Detailed interpretation of coagulation process

Date:Mar 22, 2019

The main source of wastewater in battery production is the wastewater from the battery production line cleaning slurry, the pesticide wastewater that is spilled in the slurry, and the wastewater from the ground. The amount of wastewater is about 250 to 300 tons per day, and the heavy metal ions in the wastewater exceed the allowable discharge standard by several times or even several tens of times.


First, the treatment process of battery industrial wastewater

1. Basic requirements for governance

(1) The water quality after treatment meets the first-level emission standard of DB4437-90 standard for wastewater treatment discharge.

(2) The sludge generated during the treatment process is used to make bricks, which will not cause pollution to the environment.

2, the basic principles of governance

According to the type of heavy metal ions contained in the wastewater, it is treated by a method of forming a sulfide or hydroxide precipitate. The process of generating a hydroxide is selected in view of the fact that the formation of hydroxide is easy to control and the waste is easy to utilize.

The chemical reaction of several major metal ions with the commonly used basic substances Na2CO3, Ca(OH)2, NaOH is as follows:




The hydroxides produced are usually insoluble materials, and the capacity in water is listed in Table 1. The above chemical reaction formula and solubility product indicate that a method of reducing the concentration of heavy metal ions by adding a basic substance is feasible.


3. Wastewater treatment process and main equipment

The wastewater treatment process adopts coagulation method, the coagulant is FeCl3, the pH adjuster is NaOH, and the main equipments are stirred coagulation tank, inclined tube sedimentation tank, box filter press and corresponding dispensing and dosing system. The process flow is shown in Figure 1.


Stirring coagulation tank and sedimentation tank are the key equipments affecting the particle size and sedimentation efficiency of the coagulated floc. The main design parameters are determined through experiments: the stirring time of the stirring coagulation tank is 25 min, and the end line speed of the stirring paddle is 1.2 m/ s, the minimum sedimentation rate of the particles is 2.9×10-5 m/s, the maximum slag content after coagulation is 600 mg/L, and the maximum wastewater treatment capacity is 400 m3/d. According to the above parameters, the effective volume of the stirred coagulation tank is 7m3, the diameter of the agitating impeller is 1m, the stirring speed is 23r/min, and the flat area of the sedimentation tank is 30m2, and its size is 7.15m×4.20m.捕获

Second, the results and discussion

After more than three years of operation, the above process is stable, the process is convenient, and the water quality of the discharged water reaches the DB4437-90 first-class discharge standard for wastewater treatment and discharge. The normal and efficient operation of the battery wastewater treatment process is mainly related to factors such as the pH value of the coagulation, the amount of coagulant, the strength of coagulation and agitation, etc., which are discussed below.

1. pH value and heavy metal ion content in water

The chemical reaction formula of the above heavy metal ions and OH- and the solubility product of the compound indicate whether the heavy metal ions in the wastewater can be removed to the utmost extent, depending on whether these metal ions can form insoluble hydroxides and these Whether the hydroxide can be stably precipitated, and these are closely related to the pH of the wastewater. In order to determine the optimum pH value, the relationship between different pH values and the content of Zn2+, Mn2+ and Hg2+ ions in wastewater was tested. For example, when the pH was 6.8, the contents of Zn2+, Mn2+ and Hg2+ were 65.0, 4.5 and 0.056 mg/L, respectively. as shown in picture 2.


As can be seen from Fig. 2, at a pH of 8.0, the content of Zn2+ and Mn2+ in water is high. With the increase of pH value, its content gradually became lower. At pH 9.0, the contents of Zn2+ and Mn2+ were 0.10 and 0.15 mg/L, respectively. The pH value continued to increase, and the Zn2+ and Mn2+ ions gradually increased. When the pH was 11.0, the Zn2+ and Mn2+ ions increased to 5.0 and 3.0 mg/L. The reason for this is that the hydroxides of Zn2+ and Mn2+ ions can form the corresponding coordination anions Zn(OH)-3 and Mn(OH)-3 at a large OH-ion concentration. The solubility products of these ions are 3×10-3 and 1×10-5, respectively, indicating that the solubility increases, and the content of Zn2+ and Mn2+ in the aqueous solution increases, while the Hg2+ ions do not have such properties, so the content thereof is relatively high. stable.

The test results show that the pH value is controlled at 8.5-8.7, which ensures that the metal ion content in the wastewater reaches the discharge standard, and the pH value also meets the emission requirements.

2, the effect of coagulant dosage

Using NaOH as a precipitant, when the content of heavy metal ions is low, the formed hydroxide is gelatinized, and these gels are easily broken into fine particles during agitation and material flow, making sedimentation extremely difficult. Tests have shown that the addition of an appropriate amount of coagulant can significantly improve this condition. When the total amount of Zn2+, Mn2+, and Hg2+ ions in the wastewater is between 20.0 and 80.0 mg/L, 300-400 g of FeCl3 is added per m3 of wastewater, which can significantly reduce the content of fine particles.

3, the effect of coagulation stirring speed

Stirring during the coagulation process not only causes the rapidly added NaOH to be dispersed, but also the heavy metal ions react with the OH- ions to form a hydroxide precipitate, and increases the collision chance between the particles, so that the particles aggregate and disperse simultaneously. If the agitation speed is too fast, the agitating paddles and the vigorously rotating water flow may break up the coagulated particles into fine particles, making the sedimentation of the particles difficult. Therefore, a suitable stirring strength can increase the particle size and increase the efficiency of the sedimentation tank.

Third, the conclusion

(1) Battery production wastewater containing Zn2+, Mn2+, Hg2+ ions can be treated with NaOH as an ion precipitant and a pH adjuster, and FeCl3 as a coagulant to reduce the above ion content to meet emission standards.

(2) Since the sedimentation rate of fine particles formed by using NaOH as a precipitant is slow, the use of a inclined tube sedimentation tank can significantly improve the wastewater treatment capacity and reduce the treatment site.

(3) The wastewater treatment process discussed in this paper is stable, easy to operate, and environmentally sanitized. It is a suitable method for wastewater treatment in battery plants.

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