Optimization of Cyanide Removal Process from Coking Wastewater

Coking wastewater source and water characteristics

The organic compounds in coking wastewater are complex, the main organic compounds are CODcr content 2500-4500mg/L; cyanide 10-20mg/L; DOD51200-2000mg/L; NH3-N400-1000mg/L; phenol 150-200mg/L; sulfide 6-15mg/L; oil 200-1000mg/L, pH6.5-8.5.

The source of coking wastewater is the process of pyrolyzing coal to obtain coke and gas, and recovering by-products such as tar and benzene in the production process. The water quality characteristics of coking wastewater are complex in composition.

According to the pollutants, it can be divided into two categories: inorganic substances and organic substances: inorganic substances exist in the form of ammonium salts, and organic substances are mainly phenolic compounds.

Second, it contains a lot of refractory substances and toxic and harmful substances. The third is the high concentration of ammonia nitrogen and great harm.

Problems existing in coking wastewater treatment

The phenol cyanide wastewater treatment station of the coking plant adopts the OAO process, and the discharged wastewater meets the requirements of the first-class standard in GB13456-1992 “Water Pollutant Discharge Standard for Iron and Steel Industry”; Bidding transformation.

The specific modifications are as follows:

①The biochemical section is transformed into HLA+O1-A/O2 process;

②The post-treatment is changed to the secondary dosing precipitation process;

③The new advanced treatment section is composed of ozone contact oxidation, intermediate pool, biological aerated filter, multi-media filtration, activated carbon filtration, and clean water pool.

The upgrading and renovation project was debugged and put into use, but some water quality indicators did not meet the design requirements. After multiple investigations, it was found that thiocyanate in the influent of coking wastewater was the key to the abnormal water quality indicators after the upgrading and renovation.

The influence of thiocyanide on the system was not considered in the original upgrade design, but through on-site data analysis and theoretical research, it was found that SCN- was the main factor affecting the denitrification of the subsequent biochemical treatment system.

Moreover, the biochemical degradation of SCN- requires sufficient hydraulic retention time and is directly affected by the concentration of volatile phenols in the wastewater; in addition, the products of SCN- degradation are mainly ammonia nitrogen, which increases the denitrification load.

The incompletely degraded SCN- will inhibit the activity of nitrifying bacteria, causing chain inhibition reaction of the entire biochemical system, and finally affecting the treatment effect of the entire biochemical system.

Solutions

In view of the high and unstable thiocyanate content in the phenol-cyanide wastewater, the existing system must be transformed. After technical demonstration, it is determined to add a set of pre-decyanation system on the basis of the existing technology.

①Reconstruction of aeration decyanation tank: The original initial aeration tank was transformed into an aerated decyanation tank, and an aeration pipe of 336m was added;

②Reconstruction of aeration decyanation sedimentation tank: The existing sedimentation tank 2 that has not been put into use is transformed into an aerated decyanation sedimentation tank, and 1 central drive sludge scraper, 2 sludge return pumps, and 2 defoaming pumps are added;

③Pipeline transformation: connect the effluent of the air floater to the aeration decyanation tank, the effluent of the pre-decyanation tank will flow to the decyanation sedimentation tank, the sludge at the bottom of the sedimentation tank will return to the pre-decyanation aeration tank, and the outlet of the decyanation sedimentation tank will be full. Flow to the HLA tank, and keep the pipeline from the air flotation tank to the HLA tank unchanged.

Debugging

4.1 Debugging steps

①The commissioning team transported mud water from the sedimentation tank to the decyanation tank in batches according to the previous biological bacteria culture plan of the decyanation tank, and sent a total of about 1500m3 of biochemical sludge into three times without affecting the operation of the original phenolic cyanide biochemical system. The decyanation tank, during this period, the originalization system runs stably without abnormality;

②After 2 days, about 300m3 of industrial water will be added to the pre-decyanation tank, and the corridor 3 of the pre-decyanation tank is basically full at this time. Turn on the sludge return pump in the decyanation tank and slightly open the aeration for internal circulation to ensure the uniformity and activity of the biological sludge in the decyanation tank. During the period, continue to replenish industrial water until the decyanation tank is full.

Due to the large amount of biochemical sludge in the decyanation tank, it was found that the pre-decyanation tank had too much foam and was uncontrollable at the beginning of aeration. System foam for defoaming;

③ Gradually increase the incoming water volume of the air flotation tank entering the pre-decyanation tank, and control the dissolved oxygen in the decyanation tank to be 2%-5%. During the period, the test indicators are normal, and the ammonia nitrogen, COD, and thiocyanate in the effluent of the decyanation tank have decreased. However, the alkalinity of the system also decreased to a certain extent;

④After 1 week of stability, increase the influent of the decyanation tank system to 5m3/h, and the dissolved oxygen is still controlled at 2%-5%. The detection indicators are relatively stable, but the alkalinity of the decyanation tank decreases rapidly after the aeration and nitrification reaction. , immediately add 1t of soda ash to the decyanation tank evenly, and then continuously supplement the alkali source to the decyanation system according to the alkalinity.

The pre-decyanation tank system gradually became stable, and after 1 week, the water inflow of the decyanation tank was gradually increased until it was running at full load (35m3/h).

4.2 Problems during debugging

①The concentration of biochemical sludge in the decyanation tank is relatively high, and the SV30 exceeds 60%;

②It is not easy to adjust the air volume at the beginning, and it is necessary to do a good job of dissolved oxygen control under the condition of ensuring the stability of the originalization system;

③ The foam is too much to control, and the foam overflow phenomenon occurs when the air duct is opened very small. Therefore, the air ducts of each corridor should be opened separately, only one-third of the opening at the beginning, and half of the subsequent opening;

④ The alkalinity dropped significantly after the nitrification reaction in the decyanation tank, which affected the operation of the system, and the previous plan was not well considered;

⑤ Glucose was originally planned to be added during the commissioning process. According to the actual operation of the decyanation tank, there is no need to add it. The carbon source of the system itself can meet the requirements of the decyanation tank;

⑥ Fluctuation of the water inflow index of the decyanation tank has a certain impact on the system, and the test tracking is insufficient.

5 Conclusion

In recent years, with the gradual enhancement of people’s awareness of environmental protection, my country’s environmental protection standards have become increasingly strict. The advanced treatment of coking wastewater by the nanofiltration process will produce a large amount of high-concentration cyanide-containing nanofiltration concentrated water. The cyanide removal effects of two calcium salts, Ca(OH)2 and CaCl2, in the coking nanofiltration concentrated water were compared and investigated for the high cyanide ion nanofiltration concentrated water.

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