Gold ore heap leaching contaminants wastewater main cyanide (for heavy metal ions including hand and iron cyanide complex), followed by heavy metal elements such as copper, zinc and the like, as well as arsenic, antimony, nitrite. All treatments are first done to remove cyanide, or to eliminate other harmful substances. There are many ways to treat cyanide-containing wastewater, including: (1) Alkaline chlorination (2) SO 2 ∕ air oxidation method (3) H 2 O 2 oxidation method (4) Chemical precipitation method (5) Ion exchange method (6) Natural degradation method (7) Biodegradation Law (8) Acidification-vaporization-regeneration method 1. Alkaline chlorination This first method is to use cyanide, which is the principle and the free cyanide and cyanide complexed heavy metal elements may be oxidized to chlorine or hypochlorite cyanate (CNO -). The reaction process is as follows: CN - +Cl 2 +2OH - Zn(CN) 4 2 - +4Cl 2 +10OH - If an additional step and prolonged contact with chlorine gas and the residence time, cyanate CNO - is further oxidized to nitrogen gas and bicarbonates. The reaction is as follows: 2CNO - +3Cl 2 +6OH - In this process, nitrites that are carcinogenic are also oxidized. Heavy metal elements in the wastewater are also removed by precipitation of hydroxides. If there are elements such as arsenic and antimony in the wastewater, then high-sulfur sulfate must be added to remove arsenic from the solution. Calcium hydroxide can also remove arsenic, but in this case, iron salt is generally added, and inexpensive lime is not added. This is because the pH of the external drainage must be controlled between 6 and 9. Since this method cannot remove the ferricyanide complex, and the residual chlorine content in the wastewater treated by this method is high, it has not been widely used in recent years. At present, gold ore heap leaching in China basically uses this method to treat wastewater. Depending on the source of supply, chlorine is used, and sodium hypochlorite is used. More calcium hypochlorite (bleaching powder) is used. Sodium hypochlorite is a liquid and is easily decomposed, especially in the hot season, so it is often made by salt electrolysis and directly prepared on site. Many domestic manufacturers produce sodium hypochlorite generators of various specifications. Calcium hypochlorite is a solid, convenient to transport, but it is a substance that is easily decomposed, so the storage time should not be too long. The process of the alkaline chlorination process is shown in Figure 1. Figure 1 Process of treating cyanide wastewater by alkaline chlorination The second reactor is provided to remove arsenic, antimony, etc., and the third reactor is used to remove heavy metal precipitates and other solid precipitates. The amount of chlorine added can depend on the cyanide content and nature of the wastewater. Generally, the ratio of active chlorine (effective chlorine) to the amount of cyanide in the wastewater is (5-9):1, preferably not more than 10 times. Excessive active chlorine makes the amount of residual nitrogen in the wastewater high. Once the residual chlorine is high, a small amount of bisamine sulfate is added to eliminate it. The reaction is carried out at room temperature, and the agitation time is 20 to 30 minutes to reach the discharge standard. Regarding the issue of cyanide efflux standards in wastewater, there are different regulations in different countries, and it depends on the water body to be discharged. For example, the ground water cyanide concentration specified in the United States is 0.02 mg ∕L, and the former Soviet Union and China have a prescription of 0.05 mg ∕L. In Canada, the limits of cyanide content in wastewater are different everywhere, most of them are 1mg ∕L, and some are 2mg ∕L. Second, SO 2 ∕ air oxidation method Because the alkaline chlorination method can not remove the ferricyanide complex, and the hypochlorous acid is easy to decompose and inconvenient to store, the excess cyanide is often high in the external drainage. Therefore, many factories and mines abroad use SO 2 ∕ air oxidation method to treat cyanide-containing wastewater. . The principle of the method is to combine the treatment of SO 2 and air under the catalysis of copper to make free cyanide, complex of heavy metal element and cyanide (except ferricyanide complex) at pH 8-10. ) oxidized to cyanate. The response is: CN - +SO 2 +O 2 +H 2 O Since cyanide is oxidized, heavy metal elements Zn, Cu, etc. are also precipitated as hydroxides. At this time, the ferricyanide ion precipitates in the form of copper or zinc ferrocyanide. 2Cu 2 + +Fe(CN) 6 4 - During the reaction, copper acts both as a catalyst and as a precipitant for ferricyanide ions. Therefore, the amount to be added must be guaranteed. The added SO 2 may be in a gaseous state (for example, containing 2% to 5% of SO 2 off-gas), or may be in a liquid state, and may also be used in some chemical raw materials such as sodium hydrogen sulfite or sodium sulfite. This method is used to treat cyanide-containing wastewater by the McBean gold heap leaching. The consumption of SO 2 and lime is 4g ∕gCN - respectively . The data before and after processing are listed in Table 1. Table 1 Monthly average results of wastewater treatment at McBean yard project name CN - (mg∕L) Cu(mg∕L) Fe(mg∕L) pH Before treatment 370 30 20 11.5 After processing 0.2 0.7 0.2 9.0 Third, hydrogen peroxide oxidation At present, more and more factories and mines in the world use hydrogen peroxide to treat cyanide-containing wastewater. The principle of this method is to remove cyanide and heavy metal elements under the catalysis of copper. The reaction is as follows: CN - +H 2 O 2 Cu(CN) 4 2- +4H 2 O 2 +2OH - The ferricyanide complex is also removed as copper ferrocyanide as in the SO 2 ∕ air oxidation process. When arsenic is present in the wastewater, as in the case of alkaline chlorination, it is treated with iron sulfate to precipitate it. The Hope Brook gold mine heap leaching process uses a three-stage process, as shown in Figure 2. Figure 2 Schematic diagram of the treatment process of Hope Brook gold heap heap leaching wastewater The third reactor in Figure 2 was charged with TMT-15 (tridecyltristritris) to precipitate residual copper in the wastewater. The results of the treatment are as follows: the total amount of cyanide is reduced from 311 mg ∕L to 1 mg ∕L, copper is reduced from 99 mg ∕L to 0.5 mg ∕L, iron is reduced from 8.8 mg ∕L to 0.3 mg ∕L, and zinc is from 0.63 mg ∕L. Drop to 0.1mg ∕L. H 2 O 2 was used to treat cyanide-containing wastewater in 1984. Over the past decade, more than 20 factories and mines have used this method to replace other treatment methods. The H 2 O 2 method has the following advantages: 1 After the treated wastewater, the residual hydrogen peroxide is quickly decomposed into water and oxygen; 2 easy to operate, simple facilities; 3 more cyanide removal. Fourth, natural degradation method This is a long-established method for treating cyanide-containing wastewater. Although its use is being replaced by other oxidation processes today, it still has practical value for factories and mines with large-volume tailings dam conditions. The method removes the metal ions of cyanide and its complex in wastewater by a long-term natural physical, chemical, and biological action. It includes volatilization, absorption of carbon dioxide in the air to lower the pH of the wastewater, chemical and microbial oxidation of cyanide, sunlight (mainly ultraviolet light) to degrade cyanide, chemical dissociation of metal ions and cyanide complex ions, A series of processes such as hydrolysis and precipitation. Among them, the destruction of the ferricyanide complex mainly relies on ultraviolet radiation. It has been proved that the removal of cyanide and its heavy metal element complex is caused by the oxidation and volatilization of hydrocyanic acid, dissociation of complex ions, and hydrolysis and precipitation of heavy metal elements. Obviously, the natural degradation process is related to many factors such as cyanide concentration in solution, stability of complex ions, pH of the medium, bacteria and temperature, sunlight, residence time, turbidity, and flow pattern. The ambient temperature is very important. In the summer, this process is fast, and in the winter, the reaction process almost stops. The advantage of this method is that it does not add any chemical reagents, but it takes a long time, requires a large volume of tail liquid pool, and does not completely treat high-concentration cyanide wastewater. Therefore, this method can only be used as a pretreatment method for chemical oxidation. SKS lead zinc, lead and zinc wall kernel natural degradation test method is conducted, when the concentrations of Cyanide 3 ~ 19mg / L and 30 ~ 55mg / L, the day 15% reduction of free cyanide. The degradation of cyanide and metal complex ions is slower, and thiocyanide hardly undergoes natural degradation. V. Chemical precipitation method The method is mainly used to remove cyanide from wastewater by means of added chemical reagents. Two methods have been reported. The domestic use of ferrous sulfate and lime to precipitate cyanide in wastewater is based on the action of ferrous hydroxide and cyanide formed in alkaline wastewater to form ferrocyanide precipitate. The reaction process is as follows: Fe 2 + +2OH - Fe(OH) 2 +2CN - Fe(CN) 2 +4CN - 2Fe 2 + +[Fe(CN) 6 ] 4- The ferrocyanide complex is easily converted to light blue by air oxidation, such as long-term air, and finally all converted into Prussian blue Fe 4 [Fe(CN) 6 ] 3 precipitate. The advantage of this method is that the raw materials are easy to obtain and the price is low. The disadvantage is that cyanide is not completely removed, especially for complexed cyanide. The Hemlo gold mining company abroad has adopted another chemical precipitation method. The method is to mix ferrous sulfate and copper sulfate in advance, and control the pH value of 6-7, the ferrous ions are oxidized by the ternary copper ions to form iron hydroxide, and the divalent copper ions themselves are reduced to low-priced copper Cu. + Cu 2 + Fe 2 + +3OH - The generated Cu + can be removed by precipitation of the free cyanide insoluble copper cyanide complex; due to the reduction of free cyanide in the wastewater, the complex ions of the heavy metal and cyanide in the wastewater are dissociated, and the cyanide dissociated further. Precipitated by Cu + , the reaction is as follows: 2Cu + +2CN - 2[Cu(CN) 4 ] 2- The dissociated Zn 2 + , Cu 2 + , Ni 2 + and the like are precipitated together with Fe(OH) 3 . Ferricyanide in wastewater is precipitated in the form of copper ferrocyanide 2Cu 2 + +[Fe(CN) 6 ] 4 - Lime is then added to raise the pH of the wastewater to 9.5 to 10 to ensure precipitation of nickel . Further treatment with hydrogen peroxide is carried out at a pH of 10 to finally remove cyanide. The company's Golden Giant gold mine uses this method to treat wastewater. The results are shown in Table 2. Table 2 Analysis results of treatment of heap leach wastewater by chemical precipitation method in Golden Giant gold mine Analysis Project pH CN - Cu Fe Ni Sb Mo CNS Pre-treatment content (mg∕L) 9.10 23.20 4.10 5.20 4.80 7.70 1.20 44.40 Post-treatment content (mg∕L) 9.81 0.13 0.50 0.11 0.08 1.00 0.20 24.20 Six other methods The ion exchange method, the microbial degradation method, the electrolytic recovery method, and the like are used less, and some are even in the experimental stage. As for the acidification-volatilization-regeneration recovery method, although it is used in many other fields, for heap leaching, since the total amount of wastewater is small, the use value is small.
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CNO - +2Cl - +H 2 O 4CNO - +8Cl - +Zn(OH) 2 +4H 2 O N 2 +6Cl - +2HCO 3 - +2H 2 O CNO - +H 2 SO 4 Cu 2 Fe(CN) 6 ↓ CNO - +H 2 O Cu(OH) 2 ↓+4CNO - +4H 2 O Fe(OH) 2 Fe(CN) 2 +2OH - [Fe(CN) 6 ] 4- Fe 2 [Fe(CN) 6 ]↓ Cu + +Fe(OH) 3 Cu 2 (CN) 2 ↓ Cu 2 + +4CN - Cu 2 [Fe(CN) 6 ]↓