How to treat cyanide-containing wastewater (effluents)?

Cyanide-containing wastewater refers to industrial wastewater that contains CN groups. Large amounts of cyanide-containing wastewater are generated in the production processes of various industries such as the extraction of gold, silver, and copper from non-ferrous metal ores, cyanide plating, the chemical industry, coking, and heat treatment. In particular, a large quantity of cyanide is used in mineral processing operations; the concentration of CNˉ in this wastewater is high, and it also contains large amounts of heavy metals and compounds such as thiocyanates, posing a serious threat to the quality of external water environments. There are not many methods for treating cyanide-containing wastewater in China. In the early days, methods such as the alkali-chlorine method (liquid chlorine method, sodium hypochlorite method, bleaching powder method, chlorine dioxide method, etc.), acid-chlorine method, Inko method, acidification-absorption-neutralization method, ozone method, electrolysis method, ion exchange method, activated carbon catalytic oxidation method, biological method, and pressurized hydrolysis method were used. However, in the face of different actual situations, we should choose different methods to handle them. (1) Emergency response methods for sudden pollution incidents. The simplest method is to use an aqueous solution of ferrous sulfate to rapidly convert ** in the wastewater into slightly toxic complexes; this method is also known as the chemical complexation method. The main chemical reactions are: Feso4·7H20 → Fe2+ + SO42- + 7H2O; Fe2+ + 6CN- → Fe(CN)4-6; 2Fe2+ + Fe(CN)4-6 → Fe2[Fe(CN)6], resulting in the formation of an insoluble compound of the prussian blue type. This method is characterized by simple operation and low cost, but the wastewater treated with it does not meet the **discharge standards**. In the 1970s, some domestic companies used this method, but none do so now. From the perspective of environmental safety, this method can serve as **one of the rapid remediation approaches to be used in case of sudden pollution incidents. Adding ferrous sulfate solution to water can quickly reduce the severity of the harm caused by cyanide pollutants in the water, thereby minimizing environmental damage, especially to aquatic organisms. When the mass concentration of CNˉ in wastewater is very low, this method fails to achieve good treatment effects. The selection of treatment methods for cyanide-containing wastewater should be based on specific circumstances. (II) Oxidation methods for medium and low-concentration cyanide-containing wastewater (1) Alkaline chlorination method The alkaline chlorination method is a widely used approach both domestically and internationally. High-valent chlorine oxidants are added to alkaline cyanide-containing wastewater; common oxidants include ClO2, Cl2 (in both gaseous and liquid forms), bleaching powder, sodium hypochlorite, calcium hypochlorite, chlorites, etc. In alkaline solutions, OClˉ or chlorine compounds in higher oxidation states are generally formed, **which are first oxidized to cyanates, and further oxidized to carbon dioxide and nitrogen. The main chemical reactions are: OClˉ + CNˉ + H2O → CNCI + 2OHˉ; CNCI + 2H⁺ → CNOˉ + Clˉ + H2O; 2CNOˉ + 3OCIˉ → CO2↑ + N2↑ + 3Clˉ + CO3²˻. The pH of the oxidation reaction is maintained around 1, making the process simple – after adding the chlorinating agent, only stirring is required. When the composition of wastewater is complex, the consumption of chlorine oxidants is high—typically 4 to 9 times the theoretical value. This consumption increases further when the purity of the chemical agents is low or when the thiocyanate content is high. In cases where ferrocyanide complexes are present, they are oxidized into ferricyanide complexes, which become soluble species; as a result, it becomes difficult for the treated wastewater to meet **discharge standards**. The advantage of this method is its wide availability of reagents, low cost, and minimal equipment investment. However, it causes severe environmental pollution at the work site, generates the secondary pollutant cyanogen chloride, poses a significant risk to operators, and requires large amounts of reagents; moreover, prolonged use leads to severe corrosion of the equipment. (2) The SO2–air method developed by Inco is known as the Inco process. It was invented by Inco in 1982; in this method, a mixture of SO2 and air is added to the wastewater to be treated, the pH value is maintained between 8 and 10, and under the catalysis of divalent copper ions, the synergistic action of SO2 and air is utilized to oxidize ** present in the wastewater. From the perspective of the reaction process, there are three main ways for ** to be removed: volatilization, oxidation, and adsorption by precipitates. The precipitates are formed as a result of neutralization with lime milk; their amount is substantial, and they absorb a certain amount of CNˉ. This method not only removes the vast majority of ** from the dilute solution and eliminates ferrocyanide complexes, but it also fails to recover valuable components. It is an oxidative process that consumes resources, and sulfur dioxide is another air pollutant that inevitably escapes and leaks during the formation and reaction processes. After water treatment, a large amount of cyanide-containing solid waste is generated, as well as problems related to pipeline blockages. The sulfur dioxide-air oxidation method is simple in process, features uncomplicated equipment; its treatment efficiency is generally better than that of the chlorine oxidation method (ignoring the toxicity of sulfur compounds); the reagents required are readily available, the treatment costs are low, and the investment needed is minimal. In 1984, the Changchun Gold Research Institute began researching the sulfur dioxide-air oxidation method, and industrial trials were completed in 1988. In 1991, this method was applied at the Zhaoyuan cyanide plant in Shandong; the initial concentration of the cyanide-containing wastewater was 380–400 mg/L, with a removal rate of over 9%, and a removal rate of SCNˉ ranging from 46% to 86%, achieving satisfactory results. Abroad, there are many examples of gold cyanidation plants using this method to treat cyanide-containing wastewater, with applications in countries such as Canada, the United States, and Australia. This method also has other application processes, such as the sodium sulfite method and the sodium metabisulfite method. (3) H2O2 oxidation method: H2O2 is oxidized under conditions where the pH is between 9.5 and 11, at room temperature, and in the presence of copper (Cu2+) ions as a catalyst**, resulting in the formation of CNOˉ. The subsequent hydrolysis of CNOˉ depends on the pH value. Its main chemical reactions are: CNˉ + H2O2 → CNOˉ + H2O; CNOˉ + 2H2O → NH4⁺ + CO3²⁻. Heavy metal ions form hydroxide precipitates, while ferrocyanide complexes and other heavy metal ions form ferrocyanide salts that can be removed. Metals such as copper and zinc, which exist in the form of metal-cyanide complexes, also form hydroxide precipitates once they are oxidized and removed. Excess hydrogen peroxide can rapidly decompose into water and oxygen. This method is suitable for treating low-concentration cyanide-containing wastewater. Under the H2O2 oxidation method, foreign catalysts can also be purchased directly for treatment. However, these catalysts are quite expensive, their availability is limited, and the treatment costs are relatively high; HZQ also poses certain risks during transportation and use. This method has difficulty oxidizing SCNˉ in wastewater, and the treated wastewater remains somewhat toxic. This technology was first used in 1974 by the American company DuPont to treat cyanide-containing wastewater; later, in 1984, Germany applied it at a gold cyanidation plant in Africa. In 1997, it was successfully used to treat the tailings from the acidification method at the Sanshandao Gold Mine in Shandong. (4) Ozone oxidation method: Ozone possesses extremely strong oxidizing power, with an electrode potential of 2.07 mv; second only to fluorine, it can easily break down components that other oxidants are unable to decompose. The chemical reaction mechanism of ozone oxidation treatment is: 2CNˉ + 2H⁺ + H₂O + O₃ → 2H₂CO₃ + 2O₂ + N₂. Initially, ozone reacts with ** to form cyanate; subsequent hydrolysis of cyanate yields nitrogen and carbonate ions. The ozone oxidation method is used to treat ** in wastewater. It only requires an ozone generation device; there is no need to purchase or transport any chemicals. This method enables compliance with the ** comprehensive wastewater discharge standards. No other harmful substances are added during the treatment process, thus preventing secondary pollution. No further treatment is necessary. However, industrial applications are limited due to the high cost of producing ozone with ozone generators and the difficulties in maintaining the equipment. As long as ozone generators can overcome the barriers to ozone production, their prospects for industrial use are very promising. The ozone oxidation method requires a large amount of electrical energy, making it difficult to apply in areas with limited power supply. (III) Treatment methods for cyanide-containing wastewater of high quality concentration (1) Acidification volatilization–alkaline absorption method. The acidification volatilization–alkaline absorption method is a traditional approach for treating cyanide-containing wastewater of high and medium quality concentrations; it was used by mining plants around the world in the early days. The reaction mechanism of this method is as follows: Me + 2CN⁻ + H2SO4 → MeSO4 + 2HCN↑ (where Me refers to Na+, K+, Ca2+, etc.). HCN + NaOH → NaCN + H2O. The process parameters for this method are as follows: the pH should be maintained between 2 and 3. The boiling point of HCN is 25.6°C. An inert, volatile strong acid—H2SO4—is mixed with the wastewater, and then the mixture is heated to 30°C–40°C. At these temperatures, HCN becomes highly volatile; the vaporized HCN is absorbed using a NaOH solution. The resulting sodium cyanide solution can be reused. **The absorption rate is generally between 85% and 95%. When the process conditions are well controlled, the mass concentration of cyanide in the residual liquid can be as low as 3–5 mg/L, with an average concentration of 10–20 mg/L; if the process conditions are not properly controlled, the mass concentration of cyanide in the residual liquid rises to 30–50 mg/L. The main impact of cyanide-containing electroplating wastewater comes from several heavy metal ions, while the factors affecting cyanide-containing wastewater from gold mines are more complex; in addition to the heavy metals Fe, Cu, Zn, Ag, and Au, certain acidic anions also participate in reactions. For example, the cyanide-containing wastewater from many cyanide plants contains large amounts of SCNˉ, and a precipitation reaction occurs to form white CuSCN; when the iron content is high, iron salts also precipitate. The advantage of this method is its ability to recover ** to the greatest extent possible, enabling resource recycling and achieving the highest effective utilization rate, which results in significant economic benefits. The disadvantages include high initial investment, which is difficult for some small and medium-sized enterprises to afford, complex operation procedures, and the fact that the cyanide-containing residues after treatment do not meet the ** discharge standards. For example, some large gold processing plants in Shandong province use two acidification steps followed by two alkali absorption steps, yet still fail to meet the **emission standards; further treatment of the residual liquid is required. Therefore, the acidification-volatilization–alkali absorption method is suitable for wastewater with high cyanide mass concentrations, and it performs well when used in combination with other processes. The acidification-volatilization–alkali absorption method yields good treatment results, features high resource utilization and significant economic benefits, but it requires large investment, high operating costs, and complex technical maintenance. (2) Two-step precipitation impurity removal closed-loop full recycling process (two-step precipitation method). The two-step precipitation method was developed by Changchun Gold Research Institute during the treatment of wastewater from a cyanidation plant in Shandong. This method is an efficient closed-loop, full-circulation system developed specifically for small and medium-sized gold processing plants in China that generate wastewater containing high concentrations of SCNˉ, achieving \"zero discharge\" of such wastewater. Its basic reaction principles are as follows: 2Cu²⁺ + 2SCN⁻ → Cu₂(SCN)₂↓ (white); Ca²⁺ + SO₄²⁻ → CaSO₄↓ (white); Pb²⁺ + SO₄²⁻ → PbSO₄↓ (white); H⁺ + CN⁻ → HCN. Precipitation occurs, and the vast majority of HCN remains in the solution; only a small amount of HCN gas evaporates, yet this is still contained within a closed container. In the solution, the removal rate of harmful heavy metals ranges from 80% to 95%. The acidified lean solution after precipitation contains a large amount of sulfate ions; calcium oxide (in slurry form) is added directly to neutralize it, with the pH value kept between 10 and 12, resulting in the formation of a large amount of white precipitate. After neutralization, a large amount **was reconverted to CNˉ, while a significant amount of SO2-4 was removed. After solid-liquid separation, the solution can be directly returned to the production process for recycling after adding sodium cyanide. Calculated at the prices of the late 1990s, the cost for treating 1 m3 of low-concentration liquid with a high quality level (above 2000 mg/L) is 9.26 yuan; the value of the valuable resources recovered amounts to 37 yuan, resulting in a profit of 27.74 yuan – thus the economic benefits are quite significant. The disadvantage of this method is that complete precipitation must occur in the first step, and the clarification time is long; otherwise, when alkali is added, cuprous thiocyanate re-dissolves, affecting the treatment efficiency. The process challenge is that undeposited CaS can cause valve blockage. Therefore, this process requires an extended settling time for the second step. If the problem of secondaryly precipitated CaSO4 can be properly addressed, this process will have broad application prospects. (3) Solvent extraction method: The solvent extraction method is a process for treating high-concentration cyanide solutions, which was developed by Tsinghua University in the late 1990s at a gold smelting plant in Shandong. The principle of the extraction process involves using organic amines for extraction. The lean liquid resulting from this extraction still contains large amounts of CNˉ, which is sent back into the production process for reuse. The organic phase loaded with these substances is then subjected to back-extraction using NaOH solution, thereby regenerating the organic amine extractants, which can be reused again. The aqueous solution after alkaline treatment contains only small amounts of elements such as copper and zinc, which does not affect the leaching of gold and silver; it is returned to the gold leaching process for reuse, thereby enabling the recycling of extractants and achieving a cycle for the lean solution. In this process, the concentrated waste liquid with high copper and zinc contents accounts for 1/6 of the volume of the original dilute liquid; it also requires acidification treatment, and the cumulative amount is significant, making subsequent processing complicated. Extraction agents are more expensive than inorganic reagents such as acids, bases, and salts used in other processes, resulting in higher costs. (4) Natural purification method: In containers, industrial ponds, or mine tailing ponds, the mass concentration of cyanide in cyanide-containing wastewater gradually decreases over time under the conditions of natural temperature and pressure; this method is known as the natural purification method. It includes processes such as volatilization, self-decomposition, oxidation, photochemical degradation, biodegradation, and precipitation adsorption; it is the result of complex interactions involving physical chemistry, photochemistry, biochemistry, and other factors. Therefore, the natural purification method can achieve the purpose of removal ** without the need for mechanical equipment or the addition of any chemical agents. The natural purification method has advantages such as low investment and low production costs; mining enterprises both at home and abroad use this method, primarily as a supplementary treatment approach for cyanide-containing wastewater with low concentration. Natural purification methods require sufficient purification time and an adequate area for oxygen exchange, as well as favorable conditions for gas diffusion and a good underground impermeable layer. They must be located away from environments where birds and other organisms obtain water; thus, their application conditions are quite stringent. This method is commonly used for pretreatment and post-treatment in the gold production industry; however, due to unsuitable application conditions, water pollution incidents and poisoning events occur frequently. (5) Biological treatment methods: Biological treatment methods mainly include microbial treatment and plant treatment. The biotechnologies reported in the literature are mainly microbial treatment technologies. The commonly used microbial treatment methods are mainly the activated sludge process and the biofilm process. Currently, biofilms are mainly used in the form of biofilm formation or as packing in biological filters, or tower-type biological filters are employed to treat cyanide-containing wastewater. The use of aquatic plants to treat tailing wastewater with low concentrations has also been applied; for example, a gold mine in Guangdong used aquatic plants to degrade trace amounts of ** in the tailing water, and a gold mine in the Heilongjiang River basin employed plants to reduce the levels of ** in the wastewater discharged there. In recent years, biological treatment of cyanide-containing wastewater has gradually become a major focus of research both domestically and internationally. **Although it is a **organic substance, certain microorganisms can obtain carbon and nitrogen nutrients from **; some microorganisms even use ** as both a carbon source and a nitrogen source. During their metabolic processes, they convert ** into carbon dioxide, ammonia, formic acid, formamide, etc., thereby making wastewater containing ** biodegradable. Biological methods can overcome disadvantages such as incomplete removal of metal cyanide complexes, but this approach has issues such as low treatment concentrations and limited load capacity. Biological treatment is also one of the promising methods; the key to this technology lies in developing advantageous bacterial strains capable of directly treating wastewater with moderate concentration, or in creating new treatment processes. (6) Membrane methods: Membrane methods mainly include liquid membrane methods, gas membrane methods, and membrane separation technologies. The principle of recovering ** by liquid membrane separation is mainly based on a water-in-oil type system. The solvent forms the matrix of the film; the hydrophilic and hydrophobic groups of the surfactant are arranged in an ordered manner to stabilize the film structure, while the emulsion is dispersed in a third phase to form a liquid film. After acidification of cyanide-containing wastewater, HCN can pass through the oil phase (kerosene or surfactant) liquid film to enter the inner aqueous phase containing NaOH, where it reacts to form NaCN. HCN is separated and recovered using the oil phase, and NaCN is obtained again after demulsification. In 1994, the liquid membrane method was employed by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, at the Cangshang Gold Mine in Shandong for treating cyanide-containing wastewater. Subsequently, other researchers also used the liquid membrane method to recover **. The gaseous membrane method is relatively rare; this approach is still in the laboratory testing stage for the recovery and closed-loop recycling of cyanide-containing electroplating wastewater. Currently, technologies such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis are all based on membrane separation technology. Taking the recovery of ** using hollow fiber membranes as an example, during separation, acidified cyanide-containing wastewater flows on one side of the membrane, while the alkaline solution containing the recovered ** flows on the other side. There is a chemical potential difference for HCN diffusion between the two sides of the membrane; driven by this chemical potential difference, HCN in the wastewater diffuses through the membrane’s pores into the alkaline solution, where NaOH reacts with HCN to form NaCN. Membrane separation technology is widely used in industrial water treatment, advanced treatment of urban domestic wastewater, and industrial production. In the field of cyanide-containing wastewater treatment, the combined use of membrane separation technology with other techniques holds market potential. However, its disadvantages include high investment costs, high electricity consumption, elevated overall costs, imperfect equipment, and difficulties in addressing membrane fouling. (7) Wet air oxidation and supercritical water oxidation: The principle of both wet air oxidation and supercritical water oxidation is to use oxygen as an oxidant to treat high-concentration cyanide-containing pollutants under high temperature and pressure. Wastewater treatment plants exist abroad; alone in the United States, 5 facilities use wet air oxidation to treat acrylonitrile wastewater. Research in this field started relatively late in China; some studies have been conducted on the wet air oxidation method, with orthogonal experiments being carried out using 5 main factors such as temperature and pressure. The results showed that temperature is the primary influencing factor, followed by reaction time, pH value, etc. Supercritical water oxidation is an emerging technology for treating cyanide-containing wastewater. This technology was first proposed by Modeull, and it offers significant advantages in dealing with pollutants that cannot be completely removed or thoroughly oxidized by conventional methods. This method achieves a high removal efficiency for toxic substances (over 9.9%), features a simple reactor structure and small size, has a wide range of applications, produces clean products that require no further treatment, and can operate autonomously without external heating. However, under high temperature and high pressure conditions, strict requirements are placed on the material of the equipment. In China, basic research began to develop mainly in the late 1990s, with many research reports emerging on that front. Due to the shortcomings of supercritical water oxidation technology, such as catalyst issues, problems related to corrosion under high temperature and pressure, and the formation of inorganic salt precipitates, these methods require sophisticated equipment, result in high initial investment costs and high electricity consumption. However, their advantages include thorough oxidation, no secondary pollution, compliance with emission standards, and environmental friendliness – making them the ideal goal for treatment technologies. At present, this method has not been widely adopted due to unresolved technical issues; once these key technical problems are solved, it will have great application prospects. (8) Ion exchange method: The ion exchange method is a separation technique that utilizes ion exchangers to exchange ions with those in the solution. Various metal cyanide complexes in cyanide-containing wastewater exhibit a strong affinity for anion exchange resins; therefore, anion exchange resins are generally used for the recovery of valuable metals from such wastewater. The advantage of this method is that the purified water has good and stable quality, and can be reused. Ion exchange resins have a small particle size and limited mechanical strength; it is necessary to research and develop ideal resins with high capacity and greater strength, as well as specialized, highly efficient integrated equipment. The ion exchange process is complex, difficult to operate, has high treatment costs, and yields low economic benefits. Due to the different selectivities of various ion exchange resins for different ions, it is difficult to achieve complete treatment of complex multi-ion systems. After adsorbing cyanide-containing wastewater using the existing ion exchange resin method, the residual **mass concentration remains too high; additional treatment methods are required to meet the discharge standards. Ion exchange resins require frequent regeneration, the process is complex, maintenance is difficult, and the workload is heavy. (9) High-pressure hydrolysis method and electrochemical method: The high-pressure hydrolysis method involves the reaction of CN˻ with water under high temperature and pressure to produce non-toxic ammonia and carbonates; salts of transition metals can act as catalysts for this reaction. This method is safe and effective; it can handle a wide range of mass concentration levels, yields good results, does not cause secondary pollution, is simple to operate, and operates stably. However, it requires special equipment that operates under high temperature and pressure, and the operational costs are high, which hinders its widespread use. Canada established industrial facilities for heated and pressurized hydrolysis in the early 1950s, and is currently working on optimizing the design and operation of the reactors. In practical operations, the reaction temperature is generally maintained within the range of 170°C to 180°C, the pressure is kept around 0 to 9 MPa, and the pH value of the reaction is controlled at around 10.5. There is little research and few reports in China on the pressurized hydrolysis method for cyanide-containing wastewater. Someone has studied the mechanics of *hydrolysis* under normal-pressure reflux conditions ； Condition tests have also been conducted to investigate the cyanide removal efficiency of cyanide-containing wastewater in relation to influencing factors such as pressure, temperature, and solution pH. Common electrochemical methods include electrodialysis and electrolysis. Electrolysis is a relatively mature water treatment technology. In the early days, its development was slow due to factors such as electricity costs and other expenses; it was mainly used to treat electroplating wastewater containing cyanide and chromium. In recent years, with the development of the power industry, electrolysis has increasingly become a hot topic in water treatment technology. Some people summarize the advantages of electrolytic water treatment technology as follows: during the electrolysis process, waste water can be **degraded into carbon dioxide and simple inorganic compounds, with little or no secondary pollution; it has high energy efficiency, as electrochemical processes generally take place at normal temperature and pressure; it can be used as a standalone treatment method or combined with other treatment methods as a pre-treatment step, and the electrolysis equipment and operation procedures are usually relatively simple. The disadvantages of the electrolytic method are high electricity consumption, long treatment time, the need for specialized electrolysis equipment, and high operating costs. It is not suitable for treating cyanide-containing wastewater with low concentrations, and is often used for treating such wastewater with high concentrations. This method is often limited, as it destroys **and makes it unusable when dealing with cyanide-containing wastewater from gold mines. At present, China has reached a relatively advanced level in the world when it comes to technologies and processes for treating cyanide-containing wastewater. In particular, the oxidation-based treatment techniques for such wastewater, as well as **full-circulation reuse technologies, are already quite mature in industrial applications. However, these various processes are not yet perfect and require further improvement. Cyanide-containing wastewater comes from various industries; the quality concentration, properties, and composition of this wastewater vary, which leads to differences in the methods and processes chosen for its treatment. Based on an analysis of the trends in the development of technical methods and their application in industrial processes, the following characteristics can be observed: (1) For cyanide-containing wastewater with medium to high concentrations, as well as simple wastewater, recovery processes such as acidification-based recovery, extraction, and two-step precipitation are preferred; the residual liquid is then further oxidized to meet the discharge standards. When this combined process is used, emphasis should be placed on treating the solid waste generated during the process, or selling it as a by-product. (2) For cyanide-containing wastewater with medium to low concentrations, there is a wide range of process options available. For one-step treatment methods, wet oxidation and supercritical water oxidation can be used; a commonly employed combined process is oxidation followed by activated carbon adsorption. When the wastewater is relatively clear with low levels of suspended solids and salts, pressure hydrolysis, ion exchange, and membrane separation technologies can be considered. The treated water should be reused as much as possible in the production process in order to reduce wastewater discharge. (3) For wastewater with a cyanide mass concentration of 10 ng/L or less, a combined process of biological treatment and natural purification can be employed. Wastewater treated using this method must be strictly monitored at the total discharge outlet, and only after meeting the **wastewater discharge standards** can it be released into the external water environment. In summary, the treatment of cyanide-containing wastewater should be based on the principles of clean production and sustainable development, with efforts to recover ** and precious metals as much as possible, as well as to achieve water recycling, thereby reducing or eliminating the release of toxic pollutants. Therefore, there is an urgent need to improve and refine the existing treatment processes, and to intensify research on new processes and methods in order to achieve full recycling of cyanide-containing wastewater and attain \"zero discharge\" of sewage.