activated carbon, and the upper part of the tower has enough space. Generally, the height of activated carbon sheets should not exceed1.5m. If it is too high, the air resistance will be great.
2) Gas uniformly passes through the carbon bed from the lower part of the tower and is discharged from the upper part of the tower.
3) The liquid is evenly sprayed on the carbon bed at the upper part of the tower and flows to the lower part of the tower. And can be discharged from the lower part of the tower without overflowing.
4) Provide manholes, holes for loading and unloading activated carbon and necessary observation holes.
5) Provide spraying device and drainage device for activated carbon regeneration.
6) As the regeneration of activated carbon needs to be soaked in acidic solution or other corrosive solution, all parts of the reaction tower must be strictly anticorrosion.
Three. Activated carbon wet regeneration device
The device consists of two solution tanks and an anti-corrosion pump. One tank is used to prepare pickling solution and the other tank is used to prepare copper sulfate solution. When the leaching regeneration method is adopted, the leached wastewater is returned to the tank for recycling, and the leaching is still completed under ventilation conditions. For example, decomposition products such as carbon dioxide will escape, and all storage tanks, pipelines and valves must be strictly protected against corrosion. Due to different impurity loads, the carbon bed of each oxidation tower should be regenerated separately to achieve good treatment effect.
The most ideal way is to adopt a new pretreatment method, so that activated carbon is not poisoned and does not need regeneration.
Four. Secondary treatment facility
The wastewater treated by activated carbon catalytic oxidation may contain a certain amount of suspended solids, and heavy metals may exceed the standard. The content of pollutants can be further reduced by adding a small amount of lime and other secondary treatment precipitation. The secondary treatment facilities can be specially constructed sedimentation tanks, secondary dams of tailings ponds or pretreatment devices similar to those mentioned above.
Activated carbon catalytic oxidation method is suitable for treating clarified cyanide-containing wastewater, such as overflow water from tailings pond. This treatment method is very ideal for cyanide plants with seepage-proof tailings ponds. The wastewater is self-purified by the tailings pond, and the pH value is reduced to 7 ~ 9, and the heavy metal content is very low, so it is suitable for activated carbon catalytic oxidation.
7.5.2 Activated carbon catalytic hydrolysis method
The activated carbon catalytic hydrolysis method does not need air, so its device is similar to an adsorption column. In order to ensure the cleanliness of the carbon layer, there is also a pretreatment device, but the treated wastewater is directly discharged and the activated carbon needs to be regenerated. The regeneration method is basically the same as the catalytic oxidation method. In order to reduce the treatment cost, trough equipment developed by Jin Huan Technology Development Company of Changchun Gold Research Institute can be used.
7.5.3 Stripping method of activated carbon fixed bed
Using the hydrophilicity and packing function of activated carbon surface, the packed tower of activated carbon is used as the stripping tower and as the secondary treatment of wastewater produced by acidification recovery method, so that cyanide is reduced to below 2 mg/L. Because HCN is mainly stripped, it is not necessary to adjust the pH value of wastewater. When the pH value is 2 ~ 3, the cost of stripping treatment is much lower than that of chemical method. Moreover, gold in wastewater can be recovered, killing two birds with one stone, and the treated wastewater can meet the standard by dilution or natural purification of flotation wastewater.
7.6 Practice of Treating Cyanide-containing Wastewater by Activated Carbon Method
There are only three cyanidation plants using activated carbon method in China. The following is just a brief introduction.
7.6. 1 A cyanide plant adopts the whole mud-zinc powder replacement process to extract gold. After cyanide tail is cleaned by tailings pond, the cyanide concentration is 30 mg/L. Using the patented technology of activated carbon catalytic oxidation in Changchun Gold Research Institute, cyanide is reduced to below 2mg/L after only one oxidation tower treatment, and the reaction pH is 6.5 ~ 9. The gas-liquid ratio is 80, the wastewater treatment capacity is 3m3/t h, and the wastewater treatment consumption per ton is approximately 0. 1kg hydrochloric acid, 0.05kg copper sulfate pentahydrate and 1kwh power consumption. The investment of the device is about 40,000 yuan (excluding the factory building).
7.6.2 A cyanidation plant is located in a mountainous area, and the whole mud cyanidation-zinc powder replacement process is adopted to extract gold. The cyanide tail liquid contains about 70 mg/L cyanide. After self-cleaning, the cyanide content in the tailings pond decreased to about 3mg/L, and it was treated by activated carbon catalytic hydrolysis process. After three adsorption columns, cyanide reached the standard. More than 8 kilograms of gold can be recovered every year. Because the gold-loaded carbon is incinerated. Without regenerating activated carbon, regardless of the investment cost of activated carbon, the power consumption of wastewater treatment is only 0.05kwh/m3. The problem is that it can't be used in winter, and there is still overflow water in the tailings pond for a period of time in winter.
7.6.3 A cyanidation plant in China adopts all-slime cyanidation-zinc powder replacement process to extract gold, with a treatment capacity of 50t/d, and adopts alkaline chlorination process to treat cyanidation tailings. For various reasons, the cyanide content in the drainage outside the tailings pond is often higher than 0.5 mg/L. The patented technology of activated carbon hydrolysis in Changchun Gold Research Institute is used for secondary treatment of the wastewater, and the annual recovery of gold is about 65438±0.5kg. When the cyanide content in the drainage outside the tailings pond is less than 5 mg/L, the cyanide can reach the standard after treatment, and the removal rate of heavy metals such as zinc, copper and iron is also very high. Because this technology does not use electricity and does not need special personnel to operate, its treatment cost is only the labor cost of cleaning the carbon bed regularly every day. Due to the new technology of eluting gold with activated carbon, activated carbon is regenerated, and the recovery cost of gold is only 65,438+00% of the gold price, which has considerable economic benefits.