Fluorine-containing wastewater is widely derived from the production process of fluorine chemical industry, aluminum electrolysis, glass manufacturing and other industries. Excessive discharge of fluorine will cause serious harm to the human body. At present, the high-concentration fluorine-containing wastewater is mainly pretreated by lime precipitation, and the fluorine concentration in the wastewater can be reduced to 20-60 mg/L.
In order to ensure that the effluent meets the requirements of 10mg/L stipulated in the national comprehensive sewage discharge standard (GB8978-1996), it is necessary to further use flocculation, sedimentation, adsorption and other methods for advanced treatment.
Among them, the flocculation sedimentation method is widely used in engineering because of its simple operation. The flocculation precipitation defluorination process includes two stages, one is the adsorption defluorination stage, which determines the fluorine removal efficiency; the second is the solid-liquid separation stage of the precipitation product, which determines the design and operation of the secondary sedimentation tank.
Existing studies mainly focus on exploring the influencing factors of flocculant adsorption and defluorination, including the type of flocculant, hydraulic conditions, reaction pH value, etc., but there are few studies on the solid-liquid separation performance of precipitation products.
In the previous work, the author’s research group carried out research on the recovery of cryolite and calcium fluoride from high-concentration fluorine-containing wastewater in the fluorine chemical industry by seed-induced crystallization, and found that seeds can enhance the effect of crystallization and precipitation.
On this basis, by adding seed crystals in the process of flocculation and precipitation, and using the fluorine concentration in the effluent and the sedimentation performance of the flocculation and precipitation products as the indicators, the induction and strengthening effect of the seeds on the flocculation and precipitation defluorination process was studied. The integrated equipment of precipitation reaction and solid-liquid separation has carried out advanced treatment of fluorine-containing wastewater from a certain fluorine chemical industry, and explored the feasibility of the application of seed-induced flocculation method.
Materials and methods
1.1 Experimental reagents
The simulated wastewater was prepared with sodium fluoride, and the fluorine concentration of the wastewater was 100 mg/L. The industrial wastewater was taken from a fluoridation factory in Hunan, from the mother liquor of high-concentration fluorine-containing wastewater after lime precipitation. The water quality is shown in Table 1, and the pH value is 12.4. The precipitant is prepared from calcium chloride, the flocculant is polyaluminum chloride (purity 28%), anionic, cationic and non-ionic polyacrylamide. The seed crystals are calcium fluoride particles, and the particle size ranges from 200 to 400 meshes.
1.2 Integrated reactor design
The integrated equipment for reaction and solid-liquid separation is shown in Figure 1, which can simultaneously realize the flocculation reaction and the sedimentation and separation of flocculation and sedimentation products during continuous operation.
The equipment is made of polypropylene, the size of the inner cylinder (1) is 200mm×600mm, the diameter of the outer cylinder (2) is 315mm, and the total volume of the equipment is 40L.
The inlet water (5) and (6) are added to the inner cylinder (1) from the top of the reactor, and the solution after stirring and reacting enters the outer cylinder (2) through the baffle plate (3) and realizes solid-liquid sedimentation separation, and the supernatant liquid is discharged from the water outlet. (7) Outflow, and sediments such as seed crystals stay at the bottom of the reactor to induce flocculation.
The small-scale experiment was carried out in a beaker, and precipitating agent and flocculant were quantitatively added to 1000 mL of fluorine-containing wastewater, the amount of seed crystals was controlled to be 10 g/L, and the reaction pH was 7-8. The reaction was stirred for 10 min, and the water turbidity, fluorine concentration and calcium concentration of the supernatant were measured after the reaction was completed.
The pilot test was carried out in an integrated reactor. Seed crystals and a small amount of tap water were added to the reactor in advance. During the operation, industrial wastewater and flocculant were continuously added with a magnetic pump and a metering pump, and the pH of the reaction was adjusted with hydrochloric acid. The water outlet (7) is sampled, and the pH and fluorine concentration of the effluent are measured.
Results and discussion
2.1 Simulated wastewater pilot experiment
2.1.1 Influence of seed crystals on fluorine concentration in effluent
The Ca/F ratio of the control reaction is 1.52, and the effect of seed crystals on the fluorine concentration in the flocculation effluent is shown in Figure 2. Before adding seed crystals, the fluorine concentration in the effluent after treatment with four flocculants was between 17 mg/L and 23 mg/L. After adding the seed crystals, the fluorine concentration in the effluent is significantly reduced, especially after the polyaluminum treatment, the fluorine concentration in the effluent drops below 6 mg/L. Therefore, adding seed crystals helps to improve the efficiency of fluorine removal by flocculation.
When no calcium precipitant is added, the effect of the amount of polyaluminum on the fluorine concentration in the effluent is shown in Figure 5. As the amount of polyaluminum increases, the fluorine concentration in the effluent decreases gradually. When the amount of polyaluminum is 100 mg/L, the fluorine concentration in the effluent decreases. less than 10mg/L.
However, the amount of polyaluminum in the literature under similar experimental conditions needs to reach 400-500 mg/L, which once again proves the strengthening effect of adding seeds to improve the efficiency of flocculation and fluorine removal.
In addition, the calcium concentration in the effluent basically does not change with the increase of the amount of polyaluminum. This is because the fluorine concentration in the wastewater is low, and the conditions of the calcium fluoride precipitation reaction are in the critical supersaturation region, and it is difficult for Ca2+ to spontaneously combine with F- to form calcium fluoride precipitation.
At this time, mainly relying on the coordination and adsorption of fluorine by polyaluminum, complex precipitates in the form of Al(OH)3-x·xF are generated to achieve the purpose of removing fluorine.
(1) Adding crystal seeds in the process of flocculation precipitation defluorination can significantly reduce the fluorine concentration in the effluent, improve the sedimentation performance of the precipitation product, and strengthen the defluorination effect.
(2) An integrated precipitation reactor is used to treat fluorine-containing wastewater from a certain fluorine chemical industry. Under the conditions that the pH value of the wastewater is 7.2-7.6, the wastewater flow rate is 40L/h, and the amount of polyaluminum is 200mg/L, the fluorine concentration in the effluent is stably lower than 10mg/L, the turbidity is lower than 30NTU, which meets the requirements of the national comprehensive sewage discharge standard (GB8978-1996).