Although most of the cyanide-containing wastewater in the cyanide plant has been returned to use, some of the cyanide-containing waste liquid and washing liquid and cyanide-containing residue need to be discarded. These materials must be disposed of before they are disposed of, so that they can be disposed of before they are disposed of, so as not to pollute the environment.

For the treatment of cyanide-containing residues, the cyanide is usually decomposed into non-toxic substances by chemical methods, or the cyanide in the residue is transferred to the solution and then returned to use, or discarded after being treated.

Cyanide-containing waste liquids and washings in cyanide plants, which may contain many of sodium cyanide, zinc cyanide, copper cyanide, iron cyanide, and thiosulfate cyanide and other compounds that can be cyanated. Inactive salts are usually purified by destroying the cyanide in the solution. However, the method chosen should be based on the characteristics of the factory waste solution.

Since JT Woodcock comprehensively reported 13 methods for treating cyanide waste liquid in 1954, many articles and special research reports on cyanide waste liquid treatment have been published. Although most of these methods deal with the treatment of cyanide-containing electroplating waste liquids, they also play an important role in promoting the cyanide plant waste liquid treatment process.

Nowadays, the methods suitable for the treatment of waste liquid in cyanide plant include acidification method, chlorination oxidation method, ferrous sulfate-lime method, stripping method, electrolytic oxidation method and biological purification method, natural degradation method, and hydrogen peroxide formaldehyde method. Among these methods, the acidification method is relatively simple; the chlorination oxidation method, the biological purification method and the electrolytic oxidation method have been used for a period of time, and some experience has been accumulated; the ferrous sulfate-lime method is not stable enough to remove cyanide, and Thoroughly; the law of blow-out is limited by the pollution of the atmosphere.

First, the acidification method

This method can process a solution containing a high concentration of cyanide (60×10 - 6 or more NaCN) discharged from most factories, and the free cyanide ion in the solution can be reduced to 1 × 10 - 6 .

Although the acidification process is suitable for the treatment of cyanide-containing waste liquids, it is unsatisfactory to treat cyanide-containing pulps for the treatment of carbonate-containing or acid-soluble sulfides (such as pyrrhotite). This is especially true for pulp.

Second, the natural degradation method

The natural degradation method is a purification method that utilizes the action of natural factors such as light to cause the cyanide to decompose by itself. This method was used in some large mines in Canada in the early 1980s to purify cyanide-containing wastewater. The Dome (Dome) gold ore, winter total cyanide containing waste 100mg / L, injecting liquid storage tanks, Midsummer to the total cyanide-containing waste has been reduced to 0.1 mg / L, and then discharged into the tailings dam. Although this method has low operating cost and does not consume chemicals, since the natural degradation process is very slow, even in the summer season, it takes at least 10-15 days to degrade cyanide to about 1 mg∕L. Therefore, most cyanide plants have no conditions to use this method to make the waste liquid meet emission standards.

Third, the chlorination oxidation method (also known as alkaline chlorination method)

Commonly used oxidants are chlorine, bleach solution or slurry, sodium hypochlorite solution, and the like. It is the most effective way to destroy cyanide. When chlorine is used as the oxidant, the base should be added at the same time.

The chlorination method generally uses bleaching powder (CaOCl 2 ) as an oxidizing agent, but the addition of the bleaching powder generates a large amount of sludge, and the labor for cleaning the slag is large. Although chlorine or sodium hypochlorite is used, the amount of slag is small, and the operation is convenient, but it is unsafe and produces a heavy irritating odor. The base used may be sodium hydroxide or lime milk.

The chlorination method is usually carried out under the conditions of pH 8. 5 to 11 (generally 9). When the pH is above 11, cyanate is easily oxidized by chlorine (less than 1 min) to complete the reaction to form CNO - ions:

CN - +Cl 2 +2OH - CNO - +2Cl - +H 2 O

CN - +OCl - CNO - +Cl -

Since the oxidation process is sufficient to produce CNO - ion in solution after only a thousandth of residual CN -. When the above reaction occurs at a pH of less than 8.5, a toxic CNCl gas is evolved and the reaction rate is slowed down.

CNO - ions should be further decomposed under control conditions of pH8 ~ 8.5. The decomposition reaction at this time is slower than the previous reaction, and usually takes more than 0.5h to complete:

2CNO+3Cl 2 +4OH - 2CO 2 ↑+N 2 ↑+6Cl - +2H 2 O

2CNO - +3OCl - +H 2 O - 2CO 2 ↑+N 2 ↑+3Cl - +2OH -

The amount of chlorine required for the free cyanate oxidation process is almost equal to the stoichiometric amount, but different methods produce different cyanides. In addition, since many other oxidizable substances (such as S 2 O 3 2 - and CNS - ) are present in the cyanide solution, and in order to make the reaction more sufficient, the solution should contain a certain amount of residual chlorine, so the actual chlorine The consumption is greater than the amount of chlorine required to oxidize cyanide. The theoretical amount (mass ratio) of the oxidizing agent and the base calculated from the reaction formula is shown in the following table. The chlorine in the table below is active oxygen. In calculation, liquid chlorine is calculated according to 100% chlorine, bleaching powder containing 20% ​​to 30% chlorine, and sodium hypochlorite containing 10% chlorine.

Table The ratio of the theoretical amount of oxidant and base to the mass ratio of cyanate

Cyanate

Cyanate generation stage

Final oxidation stage

chlorine

Sodium oxychloride

Calcium oxide

chlorine

Sodium hydroxide

Calcium oxide

1

2.73

3.10

2.15

6.83

6.20

4.31

It can be seen from the above table that by using the chlorination oxidation method, theoretically removing a portion of cyanide requires consumption of 6.83 parts of chlorine. However, in actual operation, due to the presence of a large amount of thiocyanate, sulfide, reduced metal ions and their compounds, and disproportionation of chlorine in the waste liquid, and in order to ensure sufficient residual chlorine in the wastewater, the actual consumption of chlorine is often as high. 1:15.

The cyanide-containing cyanide-containing waste liquid is usually treated by a batch process. This treatment method is easier to adapt to changes in water volume and cyanide concentration, and can achieve the desired treatment effect safely and reliably. When the amount of waste liquid is too large, continuous treatment is often used to reduce equipment and floor space. However, continuous treatment often does not effectively control the concentration of cyanide in the effluent. The equipment system for purifying cyanide-containing sewage with chlorine gas in a sealed reaction tank of a factory in China is shown in Fig. 1.

Figure 1 Equipment system for chlorine-containing purification of cyanide-containing sewage

1-chloro gas cylinder; 2-chlorination machine; 3- ejector;

4-mixing tank; 5- pump house; 6-rotor flowmeter; 7-reaction tank

The practice of the chlorination oxidation method proves that in order to control the concentration of cyanide in the effluent to be less than 0.05 mg ∕L, it is necessary to maintain 3 to 5 mg of residual active chlorine in the treated waste liquid. However, in view of the toxicity of chlorine itself, it must be removed by adding thiosulfate, hydrazine sulfate or ferrous sulfate to the waste liquid before discharge. When chlorine is removed by ferrous sulfate, the amount of ferrous sulfate can be added in a weight ratio of Cl:FeSO 4 ·7H 2 O = 1:32:

3Cl 2 +6FeSO 4 2Fe 2 (SO 4 ) 3 +2FeCl 3

In view of the bactericidal action of chlorine, such as the discharge of residual chlorine solution directly into the urban sewer, it is not necessary to remove chlorine.

A mine uses an alkaline chlorination method with bleaching powder to treat a tailings slurry with a pH of about 10 and a CN - average of 150 mg ∕L. Press pulp CN - 1:9 ~ 10 reaction ratio of bleach to a septic tank 1h, which may be simple cyanide (CN -) and not very stable complex cyanide [Zn (CN) 4 2 -, etc.] It is completely destroyed and can make the extremely toxic highly stable cyano complex [Fe(CN) 6 3 - , Fe(CN) 6 4 - etc.) practically non-toxic. When the tailings slurry contains CN - 159mg ∕L, the amount of bleaching powder is 2.5kg / t, and the purified clarified water containing CN - can be reduced to 0.04mg ∕L.

4. Copper ion catalyzed SO 2 air oxidation

This method was developed by the International Nickel Company (Inco) in the early 1980s. It overcomes the shortcomings of the alkaline chlorination method that cannot remove the ferricyanide complex. Except for the free cyanide, the remaining cyano-containing complex can be removed. Therefore, the removal rate of cyanide can reach 99%, and the concentration of heavy metal ions can be reduced to below 1 mg/L.

In the method, CuSO 4 is added as a catalyst at a temperature of 40 to 60 ° C, and air (or flue gas) containing SO 2 is bubbled into the waste liquid, and lime milk is continuously added to maintain the pH between 9 and 10. When the poor liquid containing 400 to 1000 mg of total cyanide is treated, the total cyanide is reduced to 0.7 mg/L after 24 hours, and the weak acid soluble cyanide is usually less than 0.2 mg. The concentration of SO 2 in the air was measured, and the oxidation-reduction potential was controlled by the calomel electrode to be +90 to 130 mV. That is, the volume ratio of SO 2 in the air is preferably from 1% to 3%. The total cyanide consumption per gram of SO 2 is 3.8 g and Ca(OH) 2 is 5.7 g.

The first manufacturer approved to use the Inco patent is the Scottie Gold Mine, which is used to treat a mixture of waste liquid and tailings slurry. After entering normal operation in September 1982, it can be used to make a slurry containing 200 mg of total cyanide. The material was reduced to 3 mg ∕L, and the heavy metal ions were reduced to 1 mg ∕L.

Five, iron ion precipitation method

This method can use sulfates or hydrochlorides of Fe 2 + and Fe 3 + . The addition of iron ions to the alkaline cyanide-containing waste liquid having a pH of 7.5 to 10 dissociates the metal cyanide ion in the solution into metal ions and CN - . Dissociated CN - and Fe 2 + -formed Fe(CN) 6 4 - , Fe(CN) 6 4 - can be separated from a small portion of Cu, Pb, Zn, Ni heavy metal ions to form Me 2 Fe(CN) 6·xH 2 O coprecipitation. In addition to Fe 3 + and CN - generated precipitate was similar, but may also generate Fe (OH) 3 precipitate. Most of the dissociated heavy metal ions such as Cu, Pb, Zn, and Ni are hydrolyzed to form hydroxide precipitates. Solution SCN - also generate Me (SCN) 2 and precipitate heavy metal ions. Due to the complexity of the reaction process, the iron ions are added in different conditions will be the CN - generate different blue sparingly soluble ferricyanide compound. For convenience, this blue precipitate is collectively referred to as "Prussian" or "Prussian blue". After the solution treated with iron ions, the residual amount of total cyanide can be reduced to 2-10 mg ∕L NaCN, which are mainly ZnCN 2 and its complex.

The Homes Gold Mine is a step of adding a step of FeSO 4 or introducing SO 2 to a cyanide-containing waste liquid while adding Na 2 SO 3 to cause two reactions. That is, on the one hand, a ferrocyanide precipitate is formed, and Na 2 SO 3 participates in the reaction to decompose cyanide into ammonia, carbon dioxide and water.

Precipitation with Fe 2 + has also been widely used in the former Soviet Union. Later studies have found that this precipitation of ferrocyanide, which has always been considered extremely difficult to dissolve, can decompose by itself under natural conditions and cause secondary pollution. This may be the reason why this method has not been widely used in cyanide plants.

JT Woodcock's cyanide plant waste treatment process is applied to a percolation leaching cyanide plant and a stirred leaching cyanide plant in Victoria, Australia.

In these two plants using different cyanidation methods, the practice of the Australian gold mining company Morning Star GMA diafiltration cyanide plant waste liquid treatment is outlined below.

The main components of the waste liquid discharged from the factory are (%): free NaCN0.049, total NaCN0.212, CaO0.038, KCNS0.028, Zn0.084, Fe0.013, Cu0.003, pH 10.7.

The processing operation is performed in a container having a capacity of 3,800 liters, and 3000 to 3,400 liters of the used waste liquid is recycled each time. After adding about 9 kg of water-soluble ferrous sulfate, the solution was circulated for about 30 minutes, and a bleaching slurry was added while circulating. Pause, with potassium iodide starch paper (with a gold - silver electrodes or other electrodes used to determine the extent of change in redox potential) amount of the excess free chlorine test. The pH of the solution should be kept at around 10 during the process, and lime adjustment can be added if necessary.

After adding sodium thiosulfate solution to remove excess chlorine, the pH was adjusted to less than 9 by adding sulfuric acid. The solution is then stopped and a small amount of brown sludge formed is precipitated. The treated waste liquid contains free cyanide ions equivalent to (2 to 11) x 10 - 6 HCN. This solution was filtered through a jute-filled filter and discharged into a river having a minimum flow rate of 680 L ∕ min at a flow rate of 0.5 L per minute. The filtered sludge is collected together, and after being treated for 3 to 4 batches, it is dried and buried.

6. Activated carbon adsorption catalytic oxidation

This method is a new process for the purification of cyanide-containing wastewater developed by the Changchun Gold Research Institute and has obtained the national patent (ZL 91104787.5). It has been successfully applied to the Dongyaoyu gold mine in Qianxi, Hebei Province, and has low treatment cost, comprehensive recovery of Au, Cu, Pb, Zn, and the discharge of the tailings dam outside the “zero discharge”.

The activated carbon adsorption catalytic oxidation process for the treatment of cyanide-containing wastewater is to add a catalyst and supply sufficient oxygen to the wastewater. The following reaction occurs on the carrier carbon to complete the oxidation and cyanide removal:

CN - + O 2 CNO -

CNO - +2H 2 O HCO 3 - +NH 3

NH 3 +H 2 O NH 4 OH

NH 4 OH NH 4 + +OH -

Since the above reaction is carried out under weakly acidic conditions, and the pH of the mine wastewater is 12 to 14, the pH is adjusted to about 7 by adding sulfuric acid to save acid and prevent HCN from escaping. The carrier carbon is first filled with the catalyst after the adsorption catalyst, and then the cyanide-containing wastewater is added, and the air is bubbled into the adsorption catalytic oxidation. Each bed volume of activated carbon, treated with 52 to 51 bed volumes of cyanide-containing 364 mg ∕L wastewater, cyanide can be reduced to 0.5 mg ∕L. The carrier carbon was washed with 6% HCl to obtain regeneration.

In 1992, the mine treated 1908m 3 of wastewater containing cyanide 250～426mg∕L, and the treated tailings contained cyanide reduced to 0.62mg∕L. After being discharged into the tailings dam, it was discharged from the tailings dam. The cyanide content in water is less than 0.00x mg/L, which is far below the national environmental standards.

The ore is treated with liquid chlorination method to treat cyanide-containing wastewater. The unit cost is 21.57 yuan, and the traffic is inconvenient, and the supply of chlorine is tight, which often affects production. After switching to the activated carbon catalytic oxidation method, the unit cost is reduced to 5.82 yuan, 70% lower than the liquid oxidation method, and a large amount of metals such as Au, Ag, Cu, Pb, and Zn are simultaneously recovered.

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