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Jiangsu Provincial Research Institute of Environmental Sciences, Zou Min
Nanjing Jiaqing Environmental Protection Engineering Company Limited, Wu Xiaoxiang
Abstract: Using the new technology and process of high-pressure pulse electrocoagulation-diatom fine earth application In 2000t/d electroplating mixed wastewater renovation project, the removal rate of Cr6+, Ni2+ and Cu2+ reaches 99.77%, 99.90% and ~100% respectively, and all the indexes meet the emission standard. This process has the characteristics of strong treatment capacity, fast speed, occupying area, convenient operation, reliable operation, investment and low treatment cost compared with the traditional chemical method.
Keywords: high-pressure pulse electrocoagulation, diatom fine earth, electroplating integrated wastewater
I. Preface
At present, the heavy metal pollution in certain areas of China is relatively serious, the reason is because of the plating plants are many and scattered, and wastewater is not high enough to meet the actual standards of discharge rate caused. Heavy metal ions such as hexavalent chromium, nickel, copper, zinc, cadmium, etc. has a strong toxicity, on people, animals and crops will cause serious harm, so the plating wastewater discharge over the standard on the environment is very serious, and must pay great attention.
Currently at home and abroad to manage electroplating wastewater using several methods, iron oxidation method of raw materials is convenient, inexpensive, but the color of the water is poor, the amount of sludge; electrolysis method of investment, power consumption, uneconomical; ion exchange method and film method of water quality, but regeneration, replacement of resins and diaphragms are complex, not easy to master. Comparison from the methods, chemical method is a more mature and reliable treatment process, but the degree of automation requires a high degree of complexity of the process, must be classified according to chromium, cyanide, acid and alkali, oil, phosphorus and other treatment and then comprehensive treatment, equipment, process flow is long. In addition, the chemical method must add a variety of agents, resulting in cumbersome operation and large amount of waste residue. In response to these problems, we propose the use of proposed high-pressure pulse electrocoagulation - diatom fine earth electroplating integrated wastewater treatment of new processes and new technologies, and has been successfully applied in Zhejiang Yirong gasoline engine parts Co.
Second, the project overview
1, process and treatment equipment
Zhejiang Yirong Gasoline Engine Parts Co., Ltd. electroplating integrated wastewater discharge of 2000t / d, the discharge standard implementation (GB8978-96) "Comprehensive Wastewater Emission Standards" in Table 1 and Table 2 in the first level of the discharge standard. The original wastewater treatment system adopts chemical process, because the electroplating production process can not divert all kinds of wastewater and the original wastewater in the high concentration of heavy metal ions, so that the original treatment system does not meet the design objectives. Table 1 shows the measured water quality indicators of the original wastewater.
Table 1 raw wastewater quality
Items pH CODCr (mg/L) Cr6+ (mg/L) Ni2+ (mg/L) Cu2+ (mg/L)
Mean 2.13~10.65 348.5 28.6 362.3 5.0
Range of change 142~871 14.5~63.6 115~985 1.18~15.7
The renovated wastewater treatment system adopts a new process of high-pressure pulse electrocoagulation - diatom fine earth, and the process flow is shown in Figure 1.
Figure 1 Process flow diagram
High-pressure pulse electrocoagulant machine is the key equipment in the process, *** add 4 sets, 3 with 1 spare, a single processing capacity of 30t/h, electrocoagulation tank net size of 3000 × 3000 × 3000mm. coagulation reaction sedimentation tank using the existing equipment, add a diatomaceous earth dry powder dosing device, and update the PAM dosing equipment.
2, the treatment effect
In January 5, 2004, the system began trial operation, December 27, 2004 -28, the environmental testing station of the renovation project for acceptance monitoring. The results are shown in Table 2.
Table 2 Water quality monitoring results of electrocoagulator outlet and total discharge
Monitoring points pH Cr6+ (mg/L) Ni2+ (mg/L) Cu2+ (mg/L)
Electrocoagulant outlet Mean 4.32~5.83 0.018 91.66 2.23
Range of change 0.014~0.020 67.4~5.83 2.23
The range of change 0.014~0.020 67.4~5.23
The range of change 0.014~0.020 0.020 0.014~0.020 0.020 67.4~161 1.60~2.85
Removal rate (%) 99.94 74.8 55.4
Total outlet Average 6.72~7.34 0.066 (Note 1) 0.68 <0.05
Range of variation 0.055~0.093 0.058~0.88 <. 0.05
Removal rate (%) 99.77 99.90 ~100
Note 1: The concentration of Cr6+ at the total discharge outlet is higher than that at the outlet of the electrocoagulant machine, which is mainly due to the release of Cr6+ from the sludge accumulating in the clearing basin, discharge outlet, and pipeline ditches.
3. Treatment cost
The direct running cost is 1.65 yuan/t of wastewater, of which: 0.5 yuan/t of wastewater for electricity, 0.7 yuan/t of wastewater for the consumption of pole plate, 0.45 yuan/t of wastewater for the cost of acid, alkali, PAM, diatomaceous earth and other chemicals. Together with maintenance and depreciation, the treatment cost is about 2 yuan/t wastewater.
Three, high-pressure pulse electrocoagulation and diatom fine earth principle
1, high-pressure pulse electrocoagulation technology principle
The technology breaks through the traditional low-voltage, high-current electrolysis method, and the use of high-voltage, low-current - high-voltage pulse electrocoagulation method (HVES). This method is the use of electrochemical principles, with the help of external high voltage to produce electrochemical reactions, electrical energy into chemical energy, a single electrocoagulation equipment can be organic or inorganic wastewater oxidation and reduction reaction, and then cohesion, floating, the pollutants from the water body to separate, can be effectively removed from electroplating wastewater in the Cr6 +, Zn2 +, Ni2 +, Cu2 +, Cd2 +, CN, oil, phosphate, and COD, SS, and the electroplating waste. Phosphate as well as COD, SS & Color.
High-pressure pulse electrocoagulation equipment is a soluble metal iron for the pole plate, wastewater into the electrocoagulation machine under the action of direct current, the aqueous solution dissociates into H+ and OH-. Electrocoagulation machine without dosing each electrolysis unit occurs as follows.
1.1 Reduction reaction removes Cr6+, chromaticity
A reduction reaction occurs on the cathode, producing hydrogen molecules.
2H+2e→2H→H2↑
This new ecology (H) has both a strong reducing ability to reduce hexavalent chromium to trivalent chromium and remove it with chromium hydroxide precipitation. For many of the oxidation state components of the pigment dyes can be reduced to colorless substances, and the waste water color removal.
Cr2O72-+6e+14H+→2Cr3++7H2O
CrO42-+3e+8H+→Cr3++4H2O
Cr3++3OH-→Cr(OH)3↓
1.2 Removal of Heavy Metal Ions
Heavy metal ions, together with OH in the electrolyzed water, form metal hydroxides. -generates metal hydroxide, forming a solid precipitate.
Cu2++2OH-→Cu(OH)2↓
Ni2++2OH-→Ni(OH)2↓
Cd2+2OHˉ→Cd(OH)2↓
Zn2++2OH-→Zn ( OH)2↓
1.3 Oxidizing reaction removes COD and CN-
Main reaction of anode plate:
Fe -2e→Fe2+
4OH- 4e→2H2O+2O→2H2O+O2↑
Anode produces new O] has a strong oxidizing ability, can oxidize organic or inorganic compounds in water, remove COD in water. anode due to the release of new ecological (O) as an oxidant, oxidizing CN-, the cyanide root will be broken.
CN-+ 2OH -2e→CNOˉ+ H2O
2CˉNO-+4OH-6e→2CO2+N2+ H2O
1.4 Removal of phosphorus
Iron electrode plate precipitated by the electrochemical action of Fe2+ is oxidized into Fe3+ and phosphate reactive precipitation, and it can be used to form *** precipitation with other metals. Formation of *** precipitation, to achieve the best effect of phosphorus removal.
Fe3+PO43-→FePO4 ↓
1.5 Coagulation to remove SS
Metal electrode plate in the anode plate dissociates Fe2+ and oxygen reaction to generate Fe3+, resulting in Fe(OH)3 precipitation.
Fe2++2OH-→Fe(OH)2↓
4Fe(OH)2+O2+2H2O→4Fe(OH)3↓
The above reaction produces Fe(OH)3 activity is very strong, and can coagulate with organic and inorganic impurities in the water to produce a plume, in order to remove suspended solids in the wastewater, than aluminum salt, Iron salt coagulant on the suspended solids in wastewater and difficult to precipitate fine ions such as coagulation to remove the effect is better.
1.6 Removal of grease and colloid floating
In the process of electrocoagulation, the anode and cathode surface constantly generate oxygen and hydrogen, and escape with fine bubbles, which can be adhered to the flocs and oils in the wastewater to make it less dense and float to the surface, resulting in air flotation. It is more effective than the traditional air flotation method with the release of air bubbles produced by dissolved air water tiny.
2, the role of diatom fine earth technology in electroplating wastewater treatment
Since the supernatant after electrocoagulation technology is affected by the pH value, there will still exist some heavy metal ions such as Ni2+, Cu2+ and so on. Generally, in order to further remove these heavy metals, the conventional method is to add alkali to raise the pH value of the wastewater to 9.5 or higher, and then add acid to adjust the pH value to 6~9. However, if the addition of diatomaceous earth treatment agent, the pH value of 7.5 or so, you can get a very high removal rate of heavy metal ions. In the actual project, the removal rate of Ni2+ in this unit reaches 99.925%, and the Cu2+ effluent is less than 0.05mg/L.
Diatoms in the process of selecting diatoms *** raw impurities are separated from diatoms, so that the diatom surface has been balanced by the formation of the unbalanced potential, the diatom extract treatment agent is added to the wastewater in microdosage in the high-speed mixing, or pumping the sewage under the rotation of the blades of the pump machine. In water treatment, after the diatom earth treatment agent is added into the sewage in small amount, under the rotation of high-speed mixing, or pumping blades of sewage, it will be dispersed in the water body instantly, the unbalanced potential of diatom surface can neutralize the suspended particles' electrically charged, so that its repulsive potential will be damaged and form mu plume with diatoms, which will produce potential neutralization and sedimentation, and then agglomerate into bigger flocs, and then precipitate to the bottom by gravity, coupled with the huge surface area of diatoms, enormous pore volume and strong adsorption force, which can adsorb metal ions onto the surface of diatoms, forming a chain structure. Formation of chain structure. Diatoms composed of non-crystalline active silica have the performance of phase aggregation and free settling into diatom flocs in the water body. Together with the flocculation effect produced by the diatom fine earth being modified, it accelerates the deposition speed of the diatom flocs, which can be sunk and separated from the water body in a very short period of time, thus playing the role of removing heavy metal ions.
Four, process technology features and advanced
(1) electroplating line of various types of wastewater can be concentrated to a wastewater conditioning tank, different from the chemical method of different wastewater treatment separately. This is due to the electrocoagulation system for electroplating integrated wastewater organic and inorganic pollutants with reduction, oxidation, neutralization, flocculation, floating in addition to the separation of efficacy, to complete the treatment of mixed wastewater.
(2) the wastewater treatment system process is short, the equipment occupies a small area, the one-time investment is small.
(3) The various types of pollutants generated by the electroplating production line has a considerable safety factor and shock load resistance. When the quality of wastewater water changes, the adjustment is flexible and quick to strain.
(4) high-pressure pulse electrocoagulation method breaks through the traditional electrolysis method of fixed use of the pole plate, the equipment can be automatically interchanged on a regular basis will be the cathode and anode plate and activation. Can tear the electrode plate passivation film, completely solved the electrode plate passivation of this electrolytic equipment at home and abroad prevailing problems. Not only the two pole plate interchangeable to extend the life of the electrode, reduce consumables, and save electricity, only 1/15 ~ 1/20 of the ordinary electrolysis method.
(5) High degree of automation. The treatment system can realize automatic control of pH, ORP, liquid level, etc. and automatic addition of acid, alkali and medicine.
(6) Low operating costs. Comprehensive treatment cost of about 2.0 yuan / wastewater, only 1/5 ~ 1/3 of the chemical and other treatment methods.
V. Conclusion and Discussion
(1) High-pressure pulse electrocoagulation - diatom fine earth electroplating integrated wastewater treatment of the new process and the new technology technology process is simple, the ability to deal with electroplating integrated wastewater, fast speed, occupies an area of the province, easy to operate, reliable operation, low treatment costs.
(2) The treated effluent not only meets the discharge standard, but also can be partially reused and production, saving water resources, with good environmental and economic benefits.
(3) From the experience of the current project operation, it is very critical to control the appropriate pH value of wastewater into the electrocoagulant machine, the optimal pH value of 3~5, pH higher than 6 electrode plate is easy to passivate, and affect the treatment effect.
(4) When the wastewater quality changes, the current of the electrocoagulant machine must be adjusted accordingly, and can only be controlled manually, so further in-depth research is needed in order to realize the full automation control to ensure that the treatment effect.