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1·Features of cyanide wastewater treatment process The cyanide wastewater treatment process includes alkaline chlorination, ion exchange, electrolysis, and other treatment methods. In the engineering practice, the two-stage alkaline chlorination treatment process is used more frequently. This method is stable, reliable, and has a low cost and is easy to control. Chlorination agent can use liquid chlorine, sodium hypochlorite, chlorine dioxide and so on. According to the comparison of empirical data, specific comparisons between the consumption of chlorine oxidants and the treatment effects are shown in Table 1.
From Table 1, it can be concluded that the chlorine content of liquid chlorine is high, but the storage and dosing equipment is complex and the safety is low; the sodium hypochlorite has low effective chlorine content, large dosage, large slag yield, and the short period of sodium hypochlorite is short. It is not easy to store; while chlorine dioxide has a high content of available chlorine, it can be prepared on site, raw materials are easy to store, and the generating and feeding equipment can be automatically detected and automatically added. It is easy to operate, high in safety, and has significant advantages. This article discusses the process of two-step treatment of cyanide-containing wastewater with chlorine dioxide in alkaline conditions.
2. Process Principles Chlorine dioxide is a strong oxidant. The oxidation process is carried out in two steps. In the first step, under basic conditions, the CN- is oxidized to chlorite and cyanate; the second step Under slightly alkaline conditions, the resulting cyanate is further hydrolyzed under the oxidation of ClO2 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 A plating production line discharges cyanide-containing waste water with a wastewater volume of 3m3/h and CN-30mg/L. A two-stage alkaline chlorination process is used. The specific process design flow is shown in Figure 1.
Fig. 1 Cyanide removal process flow chart 4 Process design parameters of each treatment unit (1) Cyanide-containing wastewater conditioning pool: Effective volume 12m3, residence time 4h.
(2) Chlorine dioxide generator: The negative pressure of the reaction reagent is self-primed into the chlorine dioxide generator and is added to the cyanogen-depleting reaction tank a through an automatic dosing device. The ratio of ClO2 is theoretically Ï(ClO2/CN-)=1.04 However, in practice, many kinds of side reactions interfere, resulting in increased ClO2 consumption. To ensure [CN-]<0.5mg/L after wastewater treatment, the actual ClO2 dosage ratio (ClO2/CN-) is 4.
(3) Cyanide-reducing reaction tank a: It adopts the method of entering and exiting the water. The effective volume is 1.25m3 and the residence time is 25min. The reaction tank was equipped with a stirrer, and the angular velocity was 166 rpm. An ORP electrode and a pH meter probe were set at the outlet position to control the dosing of lye and chlorine dioxide.
(4) Breaking the cyanide reaction tank b: The effective volume is 1.25m3, and the residence time is 25min. The middle is provided with a baffle plate, the diversion outlet is arranged at the bottom of the plate, and the upper end enters the water outlet. ORP electrode and PH meter probe are installed at the exit. The water flow rate is less than 0.1m/s.
(5) Inclined-tube sedimentation tank: The surface load is 2m3(/m2·h), the effective volume is 5.7m3, the residence time is 1.9h, the PAM coagulant is added to the coagulation reaction section, and the dosage is 2mg/L.
(6) Quartz sand filter: forced filter speed: 16m/h; filled with filter material quartz sand: d0.6 ~ 0.8 (K80 <1.25); working period: 12 ~ 24h; recoil strength: 24 ~ 48m/h; Recoiling lasted: 6-8 minutes.
(7) Clear water pool, effective volume 15m3, residence time 5h. Set the residual chlorine meter at the drain outlet.
5. Process automation control parameters This process adopts automatic control system. The specific automatic control points and control parameters are as follows:
(1) ORP electrode control device controls the dosage of chlorine dioxide generator by breaking the electrode potential of the cyanide reaction tank, in which the ORP value of the cyanide-reducing reaction tank a is kept at 300-330, and the ORP value of the cyanide-destroying reaction tank b is kept at 600. ~650.
(2) The PH meter is connected to the lye tank outlet water valve to control the pH value of the cyanide-reaction tanks a and b. According to the long-term practical experience, the optimum pH control range for the cyanogen-breaking reaction should be: the pH of the cyanogen-removal tank a is maintained at 11 to 13; the pH of the cyanide-removal tank b is maintained at 8 to 9.
(3) The residual chlorine is set at the outlet of the clear water pool. The amount of residual chlorine can reflect the degree of oxidation in the process. Through experience, the residual chlorine is controlled at about 3-6 ppm, and the CN- concentration can generally be controlled at 0.5. Below mg/L.
6. Actual operation effect The system has stable effect, and the effluent CN-content is 0.15-0.3mg/L. Practice has proved that ensuring the pH value in the cyanogen-breaking process is the key factor to achieve the treatment effect; the effluent residual chlorine monitoring is guaranteed to be effective. The key to chlorine dosing; when these two factors are strictly controlled, the CN-content in the raw water fluctuates a bit, and does not affect the stability of the effluent effect.
7. Conclusions (1) Chlorine dioxide oxidation is an effective process to remove high concentrations of water.
(2) For wastewater with concentration (in CN-meter) of 100-300 mg/L, when pH=11.2 to 12.6 and ClO2/CN-=2.28 to 5.92, the removal rate is greater than 98%, and the treatment effect is stable.
(3) The higher the concentration in the raw water, the lower the ClO2/CN- required for achieving the same removal rate.
(4) The pH value has a significant effect on the removal rate of chlorine dioxide by cyanogen removal. Under acidic conditions, chlorine dioxide has no effect on removal; under weak alkaline conditions, the oxidation rate is slow, and it takes a longer contact time to achieve higher The removal rate; when the pH is higher than 11, the contact time removal rate of 30 minutes can reach more than 96%.
Chlorine dioxide has been widely used in the fields of drinking water disinfection, medical device disinfection, and pulp bleaching because of its strong oxidizing properties and other excellent properties. It also shows strong advantages in the treatment of industrial waste water. Pay attention. NaCN wastewater will be generated during the NaCN leaching process in wet smelting and cyanide plating during mechanical processing. As cyanide-containing waste water is a highly toxic inorganic waste water, it has a strong poisoning effect on living organisms and the environment, and it will produce highly toxic hydrofluoric acid gas under acidic conditions, which directly has a fatal hazard. Therefore, it is of great practical and social significance to treat waste water containing cyanide after it has been discharged or used for production.