Treatment of cyanide-containing wastewater by chlorination

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

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 large amount of waste thiocyanate, sulphide, the reduced state metal ions and their compounds, and chlorine and disproportionation in order to ensure that there is sufficient residual waste such as chlorine, chlorine often actually consumed Up to 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 their toxic chlorine, thiosulfate must be added before discharge to the waste liquid, with ferrous sulfate or ammonium sulfate which was removed. 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.

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