Alkaline chlorination is the fastest growing method in all cyanide processes that can be used to destroy industrial wastewater. It is widely used in metal plating and gold dressing plant wastewater treatment system.
Cyanide is completely oxidized by chlorine and is carried out in two stages at different pH values. In the first stage, cyanide is formed as a cyanate salt, which is roughly equivalent to one thousandth of HCN. The first stage is carried out in two steps. The first step is the oxidation of cyanide to cyanogen chloride (CNCl), which is represented by the following equation:
NaCN+Cl 2 →CNCl+NaCl
This reaction is almost instantaneous and independent of pH. In the second step, the cyanogen chloride in the high alkalinity solution is hydrolyzed to the cyanate salt by the following:
CNCl+2NaOH→NaCNO+NaCl+H 2 O
Since cyanogen chloride is a highly volatile toxic gas, in order to complete this reaction, it is necessary to maintain the high alkalinity of the solution while avoiding the release of such toxic gases.
At pH 8.5 to 9, it takes 10 to 30 minutes to complete the first reaction, and the reaction time is shortened to 5 to 10 minutes at pH 10-11. The cyanide is oxidized to cyanic acid, which is calculated to require 2.73 parts of chlorine per part of cyanide. However, in practice, a higher chlorine/cyanide ratio is required, up to 5:1. Because part of the chlorine reacts with other oxidizing substances, for example, thiocyanates and thio salts. Thiocyanate (CNS) is important because in this step, a large amount of thiocyanate is formed in the gold smelting plant solution. According to the following equation, 4.9 parts of chlorine and 6.4 parts of slaked lime are consumed per thiocyanate.
2NaCNS+8Cl 2 +Ca(OH) 2 →2NaCNO+2CaSO 4 +8CaCl+10H 2 O
Many easily dissociated metal cyanide complexes react in a similar manner as described above. The main difference is the reaction rate and the formation of metal hydroxide precipitates. For example, zinc cyanide complex oxidation is carried out as follows:
Na 2 Zn(CN) 4 +10NaOH+4Cl 2 →4NaCNO+8NaCl+Zn(OH) 2 +4H 2 O
Only metal cyanide complexes which are less stable than iron and cobalt cyanide complexes are suitable for the chlorination process.
The second stage of the alkali chlorination process is also carried out in two steps. The cyanate is finally converted to bicarbonate and nitrogen. The first step (this step is the speed control step) is the catalytic hydrolysis of cyanate by chlorine to form ammonium carbonate:
3Cl 2 +2NaCNO+4H 2 O→(NH 4 ) 2 CO 3 +Na 2 CO 3 +3Cl 2
Advantages of alkali chlorination: (1) This method is widely used and can be used for reference; (2) the feed to be treated is alkaline; (3) the reaction is complete and the speed is appropriate; (4) toxic metal energy (5) Chlorine is easily obtained in different forms; (6) It is easy to achieve continuous operation and is suitable for intermittent operation; (7) Capital investment is low; (8) Relatively good work reliability control; (9) The first stage oxidation reaction is easily controlled under conditions in which cyanate is allowed to be discharged.
The disadvantages are mainly high reagent costs, cyanide and the like cannot be recovered, and iron and ferrocyanide are usually not destroyed.
In the presence of chlorine, this step of reaction is completed for 1.5 h, after which (NH 4 )CO 3 is rapidly oxidized to N 2 and the carbonate is converted to bicarbonate. This reaction takes 20 minutes at pH 7.0-7.5; 30-45 dishes at pH 8.0-8.5; the reaction is extremely slow at pH 10 or higher.
The oxidation of cyanide to cyanate theoretically requires 2.73 parts of chlorine and 3.34 parts of slaked lime per part of cyanide. While completely oxidized to N 2 and carbonate (3Cl 2 + 6 NaOH + (NH 4 ) 2 CO 3 + Na 2 CO 3 → 2 NaHCO 3 + 6 NaCl + 6H 2 O + N 2 ↑), a total of 6.82 parts of chlorine and 8.35 parts of slaked lime per part of cyanide are required. It is generally believed that oxidation to cyanate is satisfactory because cyanate is at least two orders of magnitude less toxic to fish than HCN. More importantly, it is possible to save cyanate and further oxidize the operating costs, thus avoiding greater consumption of the medicament.
Alkaline chlorination is widely used at home and abroad.
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