**4.1.2. Precipitation method**

The precipitation method is based on the combination reaction(s) when cations and anions in the solution combine to form insoluble ionic solid, so-called precipitate. The method can be divided to [35]:


$$2\text{ (NH}\_2\text{)}\_2\text{CO} + 3\text{ H}\_2\text{O} \xrightarrow{\text{row}} 2\text{ NH}\_4\text{OH} + \text{CO}\_2\tag{10}$$


Wet techniques of apatite preparation are based on the precipitation from solution at ambi‐ ent temperature [67]. The preparation techniques based on aqueous precipitation at moder‐ ate temperatures often lead to non-stoichiometric apatites [68]. Hydroxylapatite close to the ideal formula, can be precipitated by the addition of Ca(OH)2 to diluted phosphoric acid and complete neutralization at the boiling point [69]

$$16\text{ H}\_3\text{PO}\_4 + 10\text{ Ca}(\text{OH})\_2 \rightarrow \text{Ca}\_{10}\text{(PO}\_4)\_6\text{(OH)}\_2 + 18\text{ H}\_2\text{O}\tag{11}$$

Precipitated hydroxylapatite shows extremely small crystal sizes (hexagonal plates ~200Å sides) and large surface area from 50 to 200 m2 ·g−1.

The Eh-pH diagrams for the Ca-P-H2O system at 25 and 300°C for 1.67 mol activity of Ca and 1 mol activity of P (*a*Ca = 1.67*a*P) under the pressure of 1 bar are shown in **Fig. 8**(**a**) and (**b**), respectively. The P*a*Ca-pH diagrams, where P*a*Ca = -*log a*Ca for this system show that the pH of minimum solubility of HAP clearly decreases with increasing temperature. At each tempera‐

**Fig. 8.** Eh-pH and PaCa-pH diagram of Ca-P-H2O system at 25°C (a) and 300°C (b).

phase transition involving the reduction in unit-cell symmetry from hexagonal to monoclin‐

Ca<Sr<Ba<Pb<Cd,

and the monoclinic phases are less anisotropic but have larger thermal expansion coeffi‐

The precipitation method is based on the combination reaction(s) when cations and anions in the solution combine to form insoluble ionic solid, so-called precipitate. The method can be

**i. Direct precipitation method** is based on the reaction of neutralization and precipi‐ tation. The precipitate is then separated from the solution via filtration.

**ii. Homogeneous precipitation method** does not need precipitants because decom‐

**iii. Coprecipitation method** is initiated by the addition of precipitant to mixed-salt

**iv. Compound precipitation method** is the precipitation of stoichiometric compounds

Wet techniques of apatite preparation are based on the precipitation from solution at ambi‐ ent temperature [67]. The preparation techniques based on aqueous precipitation at moder‐ ate temperatures often lead to non-stoichiometric apatites [68]. Hydroxylapatite close to the ideal formula, can be precipitated by the addition of Ca(OH)2 to diluted phosphoric acid and

Precipitated hydroxylapatite shows extremely small crystal sizes (hexagonal plates ~200Å

The Eh-pH diagrams for the Ca-P-H2O system at 25 and 300°C for 1.67 mol activity of Ca and 1 mol activity of P (*a*Ca = 1.67*a*P) under the pressure of 1 bar are shown in **Fig. 8**(**a**) and (**b**), respectively. The P*a*Ca-pH diagrams, where P*a*Ca = -*log a*Ca for this system show that the pH of minimum solubility of HAP clearly decreases with increasing temperature. At each tempera‐

·g−1.

3 4 ( ) ( )( ) <sup>2</sup> 10 4 6 2 <sup>2</sup> 6 H PO 10 Ca OH Ca PO OH 18 H O +® + (11)

hydroxide and formed NH4OH acts as the precipitant:

( ) 70°C

posed chemical acts as the precipitant, e.g. urea is decomposed in ammonium

2 2 42 <sup>2</sup> NH CO 3 H O 2 NH OH CO +® + (10)

ic. The thermal expansion anisotropy in the hexagonal phases increases in the order:

190 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

cients in comparison with the hexagonal phases.

**4.1.2. Precipitation method**

solution.

from the solution.

complete neutralization at the boiling point [69]

sides) and large surface area from 50 to 200 m2

divided to [35]:

ture, HAP predominates in higher pH range, while Ca3(PO4)2, Ca2P2O7 and CaH6P2O9 have predominates at lower pH [21].

The stability of calcium phosphates at higher temperatures is shown in **Fig. 9**. The equation numbers refer to the following reactions [21]:

$$\text{13 }\text{CaHPO}\_4 \leftrightarrow \text{Ca}\_3\text{(PO}\_4\text{)}\_2 + \text{H}\_3\text{PO}\_4 \tag{12}$$

$$\text{3 Ca} \left(\text{H}\_2\text{PO}\_4\right)\_2 \leftrightarrow \text{Ca}\_3\text{(PO}\_4\text{)}\_2 + \text{4 H}\_3\text{PO}\_4 \tag{13}$$

$$\text{Ca}\_{10}\text{H}\_{2}\text{P}\_{6}\text{O}\_{26} \leftrightarrow \text{3 Ca}\_{3}\text{(PO}\_{4}\text{)}\_{2} + \text{CaO} + \text{H}\_{2}\text{O} \tag{14}$$

$$\text{CaH}\_{\text{g}}\text{PO}\_{6} \leftrightarrow \text{CaHPO}\_{4} + 2\text{ H}\_{2}\text{O} \tag{15}$$

$$\text{CaH}\_6\text{P}\_2\text{O}\_9 \leftrightarrow \text{Ca}\left(\text{H}\_2\text{PO}\_4\right)\_2 + \text{H}\_2\text{O} \tag{16}$$

**Fig. 9.** Temperature dependence of free energy of reaction for some calcium phosphates according to **Eqs. 12–23** for water vapor fugacity equal to 0.03 atm., except for the dashed line **Eq. 14´**, which corresponds to water vapor fugacity equal to 1 atm. [21].

**Fig. 10.** Eh-pH diagram of Ca-P-H2O system at 25°C (a), 100°C (b), 200°C (c) and 300°C (d).

For the purpose of this book the calculation of Eh-pH diagram for the solution where the concentration ofions (Ca2+, PO4 3− and OH<sup>−</sup> ) is equivalent to the system containing 5·10−3 mol·dm −3 of apatite was performed. The ionic strength (refer to **Footnote 31** in **Section 3.4.1**) of that solution enables the calculation of activity of Ca and P using the activity coefficient estimat‐ ed from modified Davies equation (refer to **Footnote 31** in **Section 3.4.1**) and the concentra‐ tion of calcium as follows: *a*Ca = 1.062·10−3 and *a*P = 1.67·*a*Ca = 6.36·10−4. If the activity of ions is used instead of its concentration, the Eh-pH in **Fig. 10** can be calculated.

192 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

–2 000 –4 000 –6 000 –8 000 –10 000

Free Energy of Reaction [J]

equal to 1 atm. [21].

0 200 400 800 1200

**Fig. 9.** Temperature dependence of free energy of reaction for some calcium phosphates according to **Eqs. 12–23** for water vapor fugacity equal to 0.03 atm., except for the dashed line **Eq. 14´**, which corresponds to water vapor fugacity

**Fig. 10.** Eh-pH diagram of Ca-P-H2O system at 25°C (a), 100°C (b), 200°C (c) and 300°C (d).

Eq. 16

Eq. 15

Eq. 12

Eq. 14

Eq. 14'

Eq. 13

Temperature [k]

1600

In this system,35 CaHPO4·2H2O is stable under ambient temperature and nearly neutral pH. Hydroxylapatite becomes stable at the pH higher than 7.5. With increasing temperature, the formation of HAP instead of CaHPO4·2H2O is more probable. Minimal solubility of hydroxy‐ lapatite is then shifted to significantly lower pH than for **Fig. 8**.

**Fig. 11.** Eh-pH diagram of Ca-P-H2O system with the concentration 20× higher than for that in **Fig. 10** at 25°C (a), 100°C (b), 200°C (c) and 300°C (d).

Other difference is a fact, that the field of stability of Ca(OH)2 starts at the pH = 13 forthe system with elevated temperature. The formation of CaHPO4·2H2O, Ca2P2O7 and Ca3(PO4)2 was not predicted.

The calculation for 20-times higher concentration **Fig. 11** than for the system mentioned above shows broadening field of CaHPO4·2H2O. Ca2P2O7 was formed by the thermal condensation

<sup>35</sup> The main difference against to the systems on **Fig. 8** and **Fig. 11** is significantly lower ionic strength.

of CaHPO4 at temperatures higher than 164°C in acidic environment and Ca3(PO4)2 precipi‐ tated from the solution at nearly neutral conditions. Hydroxylapatite again predominates at higher pH and Ca(OH)2 does not appear at higher temperatures and the pH below 14 (the same as for **Fig. 8**).

The phase equilibrium in the system CaO-P2O5-H2O was extensively studied by the solid-state reaction method under the atmospheric pressure of water vapor by VAN WAZRER [70] and in aqueous systems at temperatures lower than 100°C by BROWN et al [71],[72]. BIGGAR [73] studied the CaO-P2O5-H2O system in the temperature range from 700 to 950°C and the pressure of 1 kbar. FENG and ROCKETT [74 ] studied the system CaO-P2O5-H2O at 1000 bar with 50%wt. and 200°C (**Fig. 12**).

**Fig. 12.** Phase diagram of Ca(OH)2-Ca3(PO4)2-H2O system [21].

The **molten** (**fused**) **salts**<sup>36</sup> **precipitation method** uses the precursor mixed with low melting point salt such as NaCl, KCl, or their eutectics. Upon the melting of the mixture, reactant oxides dissolve in the salt and desired compound precipitates due to its low solubility in molten salt.

<sup>36</sup> Fused salts are widely used in many industrial processes requiring to free the limitations arising from the use of aqueous solutions. Their thermal stability and generally low vapor pressure enable fast reaction rates and ability to dissolve many inorganic compounds making them useful solvents in electrometallurgy, metal coating, treatment of by-products, and energy conversion. It is recalled that one of the most important chemicals produced worldwide, sulfuric acid, is made by the molten salt catalysis. The electrolysis of molten salt is a technique used by H. MOISSAN for the isolation of element fluorine from the melt of KF·2HF (Moissan's method is used for industrial production of fluorine). It was also used by H. DAVY to discover several new elements (sodium, potassium, alkali metals) and to prove the chlorine as a new element (originally discovered by C.W. SHEELLE who considered it as "*dephlogisticated marine acid*"). Today the industri‐ al production of Li and Na is based on the electrolysis of eutectic melt of LiCl–KCl (or CaCl2) and NaCl–KCl (or CaCl2), respectively. The production of K, Rb and Cs is based on the reduction of molten KCl, RbCl and CsCl by Na at the temperature of 600°C. Molten salt method also plays significant role in the development of energy resources, including the reprocessing of nuclear wastes, molten carbonate/solid oxide fuel cells (**Section 10.4**), and high temperature molten salt batteries. Fused alkali nitrates/nitrites are valuable materials for the heat transport and storage in solar plants. Molten salt bathes remain of large use in industry for the treatment of steel and variety of other metals as well as nonmetals, such as glass, plastics and rubber [75].

The melt is cooled down, and the salt is dissolved to yield the powder product of synthesis [75], [76].

The structure of a molten salt is characterized by an alteration of positively and negatively charged ionic solvation shells around a given ion. This arises from the predominance of Coulombic effects, which results in a strong attraction between oppositely charged species and a strong repulsion otherwise [75].

The utilization of molten salt precipitation method for the synthesis of apatites at "moderate temperatures" in the range from 500 to 700°C was also reported. Based on its principle, the method combines the advantages of thermal hydrolysis ("dry method") and the precipita‐ tion from the solution ("wet method"). As a reaction media, the chloride melt of the equimo‐ lar NaCl-KCl (665°C) composition as well as eutectic melt (390°C) in the system Li2CO3 (27)– Na2CO3 (28)–K2CO3 (45% mol.) can be used. The most probable reactions are estimated from the thermodynamic consideration as follows [22]:

$$66\text{ KPO}\_3 + 6\text{ CaO} + 3\text{ CaO}\_3 + \text{Ca(OH)}\_2 \rightarrow \text{HAP} + 3\text{ K}\_2\text{CO}\_3\tag{17}$$

$$66\text{ NaPO}\_3 + 6\text{ CaO} + 3\text{ CaCO}\_3 + \text{Ca(OH)}\_2 \rightarrow \text{HAP} + 3\text{ Na}\_2\text{CO}\_3\tag{18}$$

$$66\text{ KPO}\_3 + 6\text{ CaO} + 3\text{ CaO}\_3 + \text{CaF}\_2 \rightarrow \text{FAP} + 3\text{ K}\_2\text{CO}\_3\tag{19}$$

$$66\text{ NaPO}\_3 + 6\text{ CaO} + 3\text{ CaCO}\_3 + \text{CaF}\_2 \rightarrow \text{FAP} + 3\text{ Na}\_2\text{CO}\_3\tag{20}$$
