Author: M A Abdulbaki
Atomic Energy Commission, Hydrometallurgy Office,
P.O. Box 6091 Damascus, Syria
Received 30 November 2005; revised received 14 March 2007;

accepted 15 March 2007 The removal of fluoride from commercial Syrian wet phosphoric acid was studied by precipitation with different sodium and potassium salts under different experimental conditions: stoichiometric ratio, temperature and mixing time. The ideal conditions that give high removal, were determined. All the salts used were found to give high percentage of removal i.e. 95-97%. But, commercial sodium chloride is recommended due to economical reasons. Keywords: Phosphoric acid, Precipitation, Fluoride IPC Code(s): C01B9/08, C01B25/18 Phosphate rocks contain 3 to 4% fluoride salts and 65% of these salts go to phosphogypsum during the manufacture of phosphoric acid1. Purification of wet phosphoric acid was studied by Davister and Peeterbroeck2 by liquid-liquid extraction with a mixture of disopropylether (DIPE) and tributylphosphate (TBP) in kerosene diluent. Two third of P2O5 was extracted in four counter current stages at 5-25°C. The pure phosphoric acid was recovered from the solvent by stripping with distilled  water. A process to extract H3PO4 from wet phosphoric acid by liquid-liquid extraction using n-heptanol has been reported by McCullough3. It was possible to remove 97% of the metals and 82% of the fluoride. TVA co used methanol with ammonia to precipitate most metallic impurities in the form of MNH4PO4 (M for metallic cations). Lanoe et al.4 has claimed in a patent a procedure where strong wet phosphoric acid is pretreated and extracted with 80% of tributyl phosphate (TBP) and 20% of saturated hydrocarbon by volume in counter-current extraction at ambient temperature.

The most important process includes addition of silica gel to the acid heating under vacuum. Fluoride was recovered as H2SiF6 by washing the volatilized silicon tetrafluoride with water5.

3SiF4 + 2H2O 2H 2SiF6 + SiO2 The removal of fluoride from phosphoric acid depends on the precipitation of SiF6

2- in the phosphoric acid as fluorosilicate salts by addition of the sodium and potassium components to the phosphoric acid according to the equation, SiF6-+2Na+ → Na2SiF6
and the precipitated salts are removed by filtration6.Precipitation fluoride method for fluorosilicate salts is not widely practiced, although it appears to be a cheap and convenient method for eliminating not only fluoride, but also silicon.In the present communication the removal of fluoride from commercial wet phosphoric acid has been achieved. Experimental Procedure

Materials
A commercial wet phosphoric acid from the General Fertilizer Company GFC Homs, Syria with 29% wt. P2O5, F=24815 g/L and a density of 1.26 g/cm3 was used after being treated. Na2CO3, Na2SO4, Na2CO3, NaCl, K2SO4, K2CO3 and KCl used were of analytical grade and Na2SO4 and NaCl were of commercial grade.

Apparatus and method All precipitation tests were carried out in a beaker with a magnetic stirrer placed in a thermostat to control the temperature. The mixtures were filtered under a vacuum unit. The fluoride concentration was determined by ion selective electrode using ion selective/pH meter from Metrohm Co.Results and Discussion Effect of the salt stoichiometric ratio on fluoride removal Different stoichiometric ratios i.e. 100, 150, 200 and 250% from each salt were taken and were added to 100 g of wet phosphoric acid at a temperature of 55°C, mixing time being 30 min. The results obtained are shown in Table 1. The efficiency slightly increased after 200%wt stoichiometric ratio. So, it is enough to fix the stoichiometric ratio at 200% for other experiments.

Effect of temperature on the efficiency The effect of temperature was investigated by varying the temperature from 25 to 55°C with other conditions remaining constant (mixing time = 30 min, stoichiometric ratio = 200%). The results are shown in Table 2. It is clear that the efficiency decreases by increasing the temperature. Therefore, it is usual practice to carry out the fluoride removal at room temperature.

Effect of the mixing time on the efficiency The fluoride ions removal were carried out by varying the time of mixing from 30 to 120 min while keeping other conditions constant (T=25ºC, stoichiometric ratio = 200%). The results are represented by Table 3. Since, efficiency increases negligibly after 60 min, hence, it is appropriate to fix the mixing time at one hour.

Fluoride removal by commercial sodium chloride and sodium sulphate The commercial as well as laboratory prepared samples of sodium chloride and sodium sulphate were used for fluoride removal by adopting the previously mentioned typical conditions (P2O5=29%, F=24815 g/L, mixing time = 60 min, stoichiometric ratio = 200%, T = 25°C).

The results show that the removal efficiency is almost same for commercial and laboratory prepared sodium chloride and sodium sulphate salts (96-97%). So commercial sodium chloride is recommended which it is locally available. Conclusion From the above observations, it may be inferred that, (i) it is possible to remove fluoride ions from wet phosphoric acid with high efficiency i.e. 95-97% by precipitation using sodium and sulphate salts.(ii) the efficiency of fluoride ion removal decreases by increasing temperature. Mixing time = 60 min, T =25°C and stoichiometric ratio = 200% was found to be the most appropriate fluoride ion removal conditions. (iii) fluoride ion removal efficiency of 97% was achieved by using sodium chloride and sodium sulphate salts and it has been found to be having the highest efficiency. (iv) commercial sodium chloride is recommended for economical reasons which is locally available also.

Acknowledgement

The author would like to express his thanks and appreciation to the General Director of the Atomic Energy Commission, Dr Ibrahim Othman for his help and encouragement to carry out this research work.

References 

1 Moldovan I, Popovic N & Chivu G, The Technology of Mineral Fertilizers (British Sulphur Corp, London), 1969. 2 Davister 000000++A & Peeterbroeck` M, The Prayon Process for Wet Acid Purification, Chemical Engineering Progress (Societe de Prayon, Liege, Belgium), 1982, 35. 3 McCullough J, Chem Eng, 83(26) (1976) 101. 4 Lanoe J, Malaterre R & Morin M, Europ Pat no. 0517580B1, July 3 (1992). 5 Fathi Habashi & Farouk T Awadalla, Sep Sci Technol, 18(5) (1983) 485. 6 Zouhair Qafas, Kacem EL Kacemi, Ettaybi Ennaassia & Mohamed Chakib Edelahi, Sci Lett, (3)3 (2002).