**4. Effect of operating parameters on CaCO3 precipitation**

The solubility of CaCO3 polymorphs increases in the order of calcite, aragonite, and vaterite [26, 27]. Environmental conditions play effective roles in the crystal growth and morphological changes of CaCO3. Indeed, the nucleation and growth kinetics of CaCO3 as well as the morphology and polymorphism of the obtained precipitates are affected by various key operating parameters such as supersaturation [23, 28, 38], temperature [37, 39, 40], and pH [41–43]. These experimental parameters operate together and control the polymorphism of CaCO3. For example, it was shown that vaterite can be obtained at high superasaturations and low calcium concentrations typically below 10�<sup>2</sup> M [44], solution pH in the range 8.5–10 [45] and/or low temperatures in the range 20–40°C [45, 46].

Söhnel and Mullin [38] showed that the crystal growth rate of CaCO3 increased with increasing the solution supersaturation. Vaterite was favored in moderate (S < 6.5) to enhanced supersaturation solutions [28, 44] and smaller vaterite crystals were obtained when supersaturation increased [28]. High supersaturations in the range 4�14 prevented the transformation of vaterite into calcite and increased the crystal growth rate [16, 23].

It is worth noticing that the initial calcium concentration in the solution has an important impact on the polymorphism of CaCO3. Indeed, it was found that calcite did not crystallize at decreased Ca2+ concentrations (typically below 10�<sup>2</sup> M), and only vaterite was obtained when CaCO3 was prepared, at 25°C, by a CO2/N2 mixed gas bubbled into a solution of CaCO3 [23]. This agrees with the results of Korchef [16] who showed that vaterite was the predominant polymorph at 28°C, for a supersaturation in the range 4–14, and calcite was not detected, because of the low initial calcium concentration (4 � <sup>10</sup>�<sup>3</sup> M). Thus, for a solution supersaturated with respect to calcite (�15–62), aragonite (4–17) and vaterite (10–38), vaterite was favored for an initial calcium concentration 4 � <sup>10</sup>�<sup>3</sup> M (<10�<sup>2</sup> M), a precipitation pH in the range 8.4–8.8 and a temperature of 28°C [16].

#### *Effect of Operating Parameters and Foreign Ions on the Crystal Growth of Calcium Carbonate… DOI: http://dx.doi.org/10.5772/intechopen.94121*

The temperature and the solution pH are considered as critical parameters in controlling the kinetics crystallization, polymorphism, and morphology of CaCO3 particles. At room temperature, calcite is the most predominant phase. At temperatures higher than 50°C, CaCO3 precipitates as needles-like aragonite crystals [47–50]. At conditions of spontaneous CaCO3 formation preceded by the lapse of induction time, the increase of temperature, at fixed ionic strength, results to the decrease of the induction time and to the increase of the crystalline growth rate and also the amount precipitated [37]. Chen and Xiang [46] investigated the effect of temperature on the structures and morphology of CaCO3 crystals obtained by double injection of CaCl2 and NH4HCO3 solutions with a 1:1 molar ratio. At 3040°C, they obtained crystals of lamellar vaterite. At higher temperatures of 50–70°C, a mixture of calcite and aragonite formed, and only aragonite whiskers crystallized at a temperature of 80°C. However, aragonite was not detected when CaCO3 precipitated by mixing calcium acetate Ca(C2H3O2)2 and ammonium carbonate (NH4)2CO3 [40].

In their recent publication, Korchef and Touabi [17] studied the effect of temperature and initial solution pH on the crystallization of CaCO3 by CO2 repelling. They showed that vaterite and aragonite were the polymorphs of CaCO3 collected at 28°C and 50°C and the increase in temperature from 28–50°C leads to the acceleration of CaCO3 nucleation and crystal growth. This is was explained by higher supersaturations with respect to aragonite (in the range 40–69) and vaterite (between 12 and 21) obtained at 50°C than those obtained at 28°C. The increase of the initial solution pH accelerates the CaCO3 precipitation. Indeed, the solution supersaturation increases with increasing the initial pH of the solution. This accelerates the nucleation and growth of CaCO3. For example, at the initial solution pH 7, the supersaturation ratios with respect to calcite and vaterite are in the ranges 17–30 and 5–8, respectively. For the initial solution pH 9, CaCO3 precipitation is instantaneous and the supersaturation ratios with respect to calcite and vaterite significantly increase, i.e., in the range 51–84 for calcite and in the range 14 –23 for vaterite. However, at constant supersaturation, it was shown that the growth rate of calcite decreases with increasing pH from 7.5 to 12 [41]. This is because the surface concentration of active growth sites decreases with increasing pH of the solution. Ramakrishna et al. [42] prepared CaCO3 crystals by mixing Na2, CO3, and CaCl2 solutions injected simultaneously into distilled water at different pH values. They showed that pure aragonite needles were formed at pH 10 and a mixture of calcite and aragonite was obtained when increasing the pH from 11 to 12. At high pH values, typically greater than 12, calcite was favored [43], and at pH lower than 8, vaterite was obtained [23]. Using a constant composition method, Tai and Chen [45] showed that vaterite was the predominant phase obtained, at 24°C, in the pH range 8.510, which is comparable to the precipitation pH range obtained when CaCO3 was precipitated by the CO2 repelling method (8.48.8) [16, 17]. For solution pH lower than 7–7.5, no precipitation of CaCO3 was detected [17]. This result is of utmost importance in inhibiting CaCO3 precipitation during scale formation in water and wastewater treatment processes, as will be discussed in the last paragraph of the present chapter.
