Several major synthesis routes are used for preparing apatites: precipitation in an aqueous route, solid-solid reaction (dry route), reaction in coalesced salts, sol-gel procedure and the cements route.
Reactions in aqueous phase
Syntheses in an aqueous phase are produced via two different processes: the double decomposition and the neutralisation method. These processes are currently used for the industrial production of apatite.
The double decomposition method [Hayek E.; 1963; Trombe, 1972; Arends, 1987] consists of adding a Me cation salt solution in a controlled manner to a solution of XO4 anion salt. The precipitate is then washed and dried. This technique also provides mixed apatites (containing two different cations) with control of the Me1/Me2 ratio. The cations are introduced simultaneously into the reactor with the desired Me1/Me2 ratio, enabling segregation during precipitation to be avoided. The main drawbacks in this method stem from its method of implementation, which requires a lot of equipment and its synthesis speed which is rather slow.
The neutralisation method consists of neutralising a milk of lime solution by adding a phosphoric acid solution. This reaction rapidly provides large quantities of phosphocalcic hydroxyapatite using little equipment [Trombe, 1972; Osaka, 1991]. It is also possible to synthesise fluorapatites.
Reactions in solid phase
Synthesis by solid-solid reaction consist of heating a reactional mixture consisting of various cation and anion salts in a Me/XO4 ratio of 1.67. This mixture must be perfectly homogeneous to provide a total reaction. A phosphocalcic fluorapatite can be synthesised, for example, from tricalcic phosphate and calcium fluoride via the following reaction [Wallaeys R, 1952] :
3 Ca3(PO4)2 + CaF2 ------------------------------------- Ca10(PO4)6F2
This reaction takes place at 900°C over several hours.
In the case of a solid/gas reaction, the gas comes either from sublimation of a solid salt contained in the reactional mixture – synthesis can then take place in a closed chamber – or by external contribution in the form of flushing the reactional gas, for example.
Reactions in coalesced salts.
This method provides a close approximation of the natural conditions for synthesising certain apatites. Metallic phosphates with an apatitic structure have been prepared in this way. Crystals, more often than not excessively mixed with the initial reactive agents can thereby be obtained.
Sol-gel reactions
The sol-gel process is based on polymerising organo-metallic precursors of the M(OR)n alcoxide type. After controlled hydrolysis of this alcoxide in solution, condensation of the monomers produces oxo bridges and then an organic oxide. Progressive polymerisation of these precursors forms oligomers and then polymer, thereby increasing the viscosity. These polymeric solutions produce gels that facilitate the forming of materials (dense, transparent films, ultra-fine powders, ceramics, etc.) with numerous technological applications [Livage, 1992].
Liquid/solid reaction: the cements’ synthesis route.
Phosphocalcic cements are hydraulic mineral cements that set by means of acid-base reactions between the calcium phosphates of an acidic nature and the calcium phosphates of a basic nature to provide a phosphocalcic hydroxyapatite in a single phase.
