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Removal of fluoride from moroccan phosphate and synthetic fluoroapatites
Journal of Fluorine Chemistry 101 (2000) 69±73
Removal of ¯uoride from moroccan phosphate and synthetic ¯uoroapatites A. Laghzizil*, N. Elhrech, O. Britel, A. Bouhaouss, M. Ferhat Laboratoire de Chim.Phys.GeÂn., DeÂpt. Chimie, Fac.Sc., Rabat, Morocco Received 5 July 1999; accepted 3 August 1999
Abstract Natural phosphate is of importance to the Moroccan economy because it is a raw material for the production of phosphoric acid and phosphate fertilizers. However, toxic elements such as ¯uorine are present in the ore and cause problems in the industrial processes. In the present work, we have studied the extraction of ¯uoride from various phosphates using a steam atmosphere at various temperatures. Natural phosphates and synthetic ¯uoroapatites have been studied. A correlation between the ¯uoride mobility in the structure and its extraction has been established. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Natural phosphate; Fluoroapatites; Fluorine; De¯uorination
1. Introduction Fluoroapatite is a common source of phosphates, particularly in the Moroccan natural phosphates, because it is the most stable and insoluble of all the calcium phosphates. It occurs in a wide range of substituted forms, both biological and mineralogical [1]. Apatites are important in the production of phosphoric acid and phosphate fertilizers [2±4]. However, they may contain toxic elements which cause industrial and ecological problems [5±7]. So, large efforts are devoted to the removal of these toxic elements, among them ¯uorine. Therefore, the de¯uorination of natural phosphates will result in an improvement in the fertilizer properties of phosphates as well as economic added value and better environmental protection [6]. To date, there has been no attempt to remove the ¯uoride ions from Moroccan phosphate using thermal treatments at various temperatures and pressures. In previous works, the de¯uorination of natural phosphate in world rock (Russian phosphates) has presented some dif®culties [6±8]. Deutsch and Sarica have suggested a possibility for the substitution of Fÿ by OHÿ ions using a hydrothermal method. However, only a partial substitution is possible at temperatures higher than 10008C. Such high temperatures may lead to a decomposition of the carbonated apatite [9]. There is evidence that
* Corresponding author.. E-mail address: [email protected] (A. Laghzizil)
modi®cation of crystal growth of apatite gives several problems in phosphate industry products because their solubility and other properties can be modi®ed. The present investigation including structural studies on Moroccan phosphate (Youssou®a phosphate type) and synthetic apatite systems, is required before this stabilizing effect can be satisfactorily explained. Heat treatment under steam pressure favours the de¯uorination process. In order to model the mechanisms of the de¯uorination process, we have prepared various substituted M10(PO4)6F2 (noted as MFAp) (where M Ca, Sr, Pb and Ba), because these compounds are present in the Moroccan phosphates. Correlations between de¯uorination results on ¯uoroapatites and their physico-chemical properties have been established. 2. Materials The Youssou®a natural phosphate (Morocco) is constituted essentially of carbonated ¯uoroapatite of B-type Ca10ÿxy(PO4)6ÿx(CO3)xF2ÿx2y where 0 y x and has been provided by the Moroccan phosphate research center [5±7]. The different elemental constituents of this mineral are given in Table 1. MFAp ¯uoroapatites have been prepared by precipitation at 808C, in ammoniacal solutions containing (NH4)2HPO4, NH4F and M(NO3)2 (where M Ca, Sr, Ba, Pb). The ¯uoroapatite structure has been con®rmed by X-ray powder diffraction, infrared spectroscopy and chemical analysis [10,11]. Results are given in
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 8 4 - 0
70
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Table 1 Youssoufia natural phosphate analysisa
4. Results
Elements
(%)
Trace elements
P C S Si Ca Mg Fe Al Na F Cl
13.51 1.82 0.74 1.52 36.20 0.46 0.11 0.17 0.58 3.68 0.02
As Cd Cr Cu Sr Pb Th U V Zn Rare earths
a
ppm 7 37 306 28 1300 800 9 119 245 425 460
Moroccan phosphate research center.
Table 2. The ratios metal/phosphorus (Metal/P) which is close to 1.67 indicates that these ¯uoroapatites are indeed stoicheometric. 3. Experimental protocol Fig. 1 shows the experimental procedure used for this study. The sample was placed in a rectangular platinum crucible in a steel tubular furnace. The chamber is evacuated before introducing a measured volume of water. The pressure is ®xed by the gradual release of the water steam controlled by a release valve adjusted to the desired pressure. Treatments were performed for 2 h at variable temperatures and pressures. Several preliminary tests were made in order to optimize some operating conditions, especially the treatment time.
4.1. Natural phosphate The natural phosphate product was mainly apatite with a hexagonal structure (space group P63/m). After treatment at various temperatures, only a small displacement of the Xray diffraction peaks is observed. This corresponds to a very small change of the cell parameters at 8008C (from Ê and c 6.852 A Ê before treatment to a 9.352 A Ê Ê a 9.362 A and c 6.861 A after). However, the infrared spectra show the appearance of the characteristic vibration bands of OHÿ groups at 3530 cmÿ1 together with a band at 745 cmÿ1 related to the hydrogen-bonded H±F. For a ®xed time of treatment, the intensity of the OH vibrations and of the H±F band increase with the temperature and the pressure of treatment (Figs. 2 and 3). The presence of some OHÿ ions with Fÿ perturbs the OH band frequencies of the pure calcium hydroxyapatite at 3560 and 630 cmÿ1 respectively to about 3530 and 745 cmÿ1 for ¯uorinated calcium hydroxyapatite. The ¯uorine content was determined by a potentiometric method with a ¯uoride ion-selective electrode [12]. Fig. 4 shows the change of the de¯uorination percentage as a function of the treatment temperature and steam pressure of the natural phosphate treated. We have observed no de¯uorination below 6008C and the steam pressure has an important role, especially for values less than 2 atm. 4.2. Synthetic fluoroapatites M10(PO4)6F2 Several factors can affect the ¯uoride de¯uorination rate, especially the chemical composition. The size of the con-
Table 2 Cell parameters and chemical analysis of MFAp fluoroapatites MFAp
CaFAp SrFAp PbFAp BaFAp
Cell parameters Ê) a (A
Ê) c (A
9.375 9.772 9.755 10.104
6.875 7.200 7.245 7.693
Metal/P ratio (0.04)
% F 0.1 (measured)
% F (theoretical)
1.68 1.67 1.64 1.65
3.7 2.6 1.3 1.8
3.77 2.56 1.42 1.92
Fig. 1. Experimental protocol for the defluorination process.
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Fig. 2. Infrared spectra of natural phosphate treated at various temperatures and a steam pressure of 2 atm: (a) before heat treatment; (b) after heat treatment.
71
stituents may affect the diameter of the tunnels containing the ¯uoride ions and drastically modify its mobility. In order to clarify and to demonstrate such a correlation between the structure and the extraction rate, we have prepared ¯uoroapatites containing Ca2, Sr2, Pb2 and Ba2. X-ray diffraction diagrams obtained for different products MFAp shows that these samples are pure ¯uoroapatites. They all have the crystalline hexagonal system P63/m. The cell parameters values of a and c obtained from least square ®tting to the peak positions are summarized in Table 2. We note an increase in a and c going from Ca, Sr, Pb to Ba related to ionic radius of these cations. The de¯uorination studies on MFAp samples has been performed under the same conditions as described above. Infrared spectra of these samples after de¯uorination at 8008C and a pressure of two atmospheres, are presented in Fig. 5. The intensity of the OH vibration bands increases from Sr, Ca, Ba to Pb-¯uoroapatite. Also, higher temperatures of treatment lead to a higher OHÿ band intensity as shown in the case of CaFAp in Fig. 6. The amount of ¯uorine removed from the apatite as a function of the steam pressure for various temperatures for CaFAp is reported in Fig. 7. X-ray diffraction shows that the apatite structure is maintained through out the reaction but with modi®cation of the cell parameters. They are reported in Table 3 for the different cations studied and different OHÿ contents. The substitution of Fÿ by the larger OHÿ ions results in larger cell parameters. It can be also noted in Table 3 that the PbFAp gives the highest exchange rate of Fÿ by OHÿ ions.
Fig. 3. Infrared spectra of natural phosphate before and after treatment at 8008C as a function of steam pressure: (a) before heat treatment; (b) after heat treatment.
Fig. 4. Defluorination percentage from natural phosphate (Morocco) as a function of the steam pressure at different temperatures.
Fig. 5. Infrared spectra of synthetic fluoroapatites MFAp samples treated at T 8008C and P 2 atm.
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A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Fig. 6. IR spectra of synthetic fluoroapatite CaFAp before and after defluorination as a function of the heated temperature (P 2 atm).
Fig. 7. Defluorination percentage as a function of the steam pressure for synthetic calcium fluoroapatite CaFAp.
5. Discussions Partial de¯uorination for all the studied compounds (natural phosphate and synthetic ¯uoroapatites) has been observed. This de¯uorination has been provided by the ¯uorine analysis.
The OH vibration bands in all apatite spectra are situated in the region 3500±3530 cmÿ1. That band appears generally as a narrow absorption, whose position is indicative of the OH lattice attractive interaction, which has been attributed to the existence of H±F hydrogen bonding. This indicates the formation of the ¯uorinated hydroxyapatite Ca10(PO4)6F2ÿx(OH)x [13,14]. The reduction of the ¯uorine content in the samples when the treatment temperature and the steam pressure increase is also indicated by an increase in the a and c lattice parameters (Table 3). The replacement in the apatite structure of calcium by barium or lead increases the diameter of tunnels in apatite structure, and favours the ¯uoride mobility. Thus, we have observed that the ¯uoride extraction is greater in PbFAp than in other ¯uoroapatites. These results can be also explained by a higher polarizability of the lattice and a higher covalence of the structure in the case of Pb [12]. It is certain that the de¯uorination process depends on the physico-chemical characteristic of the apatite, in addition to the role played by the temperature and the pressure. De¯uorination rates are in accordance with the mobility studies of solid MFAp using a complex impedance method
Table 3 Physico-chemical characteristics of some synthetic fluoroapatites Fluoroapatites
SrFAp
CaFAp
BaFAp
PbFAp
Defluorination (%) (T 8008C, P 2 atm, 2 h) Cell parameters Before defluorination Ê) a (A Ê) c (A After defluorination Ê) a (A Ê) c (A Polarizability of cation (10ÿ24 cm) [12] Ionic radius of cation [16]
10.08
29.62
36.33
42.22
9.772 7.200
9.375 6.875
10.107 7.673
9.755 7.245
9.780 7.209 0.9 1.12
9.386 6.882 0.6 0.99
10.181 7.692 1.6 1.34
9.849 7.277 3.6 1.24
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
73
Fig. 8. Arrhenius plots of ionic conductivity for MFAp fluoroapatites (M Ca, Sr, Ba, Pb).
[10,15]. Ionic conductivities in these compounds follow an Arrhenius law according to nature of the cation (Fig. 8). The apparent activation energies for conduction were calculated from the slopes of the Arrhenius plots. We obtain a progressive decrease of the activation energy from CaFAp (1.86 eV), SrFAp (1.49 eV), BaFAp (1.47 eV) to PbFAp (1.36 eV). In these compounds, Fÿ ions are considered the major charge carrier in the 400±9008C temperature range, which indicated a notable mobility especially in PbFAp (increase of and decrease of Ea). The high polarizability of Pb2 and the widening of tunnels by the introduction of the big cations favour the increase of the ¯uoride mobility. Ionic conductivity in ¯uoroapatites with tunnels, the mobile ¯uoride ion will move from the occupied site to the vacant site in a one-dimensional channel. Electrical property results can be related to gas sensors and catalytic properties which relate to ionic mobility [17,18]. To summarize, we have demonstrated the temperature, the pressure effects and the polarizing power of cation M2 on ¯uoride mobility on apatite structures. Similar studies can be made on the other Moroccan natural phosphate (black phosphate) and the ¯uoride mobility can be estimated simultaneously by complex impedance measurements. 6. Conclusion The present study on the temperature and the pressure treatment clearly shows ¯uoride extraction in the 600± 8008C temperature range in the presence of steam, without destruction of the apatite structure. De¯uorination results obtained from the Moroccan phosphate and from the similar synthetic ¯uoroapatites present approximately the same behaviour. We have also established some correlation between the physico-chemical properties of different ¯uoroapatites and their de¯uorination results.
Acknowledgements The authors would like to acknowledge material support given by the Moroccan Phosphate Research Center and to thank P. Barboux (Paris, France) for scienti®c and technical help. References [1] G. Montel, G. Bonel, J.C. Trombe, J.C. Heughebaert, C. Rey, 1er Congre International des ComposeÂs PhosphateÂs, Rabat, Morocco, 1977. [2] S. Zielinski, A. Szczepanik, Pr. Zem. Chem. 65 (1986) 205. [3] J.C. Lacout, Thesis, Toulouse, France, 1983. [4] M. Veiderma, M. Poldme, K. Tonsuardu, Chem. Tech. (Leipzig) 40 (1988) 169. [5] A. Bouhaouss, M. Ferhat, N. Gharbi, J. Livage, J. Chim. Phys. 81 (1984) 73. [6] O. Britel, Thesis, Rabat, Morocco, 1990. [7] C. Trombe, G. Montel, Ann. Chim. 5 (1980) 44. [8] Y. Deutsch, S. Sarica, I. Chem. Technol. Biotecnol. 30 (1980) 688. [9] J.C. Trombe, G. Montel, J. Inorg. Nucl. Chem. 40 (1975) 15. [10] A. Laghzizil, A. Bouhaouss, M. Ferhat, P. Barboux, R. Morineau, J. Livage, Adv. Mater. Res. 1-2 (1994) 479. [11] A. Laghzizil, Thesis, Rabat, Morocco, 1993. [12] G. Charlot, in: Masson et cie (Eds.), Analyse Quantitative Minerale, 1971. [13] G.C. Maiti, F. Freud, J. Chem. Soc. Dalton Trans. (1981) 949. [14] T. Takahashi, S. Tanase, O. Yamamoto, Electrochim. Acta 23 (1978) 369. [15] A. Laghzizil, A. Bouhaouss, P. Barboux, R. Morineau, J. Livage, Solid State Ionics 67 (1993) 137. [16] E.E. Dreenwood (Ed.), Edition Ionic Crystals, Lattice Defects and Non stoichiometry, London, 1968. [17] K. Yamashita, H. Owada, T. Umegaki, T. Kanazawa, K. Katayama, Solid State Ionics 40-41 (1990) 918. [18] K. Yamashita, H. Owada, T. Umegaki, T. Kanazawa, T. Futagami, Solid State Ionics 28-30 (1988) 660.
Removal of ¯uoride from moroccan phosphate and synthetic ¯uoroapatites A. Laghzizil*, N. Elhrech, O. Britel, A. Bouhaouss, M. Ferhat Laboratoire de Chim.Phys.GeÂn., DeÂpt. Chimie, Fac.Sc., Rabat, Morocco Received 5 July 1999; accepted 3 August 1999
Abstract Natural phosphate is of importance to the Moroccan economy because it is a raw material for the production of phosphoric acid and phosphate fertilizers. However, toxic elements such as ¯uorine are present in the ore and cause problems in the industrial processes. In the present work, we have studied the extraction of ¯uoride from various phosphates using a steam atmosphere at various temperatures. Natural phosphates and synthetic ¯uoroapatites have been studied. A correlation between the ¯uoride mobility in the structure and its extraction has been established. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Natural phosphate; Fluoroapatites; Fluorine; De¯uorination
1. Introduction Fluoroapatite is a common source of phosphates, particularly in the Moroccan natural phosphates, because it is the most stable and insoluble of all the calcium phosphates. It occurs in a wide range of substituted forms, both biological and mineralogical [1]. Apatites are important in the production of phosphoric acid and phosphate fertilizers [2±4]. However, they may contain toxic elements which cause industrial and ecological problems [5±7]. So, large efforts are devoted to the removal of these toxic elements, among them ¯uorine. Therefore, the de¯uorination of natural phosphates will result in an improvement in the fertilizer properties of phosphates as well as economic added value and better environmental protection [6]. To date, there has been no attempt to remove the ¯uoride ions from Moroccan phosphate using thermal treatments at various temperatures and pressures. In previous works, the de¯uorination of natural phosphate in world rock (Russian phosphates) has presented some dif®culties [6±8]. Deutsch and Sarica have suggested a possibility for the substitution of Fÿ by OHÿ ions using a hydrothermal method. However, only a partial substitution is possible at temperatures higher than 10008C. Such high temperatures may lead to a decomposition of the carbonated apatite [9]. There is evidence that
* Corresponding author.. E-mail address: [email protected] (A. Laghzizil)
modi®cation of crystal growth of apatite gives several problems in phosphate industry products because their solubility and other properties can be modi®ed. The present investigation including structural studies on Moroccan phosphate (Youssou®a phosphate type) and synthetic apatite systems, is required before this stabilizing effect can be satisfactorily explained. Heat treatment under steam pressure favours the de¯uorination process. In order to model the mechanisms of the de¯uorination process, we have prepared various substituted M10(PO4)6F2 (noted as MFAp) (where M Ca, Sr, Pb and Ba), because these compounds are present in the Moroccan phosphates. Correlations between de¯uorination results on ¯uoroapatites and their physico-chemical properties have been established. 2. Materials The Youssou®a natural phosphate (Morocco) is constituted essentially of carbonated ¯uoroapatite of B-type Ca10ÿxy(PO4)6ÿx(CO3)xF2ÿx2y where 0 y x and has been provided by the Moroccan phosphate research center [5±7]. The different elemental constituents of this mineral are given in Table 1. MFAp ¯uoroapatites have been prepared by precipitation at 808C, in ammoniacal solutions containing (NH4)2HPO4, NH4F and M(NO3)2 (where M Ca, Sr, Ba, Pb). The ¯uoroapatite structure has been con®rmed by X-ray powder diffraction, infrared spectroscopy and chemical analysis [10,11]. Results are given in
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 8 4 - 0
70
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Table 1 Youssoufia natural phosphate analysisa
4. Results
Elements
(%)
Trace elements
P C S Si Ca Mg Fe Al Na F Cl
13.51 1.82 0.74 1.52 36.20 0.46 0.11 0.17 0.58 3.68 0.02
As Cd Cr Cu Sr Pb Th U V Zn Rare earths
a
ppm 7 37 306 28 1300 800 9 119 245 425 460
Moroccan phosphate research center.
Table 2. The ratios metal/phosphorus (Metal/P) which is close to 1.67 indicates that these ¯uoroapatites are indeed stoicheometric. 3. Experimental protocol Fig. 1 shows the experimental procedure used for this study. The sample was placed in a rectangular platinum crucible in a steel tubular furnace. The chamber is evacuated before introducing a measured volume of water. The pressure is ®xed by the gradual release of the water steam controlled by a release valve adjusted to the desired pressure. Treatments were performed for 2 h at variable temperatures and pressures. Several preliminary tests were made in order to optimize some operating conditions, especially the treatment time.
4.1. Natural phosphate The natural phosphate product was mainly apatite with a hexagonal structure (space group P63/m). After treatment at various temperatures, only a small displacement of the Xray diffraction peaks is observed. This corresponds to a very small change of the cell parameters at 8008C (from Ê and c 6.852 A Ê before treatment to a 9.352 A Ê Ê a 9.362 A and c 6.861 A after). However, the infrared spectra show the appearance of the characteristic vibration bands of OHÿ groups at 3530 cmÿ1 together with a band at 745 cmÿ1 related to the hydrogen-bonded H±F. For a ®xed time of treatment, the intensity of the OH vibrations and of the H±F band increase with the temperature and the pressure of treatment (Figs. 2 and 3). The presence of some OHÿ ions with Fÿ perturbs the OH band frequencies of the pure calcium hydroxyapatite at 3560 and 630 cmÿ1 respectively to about 3530 and 745 cmÿ1 for ¯uorinated calcium hydroxyapatite. The ¯uorine content was determined by a potentiometric method with a ¯uoride ion-selective electrode [12]. Fig. 4 shows the change of the de¯uorination percentage as a function of the treatment temperature and steam pressure of the natural phosphate treated. We have observed no de¯uorination below 6008C and the steam pressure has an important role, especially for values less than 2 atm. 4.2. Synthetic fluoroapatites M10(PO4)6F2 Several factors can affect the ¯uoride de¯uorination rate, especially the chemical composition. The size of the con-
Table 2 Cell parameters and chemical analysis of MFAp fluoroapatites MFAp
CaFAp SrFAp PbFAp BaFAp
Cell parameters Ê) a (A
Ê) c (A
9.375 9.772 9.755 10.104
6.875 7.200 7.245 7.693
Metal/P ratio (0.04)
% F 0.1 (measured)
% F (theoretical)
1.68 1.67 1.64 1.65
3.7 2.6 1.3 1.8
3.77 2.56 1.42 1.92
Fig. 1. Experimental protocol for the defluorination process.
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Fig. 2. Infrared spectra of natural phosphate treated at various temperatures and a steam pressure of 2 atm: (a) before heat treatment; (b) after heat treatment.
71
stituents may affect the diameter of the tunnels containing the ¯uoride ions and drastically modify its mobility. In order to clarify and to demonstrate such a correlation between the structure and the extraction rate, we have prepared ¯uoroapatites containing Ca2, Sr2, Pb2 and Ba2. X-ray diffraction diagrams obtained for different products MFAp shows that these samples are pure ¯uoroapatites. They all have the crystalline hexagonal system P63/m. The cell parameters values of a and c obtained from least square ®tting to the peak positions are summarized in Table 2. We note an increase in a and c going from Ca, Sr, Pb to Ba related to ionic radius of these cations. The de¯uorination studies on MFAp samples has been performed under the same conditions as described above. Infrared spectra of these samples after de¯uorination at 8008C and a pressure of two atmospheres, are presented in Fig. 5. The intensity of the OH vibration bands increases from Sr, Ca, Ba to Pb-¯uoroapatite. Also, higher temperatures of treatment lead to a higher OHÿ band intensity as shown in the case of CaFAp in Fig. 6. The amount of ¯uorine removed from the apatite as a function of the steam pressure for various temperatures for CaFAp is reported in Fig. 7. X-ray diffraction shows that the apatite structure is maintained through out the reaction but with modi®cation of the cell parameters. They are reported in Table 3 for the different cations studied and different OHÿ contents. The substitution of Fÿ by the larger OHÿ ions results in larger cell parameters. It can be also noted in Table 3 that the PbFAp gives the highest exchange rate of Fÿ by OHÿ ions.
Fig. 3. Infrared spectra of natural phosphate before and after treatment at 8008C as a function of steam pressure: (a) before heat treatment; (b) after heat treatment.
Fig. 4. Defluorination percentage from natural phosphate (Morocco) as a function of the steam pressure at different temperatures.
Fig. 5. Infrared spectra of synthetic fluoroapatites MFAp samples treated at T 8008C and P 2 atm.
72
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
Fig. 6. IR spectra of synthetic fluoroapatite CaFAp before and after defluorination as a function of the heated temperature (P 2 atm).
Fig. 7. Defluorination percentage as a function of the steam pressure for synthetic calcium fluoroapatite CaFAp.
5. Discussions Partial de¯uorination for all the studied compounds (natural phosphate and synthetic ¯uoroapatites) has been observed. This de¯uorination has been provided by the ¯uorine analysis.
The OH vibration bands in all apatite spectra are situated in the region 3500±3530 cmÿ1. That band appears generally as a narrow absorption, whose position is indicative of the OH lattice attractive interaction, which has been attributed to the existence of H±F hydrogen bonding. This indicates the formation of the ¯uorinated hydroxyapatite Ca10(PO4)6F2ÿx(OH)x [13,14]. The reduction of the ¯uorine content in the samples when the treatment temperature and the steam pressure increase is also indicated by an increase in the a and c lattice parameters (Table 3). The replacement in the apatite structure of calcium by barium or lead increases the diameter of tunnels in apatite structure, and favours the ¯uoride mobility. Thus, we have observed that the ¯uoride extraction is greater in PbFAp than in other ¯uoroapatites. These results can be also explained by a higher polarizability of the lattice and a higher covalence of the structure in the case of Pb [12]. It is certain that the de¯uorination process depends on the physico-chemical characteristic of the apatite, in addition to the role played by the temperature and the pressure. De¯uorination rates are in accordance with the mobility studies of solid MFAp using a complex impedance method
Table 3 Physico-chemical characteristics of some synthetic fluoroapatites Fluoroapatites
SrFAp
CaFAp
BaFAp
PbFAp
Defluorination (%) (T 8008C, P 2 atm, 2 h) Cell parameters Before defluorination Ê) a (A Ê) c (A After defluorination Ê) a (A Ê) c (A Polarizability of cation (10ÿ24 cm) [12] Ionic radius of cation [16]
10.08
29.62
36.33
42.22
9.772 7.200
9.375 6.875
10.107 7.673
9.755 7.245
9.780 7.209 0.9 1.12
9.386 6.882 0.6 0.99
10.181 7.692 1.6 1.34
9.849 7.277 3.6 1.24
A. Laghzizil et al. / Journal of Fluorine Chemistry 101 (2000) 69±73
73
Fig. 8. Arrhenius plots of ionic conductivity for MFAp fluoroapatites (M Ca, Sr, Ba, Pb).
[10,15]. Ionic conductivities in these compounds follow an Arrhenius law according to nature of the cation (Fig. 8). The apparent activation energies for conduction were calculated from the slopes of the Arrhenius plots. We obtain a progressive decrease of the activation energy from CaFAp (1.86 eV), SrFAp (1.49 eV), BaFAp (1.47 eV) to PbFAp (1.36 eV). In these compounds, Fÿ ions are considered the major charge carrier in the 400±9008C temperature range, which indicated a notable mobility especially in PbFAp (increase of and decrease of Ea). The high polarizability of Pb2 and the widening of tunnels by the introduction of the big cations favour the increase of the ¯uoride mobility. Ionic conductivity in ¯uoroapatites with tunnels, the mobile ¯uoride ion will move from the occupied site to the vacant site in a one-dimensional channel. Electrical property results can be related to gas sensors and catalytic properties which relate to ionic mobility [17,18]. To summarize, we have demonstrated the temperature, the pressure effects and the polarizing power of cation M2 on ¯uoride mobility on apatite structures. Similar studies can be made on the other Moroccan natural phosphate (black phosphate) and the ¯uoride mobility can be estimated simultaneously by complex impedance measurements. 6. Conclusion The present study on the temperature and the pressure treatment clearly shows ¯uoride extraction in the 600± 8008C temperature range in the presence of steam, without destruction of the apatite structure. De¯uorination results obtained from the Moroccan phosphate and from the similar synthetic ¯uoroapatites present approximately the same behaviour. We have also established some correlation between the physico-chemical properties of different ¯uoroapatites and their de¯uorination results.
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