The treatment of water containing Fluorides, has been traditionally and historically done with Lime in alkaline pH range.
The reaction is : Ca(OH)2 + 2 HF = CaF2 + H2O
The fundamental problem that exists using this technique, arises from the low solubility of the Calcium Hydroxide (around = 0,07%) that therefore requires an excess of reagent to get a complete precipitation. For against, theoric solubility of the Calcium Fluoride (Ksp = 4*10 exp-8 equivalent to
17 ppm of CaF2 or 8,3 ppm as F- ) doesn't obtain a complete removal of the Fluorides as required by wastewater discharge Limits or WHO recommendation for drinking water. Furthermore, substantial lowering in Fluoride ion, by such method requires very long contact time
The study on the formation of Aluminium complexes with the Fluorine has been developed (Garrison Sposito: The Environmental Chemistry of Aluminium–CRC Press–1989).
Experimental trials confirmed the ability of absorption of Fluoride ions on the Aluminium Hydroxide matrix due to the dimension of the ion F - that is similar to the ion OH –RELATE INTERTRADE ’s product WT-747 is a mixture of precipitated Aluminium Oxide and re-dissolution of a complex Aluminium salt containing different ligands at pH value < 1
The usual treatment with lime in slurry allows to treat concentrated flows (Fluorides > 1500 ppm) rising from washing or process wastewater where Hydrofluoric Acid (or Ammonium Fluoride) has been used. Such treatment obtains residual Fluoride content in the range of 35 – 60 ppm.
Limpid water from pre-treatment step is treated as follows :
1. Mixing of WT-747 in a well stirred reactor for a reaction time of 3 – 5 minutes
2. pH adjustment in a second reactor with Lime slurry until pH value of 7,5 – 7,8
3. Anionic Polymer solution addition to allow a formation of big flocks
4. Sedimentation in a final settling tank
PRELIMINARY LABORATORY METHOD GUIDELINE
The wastewater containing Fluorides ( more than 500 ppm) must be treated with the conventional method :
Lime addition until pH 10 – 11
Stirring for a period of 10 – 15 minutes
Addition of Anionic Polymer (10 – 20 ppm) until a complete flocculation and sedimentation
By such way it’s possible to arrive to a final concentration of Fluorides in the range 35 – 50 ppm
If residual content of Fluorides is ALREADY in such range, this pre-treatment is not required.
On the limpid liquid superficial (containing the above mentioned concentration) :
1. WT-747 is added : it’s occurs a pH lowering (to maintain the stirring for 2 minutes)
2. Lime is added until pH 7,5 – 8,5 (to maintain the stirring for 3 minutes)
3. Anionic polymer is added (10 ppm) to achieve a complete flocculation and sedimentation
The optimum WT-747 dosage has to be defined in lab tests.
Usually, the laboratory test is carried out by additions, with the same procedure, of 500 – 1000 – 1500 – 2000 – 3000 ppm and by reporting in a calibration curve the Fluoride residual concentration.
By such way, it’s possible to define the more suitable dosage.
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The treatment of water containing Fluorides, has been traditionally and historically done with Lime in alkaline pH range. The reaction is : Ca(OH)2 + 2 HF = CaF2 + H2O The fundamental problem that exists using this technique, arises from the low solubility of the Calcium Hydroxide (aro