Fluoridated apatites fluoridation reactions

Modem production of elemental phosphoras uses a technique similar to the metallurgical processes described in Chapter 20. Apatite is mixed with silica and coke and then heated strongly in the absence of oxygen. Under these conditions, coke reduces phosphate to elemental phosphoms, the silica forms liquid calcium silicate, and the fluoride ions in apatite dissolve In the liquid calcium silicate. The reactions are not fully understood, but the stoichiometry for the calcium phosphate part of apatite is as follows ... 

Several high-temperature methods leading to fluoridated apatites can be found in the literature they involve solid-gas reaction, pyrolysis or crystal growth processes. 

Fluorhydroxyapatite can be synthesised by the traditional double-decomposition method generally used for apatite precipitation. An ammonium phosphate and fluoride solution (solution B) is added, dropwise, into a hot (generally at boiling temperature) calcium solution (solution A) at a basic pH level as previously published [122,123]. Fluorapatites close to stoichiometry are obtained (a = 2, see the following reaction equation) however, a very small residual amount of OH always seems to be present. Filtration and several washing operations are necessary to remove the counter-ions. The reaction is almost total due to the very low solubility of fluorhydroxyapatites. 

Low-temperature exchange reactions have been described forfluorhydroxyapatite solid solutions [115,130,131], They generally occur in aqueous media and in most instances involve a dissolution-reprecipitation mechanism. Such reactions may be used to partly or totally modify the surface composition of ceramics or coatings. In order to observe such reactions, the resulting apatites should be less soluble than the starting compounds in the solution conditions [132], This is the case, for example, with fluoride uptake by HA. 

Fluoridated apatite crystals can grow using the dual membrane system involving on the one hand a calcium acetate solution and on the other hand a phosphate solution at physiological temperature with a pH of 6.5. lijima et al. showed that the combination of fluoride ions, added to the phosphate solution, and amelogenin (a major protein in the enamel extracellular matrix), present in the reaction space between the two membranes, controlled the transformation of octacalcium phosphate (OCP) into fine rod-like fluoridated apatite crystals with habit, size... 

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

We studied (2) the mechanism of formation of fluorapatite from apatitic tricalcium phosphate and calcium fluoride in the solid state. Overall, the chemical reaction may be written ... 

The most important sources of phosphorus are phosphate rocks containing either apatite, (a mixed fluoride-phosphate of calcium, Ca2FP04 Ca3(P04)2), or calcium phosphate itself. These yield elemental phosphorus when heated with a mixture of carbon and silica the latter forms a fusible slag with the CaO formed during the reaction, and the phosphorus formed from reduction by the carbon is distilled away from the mixture. 

Traces of fluoride can be separated from aqueous solutions by extraction with triethylchlorsilane in m-xylene The conditions were optimized and co-precipitation (for enrichment) reactions were tested Adsorption on hydroxyl apatite was found most suitable. From acidified solutions of biological materials fluoride was selectively extracted with a solution of trimethylchlorsilane in benzeneOrganic material or blood was destroyed by heating with hydrochloric acid and hydrogen peroxide in a closed distilling apparatus. The resulting distillates were extracted by triethylchlorsilane in tetrachloroethylene ... 

This reaction takes place in two steps anhydrite and phosphoric acid being formed in the fast first step. This phosphoric acid then reacts slowly - over a period of weeks - ( curing ) with further apatite producing mono-calcium phosphate hydrate. Part (ca. 10 to 40%) of the fluoride contained in the apatite is expelled in the form of gaseous... 

The first and primary protective effect of fluoride is due to its strong, spontaneous reaction with metal ions. Biologically, the most important of these ions is the calcium ion, large amounts of which interact with phosphate to form bones and teeth. Studies show that fluoride reduces apatite solubility in acids by an isomorphic replacement of hydroxide ions with fluoride ions to form fluoro-hydroxyapatite and difluoro-apatite (Fig. 16.6a). 

Sodium fluoride occurs naturally as the mineral villiau-mite, although the compound is not produced commercially from that source. Some sodium fluoride is obtained as a byproduct of the manufacture of phosphate fertilizers. In that process, apatite (a form of calcium phosphate that also contains fluorides and/or chlorides) is crushed and treated with sulfuric acid (H2S04). The products of that reaction include phosphoric acid (H3P04), calcium sulfate (CaS04),... 

These techniques are bas not only on the principle that lead-containing phosphates with the apatite structure are highly insoluble, but also that rapid reactions occur with apatite and lead ions at the sohd/aqueous solution interface [12, 13, 15, 20, 29, 48, 53, 56]. Removal of lead from aqueous solutions using synthetic hydroxyapatite gives aqueous lead concentrations below the maximum contamination level after Ih [12, 53]. Other workers [9] observed the formation of calcium-lead apatite solid-solutions after 3 mins contact between synthetic hydroxyapatite and aqueous solutions containing lead, and no lead was detected in the aqueous solution after 24 h contact. However, the efficiency of lead removal depends on the characteristics of the phosphate rock employed [15]. It has been shown that the composition and crystallinity of the phosphate influence the speed of the surface reactions [4, 44]. More highly crystalline solids have lower solubilities and dissolution rates, making the apatite less reactive [4]. The presence of fluoride in the hydroxyapatite structure decreases its solubility and dissolution rate, while the presence of carbonate decreases structural stability, and increases solubility and the dissolution rate [4, 35]. 

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