How to deal with cyanide waste?

The treatment method of cyanide lean liquid is introduced, and the purification method (alkali chlorine oxidation method, sulfur dioxide-air oxidation method, hydrogen peroxide oxidation method, ozonation method, electrolytic oxidation method, microbial oxidation method) and recovery method (acidification) are mainly introduced. Method, ion exchange method, adsorption method, solvent extraction method, membrane method) and the basic principles, advantages and disadvantages of various methods, and the importance of developing new technology.

Gold cyanide barren solution mainly produced smelter after demetallization solution (lean solution), and the filtrate vacuum adsorption tail desizing solution. The best solution for cyanide lean liquid treatment is to return to use, but the amount of return is very limited, because the dressing , leaching and washing process requires a large amount of fresh water, and when the cyanide lean liquid is recycled, impurities accumulate and affect the quality of cyanide leachate. Therefore, the discharge of part of the sewage from the gold smelter is inevitable. The ore processed by each gold smelter is different, the process used is different, and the chemical composition of the produced sewage is also different. The cyanide lean liquid usually contains CN-, CNS-, Zn2+, Cu2+, Fe2+, etc., and the mass concentration of cyanide ions is generally between 0.5 and 1.0 g/L, which needs to be strictly treated.

1 Treatment method of cyanide lean liquid

There are various treatment methods for cyanide-lean liquid, which can be roughly classified into two types: purification method and recovery (regeneration) method. Purification method uses relevant chemical reagents to destroy cyanide-containing ions to reduce cyanide content. Commonly used methods are alkali chloride oxidation, sulfur dioxide-air oxidation, hydrogen peroxide oxidation, ozone oxidation, electrolytic oxidation, and microbial oxidation. law. The recovery (regeneration) method is to recover and reuse cyanide while recovering valuable metals, mainly acidification, ion exchange, adsorption, solvent extraction, liquid membrane, and electroosmosis.

1.1 purification method

1.1.1 Alkali Chloride Oxidation

Alkali chloride oxidation is a relatively mature method for destroying cyanide in wastewater. It is widely used in electroplating plants, coking plants and gold smelters. The cyanide and metal complex ions in the cyanide-containing wastewater are oxidized to cyanate at a pH of 11 to 12, and then chlorinated to carbon dioxide and nitrogen by secondary chlorine addition. The process is relatively mature, the treatment effect is good, the application is extensive, and the treatment process is easy to realize automation; however, the cyanide can not be recovered, the treatment cost is high, and the ferric cyanide complex cannot be removed, and there is secondary pollution.

1.2 sulfur dioxide - air oxidation

The SO2 air oxidation method is also called the Inco method. Add waste liquid to the stirred vessel, pass air and SO2 (liquid or gaseous, or sulfite solution, or from combustion elemental sulfur), control the pH to 7-10, neutralize the acid generated during the oxidation reaction with lime. Soluble copper (as a catalyst) is required during the reaction.

Inco-SO2/air oxidation can decompose all cyanide including ferricyanide, which can be removed by some safe and inexpensive reagents. This method has the characteristics of less investment, quick effect, easy to master, safe and reliable, but this method is difficult to oxidize SCN-, so it is not suitable for the treatment of cyanide-depleted liquid with high content of SCN.

1.3 Hydrogen peroxide method

The basic principle of the hydrogen peroxide oxidation method is: in the presence of alkaline (pH=10-11) and the presence of copper ions, cyanide peroxide generates CNO-, NH+4, etc., while heavy metal ions pass through to form hydrogen peroxide. The precipitate is separated and precipitated, and the ferricyanide complex and other heavy metal ions form a ferricyanide complex which is removed. The process is suitable for treating low concentration cyanide-containing wastewater. The treated wastewater has low COD and no secondary pollution, but the price of hydrogen peroxide is relatively high and the treatment cost is high; SCN- is difficult to oxidize, and the treated wastewater still has certain toxicity.

1.4 Ozone oxidation method

Ozone is a strong oxidant. It is more complete than the alkali chloride oxidation process for the treatment of cyanide-containing wastewater. The mechanism for treating wastewater is: under alkaline conditions (pH 11 to 2), ozone oxidizes cyanide to form HCO-3 and N2. The dissolved oxygen in the wastewater solution after ozone treatment increases, and can be recycled to the cyanide system for recycling, which is beneficial to the dissolution of gold and improve the efficiency of gold leaching.

The ozone method is simple and convenient to operate, easy to control, high in production automation, and can produce ozone on the spot. It is of great significance for the cyanide plant with sufficient power supply and sufficient power supply; the ozone method has high purification efficiency and no secondary pollution; Ozone has high power consumption and high production costs, which limits its wide application.

1.5 Electrolytic oxidation method

The electrolytic oxidation method is mainly used for the treatment of electroplating cyanide-containing wastewater, and there are no industrial application examples at present. In the cyanide lean liquid, copper and zinc exist in the form of Cu(CN)3-4 and Zn(CN)2-4. Before the electrolysis, the first lean liquid cyanide adjust pH> 7, a small amount of salt added to the graphite as the anode, a titanium plate as the cathode, alkaline copper, zinc aqueous solution as electrolyte, through the direct current, then the output copper cathode and Zinc, accompanied by hydrogen production; CN-oxidation on the anode to CNO-, CO2, N2, Cl- is oxidized to Cl2, and Cl2 enters the solution to form HClO.

The characteristics of the electrolytic oxidation method are that the electrolysis equipment is simple, the floor space is small, the amount of sludge is small, the electrolytic tail liquid can be recycled and reused, and the purity of the metal obtained by electrolysis is high, and the production process is easy to realize automation. However, in the electrolysis process, the current efficiency is relatively low, the electric energy consumption is relatively large, and the cost is high; and the tear gas CNCl is generated, and the treated wastewater is also difficult to meet the discharge standard.

1.6 Microbial oxidation

The microbial oxidation method utilizes the biochemical properties of microorganisms to decompose cyanide, thiocyanate, and ferricyanide to form ammonia, carbon dioxide, and sulfate, or hydrolyze cyanide to formamide, while bacteria adsorb heavy metal ions. It is removed as the biofilm falls off.

The residual concentration of cyanide, thiocyanide, metal and ammonia in the waste liquid treated by this method is very low. An important feature of the method is to maintain the temperature above 10 °C at all times to maintain a reasonable rate of deacylation.

2 recycling method

2.1 Acidification

The acidification recovery method has a long history in the treatment of high-concentration cyanide-containing wastewater, and the technology is the most mature. China is also one of the countries with the highest dosage of acidification recovery equipment. The first domestic manufacturer to treat cyanide-lean liquid by acidification is Xincheng Gold Mine. The main principle of the method is to add sulfuric acid to the cyanide wastewater, adjust the pH to about 1.5, and convert CN- to HCN. Because HCN has a very low boiling point of only 26.5 ° C, it can escape from the solution at normal temperature. The escaping HCN gas is introduced into the absorber and absorbed by an alkali solution (sodium hydroxide or calcium hydroxide solution) to obtain a 20% to 30% cyanide solution, which can be recycled.

The process can maximize the recovery of cyanide, improve the effective utilization of cyanide, and reduce the production cost; but the one-time investment cost is large, the process is complicated, and the treated cyanide-containing residual liquid is difficult to meet the discharge standard.

2.2 ion exchange method

The anion exchange resin has a strong affinity for various metal cyanide complex ions in cyanide wastewater, and the cyanide and valuable metals in the wastewater can be removed by adsorption desorption.

The advantage of this method is that the purified water quality is good and stable, but the anion exchange resin has small particle size and insufficient mechanical strength; the process is complicated, the operation is difficult, and the cost is high; since the ion exchange resin is selective for different ions, the complex The process of the multi-ion system is complicated and difficult.

2.3 Activated carbon adsorption method

Activated carbon has a rich microporous structure and surface hydrophobicity, and has a strong affinity for toxic substances in water. Therefore, activated carbon adsorption has been one of the most effective methods for removing low concentrations of toxic substances in water. The adsorption of activated carbon mainly depends on its numerous pores and huge specific surface area. The adsorption process includes physical adsorption and chemical adsorption. The removal of cyanide mainly includes three ways of oxidation, hydrolysis and stripping. The main process is cyanide-containing wastewater. The cyanide oxidatively decomposes on the surface of the activated carbon with hydrogen peroxide.

The biggest disadvantage of the activated carbon adsorption method for the treatment of cyanide-containing tail liquid is that the selectivity of activated carbon is poor. When the cyanide tail liquid contains various metal cyanide complexes, it is generally not suitable to use this process; and, with cyanide liquid As the concentration of KSCN increases, the ability of activated carbon to adsorb gold is also gradually reduced. However, the desorption of activated carbon is economical and relatively simple.

2.4 Membrane method

Membrane treatment technology is a new separation treatment technology developed in modern times. Membrane separation techniques include a gas membrane method and a liquid membrane method. The application in the treatment of cyanide lean liquid is not very extensive, and the technology is not very mature.

2.4.1 Gas film method

In the treatment of cyanide tailings, the gas film method is the combined application of ion exchange resin and gaseous film. First, the cyanide is enriched with ion exchange resin, and then eluted with hydrochloric acid. The eluate passes through the hollow fiber membrane. At this time, HCN will permeate through the other side of the membrane and be absorbed by the NaOH solution on the membrane side to form NaCN. The NaCN form is recovered. The treated wastewater meets emission standards and can be returned to the electroplating plant for use as a wash water.

The method has the advantages of fast processing speed, convenient process operation, low energy consumption, small floor space and high economic benefit. Although it is still in the laboratory stage at this stage, it has shown good industrial application prospects.

2.4.2 Liquid film method

Since the liquid membrane method was first proposed in 1963, it has been successfully applied in the treatment of industrial wastewater, but the treatment of cyanide-containing wastewater is still in the laboratory stage and has not been industrialized. The liquid membrane method mainly uses the water-in-oil system. The basic principle is: firstly acidify the cyanide-containing wastewater to convert the cyanide into HCN, and the HCN enters the internal water phase through the oil phase liquid film. Neutralization with NaOH produces NaCN. HCN can be recovered by oil phase separation, and NaCN is recovered after demulsification.

This method is characterized by high efficiency, rapidity, and high selectivity, but it has not yet been industrialized.

2.5 Solvent extraction

In the late 1990s, Tsinghua University proposed a solvent extraction method for the study of high-quality concentrated cyanide-depleted liquids. The extractants used were mainly amines. In the extraction stage, the organic amine is first acidified to form an amine salt; then, the SO2-4 group in the amine salt is exchanged with the metal complex anion in the aqueous phase to cause the metal ion to enter the organic phase; The metal ions are back-extracted to return the metal ions to the aqueous phase, thereby separating, enriching and purifying the metal ions.

The advantage of this process is that the organic solvent is not lost much, there is almost no waste liquid discharge in the process, the operation is simple, and the working conditions are good; but the organic extractant used is expensive compared with the inorganic reagents such as acid, alkali and salt used in other processes. ,higher cost. This method is not yet very mature and has few studies.

3 other methods

In addition to the above methods, at the present stage, two or more combinations of processes are often used in the treatment of cyanide-lean liquid, such as oxidation destruction method + activated carbon adsorption method, ozone combined oxidation method (ozone-peroxide oxidation method, Ozone-ultrasonic oxidation method, ozone-ultraviolet oxidation method, two-step precipitation-purification method, acidification-precipitation-alkali neutralization method, and the like. There are also some new methods, such as nano titanium dioxide photocatalytic oxidation, wet air oxidation and supercritical water oxidation.

The cyanidation method has always been the most commonly used method of gold extraction at home and abroad. Gold ore generally contains other metal elements. Therefore, in addition to cyanide, the cyanide tail liquid contains metal ions such as Zn2+, Pb2+, and Cu2+. A novel adsorbent material prepared by sol-gel method using bentonite and titanium tetrachloride--bentonite-loaded nano-TiO2 material can be used to decompose CN- and adsorb metal ions such as zinc, lead and copper, and metal ions are desorbed separately. It can be recycled and the cyanide in the tail liquid can be discharged.

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