Sedimentation method for fluorine-laden wastewater treatment - chemical precipitation method

Sedimentation method for fluorine-laden wastewater treatment - chemical precipitation method
For high concentration fluorine-rich industrial wastewater, the calcium salt precipitation procedure is typically preferred, which involves the addition of lime to the wastewater to form CaF2 precipitate amongst fluoride ions and calcium ions for removal. This method boasts advantages such as simplicity of methodology, ease of treatment, and modest cost, but does present disadvantages such as struggle in achieving post-treatment effluent standards, sluggish settling of sludge, and difficulty in dewatering.
The solubility of calcium fluoride in water at 18 ℃ is 16.3 mg/L, calculated as fluoride ions, which equates to 7.9 mg/L. At this solubility, calcium fluoride will precipitate. When the residual amount of fluorine is 10-20 mg/L, the rate of precipitation formation will decelerate. When water contains a certain quantity of salts such as sodium chloride, sodium sulfate, and ammonium chloride, it will augment the solubility of calcium fluoride. Hence, the fluoride content in the wastewater treated with lime is generally not less than 20-30 mg/L.
Lime is economical, but possesses low solubility and can only be added as an emulsion. Given the fact that the CaF2 precipitate produced is enveloped around the surface of Ca (OH) 2 particles, it cannot be fully harnessed, resulting in substantial usage. When adding lime milk, even if the dosage is employed to achieve a pH of 12 in the wastewater, it can only diminish the fluoride ion concentration in the wastewater to around 15 mg/L, and the suspended solids content in the water is exceedingly high. When water contains soluble calcium salts such as calcium chloride and calcium sulfate, the solubility of calcium fluoride is diminished due to the same ion effect. Incorporating a blend of lime and calcium chloride to fluorine-laden wastewater, following neutralization, clarification, and filtration, the total fluoride content in the wastewater can be reduced to around 10 mg/L when the pH is 7-8.
To expedite the 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 tandem in the wastewater. To prevent damage to the already formed flocs, the stirring operation should be executed gently, and the resultant precipitate can be separated into solid and liquid using static separation method. At any pH, the concentration of fluoride ions diminishes as the concentration of calcium ions escalates. When the excess amount of calcium ions is less than 40 mg/L, the fluoride ion concentration rapidly decreases with the augmentation of calcium ion concentration, while when the calcium ion concentration exceeds 100 mg/L, the fluoride ion concentration fluctuates slowly with the calcium ion concentration.
Thus, when employing the lime precipitation method to treat fluorine-laden wastewater, it is insufficient to merely increase the excess amount of lime to enhance the defluorination effect. Instead, a balanced consideration should be given between defluorination efficiency and economy, so as to achieve optimal defluorination effect while minimizing the addition of lime. This is also advantageous for reducing the volume of sludge discharged post-treatment.