Excessive fluoride has a great impact on the environment and the human body. Commonly used fluoride-containing wastewater treatment methods include:
1. Chemical precipitation method
For high-concentration fluorine-containing wastewater, Fluorine industrial wastewater is generally removed by the calcium salt precipitation method, that is, adding lime to the wastewater to cause fluoride ions and calcium ions to precipitate CaF2. This process has the advantages of simple method, convenient treatment, and low cost. However, it has the disadvantages of difficulty in reaching the standard of effluent after treatment, slow sedimentation of sludge, and difficulty in dehydration.
The solubility of calcium fluoride in water at 18°C ??is 16.3mg/L, calculated as fluoride ions, 7.9mg/L. Calcium fluoride with this solubility will form a precipitate. When the residual amount of fluorine is 10 to 20 mg/L, the rate of precipitate formation will slow down. When the water contains a certain amount of salts, such as sodium chloride, sodium sulfate, and ammonium chloride, the solubility of calcium fluoride will be increased. Therefore, the fluorine content in wastewater treated with lime will generally not be less than 20 to 30 mg/L.
Lime is cheap, but has low solubility and can only be added as an emulsion. Since the CaF2 precipitate produced is wrapped on the surface of Ca(OH)2 particles, it cannot be fully utilized, so the dosage is large. . When lime milk is added, even if its dosage reaches the pH of the wastewater to 12, it can only reduce the fluoride ion concentration in the wastewater to about 15mg/L, and the suspended matter content in the water is very high. When water contains soluble calcium salts such as calcium chloride and calcium sulfate, the solubility of calcium fluoride is reduced due to the same ion effect.
Add a mixture of lime and calcium chloride to fluorine-containing wastewater. After neutralization, clarification and filtration, when the pH is 7 to 8, the total fluorine content in the wastewater can be reduced to about 10mg/L. In order to quickly coagulate and precipitate the generated sediment, commonly used inorganic salt coagulants (such as ferric chloride) or polymer coagulants (such as polyacrylamide) can be added to the wastewater alone or in combination. In order not to destroy the formed flocs, the stirring operation should be carried out slowly, and the generated precipitate can be solid-liquid separated by static separation method.
At any pH, the concentration of fluoride ions decreases as the concentration of calcium ions increases. When the excess calcium ion is less than 40 mg/L, the fluoride ion concentration decreases rapidly with the increase of calcium ion concentration, while when the calcium ion concentration is greater than 100 mg/L, the fluoride ion concentration changes slowly with the calcium ion concentration.
Therefore, when using lime precipitation to treat fluoride-containing wastewater, the method of simply increasing the excess amount of lime cannot be used to improve the fluoride removal effect. Instead, coordination between fluoride removal efficiency and economy should be considered. , so that it can have better fluoride removal effect and add as little lime as possible. This also helps reduce the amount of sludge discharged after treatment.
2. Flocculation and precipitation method
The commonly used flocculant in the flocculation and precipitation method of fluoride ion wastewater is aluminum salt. After the aluminum salt is added to the water, the complexation of Al3 and F-, the intermediate products of aluminum salt hydrolysis and the finally generated Al(OH)3(am)? alum are used to exchange ligands, physical adsorption, and sweep of fluoride ions. It removes fluoride ions from water. Compared with the calcium salt precipitation method, the aluminum salt flocculation precipitation method has the advantages of less chemical dosage, large processing capacity, and can meet national emission standards after one treatment. Aluminum salts such as aluminum sulfate and polyaluminum have good coagulation removal effects on fluoride ions.
When using aluminum salt, the optimal pH for coagulation is 6.4 to 7.2, but the dosage is large. Depending on the situation, 150 to 1000g per m3 of water needs to be added, which will cause a certain amount of chlorine in the effluent. Dissolved aluminum hazardous to human health. After using polyaluminum, the dosage can be reduced by about half, and the pH range of flocculation and precipitation is expanded to 5 to 8. The fluoride removal effect of polyaluminum is related to the properties of the polyaluminum itself. Polyaluminum with an alkalization degree of 75 is the best for fluoride removal, and the dosage is optimal when the molar ratio of F to Al in water is about 0.7.
The aluminum salt flocculation precipitation method also has obvious shortcomings, that is, the scope of use is small. If the fluorine content is large, the amount of coagulant used will be large, the treatment cost will be large, and a large amount of sludge will be produced; fluoride ions The removal effect is greatly affected by operating factors such as stirring conditions, settling time, and anions such as SO42- and Cl- in the water. The effluent water quality is not stable enough. This is related to the fact that the current understanding of the coagulation defluoridation mechanism is insufficient. Research on the flocculation defluoridation mechanism It has obvious practical significance.
The mechanism of aluminum salt flocculation to remove fluoride ions is relatively complex, and there are three main mechanisms: adsorption, ion exchange, and complex sedimentation.
(1) Adsorption. The fluoride removal process of aluminum salt flocculation precipitation is electrostatic adsorption. The most direct evidence is that the AC or PAC fluorine-containing floc adsorbs charged fluoride ions, and the positive charge is partially neutralized. Under the same pH conditions, the zeta potential is higher than that of its own floc. The body should be low. Another evidence is that when the concentration of SO42-, Cl- and other anions in the water is high, the adsorption capacity of Al(OH)3 alum flowers formed during the flocculation process for fluoride ions will be significantly reduced due to competition.
(2) Ion exchange. The radii and charges of fluoride ions and hydroxide are similar. During the flocculation and defluoridation process of aluminum salts, polyhydroxycations such as Al13O4(OH)147 added to the water and the amorphous Al(OH)3(am) formed after hydrolysis ) precipitation, in which OH- and F- are exchanged. This exchange process is carried out under equal charge conditions. After the exchange, the charge of the flocs remains unchanged, and the ζ potential of the flocs will not increase or decrease as a result. However, the OH- released during this process will increase the pH of the system, indicating that ion exchange is also an important mode of defluoridation by aluminum salts.
(3) Complexation and precipitation. F- can form 6 complexes with Al3, ranging from AlF2, AlF2, AlF3 to AlF63-***. Calculation of solution chemical equilibrium shows that when the F- concentration is 1×10-4~1×10-2mol/L In the aluminum salt coagulation defluoridation system, when the pH is 5 to 6, it mainly exists in the form of AlF2, AlF3, AlF4- and AlF52-. These aluminum-fluorine complex ions will form aluminum-fluorine complex during the flocculation process. The substances (AlFx(OH)(3-x) and Na(x-3)AlFx) or mixed in the newly formed Al(OH)3(am) flocs settled down, and the IR and XPS spectra of the flocs were finally observed. Part of the obtained aluminum fluoride complex ion AlFx(3-x) is the result of complex precipitation, and the other part may be the product of ion exchange. The information is referenced from www.codquchuji.cn for reference only