Cyanide Gold Extraction Wastewater Treatment Process

At present, the cyanide gold extraction method is one of the main mature processes for gold smelting in China. It uses cyanide solution to extract gold from ores, featuring high recovery rate, strong adaptability to ore properties, and the ability to produce gold on - site. Since the first use of cyanide solution to leach gold from ores in 1887. this method has been widely applied until now. However, cyanide gold extraction generates a large amount of toxic and harmful substances, posing a huge threat to the surrounding environment and humans. Therefore, in order to reduce the harm, it is necessary to study the treatment methods of cyanide gold extraction wastewater. A large number of researchers have summarized the treatment methods, chemical principles, and development trends of cyanide - containing wastewater, but most of them only discuss one or two methods. Therefore, this article conducts a detailed analysis of various treatment methods for cyanide gold extraction wastewater currently applied in industry, compares the advantages, disadvantages, and application scenarios of each method, which has certain guiding significance for similar applications in actual production.

I. Sources and Hazards of Cyanide Gold Extraction Wastewater

The main principle of cyanide gold extraction is that in an aerobic environment, sodium cyanide reacts with gold to form gold complexes, which are then dissolved. After that, gold can be extracted by enrichment through activated carbon adsorption or displaced by zinc powder from gold cyanide. At the same time, other heavy metals such as silver, copper, and zinc also form complexes and dissolve.

The cyanides used in the reaction and the complexes produced are all toxic and harmful substances. Sodium cyanide is easy to hydrolyze and is a Class 1 highly toxic substance, with a lethal dose of 0.10g. When cyanides leak into water bodies, it is extremely detrimental to the organisms in the water, and will cause a huge threat to humans and the surrounding environment. Therefore, the treatment of cyanide gold extraction wastewater is of great importance.

II. Main Treatment Methods for Cyanide Gold Extraction Wastewater

Alkaline Chlorination Method

The alkaline chlorination method is currently one of the most commonly used methods for treating cyanide - containing wastewater from cyanide gold extraction. It mainly uses chlorine - based oxidants to oxidize the cyanides in the wastewater under alkaline conditions, converting them into non - toxic substances. The cyanide - breaking process of alkaline chlorination is divided into two stages:

The first stage is to oxidize cyanide to cyanate, which is called the “incomplete oxidation” stage. CN⁻ reacts with OCl⁻ to first form CNCl, and then it hydrolyzes to CNO⁻. It should be noted that CNCl is highly volatile and toxic under acidic conditions. Therefore, during operation, the pH value must be strictly controlled to be in an alkaline state.

The second stage is to further oxidize cyanate to carbon dioxide and nitrogen, which is called the “complete oxidation” stage. During the cyanide - breaking process, the pH value has a great impact on the oxidation reaction. The pH value of the first - stage oxidation should be controlled at 10 - 11. and the reaction time is 10 - 15 minutes. The pH value of the second - stage oxidation should be controlled at 6.5 - 7.0. and the reaction time is 10 - 15 minutes.

A certain mine uses the alkaline chlorination method to treat the supernatant of cyanide tail slurry (with a cyanide content of 200mg/L) and the seepage water from the sedimentation tank (with a cyanide content of 5mg/L). The pH value is controlled at 10 - 11. and bleaching powder is added at a ratio of 35 - 40 times the cyanide content for mixing and stirring. After sedimentation in a thickener, the total cyanide content can be reduced to 0.1mg/L.

The alkaline chlorination method is the most commonly used method for treating cyanide - containing wastewater, and bleaching powder is the most commonly used chlorine - based oxidant. This method is suitable for treating cyanide gold extraction wastewater with high or low concentrations. It can also remove thiocyanate and cyanide - containing complexes (except for ferrocyanide complexes). The medicament is widely available, the generated waste residue is easy to filter, and the operation is simple. However, the operation environment is relatively harsh when using bleaching powder to treat wastewater. Now some enterprises use bleaching liquid or chlorine dioxide instead, which improves the operation environment to some extent. But toxic gases are generated during the reaction process, and it has a relatively large corrosiveness to equipment. The medicament cost and maintenance cost are relatively high.

Ferrous Salt Complexation Method

The ferrous salt complexation method is a treatment method for cyanide gold extraction wastewater that has emerged in recent years. By controlling the reaction pH value at 7 - 8. ferrous ions react with free cyanide and some cyanide complexes in the cyanide gold extraction wastewater to form precipitates.

Experiments have shown that generally, adding only ferrous sulfate to treat cyanide gold extraction wastewater cannot make the wastewater meet the discharge standards. Therefore, a general oxidant needs to be added to the treated wastewater for deep - cyanide removal. As long as the conditions are well - controlled, the oxidant can be directly added for treatment without separating the precipitate, and the discharge standard can also be achieved. This has positive significance compared with the traditional method of separation first and then treatment.

A certain gold smelter uses the sodium sulfide - ferrous sulfate method to treat cyanide - poor liquid. The influent has a cyanide content of 2500mg/L. After treatment, the effluent has a cyanide content of less than 20mg/L, with a removal rate of 99.2%, showing remarkable results. The subsequent deep - treatment uses the sodium metabisulfite - air method to reduce the total cyanide to less than 0.4mg/L.

The ferrous salt complexation method is a newly emerging treatment method, mainly used to treat high - concentration cyanide - containing wastewater. Its process is simple, the one - time investment is small, it is easy to operate, the medicament (mainly ferrous sulfate) is widely available, inexpensive, and easy to use. However, because the ferrous sulfate solution is acidic, when it is mixed with cyanide gold extraction wastewater, the local area becomes acidic, and there is a possibility of generating hydrogen cyanide gas. Moreover, it cannot remove thiocyanate, and the treated wastewater still needs deep treatment to meet the discharge standards.

Sodium Metabisulfite - Air Method

The sodium metabisulfite - air method is developed from the sulfur dioxide - air method. It mainly uses the synergistic effect of sodium metabisulfite and air on the cyanides in the wastewater within a certain pH range, with the catalytic effect of copper ions, to oxidize CN⁻ to CNO⁻.

If the cyanide content in the cyanide - containing wastewater is high, pre - treatment can be carried out first to reduce the total cyanide concentration to less than 100mg/L. Then, sodium metabisulfite and copper sulfate are added, sufficient air is introduced, and the pH value is controlled (generally controlled at 7 - 8), so that cyanide is oxidized to cyanate, which is then hydrolyzed to form bicarbonate ions and ammonia.

The sodium metabisulfite - air method is suitable for treating low - concentration cyanide gold extraction wastewater. The dosage of the medicament is small, the labor intensity is low, but the upfront investment is relatively large, and equipment such as blowers needs to be added. The requirements for process indicators are relatively strict, and controlling the pH value is very crucial. Copper sulfate also needs to be added as a catalyst. The reaction time is long. If the treatment is not proper, a large amount of ammonium ions will be generated, and the generated slag is not easy to filter. There is a small amount of ammonia gas generated on - site, and it has no effect on removing thiocyanides.

Hydrogen Peroxide Oxidation Method

The hydrogen peroxide oxidation method is to oxidize cyanides to CNO⁻ under normal temperature, alkaline (pH = 10 - 11) conditions, with Cu²⁺ as a catalyst, and then hydrolyze them into non - toxic substances. Complex cyanides (complexes of Cu, Zn, Pb, Ni, Cd) are also dissociated due to the destruction of cyanides in them. Ferrocyanide ions and other heavy metal ions form ferrocyanide complex salts and are removed. Finally, the total cyanide concentration in the treated wastewater can be reduced to less than 0.5mg/L.

This method is suitable for treating low - concentration cyanide - containing wastewater. The hydrogen peroxide treatment equipment is simple and easy to achieve automatic control. However, the generated cyanate needs to stay for a certain period of time to decompose into CO₂ and NH₃. The disadvantages are that using copper as a catalyst may cause copper in the discharged water to exceed the standard, the raw material cost is relatively high, thiocyanides cannot be oxidized, and ammonium ions are generated. In fact, the wastewater still has a certain toxicity. Moreover, since hydrogen peroxide is an oxidant, it has large corrosiveness, and there are certain difficulties and dangers in transportation and use.

Acidification Method

When using the acidification method to treat cyanide - poor liquid, its reaction mechanism is relatively complex, mainly including three processes: the acidification process of cyanide - containing wastewater, the stripping and absorption process of HCN gas, and the neutralization process of the stripped liquid.

(1) Acidification reaction: The cyanide - poor liquid is acidified and purified with acid. The complex cyanides in the poor liquid will form insoluble precipitates such as CuCN, CuSCN, and Zn₂Fe(CN)₆ and be removed, and at the same time, hydrogen cyanide is generated.

(2) Volatilization and absorption reaction: The poor liquid is pre - heated to about 30℃ before acidification. Since the boiling point of HCN is only 26.5℃, it is extremely volatile. Therefore, a packed tower is used as the mass - transfer equipment for the contact between the gas - liquid two - phase in the acidification method, which is easy to achieve the stripping and absorption of HCN.

(3) Neutralization reaction: Lime or liquid alkali is used to neutralize the acid - stripped residual liquid. The residual HCN molecules in the solution will be converted into the CN⁻ form. The acidification method can recover sodium cyanide from the cyanide - containing wastewater and realize resource recovery. However, it has high requirements for equipment sealing, a relatively large upfront investment, requires high - level operation skills, and equipment maintenance is difficult. There are also certain safety hazards. The wastewater generated after recovery still needs deep treatment to meet the discharge standards.

Electrolysis Method

The electrolysis method uses electrochemical redox reactions to destroy the cyanides in the wastewater. During ion electrolysis, cyanides lose electrons at the anode and are oxidized to cyanate, carbonate, nitrogen, or ammonium. Cyanate is further oxidized to CO₂ and H₂O. The main reactions are:

CN⁻ + 2OH⁻ - 2e → CNO⁻ + H₂O (24)

2CN⁻ + 4OH⁻ - 6e → 2CO₂ + N₂ + 2H₂O (25)

Electrolysis experiments using a self - made ceramic - based lead dioxide electrode rod and a stainless - steel cathode plate have proven that by using the electrolysis method to treat cyanide - containing wastewater, after 2 hours of electrolysis, the CN⁻ concentration can be reduced from 385mg/L to 58mg/L, and the Cu²⁺ concentration can be reduced from 450mg/L to 48mg/L. In addition, Hunan Zhongnan Gold Smelter uses the electrochemical method to treat cyanide gold extraction wastewater, which can reduce the total cyanide from 4g/L to 0.8g/L. The difference from the above is that both the anode and cathode plates are made of iron plates. During the operation process, not only electric energy is consumed, but also the iron plates are consumed.

The electrolysis method is mainly used to treat high - concentration cyanide - containing wastewater. The equipment occupies a small area, the process is simple and easy to control, but it consumes a large amount of electric energy, and the operating cost is higher than that of the alkaline chlorination method. The cyanide removal rate is average, and it has no effect on removing cyanide complexes.

At present, among the treatment methods for cyanide gold extraction wastewater, the alkaline chlorination method, acidification method, and sodium metabisulfite - air method are widely used. The electrolysis method and ferrous salt complexation method are newly emerging methods that have been successfully applied in industrial treatment. The hydrogen peroxide oxidation method is mainly an emergency treatment method. There are many other treatment methods for treating cyanide gold extraction wastewater, such as natural purification method, biological method, membrane separation method, ion - exchange method, etc. However, as industrial applications, they all have certain limitations and still need continuous improvement.

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