Due to its strong oxidizing properties and other excellent properties, chlorine dioxide has been widely used in drinking water disinfection, medical device disinfection, pulp bleaching, etc., and has shown strong advantages in industrial wastewater treatment. Pay attention to it.
Cyanide-containing wastewater is produced during the NaCN leaching process in wet smelting, cyanide plating in mechanical processing, and the like. Because cyanide-containing wastewater is a highly toxic inorganic wastewater, it has strong toxic effects on organisms and the environment, and it will produce highly toxic hydrofluoric acid gas under acidic conditions, which is directly fatal.
Therefore, it is of great practical and social significance to treat the cyanide-containing wastewater after it has been discharged or recycled for production.
1•Characteristics of cyanide-containing wastewater treatment process
The cyanide-containing wastewater treatment process includes various treatment methods such as alkaline chlorination, ion exchange, and electrolysis. In the engineering practice, the two-stage alkaline chlorination process is adopted, which is stable, reliable, and low in cost, and is easy to control.
The chlorinating agent may be liquid chlorine, sodium hypochlorite, chlorine dioxide, bleaching powder or the like.
The liquid chlorine has a high effective chlorine content, but the storage and dosing equipment is complicated and the safety is low;
Sodium hypochlorite and bleaching powder have low effective chlorine content, large dosage, large slag amount, and short sodium hypochlorite, which is difficult to store;
Chlorine dioxide has high effective chlorine content, easy storage of raw materials, automatic detection and automatic feeding, and easy to operate, high safety and remarkable superiority.
The following is a process for the treatment of cyanide-containing wastewater by a two-step process of chlorine dioxide under alkaline conditions.
2•Processing principle
Chlorine dioxide is a strong oxidant, and the oxidation process of cyanide is carried out in two steps.
The first step is to oxidize the cyanide ion (CN-) to chlorite and cyanate under alkaline conditions;
In the second step, under the slightly weak alkaline conditions, the generated cyanate chlorine dioxide is further hydrolyzed and decomposed into carbon dioxide and nitrogen.
The reaction formula is as follows:
2ClO2+CN-+H2O→CNO-+2ClO2+2H+
2CNO-+H2O→2CO2↑+N2↑+OH-
3• Process flow
An electroplating production line discharges cyanide-containing wastewater, the amount of wastewater is 3m3/h, CN-30mg/L, and the two-stage alkaline chlorination method is adopted.
4• Process design parameters of each processing unit
(1) Cyanide-containing wastewater conditioning tank: effective volume 12m3, residence time 4h.
(2) Chlorine dioxide generator: the negative pressure of the reaction agent is self-priming into the chlorine dioxide generator, and is added to the cyanide reaction tank a through an automatic dosing device. The ratio of ClO2 is theoretically ρ(ClO2/CN-)=1.04 However, in practice, various side reactions interfere with each other, resulting in an increase in the consumption of ClO2. To ensure that [CN-] <0.5 mg/L after wastewater treatment, the actual ClO2 dosage ratio is ρ (ClO2/CN-).
(3) Cyanide-removing reaction tank a: The water is discharged from the water, the effective volume is 1.25m3, and the residence time is 25min. The reaction tank was equipped with a stirrer, the angular velocity was 166 rpm, and the ORP electrode and the pH meter probe were set at the outlet position to control the addition of the alkali solution and the chlorine dioxide.
(4) Cyanide-reducing reaction tank b: The effective volume is 1.25m3, the residence time is 25min, the middle part is provided with a partition, the bottom of the board is provided with a diversion nozzle, and the upper end is discharged into the water end. An ORP electrode and a pH meter probe are provided at the exit. The water flow rate is less than 0.1 m/s.
(5) Inclined tube sedimentation tank: surface load 2m3 (/m2•h), effective volume 5.7m3, residence time 1.9h, PAM coagulant added to the coagulation reaction zone, the dosage is 2mg/L.
(6) Quartz sand filter: forced filtration speed: 16m / h; loaded filter quartz sand: d0.6 ~ 0.8 (K80 < 1.25); working cycle: 12 ~ 24h; recoil intensity: 24 ~ 48m / h; Backlash duration: 6 ~ 8min.
(7) Clear water tank, effective volume 15m3, residence time 5h. Set a residual chlorine meter at the drain.
5• Process self-control operating parameters
The process adopts an automatic control system, and the specific self-control points and control parameters are as follows:
(1) ORP electrode controller, the chlorine dioxide generator is controlled by the electrode potential of the cyanide reaction tank, wherein the ORP value of the cyanide reaction tank a is maintained at 300-330, and the ORP value of the cyanide reaction tank b is maintained at 600. ~650.
(2) The pH meter is connected with the effluent water outlet electric valve to control the pH of the cyanide reaction tanks a and b. According to long-term practical experience, the optimum pH control range for cyanogenation reaction should be: the pH of cyanide-removing reaction tank a is maintained at 11-13; the pH of cyanide-removing reaction tank b is maintained at 8-9.
(3) The residual chlorine meter is located at the outlet of the clear water tank. The residual chlorine amount can reflect the degree of oxidation of cyanide during the process. Through experience, the residual chlorine is controlled at about 3-6ppm, and the CN-concentration can be controlled generally. Below 0.5mg/L.
6• Actual operation effect
The system has stable effect and the effluent CN-content is 0.15~0.3mg/L. It has been proved that the pH value in the process of cyanide elimination is the key factor to achieve the treatment effect; the effluent residual chlorine monitoring is the key to ensure the effective chlorine dosage. When these two factors are strictly controlled, the CN-content in the raw water fluctuates a little, and the stability of the effluent effect is not affected.
7• Conclusion
(1) Chlorine dioxide oxidation is an effective treatment process for removing high concentrations of cyanide from water.
(2) When the concentration of cyanide (calculated as CN-) is 100-300 mg/L, when the pH is 11.2 to 12.6 and ClO2/CN-=2.28 to 5.92, the removal rate is more than 98%, and the treatment effect is stable.
(3) The higher the cyanide concentration in the raw water, the lower the ClO2/CN- required to reach the same removal rate.
(4) The pH value has a significant effect on the removal rate of cyanide by oxidation of chlorine dioxide. Under acidic conditions, chlorine dioxide has no removal effect on cyanide; under weak alkaline conditions, the oxidation rate is slow, and it is necessary to prolong the contact time to obtain Higher removal rate; when the alkaline condition of pH>11, the contact time removal rate of 30min can reach more than 96%.
(5) The chlorine dioxide treatment system is simple in process, safe in operation and high in automation.
[The following models of chlorine dioxide are suitable for use in the treatment of cyanide-containing wastewater]
1, high purity chlorine dioxide disinfectant
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2, stable chlorine dioxide solution
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3, one yuan chlorine dioxide disinfection powder
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