Chemical Precipitation Method for Treating Fluorine-Containing Wastewater

For the treatment of high concentration fluorine-containing industrial wastewater, the calcium salt precipitation method is generally used, which involves adding lime to the wastewater to form CaF2 precipitate between fluoride ions and calcium ions for removal. The chemical precipitation method for treating fluorine-containing wastewater has the advantages of simple method, convenient treatment, and low cost, but it has disadvantages such as difficulty in meeting the effluent standards after treatment, slow sedimentation of sludge, and difficulty in dewatering.

The solubility of calcium fluoride in water at 18 ℃ is 16.3mg/L, calculated as fluoride ions, which is 7.9mg/L. At this solubility, calcium fluoride will form a precipitate. When the residual amount of fluorine is 10-20mg/L, the rate of precipitation formation will slow down. When water contains a certain amount of salts such as sodium chloride, sodium sulfate, and ammonium chloride, it will increase the solubility of calcium fluoride. Therefore, the fluoride content in the wastewater treated with lime is generally not less than 20-30mg/L.

Lime is cheap, but has low solubility and can only be added as an emulsion. Due to the fact that the CaF2 precipitate produced is wrapped around the surface of Ca (OH) 2 particles, it cannot be fully utilized, resulting in a large amount used. When adding lime milk, even if the dosage is used to achieve a pH of 12 in the wastewater, it can only reduce the fluoride ion concentration in the wastewater to around 15mg/L, and the suspended solids 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.

Adding a mixture of lime and calcium chloride to fluorine-containing wastewater, after neutralization, clarification, and filtration, the total fluoride content in the wastewater can be reduced to around 10mg/L when the pH is 7-8. To facilitate the rapid coagulation and precipitation of the generated precipitate, commonly used inorganic salt coagulants (such as ferric chloride) or polymer coagulants (such as polyacrylamide) can be added separately or in combination in the wastewater. To avoid damaging the already formed flocs, the stirring operation should be carried out slowly, and the resulting precipitate can be separated into solid and liquid using static separation method.

At any pH, the concentration of fluoride ions decreases as the concentration of calcium ions increases. When the excess amount of calcium ions is less than 40mg/L, the fluoride ion concentration rapidly decreases with the increase of calcium ion concentration, while when the calcium ion concentration is greater than 100mg/L, the fluoride ion concentration changes slowly with the calcium ion concentration.

Therefore, when using lime precipitation method to treat fluorine-containing wastewater, it is not enough to simply increase the excess amount of lime to improve the defluorination effect. Instead, a coordinated consideration should be made between defluorination efficiency and economy, so as to achieve good defluorination effect while minimizing the amount of lime added, which is also conducive to reducing the amount of sludge discharged after treatment.

For more information on fluorine-containing wastewater treatment, please continue to follow the official website of Jiangsu Haipu Functional Materials Co., Ltd., a fluorine-containing wastewater treatment manufacturer.