Fluorine removal methods

1. Fluorine removal methods are as follows:

1) fluorine containing water passes through the adsorption column and fluorine is adsorbed on the surface of the adsorbent to generate insoluble fluoride.The adsorbents are activated alumina, tricalcium phosphate or bone charcoal.It is the oldest method applied to groundwater with fluoride < 10mg/L and suspended substance < 5mg/L.When the adsorbent fluorine removal capacity tends to be saturated, the need to add regeneration agent regeneration.

2) coagulant coagulant is added to fluorine-containing water to form insoluble fluoride, which is removed by precipitation filtration.The coagulants are Al2(SO4)3, basic aluminum chloride, magnesium hydroxide, etc.Suitable for raw water with high turbidity and low fluorine content.

3) ion exchange method USES the exchange capacity of ion exchange resin to remove fluorine ions from the water.

4) electrodialysis USES the selective permeability of ion exchange membrane to remove fluorine

5) reverse osmosis method USES the selective permeability of semi-permeable membrane to remove fluorine

Among the above methods, 3), 4) and 5) are applicable to brackish water and areas with high fluorine content. Reverse osmosis is currently widely used.

2. Key points of technological design for aluminum salt coagulation and precipitation removal of fluorine

Coagulation precipitation is suitable for raw water with a fluoride content of < 4mg/L. Raw water with a low fluorine content and the turbidity of the water needs to be removed at the same time.The agent should choose aluminum salt.When the concentration of CL- in raw water is high, aluminum sulfate is used.When raw water sulfate concentration is high, aluminum chloride is used;Basic aluminum chloride is used when both ion concentrations are high.

The coagulation time is 5-6min and the pH value is generally controlled between 6.5 and 7.5.Mixing tank, flocculation reaction tank, sedimentation tank and precipitation need to be designed with variable speed or intermittent precipitation, with good fluorine removal effect.

Process flow: groundwater – coagulation – precipitation – filtration, when the quality of raw water is good, can be omitted precipitation or filtration treatment unit, generally used before pump dosing, pump mixing, standing precipitation, take the upper water for drinking.

3, activated alumina adsorption and filtration fluorine removal process design

Generally, the groundwater with high fluoride content has a higher alkalinity and PH > 8.0, while PH has a great influence on the adsorption capacity of activated alumina.The adsorption capacity of activated alumina is 4-5g(F-)/kg(Al2O3) when the PH of water intake is 6.0-6.5.At PH of 6.5-7.0, the adsorption capacity was 3-4g(F-)/kg(Al2O3).Without PH adjustment, the adsorption capacity is only about 1g(F-)/kg(Al2O3).

Activated alumina is the filter material, the particle size should be less than 2.5mm, generally 0.5-1.5mm, the non-uniformity coefficient of the filter material K80≤2 when the fluoride content of raw water is less than 4mg/L, the filter material thickness should be more than 1.5mm;When the fluoride content of raw water is greater than 4mg/L, the filter material thickness should be greater than 1.8mm.When the PH of water inlet is > 7.0, the filtration rate is generally 2-3m/h, and when the PH of water inlet is less than 7.0, the filtration rate is generally 6-8m/h, and the corresponding contact time should be 20-30min. Aluminum sulfate is used as the regenerating agent, the ratio of dosage to fluorine removal is 60:1, and the content of regenerating solution is 2%-3%.During regeneration, the raw water was first backwashed from bottom to top with the washing intensity of 11-12l (s.m2), the washing time of 5-8min and the expansion height of about 50%.Then, 2% aluminum sulfate solution was used for regeneration, which was passed through from top to bottom with a filtration rate of about 0.6m/h and lasted 6-8min.After regeneration, rinse with defluorination water.The washing intensity was the same as the initial washing and lasted 8-10min.